The Early History of Radioactivity (1896-1904)

Thesis presented for the degree of Doctor of Philosophy

in the Field of History of Science

by

Stephen Brian Sinclair

Department of History of Science and Technology

Imperial College of Science and Technology University of London

May 1976

2

ABSTRACT

Both the beginning and end of this history are

ostensibly well defined. Becquerel's quiet discovery of

uranium rays came to fruition in the appearance of the

first standard textbooks on radioactivity, with their

claims for an independent subject area. One may see in

the intervening period the progressive construction of a

new bridge between physics and chemistry founded on a

coherent theory of atomic transmutation and disintegration.

My examination of the scene from the viewpoints of several

interested parties reveals an alternative picture comprising

complex linked series of discoveries, experiments and

hypotheses of various levels. At the moving boundaries of

research the results and conclusions of individuals were

always subject to reinterpretation in their adoption by

others. This study thus proceeds in the light of three

main considerations. These are, firstly, parallel

investigations in radioactivity by different workers;

secondly, contemporary related areas of physical science

such as X-rays, cathode rays, corpuscular theory; and

thirdly, the relevant concepts developed earlier, during

the nineteenth century. The introductory chapter concentrates upon the last

of these, considering some long-standing hopes and

unanswered questions concerning, for example, the

unification of matter, ether, and electricity, and the

relations between the chemical elements. This forms an

essential part of the background to radioactivity.

Chapter two describes the opening of the radiochemical

field by the Curies, following Becquerel's original

discovery, and discusses the blending of these results

with Rutherford's earliest radiation studies. The third

chapter deals with a confused phase where new observations

3

outran theory as 'emanations', 'induced' activities,

spontaneous cathode-ray emissions, and dubious

radiochemical claims combined with the ever-growing

energy problem. The fourth and fifth chapters trace

the emergence of rival theories, and the singular success

of one of these at the expense of all others.

4

ACKNOWLEDGMENTS

I wish to thank for their assistance those who

made manuscript materials available to me at the

following institutions: Cambridge University Library,

Bibliotheque Nationale, Academie des Sciences,

Royal Institution of Great Britain, Wellcome Institute

for the History of Medicine Library, Library of

University College London, Library of the Royal Society,

Bodleian Library, Science Museum Library, Imperial College

Archives.

To my research supervisor Dr. M.B. Hall of the

Department of History of Science and Technology, Imperial

College, I am greatly indebted for her patient advice

throughout the project and for her careful reading of

this thesis during its construction.

TABLE OF CONTENTS

ABSTRACT 2

ACKNOWLEDGEMENTS 4

CHAPTER 1. NINETEENTH-CENTURY THREADS 8

1. Introduction 8 Scientific revolution - summary of threads.

2. Chemical atomic theories and the unity and complex ty of the chemical elements 12

Dalton's theory - evolution and compound nature of elements - Crookes - Stokes - Lockyer - J.J.Thomson.

3. Physical theories of matter, electricity, ether 24 Maxwell - various ethers - Larmor - Lorentz -Zeeman - Mme.Curie, Rutherford - Hertzian molecule.

4. Chemical physics. Physical chemistry 32 Hertzian atom - Stoney's electron theory -electrochemistry - vortex atom - J.J.Thomson -electricity and gases - corpuscular atom.

CHAPTER 2. THE DISCOVERY OF URANIUM RAYS AND RADIOACTIVITY 48

1. Becquerel's discovery of uranium rays (1896-7) 48 X-rays and phosphorescence.- Becquerel and uranium rays - nature of the rays - S.P.Thompson - other radiations - vapours and W.J.Russell's photographic work -electrical studies.

2. Rutherford, and the Cavendish Laboratory (1894-8) 70 Hertzian radiation and magnetism - X-rays and conductivity of gases - ionic theory - ultraviolet radiation - uranium rays.

3. Pierre Curie, Marie Curie and the new radioactive elements (1890.17)-- 92

P.Curie's researches - Marie Curie - thorium rays - G.C.Schmidt - polonium - radium - atomic property -energy source, speculations.

4. Theories and trends (1896-9) 110 Source of the energy - interest in uranium rays -development of theories - atomic change.

6

CHAPTER 3. EMANATIONS AND RADIATIONS 119

1. The ma netic deflection of the Becquerel rays 1 9-1900) 119

The rays from active substances and their magnetic deflection - magnetism and radio-activity - Becquerel's 'material rays'.

2. The discovery of induced radioactivity (1899) 129 The Curies' discovery, 'la radioactivite induite' - Rutherford's idea of a thorium emanation - properties of the emanation - the production of a radioactive deposit.

3. The source of radioactivity (1900) 147

The effect of temperature change and its implications - phosphorescence, Behrendsen, and the views of Becquerel - Marie Curie's speculations on atomic change and disintegration - Rutherford and the energy of radioactivity.

4. Emanations and the X-substances (1900-1) 162

Fitzgerald, and transmutations - A.Debierne and actinium - induced radioactivity: Giesel, Hofmann, and radiolead; P.Villard - Crookes and UrX - the Curies' views - E.Dorn and an emanation from radium - Rutherford's problems with the emanations - atmospheric emanation of Elster and Geitel.

CHAPTER 4. DISINTEGRATION, INDUCTION, TRANSFORMATION 179

1. The emergence of induction and disintegration gorfes (1901-1.) 179

Ionic and emanation hypotheses of Elster and Geitel - comparison of theories - induced radioactivity; the function of the gaseous medium - radioactive water; the induction theory of Curie and Debierne - Becquerel, uranium and auto-induction - Curies' criticism - Crookes and ultra-atomic diffusion - Martin and total disintegration - J.Stark and the genesis of atoms .

2. A quantitative theory of atomic transmutation T1902)

Introduction - F.Soddy - the first joint publication: an inert gas from thorium, a possible transmutation; ThX as the source of the emanation - Rutherford and the transmission of excited radioactivity - interpretations of Becquerel's views - the second publication: thorium and ThX, transmutation quantitatively observed - the new accompaniment version of the disintegration theory, and the question of induction.

196

7

CHAPTER 5. RECEPTIONS, GENERALISATIONS, SPECULATIONS 226

1. Reception of the disintegration theory (1902-3) 226 Introduction - changing views of the Caries; the heat from radium - F.Giesel - J.J.Thomson: wind, water, and atomic disintegration - Crookes and the mysteries of radium; a wider public - creation of helium; the Curies converted - the summit of fame - physicists and chemists: a campaign well fought.

2. The mechanism of radioactivity (1903-4) 252

Speculations of physical chemists: ether, energy, electrons - Soddy and the randomness of disintegration - physical models for radioactivity - Lodge's radiation-loss hypothesis - Kelvin's atomic theories - Nagaoka's saturnian atom - Thomson's corpuscular atom.

3. Conclusion 271 Rutherford and the succession of changes - cosmical, universal, and Proutian radioactivity.

NOTES FOR CHAPTERS 1 to 5 281

BIBLIOGRAPHY 331

Ri3 RE .f% 14-nonis 361e

8

CHAPTER 1

NINETEENTH-CENTURY THREADS

1. Introduction

The discovery of X-rays by ROntgen in 1895, which led

directly to Becquerel's discovery of uranium rays, has been

hailed as marking a watershed in physics, the beginning of

a revolution. This interpretation generally invokes the

further statement that from 1880 to the turn of the century

physical science had attained a satisfactory state with few internal problems.' The latter idea, with its implication

of some form of stagnation in research, seems the more

dubious of the two. The words of Maxwell's 'Introductory

Lecture on Experimental Physics' of 1871 have been used to

support this view:

the opinion seems to have got abroad, that in a few years all the great physical constants will have been approximately estimated, and that the only occupation which will then be left to men of science will be to carry on these measurements to another place of decimals.2

That this was not 'really the state of things to which we

are approaching' is shown by his own continuation:

But we have no right to think thus of the unsearchable riches of creation, or of the untried fertility of those fresh minds into which these riches will continue to be poured...3

And although the great chemist Marcelin Berthelot in

introducing his book, Les Origines de L'Alchimie, of 1885,

wrote that the worm was then 'without mystery',4 scientists in other areas took quite the opposite view. Oliver Lodge

concluding his book Modern Views of Electricity, of 1889,. wrote:

'Conclusion' is an absurd word to write at the present time, when the whole subject is astir with life, and when every month seems to bring out some fresh aspect...5

And we will leave Lodge with a resounding word on this

particular point, in his lecture on 'The Discharge of a

9

Leyden Jar' at the Royal Institution, in the same year;

The present is an epoch of astounding activity in physical science. Progress is a thing of months and weeks, almost of days. The long line of isolated ripples of past discovery seem blending into a mighty wave, on the crest of which one begins to discern some oncoming magnificent generalisation. The suspense is becoming feverish, at times almost painful. One feels like a boy who has been long strumming on the silent key-board of a deserted organ, into the chest of which an unseen power begins to blow a vivifying breath. Astonished, he now finds that the touch of a finger elicits a responsive note, and he hesitates, half delighted, half affrighted, least(sic) he be deafened by the chords which it woara seem he can now summon forth almost at will.6

This happy view sprang from Hertz' recent effective

experimental confirmation of Maxwell's electromagnetic

theory of light. Taken at face value this opposes ideas

of a static or stagnant science at the time. And although

things were not considered to be so happy in related

fields such as spectroscopy and molecular kinetic theory,7

which were beset with problems and contradictions in the

last quarter of the century, they were always lively.

As for the question of whether a date near the turn

of the century should yet be considered as marking the

beginning of a revolution in science, this is a difficult

problem of interpretation. L.P.Williams, for example,

considers the whole lifetime of Queen Victoria (1819-1901)

as manifesting revolutionary changes in the viewpoint of

physics, biology and chemistry.8 My purpose is not to

attempt a full answer to the question, but to point out

that the understanding of the bases of, and changes in,

physical science at the time of the discovery of radio-

activity can be aided by studies of the preceding period.

These can throw light upon the science of radioactivity,

whose phenomena were interpreted in current physical and

chemical terms in its early years before it developed

some concepts of its own.

Thus an outline of chemical atomic theory and

speculations on the unity, complexity and evolution of

the chemical elements points to the Proutian and

10

evolutionary ideas which as will be seen, provided some of

the first tentative explanations of the new phenomena. So

also a thread of physical theories leads via elastic, fluid,

and electromagnetic optical ethers to Hertz' experimental

work on electric radiation. Similarly via optical electron

theories of matter to Zeeman's discovery of the magnetic

influence on sources emitting light. The former provided

the subject of Rutherford's earliest publications, which

contributed to later ideas on radioactive radiations. The

latter played a part in the development of electron or

corpuscular atomic theories. Finally to be considered is a theme in part uniting

the above threads which are mainly but not entirely

independent. This comes under the heading of chemical

physics or physical chemistry, in which a rather thin line

of physicists attempted to apply their science to chemistry;

this was complemented by a few chemists thinking in terms

of physical theory. Chemical thermodynamics does come

under this heading, but is not discussed here, since it

did not directly influence the early development of radio-

activity. But the study of the electrical phenomena and

properties exhibited by chemical substances, especially in

the gaseous state, including cathode rays, provided a vital

link between several of the areas mentioned above, as

exemplified by the work of the physicist J.J.Thomson and

the chemist William Crookes. A union of aspects of physics

and chemistry was forged strongly in the science of radio-

activity but owed much to the earlier unifying thread.

This thread itself contributed independently to experi-

mental atomic physics, for example in studies on rays of

positive electricity in the early twentieth century.

Other areas not to be discussed in this chapter will

be mentioned briefly as the story of radioactivity unfolds.

Kinetic theory of heat, and chemical thermodynamics, for

example, enter this story but briefly, although forming

part of the acceptable knowledge of most physical scientists

at the time. Some techniques and developments relevant to

radioactivity such as the work of the Becquerels and Stokes

on phosphorescence; photographic techniques for the study of radiation; electroscopes and electrometers; various

11 technical traditions of chemical analysis; discovery of the inert gases, will also be mentioned or brought out

in later discussion.

It must again be noted that the areas covered in

this chapter cannot be considered as completely independent

at any time. That the opposing processes of fragmentation

and unification appear to have occurred together during

the progress of nineteenth and early twentieth century

physical science, makes the interpretation of threads

problematical. So also does the ready transfer of ideas

among the scientists who formed something of a European

and Colonial community within which published and private

communication was abundant. Several influential scientists

lived and worked through the whole of the period of some

fifty years from the time of Faraday, the rise of chemical

spectroscopy and the creation of the periodic table of the

chemical elements, until radioactivity was established as

a school of research based on the theory of atomic dis-

integration and transmutation. These points could lead

one to place separate threads of development in the mind

of one and the same scientist; but this does not seem

impossible.

The areas discussed under the three main headings

thus point, in largely uncharted territory, towards the

scientific ideas or knowledge used by those involved with

radioactivity; the treatment is not intended to do more

than this. It may also, however, serve to provide part of

an answer to the question of how far developments of science

related to the new experimental discoveries, around 1896,

of X-rays, radioactivity, corpuscles or electrons, and the

Zeeman effect can be considered as novel.

12

2. Chemical atomic theories and the unity and complexity of the chemical elements

In reviewing7sr-Tudies on radioactivity after her

first year's work on the subject, Mme.Curie put forward

several speculations on the source of the continually

radiated energy, not yet the problem that it was to be.

One was that:

L'6nergie utilisable des substances radioactives diminue constamment. On pourrait, par exemple, rattacher la radioactivit6 a la theorie de Crookes stir 11 6volution des 616ments, en attribuant la radioactivit6 aux 616ments a gros poids atomiques, qui se seraient form6s en dernier et dont l'6volution ne serait pas encore achevee.10

Although this does not seem to be the hypothesis

she favoured (nor was it accepted by Crookes himself at

the time), that Crookes' idea of more than a decade

earlier could be brought up in this way indicates its

currency during the last years of the nineteenth century.

It turned out to be nearer the mark than either had

supposed.

In a similar way, and perhaps more important for

radioactivity, J.J.Thomson first announced his demon-

stration of subatomic corpuscles in a lecture at the

Royal Institution on 'Cathode Rays' in 1897.11 Here he

referred to the earlier suggestions by William Prout,

unnamed chemists, and Norman Lockyer on the constitution

of the chemical elements:

The assumption of a state of matter more finely subdivided than the atom of an element is a somewhat startling one; but a hypothesis that would involve somewhat similar consequences - viz. that the so-called elements are compounds of some primordial element has been put forward by various chemists. Thus, Prout believed that the atoms of all the elements were built up of atoms of hydrogen, and Mr. Norman Lockyer has advanced weighty arguments founded on spectroscopic consideration, in favour of the composite nature of the elements.12

Such hypotheses were thus neither dead nor forgotten

at the end of the nineteenth century, though their origin

lies at its beginning or earlier, with ideas of the unity

13

of all matter predating the atomic theory of Dalton, and

running parallel to its early development. Knight13 has shown how the alternative point-atomism of Boscovich was

entertained by Davy and Faraday, and illustrates the

debatable character of atomic hypotheses from their time

to the 1870's. Dalton's atomic theory required that there be as many different types of true atom as undecomposable

elements. Of these there were about 30 at the time,

rising steadily to about 75 by the end of the century.14

Prout's hypothesis implied that each Daltonian atom

of an element consisted of a number of combined atoms of

hydrogen. And the nearness of atomic weights to multiples

of H=1, i.e. to whole numbers, became clearer throughout

the century. But as analytical results became more certain,

so too did the discrepancies, with the atom of chlorine

obstinately weighing about halfway between 35 and 36

hydrogen atoms. Prout's hypothesis itself, and the saving

device of using 0.5, or smaller, as the basic unit provoked

strong criticism. But as Farrar, for example, has shown,15

modified Proutian ideas developed in the second half of the

century gaining much from analogies with related scientific

areas. The existence and behaviour of organic radicals

showed that compound units could behave in a quasi-atomic

manner; and homologous series had Proutian implications

which were soon taken up. Here two substances, analogous

to C and H in organic chemistry, are required, rather than

the usual single substance of Prout's hypothesis. Then,

about 1860, independent generalisations were impressed

upon the scientific public, and their combined influence

was to be of considerable importance.

The attainment of consistent atomic weights was

dependent upon the acceptance of the remarkable suggestion

by Avogadro, revived by Cannizzaro in 1860, that equal

volumes of gas contain equal numbers of particles, regard-

less of the mass or nature of these. Also essential was

the related postulate of double atoms, like 02, in gaseous

elements, which entailed the problem of the affinity of

like atoms and its implications. Discussions of chemical

philosophy, the different kinds of atomism, and the problems

14

of chemical affinity in the first half of the century

have been provided by Levere.16 Only after 1860 could

the patterns and families of chemical elements be put

together to form the great generalisation of the

periodic table. This, together with the evolutionary

ideas expressed in Darwin's Origin of Species (1859)

and the rise of spectrum analysis in the 1860's,17

provided the elements which appear in newer 'Proutian'

speculations from the 1870's to the turn of the century.

William Crookes, already well known for his work on

the radiometer, high vacua, and inorganic analysis, among

other things, combined the points just mentioned with

others deriving from geology or mineralogy. In his long-

remembered address to the Chemical Section of the British

Association18 in 1886 he remarked that: The array of elements cannot fail to remind us of the organic world. In both cases we see certain groups well filled up, even crowded, with forms having among themselves but little specific difference. On the other hand, in both, other forms stand widely isolated. Both display species that are rare; both have groups that are widely distributed - it might be said cosmopolitan - and other groups of very restricted occurrence. Among animals I may mention as instances the Monotremata of Australia, and among the elements the metals of the so-called rare earths.19

Biological evolution was considered to be an unceasing

process from the remote past and still actually occurring

in the present; but Crookes denied this implication for

the evolution of chemical elements:

The analogy here suggested between elements and organisms is indeed not the closest and must not be pushed too far ... Nor would I for a moment suggest that any one of our present elements, however rare is ... in process of extinction, that any new element is in the course of formation, or that the properties of existing elements are gradually undergoing modification. All such changes must have been confined to that period so remote as not to be grasped by the imagination ... The epoch of elemental development is decidedly over...20

This could have been written as a reply to Mme.Curie's

suggestion of the evolution of the heaviest atoms thorium

15

and uranium a decade later; it does indeed seem to express

his opinion of any such suggestions for radioactivity for

most of the period 1898-1904 during which Crookes worked

and wrote on the subject. It is true that at the beginning

of his speech he spoke favourably of Norman Lockyer's

dissociation hypothesis, set out during the previous

decade:

Mr. Norman Lockyer has shown, I think on good evidence, that, in the heavenly bodies of the highest temperature, a large number of our reputed elements are dissociated, or as it would perhaps be better to say, have never been formed. Mr. Lockyer holds that 'the temperature of the sun and the electric arc is high enough to dissociate some of the so-called chemical elements, and give us a glimpse of the spectra of their bases'.

But Crookes, apparently lining up with a majority of

chemists, spoke against this view with the question:

Is there, then,in the first place, any direct evidence of the transmutation of any supposed 'element' of our existing list into another, or of its resolution into anything simpler?

To this question I am obliged to reply in the negative ... The highest temperatures and the most powerful electric currents at our disposal have been tried, and tried in vain.21

However, he considered the mineralogical association

of like elements, and chemical periodicity, to be indirect

but undeniable evidence of a former though now frozen

Evolution. This had begun from an original 'protyle',

possibly 'helium' and developed into an oscillating

periodic table linking temperature of formation with

atomic weight, atomicity, electrical, and magnetic

properties; as temperature slowly decreased the lighter

elements were first formed, and finally thorium then

uranium.

These ideas are best not considered as purely

speculative, for they were closely linked to the experi-

mental chemistry of the time via Crookes' work on the

rare earths. With some modification these notions were

repeated over the following few years,22 although not

highly regarded by other chemists.23 The delicate

weighings required in chemical analysis had led Crookes

16

to the radiometer effect24 ten years earlier. This in

turn led to the study of rarefied gases, cathode rays,

and the 'fourth state of matter', and thence back to

'radiant matter spectroscopy' as a novel aid to chemical

analysis. The experimental thread is here easier to

follow than his ideas on the molecular or atomic structure

of matter; by 1886 Crookes had formulated an unusual

interpretation of the new spectroscopic properties of

some rare earth elements.25 Whereas radiant matter

spectroscopy indicated five fractionated components of

yttria for example, the ordinary spark spectrum and the

chemical properties of the five were identical, indicating

but a single element. He speculated:

that the structure of a chemical element is more complicated than has hitherto been supposed. Between the molecules we are accustomed to deal with in chemical reactions and ultimate atoms as first created, come smaller molecules or aggregates of physical atoms; these sub-molecules differ one from the other, according to the position they occupied in the yttrium edifice.26

The alternative required an element for each spectrum,

five new elements for yttria alone. The apparent identity

of the ordinary spark spectra of the new components

occurred because in 'the intense heat of the electric

spark, the little differences of molecular arrangement

vanish'. His 'compound molecule explanation' was supposed

to apply generally, for 'had we tests as delicate for the

constituent molecular groups of calcium' this too might

be resolved into simpler groupings.27 In this he seems to

stand not far from the dissociation hypothesis of Lockyer;

both have similarities to the theories of some physicists.

Crookes' query 'whether there is an absolute

uniformity in the mass of every ultimate atom of the

same element. Probably our atomic weights represent a

mean value ...'28 may or may not derive from Liveing's

earlier admission that considering the thousands of iron

spectrum lines he was 'almost driven to ascribe them to

a mixture of differing molecules, though we have as yet no

independent evidence of this'.29 The above statements

indicate some of the difficulties of the terms used at that

17

time, with Crookes or his reporters employing 'ultimate

atoms' in two different ways, also 'physical atoms' as

synonymous with 'sub-molecules' and 'smaller molecules',

which are different from chemical 'molecules'. Liveing,

however, may have meant aggregates of what had been

defined as chemical atoms.

The proposition that the chemical elements or atoms

might be complex seems to have been far less controversial

than its extension to an actual dissociation of the

elements; some evidence pointed only to the first statement,

but spectroscopic observations were taken as pointing to both. For example, G.G.Stokes the Cambridge physicist

early considered dissociation. His publications span the

years 1840-1902, and he communicated with scientists

ranging from Faraday at the beginning of this period to

Crookes, S.P.Thompson and Henri Becquerel on radioactivity

at its end; his rising reputation had been aided by work

on mathematical optics and fluorescence.30 In 1854 he

wrote to William Thomson (later Kelvin)31 concerning the

'enormous length' of the line-spectrum obtained from

electric discharge between metal points, compared to the

spectrum then of greatest range - that of the sun:

I cannot help thinking that decompositions of a very high order may be going on in such an arc (the voltaic arc I mean) and that a careful examination of these lines may lead to remarkable inferences respecting the bodies at present regarded as elementary. There is nothing extravagant in this supposition: few chemists I imagine believe that the so-called elements are all really such.

Now it is quite conceivable that chemically pure metals should agree with compounds of sodium in giving the bright line D. If this were made out I should say that perhaps these metals were compounds of sodium, but more probably they and sodium were compounds of some substance yet more elementary.32

Stokes was most cautious on such matters in publications;

he later found fault with Lockyer, who according to his

most recent biographer, was not.33 The inferences of Lockyer, first made public in 1873, and based on

comparisons between stellar, solar, and laboratory34

spectra, developed into a comprehensive scheme of

18 dissociation of the elements. Stokes wrote in 1876

concerning Lockyer's Preliminary Note to the Royal

Society on the 'Compound Nature of the Line-Spectra of

Elementary Bodies' saying that the simplification of the

calcium spectrum observed in the sun might well be due

to variations with temperature of the relative intensities

of the various molecular vibrations existing and observed

in undissociated calcium:

Hence, while I regard the facts you mention as evidence of the high temperature of the sun, I do not regard them as conclusive evidence of the dissociation of the molecule of calcium.35

The term 'molecule of calcium' used in this context

indicates the continued acceptance of some kind of

complexity of the element, but is otherwise not very clear.

This is perhaps an intentional reserve: Stokes was familiar

with chemical practice and nomenclature.36 The usage is

comparable with the similar expression of a structured

'molecule of uranium' used twenty years later in Stokes'

comments on the origin of the Becquerel rays.37

Writing to Lockyer in 1879, and again criticising his

inferences, Stokes made it clear that he did accept the

compound nature of the elements and believed the view to

be generally favoured:

the question observe is not, Are the elements compound bodies? But, has any satisfactory evidence been now obtained that they are compound bodies? You would, I imagine, find plenty of chemists, from Prout downwards, who would regard it as most probable that they were compounded. I may say that, in common I suppose with multitudes of others, I have long supposed for my part that they were.38

As for Lockyer's evidence the effect of impurities had

not been ruled out; and Dumas' arguments on atomic weights,

on which Lockyer had asked Stokes' opinion, were not

strong. The importance of impurities here is that Lockyer's

thesis of increasing dissociation from laboratory flame,

arc, and spark to stellar temperatures relied upon the

existence of common lines exhibited by different elements

- these indicated common simpler components of the so-called

elements. The actual presence of remaining traces of each

element in a chemically separated pair would give such

19

common lines. Lockyer's development of new techniques

for determining which lines were caused by impurities and

which were truly common to different elements, and his

controversies with some chemists over this, have recently

been described by Brock,39 McGucken,40 and Meadows.41

Besides the difficulty with impurities which always proved

a danger to interpretation there was the question whether

common lines were not accidental i.e. meaningless, or

simply very close. The latter was less likely to occur

with greater dispersion of the spectra. Liveing and Dewar

in collaboration conducted detailed comparisons of some of

the relevant metallic laboratory spectra and having caused

the majority of coincidences to vanish they published

conclusions highly critical of Lockyer's hypothesis.42

By 1885 the opinions of chemists, physicists and astronomers

tended towards the view that the dissociation hypothesis

did not hold water; but Lockyer considered that it fitted

astronomical observations such as those on the heights of

element lines in the sun and on the differing metallic

lines present in the spectra of stars of different

temperatures. He continued to publish books and articles

setting out or invoking the hypothesis, publicly and

privately put up spirited defences, and in so doing perhaps

incited others to technical advance. In later publications

such as The Chemistry of the Sun, 1887; 'On the Chemistry

of the Hottest Stars', 1897,47—;nd Inorganic Evolution,1900,

he was able to drum up supporting opinions for his work

from several chemists. But even some of these, for example

Berthelot, Brodie and Crookes were, or had been, critics

of the dissociation hypothesis.

This seems to fit in both with a general rejection

of his evidence and with Stokes' analysis of 187944 that

'multitudes' of chemists and others considered elements

to be most probably compounded. One could say that a

continuing belief that elements were complex or compounded

and occasionally even dissociated in some way, remained

in the last decades of the nineteenth century, despite

Lockyer.

The literature of this period shows a continuing

20

interest in periodic tables; discussion of Prout's hypo-

thesis with regard to atomic weights was still alive and

had always implied more than mere numerical juggling.

Rayleigh (J.W.Strutt) in his Presidential Address to the

Physics Section of the British Association in 1882,

introducing his plans for the redetermination of gas

densities, said:

The other subject on which, though with diffidence, I should like to make a remark or two is Prout's Law according to which the atomic weightsEir the elements, or at least many of them, stand in simple relation to that of hydrogen.

Some chemists think this speculative, but:

Others, impressed more by the argument that the close approximation to simple numbers cannot be fortuitous, and more alive to the obvious imperfection of our measurements, consider that the experimental evidence against the simple numbers is of a very slender character.45

The gas density determinations which Rayleigh thought

would settle the question led instead to the discovery by

Ramsay and himself of an entire new group of elements, the

inert gases. Those discoveries of 1895-8 perhaps owed

something to the periodic table,46 caused in return several

further modifications of periodic tables by Crookes, Stoney

and others, and were vital for Rutherford's interpretation

of radioactive emanations three or four years later. In

1901 R.J.Strutt (Rayleigh) then working on cathode and

radioactive rays published 'On the tendency of the atomic

weights to approximately whole numbers'47 restating his

father's views of two decades earlier, and estimating a

thousand to one probability against the randomness of the

current atomic weights. His conclusion that there must be

some law behind this indicates the continuing belief.

More remarkable than this however are views expressed

by the chemists who considered that they had effectively

demolished Lockyer's dissociation hypothesis. Liveing

believed that the simpler spectral patterns might occur . 'like the overtones of a string',48 we have seen that he

attributed more complicated spectra to the complexity of

the chemical elements, which might be of an aggregate type.

21

This was shortly after his and Dewar's first hostile and

effective criticisms of 1880-1 against Lockyer's ideas on

dissociation. In spite of this, Liveing in his Address

of 1882, providing further speculations on Prout's

hypothesis at this meeting, asked 'Why may not the

chemical elements be further broken up by still higher

temperatures? A priori and from analogy such a supposition

is extremely probable'.49 Thus both the complexity and

dissociation of elements were considered feasible by

Liveing. Dewar's views expressed six years later in a

lecture on 'Phosphorescence and Ozone150 agree with this:

In this experiment ozone is formed by the action of a high temperature owing to the dissociation of oxygen molecules and their partial recombination into the more complex molecules of ozone. We may conceive it not improbable that some of the elementary bodies might be formed somewhat like the ozone in the whole experiment, but at very high temperatures, by the collocation of certain dissociated constituents and with the simultaneous absorption of heat.

This seems to mean that some elements had been formed by

the dissociation then reassociation of others; it may be

an exception -to the view51 that the only reasonable opinion

at the time took the result of intense heat to be

dissociation only.

Some clarification of the ramifications of chemical

atomism seems to have occurred by the end of the next

decade as relevant studies on cathode rays and radio-

activity were beginning to develop. However, Liveing's

conclusions 'On the Flame-Spectrum of Mercury and its

Bearing on the Distribution of Energy in Gases' of 189852

displays a stronger debt to spectroscopy and kinetic

theory. The significance of mercury here stemmed from

its status as an element whose vapour was definitely

monatomic and not aggregated; this was well established

by chemical experiments on combining weights and vapour

density, and by gas kinetic theory. A difficult deter-

mination53 of the velocity of sound in this vapour

indicated the high ratio of specific heats expected for

a monatomic gas which could store none of the absorbed

heat energy internally. These corroborating results

22

were difficult to reconcile with spectroscopic observation

and theory, which took the complex spark spectrum of

mercury vapour to be caused by a multitude of internal

vibrations. Liveing however regarded:

the production of spectra by an electric discharge as essentially a different process from the production by heat ... a great many rays are given out by various elements in an electric discharge which have never been observed to result from mere heating.54

This point seems to distinguish Liveing's view from

Stoney's electron theory of 189555 and from other

electrical theories of spectra, to be discussed.56

Liveing's studies of the non-electrical excitation of

spectrum lines in a high temperature flame, where chemical

combination also could not occur, gave him a means of

easing the problem of mercury. He considered that:

heat ... is, in part, transformed into vibratory motion which affects the ether; and the true inference from the ratio of the specific heats appears to be, that, at the temperature at which this ratio was measured, the amount of heat converted into vibratory motion is very small...57

He identified the gaseous mercury 'molecules' of physical

kinetic theory with 'chemical atoms', and remarked:

It is possible that a chemically monatomic molecule may have, though it is not probable that it really has, a simpler constitution than a chemically complex molecule, and so may have not so many degrees of freedom as the latter, but still a plurality of degrees.58

Thus Liveing shows a clear usage of the terms of kinetic,

chemical, and spectroscopic theories, and illustrates his

current picture of gaseous molecules composed of one or

more chemical atoms, which themselves have constituents

capable of complex vibration. But evidently the principle

of equipartition of energy among all degrees of freedom

has been tacitly modified or sacrificed. This principle

was one of Kelvin's 'clouds' over the dynamical theory of

heat of 1901.59 In discussing the problems of 'practically

monatomic' gases he attempted in a confused way to disperse

the cloud by postulating 'satellites' of the atoms, with

far smaller mass, which could be the 'ions' of J.J.Thomson.

It is possible that Liveing had been similarly influenced

23

by the chemical atomic theory which Thomson founded in

1897 upon his new discovery of subatomic material

particles. If so, then according to Thomson's later

account60 he was one of very few at this time.

Thus we see that a continuing theme involving the

notions of unity and complexity of the chemical elements,

.based on observational and experimental evidence, runs

into the era of cathode rays and radioactivity. But

questions concerning J.J.Thomson's choice of Lockyer for

support in 1897 are interesting and difficult. If Thomson

took note of Liveing's views, and they did meet at the

Cambridge Philosophical Society, and from 1893 at the less

formal Cavendish Physical Society,61 one might have

expected him to be aware of the past and present low

reputation of the relevant parts of Lockyer's work.

Thomson could indeed have quoted the words of Liveing

himself on the complexity of the elements, as considered

above. Stokes too, who had discussed cathode rays and

X-rays with Thomson62 in 1896 or 1897, shared Liveing's

opinion. If only the most recent work of Lockyer63 were seized upon, ignoring as many did64 his earlier studies,

there were again strong criticisms by the reputable

Schuster.65 The latter suggested at the Royal Society

discussion meeting of March 25th 1897 that the difference

between stars emitting hydrogen and metal spectra for

example was as well explained by differences in density

and convection of layers of ordinary elements as by

dissociation. For proof of dissociation, he stressed,

traces of other elements from the electric discharge

between iron poles must be found. But Lockyer had already

blundered, along these lines,66 and surely no one seriously

expected the challenge to be met.

Thomson's first brief announcement of 'a state of

matter more finely subdivided than the atom of an element'

was made at a Royal Institution Lecture at the end of the

next month. He later recalled in his autobiography67 that

its reception was poor, and that it was perhaps not taken

seriously. Such a response is understandable when one

considers that he claimed quite clearly to have produced

24

by electric discharge not just the traces of other

elements from iron, as recently demanded of Lockyer by

Schuster, but a common material component of all chemical

elements.

The corpuscular atomic theory of Thomson was

developed during the time of the young Rutherford's work

and collaboration at the Cavendish Laboratory on closely

related areas. This was to be essential to both men's

understanding first of uranium rays and then of radio-

activity. We shall see that Thomson's ideas of the

complexity of the chemical elements were of long

standing, and founded on physical theories of ether,

electricity and matter. He was able to take up part of

a long thread of chemical Proutian ideas and to weave it

into his own physicists' theory of atomic structure.

Others too who worked and thought upon radioactivity saw

the significance of these ideas.

3. Physical theories of matter, electricity, ether

When Hertz began his magnificent experiments on electric oscillations, there were many theories of electrical action. When he had finished them there was only one, Clerk Maxwell's.

So said J.J.Thomson68 in 1894, shortly after the untimely

death of Hertz. This was the year of Rutherford's first

publication on magnetism and electric oscillations.

In the 1860's Maxwell had developed a unified theory

of optics and electromagnetism involving the explicit

provision of a single ether through which both kinds of

effect were considered to be transmitted. But this was

slow to be accepted. As Schaffner has recently commented69

at the time there were not only competing theories of

electromagnetic action but a number of non-electrical

optical theories. While optical and electrical theories

25

came closer together in the minds of many scientists

after the experiments of Hertz interactions with a third

theme of electrical chemistry had already been increasing

during the 1880's. The physical side will be discussed

in this section in so far as it can be considered

separately; both physicists and some chemists during

this period were prone to look back to Faraday as an

earlier master of chemistry, electrochemistry, and

electromagnetism.70 Aspects of all these areas became

more strongly united in the 1890's as spectroscopy,

physical theories of the chemical atom and studies on

radioactivity progressed.

Maxwell in his Treatise on Electricity and Magnetism

of 1873 went so far as to write 'before I began the study

of electricity I resolved to read no mathematics on the

subject till I had first read through Faraday's

Experimental Researches on Electricity'.71 Maxwell

considered that he had combined the methods of the German

school of 'electricians and mathematicians', which

involved 'action at a distance impressed on the electric

fluids', with the more pictorial ideas of Faraday which

involved 'real actions going on in the medium'. Faraday

however had earlier expressed doubts as to the existence

of such a medium72 and it was found to be a problematical

concept, if a valuable one, even after Maxwell's work.

The problems are indicated by the variety of ethers73

which in the nineteenth century were used in conjunction

with physical principles to explain optical and

electrical phenomena.

R.T.Glazebrook, senior demonstrator at the

Cavendish Laboratory, in his major 'Report on Optical

Theories' to the British Association in 188574 discussed

the major optical ethers and their relations with matter

with regard to their explanations of reflexion, refraction,

dispersion, diffraction, polarisation and other phenomena.

None was without its flaws, but:

The electro-magnetic theory, if we accept its fundamental hypothesis, is thus seen to be capable of explaining in a fairly satisfactory manner most of the known phenomena of optics.

26

The great difficulty is, as we have said, to account for the properties which the medium must have in order to sustain electrical stresses.75

Illustrative of the requirements of what may have been

a mainly British viewpoint, Glazebrook saw a similar

difficulty:

of realising mechanically what electric displacement is, of forming for oneself a physical idea of a change of structure in some medium of unknown properties which shall obey the laws implied by the various equations satisfied by the components of electric displacement. 76

The earlier conflict between the rigidity required for

transmission of light waves, and the fluidity necessary

for free passage of the planets, had been eased by Stokes'

view that these properties could be compatible in a

medium both of small density and low rigidity. But to conceive of such a rigidity existing in an ether which

was like an elastic solid, would not be sufficient to

account for its capability of sustaining known electrical

stresses. The solution might be to have a non-rigid,

fluid ether, with a quasi-rigidity conferred upon it by

filling it with vortices, in the form of filaments or

rings. In Glazebrooke's opinion this could explain

transmission of transverse waves and maintenance of

electric stress, whilst electric and magnetic polarisation

would then consist of definite arrangements of the fila-

ments or rings.

Similar views were held by some others at the time.

Earlier in 1885 G.F.Fitzgerald had written77 from Dublin to

J.J.Thomson with detailed suggestions of the 'infinite

possibilities in a vortex-sponge' either of 'ring vortices

i.e. molecular' or of filaments78 for explanations of electrostatic and electromagnetic phenomena. J.J.Thomson

too in the previous few years had been developing an ether

theory involving vortices, attempting to explain not only

these physical phenomena, but also chemical matter and its

properties, as will be seen in the following section. But

the adherents to electromagnetic-optical ether theories

were not great in number and were probably mostly British,

until the 1890's.

27

Included in this school, Joseph Larmor, at Cambridge,

developed a theory of optics and electrodynamics starting

from a single ether and using a form of the Principle of

Least Action expressed in terms of potential and kinetic

energy. By 1893 he had developed 'A Dynamical Theory of the Electrical and Luminiferous Medium',79 and 'a method of evolving the dynamical properties of the aether from a

single analytical basis':

We shall show that an energy-function can be assigned for the aether which will give a complete account of what the aether has to do in order to satisfy the ordinary demands of Physical Optics; and it will then be our aim to examine how far the phenomena of electricity can be explained as non-vibrational manifestations of the activity of the same medium.80

Apparently for reasons concerning an explanation of

the forces between permanent magnets, Larmor modified his

view in a later addition entitled 'Introduction of Free Electrons'.81 He considered these electrons as 'electric

centres' or 'nuclei of radial rotational strain', having

adopted Stoney's expression for the electrolytic unit

charge, and had a few words to say on the electrical

nature of chemical energy and spectra.

In another of the relatively few references to

recent experimental evidence Larmor stated that

J.J.Thomson had informed him of his determination of the

velocity of the negative rays in vacuum tubes. This

phenomenon Larmor saw as the projection of free electrons

of purely electrical inertia. Our brief mention of studies

of cathode rays is reserved for the next Section; these

cannot however be entirely separated from mathematical

physical theories, as Larmor's interesting interpretation shows.

The ether electron theory of Larmor was shortly

preceded by a differing and now much better known

electromagnetic-optical theory of electrons - that of

Lorentz, published in 1892.82 Hirosige83 and McCormmach84

have described the structure and development of this theory

in some detail. Lorentz here seems not to have unified

the entities of matter, ether and electricity as strongly. as Larmor and others, but sought explanations in terms

28

of ponderable matter and a static ether connected via

electrically charged particles, to which the usual

principles of Newtonian dynamics and of energy are

applied. The positive or negative charge was considered

to be fundamental and was not explained further in terms

of ether in the manner of some British physicists. The

fundamental charged particles were supposed to be spherical,

to possess mass and weight, to be contained within the

ponderable molecules of which all matter consists, and to

form the sole connection with the co-existing ether.85

One cannot say that the possibilities for chemical or

electrochemical explanations were explored, though Lorentz

acknowledged his debt to Helmholtz and Weber who had had

some such interests in previous years.

Lorentz' theory of 1892 and 189586 did however

contribute indirectly to J.J.Thomson's first exposition

of a corpuscular chemical atomic structure in 1897 via

the discovery of Zeeman announced a few months earlier.

Zeeman87 stated that he had used Lorentz' theory to give

an actual estimate of the charge to mass ratio of the

charged particles whose oscillations were taken to be

the cause of the etherial vibrations constituting bright

spectral lines. The discovery of the widening of the

yellow sodium doublet under magnetic influence - the

'Zeeman effect' - and the quantitative use of Lorentz'

theory gave a charge to mass ratio which Thomson noted88

as agreeing with that of his cathode ray corpuscles.

Electronic or corpuscular atomic spectroscopy was developed

rapidly after this time, but not to any great extent by

Thomson nor by the workers on radioactivity, though its

conclusions remained relevant.

Lorentz saw his electron theory as deriving in part

from Maxwell, Weber and Helmholtz. Larmor too gave credit

to these scientists and to an Irish school of mathematical

physicists including MacCullagh and Fitzgerald. Both of

these later proponents of an electromagnetic electron

theory shared the general view of the importance for their

work of the striking experiments of Hertz a few years

earlier. Hertz informed English readers in a collection

29

of his papers, Electric Waves being Researches on the

Propagation of Electric Action with Finite VelocitTg9

that his experiments had been guided by Helmholtz' theory.

This normally invoked Newtonian action-at-a-distance, but

in a limiting case gave some results similar to Maxwell's

with regard to the speed of propagation of electrical and

magnetic quantities. Hertz devised simple apparatus for

production and detection of electrical effects in the air,

and demonstrated standing waves, reflection, refraction,

polarisation, and a variety of measureable wavelengths,

of the order of metres. His work of the late 1880's was

accepted by physicists as an impressive demonstration of

the validity of a unified optical-electromagnetic ether.

This led to the fairly rapid publication not only of

learned articles on these lines but of textbooks of

various kinds.

The use of these books by young scientists shortly

to become involved in investigations of the radiation from

uranium is of interest, and may be significant for inter-

pretations of radioactivity. Paul Drude's Physik des

Aethers auf elektromagnetischer Grundldge, published in

1894,90 was used by Marie Curie91 perhaps for her earliest

researches on magnetic properties of tempered steeis.92

Its brief explanation of fluorescence and phosphorescence93

appears to be essentially a reinterpretation and development

in electromagnetic terms of Stokes' ideas of some forty

earlier.94 Drude represented the molecule of a body as a

closed wire circuit whose natural electric vibrations are

doubled in wavelength on changing to a linear form under intense excitation. This could also account for thermo-

luminescence, so-called by E.Wiedemann, where heating

alone causes characteristic luminosity in some substances.

Radioactivity was at first vaguely interpreted in terms of

phosphorescence by Becquerel, then by Mme.Curie in terms

of fluorescence, a rapid re-emission of received rays.

Drude's book shows the electromagnetic view of optical

phenomena generally accepted at the time; and he refers95

to recently published textbooks by Boltzmann96 and

Poincare,97 which professed some differences in their

30

approach, and which were also available to the student.

E.Wiedemann, who aided the work of G.C.Schmidt on

fluorescence and the new uranium and thorium rays in

1895-9, shared Drude's electromagnetic view of

fluorescence. In a letter98 to Stokes early in 1896

concerning his own and Dr.Schmidt's work on fluorescence

of metal salt solutions and metal vapours he indicated

his view that even with monatomic gases the illuminating

mechanism was more complex than commonly supposed, that

the emission from mercury vapour contradicted the kinetic

theory of gases, and that the explanation might lie in

supposing the molecule comparable to a type of oscillatory electrical circuit.

Thus we see that the view of the 'molecule' behaving

as an electrical system was developing soon after the

work of Hertz of 1888. The identification of this

electrical molecule with the chemical atom may be inherent

in Wiedemann's letter; so also may the not uncommon idea •

of complexity within the chemical atom; but these ideas

were not explicitly discussed, nor are they suggested in

Drude's book. Whether or not the idea of an electrically

composed chemical atom occurred to Schmidt or Marie Curie

before or after J.J.Thomson's corpuscular atomic theory,

neither appears to have clearly used the idea for the

early interpretation of radioactivity.

The conception of an electrically constituted

chemical atom appears more definitely in the textbooks

of 0.Lodge and J.J.Thomson cited by the young Rutherford

in his first published researches, of 1894.99 Thomson,

having edited the third edition of Maxwell's Treatise on

Electricity and Magnetism,100 published a considerable

supplementary volume.101 Besides detailed treatments of

electrical oscillations and Hertzian electromagnetic

radiation to which Rutherford referred specifically102

there is a description of Thomson's own development of

Faraday tubes of force for the understanding of chemical

combination103 and the passage of electricity through

gases.104 This approach dates from the early 1880's and

contains clear allusions to the etherial-electrical

31

construction of chemical atoms.105

The Modern Views of Electricity of Oliver Lodge of

1889106 was one of the many Maxwellian books published

soon after Hertz' experiments. Here Lodge gave non-

mathematical accounts of electricity, magnetism and

radiation. His explanations were based on a single fluid

ether 'a continuous incompressible perfect fluid filling

all space' possibly consisting of 'interlaced vortex

filaments like a sponge' and were illustrated with one

of the most thoroughly mechanical or machine-like

depictions of its motion yet produced. Air and other

dielectrics had interlocked wheels, with cogs upon them,

to represent their etherial molecular structure, but

'in a metallic conductor the gearing is imperfect; it is

a case of friction-gearing with more or less lubrication

and slip, so that turning one wheel only starts the next

gradually'.107 The oscillatory charge and discharge of a

Leyden jar, much discussed after 1888, he illustrated by a device of weights and pulleys balanced by elastic strings,

with sliding joints accounting for the residual charge

effect.

Despite the popular exposition in parts,108 Lodge

had many points to make in this book and elsewhere which

were found to be valuable. His discussion of rapidly

varying magnetic fields, and the 'skin' effect of currents

starting from the outside of a conductor, for example,

were noted by Rutherford109 who may also have noticed

some suggestive comments on phenomena explained by the

electrical nature of chemical atoms.110

These then are some of the areas developed by

physicists in the last decades of the nineteenth century,

which form a background to the understanding of the

younger physicists beginning their researches in the

1890's.

32

4. Chemical physics. Physical chemistry Aspects of chemistry and of physics in the 1880's

and 1890's have so far been discussed. Perhaps even more

important than either of these from our point of view

were the attempts at applying physical theories to

chemistry, together with the study of areas considered

to be intermediate between the two, which became stronger

during this period.

Lodge's book of 1889 contains allusions to aspects

of chemistry which are clear, if brief. He speculated as

to whether the etherial 'whirls' or wheels might represent

not simply electricity but 'atoms',111 and seems generally

to have been prepared to use the word atom in its chemical

sense. For example, in a chapter on the 'Mechanism of

Electrical Radiation' the emission of light, understood

as an electromagnetic phenomenon, is attributed to

oscillation of the unit electrical charges whose existence

he took electrolysis to have demonstrated:

It can be calculated that the oscillation of an atomic charge would give rise to only ultra-violet rays. It is probably because these ultra-violet rays synchronize with the period of vibration of atomic charges that they have such extraordinarily powerful chemical effects...112

Lodge was one of the first to attribute the effect of heat

in producing definite spectra of chemical substances to

the thermal, hence mechanical, oscillation of the charged

atoms in a molecule:

Under the influence of heat the components of the molecule are set in vibration like the prongs of a tuning fork, the rate of vibration depending on and being characteristic of the constants of the particular molecule. The atoms being charged, however, their mechanical oscillation is necessarily accompanied by an electric oscillation, and so an electric radiation is emitted and propagated outwards...113

Lodge's suggestion as to the explanation of phosphorescence

involved 'atoms receiving indirectly some of the ethereal

disturbance, and so prolonging it by their inertia, instead

of leaving it to the far less inertia of the ether alone'.114

He referred115 to the recent researches of Hertz, Ebert,

Wiedemann and others on electrical effects of ultra-violet

light, and attributed these to the same cause as the

33

chemical effects of light; this was to be clarified by

J.J.Thomson and his associates C.T.R.Wilson, E.Rutherford

and others at the Cavendish Laboratory in the next few

years and by the German physicists J.Elster and H.Geitel,

who were to develop strong interests in radioactivity.

We recall that about five years after Lodge's account

Drude, whose book of 1894 was consulted by Marie Curie,

qualitatively compared the 'molecule' to an oscillatory

electric circuit. Now although Lodge seems to have

accepted that the mechanical oscillation of fixed atomic

charges was the source of the luminosity caused by heating,

he also suggested what appears to be an alternative idea

that 'those short ethereal waves which are able to affect

the retina, and which we are accustomed to call "light",

may be really excited by electrical oscillations or

surgings in circuits of atomic dimensions'.116 Rough

estimates for a single loop of wire showed that this

circuit would have to be of atomic dimensions to give

frequencies of the right order.

G.J.Stoney, however, in 1891 made a distinction

between the two ideas, and specifically criticised not

Lodge's version, but the suggestion that discharges

between molecules could be the source of spectra:

The lines of the spectrum of a gas are due to some events which occur within the molecules, and which are able to affect the ether. These events may be Hertzian discharges between molecules that are differently electrified, or they may be the moving about of those irremovable electric charges, the supposition of which offers the simplest explanation of Faraday's law of electrolysis ... Several considerations suggest that the source of the spectral lines is to be sought not in the Hertzian discharges, but in the carrying about of the fixed electric charges, which, for convenience, may be called electrons.117

This statement was reaffirmed and extended in 1894118 when

he wrote that 'the motions going on within each molecule or

chemical atom cause these electrons to be waved about in

the luminiferous aether' and that 'the only other

conceivable source of these spectra is excluded, viz.,

Hertzian undulations consequent upon electric discharges

within and between the molecules'. This exclusion was

34

owing to Fitzgerald's estimate that the frequency would

be higher than any known part of the spectra of gases.

The proportionality of the wavelength emitted to the

geometric mean of the capacity and self-inductance of a

conducting circuit was the vital relationship for Lodge's

calculation,119 but that accepted by Stoney and leading

to such different conclusions is at present not clear.

The cause of radiation adopted is of interest for its use

of the electrolytic 'electron' entity, a term which was

coined by Stoney himself at about this time.120 Considering

its importance in the physical science of the next decade

it is valuable to see briefly how this had arisen.

A mathematical physicist with strong interests in

kinetic theory, spectroscopy, and the chemical periodic

table, G.J.Stoney had in 1871 provided a quantitative

explanation of line spectra.121 The harmonics of a

vibrating string were considered as the basis of a

mathematical comparison, but the quantitative aspects

were refuted by A.Schuster by the end of the next

decade.122 During these years however, Stoney began to

produce the basis of his electron theory which provided explanations in some areas of contact between physics and

chemistry in the 1890's. A paper read to the British

Association in 1874123 and published only in 1881,

entitled 'On the Physical Units of Nature'124 combined

chemical atomic theory with Faraday's Law to give the

required Physical Unit of electricity:

And, finally, Nature presents us, in the phenomenon of electrolysis, with a single definite quantity of electricity which is independent of the particular bodies acted on. To make this clear I shall express 'Faraday's Law' in the following terms, which, as I shall show, will give it precision, viz.:- For each chemical bond which is ruptured within an electrolyte a certain quantity of electricity traverses the electrolyte which is the same in all cases.125

Stoney here defined and discussed the chemical atom, gaseous

molecule, and the 'hands or feelers which each atom has and

which by grappling with the hands or feelers of other atoms,

establish bonds between them'. He seems not to say that

the bonds are purely electrical in nature, but stresses

35

that in electrolysis 'a definite quantity of electricity

traverses the solution for each bond that in separated'.

Helmholtz' emphasis in a similar discussion in the same

year was somewhat different as will be seen. The

definite charges were employed by Stoney in a new

explanation of spectral series of doublets, and triplets,

about 1890. This involved mathematical analysis of

waves emitted by charges following orbits within a

molecule, this consisting of one or more chemical. atoms

which may possibly be vortex atoms.127 What became of

radiation produced by rotational and vibrational motion

of molecules, which they possess according to kinetic

theory, is not explained but this was a problem not only

of Stoney's theory.

By 1895 he had combined this electron theory with

the kinetic theory of gases to account qualitatively for

several phenomena of chemical physics such as phosphor-

escence, gas absorption spectra, luminosity in certain

chemical reactions, and the ratio of specific heats of

gases with complex molecules. His explanation of

phosphorescence is of interest with regard to Becquerel's

understanding of the newly discovered uranium rays in

terms of this phenomenon six months later; the latter

did not suggest details of mechanism, electrical,

electronic or molecular.

Stoney classified vibrations within molecules

according to the ease with which they exchange energy

with translational motion, upon collision. A motions

are immediately affected, C motions are unaffected by

collisions, and B motions are intermediate:

Thus, when a phosphorescent body has been exposed to suitable light, t is an electron associated with Bb motions18 that is primarily acted on by the aether.129

When an electron is associated with more rapidly

exchanging Ba motions or events, etherial vibrations

received are transferred rapidly to translational

motion, phosphorescence does not occur, the temperature

rises and etherial undulations cease 'in other words,

the gas is one that has an absorption spectrum:130

126

36

Extending this kind of explanation he noted that'the

number of electrons within an atom may be greater than

its place in Mendeleeff's table would seem to suggest'

as shown by the chemical behaviour of potassium and bonds

between molecules in crystals. The electron is now taken

to be the principal factor in any chemical bond, and

formulae such as H.CiC.H for acetylene, with one electron

per bond, are given. He suggested that 'it is when excited

by chemical reactions that electrons produce their most

conspicuous luminous effects' which included the luminosity

in electric discharge tubes.131 Though it is uncertain

whether the electron theory of Stoney of 1895 and earlier

was directly used by those studying radioactivity, the

publications were readily available, and similar ideas

were shared by others. Among these Arthur Schuster wrote

not of the 'electrons within an atom' as had Stoney, but

of electrons 'moving along the surface of an atom' about

positions of equilibrium, to account for line spectra.132

Larmor's rotational ether electrons and their applications

have been mentioned, and J.J.Thomson's ideas were akin to

these at the time. In Germany similar developments were

occurring; Ebert's paper on 'Heat of Dissociation

according to the Electrochemical Theory' of 1894133

provided quantitative evidence that the energy of chemical

bonds was purely electrostatic and not 'specially chemical'.

He credited Helmholtz with the first statement, in 1881,

of an electrolytic atomic charge. Stoney wrote134 to

claim priority over Helmholtz on this point, and over

Ebert for his auggestion135 that 'motions going on within

each molecule or chemical atom cause these electrons to

be waved about in the luminiferous ether' to produce

spectra.136 The suggestion that the luminous radiation

produced by friction, disruption of a crystal, and chemical

reactions could be understood in terms of electrons which

'are started into activity'137 is of interest; for

Rutherford struggled with this kind of idea, as did others,

in studying the properties of radioactive emanations and

their radiations some five years later.

37

It is to be noted that such a union of chemical and

electrical science had developed comparatively recently;

C.A.Russell has described the Faraday Lecture delivered

by Helmholtz to the Chemical Society of London in 1881138

as marking the beginning of 'The Renaissance of Electro-

chemistry', after the subject had suffered thirty years

of disrepute.139 The claims of Stoney are ignored by

Russell, and with justification, for although prior by

some months, his statements of 1881140 seem not to have

had the same influence nor to have gone as far as those

of Helmholtz towards a purely electrical theory of

chemical combination. Indeed at this time Stoney seems

to have gone little beyond Maxwell's comments of 1873

that electrolytic phenomena necessitate the assumption

of definite molecular charges,141 but that 'chemical

combination is a process of a higher order than any

purely electrical phenomenon'.142 On the other hand

Helmholtz in 1881 while admitting 'other molecular forces'

did stress that in all compounds 'the very mightiest among

the chemical forces are of electric origin. The atoms

cling to their electric charges, and opposite electric

charges cling to each other...'143 That there was arising

interest by the 1890's in the subject of electrochemistry,

together with the study of colligative properties such as

osmotic pressure, is shown by the publication of experi-

mental and theoretical papers, some controversial, by

many authors.144 Oliver Lodge provided a report 'On

Electrolysis' for the British Association in 1885145 at

the urgent request of H.E.Armstrong, President of the

Chemical Section, who found Helmholtz' ideas of chemical

affinity not proven. Lodge's words show a not entirely

willing interest in the subject for 'though convinced of

the immense importance' of its study, he considered it had

'the somewhat repulsive character attaching to any

borderland branch of science - in this case not wholly

physics nor wholly chemistry'.146 Other scientists saw

this subject's importance but did not take Lodge's view

of borderland branches. Some physicists, and chemists,

though few in number found the investigation of such areas

38

rewarding. And it is true that most of the older

scientists who had sufficient interest to publish on the new subject of radioactivity in the first few years

of the twentieth century, were those studying such border

areas during the last decades of the nineteenth. Among

chemists were Armstrong, Crookes and Mendeleef; physicists

include Becquerel, Kelvin, Lodge, Stoney, Schuster and

J.J.Thomson.

The work of J.J.Thomson is of particular significance

for radioactivity both with regard to his own interpretations

of its problems, and to his influence upon Rutherford,

before, during and after the latter's three years (1895-8)

at the Cavendish Laboratory.

From early in his scientific career Thomson developed

strong interests in Maxwell's electrical theory and in the

application of such physical theories to aspects of

chemistry, especially with regard to the electrical

properties of gases. By 1894, reporting to the British

Association on 'The Connection between Chemical Combination

and the Discharge of Electricity through Gases'147 he

could conclude that his experiments:

give hopes that the study of the passage of electricity through gases may be the means of throwing light on the mechanism of chemical combination. The work of chemists and physicists may be compared to that of two sets of engineers boring a tunnel from opposite ends—they have not met yet, but they have got so near together that they can hear the sounds of each other's works and appreciate the importance of each other's advances.148

These hopes, shared by others at the time, were soon to be

fulfilled. However, Thomson's more ambitious ideas of the

physical structure of chemical atoms date back at least

as far as the early 1880's. These derived from the vortex

atom theory of Sir William Thomson149 who had taken

Helmholtz' mathematical treatment of vortices in a perfect

fluid (1858) to represent chemical atoms composed of fluid

ether in motion;150 the former had been impressed by Tait's

39

smoke-ring demonstrations (1867). Permanence, indestruct-

ibility, the gas laws, and many possible modes of

vibration for spectra, were all accounted for quantitatively

or mainly qualitatively by single, linked, or knotted

vortex rings. The explanation of gravitation and the

inertia of matter was problematical as Maxwell's

resurrection of Le Sage's theory indicates.151 Maxwell

pointed out that this theory had the flaw of predicting

a temperature rise of the material bodies involved due to

the impact of the etherial corpuscles which were supposed

to cause the net gravitational force. Kelvin in 1881152

said that he would not be satisfied with the vortex atom

theory until chemical affinity, electricity, magnetism,

gravitation, and inertia could be explained by it. He

pointed to the insoluble contradiction between the isotropy

of gravity and the anisotropy of crystals; he was evidently

also unable to develop the chemical aspects of the vortex

atom theory. And by about 1890 he seems to have rejected

it in favour of Boscovichian explanations of physical and

chemical properties; Kelvin now used action-at-a-distance

force laws between atoms of matter - meaning chemical

subatoms e.g. H = (h h) - and atoms of an electric fluid.153

Attempts to find unified explanations of the properties of

matter, electricity and luminiferous ether were not so

strong in Kelvin's thought as in that of some other British

scientists, from the 1890's through the early period of

radioactivity.

J.J.Thomson was awarded the Adams prize of 1882 at

Cambridge for 'A general investigation of the action upon

each other of two closed vortices in a perfect incompressible

fluid'. His mathematical treatment was published in the

following year as A Treatise on the Motion of Vortex Rings154

with some significant additions on chemical applications.

Referring to Kelvin, as he did on many points, Thomson

introduced the study by pointing out that the vortex ring

'possesses many of the qualities essential to a molecule

that has to be the basis of a dynamical theory of gases',

it is indestructible and indivisible; 'the strength of the ring155 and the volume of liquid composing it remain for

40

ever unaltered', and rings 'will retain for ever the same

kind of be-knottedness or linking'. It possesses kinetic

energy by virtue of translational motion, and 'it can also vibrate about its circular form, and in this way

possess internal energy', which was promising for

explanations of heat and radiation. That the treatment

was almost entirely kinematical, once having accepted

Helmholtz' hydrodynamics, was taken to indicate its more

fundamental nature compared with the ordinary 'solid

particle' kinetic theory which required the assumption

of repulsive forces.156

Thomson developed Kelvin's vortex atom version of

gas kinetic theory as far as a derivation of Boyle's law

and gave some suggestions for possible experiments to

decide between this and the ordinary kinetic theory;157

it is not known whether these were tried. Applying these

results to gaseous chemical compounds Thomson assumed

that atoms combined in the manner of the association of

two vortex rings of equal strength, when one overtakes

the other. In this case, providing their dimensions are

compatible, the hinder one passes through the one in

front, they do not separate, but continue to circulate

in and out of one another; a transverse section would

show two separate circles rotating about a point midway

between them. The disturbance of neighbouring rings

would alter the radii and cause a brief separation

resulting in continual change of partners as in the

theory of Clausius and Williamson. The ratio of paired

to free time is of importance with respect to whether

combination occurs or not, and this factor might link

the chemical strength with the dielectic strength of a

gas. His attempts to extend the results for linked

columnar and, mathematically similar, ring vortices to

chemical bonding and atomic structure is of considerable

interest. Firstly, the kind of linking considered was not

in the manner of a chain, but like the strands of a

twisted rope, thus:

take a cylindrical rod and describe on its surface a screw with n threads ... bend the rod into a circle and join the ends, then each of the n threads

41

of the screw will represent the central line of the vortex core of one of the n equal linked vortices...158

The vortices could be independent twisted rings, or if

not joined end to end there could result 'an endless

thread with n loops'. He assumed that the atom of each

element was composed in either of these similar ways, of

a number of rings, or a single ring. It was demonstrated

that six rings (or columns), or less would maintain a

stable motion; the transverse section would show, for

example, six separate circles on the points of a hexagon

moving around the midpoint of the hexagon;159 this he compared with Mayer's experiments on the stable arrange-

ments of thin vertical magnets floating in water. Thomson

used the same analogy in 1897 when his corpuscular atomic

theory was first set out.160

The picture of chemical combination is difficult to

discern from Thomson's descriptions and is in need of clarification.161 The combination of two atoms each of a

single ring is described above, and it is this kind of

circulating and mutual overtaking motion which applies

both to the linked rings within a single complex atom, and

to the associated rings in a molecule of two or more atoms.

Thus when two of the complex atoms have combined to form

a molecule, the vortices form a unified system; should

there be a total of more than six rings, they must group

into 'primaries' consisting of six or fewer 'secondaries'.

A transverse section of any molecule would show single

circles, or groups of circles (primaries), arranged on

each point of a polygon. Each primary group rotates about

its own point, and the polygon itself rotates about its

centre.

An explanation of chemical valency, the main burden of

the later additions to the essay, relied on the simplifying

assumption that the strengths of all the rings composing an

atom are equal. This led to the result that for atoms to

form a stable molecule, the strengths of the primaries must

be equal. This determines valency, and Thomson gives as an

example an atom of two rings combining with an atom of one

ring:

42

since for stability of connection, the strength of all the primaries which form the components of the compound system must be equal; the atom consisting of two links must unite with molecules containing two atoms of the one with one link.162

Thus the number of linked rings per atom is taken to be

the fundamental valency. That HO is far less stable than H2O follows from the implicit supposition that the three

rings of HO could not arrange themselves symmetrically

in section. With the assumptions firstly that the linked

rings of a single complex atom can break in various ways

into two or more primary groups, and secondly that the

single rings of monovalent atoms can join closely into a

compound primary, again with a maximum of six rings per

primary, apparent variable valency was explained. Water

was probably not H-H-O, with three primaries and oxygen a

monad, but more likely H2-0 with only two primaries and

oxygen divalent. Hydrogen peroxide would then be H2-0-01 with three primaries, each of two secondaries. Thomson's

further development of the theory in the following year,

1884,163 provoked a strong reaction from the physical

chemist Ostwald,164 which was perhaps a marginally better

reception than being ignored.

During the following decade Thomson published no

further descriptions of detailed atomic structure, and

attempted to apply more general methods such as physical

dynamics and the thermodynamics of Gibbs to aspects of

chemistry.165 The most interesting aspect of his work from

the point of view of our study was that on the relation

of electricity with chemical combination in the gaseous

state, which formed a major part of his researches. In his

paper 'On a Theory of the Electric Discharge in Gases'166 published shortly before the Treatise of 1883 Thomson made

an assumption strongly unifying matter and electricity -

both were seen as manifestations of the same ether:

Let us now suppose that we have a quantity of gas in an electric field. We shall suppose, as the most general assumption we can make, that the electric field consists of a distribution of velocity in the medium whose vortex-motion constitutes the atoms of the gas.167

The attempt was then described, to relate the electric

strength, chemical stability, temperature, and pressure of

43

a gas to one another, together with some quantitative

calculations and suggestions for experiments. He stressed

the intimate connection which he saw between electrical

conduction and chemical action:

Thus, according to the view we are now discussing, chemical decomposition is not to be considered merely as an accidental attendant on the electrical discharge, but as an essential feature of the dis-charge, without which it would not occur.168

This was a view which Thomson maintained, extended, and

studied experimentally for the next decade, and although

he did not develop the vortex atom theory further during

this period, in which its original proponent Kelvin came

to reject the idea totally,169 his explanation of the

electric field in terms of fluid motion became more

detailed. Thomson's description of the field in terms of

'tubes of force' of 1891,170 the theory of electrical

oscillations, and descriptions of the experimental and

theoretical work on the discharge of electricity through

gases, were treated in his Recent Researches in Electricity

and Magnetism, of 1893;171 these were cited by Rutherford

in his first papers, on magnetism and electromagnetic

radiation, during the next two years.

Each tube of electrostatic force is of unit strength,

starting on a unit electrolytic positive charge and ending

on a negative one, or else forming a closed ring.172 They

consist of vortex columns or filaments in the ether whose

kinetic energy constitutes the potential energy of the

electrostatic field; magnetic effects are produced by their

lateral motion, which also constitutes the propagation of

light in a quasi-corpuscular fashion.173 Thomson's theory

of chemical combination of 1893, with its hints of subatomic

structure, was purely electrical - an extension of his views

of 1883 and a modification of the theory of Helmholtz of

1881.174 All unclosed tubes join pairs of atoms, which are

considered to be chemically combined if the tube is of

molecular dimensions, but chemically free if the tube is

long: 'when a tube falls on an atom it may modify the

internal motion of the atom and thus affect its energy',175

which accounted for the differing affinity of atoms for

electricity postulated by Helmholtz.

44

'Now the laws of Electrolysis show that the number of

Faraday tubes which can fall on an atom is limited; thus

only one can fall on an atom of a monad element, two on

that of a dyad and so on'.176 The atoms in chemically

saturated compounds can receive no more tubes so that

each end of an unclosed tube always falls on a free atom.

The existence of free electricity in electrolytes, gases,

and metals too, therefore always requires free atoms and

hence chemical decomposition. Thomson gave diagrams of

the arrangement and movement of the tubes between atoms in a gas,177 but did not explain how the vortex tubes of

force were linked to the 'internal motion of an atom',

nor what form he now conceived this motion to take.

However, in his paper on 'The Relation between the

Atom and the Charge of Electricity carried by it',178

published in 1895 at about the time of the arrival of

J.A.McClelland, E.Rutherford and J.S.Townsend to study at

the Cavendish, Thomson gave something of a picture of an

electro-chemical vortex atom theory. The atom seems to be

differently constituted from that of 1882-3 and appears

incidentally to suggest an explanation of the directional

nature of valency, which the earlier theory did not:

Now let us consider the atoms on which these tubes end. Let us suppose that these atoms have a structure possessing similar properties to those which the atoms would possess if they contained a number of gyrostats all spinning in one way round the outwardly drawn normals to their surface.179

Within two years Thomson had constructed the first

corpuscular atomic model. His support for this theory

in some way involved Lockyer's dissociation hypothesis,

Lorentz' electron theory and the newly discovered Zeeman

effect, as we have seen. But the attainment of such a

theory depended not only upon the discovery of the charged

material 'corpuscle' by means of improved experiments on

the cathode rays, but on remnants of the vortex theory

of the chemical atom.

This is illustrated by others who had developed

electrical and vacuum techniques sufficiently to obtain

results similar and perhaps prior to those of Thomson,

but did not make such significant use of them at the time.

45

The publications of W.Kaufmann180 and E.Wiechert181 in

1897 are notable in this respect. From his experimental

results Kaufmann drew only the limited conclusion that

if the cathode rays were particles then their charge to

mass ratio was 107, which was unexpectedly large. Wiechert

obtained a similar e/m value; he went further than Kaufmann

by assuming that the rays were in fact particles, and that

they possessed the electrolytic charge. He thus estimated

their mass to be of the order of 1/1000 of the lightest

atom. Wiechert continued to concentrate on mathematical

electrodynamics and did not develop the chemical impli-

cations.182

It is generally stated that Thomson's first conception

of a subatomic corpuscle dates from about the beginning of

1897, perhaps after knowing of Zeeman's new experimental

results.183 This was closely linked with his experiments

showing that the magnetic deflection of cathode rays was

independent of the gas in which they were produced,184 as

well as mean free path, velocity, and other considerations.

However, Thomson's continuing interest in chemical atomic

structure throughout the experimental developments has

previously not been clearly brought out. We have already

noted his theory of electrical conduction of all kinds

published in 1895, which involved vortical tubes of force

linking atoms containing gyrostats; another aspect of

Thomson's chemical ideas soon became evident. In April

1896, reviewing experimental advances on the recently

discovered Ontgen and Becquerel rays,185 he several times

slightly misstated Rontgen's result that for a series of

metals the absorption of the X-rays is in the same order

as their density.186 For example, he mentioned 'Rbntgen's

discovery of the close connection between the absorption

of these rays and the atomic weight of the absorber'.187

Either version fitted the experimental results but Thomson's

statement may be significant for he also suggested that the

absorption of these rays, considered to be of very short

wavelength, was caused by Proutian primordial atoms. The

expression 'atomic weight' was indeed correctly replaced by

'density' in the report188 of the Rede Lecture given some

weeks later, but in this we read:

46

There seems no simple relation between the density of a body and its transparency to visible radiation or electrical vibration; in the case of the Wintgen rays, however, it seems the greater the density the greater the opacity. This appears to favour Prout's idea that the different elements are compounds of some primordial element, and that the density of a substance is proportional to the number of primordial atoms; for if each of these primordial atoms did its share in stopping the Wintgen rays, we should have that intimate connection between density and opacity which is so marked a feature for these rays.189

That Prout's hypothesis could be brought up on such

evidence indicates a predilection for the idea in 1896.

The statement was made with reference only to X-rays,

seen as radiation in terms of tubes of force,190 and not

to cathode rays, seen as particles with etherial effects.

However, in his well-known paper of 1897191 Thomson

laid stress upon the distances penetrated by the cathode

ray particles without mentioning X-rays: 'Now Lenard

found that this distance depends solely upon the density

of the medium and not upon its chemical nature or physical state'.192 By this time Thomson considered the cathode ray corpuscles to consist of 'a substance from which all the

chemical elements are built up'193 and in imagining this

building194 he considered the possibilities of Mayer's

magnets more deeply than with the vortex atom of 1882-3.

In the atom the material corpuscular components, whose

charge might be much greater than the electrolytic charge

(Stoney's electron),195 were held together by tubes of

force, many for each corpuscle. Only a single stray unit

tube would bind for example a hydrogen to a chlorine atom,

giving HC1. He did not discuss the existence of vortices

within a corpuscle to which the tubes are connected nor,

later, the possible gyrostatic nature of the unit-charge

corpuscles which he soon accepted. Thomson's stress, and

defence, of the material nature of the corpuscles seems

compatible with some kind of etherial vortex explanation

of matter, electricity and light. But Thomson had said

that such a theory 'cannot be said to explain what matter

is, since it postulates the existence of a fluid possessing

inertia'.196 A purely electromagnetic explanation of matter,

47

in which studies of the radioactive rays played a vital

part, was to develop by 1901.197

As Thomson struggled with experiments and Proutian

ideas involving X-rays and cathode rays in 1896, and was

taking an interest in the Becquerel rays, Rutherford

wrote home from Cambridge:

I am working very hard in the Lab. and have got on what seems to me a very promising line - very original needless to say. I have some very big ideas which I hope to try and these, if successful, would be the making of me. Don't be surprised if you see a cable some morning that yours truly has discovered half-a-dozen new elements, for such is the direction my work is taking. The possibility is considerable but the probability rather remote.198

Rutherford's known laboratory notes199 and his published

papers on the newly adopted subject of the electric

properties of gases exposed to radiation - X-rays,

uranium rays, and ultra-violet light - give us obscure

clues as to the nature of these new ideas. We have,

however, noted the long-standing interest of his Professor

in chemical atomic theory. We shall see that Rutherford

shared this interest and how what may have been no more

than a light-hearted prediction became fulfilled and

exceeded through radioactivity. But the existence of the

first new chemical element arising from these studies was

proposed in France within two years of Rutherford's boast -

and then he did not believe it.

48

CHAPTER 2

THE DISCOVERY OF URANIUM RAYS AND RADIOACTIVITY

1. Becquerel's discovery of uranium rays (1896-7) From ROntgen's famous experimental discovery of X-rays

stemmed Henri Becquerel's discovery of uranium rays.

Within days of ROntgen's paper 'On a new kind of radiation'1

many scientists were using the readily available Hittorf,

Lenard or Crookes vacuum tubes to verify, and advance

studies on the new phenomenon. The most spectacular aspect

of the rays was their capability of penetrating certain

solid materials to produce fluorescence or photographic

action on an adjacent screen or plate, giving images for

example of bones within the living body. Most of the

thousand articles and fifty books published on the subject

during the year of 1896 were concerned solely with such

medical possibilities.2 However, there were several points

for physicists to consider, the question of the source of

the rays being an important one. Their discoverer pointed

out that they appeared to come from the most brightly

fluorescing part of the glass of the vacuum tube,3 and it

was this statement, though not at first hand, which came

to set Henri Becquerel on a somewhat novel line of research.

It is my intention firstly to describe and discuss the

experiments and hypotheses which constitute Becquerel's

discovery of uranium rays and to indicate some of the

neglected background to those studies. This is followed

by an account of other varied and novel researches

emerging during the period 1896-7 which may provide an

improved interpretation of the contemporary attitude to

the Becquerel rays.

Unlike the events leading to the discovery of X-rays,

which remain obscure,4 the beginnings of the experimental

work with which we are now concerned were clearly described

by Becquerel, some seven years later,5 leaving only a few

questions in doubt:

Dans la seance de l'A.cadgmie des Sciences du 20 janvier, au moment ou M.H.Poincarg venait de montrer les premieres radiographies envoy6es par

49

M.ROntgen, je demandai a mon confrere si l'on avait determing quel etait, dans l'ampoule vide productrice des rayons X, le lieu d'emission de ces rayons. Il me fut repondu que l'origine du rayonnement etait la tache lumineuse de la paroi qui recevait le flux cathodique. Je pensai aussit8t a rechercher si l'emission nouvelle n'etait pas une manifestation du mouvement vibratoire qui donnait naissance a la phosphor-escence, et si tout corps phosphorescent n'emettait pas de semblables rayons. Je fis part de cette idee et de ce projet a M.Poincare...6

The weekly report of the Academy for 20th January in fact

contains only a brief note of the exhibition of some of

the earliest X-ray photographs; H.Poincare,Professor of

mathematical physics at the Sorbonne, first published the idea ten days later.7 The experimental connection between fluorescence and X-ray emission was eroded throughout

the year, as it was shown that the point of incidence of

the cathode rays upon solid objects was the primary source,

that rays were not emitted from other brightly glowing

parts of the glass tube, and that metals could emit X-rays

unaccompanied by visible fluorescence.8 However, such a

connection was under active consideration at the beginning

of 1896, and whilst J.J.Thomson at Cambridge reported

failure to obtain penetrating radiation from fluorescing

glass, gases, and luminous paint,9 others, in France,

announced success.

Henri Becquerel was the third of four successive

members of the family to be Professor of Physics at the

Natural History Museum in Paris; his father Edmond and

himself were well known for their work on various aspects

of physics, particularly phosphorescence.10 G.G.Stokes had coined the term 'fluorescence' for emission of light

only during the time of irradiation, and considered

'phosphorescence' which continued after irradiation had

ceased, to be of a different nature. Edmond Becquerel,

however, insisted on an experimental continuity between

the two, using his rotating 'phosphoroscope' of 1858 for

timing short durations. Whilst he always used the term

'phosphorescence',11 some maintained Stokes' distinction,

and others continued to use the terms indiscriminately.12

During the last quarter of the nineteenth century

50

the subjects of phosphorescence and fluorescence had

provided opportunities for a considerable effort in

experimentation upon a large number of inorganic and,

in the 1880's, organic substances, as solids, solutions

or gases. Studies included examinations of the relations

between the wavelengths and intensities of absorbed and

emitted light, the rise and decay of intensities with time,

and the marked effect of traces of metal compounds in

solids. Following Stokes' discovery of fluorescence, and

his explanation of the phenomenon in terms of vibrations

of molecules, in 1852,13 E.Lommel then others in Germany

developed mathematical theories of oscillations of part-

icles, natural, forced and damped, which partly agreed

with the observed phenomena.14 A clear picture of the part

played by studies of luminescence in the development of

physical science has yet to be created; this does not

concern us here, but we have noted that several scientists

who were linked with radioactivity entertained qualitative

electrical atomic-molecular views of phosphorescence in

the 1890'8.15 E.Becquerel who is said to have dominated

the experimental field16 until about 1880 put forward only

qualitative explanations of his results;17 his son Henri

continued the researches in a similar way. Some of the

latter's comments concerning uranium salts are of interest

in throwing some light on the way in which he understood

the phenomena of phosphorescence, which came to be so

closely linked with the new uranium rays in 1896. In

discussing the 'Relations entre l'absorption de la lumiere

et 1'6mission de la phosphorescence dans les compos6s

d'uranium' in a paper of 188518 he concluded that the

compounds of uranium are in 'un etat moleculaire' such

that they exert a selective absorption of harmonically

related wavelengths of light, and that some of these

compounds in the uranic series also emit harmonically

related bands of a longer wavelength; some bands were

common both to absorption and emission spectra, which

suggested that the property 'de vibrer a l'unisson' might be the actual cause of absorption. He noted that the

broad green band of the emission spectrum of incandescent

51

uranium vapour was very close to an absorption band

characteristic of uranous salts. Perhaps he considered

that the emission of light, or of certain bands by uranium

vapour might be due to its molecular state, but Becquerel

gave no detail. Six years later, in 1891, his views 'Sur

les lois de l'intensite de la lumiere 6mise par les corps

phosphorescents119 were that the laws of rise and decay

of luminous intensity could be related to an equation of

simple harmonic vibration, modified by taking into account

the loss of energy of the oscillating molecules via the

'ether intermoleculaire', which loss was taken as proportion-

al to the square of a velocity term. And in a note of the

same year 'Sur les differentes manifestations de la phos-

phorescence des mineraux sous l'influence de la lumiere ou

de la chaleur'20 Becquerel speculated on the mechanism by

which the 'conservation indefinie dans les corps' of the

energy of phosphorescence-by-heat (later termed thermo-

luminescence) might be achieved. Was the substance in a

state comparable with magnetism, or:

La deperdition d'energie est-elle continuellement compensee? Ce sont des questions que l'on ne saurait decider actuellement et sur lesquelles les etudes ulterieures apporteront peut-kre quelque lumiere.21

He did not publish further on this point himself; his rival

E.Wiedemann in conjunction with G.C.Schmidt had provided

the beginnings of a chemical explanation by 1895.22

Becquerel's studies of the connection between the new X-rays

and the phenomenon of phosphorescence led him to his great

discovery of uranium rays and to questions of energy which

were even more difficult to answer. Henri Becquerel's first publication for some three

years was again on the subject of phosphorescence. In his note of 24th February 1896 'Sur les radiations emises par

phosphorescence'23 he cited not Poincare but Charles Henry,

of the gcole Pratique des Hautes Etudes, Paris, with whom

there were to be controversies later that year.24 Henry

in his paper on 'Augmentation du rendement photographique

des rayons ROntgen par le sulfure de zinc phosphorescent'25 showed that phosphorescent zinc sulphide placed in the path

of the rays from a Crookes tube intensified the photographic

52

effect of rays penetrating aluminium. Ho quoted Poincare's

query: Ne peut-on alors se demander si tous les corps dont la fluorescence est suffisamment intense n'emettent pas, outre les rayons lumineux, des rayons .X de Röntgen, quelle que soit la cause de leur fluorescence...26

and considered that he could answer it in the affirmative.

This statement of Poincare, repeated by Henry, does contain

the point claimed as his own by Becquerel in 1903,27 but

one cannot thereby deny this claim. Becquerel also mentioned

Niewenglowski's announcement of the previous week28 that

the sulphide of calcium as well as zinc produced penetrating

rays during irradiation merely by sunlight. Becquerel extended these observations to some unnamed phosphorescent

substances and in particular to uranium salts whose brilliant

phosphorescence of short duration was well known. The

techniques which were involved, simple though fraught with

pitfalls, are illustrated by Becquerel's description of

some of his first experiments.29 He wrapped a Lumiere

bromide-in-gelatine photographic plate in two sheets of

dense black paper, light-proof for a day's exposure to sun-

light. Upon this was placed a crystalline lamella of

potassium uranium sulphate, with a glass slip interposed

to avoid the chemical effects of vapours. Exposure to

sunlight for several hours to produce the required pen-

etrating phosphorescence, followed by development of the

plate, revealed a blackened area. Becquerel reported his

success in obtaining images of metal objects by placing

these between the phosphorescent source of the penetrating

rays, and the sensitive plate. This had previously been

achieved only by means of X-rays. The fortunate experiments made by Becquerel during

the following week and announced at the next meeting of

the Academie on 2nd March,30 were immediately seen by him-

self as important, and can now be interpreted as a sig-

nificant marker in the initial stages of Becquerel's

research. He firstly described experiments showing that

photographic action occurred after excitation by reflected,

refracted, or diffused sunlight, and through aluminium and

copper foil as well as black paper, and then stressed his

53

view of the importance and the unusual nature of some new

observations.

Combinations of a lamella of the double sulphate of

uranium and potassium, K(UO)SO4.H20, aluminium sheet, and

photographic plate prepared for irradiation on Wednesday

26th and Thursday 27th February were never thus treated,

owing to the intermittent character of the sunlight during

these days. They were placed in a drawer, and then dev-

eloped on Sunday 1st March.31 Expecting weak images, instead

he found very intense ones, and immediately concluded that

the action must have been occurring in darkness, 'l'action avait da continuer a 1'obscurit6'. He found experimental confirmation that day. Three lamellae, one placed directly

on a photographic plate, the others with glass or aluminium

sheets interposed, were left for five hours in a cardboard

box inside another cardboard box, in a drawer. The whole

operation was performed in the dark-room. Becquerel found

images, weaker with the aluminium, exactly as when the

crystals were irradiated by sunlight. His interesting,

but tentative, hypothesis was that the effects might be

due to invisible radiations 'emises par phosphorescence'

but of a duration 'infiniment plus grande' than that of

visible phosphorescence, known to be but 1/100 sec. for the salt used. He considered that he had found 'un nouvel ordre de phenomenes'.

Difficulties were seen when Becquerel repeated his

conclusions at the Physical Society meeting, later that

week, adding that certain phosphorescent crystals did not

give the rays. For if the effects were due to radiations

of shorter wavelength than those from the sun as Becquerel's comparison with Lenard and Röntgen rays was taken to imply, Stokes' law of phosphorescence would be contradicted; it

was therefore suggested that the new penetrating rays might instead be of a longer wavelength.32 Experimental invest-igation and comparison of his new rays with X-rays and an examination of the relationship with visible phosphorescence

was Becquerel's way forward and away from the heavy crit-icisms levelled at the work of G.Le Bon on Ilumi6re noire'P His remarkable results, announced within days34 and standing

54

unchallenged for two years, appeared to provide a nearly

complete answer to the question of the nature, if not the

source, of the new rays, and furthermore to clarify the

understanding of X-rays. Placing the salt within the vessel of a gold-leaf

electroscope which was shielded from electric radiations35

by a metal screen, and against ultra-violet rays by yellow

glass, Becquerel showed that the new rays caused the diss-

ipation of positive and negative charges at equal rates.

Thus applying this electrical and more quantitative tech-

nique he found that the time of collapse of the leaves was

proportional to the thickness of an interposed aluminium

screen. These were important properties, which X-rays

were known to possess by this time, so that the new rays

were evidently similar to X-rays. By means of a steel plane

mirror, and a concave hemisphere of tin, diffuse photo-

graphic images were obtained. Becquerel took these results

to indicate a definite, if diffuse, reflection of the rays,

in conjunction with more striking experiments. Glass tubes

filled with a powdered phosphorescent substance were

attached perpendicular to a glass sheet which was placed

upon the photographic plate. The resulting image, which

was a clear circle with a black disc within, surrounded by

a blackened area, proved that the invisible rays were both

reflected and refracted in the same way as light. These

experiments were later found to be repeatable,36 but

Becquerel's interpretation was to be rejected in a little

over two years37 when the images were attributed to

'secondary' radiation. These results were soon to be

joined by some which are dubious or inexplicable and by

others which stand to this day. Becquerel's survey of

phosphorescent minerals showed that several uranium salts

and two calcium sulphide specimens gave rays capable of

penetrating 2mm.of aluminium but that other phosphorescent

substances failed to give penetrating rays. Perhaps, he

thought, the phenomenon could be likened to visible

'thermoluminescence'; the latter involved earlier excit-

ation, then an emission of visible radiation upon gentle

heating at a later time.

55

The calcium sulphide specimens had vanished from

Becquerel's list of emitting substances by the time of his

fourth report delivered two weeks later on 23rd March.

How they had come to be included at all is not clear. He

remarked that no form of thermal or electrical excitation

could re-excite the emission of invisible radiation from

these still highly phosphorescent substances and that

Troost, Professor of General Chemistry at the University

of Paris, in following Becquerel's first experiments, had

experienced a similar effect.38 Thus only uranium salts

were now left.39 Their emission of penetrating rays, still

continuing after fifteen days, was reported by Becquerel

to be strongly intensified after illumination by electric

spark or arc, slightly intensified by daylight, but

imperceptibly by the light of burning magnesium, as

determined photographically; this latter method did however

cause excitation as determined electroscopically. His

conclusion was that the emission of invisible rays was a

kind of phosphorescence, excited by certain radiations,

but not closely connected with ordinary visible phosphor-

escence nor fluorescence. From the modern point of view,

there should have been no change in the intensity of the

radiation by these means. Perhaps he was misled by a

belief that the new rays ought to be capable of excitation

in the manner of ordinary phosphorescence, combined with

the flexibility of interpretation which his qualitative

experimental methods allowed. It was to become apparent

during the following year that such an increase in intensity

could not be produced; this was to place the phenomenon

further beyond accepted explanations. But even as they

were announced the problems of reconciling his conclusions

with Carnot's principle, as well as Stokes' law, were

recognised and discussed for example at the meeting of the

Physical Society which Becquerel addressed at this time.40

In his fourth note to the Academy41 Becquerel was

able to confirm the distinction between the invisible and

visible phosphorescence by the remarkable procedure of

melting a crystal of uranium nitrate in its own water of

crystallisation, when it ceased to exhibit phosphorescence

56

or fluorescence, and then allowing recrystallisation by

cooling, while excluding all luminous excitation. There

could now be no emission of visible radiation, but the

photographic effect of the invisible rays was as strong

as ever.

Becquerel was now turning towards a study of the

nature of the radiations; his final attempts at finding

their source were highly, if not completely, successful,

and are described in his next two papers. On 30th March42

he noted that 'un nouvel exemple d'ind6pendance entre les

deux ph6nomenes d'6mission' was provided by the invisible

rays detectable from the non-fluorescent solution of

uranium nitrate; and further, that visibly phosphorescent

sulphides could not be induced to emit invisible penetrating

rays, upon excitation with X-rays. After a seven-week

silence, awaiting Moissan's latest preparations of pure

metallic uranium,43 the purpose of Becquerel's sixth note

to the Academy was to announce the 'Emission de radiations

nouvelles par l'uranium metallique'.44 The inorganic

chemist Henri Moissan had turned from his successful

researches on fluorine towards high temperature studies

of boron, carbon and diamond, and silicon, in the 1890's;

he had applied his newly developed electric furnace of

189245 to the problem of preparing pure samples of .

refractory metals such as zirconium, chromium, manganese,

tungsten, molybdenum, vanadium, titanium and uranium,46

whose melting points lie above about 2000 degrees C. He

achieved the first fusions of some of these, and also

examined the carbides and carbon solutions of these and

other metals, including uranium.47

Thus Becquerel found that it was not only the salts

of uranium, but the element uranium itself from which the

rays were emitted. Uranium carbide, commercial uranium

powder and crystallised and cast uranium metal gave

stronger photographic and electrical effects than potassium

uranium sulphate; the charged leaves of an electroscope

collapsed about four times faster with uranium metal, at

rates of the order of 1 to 8 degrees per minute of time. Salts stored in a double lead box shielded from all

57

exciting radiation continued to emit radiation at a very slowly decreasing intensity. The excitation of the

emission above this level upon irradiation by the sun or

more effectively by electric arc or spark was, strangely,

confirmed; the level of emission declined to the normal

within hours. These results involving decay or excitation

were not corroborated by other workers, and were soon

forgotten, but at the time they agreed with Becquerel's

conclusions concerning the nature of the new phenomenon.

Without describing a possible mechanism he stated that

this was the first example of a metal exhibiting invisible

phosphorescence. Hence 'radiations uraniques' and 'rayons

uraniques' he named the rays in his next communication

some months later.48 Their intensity, he now cautiously

admitted, seemed hardly to have changed in eight months,

which fact was completely outside ordinary phosphorescent

phenomena: 'on n'a pu reconnaitre encore ou l'uranium

emprunte l'energie quill emet avec une si longue persist-

ance'.49 Becquerel had not solved this problem by the time of his last paper of the series, delivered six months

later in April 1897;50 the problem of the energy source was to worsen during the next seven years, to reach crisis

proportions in the view of some scientists.

But by 1897 Becquerel was concentrating more upon the nature and properties of the invisible rays; these studies

formed an important part of his discovery in the opinion

of fellow physicists. We have seen that by 9th March 1896

Becquerel's experiments indicated that the new rays possess-

ed the properties of penetration, and discharge of elect-

rified bodies, in common with X-rays, but were reflected

and refracted like visible light. On 23rd March51 he

reported qualitative confirmation of refraction, using a

glass prism and a linear source of the rays consisting of

a glass tube of lmm. diameter filled with crystalline

uranium nitrate. A further comparison between X-rays and

the new rays was the proportion absorbed by the same screen.

Becquerel found that X-rays were weakened four times as

much as the new rays, and took this to indicate that the

two kinds of radiation differed in wavelength. As for the

58

cause of the dissipation of electric charges by both

radiations, Becquerel professed ignorance, but noted

that the gas appeared to become conducting. Others with

previous experience in this area were working somewhat

more successfully on this problem at the time, as will be

seen. Becquerel's next note to the Academy was specifically

'Sur les proprietes differentes des radiations invisibles

emises par les sels d'uranium, et du rayonnement de la

paroi anticathodique d'un tube de Crookes'.52 He confirmed

unequal absorption of these radiations by different sub-

stances, both photographically and electroscopically, the

new rays being generally the more penetrating. Also, that

the new rays were non-homogeneous, as others had found for

X-rays, by interposing screens between source and electro-

scopic detector. The most strikingly successful of

Becquerel's experiments, so it seemed, was his attempt to

detect polarization of the rays, using crossed and parallel

tourmalines, with which Riintgen had obtained negative results for X-rays. Using the double sulphate of uranium

and potassium as the source of the rays, Becquerel found

that plates developed after sixty hours showed clearly

stronger intensities for parallel tourmaline crystals.

Becquerel took this to show that the invisible rays

suffered double refraction and polarisation of the two

refracted rays, followed by unequal absorption of the

differently polarised rays by the second tourmaline.

Although never repeatable and still inexplicable, these

results were eagerly accepted without public question

until 1899. A reviewer of 1898 commented that Becquerel's

demonstrations of the reflection, refraction, double

refraction, and polarisation of uranium rays showed that

'there can be no reasonable doubt that they are short

transverse ether waves';53 their possesion of all the

properties of X-rays indicated that the latter were similar.

Indeed, the view that these studies of uranium rays were

valuable in throwing light upon the nature of X-rays had

been expressed early in 1896. Some three weeks after

Becquerel's announcement of the polarisation of uranium

rays, J.J.Thomson's comment appeared in print:

59 The radiation from the uranium salts is thus intermediate in properties between ordinary light and Riintgen rays; and as there can be no question but that this radiation consists of transverse vibrations, inasmuch as it can be polarised, it affords presumptive evidence that the RCintgen rays are also due to transverse vibrations.54

J. Perrin made similar points,55 so too did G.G.Stokes

whose transversal irregular ether pulse theory of X-rays

was accepted by many physicists.56 As the most crucial of

Becquerel's results on the properties of the uranium rays,

and the conclusions drawn from them were shown to be false,

X-rays too began to be seriously considered in a different

light;57 but that is another story.

Despite the unreliability which was soon shown to

exist in some of his experimental work, Becquerel's

achievement in arriving at a demonstration of the existence

of uranium rays was of a remarkable nature. This is high-

lighted by the parallel studies by some of his contempor-

aries, which were seen as being related to Becquerel's

rays. Some of these studies proved to be very short-lived,

others were more reliable and came to be distinguished

from those on uranium rays, with varying degrees of

difficulty and rapidity. We have already noted that

Becquerel and Troost had obtained photographic effects

which they interpreted as being caused by rays penetrating

black paper and aluminium after issuing from phosphorescent

sulphides; these results stood only for a few weeks, though

they led to the discovery of uranium rays. It is notable

that S.P.Thompson, in London, followed a closely similar

path; like Becquerel he cited C.Henry on the augmentation

of X-ray photographs by phosphorescent zinc sulphide58 and

found that phosphorescent substances emitted such rays on

their own account. The merits of his claim59 to an

independent discovery of the rays from uranium salts

have been discussed;60 his explanation of the effect was

the same as Becquerel's; he coined the name 'hyperphos-

phorescence' but this was little used. Like Becquerel he

60

had found that 'a phosphorescent substance such as

sulphide of Barium' during and after illumination,

emitted, besides the visible radiation of phosphorescence,

rays which were like X-rays in being invisible, penetrating

aluminium sheets and producing a photographic effect. On

communicating this to Stokes,61 the authority on phosphor-escence, he was advised first not to delay publication

then, a few days later, that he had 'already been antici-pated'62 by Becquerel in papers earlier that month. Thompson63 specified uranium salts only after he had seen

these papers and had been notified by Stokes that the French

scientist had attributed the radiation to metallic uranium64 He asked Crookes for a specimen of uranium metal; the latter

was examining the efficacy of metals as radiators of X-rays

under the impact of cathode rays, and replied:

I have my metallic uranium in a vacuum tube at present, testing it against platinum as a radiator of the unknown X. So far it is decidedly the better of the two. I have another small piece which will be disengaged tomorrow, and then you shall have it.65

Uranium in metallic form was not readily available, though

H.Moissan provided some specimens; Crookes reported that

he had to discontinue these experiments owing to lack of the metal;66 there was no difficulty in obtaining compounds.

No doubt some of Becquerel's experiments of 1896 were

repeated by Stokes or others at Cambridge, Crookes or

S.P.Thompson, G.Le Bon in France,67 or by German physicists.

But no-one published a denial of the vital proof of double

refraction and polarization during that year nor the next.

The results of S.P.Thompson, Becquerel and Troost,

who had found that penetrating rays were emitted from

phosphorescent sulphides, were obtained by the photographic

method of detection; indeed the purpose of these experi-

ments, and those of C.Henry, was to obtain photographic

images. Becquerel from the first realised the danger of

chemical action from vapours and took precautions against

this; others did not. He knew that substances opaque to

visible light could be transparent to Hertzian waves,

infra-red and ultra-violet light, which might produce

photographic or electrical effects, and took measures to

61

avoid the possibility of confusion with uranium rays.

Others, in the rush to publish on X-rays and similar

phenomena during 1896-7, may have neglected any of these

points, and produced photographic results with conclusions which were soon proved false.

A few scientists thus took up Becquerel's work with

misguided enthusiasm finding Becquerel rays where we see

none now. H.Muraoka of the Physical Institute, Kyoto,

Japan, in his paper on the light from glow-worms,68

published towards the end of 1896, claimed that in addition

to visible light these worms emitted rays similar to those

of Becquerel in being capable of penetrating metals. The

photographic plate was blackened only in proximity to the

cardboard of the container, and the author explained this

by a concentration effect of the cardboard upon the glow-

worm rays. A partial denial of these results, attributing

some of the effects to vapours, in the following year is to his credit.69 In similar fashion, W.Arnold70 attributed

the action of certain substances such as metallic sulphides,

uranium salts, and retene on photographic plates to

'Becquerelstrahlen'. And A.F.McKissick reported from the

Alabama Polytechnic Institute, U.S.A., a similar success

in his search for phosphorescent substances which emitted

the Becquerel rays. He examined uranium salts but found

sugar to be the best emitter.71 P.Spies72 and F.Maack73

reported intensification of the photographic effect of

the rays from uranium by the interposition of certain sub-

stances; we may suppose that secondary radiations or

chemical vapours from these substances could have produced

such an effect. These results, essentially extensions of

Becquerel's work, were not developed further. But Gustave

Le Bon considered that his own researches on radiation

constituted a branch of study conceived independently of

and prior to that of Becquerel and that Becquerel's work

was encompassed within his own.74 He was supported in this

by P.de Heen, Professor of Physics at the University, and

Director of the Institute of Physics, Li6ge, Belgium.75 Le Bon had previously published on psychology,76 and he is considered by social scientists as a serious contributor

62

to that field.77 However, from the time of his first

announcement concerning an unknown type of 'lumiere noire'78

he was heavily criticised by G.H.Niewenglowski,79

A. and L.Lumiere,80 Perrigot81 and Becquerel82 at the

Academy; these scientists were either unable to repeat the

experiments or attributed the photographic effects of the

new penetrating rays, which Le Bon supposed were emitted

by solid substances after irradiation, to red or infra-red

rays. Le Bon did not accept most of the criticisms and

continued to publish.83 He extended his studies from the

photographic to the electric actions, and concluded that

since all bodies when acted upon by light produce effects

similar to but smaller than those of uranium, this was but

one instance of a general phenomenon of penetrating radi-

ation.84 After reopening and extending discussions of 'lumiere noire' when radioactivity had become more important

in 1900 he was again criticised, this time by P.Curie,85

who pointed out that all the characteristics of 'lumiere

noire' could be accounted for by the well-known properties

of 'rayons calorifiques infra-rouges'. In his writings

on the universal dissociation of matter and emission of

material particles86 Le Bon seems to have expressed views

similar to the idea of a general radioactivity of all

matter which, as will be seen, some workers on radio-

activity entertained during and after 1903. Although

Le Bon's books were popular from 1905, no reputable student

of radioactivity regarded his work as significant. However,

the criticisms of Le Bon's early photographic work were

not quite clear-cut. For in 1905 Rutherford stated not

that the effects were due to infra-red or red radiations87

but 'that there seems to be little doubt that the effects

are due to short ultra-violet light waves'.88

Other possible causes of photographic effects were

made clear during 1896, and should perhaps have been seen

by Arnold, Muraoka, McKissick and others previously

mentioned, as a warning. For R.Colson of the Conservatoire

des Arts et Metiers, in 1896-7 pointed to a decomposition

of the salts in the plate by such causes as mechanical

pressure, chemical actions especially when damp, warmth .

when damp, intense infra-red radiation, very feeble light

63

acting for a long period, as well as visible and ultra-

violet light, and X-rays.89 Freshly cleaned zinc surfaces

blackened the sensitive plate in air or in vacuo, as did

magnesium, but aluminium did not. After attempting

experimentally to determine whether effects were due to

'une radiation ou une emanation' Colson pointed to

E.Demar?ay's detection of the vapour of metallic zinc at

temperatures as low as 184 degrees C. and attributed the

effect to metal vapours which could penetrate some materials.90 These findings may explain some of the

dubious experimental results of 1896-7; they parallel to

some extent the work of Dr. W.J.Russell, F.R.S., Lecturer

in Chemistry at St.Bartholomew's Hospital, 'On the Action

exerted by certain Metals and other Substances on a

Photographic Plate'.91 This line of research is partic-

ularly significant since it was thought to be related to

uranium radiation both before and after the discovery of

the radioactive 'emanation' from the metal thorium.

Rutherford took pains to make the distinction clear92 but

was not immediately successful in persuading all scientists

of this. Russell began his paper by explaining that being

in possession of uranium compounds used for spectroscopic

examination some years before, he had repeated 'some of

the very important experiments which Becquerel has made

with these compounds%93 Russell referred only to the photo-

graphic work, ascertained that no luminous excitation was

necessary over seven months, and noted that specimens kept

in the dark seemed if anything slightly more effective in

their action. In addition to this he found that a perforated

zinc screen, intended to show up the effect of a card painted

with yellow oxide of uranium, gave instead an image which

was the reverse of that expected: the greatest action occurr-

ed beneath the zinc. He was able easily to repeat this,

with variations:

so that the only explanation of the action was that the zinc itself must be able to effect a change of the same kind as the uranium, at all events to act on a photographic plate.94

Russell went further than Colson, in showing that a variety

of substances, though not all, produced an effect through

64

many bodies, but not through glass, even the thinnest.

As for metals and alloys, which had to have bright

surfaces: 'The following is a rough list of active

metallic bodies approximately in the order of their

activity: mercury, magnesium, cadmium, zinc, nickel,

aluminium, pewter, fusible metal, lead, bismuth, tin, cobalt, antimony'.95 Zinc salts, and some metals were not active'; a vague correlation with the 'electrical

series' was suggested. Russell's findings appeared to

have extended Becquerel's results; although he did not

make the point, these may have tended to cast some doubt

upon the latter's original experiments. For Becquerel's

descriptions of the arrangements which produced photo-

graphic effects through metal screens, had not explicitly

excluded the effects which the screens themselves were now

shown to have. Furthermore, the strawboard pill-boxes,

used as containers for the uranium salts being examined

photographically, were found to be more active than the

contents; woods and varnishes were also more active than

uranium. Photographic plates laid face upwards in a

cardboard box for a week were 'very appreciably affected',

but were protected by a glass screen. Perhaps Russell

had in mind the fact that Becquerel placed his arrangement

of salt-screen-plate in a cardboard box within a wooden

drawer for long periods. For copal varnish the cause was

definitely attributable to a vapour, but this seemed unlikely for strawboard:

Still more interest attaches to the action of the metals; do they emit a vapour so delicate in constitution and in such a quantity that it can readily permeate celluloid, gelatine &c., and produce a picture of the surface from whence it came, or is it a form of energy (possibly what has been called dark light) that these bodies emit? Zinc kept and polished in the dark loses none of its activity.96

However, Russell did state that the action through glass. 'proves that there is at least a marked difference between

the action exerted by metallic uranium and that by zinc

and other metals'.97

The close link between Russell's and Becquerel's

findings was evidently still felt by W.Crookes over a year

65

later, in September 1898:

It now appears that some bodies, even without special stimulation, are capable of giving out rays closely allied, if not in some cases identical, with those of Professor RUntgen. Uranium and thorium compounds are of this character, and it would almost seem from the important researches of Dr.Russell, that this ray-emitting power may be a general property of matter, for he has shown that nearly every substance is capable of affecting the photo-graphic plate if exposed in darkness for a sufficient time.98

But by this time the connection had loosened considerably.

C.T.R.Wilson, pursuing his experiments at Cambridge on

condensation nuclei produced in gases by radiation, showed

that a uranium salt strongly influenced the condensation

of water vapour in a glass chamber, from within its

stoppered glass container wrapped for hours in tinfoil.

This proved that tinfoil was transparent to the agent

influencing condensation, and that the uranium salt

'continues to be active when kept in the dark'. Thus, a

few months after the publication of Russell's paper, Wilson

supposed 'There can be little doubt therefore that the

effects on the condensation are really due to the radiation

studied by Becquerel'.99

A further clarification from the Cavendish Laboratory

came with J.J.Thomson's note 'On the effect of zinc and

other metals on a photographic plate'.100 In this he credited

Stokes with the suggestion, at an earlier meeting of the

Cavendish Physical Society on Russell's paper, that a blast

of air between photographic plate and source of action might

distinguish clearly between radiation and vapour. The

distorted images obtained in this way showed that vapours

were the cause. Russell was invited to deliver the Royal

Society's Bakerian Lecture of 1898, which he did, on the

subject of the photographic actions,101 now attributing

the actions to vapour of some kind; at the time of Crookes'

Presidential Address of 1898, Russell considered that the

effects of metals were due to the surface formation of

hydrogen peroxide vapour.102 He continued these studies,

which became largely separated from radioactivity,

producing interesting pictures of substances in the dark.103

66

A later chemical author agrees with Russell's explanation

of the effects of metals, and describes his research as a

'classic work on the subject'.104 Perhaps Crookes was not quite up to date in his linking of Becquerel rays with

Russell's findings, towards the end of 1898, but his was

not the last statement of this kind as will be seen.

The clearest distinction was pointed out by G.C.Schmidt

who had concluded that thorium emitted a radiation with some

properties in common with uranium rays.105 Equally signifi-

cant was his statement that the other substances, mentioned

by Arnold, Pellat, Colson, Russell, Muraoka and Henry with

regard to their photographic effects, were not analogous.

Schmidt considered the most important property of the rays

to be the electrical effect rather than the photographic,

and supposed that 'Diese beiden Eigenschaften gehen also nicht Hand in Hand'.106

The electrical properties of emitting substances were

henceforth always to occupy a more important place in

investigations than the photographic. This was due not

only to Schmidt - others too showed an interest in this

aspect of the new rays, in 1897. Becquerel himself had

discovered the effect and moved very much in the direction

of electrical studies in his last three Notes, before

temporarily leaving the subject.107 C.T.R.Wilson's work

on uranium rays was related to studies of the electrical

properties of gases at Cambridge, and his fellow research

student E.Rutherford was also interested in the electrical

properties of uranium rays in 1897.108 Certainly the dis-

charge of electricity produced by the rays gave quantitative

measures which could be correlated with their intensity.

The photometric estimation of relative blackening of

photographic plates was less reliable, and far more time-

consuming, though used for several years by W.Crookes,

for example.

Once Becquerel began to use the electroscope as a

means of studying the rays, he obtained clearer indications

of their properties, if not their cause. Towards the end

of 1896 Becquerel in employing the electroscope found

that the rays were not homogeneous and were absorbed to

67

different extents by different materials. He was able to

extend to uranium rays J.J.Thomson's demonstration that

temporarily conducting gas could be drawn off after

irradiation with X-rays.109 And in 1897 Becquerel reported

that the speed of collapse of the electroscope leaves under

the influence of uranium rays was proportional to the

square root of the density of the surrounding gas whose

pressure was varied:110 J.J.Thomson had explained the

conductivity of gases in electrolytic-ionic terms but

Becquerel gave no theoretical account. Confirmation of Becquerel's results on the penetration,

duration, photographic and electrical properties of the

rays was provided by J.Elster and H.Geitel in their paper

'Veber Byperphosphorescenz'111. They, too, noted that the

source of the energy was still completely obscure,'noch

vollsthndig dunkeln'. The main aim of this paper was,

however, to investigate whether the charge loss of the

photoelectric effect, studied for several years by the

authors, might be attributed to conductivity produced in

the surrounding air by a hyperphosphorescent emission of

invisible rays. But photoelectrically sensitive substances

were found to emit no electrically detectable invisible

rays and furthermore highly hyperphosphorescent uranium

salts were not photoelectrically active. Hence they con-

cluded that the photoelectric phenomenon could not be

explained by an emission of rays of the kind exhibited

by metallic uranium and its salts. Similarly, without

providing enlightenment as to the processes involved,

E.Villari112 confirmed some of Becquerel's results

including the diminution of electrical conductivity of

gases at lower pressures, as well as Kelvin's work on

the electrical equilibrium found to exist between uranium

and any metal in an adjacent position.

Kelvin's researches on uranium formed a part of the

Glasgow experimental studies on the electrification of

air which began in 1889 and were themselves a revival of

earlier investigations of atmospheric electricity.113

Descriptions of experiments on the electrification of air

and other gases by flames, by bubbling through liquids,

68

and by electrified needles, and on the diselectrification

of air by metal gauze 'filters', using the quadrant

electrometer, were published under the names of Kelvin,

M.Maclean, A.Galt and others during 1894-5. These

Glasgow physicists took up studies of the temporary

conductivity produced in air by X-rays as found by a

number of authors early in 1896,114 extended these to the

effects of ultra-violet light and of uranium, and published

their results in the first half of 1897. The experiments

specifically on uranium 'Electric Equilibrium between

Uranium and an Insulated Metal in its Neighbourhood'1115

'Experiments on Electric Properties of Uranium',116 and

'On, the Electrification of Air by Uranium and its

Compounds'117 were performed with a disc of the metal 5 cm.

in diameter and fr cm. thick obtained from Moissan by about

February 1897. The authors firstly confirmed Becquerel's

results on the diselectrification of electrified bodies

and showed that the affected air acted like water in

allowing a pair of dissimilar metals to develop an e.m.f. between themselves. They showed empirically, with even

less theoretical discussion than in Becquerel's public-

ations during the same few months, that aluminium was

'transparent to the uranium influence'. This 'influence'

produced saturation currents i.e., not increasing with

increased voltage, in a variety of gases at different

pressures. The leakage or current at higher pressures

was approximately proportional to the pressure and at the

lower ones to the square root of the pressure. This

latter result, together with the existence of saturation

currents, had been important factors in the development

of J.J.Thomson's electrolytic theory of conduction in

gases during 1896. Kelvin and his associates had followed

the literature118 but were very reserved in their attitude

to this theory. This may be due in part to their being

on the opposite side of the contact electricity debate

from J.J.Thomson; much of Kelvin's work on conduction in

gases seems to have been aimed at resolving the disagree-

ment.119 Kelvin had three years earlier indicated his

acceptance of the idea that a molecule of a gas could be

69

charged with electricity,120 which implied a criticism of

Thomson's electrolytic-ionic theory of 1893,121 and it is not clear if Kelvin was still thinking in terms of charged

molecules in April 1897. But by May he does seem to have

inclined towards an electrolytic gas-conduction theory.

This, however, was not linked to the considerable

quantitative results obtained; neither he nor his associ-

ates developed this theory beyond the brief speculations

of a Royal Institution Lecture.122 Kelvin closely linked

his comments on conduction in gases with the electrical

phenomena exhibited by uranium. The source of the energy

for the 'quasi electrolytic phenomena, induced by uranium

in air' was a problem:

We may conjecture evaporation of metals; we have but little confidence in the probability of the idea. Or does it depend on metallic carbides mixed among the metallic uranium? I venture on no hypothesis.123

For by the time of this lecture he had accepted both

Becquerel's proof of the emission from uranium of a

radiation 'of the same species as light' and his comparison

of the phenomenon with phosphorescence. Kelvin's newly

adopted view of normal electrolysis was published within

days of the lecture; it involved a one-fluid electrical

modification of his Boscovichian chemical atomic theory.

A single chemical atom was equivalent to one of Kelvin's

'ponderable atoms', unlike the theory adopted in 1896.124.

In the new theory each atom is of a definite radius, and

contains a few detachable point atoms of pure electricity

called 'electrions'; force laws between atoms and electrions

apply. J.C.Beattie continued experiments on 'Leakage of

Electricity from Charged Bodies at Moderate Temperaturesli25

investigating 'what becomes of the electricity which leaks

away from an insulated body in certain conditions',126

without clearly distinguishing the effect of uranium salts

from that of white phosphorus and various heated salts;

he made no explicit use of the electrolytic theory of

conduction through gases which was becoming accepted by

1899. Kelvin seems to have renewed his interest in

uranium rays only from 1903 when the subject of radio-activity was exciting widespread interest; his explanations

70

were expressed in terms of the electrion-atom theory of

1897. Ernest Rutherford and the Curies, leaders in that

field by 1903, had in 1897 each begun its study with

quantitative electrical investigations of the problematical

phenomenon of uranium radiation. An examination of

Rutherford's earlier researches may help us to understand

the background to his particular success.

2. Rutherford, and the Cavendish Laboratory (1894-8)

That Ernest Rutherford successfully pursued experi-

mental researches in three branches of physics during the

period 1893-8 is well known. 127 Although these three branches required somewhat different experimental techniques,

it should be pointed out that there were underlying connec-

tions between the physicist's understanding of magnetism

and Hertzian radiation, the behaviour of gaseous matter

under the influence of various radiations, and uranium rays.

We have seen how relevant aspects of chemical and physical

theories developed in the preceding period of the nineteenth

century128 and we shall now illustrate some of the problems

and ideas which the young Rutherford considered as he moved

towards his major studies in radioactivity.

His first publication, of research performed at

Canterbury College, Christchurch, of the University of

New Zealand, was on the 'Magnetisation of Iron by High-

frequency Discharges,129 on which subject there were but

a few conflicting comments in the literature, on the

effects of introducing iron components into electrical

circuits. Rutherford's highly competent, and sometimes

ingenious experimental techniques enabled him to show that

71

iron was indeed strongly magnetic for frequencies greater

than 100 million per second. He found the magnetic effect

of the leyden jar discharge upon steel wires to be propor-

tional to their diameters, not their areas, which confirmed

that their magnetism was confined to a thin skin. The

chemical means used to examine the depth and nature of

the magnetised skin appears to be quite original. The

method developed was one of magnetometer measurement during

controlled dissolution of the surface by nitric acid, after

calibration by dissolving a uniformly magnetised wire.

Rutherford's results showed that on moving inwards from

the surface, the magnetometer deflection decreased to zero,

changed direction, rose to a maximum then returned to zero.

The depth of penetration, of order 1/100 in. was proportion-

al to the maximum current passed, and the magnetisation

always consisted of an outer layer, and an inner thicker

layer magnetised in the opposite direction. Rutherford

considered that these layers represented the first two

half-oscillations of the exponentially decaying sine curve

of the leyden jar discharge.

The effect of the leyden jar and Hertz' dumb-bell

discharge in lowering the saturated magnetisation of iron

whatever the direction of the discharge, which Rutherford

discovered in the course of these early studies, was soon

to serve as the basis of one of the earliest magnetic

detectors of Hertzian waves. This effect also gave a clear

experimental demonstration that the discharge was oscill-

atory in nature as theory predicted; further experiments

indicated a very rapid decay of intensity. He considered

that:

The subject of the decay of the amplitude of the vibrations of a leyden-jar discharge is of considerable interest, especially in connection with the resistance of spark gaps and the radiation of energy into space.130

For terms representing each of these entered into the

complete discharge equation. We recall Oliver Lodge's

idea of a Hertzian-oscillator chemical atom131 in a work

to which Rutherford makes reference in this paper132 of

1894. It is uncertain whether Rutherford had this partic-

ular idea in mind during the course of these studies, but

72

he does seem to have been thinking in chemical and mole-

cular terms. His method of chemical removal of the sur-

face magnetisation is itself remarkable; it also bears a

striking similarity to some of his crucial experiments on

radioactivity, about five years later.133 The experiments

described above led him to the conclusion that:

iron may be shown to be strongly magnetic for the highest frequencies yet obtained. If the molecules of iron can follow the changes of magnetic force, which is reversed 1,000,000,000 times per second, there can be very little magnetic viscosity, and the molecules must move as freely as when under the influence of an alternating current of 100 per second.l34

'Magnetic Viscosity! was the subject of Rutherford's

second publication;135 for this he devised an ingenious

falling-weight timing apparatus for obtaining series of

definite small time intervals of less than 1/100,000

second. Measurements of the rise and decay of induced

magnetic forces in iron and steel were compared for rapid

and slow cycles. The considerably differing curves

obtained for rapid and for slow cycles indicated 'quite

appreciable magnetic viscosity' for iron and steel at

frequencies of 1,000 per second. This does seem to have

contradicted the conclusions of his previous paper where

the use of frequencies of 100,000,000 had shown that

'the molecule of iron can swing completely round in less

than a hundred-millionth part of a second'. He found

'the interpretation of the results very difficult' and

attributed the discrepancy to a possible variation with

frequency of the force required to cause this rapid

swing.136 That this was not the only way of understanding

magnetism at this time is shown by the considerable and

perhaps better known researches of P.Curie, who viewed

molecular magnetic theories with disfavour, preferring a

kind of 'phase' or state-of-matter explanation, as will

be seen.137 But Rutherford, as well as those whom he cited

in these first two papers, mainly 0.Lodge and J.J.Thomson,138

show no signs of this. The work of the latter contains

interesting depictions139 of the way in which the rotations

73

of a molecule, composed of atoms arranged in a particular

fashion, could produce permanent magnetism by continually

disturbing the tubes of force surrounding the molecule;

a 'shearing' of positive and negative tubes past one

another would give no electrical effects, only magnetic.

The former, Lodge, as shown above,140 explained magnetism

in terms of etherial cogged wheels. Thomson also gave

explanations of radiation, chemical combination, electrical

conduction through gases, and other phenomena in terms of

chemical atoms, molecules, and the etherial vortical tubes

of force where possible. Rutherford may well have read Thomson's earlier works,

which were much involved with unified physical and chemical

explanations, as has been seen. Indeed, Rutherford later

told Rayleigh that while still in New Zealand he had read

everything that J.J. had written.141 This was a considerable

amount; by 1895 Thomson had published some fifty papers and

four books. It was perhaps because he was familiar with

and impressed by this quantity of material that Rutherford

apparently expected to find him somewhat 'fossilized'142

at their first meeting; Rutherford was aged twenty-five

and Thomson only thirty-eight at the time. Perhaps he had

classed Thomson with Kelvin (1824-1907) and Rayleigh (1842-

1919) who were older, and whose works he had also con-

sulted.143

On the basis of his experimental work on magnetism

. Rutherford was awarded an 1851 Exhibition Science Scholar-

ship as a graduate of the University of New Zealand. A

change in the Cambridge University regulations144 enabled

him to become the first non-Cambridge graduate to start

work for a postgraduate degree at the Cavendish Laboratory.

It is not clear, however, whether the new two-year degree

played any part in his decision to work with Thomson.145

During his first term at Cambridge, beginning in October

1895, Rutherford continued his work on magnetism, extending

it particularly in developing a sensitive magnetic detector

of electromagnetic radiation. Marconi was developing the

more successful 'coherer' type of detector for long-distance

signalling at about this time. By means of its demagnetising

74

effect on a bundle of magnetically saturated iron needles,

Rutherford was able to detect the radiation, through brick

walls, at distances up to half a mile,146 and succeeded in

impressing the scientific and wider circles at Cambridge

with his demonstrations. This work is summarised in his

paper on 'A Magnetic Detector of Electrical Waves and

some of its Applications' published in 1897147 by the Royal

Society with the usual delay. This included an account of

the studies on surface magnetism performed at Canterbury

College which provided the basis of the detector; but the

studies on magnetic viscosity of difficult interpretation,

as well as speculations on the nature of the molecular

motions involved in magnetism, were omitted. By the time

this paper was presented to the Royal Society, in June 1896,

Rutherford's attentions were already concentrated upon

phenomena connected with the newly discovered X-rays;

their discovery was some four months old when he wrote

in April:

I am working with the Professor this term on Rantgen Rays. I am a little full up of my old subject and am glad of a change. I expect it will be a good thing for me to work with the Professor for a time.148

He thus exchanged the study of one form of penetrating

radiation for another, and there was a further interesting

connection between Rutherford's 'old subject' and his new

one. For in mid-1896 J.J.Thomson saw the conductivity

produced in gases by X-rays in terms of a magnetic molecular

analogy. His work with Rutherford in the next few months

was to change this; to understand the significance of this

change one must look to Thomson's views of the nature of

the electrical conductivity of gases and the light thrown

upon the subject by X-rays during 1896. In doing so we

shall see that the term 'ion' was commonly used by Thomson

in his earlier electrolytic theory of electrical conduction

through gases, and that some quantitative aspects of this

theory were under active consideration before 1896, although

the 'ionisation' theory of Thomson and Rutherford developed

during this year was an advance on all previous work. In

the Recent Researches of 1893, which contains a lengthy

75

chapter on the 'Passage of Electricity through Gases'149

Thomson repeated his view of 1883 that:

chemical decomposition is not to be considered as an accidental attendant on the electrical discharge, but as an essential feature of the discharge without which it could not occur.150

And he was able on this basis to give a 'working hypothesis'

of the 'very complex and very extensive phenomena' of the

discharge tube,151 upon which we can only touch here. At

this time his view of the cathode rays, identified as

bluish lines causing phosphorescence of glass, was that

owing to their magnetic deviability and other properties

they must be charged particles and not the purely etherial

phenomena as suggested by some German physicists. However,

these were not 'molecules' as W.Crookes had called them,

but the free, and necessarily charged atoms from dissoc-

iated molecules; negative atoms in the neighbourhood of

the cathode were strongly repelled.152 However, these

negative rays or cathode rays 'play but a small part in

carrying the current through the gas', deduced partly from

the fact determined by Thomson using a rotating mirror

method that the luminosity in the tube travels in the

opposite direction and 'with an enormously greater velocity

than we can assign to these particles'.153 Instead, he

took the bulk of the current passing through a gas to be

carried electrolytically, somewhat in the manner of the

theory of Grotthus for conducting solutions, which involved

complete chains of associated molecules bridging the two

electrodes.154 Rough calculations155 of the electrostatic

force between the hydrogen atoms in a molecule showed that

'the separation of the atoms cannot be effected by the

direct action of the electric field upon them'.156 But

the existence of chains of polarised molecules, broken up

into short lengths by collisions, would ease the separation

of an atom from the molecule at the end of a chain. The

high velocity of the luminous discharge was explicable by

the jumping of the ends of the successive unit tubes of

force along a chain of molecules, at a far greater velocity

than a moving charged atom.157 Also explained in a similar

way was the stratification, whose non-luminous portions

76

were seen as parallel Grotthus chains; the luminous areas

were at their ends, where atoms were being detached.

This view of electric discharge through gases, within

the fairly narrow limits of pressure involved here, was

repeated at the British Association meeting of 1894158 and

extended as far as a quantitative estimate of the 'very

small number of charged ions' necessary to make rarefied

gases the 'exceedingly good conductors of electricity'

which they were observed to be. By the conductivity 'we

could easily detect the presence of free ions though they

only amount to one part in 7000 of the total gas'.159 The

effect of water in facilitating chemical combination and,

as Thomson found, the electrical discharge, suggested that

its presence might facilitate the formation of the aggregates

of molecules thought necessary for the discharge 'by

supplying nuclei round which they may condense'.160

C.T.R.Wilson took up a related experimental study, touched

upon by Thomson,161 of the effect of various nuclei on the

condensation of water vapour, as the latter's last comments

on electrical conduction by gases, before the discovery of

X-rays, were published. In an article 'On the Electrolysis

of Gases'162 Thomson described his use of the spectroscope

to detect qualitatively 'the movement of the ions in

opposite directions along the discharge tube', and the

resulting decomposition of hydrogen chloride and other

gases; he drew the interesting conclusion that positively

and negatively charged hydrogen atoms exhibited different

spectra. At the end of 1895, shortly before the discovery

of X-rays, Thomson's discussion of 'The Relation between

the Atom and the Charge of Electricity carried by it'163

shows the strength of his continuing interest in the

electrical properties of gases. He supposed that the ions

of gaseous electrolysis do not have the same persistency

of sign as in the electrolysis of solutions; evidently the

two kinds of electrolysis were different and such an

assumption helped to explain this; E.Wiedemann and

G.C.Schmidt performed gaseous electrolyses, and discussed

these differences in atomic-molecular terms.164 Thomson

alone invoked vortex explanations: he supposed that the

77

attraction of atoms for electricity was related to the

arrangement of the etherial vortices within atoms, and

the emergent unit tubes of force.165 His speculation as

to the mechanism of chemical combination of hydrogen and

chlorine, considering the necessity of a third substance

for this and many reactions, again involved his idea of

the association of molecules, which always remained

valuable and flexible. In this case, for example, assoc-

iation occurred to facilitate interchange of electrical

charges, but not to 'free' an atom; the gas did not become

conducting during the course of the chemical combination,

showing that ions, easily detectable by this property, were

absent.

During the first months of 1896, the new X-ray photo-

graphy was being tried at the Cavendish Laboratory, and

probably at every other physical laboratory in Europe.

By the end of February Rutherford wrote home that he was

already tired of it;166 but there was more in X-ray studies

than this, for by then Thomson and J.A.McClelland were

engaged in the investigation of one of the few properties

of the rays to which Rdntgen had not at first laid claim167

By 29th January Thomson168 had found that the rays caused

a dissipation of electrostatic charges of either sign.

Conductivity produced in the surrounding air, or in any

solid dielectric, was pronounced as the cause within days169 This was different from the effect of ultra-violet light,

which caused the dissipation of negative charges on clean

metal surfaces, as Elster and Geitel had shown.170 Thomson

stressed his conclusion that 'all substances when trans-

mitting these rays are conductors of electricity' and

repeated his view that such conductivity in any substance

occurred 'by a splitting up of its molecules'. This was

a novel link between radiation and the electrical conduct-

ivity of gases which led to advances in experiment and

theory. It was not necessary for the gas to be rarefied,

heated or electrically stressed, and investigations could

now be made over far wider ranges of conditions than in the

discharge tube. Continuing studies during 1896-7 provided

a more quantitative understanding both of radiation and of

78

the molecular electrical structure of gases. And the views

of Thomson were to undergo some modification as experimental

studies developed.

By March 1896 he was able to give an account of con-

siderable progress in a joint paper with McClelland 'On the

Leakage of Electricity through Dielectrics traversed by

Rontgen Rays'.171 An important experimental result was the

proportionality of the leak or current to the square root

of the pressure of an irradiated gas. Now from the standard

kinetic theory of the dissociation of gases 'the number of

ions is proportional to the square root of the pressure';

hence the conductivity of the gas was itself proportional

to the number of ions present but independent of their

mean free path and velocity. The most significant results

emerged from investigations of the dependence of the current

upon the voltage applied: unlike electrolytic solutions,and

irradiated solid dielectrics, Ohm's law was not obeyed for

irradiated conducting gases. Instead a maximum current

was always obtained at a low voltage; this current could

not be exceeded even for large increases of the voltage,

'a very remarkable and characteristic property of the

conductivity produced by these rays in a gas'.172 This

agreed with the pressure-current relationship in showing

that the conductivity depended only upon the number of

ions present and not upon their velocity when the maximum

current flowed. Although the explanation of the mechanism

involved was not expressed very clearly by the authors,173

if one takes into account Thomson's ideas on conduction

through gases during previous years, discussed above,174

an interpretation is possible. The authors supposed that

the effect of the X-rays was continually to produce 'chains

of molecules or aggregations of some kind'. As we have

seen, the assumption was that these could readily release

the free atoms essential for conduction. The aggregates

produced by X-rays were supposed to be of such a kind that

'the component atoms with their electrical charges could

rearrange themselves with facility; the time T required

for this rearrangement being independent of the intensity

of the electric field'. This, Thomson seems to imply,

79

would happen spontaneously to each chain, once formed,

in the absence or presence of any electrie field. Each

rearrangement would effectively transfer a definite

quantity of electricity from one end of the chain to the

other; but in the absence of an electric field the net

effect of many randomly orientated transfers would be zero.

When a weak field is applied a proportion of the aggregates

become 'polarised' in a manner 'analogous to that of the

molecular magnets in a piece of soft iron under an external

magnetic field'. With some chains similarly orientated,

the spontaneous transfers of electricity would produce a

current. As the voltage increases, more chains become

aligned and the current rises, but 'as soon as all the

chains get pulled into one direction the current will

reach a maximum value and be independent of the electro-

motive force'.175 This electrolytic mechanism is seen to

differ from that for ordinary electrolysis generally

accepted now, and probably by many then; in electrolysis

of solutions, conductivity depends largely upon the

(measurable) velocities176 of the aggregates of ion plus

neutral molecules.

Thomson's next major paper on the subject, published

after a period of more than six months and now in con-

junction with Rutherford, contained modified views of

conduction through gases, which were more closely allied

to the accepted mechanism of the electrolysis of dilute

solutions. And the continuation of this research by

Rutherford enabled Thomson by the end of 1896 to publish

for the first time an account of a mechanism by which

X-rays could produce the particles assumed to be res-

ponsible for electrical conductivity in the normally

insulating gases. It is to be noted that the use of the

electrical conductivity produced in a gas by radiations as

'a very sensitive and convenient measure of the intensity'177

did not at first depend on any particular ionisation theory.

However, all later explanations, both scientific and

historical, from 1898 to the present, of this means of

determining the intensity of X-rays and radioactive

radiations are given in terms of the theory of Thomson

80

and Rutherford of September 1896.178

There are some interesting clues as to the path of

the development of this theory from March to September.179

In April 1896, as Rutherford left his study of Hertzian

radiation, and magnetism, and began the experimental work

on X-rays and electrical conductivity, Thomson wrote to

Nature on 'The RUntgen Rays'.180 His brief comment on the

'saturation' conductivity produced by X-rays in gases, was

that:

The relation between the rate of leak and the potential difference thus exhibits the same general features as that between the magnet-isation of a piece of soft iron and the magnetising force.181

In his Rede lecture in June Thomson indicated that he still

supposed that the conductivity was caused by the trans-

mission (not absorption) of the X-rays;182 he again used

the magnetic analogy for the more detailed voltage-current

curves obtained by Rutherford and himself: 'When the rays

are strong, the curve is like that of soft iron; when the

rays are weak, it is like steel'.183 A major objective of

the work was clearly the attainment of an understanding of

the detailed mechanism of the conduction process; it may

or may not have been a deliberate attempt to ascertain

the dimensions of the aggregates or chains of molecules

assumed to be involved, which led to the observation,

again not agreeing with Ohm's law, that:

In some experiments recently made by Mr. Rutherford and myself, we found that using a constant potential difference the rate of leak was smaller across a very thin plate of air than across a thicker one; it thus appears that the process of conduction through a gas is one that requires a considerable amount of room.l84

Perhaps the researchers entertained the idea that the

dimensions of the aggregates or chains of molecules

involved might be those found in the very different

conditions of the striations of the discharge tube. These

were of the order of one millimetre in width and were thus

composed of millions, or many thousands, of molecules.

It was not made clear how these are produced by radiation,

nor how a confined space restricts the transfer of elec-

tricity. A week after Thomson's Rede lecture of June 1896

81 Rutherford wrote of his struggle with this research:

'My scientific work is progressing fairly well but it is

rather a difficult subject I am on at present'.185 But

within a further three months many of the problems of

this area had been eased into a modified theory.

The explanation expounded in Thomson's and Rutherford's

joint paper 'On the Passage of Electricity through Gases

Exposed to Rontgen Rays'186 remained one of electrolytic

conduction occurring by means of aggregates produced in a

gas by the radiation; but series of experiments had now

clarified the nature of these aggregates. A progressive

step in the experimental work had been the piping of the

gas from the point of irradiation into a separate vessel

for examination of its conductivity. Removal of the

conductivity by certain filters187 showed the 'coarse

character' of the conducting entity within the gas,

which we can see fitted with Thomson's thinking over the

previous few years. Most important in understanding the

nature of this entity was the 'very suggestive result'

that the conductivity produced by irradiation could be

greatly diminished or entirely destroyed upon application

of an electric field, of a few volts potential difference,

across the gas as it passed along the tube before

reaching the vessel in which the leakage was tested.

The electric field was applied by inserting a central wire

within a metal tube inside the tube along which the gas

passed. On placing a glass tube over the central wire,

thus maintaining the field but preventing the current, they

were able to conclude that:

the peculiar state into which a gas is thrown by the ROntgen rays is destroyed when a current of electricity passes through it. It is the current which destroys this state, not the electric field.188

This gave a simple explanation of 'saturation':

the maximum current will be the current which destroys the conductivity at the same rate as this property is produced by the RUntgen rays.l89

This could still have agreed with the earlier theory in

which the polarization and orientation of aggregates or

chains was assumed. What was new, however, was the

82

assumption that what they called 'conducting particles'

were actually electrically charged', and that the velocity

of translation played a vital part in conductivity. This

implies a different mechanism for conduction from that

envisaged previously for the chains or aggregates had

themselves been supposed to form the electrolytically

conducting path. But this was not mentioned in the paper

published in November; the authors now explicitly placed

their explanation of conductivity in line with that accepted

for the electrolysis of dilute solutions:

We shall find that the analogy between a dilute solution of an electrolyte and gas exposed to the ROntgen rays holds through a wide range of phenomena, and we have found it of great use in explaining many of the characteristic properties of conduction through gases.190

Although the ability to impart a charge to a gas,191 and

the absence of polarization192 seem to be properties

outside the analogy, the qualitative and quantitative

explanations it provided were clearly 'of great use'.

The authors were able to derive an important quantitative

relationship equating the rate of increase of the number

of charged particles with the difference between the rate

of production by the X-rays, and the rate of destruction

both by recombination (proportional to n2) and by the

passage of current. As in ordinary electrolytic theory,

this current was expressed in terms of the 'sum of the

velocities of the positively and negatively electrified

particles'.193 These equations accounted for the higher

resistance of thinner layers of gas, but without quantit-

ative agreement. Close correlations between the equations

and experimental results were however very successfully

attained for the voltage(E)/current(i) curves for various

differently irradiated gases, which rose to a limiting

value of i according to an equation of the form A - i = B.i2/E2 . The value of the limiting current

gave easily an estimate of the proportion of the gas

electrolysed as 1/(3x1012):194 compare Thomson's similar

calculations on gaseous 'free ions' in discharge tubes,

of 1894.195 Using the curves Thomson and Rutherford were

83

able roughly to estimate the time of spontaneous diminution

of the number of particles to one half after the rays had

ceased, at about 1/10 sec., a precursor of half-life

estimates for radioactive gases. And using this they

arrived at a first estimate of 0.33 cm./sec. per volt/cm.

for the sum of the velocities of the oppositely charged

particles, upon which the current depended. This was

'very large compared with the velocity of ions through

an electrolyte' but small compared to the 50 cm./sec. for

'an atom of a gas carrying an atomic charge' which implied,

as had been assumed, 'that the charged particles in the

gas exposed to the ROntgen rays are the centres of aggreg-

ation of a considerable number of molecules'.196 It seems

possible that a distinction was already being made between

'charged particle' and 'aggregation' implying a certain

kind of mechanism, but this was made more explicit as

Rutherford continued the research. It was soon after the

reading of this paper to the British Association that he

wrote home of the possibility of discovering new chemical

elements.197

Thomson in a note appended to Rutherford's next

publication, dated December 1896, 'On the Electrification

of Gases exposed to ROntgen Rays, and the Absorption of

Rontgen Radiation by Gases and Vapours1198 gave the first

description of the way in which the radiation might cause

the conductivity of a gas. Thomson supposed that the

moving tubes of force comprising the radiation produced

charged particles by 'dissociatiOn of one molecule, or

production of one positive and one negative ion'; this

implies that aggregation is subsequent to dissociation.

Rutherford's delicate experiments on the absorption of the

rays had shown that gases which were good conductors of

electricity under irradiation were also good absorbers of

the radiation. This suggested for the first time, although

it was still not stated explicitly other than in Thomson's

note, that it was not the transmission but the absorption

of the rays which resulted in conductivity. Rutherford's

bare statement that 'Experimentally it was found that the

rate of leak of a gas is proportional to the intensity of

84

radiation at any point'199 is enigmatic. For until this

time the expressions he italicised had been taken as .

practically synonymous; the rate of leak was the only

measure of intensity. It may be that his experiments

showing that the rays 'appeared to emanate in all directions

from the anode'200 led him to suppose that the intensity

diminished according to an inverse square law in the

manner of light from a point source. Once this was assumed

- and the tentative opinion was that X-rays were pulsations

or vibrations of a similar kind to light - an experimental

demonstration that the rate of leak was proportional to

the inverse square of the distance from the source could

have given rise to the conclusion as stated. Whether or

not this is so the statement itself marks an important .

clarification in his understanding of the phenomenon.

The process was now seen as an absorption of the radiated

energy by gas or vapour, with the resulting production of

a small number of pairs of oppositely charged ions, around

which aggregation occurred. J.J.Thomson added that one

Faraday tube would be removed from the radiation for each

molecule dissociated; but Rutherford never expressed

himself in terms of these tubes in publications.

Thus outlined, the theory was developed experimentally

during the next few years by several research students at

Cambridge. Rutherford concentrated on this field for his

remaining two years here, at first working on the separation

and examination of the oppositely charged ions existing in

conducting gases. Preliminary experiments201 showed that

a gas in the conducting state could be made to acquire a.

net charge, and this was attributed to an excess of ions

of one sign over the other. At this time Kelvin and his

associates were interested in such points and may have

attributed this to the acquisition of electrical charge

by the molecules; J.Perrin in Paris accepted an ionisation

hypothesis,202 but the Cavendish school appears to have

been far ahead in experiment and theory.

In Rutherford's next publication on 'The Velocity and

Rate of Recombination of the Ions of Gases exposed to

Rlintgen Radiationc203 dated July 1897, he described an

85

ingenious method of timing the passage of ions of one

sign: only half of the gas, between plates 16 cm. apart,

was irradiated the rest was screened so that ions of one

sign would have to travel through 8 cm. of non-conducting gas before arriving at the oppositely charged plate to

produce a rapid deflection of the connected electrometer.

What was perhaps at first unexpected was the result that

the velocity of positive and negative ion always appeared

to be equal, not only in elementary gases such as hydrogen,

but in compounds with asymmetric molecules such as hydrogen

chloride. Furthermore the hydrogen ion velocity was

different in different gaseous hydrogen compounds.

John Zeleny, however, in his discussion 'On the ratio of

the velocities of the two ions produced in gases by ROntgen

radiation; etc.'204 soon afterwards demonstrated experi-

mentally that negative ions generally possessed a slightly

higher velocity; these researches show the ingenuity with

which the ions were manipulated and the reality with which

these researchers endowed them.

The equality assumed by Rutherford of the velocities

of gaseous ions certainly did not apply in the electrolysis

of solutions where each ion possessed an individual mobility.

That the observed velocities depended more upon the gas

used than the nature of the dissociated ion gave rise to

the hypothesis that the size of the cluster of molecules,

formed around the central charged particle, was determined

only by an equilibrium between intensity of bombardment by

surrounding gas molecules and the magnitude of attraction

provided by the central charge of the ion. A comparison

of the observed velocity of the ion, for example 10.4

cm./sec. for hydrogen gas, with that of 340 cm./sec.

calculated for a molecule of hydrogen carrying an atomic

charge, gave estimates of the sizes of the carriers

involved. Rutherford found more moderate sizes than

formerly may have been supposed; in the present example

5.5 molecular diameters for the hydrogen ion in hydrogen gas.205 The understanding of the electrical structure of

gases was to be an essential feature of Rutherford's

future studies of radioactivity, both experimentally and

86

theoretically. For example, his experimental investigations

on the 'decay' of after-conductivity, by 'blowing' and

static methods, confirmed the relationship dn/dt = a.n2

for the disappearance of the ions. His observations

agreed with the theoretical curve relating the declining

number of ions to the time, and with the theoretically

calculated time T taken 'for the number of conducting

particles to fall to half their number', given by the

equation T = 1/N.a ; N is the maximum number which

depends on the intensity of the radiation, a is a different

constant for each gas, T was found to be of the order i sec.

The superficial similarities with studies of the decay of

radioactivity on which Rutherford was to work some two

years later in 1899 are striking. But the deeper and more

complex links between his studies of radiations, the mag-

netic and electrical properties of matter, and uranium

rays, will become more evident. Rutherford indicated in

his paper on ions in gases exposed to X-rays, dated July

1897 and now under discussion, that experiments on uranium

radiation had already begun, and that the ions produced

in gases by this means were the same as with X-rays.206

We can see that the subject of uranium rays was considered

to be of note at the Cavendish Laboratory; for in 1896

its Professor stated that he found Becquerel's discovery

'exceedingly interesting'207 and that he had obtained

photographic effects by means of uranium salts.208

G.G.Stokes also showed an interest in this new form of

phosphorescence during 1896-7.209 In July 1897 Rutherford

stated his interest in uranium rays and announced prelim-

inary results, as has been stated; in October C.T.R.Wilson

announced his confirmation of these by the cloud-

condensation method; then Stokes at the Cavendish Physical

Society, and J.J.Thomson at the Cambridge Philosophical

Society in November discussed the implications of

W.J.Russell's article.210 But it was early in 1899 before.

Rutherford's promise of further results made in 1897211

was fulfilled.

In the intervening period he published one paper on

87

the problematical subject of 'The Discharge of Electri-

fication by Ultra-violet Light'.212 This was previously

understood, and confirmed in this paper, as an effect

produced mainly at metallic surfaces and not within the

volume of the gas; the effect of X-rays was the opposite

of this. Ultra-violet light discharged negatively

electrified metals and caused zinc and some other metals

to acquire a positive charge. Rutherford cited213 some

of the literature on the subject from the ten years of

its history into which we cannot go deeply here. He

mentioned only the theory of surface disintegration,214

and not Lodge's supposition that the effects probably

'depend on some synchronised disturbance set up in the

air ... in contact with the substance, a disturbance

resulting in some kind of chemical action'.215 Rutherford's

main concern in this paper was to investigate the nature

of the carrier of the current. Using the kinds of experi-

mental technique developed at the Cavendish Laboratory

during the previous two years, he found that the current

was carried by free gaseous ions, of negative charge only,

and not by particles of metal. The use of a variety of

different metals, from lead to sodium amalgam, showed that

'the velocity of the carrier is independent of the metal

on which the light falls%216 This indicated that the

carrier was produced not from the metal itself but from

the gas near its surface. To provide a mechanism for the

phenomenon was no doubt one desired object of this research,

but none was published at this time.

For the origin of uranium rays, an equally difficult

subject, Rutherford did suggest the outline of a mechanism.

A study of this radiation was the subject of his most

substantial paper then published: 'Uranium Radiation and

the Electrical Conduction Produced by It'217 was dated

September 1898, the month in which the young scientist left

Cambridge to replace Callendar as Macdonald Professor of

Physics at McGill University, Montreal. The suggestion

adopted by Rutherford218 was not his own, though it may

have relied to some extent on his results. It had been

put forward by J.J.Thomson at the beginning of 1898 in a

88

note 'On the Diffuse Reflection of ROntgen Rays'.219

Thomson pointed out that these diffusely reflected rays

were, like uranium rays, similar to X-rays but less

penetrating. He mentioned the experiments of Sagnac on

secondary rays emitted by metallic surfaces, as did

Marie Curie a few months later in relation to her own

speculation as to the origin of uranium rays.220 Thomson

supposed that secondary rays were produced during the

ionisation of the molecules of the material, solid, liquid

or possibly gas, by the incident X-rays. He provided a

diagram showing how the tube of force joining the atoms

in a molecule could be broken by the influence of an

incident radiated tube, and concluded that owing to the

rapid movement of tubes during the course of dissociation:

Ionization (if sudden) may thus be expected to give rise to rays having properties similar to those of the secondary Ontgen rays. 221

As for uranium rays:

It seems not impossible that in the case of a complicated structure like the uranium atom regrouping of the constituents of the atom may give rise to electrical effects similar to those which occur in ionization and might possibly be the origin of the uranium radiation.222

We recall that Thomson had made his first announcement of

the subatomic cathode particle in April 1897 and had fully

set out his theory of the corpuscular chemical atom in a

publication of October 1897. The suggestion concerning

regrouping of the corpuscles constituting the uranium

atom can be considered as the first published statement

that the emission of uranium radiation is a property of

the atom; it precedes those of Marie Curie and G.C.Schmidt

made later in 1898.223

But this kind of idea was not new: as the previous

Chapter shows, some physicists and chemists attributed

the emission of characteristic radiations, but of longer

wavelengths or different wave forms, to internal vibrations

of chemical atoms. However, the energy required for the

emission of atomic spectra was known to be positively

provided, in obvious ways. Becquerel had clearly stated

the problem of the source of the energy of the uranium rays

89

and Rutherford, who cited eight of Becquerel's nine papers

as well as later French authors on the subject, saw this

as a question to which there were now some answers. An

interesting point which emerged from Rutherford's

investigations of 1898 was that the rays from uranium

consisted of two distinct portions. He considered that

these beta and alpha rays were comparable with X-rays and

secondary X-rays respectively, and speculated that the

alpha rays might thus be produced at the surface of the

active substance by the supposed primary beta radiation.224

Rutherford quoted Thomson's idea that a rearrangement of

the constituents of the uranium atom could give rays

similar to those produced by the sudden ionisation of a

gas.225 But he tacitly modified this as required by his

experimental results by insinuating that such rays from

gases were similar to soft primary X-rays rather than to

the less penetrating secondary X-rays which Thomson had

suggested. The existence of two kinds of uranium X-ray

could thus be understood, but only in part. For in his

opinion 'The cause and origin of the radiation continuously

emitted by uranium and its salts still remain a mystery'.226

Rutherford was able to ease this with the comment that on

account of the smallness of its energy 'the radiation

could continue for long intervals of time without much

diminution of internal energy of the uranium',227 but such

relief was only temporary. The question of the possible

diminution of this radiation proved to be an important

point in the struggle to understand the subject . as the

mystery deepened and widened during the next few years.

Rutherford's stated aim of 1898 was a study of

'Uranium Radiation and the Electrical Conduction Produced

by It'.228 His repetition of Becquerel's experiments, but

with entirely negative results, led him to the. firm con-

clusion that this radiation was neither refracted nor

polarised;229 Becquerel had to agree.230 And by success-

ive interposition of metal foils Rutherford came to his

conclusion that the radiation was 'complex' consisting of

at least two distinct types each approximately homogeneous,

one readily absorbed (alpha) and one more penetrative

90

(beta),231 each of about the same coefficient of absorption

in gases as X-rays.232 He may have owed something both to

Becquerel,233 who had indicated the complexity of the

uranium rays, and to J.J.Thomson and J.A.McClelland234 who

had used metal foils in demonstrating the considerable

variety and complexity of X-rays from different bulbs.

Significantly, however, Rutherford found that different

compounds of uranium gave rays of the same composition,

as indicated by the foil method and by his own more sen-

sitive method of absorption in gases. Despite the spec-

ulation that the alpha rays were secondary to the beta,

which Thomson may never have accepted,235 this result seems

to be equivalent to a demonstration of the emission of a

definite spectrum consisting of two main components. It

may possibly have been taken by the Cavendish researchers

as a further indication that the property of emission

belonged to the uranium atom. This latter view, also

suggested by Marie Curie and G.C.Schmidt in 1898, had

been put forward by each in connection with their indep-

endent pronouncements that thorium was the only other

element giving similar spontaneous radiation. Rutherford's

brief examination236 of the thorium rays showed, in spite

of some capricious but interesting variability in readings,

that they were of a different penetrative composition from

uranium rays; these were the problems he took to Canada.

With regard to the discovery of thorium rays he mentioned237

only Schmidt, who had in fact been slightly earlier than

Marie Curie in publishing the discovery. One paper of

Marie and Pierre Curie was cited by Rutherford238 but only

to criticise seriously their conclusions. Owing to the

ready absorption of the alpha rays by any material,

including that of the emitting substance itself, Rutherford

noted that 'the rate of leak due to any uranium compound

depends largely on its amount of surface'. Thus the state

of division of the layers of powdered salts used made it

'difficult to compare the quantity of radiation given out

by equal amounts of different salts'.239 Now such a com-

parison had been the very means by which the Curies had ,

firstly come to suspect, and secondly to begin to isolate

91

chemically a new element; its characteristic, they said,

was its great power of radiation compared with uranium.

Rutherford dissolved a crystal of uranium nitrate

in water and allowed it to evaporate so as to deposit a

very thin layer of the salt.. This simple exercise gave

a higher than normal leakage due mainly to alpha radiation,

which had the greatest electrical effect. 'It is possible'

he wrote: that the apparently very powerful radiation obtained from pitchblende by Curie may be partly due to the very fine state of division of the substance rather than to the presence of a new and powerful radiating substance.240

Although the Curies had noted241 the effect of the thick-

ness of the layer of salt for uranium and thorium rays,

they had published no analysis of the composition of the

rays, nor had they developed studies on their electrical

effects in gases. With his deep experience in these areas

at the Cavendish Laboratory, surely Rutherford could not

have erred on this important point: but wrong he was.

His success came with thorium; the Curies found that

studies of their new elements polonium and radium went

from strength to strength.

92

3. Pierre Curie, Marie Curie and the new radioactive elements (1890-8)

Towards the end of 1898 Rutherford disagreed with the

Curies over the existence of a new radiating element; he

had lost this point even before his paper appeared in print

early in 1899. But these parties were to disagree on more

complex issues concerning radioactivity from 1901 to 1904

and their approaches appear to have differed well before

this time. One could say that they were near to being

adherents of different schools of scientific thought.

The main point of distinction was the attitude towards the

various mechanical, molecular and etherial models. These

as we have seen were applied in considerable variety for

explanations in physical science. But a tradition which

can be called 'positivist'242 cast doubt upon the validity

and even the utility of such models, in the last decades

of the nineteenth century. While all scientists saw the

attainment of general laws as a vital part of scientific

progress some were sceptical of the models which others

made their goal and considered the seeking of general

relationships to be the sole objective of science. There

was however a variety of opinion between these views;

Maxwell for example in considering his attempt 'to imagine

a working model' explaining the rotational character of

magnetic and optical phenomena wrote that:

The problem of determining the mechanism required to establish a given species of connections between the motions of the parts of a system always admits of an infinite number of solutions. Of these, some may be more clumsy or more complex than others, but all must satisfy the conditions of mechanism in genera1.243

And W.Ostwald appears to have changed from an atomistic

understanding of chemistry and physical chemistry to a

completely anti-atomistic and purely energetic or thermo-

dynamical approach, in about 1890.244 A fuller discussion

would lead us into areas of the philosophies of nineteenth

century scientists but I wish only to indicate that

P. Curie tended to be critical of atomic or molecular

models and sought explanations rather in terms of general

laws; he was able in this way to provide a lasting

93

contribution to aspects of experimental science in

providing such laws apparently without recourse to models.

In his earliest series of researches, published

jointly with his older brother Jacques during 1880-2 on

piezo-electricity of crystals,245 it is difficult to

discern any leaning towards either of the two approaches;

both were clearly valuable in this case. The research

was performed at the Mineralogy Laboratory of the Sorbonne

whilst both brothers were 'preparateurs'; Friedel was

director here,246 and had himself worked on pyroelectricity

- the production of electrical polarity in crystals upon

change of temperature.247 Crystals possessing axes with

dissimilar extremities, i.e. hemihedral with inclined faces,

exhibited this polarity at these extremities. The Curie

brothers claimed the discovery of a new, but related, way

of producing electrical polarity - by varying the mechanical

pressure, applied along these axes; this was later named

piezo-electricity. Some indications of the production of

electrical effects by mechanical treatment of crystals had

been known for many years248 but the experimental work of

the Curie brothers was a considerable advance. They

clarified the quantitative nature, symmetry and reversi-

bility of the effect using tourmaline and quartz crystals.

When the collaboration of the brothers ended in 1883,

with Jacques taking up the post of 'Maitre de Conf6rences,

at the University of Montpellier249 and Pierre becoming

'Chef des travaux de Physique' at the new Ecole municipale

de Physique et de Chimie industrielles in Paris, each

retained an interest in the physical and geometric properties

of crystals. During the decade 1883-93 Pierre Curie per-

formed little experimental research but continued studies

of aspects of piezo-electricity.250 These led to the

development of the kind of electrometric apparatus by

means of which Pierre and Marie Curie were able to measure

the currents of order 10-11 amps produced by uranium,

thorium, and the new radioactive elements which they were

later to discover.

It was during the period 1883-93 that Pierre Curie

seems to have moved away from molecular explanations. In

94

1881, the discussion 'Sur les phgnomenes electriques

de la tourmaline et des cristaux h6miedres a faces

inclinees1251 was conducted in molecular terms. One

cannot say whether the views expressed belonged to

Jacques or Pierre Curie or to C.Friedel; perhaps they

were common to all. The authors attributed pyroelectricity

and piezo-electricity to the same cause - a contraction or

dilation along a particular axis. Discussing a deeper

structural origin they expressed disagreement with a view

which likened the rows of molecules in a pyroelectric

crystal to a thermo-electric pile. Here, a set of success-

ive cones of copper and bismuth, for example, exhibited the

required momentary polarity on change of temperature. They

considered that a better explanation of the separation of

charge by pressure, and of the particular symmetry of the

phenomenon, was provided by the hypothesis of the permanent

polarisation of the molecules in a crystal, with the end

faces normally maintained in the neutral state by a layer

of electricity 'condensee sur la surface': '1'idee que

les molecules sont polarisees est en parfait accord avec

ce fait que l'electricite ne se montre libre sur les

bases'.252 They concluded the discussion by commenting

that the 'extremite aigue' of each molecule in these

crystals was permanently negatively charged with respect

to its base, and that 'la forme de la molecule paralt

avoir l'influence preponderante'. Pierre Curie seems never again to have published

favourable comments on any molecular mechanism put forward

to explain physical phenomena. His interest in piezo-

electricity remained,253 but as others entered the field

in the 1880's and 1890's he inclined more towards a study

of the geometrical symmetry involved.254 Other authors

had considered symmetry in physical science in a vague

manner255 but Curie developed these studies in a systematic

and original way in applying the criterion of symmetry, as

used for the classification of crystals in mineralogy, to

the phenomena of physical science.256 In his paper 'Sur

la symetrie dans les phenomenes physiques. Symetrie d'un

champ electrique et d'un champ magnetique'257 Curie applied

95

considerations of symmetry to a variety of electrical,

magnetic, optical and thermal phenomena without the use

of mechanical or etherial models. He classified phenomena

by their symmetry, and used the proposition that the

existence of a characteristic dissymmetry was a necessary

requirement for the production of an effect by a cause.

This indicated which phenomena could and which could not

exist. As Curie himself pointed out, thermodynamics

provided a different and more quantitative indication of •

possibility.258 And we note that other scientists on

occasion used dynamical principles, without mechanical

hypotheses, to develop equations whose terms they supposed

might correspond to some phenomenon existing or un-

discovered.259 It seems that the consideration of symmetry

was probably the least important of these three general

methods of physical science in the nineteenth century;

but this is perhaps not so later, in the twentieth.26;)

While developing his theoretical ideas on symmetry,

Pierre Curie also performed experimental research. His

paper on 'Propriet6s magnetiques des corps a diverses temperatures'261 appeared in 1895, the year after his

publication on symmetry considerations in physics.

Although there was no specific application of the principle

to magnetism, the generality of Curie's approach to the

interpretation of his results contrasted with the usual

molecular view of magnetism. We have seen how Rutherford

understood hysteresis in terms of molecular magnets and

how he sought to follow the rapid motion of these in an

oscillating field. Curie, on the contrary, looked to

functions of physical state as an analogy by which to

explain his experimental results and touched but briefly

upon molecular theories.

Faraday had found that all bodies exhibited magnetism,

distinguished the three varieties of this property, and

had noted that iron, strongly magnetic (ferromagnetic)

at normal temperatures, became weakly magnetic (para-

magnetic) at high temperatures. Curie now showed that

diamagnetism, which was possessed by most or perhaps all

substances, did not vary with temperature. This result

96

pointed to the independence of diamagnetism from the other

kinds of magnetism; on the other hand he was able to

demonstrate by several series of measurements at different

temperatures that ferromagnetism and paramagnetism were

closely related. All ferromagnetic bodies were progress-

ively transformed, on heating, into paramagnetic bodies.

Both the inverse relationship between absolute temperature

and intensity of magnetisation of paramagnetic substances

which Curie demonstrated experimentally,262 and the curves

of transformation of ferromagnetic substances, were com-

pared in some detail with gas-liquid phase phenomena.

For paramagnetism, likened to the gaseous state, the

corresponding equations I = A.H/T and

D = (l/R).P/T possessed interesting similarities.263

The analogy was further supported by the I/T curves for

paramagnetic-ferromagnetic transformations, which were

very similar to the continuous D/T curves for gas-liquid

transformations near the critical temperature, as deter-

mined for carbon dioxide by Amagat.264 In short, the

functions f(T,H,T) = 0 and f(D,P,T) = 0

possessed strong though not complete similarities.265

This comparison led Curie to his single comment on a

molecular analogy between magnetism and the condensation

of fluids; the rapid augmentation of magnetic intensity

in weaker fields as the temperature falls may occur

'quand l'intensite d'aimantation des particules magngtiques

est asset forte pour qu'elles puissent rgagir les unes sur

lee autres'.266

His analyses of the results on magnetism of 1895 show

just the kind of minimal mechanical or structural explan-

ation which Curie was to give for radioactivity. Marie

and Pierre Curie were in 1898 among the first to describe

uranium radiation as an atomic property yet they avoided

any deeper public discussion of the mechanism of this

atomic radiation; nor did they describe, as did others,

the kind of structure an atom might have in order to

possess such a capability. As will be seen Pierre Curie

considered radioactivity in terms of a general analogy

with the transmission of heat in its various forms.

97

Manya Sklodowska had arrived in Paris in 1891 at

the age of 24 and became one of few women science students

at the Sorbonne; she followed her elder sister Bronya who

had come to Paris from Poland to study medicine, about

five years earlier.267 Manya, or Marie, was awarded the

degree of Licence es Sciences Physiques in 1893 and the

same in mathematics in 1894.268 Then, after her marriage

to Pierre Curie in 1895, she passed the examination

requirements to become 'Agregee de 1'Enseignement

Secondaires des Jeunes Filles' in mathematics in 1896.269

Her first research was of a partly industrial nature,

performed for the Societe d'Encouragement pour l'Industrie

Nationale de France, on the magnetic properties of tempered

steels of different types from various steelworks in

France.270 The magnetic measurements involved were somewhat

similar to those of her husband's thesis on magnetic

properties at different temperatures published in 1895;271

his knowledge may have been helpful for her work. Although

chemistry does not figure among her academic qualifications

she had apparently followed courses in chemical analysis

whilst in Poland.272 These may have aided her in deter-

minations of the composition of the steels as well as in

her new subject of research uranium radiation, which

quickly developed into the wider field of radioactivity.

It is largely upon her work with radioactive substances

and the discovery of the element radium that Marie Curie's

present fame rests. The beginnings of her popular renown

came in 1903 when spectacular developments in radioactivity

were very much in the public eye; public honours accumulated

considerably during 1903-4.273 The tragic accidental death

of Pierre Curie in 1906 aroused great public sympathy for

the widow; she was then appointed Assistant Professor at

the Sorbonne taking the place of her husband in the Chair

of Physics created for him in 1904. This was an unpreced-

ented appointment for a woman as was her full Professorship

in 1908.274 Her second Nobel Prize, on this occasion for

Chemistry and not shared, and her involvement in the

matrimonial separation of P.Langevin,275 both in 1911,

afforded continuing public interest. Work with the first

98

mobile medical X-ray machines in the War of 1914-18267

and the inseparable association of her name and that of

her daughter277 with the ever more important radium and

radioactive elements seal Marie Curie's lasting fame.278

Nevertheless, it must be pointed out that the theoretical

explanation of radioactivity to which the Curies adhered

during the period now under consideration, did not agree

with that developed by Rutherford and which is now accepted.

Her claim279 of priority for the transformation-

disintegration theory, made in 1906, is open to doubt:

Cette hypothese se trouve parmi celles qui ont ete indiquees par M.Curie et moi d6s le debut de nos recherches sur la radioactivite. Mais elle a ote surtout procisee et developpee par Rutherford et Soddy, auxquels elle est, pour cette raison, generalement attribuee.280

For although some aspects of the theory of 1906 were

present among the several speculations put forward in

her earlier publication of January 1899281 to which she

referred this claim, it will be shown that these were

shared by others. And she omitted to mention the Curies'

strong opposition to Rutherford's and Soddy's theory,

during the important intervening period from 1901 to 1903.

But we hope to look more deeply into such points in later

discussions.282 Our interests in the remainder of this

Section are to follow the work of the Curies into their

studies on radioactivity, to provide a much-needed dis-

cussion of their earliest theories and speculations, and

to examine their conclusion that the emission of radiation

by uranium is an 'atomic property'.

Marie Curie tells us that her, and Pierre's, first

interest in uranium rays dates from the second half of

1897, at the time when her magnetic experiments were

complete, and when she was seeking a subject on which to

begin research for a doctoral thesis.283 Becquerel's last

paper on the subject for some time was read to the Academie

in April of that year284 but there is no indication of

direct communication between Becquerel and the Curies until

later, in 1898. Besides one possibility, that Marie Curie

came upon the subject simply by reading about it285 there

99

are several possible links with contemporary scientists

interested in the subject. That some part was played by

Pierre Curie in her decision to investigate this area was

later indicated by Marie.286 The Curies, particularly

Pierre, attended meetings of the French Physical Society287

and may have heard Becquerel's reports of his researches.

J.Perrin and G.Sagnac each reviewed Becquerel's work in

1896,288 and were acquainted with the Curies, possibly as

early as 1897; there was also the long-standing friendship

between Pierre Curie and Ch.Ed.Guillaume.289 The latter's

interest in uranium rays is shown by his discussion of the

energy problem at a Physical Society meeting, after one of

Becquerel's reports in 1896.290 The researches of Becquerel

were in any case well known in scientific circles in 1896-7.

Marie Curie's experimental work on uranium rays291

began in December 1897 in accomodation at the Ecole

Municipale de Physique et de Chimie industrielles where

her husband worked. She achieved more quantitative est-

imates of the electrical intensity of the radiation than

those of Becquerel by means of an apparatus which made use

of the piezo-electric effect studied by P. and J.Curie in

the 1880's. A layer of powdered uranium-bearing material

placed on a charged metal plate caused electrical leakage

across an air gap and a rising accumulation of charge on

a parallel plate which was connected to an electrometer.

This charge was continually balanced by adding successive

weights, by hand, to a piezo-electric quartz thus main-

taining a more or less null deflection of the electrometer.

The apparatus was calibrated by means of a known. charge

and the current flowing could be calculated from the

weight/time ratio; this method of measuring small currents

had been described in the thesis of J.Curie.292 It gave

results which Marie Curie claimed to be accurate to 2%

of the values of the minute currents of order 10-11 amps

involved during the first few months of the work.293

Problems of quantitative accuracy arose later when large

weights had to be added in a short time to compensate for

currents produced by the intensely emitting substances

which the Curies were to discover.

100

If uranium radiation were a kind of short-wavelength

or X-ray phosphorescence, an Becquerel supposed, it should

diminish in time, even if slowly, and should be excited by

irradiation. If there were similarities with the storage

of light shown by thermo-luminescence then heating should

have some effect. These points which had occurred to

Becquerel and others were probably in Marie Curie's mind

when she began by seeking the effects of heating, and of

irradiation by light and X-rays, upon uranium. The

intensity of the uranium rays, on re-examination with the

sensitive apparatus after treatment as above, remained

always unchanged.294 Her lack of success in finding a

straightforward answer to the question of the origin of

the radiation was compensated by an important discovery

as her research turned towards other materials. On

surveying as complete a list as possible of other metals

or their compounds she found that thorium too emitted

rays of the same order of intensity as uranium. Her first

publication on uranium rays 'Rayons 6mis par les composes

de l'uranium et du thorium'295 shows that she had noted

that the only two elements exhibiting this property

possessed the greatest atomic weights; she had in addition

linked this point with current research on X-rays and

their secondary rays to give something of a theory of the

origin of uranium and thorium rays.

Working independently of Mme.Curie and at about the

same time, G.C.Schmidt also surveyed many materials,

discovered that thorium emitted similar electrically

detectable radiation to that from uranium, and claimed

priority.296 With E.Wiedemann at Erlangen he had earlier

studied experimentally various kinds of fluorescence,

phosphorescence and thermoluminescence; he had discussed

the theoretical basis of these phenomena in terms of

vibrating ether envelopes around molecules, vibrations of

atoms and their valency charges, and ionisation or definite

chemical separation of atoms in the molecule.297 Schmidt

sought a relationship between three phenomena: the photo-

graphic effects of various substances, including uranium,

as described by Colson, Russell, Muraoka and others; the

101

electrical conductivity produced in gases by uranium, and

lately thorium, and their compounds; and the photoelectric

effect involving a loss of negative charge or an acquisition

of positive charge by certain metals and minerals upon

irradiation with light of certain kinds.298 He stated that

he had followed the work of Elster and Geitell who had

shown experimentally both that metals exhibiting photo-

electricity did not emit radiations electrifying the air

in the manner of uranium and that uranium salts were them-

selves not photoelectrically sensitive.299 Schmidt used a

modification of the apparatus used by Elster and Geitel300

for some earlier studies on the photoelectricity of

minerals to show that thorium compounds too gave no

increase in air conductivity when irradiated with light.

We can see that this would indicate that thorium radiation

was neither of photoelectric origin nor of a kind of phosphoresence which could be excited by the light employed.

Furthermore, none of the many photographically active

substances studied in 1896-7 proved to be electrically

active. Only uranium and thorium rays possessed both

electric and photographic properties, and Schmidt noted

the similarities of these radiations in a fuller account

of the research, of 1898:301 thorium rays were reflected

and refracted but not polarised; Becquerel's conclusion

that uranium rays exhibited all three of these properties,

still stood; and it was accepted that X-rays exhibited

none of these properties. Schmidt's only hint of an

explanation of the photographic and electrical properties

of the radiation was that 'Es scheint als ob dieselben an

das hohe Atomgewicht Uran = 240, Thorium = 232 gebunden sind' .302 Although it has a place of importance in the

progress towards the modern theory, there was no reason

for Schmidt to attribute great significance to this

interesting point. If he had written that the radiation

came from within heavy atoms this would not have been a

novel conclusion at the time. For E.Wiedemann believed

that Schmidt's spectroscopic work on fluorescent vapours

indicated that a single atom of a metal could behave as

an electrically complex emitter of electromagnetic

102

radiation.303 Having provided facts and conclusions vital

to other scientists, Schmidt left the subject with

Becquerel's question of the origin of the energy unanswered

and continued his research on the complex problems of

electro- and photo-luminescence.

Marie Curie was working along the same lines as

Schmidt but had made much more of the study than he by

the time their first papers on the subject were published

in the spring of 1898. In her first paper304 she clearly

assumed, in agreement with Becquerel's conclusion for

uranium, that the property of emission of the new radiations attaches to the elements themselves. This enabled her to

dismiss phosphorus from the active class, although the

white allotrope produced a strong air conduction effect,

since compounds of this element were not active.

Phosphorus never really became involved with radioactivity

though the cause of the conductivity produced by it,

whether due to ions, fumes, or radiation, continued to be

a point of discussion into the twentieth century.305 It

is not clear whether she was aware, as Schmidt was, of

Russell's work on the photographic activity of metals

and organic substances, but Mme.Curie seems to have •

examined only inorganic specimens. She noted that the

activity of all minerals could be attributed to the presence

of an active element, namely uranium, thorium, or one of

three weakly active rare earth elements,306 and that uranium

salts were active 'd'autant plus qu'ils contiennent plus

d'uranium'; her published list of activities, however,

shows no quantitative proportionality between uranium

content and activity. What was 'tres remarquable' were

the activities of the minerals chalcolite and pitchblende,

52 and 83 respectively, compared with impure uranium's

reading of 24. The reader, having been led thus far, was

presented with the surprising deduction that 'ces mineraux

peuvent contenir un element beaucoup plus actif que

l'uranium', and it seems that 'element' here does have

the meaning of a definite chemical element. Certainly

Rutherford could not accept this conclusion; as has been

noted307 he attributed the effect to the large surface

103

area produced by finely powdering the substance. But

Marie Curie was aware of this kind of effect, had realised

that only the surface layer of uranium emitted the rays

and that thorium differed in so far as its deeper layers

contributed to the radiation; the important factor of

consistency of readings no doubt satisfied Pierre Curie,

who was always wary of premature publication. Further

evidence came from the preparation by Debray's method of

an artificial chalcolite308 which was found to possess

an activity no greater than that of an ordinary uranium

salt.

It seems that at the time of this initial publication

Mme.Curie, and perhaps P.Curie, had decided that the

emission of the penetrating radiation was a fundamental

property of an element independent of its chemical and

physical state. Becquerel had already said this, but

Marie Curie's suggestion of a new element seems to involve

the further step of assuming that the magnitude of the

conducting effect of the radiation was specific to the

emitting element. This, without further explanation, she

called the 'activite' of the element or compound. Certainly

the high readings of some minerals comprised the only

evidence given for the existence of a new element in the

Curies' paper cited by Rutherford,309 which was Marie Curie's

second publication on the subject.310 The information that

chalcolite and pitchblende emitted a radiation qualitatively

different from that of either uranium or thorium compounds,

as determined by the fraction transmitted by an aluminium

sheet, had indeed been given in the first note but without

interpretation. Such a qualitative difference may appear

to be evidence for the existence of a new element, as good

as, or better than a high activity; it seems more akin to

the emission of the visible radiation of a distinct spark

spectrum which constituted the most acceptable evidence

and was the chemist's 'court of final appeal'311 for the

proponent of a new element. Perhaps she felt that the

considerably more penetrating nature of the rays from a

thick layer of thorium oxide, compared with a thin layer,312

impaired the value of this as a criterion for identification

104

of the emitting element. But it is possible that qualit-

ative differences in the radiations may have played some

part in Marie Curie's bold deduction of the presence of

a new element.313 For the consideration of such differences

seems to be involved in her theoretical explanation of the

origin of the radiation. G.Sagnac, in the months before Mme.Curie's first

publication on the rays of uranium and thorium, had been

continuing earlier work of others and himself on X-rays by examining their effect in causing gases to become elect-

rically conducting in the presence of metal plates.314 From

several series of experiments he concluded that X-rays

entering or leaving solid materials always produced easily

absorbed secondary rays or S-rays. Thus the observed

conductivity or ionisation produced in a gas by X-rays is

the sum of the effects of the X-rays and S-rays; this

differed from Perrin's view of 1897 that the additional

conductivity resulting from the introduction of a metal

plate arose from an ionisation occurring at the gas-metal

interface struck by X-rays.315 Sagnac examined the S-rays

from a variety of substances, using fluorescent, photo-

graphic, and electroscopic means of detection. By the

beginning of 1899316 he saw the absorption and emission

of X-rays and S-rays by chemical elements in terms of

their similarity to normal spectroscopic absorption bands

though the comparison was not a clear one; and it seems

that this idea of an incipient X-ray spectroscopy was

being formed during the previous year. In her initial paper of 1898 Mme.Curie mentioned the

researches of Sagnac and pointed out that the properties

of uranium and thorium rays are 'tres analogues' to those

of the secondary X-rays; the researchers at Cambridge had

also taken up this analogy, as we have seen.317 MarieCurie

tells us that she had herself examined the secondary rays

from uranium, pitchblende and thorium oxide and had found

these to have a greater discharging effect than those from

lead.318 Her proposed explanation of uranium and thorium

rays was that:

105

Pour interpreter le rayonnement spontane de l'uranium et du thorium on pourrait imaginer que tout l'espace est constamment traverse par des rayons analogues aux rayons de Rantgen maisbeaucoup plus penetrants et ne pouvant gtre absorbes que par certains elements a gros poids atomique, tels que l'uranium et le thorium.319

It was well known that bodies of greatest density, or

atomic weight, absorbed X-rays most effectively. Marie

Curie was now postulating the existence of a highly

penetrating radiation which was absorbed and then re-

emitted in less penetrating form by the elements of

highest atomic weight only. Why lead, with an atomic

weight of about 210, should show no sign whatever of

activity whereas thorium (230) was more active than

uranium (240) was not explained; however, the relatively

high activities of some minerals may have suggested that

the atomic weight of the new element might be greater

than 210. Mme.Curie's theory of the origin of uranium

and thorium rays was disposed of by her own hand, and

those of others, by the end of the year 1898; nevertheless

her suggestion of the existence of a new element proved

to be doubly justified.

Following her original suggestion Marie Curie was

joined by Pierre Curie in the ensuing chemical search for

the new active element and the couple were advised and

aided by G.Bemont, Pierre's counterpart in chemistry at

the Municipal School. After three months they were able

to report to the Academy through Becquerel, 'Sur une

substance nouvelle radioactive contenue dans la pechblende 1320

and went so far as to give a name to the new element, though,

as they noted, E.Demar9ay was unable to confirm it spectro-

scopically. They had started with the mineral pitchblende,

two and a half times as active as uranium, and applied the

usual successive dissolution and precipitation techniques

of inorganic chemical analysis. After each operation the

more active portion, measured with the piezo-electric

electrometer, was selected for further analysis. Finally,

after separations which were effective though incomplete

the highest activity resided with the element bismuth.

Continuing fractional dissolution and reprecipitation

106

gave slowly increasing activities, but sublimation of the

active bismuth sulphide gave a substance of the highest

activity, 400 times that of uranium. As is well known,

Marie Curie in this publication named the possible new

element 'polonium' after her country of origin. Polonium

obstinately refused to exhibit a spectrum, and had a

chequered history;321 however an element of this name

survives today. Most of the products set aside in the

pitchblende analysis must themselves have been active -

a number of quite novel chemical avenues could now be

followed. By the end of the next academic term, in

December 1898, the Curies and B6mont showed how successful

a path they had trodden by announcing the discovery of a .

second new radioactive element, on this occasion with far

stronger evidence to support their claim.322

The direct inorganic analysis guided by continual

electrical measurements led to barium chloride of activity

60 times that of uranium; the assumption was that a new

active element chemically similar to barium was present.

A fractionation procedure of dissolution in water and

partial precipitation by alcohol produced progressively

more active precipitates. The authors noted that this was

evidence of the existence of a new active element whose

chloride possessed different solubility characteristics

from those of barium chloride. They were able in this way.

to attain a substance of an unprecedented activity of 900,

before the materials ran out.323 100 kg. of pitchblende

residues from Joachimsthal (lacking uranium which had been

extracted for use as a colouring material) had already

been acquired.324 The strongest evidence for a new element

which they produced at this stage was provided by the

rare-earth spectroscopist E.Demarpy325 who stated that

he had actually found a new line in the spark spectrum

of the active substance. And the Curies and Bemont noted

that 'L'intensite de cette raie augmente done en meme

temps que la radioactivite'326 which constituted 'une

raison tres serieusel for attributing the new line to

the radioactive portion of the specimen. They named the

new element 'radium'. A possibly marginally greater

107

atomic weight of the active barium, compared with ordinary

barium, was obtained by determining the chlorine in the

anhydrous chloride. The expectation of an atomic weight

greater than that of lead was indeed to be realised.

All of the evidence, radioactive, spectroscopic and

gravimetric, was fully confirmed within four years.327

In the original paper on radium of December 1898 the

Curies described radioactivity for the first time as

'une proprietb atomique, persistant dans tous les tats . chimiques et physiques',328 they henceforth persisted in

the use of this expression, throughout the controversies

in the next few years concerning the origin of the phen-

omena. Just as 'atomic weight' need mean no more than

'relative combining weight' the expression 'atomique'

may here mean no more than 'elemental'. The authors

elaborated no further, perhaps with good reason.

Marie Curie's review article on 'Les Rayons de

Becquerel et le Polonium'329 appeared shortly after the

announcement of the discovery of radium, contains no

mention of that substance, and was therefore probably

written between July and November 1898. Its contents

give us an indication of the meaning of the description

of the emission of rays as an atomic property. One of

the reasons which Marie Curie gave for excluding white

phosphorus from the class of radioactive substances,

despite large readings on the measuring apparatus, was

that it was active neither when in chemical combination

nor as the red allotrope so that 'on ne retrouve done

pas le caract6re d'activit4 atomique independante des

6tats physiques et chimiques';330 this is similar to the

statement made at the December meeting of the Academy.

But in the earlier review we are also told that since

uranium exhibits constant readings, independent of its

chemical or physical state, its radiation appears

'comme une propriete mol6culaire, inherente a la mati6re miime de l'uranium'.331 A possible origin of the rays

which could be described without contradiction as both

atomic and molecular might be the Ur-Ur bond; chemical

valency in general might also be described thus; but

108

Mme. Curie did not discuss such points. She was never to

describe the phenomenon as molecular after 1898. But

others were to do so with attendant difficulty or con-

fusion surpassing that found here.

Marie Curie's speculations of 1898 were aimed more

at the problem of the 'Degagement d'energie par lee corps . radioactifs',332 she considered several possibilities for

the source. The hypothesis that the radiation was a

phosphorescence of long duration previously excited by

light she dismissed firmly; we have seen that Becquerel

had moved in this direction in 1896. The possibility,

briefly mentioned, that the radiation 'est une emission

de matiere' accompanied by a loss of weight of the active

substances333 seems suggestive of future work and remin-

iscent of W.Crookes"radiant matter' in discharge tubes;

but she may have meant no more than a release of vapour.

This is shown by the heading 'Emission de rayonnement lie

a un etat chimique de la mati6re radiante' under which

she now discussed334 the photographic effects studied

by Colson and Russell in terms of the release of vapours.

Thirdly she wrote that the source of the energy could

come from the evolution of the elements in the manner

suggested by Crookes - the elements of greatest atomic

weight could still be in the process of formation. Much

was left unexplained concerning the original protyle and

the production of the radiation; Crookes was himself

thinking of the problems of radioactivity at this time

but in other ways. Fourthly, Mme.Curie repeated more

clearly her own earlier theory, claiming that there was

nothing improbable in supposing that 'l'espace est le

siege de transmissions d'energie, dont nous avons aucune

idee'335 and that these ultra-X-rays might be transformed

into detectable, less penetrating secondary radiation by

heavy atoms. However, the sun could not be the source

of such rays, since the interposition of the whole body

of the earth would surely absorb a proportion of these

and cause a reduction in the intensity of uranium rays,

yet the midnight and midday readings were equal. She was

more concerned to argue that although such a theory was

109

in accord with the principle of Carnot there were author-

ities who held that such a principle need not apply 'avec

un mecanisme tree petit'. Mentioning the opinion of

Helmholtz on this point, Maxwell's 'demon' of the kinetic

theory of gases, and the work on Brownian motion of

L.G.Gouy her final comment was that:

Dans cette maniere de voir, le rayonnement de Becquerel pourrait 8tre consid6re comme un reflet des mouvements non coordonn6s de molecules matgrielles.336

Thus descriptions of the phenomena as atomic or molecular

were each consistent with such an external energy supply.

Her connection of the radiation to random molecular

motion has implications which were not followed up:

G.G.Stokes, J.J.Thomson and others at this time tentatively

accepted a theory that X-rays consisted of irregular ether-

pulses; but she made no mention of these scientists nor

did she state which, if any, of the five possible theories

she preferred.

Between the times of writing and publication of Marie

Curie's review others indicated their interest in the

growing problem of the energy source, heightened indeed

by the Curies' discovery of highly active substances.

William Crookes independently put forward almost the

identical molecular explanation of the origin both of the

energy and of the attendant phenomena of radioactivity.

And J.Elster and H.Geitel had gone so far as to devise

and perform experiments which cast doubt both upon

Crookes' theory and on Marie Curie's alternative of

unknown radiations from space.337

110

4. Theories and trends (1896-9)

In his address as President of the British Association

in September 1898 William Crookes338 covered a wide variety

of topics as expected, from the beneficial effects of

chemical fertiliser on the world food problem, to the

continuing controversy over the psychic researches whose

validity he accepted. As for uranium rays his early

interest in these seems to have been increased by

Mme.Curie's announcement of a new active element. In his

account of recent development in physical science Crookes

devoted some time to a discussion of the 'radiant activity'

of uranium, thorium, and the new body which possessed this

activity in 400-fold degree, polonium: 'like uranium, it

draws its energy from some constantly regenerating and

hitherto unsuspected store, exhaustless in amount'.339

With regard to the 'haunting problem' of the nature of

this store Crookes adopted the view of proponents of the

kinetic theory of gases, such as Johnstone Stoney, who

believed that the energy of molecular motions might be

made available, as with Maxwell's imaginary demons,

contrary to 'accepted canons'. In this Crookes independ-

ently made the same suggestion as had Marie Curie. But

he went into detail, in a manner which she never adopted,

by suggesting a mechanism involving the atoms of the

elements by which the emission of rays allied to X-rays

could spontaneously and perpetually occur. Crookes

pointed out that faster moving molecules were separated

from slower ones in the case of the evaporation of a

liquid and in the separation by diffusion of a lighter

from a heavier gas: Let uranium or polonium, bodies of densest atoms, have a structure that enables them to throw off the slow moving molecules of the atmosphere, while the quick moving molecules, smashing on to the surface have their energy reduced and that of the target correspondingly increased. The energy thus gained seems to be employed partly in dissociating some of the molecules of the gas ... and partly in originating an undulation through the ether, which, as it takes its rise in phenomena so disconnected as the impacts of the molecules of the air,

111 must furnish a large contingent of light waves of short wave-length. The shortness in the case of these Becquerel rays appears to approach without attaining the extreme shortness of ordinary Rtintgen rays.340

Kinetic theory indicated a large supply of energy,

translational and vibrational, contained in the air.

Crookes' and Marie Curie's speculations were within

months subjected to experimental tests not of their own;

and each theory was found wanting.

J.Elster and H.Geitel, scientific collaborators in

Wolfenbuttel, had throughout 1896 attempted to determine

whether the electrical effects produced by uranium were

related to photoelectricity.341 In experiments, followed

by G.C.Schmidt for thorium,342 they showed that no rel-

ationship existed: uranium salts gave no photoelectric

effect, and photoelectrically sensitive metals did not

emit invisible radiations. Nearly two years later

Crookes' address of September 1898 which they read in

Nature343 induced them to compose and send in their

paper 'Versuche an Becquerelstrahlen'344 before the end of

that month. Reporting experiments begun, in part, earlier,

they noted that they too had considered the surrounding

air to be a possible supplier of the radiated energy; not

as Crookes imagined but by means of a chemical reaction

between one of its constituents and the uranium salt used.

If the air were the source then a decrease in its pressure

should reduce the intensity of the radiation. As they

remarked, Beattie and de Smolan at Glasgow had shown that

the conductivity produced by Becquerel rays in air indeed

decreased steadily to a very small value as the pressure

of the air was reduced. Unlike the Glasgow researchers,

Elster and Geitel interpreted this result in ionic terms-

fewer of the ions required for transport of electricity

would be produced in a rarefied gas.345 Thus the reduction

of the air pressure should give a lower electrical reading

whether or not the intensity of the radiation diminished.

They therefore placed the radiating substance in a vessel

connected to a vacuum pump and examined the rays emerging

through an aluminium window. No significant variation of

112

intensity was to be found using the photographic method

with a long period of exposure - not a reliable quant-

itative method. This procedure was apparently employed

prior to Marie Curie's first publication on the subject

in April 1898; after this Elster and Geitel improved it

by using the more active natural pitchblende, enabling

electrical measurements to be made. These confirmed the

conclusion that the radiating activity was unaffected by

the surrounding air. We know that Rutherford, with his

deeper studies of the conduction of electricity through

irradiated gases, had already assumed as much; this is

shown in his paper on uranium rays,346 then not yet

published, in which experiments using a variety of gases

and pressures were described. Elster and Geitel admitted

that their result did not entirely demolish Crookes'

hypothesis, for even the best vacua contained millions

of molecules which might still supply sufficient energy

for radiation and ionisation. Crookes published his views

in France soon afterwards347 and was to maintain these

even when the energy problem became much greater in the

following years.

Elster and Geitel indicated348 that by July they had

also arranged to test Marie Curie's theory of April 1898.

They believed that no material could be completely trans-

parent to any radiation *and suggested that a thickness of

more than one hundred metres of solid rock would surely

absorb to a noticeable extent the ultra-X-rays from space

imagined in Mme. Curie's hypothesis; this should affect

the 'secondary' or uranium rays. Using the same portable

electroscope and the same piece of pitchblende they

recorded no difference in readings when the apparatus

was taken 300 metres below ground into a mine. A photo-

graphic experiment carried out for them at the bottom of

the Schact Kaiser Wilhelm II, 852 metres deep, also showed

no variation of intensity. Their conclusion was much firmer

than with Crookes' hypothesis:

Nach diesen Versuchen erscheint uns die Hypothese der Erregung der Becquerelstrahlen durch andere im Raume prLexistirende Strahlen im hiSchsten Grade unwahrscheinlich.349

113

However, they were able to confirm the Curies' chemical

extraction from pitchblende of a highly active substance.

This, following the discovery of thorium rays, appears to

have been seen in 1898 as the most important experimental

result in the field since the original researches of

Becquerel.

The development of the subject of uranium rays by

Marie Curie during the early part of 1898 has been regarded

as marking the conclusion to a period in which interest

in these rays had declined to a low ebb partly because of

their submersion in a 'morass' of other radiations.350

This obscurity, as has been shown, was complemented by a

confusion of uranium rays with the Russelleffect. It is

true that in France there was a period of several months

in 1897 when Becquerel had turned to studies of the Zeeman

effect, and Marie Curie had not yet taken up her investi-

gations. But it may be pointed out that elsewhere in

Europe one can trace something of a continuing concern

with the subject during 1896-8. As we have seen, in 1896

several French physicists showed an interest in uranium

rays, which is indicated by their published reviews and

discussions. In England J.J.Thomson's351 comments on the

subject were followed by some experimental studies of E.Rutherford and C.T.R.Wilson in 1897. G.G.Stokes too

discussed the phenomenon with S.P.Thompson in 1896.

Stokes published a theoretical explanation of the radiation

in mid-1897352 and during this year he discussed at the

Cavendish Physical Society the work of W.J.Russell on the

photographic effects of metals; J.J.Thomson reported this

in October 1897.353 Stokes also discussed the subject in

communication with Crookes who had the phenomenon in mind in February 1898.354 Crookes had been attempting experi-mentally to obtain a radiometer effect from uranium

radiation in August 1897; but the definite results obtained

were attributed to temperature differences.355 Kelvin and

others at Glasgow published their researches on the

electrical effects of uranium in 1897. And one can trace

a line of minor interest, if not of continuing experimental

work, in Germany, where the publications of Elster and

114

Geitel of 1897, were followed by G.C.Schmidt's studies

on thorium rays. These, together with the distinction

he made between the effects of the new radiations, of

photographically active substances, and of photoelectricity,

were announced in February 1898.356 And J.J.Thomson in the

previous month made his important speculation as to the

origin of uranium rays.357 One can thus see an underlying

interest in uranium rays which was to be considerably

excited by Marie Curie's announcement of the discovery

of polonium in mid-1898. As studies of uranium rays and radioactivity developed

during the period 1896-9 the various theories which were

put forward can be seen as more or less related to

Becquerel's early description of the phenomenon as a metal

phosphorescence after he had isolated uranium as the source.

G.G.Stokes, the British authority on phosphorescence and

fluorescence, in advising S.P.Thompson concerning the

latter's discovery in February 1896358 of the emission of

penetrating rays from various chemical compounds, also

provided at this time a mechanical molecular explanation. Stokes took the rays to be like X-rays 'transversal

vibrations of excessive frequency' and likened their

emission to the phenomenon of 'calorescence' in which

heat radiation of high intensity could raise a body to

incandescence. Stokes regarded fluorescence 'as a

disturbance extending from more limited to more extensive

molecular groups'359 but calorescence and Thompson's new

phenomenon360 appeared to be the reverse of this:

I look on calorescence as an agitation passing from wider to more minute molecular groups. In your discovery, I think we have something of the nature of calorescence; only that whereas in Tyndall's work the disturbance was excited in the first instance in wider molecular groups, in yours the 'wider groups' are already something like the chemical molecules of the peculiar substance.361

It follows from Stokes' explanation at this early stage

that if the wider vibrating groups are something like

chemical molecules then the smaller groups should be

something like the atoms in a molecule. That Becquerel

was soon able to trace the emission to uranium metal

115

might have led Stokes subsequently to such a conclusion

concerning uranium atoms. But in his Wilde Lecture

'On the Nature of the Röntgen Rays' delivered in July

1897362 he was evasive on this point, while incorporating

most of the new evidence, and expounding a modified theory

of the nature of X-rays. In order to explain the perpetual

emission of rays by uranium without the necessity of

irradiating the metal Stokes told his audience: My conjecture is that the molecule of uranium has a structure which may be roughly compared to a flexible chain with a small weight at the end of it.363

Natural vibrations travelling from the head to the tail of

the molecule would produce ether vibrations 'not of a

regular periodic character'. Stokes still saw X-rays as

transverse disturbances of the ether; however, these were

now characterised not by an extremely short wavelength but

by consisting of completely irregular pulses. Uranium rays

would thus lie between visible light and X-rays in their

regularity. The chemical implications of imagining such

a 'uranium molecule' are not discussed.

Stokes entertained this kind of explanation of radio-

activity at least until 1900 without publishing further on

the matter. Towards the end of 1899 he sent Becquerel a

copy of his Wilde Lecture and discussed theoretical ideas

of radioactivity; Stokes maintained an explanation in

terms of a comparison with the normal visible phosphores-

cence of uranium compounds. He considered that both

phenomena pointed to 'the existence of a molecular group

which is roughly speaking isolated, in the sense that

vibrations going on in it are not very quickly communicated

to the neighbouring structure'.364 Although, as Becquerel

had replied,365 the expected effect of temperature change

on the emission failed to appear Stokes continued to

entertain what he called the 'wagtail' theory. When the

self-luminosity of radium came to his notice in 1900 he

corresponded with W.Crookes concerning experiments which

might distinguish between his wagtail theory and Crookes'

'bombardment hypothesis' 366 and continued the discussion

as the phenomena and ideas became more complex in 1901.

116

We have seen that Stokes attempted to explain uranium

rays in terms of molecular vibrations akin to those assumed

for phosphorescence, that his ideas changed little during

the period 1896-1900, and that they began independently

of Becquerel. By the end of 1898 Marie Curie had explicitly

rejected the earlier statement of Becquerel that the phen-

omenon might be a long lasting invisible phosphorescence,

together with S.P.Thompson's label of hyper-phosphorescence.

It is notable however that her own first analogy of uranium

and thorium rays with secondary X-rays seems similar to the

phenomenon of fluorescence. Apart from Kelvin's guess in

1897 that the effects of uranium might be due to carbides367

two other possible sources of the rays were seriously

considered during the first three years of their investi-

gation. Firstly there was the atomic-molecular hypothesis

independently suggested by W.Crookes and Marie Curie

involving the impact of gaseous molecules upon sensitive

atoms which then produced radiation. Secondly, following

J.J.Thomson's setting out of corpuscular atomic structures

towards the end of 1897 there was his suggestion that some

kind of rearrangement of the constituents of a 'complicated

structure' like the uranium atom might be the source of the

rays. Remarkable speculations which fitted with this latter

view were soon published in Germany.

Rutherford had tentatively adopted368 the above

explanation provided by Thomson. The former's new colleague

at McGill University, R.B.Owens, attempted in 1899 to

incorporate experimental results on the non-homogeneity of

thorium rays and similar radiations into this theory. He

wrote:

Certainly it would be difficult to formulate a theory for the production of such rays which would account for only a particular number of kinds being produced. If x-rays and the radiations from uranium, thorium, polonium, &c. are disturbances in the aether occasioned by the internal motion of certain constituent parts of the atom, as had been suggested, it might be expected that such disturbances would shade off with some degree of regularity from a more intense to a less intense kind...369

But it was left to Elster and Geitel370 to put forward in

117

1899 speculations on the chemical implications of radio-

activity; these may have influenced others, in a manner

not previously brought to light.

Following their dismissal of Mme.Curie's first

hypothesis of the source of the energy of uranium rays

and their doubts as to Crookes' molecular bombardment

hypothesis, Elster and Geitel added further experimental

evidence, all negative, some of which had been previously

obtained by others but not published. Without definite

influence upon the radiation from pitchblende were sunlight,

Lenard rays, temperature changes, and being kept in darkness

for months. They noted the remarkable luminous effect of

the Curies' radium on a fluorescent screen. Their conclusion

was that since the emission of energy from all compounds of

an element could not result from a chemical reaction the

source must be the atoms themselves of the elements concerned.

Now this was similar to what the Curies had said in 1898.

And their further speculation that the atom of a radioactive

element behaves like a kind of unstable molecule emitting

rays on returning to a stable state is no more than

J.J.Thomson and E.Rutherford had written shortly before.

Elster and Geitel may or may not have derived this idea from

Thomson's publication of January 1898 or Rutherford's of

January 1899; Elster had replied to Rutherford's request

for advice on demonstration apparatus and indicated that he

had read Rutherford's paper on 'Uranium Radiation etc.' at

the time of writing, 10th February 1899.371 Elster and

Geitel's final deduction in the paper of 1899 was that the

change of a substance from an active to an inactive state

might necessitate a change of elementary properties: Der Gedanke liegt nicht fern, dass das Atom eines radioactiven Elementes nach Art des Molecules einer instabilen Verbindung unter Energieabgabe in einen stabilen Zustand Ubergeht. Allerdings Warde diese Vorstellung zu der Annahme einer allmAhlichen Umwandlung der activen Substanz zu einer inactiven nbthigen and zwar folgerichtiger Weise unter Aenderung ihrer elementaren Eigenschaften. 372

This conclusion says, almost in so many words, that a

transmutation of one chemical element to another of diffe'rent

properties should be taking place as the radioactive

118

radiations are emitted. Rutherford had considered that the

energy of the emission from uranium was so 'extremely small'

that radiation could continue for long periods 'without

much diminution of the internal energy of the uranium'.373

This avoids Elster and Geitel's conclusions by emphasising

the smallness of the effect. Now since the existence of

the highly active radium could not safely be denied in 1899

Rutherford could possibly have entertained these speculations

of the German scientists, which indeed could have followed

from his own; it is probable that he knew of them374 and

that he came to do so whilst studying the strange properties

of thorium rays at McGill University in 1899.

We have seen that at this time Rutherford accepted

the idea that the ether vibrations emitted by radioactive

substances could be attributed to rearrangement of the

corpuscles constituting the chemical atom and that he

probably knew of the speculation that atomic transmutation

might be occurring. It has also been noted that one of the

speculations put forward by Marie Curie, published early in

1899 and upon which there appears to have been no later

comment but her own, was that the heaviest elements may be

in process of evolution. All ideas on the source of the

radiations, on their energy, and nature, were to be com-

plicated by two discoveries announced towards the end of

1899. One of these, that Becquerel rays could be magnet-

ically deflected, showed the rays possessed an unexpected

property which linked them more closely to cathode rays

than to X-rays. And cathode rays were thought by

J.J.Thomson, his disciples, and some others, to consist

of the material particles which constituted the chemical

atom. But the increasing complexity of the new phenomena

allowed no easy conclusions.

119

CHAPTER 3

EMANATIONS AND RADIATIONS

1. The ma netic deflection of the Becquerel rays 1 9 -1900)

The chemical isolation of highly active substances

by the Curies in 1898 provided new opportunities for

physicists and chemists to pursue experimental studies.

One cannot say whether G.C.Schmidt, E.Rutherford or

others interested in the subject would eventually have

hit upon the existence of the new active elements but

once the Curies had opened this field others soon followed.

In this Chapter we shall trace the complex form of the

development of radioactive studies during the period

1899-1901. It will be seen that the discovery of the

new highly active substances heralded the first magnetic

deviation of radioactive rays. This had far reaching

theoretical implications particularly, in combination

with further new discoveries discussed in Section 2, for

radioactivity. F.Giesel, chemist at the Buehler quinine manufacturing

company in Braunschweig, was one of those who had studied interest in new X-ray fluorescence and photography.

1 His nterest

in the related area of radioactivity began by January 1899

when he spoke in the discussion of Elster and Geitel's

paper at Brunswick.2 During this year he followed the

analytical procedure of the Curies and Bemont to prepare

polonium, and claimed to have independently discovered

active barium compounds which possessed the novel property

of spontaneously illuminating a fluorescent screen;3 he

was able to provide Elster and Geitel with such a highly

active sample.4 Giesel had obtained his uranium residue

starting material from De Haan, chemical manufacturers of

Hanover5 and the first commercial radium-barium samples

were soon advertised by this company6 to whom Elster

advised Rutherford to apply.7 The physicists Elster and

Geitel used an active sample provided by Giesel to deter-

mine whether the electrical conductivity known to be

120

produced in air by Becquerel rays could be altered by a

magnetic field.8 A peripheral idea involved in their

apparently successful attempt led Giesel himself to a

crucial discovery.

It had been known for some years that the electrical

conductivity produced in a gas, by glowing metals for

example, could be suppressed by the application of a

magnetic field whose direction did not coincide with that

of the current.9 The explanation, if based on the assumption

that molecules, ions, or particles of any kind carried the

charge, was that the lateral force suffered by these current-

carrying particles in the magnetic field would deflect them

out of the line of conduction. A marked reduction in con-

ductivity was indeed obtained, but more interest attaches

to the control experiment which Elster and Geitel devised

to ensure that it was not a deflection of the rays them-

selves which gave the observed effect. The Becquerel rays

were thought to be similar to secondary X-rays but there

was nevertheless the possibility that the rays might be

magnetically deviable like Lenard or cathode rays. The

property possessed by Giesel's very active barium salts

of exciting phosphorescence in a screen was valuable for

the straightforward experimental arrangement of Elster

and Geitel. They placed the active substance in an evacuated

glass vessel; the emitted rays passed both through the glass

walls and an aluminium plate upon which rested a barium

platinocyanide screen 1.5 cm. distant from the source.

The visible phosphorescence excited in the screen was un-

altered when their iron horseshoe electromagnet was switched

on. They concluded that the rays were undeflectable by a

magnetic field and hence different from cathode rays; all

of the properties of the Becquerel rays were thus comparable

with those of the X-rays. Others also considered this

point. In France the Curies had earlier, in 1898, similarly

obtained negative results, unpublished, in seeking an effect

of magnetic and electric fields on radioactive rays.10 And

Becquerel later stated11 that towards the end of 1899 he

too had been seeking independently some such influence,

and had in fact found one: on placing a fluorescent screen

121

at one pole and an active sample at the other pole of a

magnet, the application of the field produced a con-

centration of the fluorescent patch into a smaller area.

All experiments on the subject of radioactivity bore

repetition at this time, autumn 1899. Thus Giesel

repeated12 Elster and Geitel's experiments in simpler

fashion without a vacuum but with a more powerful magnet.13

Using a phosphorescent screen placed upon the poles of a

vertical horseshoe magnet and an active freshly prepared

polonium specimen placed beneath the screen he became the

first to succeed in obtaining quite definite results.

Upon switching on the magnet, the luminous spot was

displaced in a blurred fashion but in a definite

direction in relation to the field. Giesel also produced

variously shaped images by the photographic method; exposures

of up to ten minutes were all that was required for a fixed

record of the effect. Having achieved these results Giesel

at first published no interpretation, but various explan-

ations were not long in coming. Two privatdozenten at

Vienna who also followed the work of Elster and Geitel

moved towards a curious misinterpretation of their own

results and expressed an interesting if short-lived

speculation bearing on the source of Becquerel rays.

S.Meyer and E.von Schweidler extended their work on

the magnetic properties of the chemical elements, which

included correlations of atomic magnetism with the periodic

table, to a study of the magnetic properties of radium

preparations and the rays emitted by these.14 They used

both Curie and Giesel barium-radium and bismuth-polonium

preparations placed 12 cm. from the air-gap through which

the rate of electrical discharge was measured electros-

copically. On applying the magnetic field this rate was

considerably reduced; the Curie polonium however was un-

affected. The results of varying the intensity of the

field indicated that at least two different effects

operated to reduce the conductivity of the air-gap. They

provisionally accepted the statement of Elster and Geitel15

that the rays were not deviated by the magnet, without

knowing that Giesel had proved this wrong, and speculated

122

that the magnetic field might act directly upon the radium

in reducing its radiation:

so ware die Ursache diener Erscheinung nur in einer direkton Beeinflunnung der Emission der Substanz selbst zu suchen wenn eine Ablenkung der Strahlen nicht stattfande.16

In a matter of days they sent off a second paper now

mentioning Giesel's work on the magnetic deflection of

rays not yet available in print which they had confirmed

for themselves. With their powerful electromagnet they

had been able for example to bend the rays in a tight

semicircle back to the screen upon which the active

substance stood; they noted that the direction of the

effects was entirely similar to that of the negatively

charged cathode rays.17

However, their initial conjecture that the magnetic

field might influence the emitter itself, ill-founded and

temporary though it was, gives us an indication of one

way in which radioactivity could be understood towards

the end of 1899. The possibility that a magnetic field

might affect the property of radioactivity was not lightly

to be discounted. It is true that none of the various

attempts to influence radioactivity by a variety of

physical and chemical means had given any positive result

and that magnetism was to remain on this list. Yet it is

interesting to note that an effect somewhat analogous to

that at first assumed by Meyer and von Schweidler had

recently been detected. Zeeman's discovery made at the

end of 1896 was accepted as demonstrating that a magnetic

field could influence the source of atomic spectra,

altering the frequency and direction of vibration of the

radiation. J.J.Thomson had used this discovery to support

his corpuscular atomic theory of 1897.18 In 1899, when the

Continental workers were studying the effect of magnetism

on the new atomic property of radioactivity and upon

Becquerel rays themselves, he was defending this developing

theory against criticism.19 In his paper 'On the Masses

of the Ions in Gases at Low Pressures'20 he insisted on

the corpuscular structure of the chemical atom and,

probably arguing against the view that all observations

123

could be explained by means of a small number of free

electrons or valency charges, he again pointed to Zeeman's

discovery, now extended. Since many spectral lines

exhibited an effect of the same order each atom must

contain many corpuscles despite the fact, shown by e/m

estimates for ions in gases, that very few could be

removed.21 At this time Thomson considered that Becquerel

rays themselves originated in the motion of these

electrified particles. Although the hope of finding

evidence to support the theoretically possible link between

magnetism and radioactivity apparently remained with him

for several years22 the negative interpretation of the

experimental results of 1899 remained unchallenged.

Thomson believed that both chemical and electrical actions

essentially involved 'the splitting up of the atom, a part 2 of the mass of the atom getting free and becoming detached'.3

The positive discovery that radioactive substances did not

simply radiate soft X-rays but released streams of subatomic

corpuscles turned out to be of extraordinary interest to

those such as Rutherford who entertained Thomson's ideas.

However, R.J.Strutt later wrote24 that he clearly

remembered that this striking interpretation of Giesel's

results was at first acceptable neither to Thomson nor

Rutherford on account of the high penetration of these

radioactive rays. The former wrote to the latter:

I see Giesel makes out that the radiation from polonium is affected by a strong magnetic field, if this is so it might be worth while trying whether your emanation from thorium were so affected.25

Rutherford replied in January 1900:

The results of Giesel & Becquerel are very interesting and remarkable. I expect the 'emanation' in thorium is also true for polonium & radium when prepared in a special manner & that the deflection due to the magnetic field is due to the action on a charged particle cast off from the active body.26

Within a few months Rutherford indeed saw the phenomenon

as a direct emission of cathode rays and had made certain

deductions from this. He had previously adopted a tentative

comparison of the beta and alpha uranium rays with X-rays

124

and the secondary radiation produced therefrom27 but had

given no decision between Thomson's soft X-ray or Sagnac's

Lenard-ray view of the secondary radiation. The demon-

stration during this period by P.Curie and G.Sagnac28 that

secondary X-rays in fact carried a negative charge may

have provided Rutherford with indirect evidence for the

new view of radioactive rays. The phrase 'secondary

X-rays' which he used29 to describe the non-deflectable

radium rays may thus have acquired the new meaning of

actual X-rays produced as a result of the impact of cathode

rays. Yet this slightly modified causal link between the

two known types of radioactive radiation was soon to be

completely broken. Extensions made by Becquerel to his

magnetic results provided direct evidence of the nature

of the deflectable rays during the first part of 1900.

By June of that year Rutherford was able to write that

the property possessed by some active substances of

'naturally emitting a kind of cathode rays'3° did not

contradict Thomson's theory of regrouping and vibration

of the constituents of the atom. But we note that such

a mechanism had been intended to explain the production

only of electromagnetic radiation. Rutherford's assumption

that the radiations in question were 'cathode rays of low

velocity',31 which agrees neither with Becquerel's con-

temporary studies nor with Rayleigh's later comment on

penetrations, illustrates the difficulties of this novel

aspect of radioactivity.

The successful magnetic deviation excited an interest

corresponding to its initial problematical nature; it gave

a further impetus to investigations of the rays themselves.

A number of physicists concerned with the cathode rays,

such as R.J.Strutt (later Lord Rayleigh), P.Villard, E.Dorn,

W.Kaufmann, used samples of radioactive substances in their

researches from about 1900, with the attainment of sig-

nificant results pointing towards an electromagnetic theory

of matter by 1901. And the effect of the magnetic discovery

upon the French students of radioactivity was considerable

not only in exciting the great activity in investigating

these rays which arose during 1900 but also in the changes

125

of view which the rapidly developing experiments helped

to produce. One can, for example, interpret the discovery

of the emission of cathode rays as marking the quiet beg-

innings of a severe conflict of the ideas of P.Curie both

within himself and against his colleagues. In the dis-

cussion following a report to the French Physical Society32

in which Becquerel announced his successful deviation of

the rays by an electrostatic field, further demonstrating

their similarity to cathode rays, P.Curie spoke. He thought

it surprising that radium should emit radiations having the

properties of cathode rays as well as X-rays since it was

generally agreed that while X-rays were the propagation of

a disturbance cathode rays were a flux of ponderable matter.

Despite his own considerable researches on the electrical

nature of the radiation, after two years he found himself

forced to reject this interpretation of cathode rays. This

was in reaction against views, implied by the emission of

subatomic masses, which several scientists began to develop

with growing supporting evidence following the magnetic

deflection of radioactive rays.

By March 1899 Becquerel, re-entering the field, had

accepted Rutherford's point published in January that

uranium rays did not behave like ordinary light with

regard to polarisation, refraction and reflection; they

were more like X-rays.33 Becquerel continued investigations

of the nature of these rays by examining their penetration,

secondary radiation, and the possible effects of magnetic

and electric fields. It was the work of this scientist,

who was fortunate in having access to the Curies' most

active specimens, which contributed perhaps most to a

clearer understanding of the nature of the rays during

the year following their first deflection. Becquerel's

initial attempts to obtain an electrostatic deviation

during 1899 failed as he perhaps expected since the field

applied was known to be insufficient to deflect ordinary

cathode rays.34 But his magnetic studies35 soon gave highly

significant results. The Curies were first in making a

valuable correlation of three aspects of the rays namely

deviability, penetration, and conductivity. Their polonium

126

radiation was easily absorbed and remained undeflectable

and making use of radium they were able to show that

whilst the deviable portion of magnetically analysed rays

was more penetrating than the non-deviable portion, it

contributed little to the total radiation as measured

electrically.36 Becquerel immediately moved the study

to a more quantitative level. He pointed out37 that if

the radium rays consisted, like cathode rays, of electrically

charged material particles then the well-known equation38

H.R = v.m/e held true. H was known, R was found by

photographic impression; this gave a value for v.m/e of

the same order as determined for the cathode rays by

J.J.Thomson, W.Wien, and P.Lenard. Becquerel deepened the

correlation between deviability and absorption: he found

that the interposition of screens before the detecting

plate gave a kind of absorption spectrum in which the most

easily deviated rays were the most easily absorbed.39 He

expanded the point in a paper 'Sur la dispersion du

rayonnement du radium dans un champ magnetique'.40 The

sharpness of shadows was unimpaired on covering the radium

source with an aluminium screen which indicated that

aluminium was truly transparent to the rays and not a

re-emitter of secondary radiation. Becquerel noted minimum

values of HR for the radium rays (dispersed as well as

deviated by the magnetic field) to which screens of various

materials and thicknesses behaved as transparent; to rays

having HR below a certain value, all screens were opaque.

It was still debatable at the time whether or not the

aluminium screen used to allow Lenard rays to escape from

the discharge tube acted as a window; penetration of

opaque bodies by cathode rays seemed unlikely to Crookes,

for example, at the end of 1900.41 J.J.Thomson's answer

in 1896 to Lenard's claim of 1895 that ether waves, but

not material particles, could penetrate a metal window

seems to have been that X-rays together with a re-emission

of charged particles were responsible for the observed

effects.42 From Thomson's discussion of 189743 one might

deduce, considering the cm. penetration of cathode rays

into ordinary air, that aluminium might not function as a

127

window if thicker than about 0.001 mm. Becquerel's

aluminium screen was 0.2 mm. thick; if the particles

were all of the same charge and mass his experiment

would seem completely to correlate velocity with pen-

etration. A reviewer in an article on 'Becquerel rays.

Confirmation of the materialist theory of the deviable

rays of radium' seems to have been first to publish the

clear inference that 'the particles which strike the plate

furthest from the source will be those possessing the

greatest speed, and it is natural that they should also

be the most penetrating'.44 Radioactive studies in 1900

thus lent temporarily a sharp clarity to the projectile

character of the cathode rays. R.J.Strutt staked his claim

in the matter writing to Nature45 from the Cavendish

Laboratory that the magnetically deviable rays from radium

exhibited, like cathode rays, a coefficient of absorption

very approximately proportional to the density of a series

of materials ranging from solids to gases, but that this

coefficient was only about 1/500 that of the cathode rays.

Hence 'One must suppose either that the particles consti-

tuting them are much smaller, or that their velocity is

much greater'. Becquerel's results together with the

Curies' demonstration that radium exhibited a continuous

emission of negative electricity, spontaneously acquiring

a high positive charge,46 provided clear evidence that

radium emitted negatively charged particles of matter.

That the question of the size of these was not simple is

further demonstrated by the suppositions of the Curies

themselves. They thought it reasonable to infer that

'le radium est le siege d'une emission constants de

particules de matiere electrisee negativement'47 which

could traverse either conducting or dielectric screens

without becoming discharged. However, one can see that

the Curies at least were not yet thinking in terms of

subatomic corpuscles for, knowing the rate of loss of

charge, they gave in March 1900 what was probably the

first published estimate of the loss of mass involved on

the assumption that e/m was the same as in electrolysis..

They did not say how these particles could penetrate screens.

128

Such an emission of negatively charged atomic or molecular

masses gave an estimated weight loss amounting to

3 milligram-equivalents in a million years. Attempts to

detect such losses from ever more active specimens were

to occupy the Curies and some others during the next few

years. However, the Curies' view of the rays soon altered

in a way perhaps similar to that of Rutherford.

This shift in opinion followed the current rapid

experimental progress. For later in that month Becquerel

effectively consummated the study by his 'Deviation du

rayonnement du radium dans un champ electrique'.48 He was

thus able to confirm that the projected particles were by

no means atomic in mass; the variability of this mass was

not yet an experimental question. Becquerel's photographic

method provided measurable and reversible deviations of

rays passing through a narrow slit in a small lead container;

the quantity e/mv2 could thus be calculated. As we have

seen the value of v.m/e could be found by magnetic deviation

experiments so that values of v and e/m could now be

separately determined. The 'point delicat' was to ensure

that the values obtained from magnetic and electric experi-

ments both applied to the same part of these deviable rays,

which exhibited a considerable range of dispersion.

Becquerel achieved this, as far as possible, by selection

of the rays using partially absorbing screens. Thus he

obtained values for v of 1.6 x 1010 and for e/m of 107

for a portion of the deviable radium rays, similar indeed

to cathode rays. For the latter Becquerel cited values

of v up to 0.81 x 1010. He made no explicit comment on

the possible subatomic nature of these particles, nor on

their uniquely high velocities, but estimated the rate of

decrease of mass which would be caused by their loss as

far lower than that of the Curies; one would expect this

from the emission of particles lighter than atoms but

carrying the same electrolytic charge. From the value

of mv2 for the particles Becquerel estimated that the power

emitted by the rays was a few ergs per square cm. per sec.

Such a value was very soon shown to be excessively low:

this portion of the radium radiation carries but a fraction

129

of the total radiated energy. Nevertheless, the experi-

mental deflection of these rays made a permanent impression

upon all of the various views of the source of radioactivity,

as elaborated below in Section 3. Even while this radiant

success was in progress Rutherford came to attribute his

own interesting results on thorium rays to something more

than a radiation - to the release of a material gas-like

substance which he took as the cause of another new phen-

omenon, 'excited' or 'induced' radioactivity. The nature

of induced radioactivity was to become. an area of partic-

ular disagreement between the Curies, Becquerel, Rutherford,

and others, but also a point of progress.

2. The discovery of induced radioactivity (1899)

Yet another branch of radioactive research unfolded as a

series of novel experimental results began to appear.

In parallel with magnetic studies, several scientists

worked in this new field on similar lines, at first indep-

endently. In some cases the extraction of the new intensely

active substances was an essential factor, in others ordinary

compounds sufficed. The discovery of the radioactivity

induced by radium and polonium was made by the Curies

towards the end of 1899, and the title of their note 'Sur

la radioactivite provoquee par les rayons de Becquerel'49

indicates their initial view that this effect was due to

the direct incidence of radiation. They found that samples

of polonium and radium with activities 5,000 to 50,000 relative to uranium could produce, in all substances tried,

130

temporary activities ranging from 1 to 50, the higher

values being obtained with longer exposure to the rays;

the effects diminished to about one tenth in two to three

hours. The new phenomenon which they named 'radioactivite

induite' created many scientific problems for the Curies.

As they stated, the value of the activity produced seemed

not at all to depend on the nature of the surface made

active. However, they were unable to explain this fact.

Could the effect be due to vapour or to the deposition

of dust particles from the active substance? They thought

that the steadily declining induced activity could not be

attributed to non-volatile particles of the activating

radium-barium chloride for the activity of the latter did

not so decline; furthermore, washing the activated surface

with water should remove this soluble salt, but the induced

activity remained unaffected by the process. Neither could

it be attributed to a vapour, for an activity was apparently

produced by radiation which had passed through the aluminium

window of a metal box within which the activating specimen

was sealed. Rutherford at this time did in fact attribute

induced radioactivity to an active gas-like substance

which could penetrate metal screens. E.Dorn shortly after-

wards in 1900 agreed50 that some kind of active gas was

responsible, but pointed to the possible presence of pores

in thin metal screens. One must note that the problem may

also have been complicated by external traces of radium as

contemporary scientists may soon have realised. However,

the Curies in 1899 concluded that 'Le phenomene de la

radioactivite induite est une sorte de rayonnement

secondaire du aux rayons de Becquerel',51 though dist-

inguished by its longer duration from the direct emission

of secondary rays. Becquerel evidently considered that

this fitted with views he had held since 1896 and he

commented52 that the new phenomenon should be placed

alongside the production of secondary rays of low pen-

etration by thorium and uranium rays; this he considered53

had led to the earlier false deductions by himself and

Schmidt that diffuse reflection of these rays took place.

Thus induced radioactivity and the emission of secondary

131

rays were respectively similar to phosphorescence and

fluorescence. Becquerel did not say how such an analogy

could explain why different substances exhibited the same

magnitude and duration of induced activity. The Curies

themselves merely expressed surprise at this and relegated

to a footnote their suggestion that the condition of the

surrounding air might have some influence in causing

irregularities in the induced effect. Similar points

had already been noticed by Rutherford in his work on

thorium, and had played a vital part in his own sharply

differing interpretation of 'excited radioactivity'.

Whilst Rutherford was studying the electrical effects

of uranium radiation at the Cavendish Laboratory in 1898,

G.C.Schmidt and Marie Curie had announced the discovery of

a similar radiation from thorium. Rutherford in his paper

on uranium rays54 wrote of his own attempts to study the

new thorium radiation. It appeared to be complex and of

a different kind from that of uranium as shown by its

penetration of aluminium screens. Unlike uranium salts

thick layers of thorium nitrate gave a greater proportion

of penetrating rays than thin layers but 'On account of

the inconstancy of thorium nitrate as a source of radiation,

no accurate experiments have been made on this point'.55

The rate of leak varied 'very capriciously'. This was the

problem which Rutherford took to Canada towards the end

of 1898. In his first preliminary paper from McGill

University some eight months later he again noted that

thorium was far from exhibiting the constant radiation

which was such a notable property of uranium:

The inconstancy of the radiation from thorium oxide was examined in detail, as it was thought it might possibly give some clue as to the cause and origin of the radiation emitted by these substances.56

It was R.B.Owens, Macdonald Professor of Electrical

Engineering, working with Rutherford on the thorium rays

in 1899, who found the first clue as to the variability

132

of thorium radiation readings - the marked effect of

slight currents of air.57 With thorium placed in a closed

box the measurements remained constant, but on opening a

door in this box the readings were consistently lower,

and were further diminished if somebody opened or closed

the laboratory door; the readings recovered on standing

for some hours in the closed box. The authors mentioned

that they had performed a large number of experiments,

for example examining the effects of blowing air over the

surface of the material, but that they had found 'no clue'

as to why the oxide of thorium should exhibit the phen-

omenon. The explanation which they provided is fascinatingly

unclear. The effect of an air current was not due to

removal of the ions but due to its action at or near the

surface of the substance:

It appears as if in the pores of the thick layer of thorium oxide some change takes place with time, which increases the intensity of the radiation, and if the result of the action is continually removed, the intensity of the radiation is diminished.58

In his next paper on 'Thorium Radiation'59 Owens detailed

how they had overcome the variability problem to study

what appeared to be the true penetration characteristics

of thorium rays. He concluded that these were composed

of many distinct types which was consistent with the

suggestion that 'the internal motion of certain constituent

parts of the atom' produced such 'disturbances in the

aether'. We shall examine his expansions and enigmatic

explanations of the original air-current phenomenon, as

well as Rutherford's more successful alternative.

Necessarily to anticipate this discussion, here are

Rutherford's words of introduction to his description of

'A Radioactive Substance emitted from Thorium Compounds': 60

In addition to ... ordinary radiation, I have found that thorium compounds continuously emit radioactive particles of some kind which retain their radioactive powers for several minutes. This 'emanation', as it will be termed for shortness, has the power of ionizing the gas in its neighbourhood...61

Two points turned. out to be most important for the

continuation of this research towards the successful

133

theory of 1902-3, both of which became the subject of

dispute. These were firstly that the 'emanation' from

thorium compounds consisted of a material vapour which

was not thorium vapour, and secondly that this emanation

produced, upon any solid body, a radioactive deposit of

a distinct chemical nature. This deposit could be con-

centrated upon any object negatively charged.

Since the discovery of the thorium emanation, I have always taken the view that the emanation consists of matter in the radioactive state present in minute quantity in the surrounding gas.62

Thus wrote Rutherford a few years later in 1903 when his

theory was widening rapidly in scope. We note that this

statement is borne out by the earlier publications, and

is not tautological in the later context. The vital

conception of a material emanation seems to have appeared

during the period May to July 1899 and that of an active

deposit probably during that period also.63 Although

Rutherford chose to discuss each aspect separately in his

publications, he evidently studied these in close parallel.

This is indicated for example by Thomson's reply to

Rutherford's communication with Owens, who was spending

the summer of 1899 at the Cavendish Laboratory. And here

began an infrequent correspondence between Thomson and

Rutherford which seems to have developed into something

of a disagreement, lasting until 1903. The views of the

former fluctuated somewhat, as will appear; but some of

his information, theoretical ideas and suggestions for

experiments, though historically neglected, clearly

influenced Rutherford. Thomson first commented on the

points which he thought were of particular importance or

difficulty:

I have today been reading the paper on thorium oxide you sent to Owens - the results are certainly very surprising. The points that struck me most were the effect of the air currents, & the necessity of the plate which is to be made active being negatively electrified. It seems to me that it would be worth while trying blasts confined to various strata between the plates ... It seems to me that it might be urged that the effect was due to the gas close to the surface of AB being very intensely ionised & giving out a kind of radiation which produced a phosphor-escent effect on the plate...64

134

Another point which seemed to 'want settling' was what

part the negative charge on a plate played in its

activation. Thomson suggested that clarification might

be achieved by the interposition of a metal gauze,

positively charged. This would prevent a positively

charged 'emanation' drifting across to be deposited upon

the negative plate, but would not prevent irradiation of

this plate. Perhaps in response to these comments

Rutherford performed further experiments on the connection

between emanation, induced activity and electric charge

during the next few months and was able to publish some

answers. With regard to the emanations Thomson continued: It is remarkable that the emanation should stand bubbling through strong 1-4SO4 & yet this substance should destroy the activity of a plate. As the emanation moves so slowly it presumably is large. Hence it must get through the Al. foil by some chemical or quasi chemical process like the 11,01 in Russell's experiments. If this is the ease perhaps gilding or silvering the aluminium might make it opaque to the emanation. I see you tried some experiments & did not get a cloud by expansion - did you use Wilson's apparatus for getting very sudden expansions...

The use of this apparatus could indicate the size of the

particles of emanation which might be larger than ordinary

ions; Wilson had obtained a fog produced by 'something

given off from metals'. Thomson's closing question on

the subject was 'If the active plate is very highly

polished can you see any trace of an alteration of surface

under the microscope'.65

Rutherford withheld discussion of this last point

and of the nature of induced activity for his second

major paper on thorium; his first, dated September 1899,

dealt almost exclusively with the emanation. Now Owens

working with Rutherford had by mid 1899 obtained various

interesting experimental results with thorium rays. In

his paper on 'Thorium Radiation' he described his attack

on the early problem of fluctuating readings. By main-

taining a very still condition of the air he was able to

conduct an examination of the thorium rays by their

penetration of aluminium screens. His readings told him

that the radiations consisted of at least two different

135

penetration types, only the most absorbable kind being

homogeneous. He thought that such complexity fitted

with the theory of internal atomic vibrations, as has

been noted. Investigations of the air effects themselves

he treated separately in ingenious experimental style.

For example, removal of the air gave marked reductions

in conduction; on the other hand agitation of the air

within the chamber by means of fitted vanes produced small

increases in readings. His explanation of this was that

the radiation from within the compounds 'changes the nature

of their surfaces, forming in the neighbourhood a more

active material'66 which could be removed; it is easy to

read Rutherford's 'emanation' into this part of the study.

In a set of related experiments Owens found that

using sheets of ordinary writing paper as screens over

the substance gave 'very curious results'. A thin layer

of thorium oxide gave a regular absorption curve, as with

aluminium foils, indicating the homogeneity of a portion

of the rays. But with a thick layer of oxide, not only

did fifteen successive sheets of paper fail to diminish

the current below the 50% reduction produced by the single

first sheet, but 'a very considerable time was required

for the current to come to a steady value as successive

layers were added'. In his opinion:

The explanation may possibly be that the penetrating radiations from a thick layer of the oxide in passing through the paper causes it to give off a secondary radiation comparable in its ionising effects to the more absorbable kind that fails to get through.67

The account thus expressed in terms of direct radiations

only was evidently neither complete nor satisfactory.

Rutherford took over the research entirely as Owens

left for Cambridge in the summer of 1899 with many

questions unanswered. Some five months after this, having

reached a second point of publication, he wrote to his

fiancée: I sent off on Thursday another long paper for the press which is a very good one, even though I say so, and comprises 1000 new facts which have been undreamt of.68

136

It is of interest to outline these facts, and the related

experiments, and hypotheses of different levels. The

letter in July from Thomson, who communicated the public-

ations to the Philosophical Magazine, shows that Rutherford

was already using his new emanation theory to good effect.

By this means he was able coherently to explain several

lines of experiment, namely, fluctuations in readings due

to air currents, the differences between thin and thick

thorium oxide layers with regard to this effect, and the

time-dependent paper screen results. Thus Rutherford

dropped Owens' suggestion of secondary rays in connection

with paper screens together with his conclusion that the

radiation was complex. 'At first sight' he wrote:

it appears as if the thorium oxide gave out two types of radiation, one of which is readily absorbed by paper and the other to only a slight extent.69

However, the curious results:

receive a complete explanation if we suppose that, in addition to the ordinary radiation, a large number of radioactive particles are given out from the mass of the active substance. This 'emanation' can pass through considerable thicknesses of paper.7O

In a manner very similar to that of his experimental

researches at Cambridge some three years earlier on the

ionisation of gases, which extended continuously to the

period now under consideration, Rutherford effected con

siderable clarification and progress. By using a slow

current of air he removed the *conducting gas from its

source for examination in a separate vessel. His recent

studies told him that:

If the ionised gas had been produced from a uranium compound, the duration of the conductivity, for voltages such as were used, would only have been a fraction of a second.71

Yet his electrometer showed that the gas withdrawn from

thorium oxide remained conducting for up to ten minutes.

The radiating particles, or emanation, whose presence in

the gas was taken as the cause of the conductivity, passed

unchanged through cotton wool, water and acids, unlike

ordinary ionised gases.72 As with Russell's photographic

actions the emanation passed through foils of metal but not of mica. However, Rutherford stressed that hydrogen

137

peroxide vapour had 'purely a chemical' action on the

photographic plate and failed to ionise, and thus make

conducting, the gas carrying it. He stated that the

radiation from the emanation, not the emanation itself,

caused both electrical and photographic actions; though

it appears that source and radiation had not been experi-

mentally separated.

One of the most interesting points concerning this

radiation from the emanation was its exhibition of a

rapid, regular 'decay' with time. The conductivity of

the carrying gas, taken as the measure of the 'intensity

of the radiation emitted by the radioactive particles',73

declined exponentially falling to half the initial value

in about one minute. From the phrasing of the discussion,

and on consideration of his later researches74 it seems

that Rutherford assumed it to be the radiation from each

individual particle of emanation which 'diminishes in a

geometrical progression with the time'. Ionic and

emanation theory fitted certain experimental observations

well: the air surrounding an envelope of paper enclosing

a thick layer of thorium oxide was drawn continuously

into an attached vessel effectively fitted with electrodes

and electrometer. The readings, starting at zero, increased

gradually and after a few minutes reached a steady value

of electrical conduction. The electric current decay curve

obtained after stopping the airflow matched the asymptotic

growth curve perfectly. Rutherford was able to account for

the latter as a balance between the increasing current

caused by a constant supply of new radiating centres which

had diffused through the paper from the thorium, and the

decreasing current due to the geometric decay of intensity.

Rutherford boldly expressed the rate of decay in the form

dn/dt = -A.n , where n was the number of ions. The

observed growth curve was indeed of the theoretically

deduced form i/I = 1 - e-A.t , where I is the maximum

current, attained at the steady state.75 This is the first

example of the growth and decay curves later to be assoc-

iated by Rutherford and Soddy with all radioactive sub-

stances. Here also was the first recognition of a decrease

138

in the radiation from what, to Rutherford, was a definite

radioactive substance. This perhaps added strength to his

year-old suggestion regarding the energy of uranium rays76

that these should eventually die away; a view already in

conflict with the French scientists' acceptance of the

fundamental constancy of this atomic property.

One may ask what significance can be attached to his

repeated statement77 'that the curve of rise of the current

is similar in form to the rise of an electric current in a

circuit of constant inductance', which seems also to imply

a similarity in the current decay curves. Was he attempting

to say something new about the emanation, about electricity,

or making a merely algebraic comparison? The first of these

possibilities is evidently in some way true. As for the

second, it is remarkable that the newest view of electrical

conduction in metals visualised its mechanism as a kind of

diffusion process somewhat analogous to that of ionic

conduction in gases. About six months after Rutherford

wrote his paper on thorium emanation J.J.Thomson divulged

'Some speculations as to the part played by corpuscles in

physical phenomena' to the 'wider circle' of the readers

of Nature.78 He pointed out that the recent demonstrations

by Giesel, Curie and Becquerel of the magnetic deflection

and electric charge of the rays from radium demonstrated

the presence of corpuscles in this substance. Thomson'.s

major point was an explanation of the electrical conduct-

ivity of metals in terms of a gas-like diffusion along

the wire of subatomic negatively charged masses or

corpuscles, temporarily dissociated from the fixed

'molecules'. He considered that this had consequences

for the relationship between electrical and thermal

phenomena. The older theory of jumping tubes of force

was not explicitly mentioned here. Now Thomson in his

earlier letter to Rutherford of July 1899 which contained

the comments on the emanation quoted above, did not mention

the theory of conduction in metals but only the work on

conductivity in gases:

I am inclined to think that at low pressures negative electricity is always carried by the small corpuscles however the electrification

139

may be produced - while the positive charge remains on the big atom - this idea leads to very interesting views as to the structure of the atom.79

In the published note Thomson recalled his first demon-

stration of the existence of corpuscles in cathode rays

in 1897 and added that 'Ever since then I have indulged

in speculations' concerning their presence in ordinary matter.80 But it is not clear whether Rutherford knew

of the corpuscular or electron-gas theory of metallic

conduction nor whether his own researches of 1899 had

any bearing upon it. If one seeks an influence in the

opposite direction the question arises whether Rutherford's

identification of formulae involved the notion that each

radiating particle of emanation behaved like a miniature

or atomic version of the radiating electric circuits

which he had studied during 1894-6. He was still concerned

with the subject and it was very much in his mind during

the month in which he wrote up the paper on thorium eman-

ation, as shown by his letter to Mary Newton:

I am giving a course of postgraduate lectures this year on Electrical Waves and Oscillations which will give me a good deal of trouble to arrange. This is the first thing of this kind ever done here and rather surprised them when I suggested it.81

Although his explicit identification of the formulae may

be no more than a mathematical guide, our discussion

indicates connections between radioactivity, and former

and contemporary electrical studies. The same points

apply to Rutherford's publication on the excited activity

with its much slower decay; this is shortly to be discussed

along with the accounts of possible mechanisms which he

fortunately detailed. It may be noted that Pierre Curie

was soon to develop explanations of the phenomena on the

basis of a different algebraic comparison, firm to the

point of analogy, between the observed decline of radio-

activity and the fall of temperature due to loss of heat

energy from a cooling body.

Two questions posed by these studies of the emanation

concerned its origin and its nature; the need to answer

these served as a stimulus to experimental research.

140

The emanation seemed to be produced spontaneously and at

constant rates which were different for differing thorium

compounds. Rutherford's conversion of the nitrate to the

oxide by gentle heating produced a considerable increase

in the current due to the emanation, and the direct

radiation increased proportionately. On the other hand,

maintaining the temperature at a white heat caused a

steady decline in the discharge due to the emanation

down to 1/20 of the initial rate for the oxide, apparently

without so diminishing its direct rays. Rutherford provided

no explanation at this time but kept the problem in mind.

What then was the nature of the emanation, of the

radiating particles comprising it? As for the size of these

particles, the fact that they passed unaffected through

cotton wool and metals was against the possibility that

they were a fine dust, as were also the results of water

vapour condensation experiments; Thomson's suggestion

regarding Wilson's more powerful method was mentioned by

Rutherford82 as a future possibility, but he may not have

tried it.83 The results indicated that the particles

constituted 'a vapour given off from thorium compounds'.

Could it be the vapour of thorium metal? There was 'reason

to believe that all metals and substances give off vapour

to some degree'.84 As Rutherford knew, this might be the

vapour of the metal or substance itself, or of hydrogen

peroxide. We have noted the dismissal of hydrogen peroxide

vapour85 on the grounds of its direct chemical photographic

action and its lack of conductivity; other differences can

also be seen, such as the need for a clean unoxidised

surface to obtain the Russell effect with metals which

contrasted with the production of an emanation by the

oxide and compounds of thorium. That the emanation might

be the vapour of thorium itself was a possibility which

he thought did agree with its declining radioactivity.

And he again pointed hopefully to future experiments

which were in this case actually completed and published

more than a year later, though not with thorium; these

were measurements of the rate of interdiffusion of the

emanation into other gases to determine its density and

141

molecular weight.86 But even without such evidence he felt

able to make the cryptic but significant comment that 'The

emanation from thorium compounds ... has properties which

the thorium itself does not possess'.87 Rutherford's know-

ledge of such properties was a part of his discovery that

by association with the emanation any surface whatever

could be made to emit radioactive rays, different from

those of uranium or thorium in being more penetrating.

The surface activity lasted for several days; it could

be concentrated upon a negatively charged body. Now the

particles of emanation were themselves electrically neutral.

Their discharging effect was entirely uninfluenced by

application of the electrostatic field which, as Rutherford

had successfully demonstrated in 1896,88 would create a

current and thus destroy the ionic conductivity of a gas.

We can see that this should dispose of the hypothesis

that the particles of emanation were themselves simply

clusters of molecules of the surrounding gas about ions,

though Thomson later adopted the idea for a time. Hence

Rutherford, whose experiments were still in progress,

assumed in September that it was 'the positive ion produced

in a gas by the emanation' which possessed 'the power of

producing radioactivity in all substances on which it

falls' .89 But he had progressed beyond this view by the time

of completion of his next publication on the 'Radioactivity

Produced in Substances by the Action of Thorium Compounds,90

in November 1899. Rutherford appears to have retained the

opinion that the emanation was not thorium vapour without

providing further evidence. It is tempting to interpret

his experimental results in terms of the later supposition

that this new 'excited' radioactivity was itself due to a

third material substance. Even if he entertained the idea

at this early date Rutherford did not mention it in his

publications, preferring other hypotheses; yet the evidence

seems suggestive. Naturally adopting the measurable

characteristics of its emitted radiation as a means of

identification, Rutherford noted that the rays from the

excited activity were of a longer duration than those of

142

the emanation and more penetrating than thorium or uranium

rays. Another distinction was the chemical one: to repeat

Thomson's earlier comment of July, 'it is remarkable that

the emanation should stand bubbling through strong H2SO4

& yet this substance should destroy the activity of a

plate'. But Rutherford in his sections on chemical and

mechanical actions on the radioactive surface91 now stated

that the induced or excited activity was not in fact so

destroyed but had been simply dissolved from the plate

by this acid, or by hydrochloric acid, in which it was

afterwards to be found; other reagents such as nitric acid

or caustic soda had far less effect. He reported without

comment the interesting observations that a microscopic

examination (suggested by Thomson) revealed no surface

changes although the intense activity could be removed by

scraping; and that raising the temperature to white heat

had little effect.92 These chemical and mechanical

approaches were to enjoy an early and successful future

development with the aid of Frederick Soddy, a trained

chemist; they also bore a resemblance to Rutherford's

past researches on surface magnetism93 performed five years

earlier. And his demonstrations that the intensity of the

induced activity was roughly proportional to the amount of

emanation, by passing the emanation along a tube containing

a series of negative electrodes, were very similar in

techniaue to earlier studies of the duration of the conduct-

ivity of ordinary ionised gases.94 The experimental and

theoretical continuity is also illustrated by other experi-

ments, by the strong element of explanations in terms of

the mechanical interplay of ions and other particles, and

by the lasting links with the researchers of the Cavendish

Laboratory. Rutherford summarised his weighty paper by considering

three possible explanatory mechanisms of the phenomena of

induced radioactivity, one of which he adopted. The hypo-

thesis that the excited radiation was a kind of phosphores-

cence produced in direct response to thorium radiation

could explain neither the production of activity outside

the incidence of the radiation nor the concentration upon

143

a negative electrode; he therefore dismissed it. Too

abruptly perhaps, for Rutherford made no mention of the

different phosphorescence hypothesis, with possible

experimental tests, which Thomson had privately suggested

in July. The latter supposed that gas molecules close to the thorium surface might become so intensely ionised that

they emit a type of radiation capable of producing a

phosphorescent effect on the plate; Thomson had not forgotten

this by 1903, when he published a similar explanation of

induced radioactivity as one possibility.95 It is clear that the identity of the induced radiation whatever the

material of the excited surface militates against this.

A second idea was that the positive ions produced in

the surrounding gas by the rays from the emanation particles

could deposit upon any surface; certainly there was no

problem here in accounting for concentration by an electric

charge, nor an identity of deposit. This had probably been

Rutherford's favoured view in September; now he wrote that

the hypothesis 'at first sight seems to explain many of

the results'.96 In consequence of the intense rays

(presumably of X-ray type) close to a radiating particle

of emanation, ions may not only be produced but 'the charges

on the ions set in violent vibration'. The observed surface

effects would occur as the ions 'gradually dissipate the

energy of their vibration by radiation into space'. What

became of the negative ions was not stated. However,

observations at low pressures, where even a large negative

charge in fact failed to concentrate the activity, were

not explained by this theory. And testifying against it

was the fact that an increase in the distance between the

emanating thorium layer and the plate to be activated from

3 mm. to 3 cm., which gave the expected higher current

attributable to the creation of more positive ions, yielded

no increase in the plate's activity.

Rutherford made his decision: 'The theory that the

radioactivity is due to a deposition of radioactive particles

from the thorium compounds affords a general explanation

of all the results'.97 The emanation itself was thus dep-

osited. He overcame the difficulty of explaining how

144 this neutral substance acquired the charge necessary for

movement to a negative electrode by supposing that positive

ions, generally present in excess in gases due to their

lower velocity, diffuse to the surface of the emanation

particles. At low pressures a lack of ions, indicated by

the observed small currents, would allow active neutral

particles to diffuse to the sides of the vessel unaffected

by the negative electrode. The implication of these

statements seems to be that excited activity consisted

of deposited particles of the emanation possibly sometimes

attached to the positive ions of various gases. But did

larger particles such as dust or smoke which Rutherford

knew caused rapid ionic recombination,98 acquire the

positive charge that they apparently should according to

this hypothesis? And why did the rate of decay of radiation

become some 660 times slower when the gaseous particles

were deposited, though obeying the same law; why were

there marked differences in chemical behaviour, for example

with sulphuric acid; did these point to the existence of

a third radioactive substance? Rutherford did not say so.

And he avoided any description of the nature of the

particles of emanation themselves, and of the mechanism

of their production. These were some questions which

Rutherford's discoveries of '1000 new facts' appear to

have posed. Research students and collaborators were to

aid him in answering them, as new information arriving

from Europe created still more problems.

In a late footnote to his paper on induced radio-

activity, Rutherford remarked that he had .received the

note of 6th November in which the Curies had announced

their discovery of 'radioactivite provoquee par les

rayons de Becquerel',99 an effect produced by the new

substances whose activities had now reached a level

50,000 times greater than uranium. Rutherford stated

that Curie had, without electrical studies, attributed

the results to a kind of phosphorescence excited by the

radiation.I00 He pointed out that 'in the case of thorium

the author has shown that such a theory is inadmissible'

so that further comparisons were required. The magnitude

145

of the activity induced by radium of several hundred

uranium units, as reported by the Curies, appears to have

been similar to Rutherford's estimate for the maximum

electrically concentrated activity produced by thorium.101

The latter's suggested general comparison between radium

and thorium was taken up directly by E.Dorn and by his

student F.Henning, in Halle.

In December 1899 Thomson wrote to Rutherford that

he had sent on this second paper on thorium to the

Philosophical Magazine. He also told him of Giesel's

magnetic deflection of polonium rays, whose implications

we have considered. Thomson found Rutherford's results

'exceedingly interesting' but saw fit to suggest a

different mechanism for the acquisition of the emanation's

charge, which in effect reversed the positions of emanation

and ions: the idea that I got on reading the experiments was that the radio-activity was due to thorium vapour or emanation which was carried by the + ions - I mean that the emanation in the field tended rather to condense round the positive ions than the negative ones, as we might expect an electro-positive substance to do.102

Evidently Thomson was not satisfied that the emanation

was not thorium vapour - a metal should tend to attach

itself to positive ions. And others' opinions of

Rutherford's results were not quite so straightforward

as has been made out. John Zeleny had been at the

Cavendish with Rutherford under Thomson, who had written

to Rutherford in 1898 when both students had left the

Cavendish, 'I hardly think at present that we have any

in the new lot as good as Zeleny & Wilson the amount of

glass they break at present is appalling'.103 Zeleny

had been first to announce, in that year, that the negative

ion in a gas generally travelled with a slightly higher

velocity than the positive.104 Indeed, he had made an

observation, which seems unfavourable to Rutherford's

emanation-charge theory of 1899, that a metallic body

suspended in a current of ionised air generally acquired

a negative charge, the air being left positive.105 Zeleny

attributed this to the differing ionic velocities. He

146

explained that such a body could acquire the positive

charge sometimes observed only if another metal were

immersed in the same quasi-electrolytic medium. This

fits with Thomson's view. An implication of Rutherford's

hypothesis seems to be that the emanation behaved not as

suspended particles but as a part of the gas. However,

it was Thomson who was soon to argue in this way, on the

basis of fresh evidence. In March 1900, soon after reading

Rutherford's papers 'with very great interest', Zeleny

wrote 'I am about ready to believe that most anything is

possible. The facts you present are certainly strange'

but 'more light on the nature of those things would be of

still greater value'. He said he would be 'interested in

your work on the energy for producing an ion. I see that

you are in for getting ahead of J.J.'.106 Rutherford had begun experiments to this end towards the close of 1899

partly if not wholly in order to clarify the question of

the energy of radioactivity. His associated speculations

are of very great interest to us, as are contemporary

discussions by others. However, Zeleny's latter suggestion

turned out to be wide of the mark with regard to Rutherford's

attempt to quantify the energy required for producing an

ion, as the following account will show.

147

3. The source of radioactivity (1900) Attempts to quantify the energy emission from radio-

active substances, together with a more advanced under-

standing of the radiations, led to interesting speculations

and anticipations. These are worthy of examination for

the improved perspective of radioactivity which they

provide. To that end we will consider formerly neglected

discussions set out by researchers in the field as their

experimental studies developed from the end of 1899

through 1900. Generally during this period theories were

as easy to knock down as to set up; Rutherford indeed

found this to be the case with the hypotheses which he

preferred. Nevertheless, even at this early stage the

view that atomic change was the source of radioactivity

began to appear in sharper focus.

The earliest explanations of radioactivity were

perforce qualitative; they likened the phenomenon either

to fluorescence or to a long-term phosphorescence. The

respective problems of the origin of the initiating

radiation or of the nature of the required ancient store

of energy, among other difficulties, had not been solved.

The subsequent suggestion, made most clearly by Thomson

in 1898, that a subatomic rearrangement might be involved,

had implications not to be fully realised until further

chemical researches, to be discussed, had come to fruition.

However, we may recall that the physicists Elster and

Geitel had early in 1899 already expressed their willingness

to sacrifice, in theory, the chemical atom to provide the

energy of radioactivity. Their experiments demonstrated

the inadequacy of Crookes' and Marie Curie's hypotheses,

each of which required external, though differing, energy

provision. These results confirmed the atomic nature of

the phenomenon and induced their proposal that the radiated

energy might come from a transmutation of the chemical

elements.107

Such a suggestion did not escape strong criticism

put forward with the support of apparently positive

experimental evidence. Both phosphorescence and all

chemical reactions were known to be highly dependent

148

upon temperature; E.Wiedemann and G.C.Schmidt108 had

indeed supposed that these two types of phenomenon were

closely related. In turn re-linking these processes with

radioactivity, 0.Behrendsen published his 'Beitrgge zur

Kenntniss der Becquerelstrahlen',109 which exhibit both

an understanding of and a direct challenge to Elster and

Geitel's solution to the 'Energiefrage'.110 The mode of

attack on his contemporaries' views was a study of the

effect of temperature upon radioactivity. Behrendsen

used a gold-leaf electroscope to determine the intensity

of the radiations emitted from specimens placed within

the electroscope vessel and an apparatus by which the

entire electroscope could be cooled. He satisfied himself

that pitchblende and its active sublimate each gave

radiation which increased steadily with temperature from

-50 to +130 degrees C. and that Moissan's uranium metal

exhibited a decreased intensity only on cooling. However

he considered that convection of the ionised air on

warming might have influenced the result with uranium.111

Raising the temperature of pitchblende to red heat

markedly decreased the intensity. This he attributed to

a chemical decomposition, which illustrates his general

assumption, or conclusion, that radioactivity was the

manifestation of ordinary interactions between atoms and

molecules. The slow conversion of a complex compound

such as pitchblende, which contains uranium, the Curie

elements and acids, into a more stable one, should indeed

be influenced by temperature variations; radioactivity

was to be compared with thermoluminescence. The Curies

and Elster and Geitel, he wrote, are of the opinion that

Becquerel rays were in fact not the result of any chemical

change so that the emission was an 'Atomeigenschaft' of

the radiating element. They had said that the atom of a radioactive element might itself be constructed like a

kind of molecule, able to emit rays while being trans-

formed into a more stable entity.112

Gegen diese Ansicht scheint mir vor allem zu sprechen, dass die Annahme eines instabil gebauten 'Atomes' nicht mit dem Atombegriff als solchem sein diarfte.113

149

Retaining his conception of the atom, and assuming the

energy to be stored within the material, Behrendsen

supposed it possible 'dass die Atome der radioactiven

elemente die Fghigkeit besgssen, miteindander und auch

mit fremden Atomen zu instabil gebauten Molecalen

zusammen zu treten' like the allotropes of sulphur or

selenium. This interpretation fitted with his thermal

results. However, in his argument114 that the salts of

uranium gave a greater total radiation than the metal

itself, which should dismiss the Ur atom as the source,

he included the visible. Now Becquerel's original discovery

had involved the creation of a clear distinction between

uranium rays and ordinary phosphorescence. Behrendsen's

paper thus may only have served to spread wider the

suggestions of those whom he criticised. His experiments

and conclusions were put in further doubt on several counts

as scattered studies of the effect of temperature continued.

F.Himstedt's report one year later 'Uber Versuche

mit Becquerel- und mit ROntgenstrahlen'

l 5 noted the view

which attributed the energy of radium to a slowly proceeding

chemical reaction. An investigation of the influence of

cooling upon the intensity of the rays was therefore of

considerable interest. His results however gave no support

to such a theory. For whilst luminous paint was completely

extinguished at the temperature of liquid air radium

radiation, as measured with an electrometer, showed but

a small diminution which itself might be attributed to

additional absorption of the rays by the liquid air.

Himstedt's conclusions were more in accord with the

conclusions of the earliest students of radioactivity

than with Behrendsen's. Others had considered this approach to radioactivity

as worthy of investigation. Before radium became

important, Rutherford had measured the activity of

uranium at 200 degrees C. but found little difference

in the rate of discharge. His opinion was that 'The

results of such experiments are very difficult to interpret,

as the variation of ionization with temperature is not

known'.116 Marie Curie had found that the activity of

150

uranium was always unchanged on returning to room

temperature after heating or cooling.117 G.G.Stokes, who entertained what can be called a 'sub-phosphorescence'

explanation of radioactivity, had corresponded with

Becquerel in August 1899 concerning uranium rays.

Stokes thought it 'very probable that the efficiency of

a substance (suppose metallic uranium) which emits them

would depend very materially upon its temperature'.118

Becquerel replied119 with a description of experiments

using uranium which he had performed two years earlier

but had not published. These involved arrangements

similar to those described by Behrendsen. Becquerel on

the contrary cautiously remarked that the photographic

method of detection was itself sensitive to changes in

temperature, so that further study was required. He

similarly noted that an electrometric method gave only a

small difference between +100 and -20 degrees, which

might be attributable to air currents of convection, and

that smaller changes in temperature appeared to have no

effect. These attempts to find a thermal influence upon

the radiations from active substances were continued by various scientists.120 The generally negative results were more than once cited positively, as theoretical

debates intensified during the period 1899 to 1903.

But these five years saw at their beginning the

discovery of the new phenomena of the radioactive

vapour-like 'emanations' released from thorium, then

radium, then actinium, and the development of an under-

standing of these. Rutherford in his earliest publications

on thorium emanation, issued in 1900, noted that temperature

here had a definite effect: he considered that heating the

thorium oxide source greatly reduced the rate of production

of emanation.121 Behrendsen however insisted on his earlier

conclusions, now with regard to 'Das Verhalten des

"Radiums" bei tiefer Temperatur' .122 His experimental

diminution of the electrometric intensity of radium

radiation to half value at the temperature of liquid air

led him to state that this behaviour was precisely similar

151

to that of visibly phosphorescent bodies. He thought

that the spontaneous decrease in the radiation from

Giesel's polonium confirmed the comparison. Yet con-

temporaries probably saw his case with radium to be

weakened by new discoveries. The complex nature of the

composition of the rays was becoming clearer during 1900;

temperature experiments ought therefore to be more dis-

criminating in this respect. And a preliminary point in

another direction is Elster and Geitel's note of January

1900 that radium released a volatile constituent on

heating;123 this may have been seen merely as comparable

with the sublimation of polonium from bismuth. Their

paper partly anticipates Dorn's description later that

year of a gaseous radioactive emanation associated with.

radium.'24 Future attempts to clarify the source of

radioactivity by seeking an effect of temperature were

compelled to take the emanations into account.

It is a sign of the complexity of changes in the

interpretation of radioactitrity occurring during the year

1900, that developments of Becquerel's views, which we now

consider, were produced largely by factors other than those

just described. Neither his experiments giving negative

or inconclusive effects of temperature change, nor the

discovery of the emanations, appear to have been immediate

influences in his definite shift from the original phos-

phorescence analogy of 1896-9. Instead, experiments on

the radiations themselves which indicated that they

consisted not only of ether vibrations but also of an

actual emission of material particles, seem to have been

most significant. Thus in March 1899, after he had

accepted that uranium rays were not after all like light,

Becquerel in his 'Note sur quelques proprietes du

rayonnement de l'uranium et des corps radio-actifs'125

considered again the energy of radioactivity. If uranium

did not lose energy in producing its rays then this metal

might be in a special state like that of the iron in a

magnet maintaining a field around it through which it

could create the observed effects by the transformation

of some external energy source. However, he accepted the

152

Curies' view that photographic and phosphorescent actions

of the new highly active substances did in fact constitute

a spontaneous release of energy 'dont on ne voit pas la

source ailleurs que dans la substance radio-active'.126

Since this was of small magnitude:

it ne serait pas contraire a ce que nous savons sur la phosphorescence, de supposer que ces substances ont une reserve d'energie relativement consid6rable qu'elles peuvent 6mettre, par rayonnement, pendant des annees, sans affaiblissement sensible; toutefois i1 n'a pas etc possible de provoquer par des influences physiques aucune variation appreciable dans l'intensite de cette emission.127

The communication by Becquerel of his unpublished temperature

experiments and earlier papers to Stokes was sufficient to

make the latter drop his phosphorescence-radioactivity link

by September 1899.128 Yet Becquerel himself apparently

retained the idea till November. This is indicated by

his application of the terms phosphorescence and fluores-

cence respectively to the new temporary induced radio-

activity and to secondary rays.129 But the analogy was

not to be stretched beyond this point.

Becquerel's new corpuscular view of the rays from

radium led him to a calculation of the energy and the

matter carried off in this way. His respective estimates,

announced in March 1900, of 1 mg. in 109 years and a few

calories per cm.2 per year from a radium sample, were

sufficiently small to allow the continued belief that

the radiated energy 'peut titre empruntee a la matiere elle meme 130' without measurable loss of weight. In the con-

cluding comments of his paper read at the International

Congress in Paris in August Becquerel cited the same

estimates and gave something of an expansion of his views.

Since the loss of mass involved was immeasurably small

n'y avait aucune contradiction entre la spontangite

du rayonnement sans cause apparente, et le principe de la

conservation de 11 6nergie'.131 The apparent assumption of

an equivalence of mass and energy seems not to alleviate

the contradiction with Carnot's principle, but he made

no mention of this. His final word here was that 'Le

153

phenomene d'emission materielle pourrait etre du meme

ordre de grandeur que l'evaporation de eertains matieres

odorantes'.132 Becquerel's firmest expression, vague

though it was, of this approach appeared in a review paper

on 'The radio-activity of matter' published in Nature some

six months later, in February 1901.133 A 'material emission'

of the order of 'certain scented substances' had now become

'the first cause of the observed phenomena'. Becquerel

mentioned aspects of Rutherford's 'very penetrating

"emanation"' from thorium without making it clear that

the emanation behaved like a vapour: he had not yet taken

the opportunity of fitting this with his 'material emission'

hypothesis. Yet towards the end of that year, by con-

sidering both of these in combination with the 'new

horizons'134 opened up by developing studies on induced

radioactivity, Becquerel arrived at his own important

unifying theory of atomic disintegration.135

Such developments were followed by some scientists,

but not by all. F.T.Trouton writing to Nature from the

Physical Laboratory, Trinity College Dublin, in March 1900

put forward a 'Suggested Source of the Energy of the

"Becquerel Rays"'136 which seems very similar to the magnetic

analogy mentioned and rejected by Becquerel himself one

year earlier. Trouton suggested that because of the

difficulties arising from the supposition of a continuous

release of energy from the active material 'the possibility

should be kept in view of the real source of the energy

being found in the things themselves in which these effects

are manifested'. Including in the expression what others

called 'rays' he wrote that 'the emanating influence would

be looked upon rather in the light of lines of force than

as a wave propagation'. Ionisation and other effects

'would then be viewed as due to what might be called a

Becquerel field of force'. The Curies' observation that

a phosphorescent screen became 'exhausted' after a time,

but could be rejuvenated by exposure to light, he inter-

preted as an indication that some or all of the energy

originated in the screen itself. According to his magnetic

analogy 'forces should exist between the acted-on substance

154

and the source of the "Becquerel Rays"'. One cannnot say

that these ideas were influential, but they serve as an

epitome of one undercurrent of conjecture on the subject

during and beyond137 the period under consideration. It

is interesting to note that W.Crookes earlier138 and

P.Curie with A.Laborde later, each sought experimentally,

and unsuccessfully, just such a force; they, however,

were thinking in terms of the pressure of radiation truly

emitted from the radioactive source. The French scientists

came to a remarkable and quite different discovery139 in

the attempt. Like all active researchers in the field, the Curies

seem always to have regarded 'radioactivity' according to

its name, which they had coined. But it is perhaps an

indication of the appeal of the interpretation described

above that the Curies, shortly after their discovery of

induced radioactivity, still found it necessary to stress

the point that energy was being released. Such was the

conclusion of their note of 20th November 1899 on the

'Effete chimiquesproduits par lee rayons de Becquerel'.140

This may be the research which prompted Trouton's paper

for the Curies here described their observations with the

fluorescent barium platinocyanide screen. The colouration

of glass and particularly the newly observed (by the spectroscopist E.Demargay) transformation of oxygen into

ozone by radium was 'une preuve que ce rayonnement represente

un degagement continu d'6nergie'. Trouton's comment con-

cerning radioactivity that 'we have no conclusive evidence

that the effects are those of waves',141 which may be

associated with the recent demise of Becquerel's earlier

description of the rays, remained true for at least a

decade. However, the Curies as well as others accepted

the view that both particles and ether vibrations were

involved. Following the successful deflection of a portion of

the radium rays, their detection of the spontaneous

acquisition of a positive electrostatic charge by an

insulated specimen of radium told the Curies that this

element continually lost negative electricity.142 'Or,

155

jusqu'ici' they wrote 'on n'a jamais reconnu l'existence

de charges electriques non liees A la matiere ponderable',

hence radium must be the seat of a constant emission of

negatively charged material particles. They cited the

work of J.Perrin who had detected the transport of a

similar charge by the cathode rays but the Curies made

no reference to J.J.Thomson's related conclusions that

these particles were smaller than atoms. Indeed, their

estimate of the continuous loss of mass which should

result from the emission was based upon an assumed e/m

ratio 'le meme que dans l'electrolyse'; thus in March 1900

they took the particles to be atomic in size. A few weeks

after their note was read Becquerel demonstrated an almost

total similarity between the deviable radium rays and

Thomson's corpuscular cathode rays.143 Becquerel's studies

may have been a cause of their acceptance, as far as it

went, of the subatomic corpuscular view of radium rays.

For in Marie Curie's review paper on 'Les nouvelles

substances radioactives' read in June 1900,144 and in the

Curies' joint paper presented to the International Congress

in Paris two months later,145 discussions on the origin of

the energy of radioactivity centred on two possibilities

neither of which involved the emission of atomic particles.

At the Congress they squarely put the question 'Quelle est

la source de l'energie des rayons de Becquerel? Faut-il

la chercher dans les corps radioactifs eux-mgmes ou bien

a l'extorieur?'146 and answered it by suggesting that one

group of radium rays might be secondary to the other.

Support for this view may have been derived from current

researches by P.Curie and G.Sagnac, and E.Dorn which

showed that secondary X-rays, easily absorbed, were them-

selves cathode rays; 147 this implied an apparent symmetry

between the production of X-rays and cathode rays. Despite

the refutation by Elster and Geitel148 the Curies extended

Marie's earlier speculation, and suggested that the primary

Becquerel rays, whichever these were, might themselves be

secondary to some unknown exterior radiation, again with

the rider that this would be contrary to Carnot's principle.

156

Yet their ideas had developed, and discussions were

couched in slightly different terms from those of the

previous years. For the Curies now debated for the first

time an alternative hypothesis which was based upon

J.J.Thomson's view of cathode rays as subatomic particles;

it involved the discovery, to which they had themselves

contributed in respect of the electric charge, that radium

emitted such particles. Although Marie Curie, in her June

reviewl49 made the disclaimer that the origin of the

radiation from radioactive bodies was no better understood

than when uranium rays alone were known, yet she evidently

felt able to express one qualitative hypothesis fully and

clearly; from this one may surmise, despite later events,

that she perhaps even considered it favourably. Thomson,

she explained, had shown that if one took the cathode rays

or 'matiere radiante' of Crookes to be electrically

charged material particles then 'ces particules transportent

a poids egal 1000 fois plus d'electricit6' than hydrogen

in electrolysis. Thomson had concluded that 'Ce ne seraient

done meme plus les atomes libres de la chimie, mais des

sous-atomes bien plus petits encore, et anim6s de vitesses

prodigieuses'.150 Radium itself therefore seemed to behave

like a spontaneously excited cathode. Thus Marie Curie

arrived at her first speculation on the disintegration of

matter. Although the loss of mass would be too small to

detect, 'La matiere radioactive serait done de la matiere

oil regne un etat de mouvement interieur violent, de la

matiere en train de se disloquer'.151

Setting out the chemical implications more clearly than

had J.J.Thomson, for example in his 'Speculations as to

the part played by corpuscles in physical phenomena'

published the previous month,152 and with the advantage

of a year and a half's new evidence over the conjectures

of Elster and Geitel on radioactivity and atomic change,

she wrote: La theorie materialiste de la radioactivite est tree seduisante. Elle explique bien les ph6nomenes de la radioactivity. Cependant, en adoptant cette theorie, it faut nous rosoudre a admettre que la matiere radioactive n'est pas a un 6tat chimique ordinaire; les atomes n'y sont pas constitue's a

153

157

11 6tat stable, puisque des particules plus petites que l'atome sont rayonn6es. L'atome, indivisible au point de vue chimique, est divisible ici, et les sous-atomes sont en mouvement. La matiere radioactive eprouve done une transformation chimique qui est la source de l'energie rayonnee; mail ce n'est point une transformation chimique ordinaire, car lee transformations chimiques ordinaires laissent l'atome invariable. Dans la matiere radioactive, s'il y a quelque chose qui se modifie, c'est forcoment l'atome, puisque c'est a l'atome qu'est attachee la radioactivite.154

Here indeed can one see the elements of a theory involving

both atomic disintegration and transformation. It has not

generally been appreciated that Marie Curie expressed such

a view at this time. The fact that she did so illustrates

the early trend; but from this the Curies turned away.

The ideas proposed by Marie Curie were readily taken up,

or independently conceived, by others in the following year

or two;155 Rutherford's experimental researches developed

along a path different from the Curies', but he may have

entertained such thoughts. However, by this time, June

1900, Rutherford had been concerned to point out that the

emission of corpuscles or cathode rays was not a property

of all radioactive substances. Mme.Curie knew that her

polonium, though highly active, gave none of the deviable

rays;156 she did not mention this in her review paper;

it was not to be her only theoretical problem with

polonium.157

In his joint paper with the Demonstrator R.K.McClung

on the 'Energy of Rdntgen and Becquerel Rays, and the

Energy required to produce an Ion in Gases'158 Rutherford

remarked that Giesel's polonium emitted deflectable rays

but Curie's did not,159 that Becquerel had 'found no trace

of magnetic action in uranium radiation',160 and that

magnetic experiments at McGill had deflected neither

uranium nor thorium rays. The point was clearly made:

This emission of rays similar in character to cathode rays of low velocity is very remarkable, but does not seem to be a necessary accompaniment of a radio-active substance ... The rays which are deflected by a magnet seem to be present or absent according to the mode of preparation of the substance, and depend possibly on the age of the specimens.161

158

This illustrates a difference in emphasis from that of

Mme.Curie. Rutherford tells us that the aims of the

study were to determine experimentally the energy required

to produce an ion in a gas, 'and to deduce from it the

energy of the radiations emitted per second by uranium,

thorium, and other radio-active substances'.162 An inter-

mediate, quantitative study of X-rays was the basis of his

method. By January 1900 he had obtained a measure of the

small heating effect of X-rays playing upon a specially

absorptive platinum bolometer to an estimated accuracy of

2%, as he helped McClung to master the complex techniques.163

The assumptions were that X-rays manifested their energy

as a heating effect when absorbed by metals,164 but as an

ionisation-conductivity effect when absorbed in a gas.165

Comparing these, and knowing J.J.Thomson's recent value

of the charge on an ion, Rutherford calculated the energy

required to produce an ion in the gas; conductivity

measurements provided the required estimates of the

energy released by radioactive substances. Referring to

his own and others' work on ionisation Rutherford argued

that 'the same energy is required to produce an ion

whatever the gas:166 This conclusion played a part in

his speculations, the more fascinating for their confusions,

upon the structure of the matter from which ions were

produced; these speculations in turn had implications

for the source of radioactivity. He implicitly adopted the purely electrical view of

atomic and molecular bonding:167 from the experimental

value of 3.8 x 10-10 erg for the minimum energy required

to 'produce a positive and a negative ion from a neutral molecule ... against the forces of electrical attraction',

he calculated the distance between ions before separation

as r = 1.1 x 10-9 cm. Rutherford took this to be a

significant value supporting Thomson's ionisation theory.

For 'The average diameter of an atom, calculated from

various methods, is about 3 x 10-8 centim.' which was

'very much greater' than the above ionic distance 'in

a molecule'.168 Now Kelvin in one of his later lectures

on 'The Size of Atoms, had in 1883 apologised:

159

I speak somewhat vaguely, and I do so, not inadvertently, when I speak of atoms and molecules. I must ask the chemists to forgive me if I even abuse the words and apply a misnomer occasionally.169

One reason for this seems to be that none of the phenomena

of optical dispersion, contact electricity, capillarity,

gas viscosity and diffusion used for the estimates specifi-

cally involved the atom of chemical combination. Rutherford

made no such apologies for his variable usage of the terms

atom and molecule; sometimes it seems quite clear. Part

of his aim may indeed be construed as the making of a case

for Thomson's corpuscular theory of matter against 'the

atomic theory, as ordinarily understood',170 radioactivity

being the test. We know of Rutherford's open attack to

this end upon 'the chemists' in 1901.171 He considered in

1900 that his own results on radioactive energy, and those

on the emission of 'a kind of cathode rays' from active

substances, indicated 'that the present views of molecular

actions require alteration or extension'.172 However, apart

from the employment of such imprecise expressions his

explanations were acknowledged as being unable to cover

the experimental results. According to Rutherford, Thomson

considered 'that an atom is not simple, but composed of

a large number of positively and negatively charged

electrons';173 we note that the latter had not publicly

stated this exact view. The problem of the positive charge

was a difficult one for it appeared that only negative

electrons or corpuscles were detectable experimentally;

Thomson's usual supposition from the first was that the

space which the corpuscles occupied behaved somehow as

if it possessed a positive charge. Nevertheless, for the

purpose of calculating theoretically the energy within an

atom which might be available for the manifestations of

radioactivity both Thomson and Rutherford appear to have

assumed a structure consisting of pairs of oppositely

charged electrons.174

Rutherford and McClung measured the energy emitted

from uranium oxide as 0.032 calories per year per gm.

and found that from thorium oxide somewhat greater.

160

Becquerel's earliest work told them that radioactivity

persisted unaltered for years and 'appears to depend on

the uranium molecule alone, and not what it is combined

with';175 this is a phrase in which 'atom' would seem to

be more suitable than 'molecule' particularly since

chemical combination was in question. The energy

measurements induced Rutherford's clear dismissal of

normal chemical change as the source: 'It is difficult

to suppose that such a quantity of energy can be derived

from regrouping of the atoms or molecular recombinations

on the ordinary chemical theory'.176 Now his estimate of

at least 300,000 calories emitted per gramme of uranium

over ten million years involves assumptions and calcul-

ations not explicitly revealed. One can however infer

some of these, knowing his view that 'a greater concen-

tration or closeness of aggregation of the components of

such a complex molecule'177 as that of uranium 'would

possibly be sufficient' to supply the required energy,

and that these components were positive-negative electron

pairs. There should be 1000 (or 500?) such pairs, taken

to be existing independently within, and in fact constit-

uting the hydrogen atom; hence 200,000 per Ur atom. Rutherford's measured energy of ionisation being about

2 x 10-10 erg, the energy per Ur atom is thus 4 x 10-5 erg.

Using Avogadro's number178 the energy per gramme of Ur is

about 2 x 1019 ergs or 5 x 1011 calories. If we assume

that, say, 0.1% of this becomes available for radiation

during the attainment of a greater 'closeness of

aggregation', the resulting 5 x 108 calories, at the

measured emission rate of 0.032 calories per year, would

last about 1010 years. Rutherford in fact stated his

supposition that uranium had already been radiating for

107 years. It is therefore reasonable to suppose that

he went through some such procedure as the rough calcul-

ation above. One interesting point implicit in

Rutherford's assumptions is that the atomic-ionic energy

contained in unit weight is the same (5 x 1011 cals per

gramme) for all elements. But what did this imply for

the element radium?

161

The Curies' statement that they had used radium

specimens 100,000 times more active than uranium told

Rutherford that about 3,200 calories per gramme would be

released per year. We have seen how Becquerel explained

his far smaller estimate. But Rutherford, try as he

might, could find no plausible answer to his own larger

problem: It is evident that, unless energy is supplied from external sources, the substance cannot continue emitting energy at such a rate for many years, even supposing a considerable amount of energy may possibly be derived from rearrangements of the components of the molecule.l79

This statement may be no more than the rejection of an

ordinary chemical or phosphorescent supply, depending on

the meaning of 'molecule'. However, he made his view

entirely clear that an electronic source though 'many

thousand times greater' was still insufficient:

The energy that might possibly be derived from regrouping of the constituents of the atom would not, however, suffice to keep up a constant emission of energy from a strong radio-active substance, like radium, for many years.180

A direct comparison with our calculation for uranium

suggests why not, though Thomson in 1903 theoretically

extracted more than enough energy from this source. As

for the third possibility that 'the radio-active substance

in some way acts as a transformer of energy' from its surroundings, he thought that 'this does not seem probable

and leads us into many difficulties'.181 Rutherford's

last word for some time on the problem is enigmatic:

It is of importance that experiments to test the constancy of a powerful radio-active substance, like radium, should be carried out at definite intervals. If the radiation should keep constant from year to year, it would be strong evidence that the energy of the radiation was not derived at the expense of the chemical energy of the radio-active substance.l82

It seems that if the activity remained constant the source

must be external. If the activity in fact declined, this

should indicate a chemical source for the energy - but of

what kind? Direct observation was not to be the way in

162

which the apparent constancy of the radiations from

radium, uranium, and thorium was eventually broken

down. In the prelude to this much-disputed advance,

the mysterious thorium emanation with its rapidly

declining activity played a part, as did quite novel

studies on the chemical side of radioactivity which

were now growing apace.

4. Emanations and the X-substances (1900-1)

In May 1900, shortly after F.T.Trouton sent his

letter concerning radioactivity from Trinity College

Dublin to Nature, the better known physicist

G.F.Fitzgerald wrote from the same address to

Rutherford. Expressing his interest in the experiments

on thorium emanation Fitzgerald remarked: that Debierne says he has isolated a substance he calls actinium which he thinks is the active material in the thorium experiments. This actinium gives out something that is magnetically deflected, but I am not sure that this is not almost always present to a small extent in all these cases and that it is merely the very powerful ones in which it has been observed.183

Was the well-known activity of thorium then not its own?

And was the emission of corpuscles or 'disembodied

electrons'184 a general property of radioactive substances

after all? I shall in this Section discuss questions like

163

the first of these as they began to be asked with

increasing persistence during the year 1900. In exploring

the expanding web of experiments, ideas, and communications

concerning the precise chemical nature of radioactive

materials we shall see how the notion of radioactive

induction was invoked and extended by some with a view to

explaining the observations. The new chemical-radioactive

studies can be seen as a vital step towards the more

successful transmutation-disintegration theory of 1902-3,

or alternatively as providing a large amount of empirical

information which helped to broaden the scope of theoretical

explanations of radioactivity.

Outside the confines of radioactive studies, though

loosely related to them, there was no lack of discussion,

continuing from the earlier period through 1900, regarding

the possible transmutation of the chemical elements.

Lockyer repeated some of his old astronomical-chemical

arguments in a new book on Inorganic Evolution.185 On the

physical side, Fitzgerald's note on 'The Theory of Ions'

appeared in Nature in September.186 He viewed favourably

the varied evidence for an electronic structure of all

matter, and hence thought that there seemed to be 'no

impossibility in the dreams of the alchemist, and an

element of one kind may some day be transmitted into

that of another'. No mention of radioactivity was here

made by Rutherford's recent correspondent. Discussions

of purely chemical transmutations at this time, and earlier,

are exemplified by F.Fittica's claim and C.Winkler's

refutation 'On the alleged transformation of phosphorus

into arsenic',187 followed by Fittica's rejoinder 'On the

transmutation of phosphorus into arsenic and antimony'.188

W.Crookes corresponded with the American chemist Dr.Etmens

concerning the latter's claim of 1897, to have converted

silver into gold by hammering; Crookes thought it doubt-

ful.189 And at the end of 1898, before the American

Chemical Society, F.P.Venable on general grounds vigorously

attacked 'so dangerous a doctrine' as that of a changing

chemical atom, which had been suggested to explain variable

valency. His view was that:

164

It will be an unfortunate day for chemists when the unchanging atom is given up. Chaos will indeed enter into all of our theories when this, the foundation rock, is left at the mercy of every shifting tide of opinion and can be shaken by all manner of unfounded hypotheses.190

It is possible that a considerable proportion of chemists

clung to the atomic theory in this way. By 1900, radio-

activity had already begun to impinge upon this view; the

phenomena evidently required new concepts of one kind or

another to explain them. With the Curies' guidance, the chemist A.Debierne

followed another line of inorganic analysis in the

pitchblende residues. In October 1899 he reported 'Sur

une nouvelle mati6re radio-active'191 which appeared to

be chemically identical to the element titanium and

comparable in its high radioactivity with radium

(100,000 Ur). By April 1900 he had taken the step of

naming a new element, 'Sur un nouvel element radio-actif:

l'actiniumi.192 At this later date, however, Debierne

associated his perhaps questionable193 new element in its

chemical properties with thorium. He noted that his

actinium, which emitted magnetically deflectable rays,

caused very weakly the permanent induced activity dis-

covered by the Curies. Now the latter had published

descriptions only of a temporary induced radioactivity.

However, without discussing duration, they had indeed

been first to pose the vital question of whether 'la

radioactivity, en apparence spontanee, n'est pas pour

certaines substances un effet induit'.194 Debierne was

one of those who followed such a suggestion and who also

becelme enmeshed in the connected problem of whether certain

ostensible radioactivities should be attributed to active

impurities. Debierne's approach is illustrated by his

conjecture that thorium, weakly active, might owe its

activity to traces of a chemically similar foreign

substance like actinium, and by his opinion195 that

Rutherford's results with thorium also might lead to

this conclusion. Study of Rutherford's publications on

thorium emanation has revealed no grounds for the

165

statement; he himself could see none.196 Nevertheless,

Debierne announced that he intended to start with thorium

compounds themselves, rather than with mineral residues,

and to separate from these either an inactive thorium or

the strongly active foreign substance, actinium. Perhaps

Debierne expressed his intention in the alternative form

because he realised that the preparation of an inactive

thorium would be an impossibility if it owed its activity

to actinium, and if this substance really induced a

permanent radioactivity upon adjacent materials. Several

scientists struggled among the confusions between induction,

element, and impurity.

F.Giesel's experimental studies ran parallel to and

sometimes ahead of those in France. In August 1899, two

or three months before announcing the first magnetic

deflection of the rays, he published 'Einiges Uber das

Verhalten des radioactiven Baryts and Uber Polonium'197

which effectively began a new line of research; though

at first he provided no theoretical interpretation of

his results. Firstly, the activity of radium was not

constant; after crystallisation it rose from a very low

level as indicated by a fluorescent screen to a constant

maximum during the course of days or weeks.198 Secondly,

the activity of a solution of the chloride gradually

faded. And his polonium sulphide precipitates had

completely lost their radioactivity after two months.

Though making no claims, Giesel at the end of 1899 seems

to have been the first explicitly to describe a new method

of induction, the artificial activation of salts by

admixture in solution. His preparation in this way of an

artificially active bismuth199 had significant consequences.

For this seems to be the beginning of his long-held belief

that polonium was merely induced bismuth. Debierne too

made progress in experiments on this new form of induction

and cited Giesel and Rutherford in his note 'Sur du baryum

radio-actif artificiel'.200 Debierne considered that the

far stronger effects which he found with the solution

method were due to the more intimate contact of the sub-

stances concerned. Ordinary barium, precipitated as

166

sulphate from a solution containing actinium, turned out

to have an activity several hundred times that of uranium;

this could then apparently not be altered by chemical

reactions. His somewhat surprising conclusion was that:

The radioactivity of barium rendered active by contact is an atomic property in the same way as that of radiferous barium, since it persists in all chemical transformations.201

The artificially active barium could, like radium, be

concentrated to comparably high activities by crystall-

isation, but Debierne held its lack of a spectrum to be

one of the 'important' differences between the two. The

fact that its activity diminished spontaneously to one

third in three weeks provided a further distinction from

both radium and actinium, each of whose activity steadily

increased to a maximum after preparation. Its vanishing

radioactivity did not deter him from claiming to have

produced:

par induction un baryum radio-actif, qui se distingue nettement du baryum et du radium et qui se presente comme un terme interm6diare entre ces deux elements.202

He was sure that his results were not due to traces of

actinium or radium. However, perhaps partly in his own

defence, Debierne accused B.von Lengyel of committing this

very error. For the latter in his report from the

Chemical Laboratory of the University of Budapest 'On

radio-active barium',203 had stated that he found uncon-

vincing all of the evidence for new chemical elements

based on radiatiom measurements. Even in the case of

radium atomic weight and spectroscopic determinations

were not sufficient. The experimental preparation of an

artificial radium or of a radioactive barium would tend

to show that radium was not an element; in his attempt

to do this von Lengyel considered himself successful.

He claimed that the active barium compounds extracted

from a melt of uranyl nitrate and barium nitrate crystals,

exhibited in qualitative fashion the increasing radio-

activity possessed by the radium of others:

It appeared that one can transform ordinary barium into a radio-active form which apparently

167

possesses all the properties of radio-active barium observed by different experimentalists.204

This conclusion was immediately criticised by F.Giesel

in his note 'Veber radioactives Baryum and Polonium'.205 He stated that uranium nitrate owed much of its radio-

activity to highly active impurities such as radium and

that these would simply have contaminated Lengyel's

barium. It is notable that the constancy of uranium rays,

so important for radioactivity up to this time, was by

implication discarded by Giesel as casually as that of

thorium had been by Debierne. With regard to active

'impurities' the subject was interconnected in such a manner that by saving radium Giesel seems in a way to

have forfeited uranium. Whether he was aware of this it

is not clear. On the other hand, he considered that the

Curies' claims for polonium were weakened by a different

factor, by their own discovery of radioactive induction.206

As yet another new element came into contention

Giesel exhibited the same cautious tone. Both the poss-

ibility of radioactive induction and of contamination by

active traces played a part in his continuing criticisms

of the assertions of K.A.Hofmann. From the chemical

laboratory of the Academy of Science in Munich, Hofmann

and E.Strauss in November 1900 announced their discovery

of several new radioactive elements. One was close to

lead in its chemical properties, 'das radioactive Biel',

'radiolead'; the others were rare earths.207 Within weeks

Giesel pointed out that traces of radium, polonium or

actinium, chemically undetectable, were the likely causes;

similarly Debierne's artificially active barium precipitated

from actinium solutions had merely carried down a trace

of the latter element, undetectable but for its radiation208

Hofmann and Strauss concentrated their researches upon the

radiolead; their determination of a chemical equivalent

of 65, hence anatomic weight of 260, seemed highly

significant (lead 207)209 but the value was never

confirmed210 and the statement was not repeated. Then

in February 1901 they claimed, apparently in reply to

Giesel, that qualitative chemical tests had ensured the

168

absence of all three new active elements from their

radiolead; this missed the latter's point. In the face

of criticism these incipient radiochemists appear to

have sought all possible means of qualitatively identi-

fying their materials. The title of their paper, 'Veber

die Einwirkung von Kathodenstrahlen auf radioactive

Substanzen',211 indicates one such method. The idea was

not new, but their attainment of seemingly definite

results had been anticipated, in part, only by one other

scientist.

During the preceding summer, P.Villard had spoken

of the induced radioactivity of bismuth;212 he had extended

the Curies' discovery of temporary induced radioactivity

by producing a similar though weak effect in bismuth by

the action of cathode rays alone. Villard expressed the

hope that researches using the vacuum tube could lead more

readily than solution studies to a simple explanation of

radioactivity. We note Villard's view, which appears to

relate current researches directly to Prout's hypothesis,

that the cathode rays or radiant matter of the Crookes

tube consisted not of sub-atomic particles but of the

lightest element hydrogen.213 His work in 1900 on the

penetration of gaseous hydrogen immediately influenced

the ideas of Crookes himself on radioactive rays;214 while

that on induced activity was taken up in 1901 at the

Cavendish Laboratory in the form of researches 'On a kind

of Radioactivity imparted to certain Salts by Cathode

Rays'.215 These studies appeared to be of theoretical

importance for radioactivity at least until 1903.

Hofmann however made no reference to Villard's work

as he explained early in 1901 that by the influence of

cathode rays he could revive the activity of radiolead.

This, unlike radium radiation, had vanished entirely in

a few months; the renewed activity lasted for several

weeks. He considered that these observations constituted

conclusive evidence, in addition to the chemical tests,

that polonium too was absent; for the similarly declining

radiation of this substance could not be restored.

Iiofmann gave a vague theoretical explanation of such

169

effects in terms of vibrations of small wavelength excited

within the metal atome.216 This could not have impressed

Giesel, for his extensive experiments with the radiolead

kindly sent to him by Hofmann confirmed none of its

distinguishing features217 not even the effect of cathode

rays. Furthermore, he turned the idea of induction

against Hofmann by precipitating an articifially radio-

active lead sulphide of declining activity from a solution

containing radium.218 The dispute continued into 1904,

with claims,219 rejections,220 and further claims,221 to

have found a distinguishing feature. By 1903, in order

to back his assertion of 'Radio-active lead as a primarily

active substance'222 Hofmann claimed to have characterised

this material by analysing its radiation, despite his

employment of techniques unrefined compared to those of

some physicists. He still assumed that the rays could

directly induce radioactivity into other substances.

According to the theory developed at that time, and now

accepted, each radioactive element is distinguished both

by the character and by the rate of decline of its rays,

but not by any inductive property. Experimental problems

of isolation and identification certainly continued to

exist within the area covered by this theory. Nevertheless,

the disagreement concerning Hofmann's preparations was

settled more or less in his favour during the following

few years. Shortly before the radiolead dispute began William

Crookes read a paper on 'Radio-activity of Uranium'223

which was to have a more direct influence than those

discussions upon the development of the understanding of

radioactivity. It was he who in May 1900 first pointed

to the problem of entrainment of radioactive traces on

precipitates, a legacy of his own spectroscopic studies

of the rare earths: a substance present only in traces

tends to follow the analytical reactions of the bulk

material even if chemically dissimilar. Crookes did not

mention radioactive induction though he probably read of

it in Debierne's papers on active titanium and actinium,

170

which he cited.224 Confused though he admitted himself

to be during the following year,225 on account of the

results of others together with his own difficulties with

radioactive fractionations,226 Crookes seems never to

have entertained the possibility of direct induction;

perhaps entrainment and diffusion made it seem unnecessary.

His laboratory notebooks record some attempts to prepare

radium and polonium from pitchblende in October 1898,227

to activate barium using uranium solutions in January 1900,228

and the vital fractionation of uranium in the following

month.229 Crookes tells us that his original intention was

simply to purify uranium for use as a photographic standard

against radium or polonium preparations.230 But on appli-

cation of a known purification method the portion of uranyl

nitrate dissolving in ether turned out, unexpectedly, to

be entirely inactive photographically while the aqueous

layer, normally discarded, gave a strong effect. He

directed his remarks against the Curies' statement that

the activity of uranium or thorium was a constant property

of the metal independent of its state of combination.231

Crookes had shown on the contrary that the radioactivity

exhibited by uranium belonged not to itself but to quite

another substance which he designated UrX. This conclusion

was supported by marked differences in photographic

intensity between various commercial samples.232 Thorium

too had begun to separate into portions of different

activity upon fractional crystallisation; he remarked upon

Debierne's supposition that actinium was the true source

of thorium rays. It appears that this notion became

persuasive for a time. Crookes informed P.Curie of his results and suggested

on the basis of extractions from a barium-free pitchblende

that radium might not really resemble barium.233 Curie

replied that he had not seen Crookes' paper but that 'it

is absolutely certain that the substance you have extracted

from your mineral is not radium'. Curie's answer was

actinium: he noted that Debierne had detected it in

commercial uranium specimens and had already succeeded in

decreasing the activity of these; and he believed that

171 the results of von Lengyel and Giesel234 of May and June

could also be explained by the presence of this material.235

In his reply, Crookes did not dispute the point;236 indeed a statement made in 1902 shows his acceptance of the

identity of his UrX and Debierne's actinium.237 By that

time the influence of Crookes' publication had run through

Becquerel and Baskerville to Soddy and Rutherford.

The American chemist C.Baskerville, in his paper 'On

the existence of a new element associated with thorium'

delivered in August 1901238 cited Crookes on UrX and

thorium. Baskerville too, like Debierne before Crookes,

thought that thorium's activity might possibly lie with

actinium; he noted that Rutherford's results on the radio-

activity induced by thorium made photographic comparisons

difficult, but discussed this no further. Five years

earlier he had used the designations Th and ThX for the

two separated elements.239 These results and symbols

featured in the future progress of the science though

this belief in the ubiquity of actinium did not.

Becquerel too had wished to prepare a pure uranium,

in his case for (successful) attempts to deviate magnetically

its rays. In a 'Note sur le rayonnement de l'uraniume of

June 1900240 he drew from the studies of Debierne on

actinium in thorium and von Lengyel on the activation of

barium by uranium. Becquerel described an actual lowering

of uranium's activity by two successive additions of

barium chloride to its solution, each followed by precip-

itation of the barium as a now active sulphate. He under-

stood apparently independently of Crookes that entrainment

of an active material, perhaps actinium occurred here but

concluded that uranium did emit a radiation of its own.

Becquerel was not so sure of this after a series of 28

entrainments performed241 during the following weeks

yielded continual diminutions in uranium's activity,

successively smaller, with some irregularities. He

acknowledged Crookes' attainment of a completely inactive

uranium nitrate, to the Paris Congress in August.242 It

was Becquerel's recognition during the following year of

the self-recovery of this activity which produced a leap

172

forward in radioactive research.

As the Curies saw the position in August 1900243

the experiments of Millard on activation with cathode

rays showed that one could 'creer la radioactivite sans

faire intervenir une substance radioactive';244 Debierne's

'baryum active' by actinium was unaffected by chemical

change, 'son activite est donc une propriete atomique'.245

They thus considered radioactivity to be an atomic property

which could be induced upon the atoms by an external agent;

the source of the energy released, once this property of

radiating had been acquired, was another question.246 As

for Crookes' work on UrX and inactive Ur the defence

against the implied criticism was clear. It was an assoc-

iated element, probably actinium, to which the radioactivity

of uranium must be ascribed. Owing to the difficulty of

obtaining this element free from actinium uranium would

simply have 'l'apparence d'un element atomiquement radio-

actif'; there was no 'contradiction avec l'idele que la

radioactivite est une propriete atomique' .247 It may appear

however that the underlying evidence for this idea was now

in considerable disarray. Moreover, the Curies' notion of

radioactive induction appears to have been at a point of

transition. Rutherford's results on thorium emanation and

its electrically sensitive inducing effect, which they were

unable to repeat in regular fashion with radium,248 together

with solution studies, had probably caused the Curies to

discard direct radiation as a simple cause of induced radio-

activity. Their readers waited until the spring of 1901

for the appearance of a coherent alternative.

The physicist E.Dorn at Halle, having recently

published on X-rays and radium rays, had already attained

some experimental success with radium induction before the

time of the Paris Congress. To radium and polonium he

applied the techniques used by Rutherford upon thorium.

With French and de Ha?n German samples he succeeded, where

Rutherford could not, in demonstrating that radium and to

a small extent polonium released a gas-like emanation with

the inducing property. His discussion 'Veber die von den

173

radioactiven Substanzen ausgesandte Emanation'249 indicates his use of Rutherford's term. Dorn also accepted the view

that the cause of induced activity was the deposition of

emanation.250 Yet he may not have grasped or adopted this

completely for in his reported experiments, some of the

earliest, on the electrolysis of radioactive solutions,

he applied the Curies' expression 'secondary activity' to

activated electrodes without visible deposit.251 And he could not understand why a wire sealed in a glass tube

after activation by radium should lose its activity in a

day, unless the glass were in fact permeable to the

emanations in an undetectable degree.252 The different

rates of decay showed him that there were qualitative

differences between the emanations from thorium and active

barium, and between the secondary activities. His discovery

of large, if irregular, increases in electrometer readings

upon moistening thorium oxide or radium specimens seemed

to him of particular interest; radioactivity was thereby

placed in close connection with a 'physikalisch-chemischen

Prozess'.253 Dorn's student F.Henning continued electrical

researches upon the emanations and upon aqueous solutions

during the following year, 1901,254 but drew no significant conclusions; he seems to have thought that the particles

of emanation could disappear spontaneously in the air.255

We recall Rutherford's assumption that the intensity of

radiation from each particle slowly declined.

On the basis of research upon the emanations, only '

Rutherford and his collaborators were to make progress.

His study of the energy of radioactivity, completed by

mid-1900, was problematical in various ways. Dorn had

accorded due acknowledgement to Rutherford regarding

emanation studies and now demanded256 the same from the

latter regarding the energy of X-rays; priorities were in

fact given, in the full published paper.257 Happily, Dorn

in his 'Bemerkungen au der Mitteilung von Rutherford und

Mc. Clung Tiber die Energie der Becquerel- und

ROntgenstrahlen etc.'258 agreed with the values therein.

But J.S.Townsend, as we have seen, disagreed with estimates

used at a succeeding stage of Rutherford's reasoning towards

174

the energy of radioactivity. The question of its source

seems not to have had an answer, nor was there any clear

direction of research which might lead to one. The same

was not quite true of the emanations: to the standing

questions of their nature and means of production

Rutherford redirected his attention. His examination of

the 'Einfluss der Temperatur auf die "Emanationen"

radioaktiver Substanzen',259 dated March 1901, constitutes

an extension of his researches of 1899 upon thorium.260 He reported that the rate of production of emanation from

both thorium and radium increased steadily with temperature

up to a red heat, above which it was almost destroyed, not

to be restored. This latter statement was to be very

significantly revised within the following months. Somewhat

like the Curies, Rutherford noted that there were irregul-

arities in the production of induced radioactivity by

radium emanation; it could be confined less readily to a

cathode than thorium emanation especially when provided,

in large amounts, by heating the radium. Rutherford con-

cluded, adopting an expression similar to that of Dorn whom

he cited, that the two emanations were probably produced by

a chemical process 'einem chemischen Vorgang im Material'261 J.J.Thomson was prepared to say much more than this

concerning the emanations, though not as yet in print. His

reply of 12th April to Rutherford's enquiry262 concerning

Tait's Chair at Edinburgh contains a comment which may seem

familiar in the context of the contemporary chemical-

radioactive researches:

I suppose you have seen Debierne's work on actinium, a substance which is closely associated with thorium, and which has extraordinary powers of producing induced activity; do you think there could have been any of this in your experiment on the thorium radiation?263

This was Rutherford's second private warning concerning

actinium; one of the main tasks of the chemist whose help

he enlisted in the following months was to see whether

the emanation really came from thorium, or not. As for the

emanation itself Thomson now thought he knew 'pretty clearly'

its nature and the mechanism which produced its observed

175

electrical properties. We have seen264 that he had

reversed Rutherford's explanation both of the concentration

of induced activity or emanation upon a cathode, and of

the failure of this at a low pressure. Rutherford had

postulated an excess of positive ions clustered around

the emanation particle, Thomson the attachment of electro-

positive emanation particles around positive ions. The

failure of electrical concentration in rarefied gases was

attributed by Rutherford to a scarcity of air molecules

and hence ions; this outweighed the increased mobility of

a charged particle. Thomson needed far fewer ions than

Rutherford both in his old account of 1899 and in his new

one of 1901: I was much interested in your paper on the effect of temperature on the emanation which I was reading last night. I think your experiments show pretty clearly what the emanation is - does not the following view explain most of the effects - suppose that thorium or radium gives out a gas (the emanation) & that this gas is radio-active in the same way as radium i.e. by giving out negatively electrified corpuscles - the effect of this emission of corpuscles will be that the particles of the emanation will behave as if they had a slight + charge. The equivalent charge will be only slight because though the emission of the corpuscle will momentarily leave the emanation with a + charge this charge will soon be neutralised by a negative ion from the surrounding ionised gas The equivalent charge ought to be less at a high pressure than at a low (or rather at a very low pressure) for the smallness of the charge is measured by the quickness with which it is neutralised & at a very low pressure there would be few ions to do this. I should not expect the effect of pressure to be great until the pressure got very low as until then the diminution of the number of ions would be compensated for by their increased mobility.265

Thomson was perhaps first thus to combine the eighteen-

month old knowledge of the corpuscular nature of the rays

with emanation studies. His account entered strongly into

Rutherford's publications266 after initially suffering

setbacks. One of these appeared in conjunction with the

justification of Thomson's description of each emanation as

a 'gas', which Rutherford announced within weeks, in ful-

fillment of a promise of 1899.267 Rutherford's experimental success as he began to answer the second major question

regarding the emanations - their precise nature - owed much

176

to radium emanation, effectively unknown in 1899; its

radiation lasted for days, rather than the minutes of

thorium emanation. His paper with Miss H.T.Brooks268

on 'The New Gas from Radium'269 and his note to Nature

on 'Emanations from Radio-active Substances', of May 1901,270 describe the diffusion experiments on radium

emanation which now gave a value for its molecular weight.

This, being between 40 and 100, excluded the vapour of

radium. 'We must therefore conclude that the emanation

is in reality a heavy radioactive vapour or gas'.271

The correct modern value of 222272 certainly would not have excluded the vapour of radium; nevertheless, the

conclusion held firm. Of great interest are two points

which Rutherford briefly made in the closing statement of

both papers, each of which weakened previous hypotheses.

Firstly, the emanation emitted a radiation 'apparently

similar in character to easily absorbed ROntgen rays' and

presumably not the charged deviable rays required by

Thomson; Rutherford was later273 to make sure of this, with

important consequences. Secondly, this emanation 'in some

way manufactures from itself a positively charged aabstance,

which travels to the negative electrode and becomes a

source of secondary activity'.274 This statement is

apparently not entirely consistent with Rutherford's and

Thomson's previous notion that it was a deposited layer

of the emanation itself, positively charged by association

with ions of any surrounding gas, which produced the

excited radiation. Had the differences between emanation

and deposit in their chemical behaviour, and the reality

of the new gas, convinced Rutherford of the occurrence of

a second strange chemical process? It was months before

radiochemical studies on thorium began in earnest at

McGill, and nearly a year before his first and barest

hint of an observed transmutation appeared in print. But,

considering the background of speculation in the field,

we are perhaps entitled to ponder privately upon Rutherford's

ideas of May 1901; though at this time he would only say

that 'Space is too short to enter into the interesting

177

question of the possible explanation of these complicated

phenomena'.275

The researches made by Elster and Geitel during

1900-1 came to stand between Rutherford and Thomson in

1902; praised and used as support by the former, they

led the latter into criticisms and doubts.276 J.Elster and H.Geitel maintained their early interests in radio-

activity; they were pleased to incorporate Rutherford's

ideas on emanations and active deposits into another of

their areas of study, the electrical phenomena and

conductivity of the atmosphere. A slow spontaneous

increase in the natural conductivity of an enclosed

portion of air over several days led them, towards the

end of 1900, to suggest that natural atmospheric conduct-

ivity might not be produced by solar radiation as formerly

supposed.277 Instead, the rays from traces of emanation

in the air, and from the resulting induced activity on

the walls of a containing vessel might be the cause.

They found observations to support this view, again going

underground. Here they found abnormally high conductivities

in the air from caves and cellars278 where there was no

possibility of contamination from their laboratory. And

the final comparisons, made by October 1901, with

Rutherford's results on the emanations were the concen-

tration of induced activity upon a negatively charged wire

simply placed in the open air,279 and the removal of this

activity from the wire by mechanical or chemical means,280

where it then continued to decay. They did not explicitly

discuss the important question of whether the active layer

was a deposited material, or the surface itself put in a

radioactive state; but the answer implied by their exper-

ments and discussions would seem to be that it was both.

For they used specific methods directed at metals to

178

remove the activity from their surfaces,281 and yet stated that the production of surface activity was

the same for a variety of substances.282 The problem-atical theory which they devised to encompass these

results was one of creation and disintegration of

radioactive matter: though unusual in certain respects,

it was not the only one of its kind.

179

CHAPTER 4

DISINTEGRATION, INDUCTION, TRANSFORMATION

1. The emergence of induction and disintegration THiiories (1901=7)

Dispersed among the various works on radioactivity

published during 1901-2 all of the pieces which were

shortly to coalesce into a coherent theory may be dis-

cerned; later priority claims testify to this. Also

present were other persuasive concepts which were to

handicap their employers greatly; but this only became clear after the event.

At the end of 1901 the physicists Elster and Geitel

provided discussions which seem typical of the period in

being both suggestive and incomplete. Their comments

were contained in a paper whose main purpose was to des-

cribe 'Recherches our la radioactivite induite par l'air

atmospherique1.1 During the past year they had linked

their discovery of atmospheric-induced radioactivity with

Rutherford's conclusion that the emanations and induced

activities of thorium and radium were distinct materials.

But now the German workers were much attracted by an

alternative view which did not require the existence of

special substances. Ironically, this occurred just at

the time when Rutherford and Soddy were making great

progress on that basis. Though it had obvious weak areas

Elster and Geitel's thesis was persuasive. For it pointed

towards a fundamental explanation not only of the universal

induced atmospheric activity but of natural radioactivity

also. All gases, they thought, possessed an ionic

constitution and could thus provide positive ions which

's'unissent aux electrons nogatifs du conducteur blectrise'.

The result was 'une sorte de combinaison instable qui se

de-bruit par l'6mission des electrons, c'est-6.-dire par la

production de rayons de Becquerel' as required.2 Accordingly,

induced activity would be due to a temporary compound

derived both from the surrounding gas and the metallic

anode. Their approach towards a theoretical advance was

180

to ask 'd'une maniere generale si l'on peut distinguer

l'une de l'autre les radioactivites primaires et induites'.3 In order to assimilate these two phenomena they argued

from the 'lois d'energie', as they had in 1899, that the

apparently permanent activity of the elements uranium,

radium and thorium in fact suffered an imperceptibly slow

decline. The particularly rapid decay of induced radio-

activity was simply attributed to the 'tres petite quantite

de matiere recueillie'. Of great interest is their idea

that this material was actually in the course of creation

- that one might be witnessing 'la veritable elaboration

d'une substance active'. The implications of this for

uranium and the other active elements are fascinating

indeed, but were not discussed. On the other hand they

ascribed the observed decay of induced activity to an

emission of electrons by which the material 'se detruit'

or 'est tres vite ramenee a l'etat indifferent'. Whilst it may be described as a theory of disintegration this

account skips over the intermediate chemical stages so

crucial for other workers. Even with the limited dis-

cussions which Elster and Geitel provided there were certain problems some of which were mentioned and some not.

The theoretically implied but unobserved effect of dilution

or quantity upon the rates of decay falls into the latter

category. In addition their statement that the Curies'

radium-induced activity, excited 'par le contact immediat'

or via fluid media, 'n'est point une propriete durable'4

seems to admit two distinct types of induction. And in

the open admission that their theory could not account

for the extraordinary high atmospheric activities observed

in certain caves may be seen their reason for not completely

rejecting Rutherford's special emanation hypothesis.

Having failed to produce the predicted induced activity

by attracting the negative ions of the air on to a pos-

itively charged wire5 their studies of the different

natural conductivities of air from caves and from the

laboratory finally convinced Elster and Geitel of the

existence of a primarily radioactive gas in the air.6

181

But by then in mid-1903 such phenomena had already been

absorbed into a disintegration theory more successful

than theirs.

That others were moving in that direction towards

the end of 1901 is indicated by Elster and Geitel's

remark that their 'idee se rapproche beaucoup' with the

analogous considerations presented recently and independ-

ently by W.Nernst and H.Becquerel. The comparisons are

interesting but not simple. Nernst gave only the briefest

comments on radioactivity to conclude his discussion

'Veber die Bedeutung elektrischer Methoden and Theorien

fUr die Chemie'.7 He supposed that electrons escaping from

their dynamic equilibrium with metallic elements constituted

the Becquerel rays. And whilst Becquerel himself would

certainly have agreed with Nernst that uranium rays consisted

in part of electrons yet on the basis of his own experiments

he postulated a much more severe dissociation of the active

material. In advance of my examination of his and other

theories of radioactivity it may be helpful roughly to

classify these according both to the kind of disintegration

envisaged and to the use made of the notion of induction

which can now be seen as a completely false trail. Elster

and Geitel's indifferent attitude towards induction has

already been noted. They shared with G.Martin and

H.Becquerel the idea of a complete or destructive atomic

disintegration. The latter was the only one of these to

employ the conception of induction by contact in his

explanations. W.Nernst, J.Perrin, W.Crookes and

J.J.Thomson all seem to have believed or implied that an

atom or molecule lost sub-atomic electrons or corpuscles

only to pick up others from the surrounding material, so

restoring the original situation. On the other hand

Becquerel, J.Stark, and Rutherford and Soddy viewed the

dissociation of radioactive atoms or molecules as passing

through a series of irreversible steps. The latter two

scientists in collaboration argued most forcefully con-

cerning the chemical consequences of such a process.

The case of the Curies is an interesting one. At

the time of her lecture of mid-1900, which contained

182

lively discussions on the relationship between atomic

change and corpuscular emission, Marie Curie would appear

to fit moderately well into the last group of the above

scheme. Within a year however all such considerations

had been effectively shelved. Reasons for this are

uncertain, but one can point again to the unresolved

contradictions regarding the non-corpuscular rays of

polonium. It is also just possible that one or both of

the Curies came to realise the chemical implications of

Marie Curie's speculations only gradually. In any case,

P.Curie and A.Debierne were able to work experimentally

towards an apparently superior theory uniting both

inductive and radiative phenomena. Their first step,

however, involved the partial disconnection of a prematurely

formed link between these two. In a paper of March 1901

'Sur la radioactivite induite provoquee par les eels de

radium'8 they described experiments performed with thorium,

radium and actinium in sealed vessels, which proved con-

clusively that radioactivity could be induced without

direct irradiation. Significantly, the hint of a remaining

bond can be seen in their comment that polonium was an

exception in producing neither induced activity nor the

deviable radiation, two facts which might be related.

They stated that it was too soon to accept Rutherford's

theory of a diffusing particulate radioactive emanation

since other equally satisfactory explanations could be

formulated. But they were not prepared to say what these

were admitting only that 'La radioactivit6 induite se

transmet dans l'air de proche en proche' from source to

object and insisting that this process might be connected

with the deviable radiation. The adoption and interpretation by Rutherford of the

results described in P.Curie's and Debierne's succeeding

note 'Sur la radioactivite induite et les gaz actives par

le radium',9 read a few weeks after their first, exemplifies

the closeness of and the differences between the paths

followed by the students of radioactivity. Two months

later Rutherford described the electrical examination of

183

what he called 'The New Gas from Radium'10

whose moderately

high molecular weight he had estimated with the aid of a

standard diffusion method; induced activity was caused by

'a positively charged substance' somehow manufactured from

this gas. His approach may be contrasted with the French

scientists' suggestion of a gas activated lox radium and

with their experimental attempts to grasp the role of the

medium through which induced activity was transmitted.

Curie and Debierne found that induced activations were

unaffected by the use of different gases or by evacuation

down to 1 cm. mercury. But in a high vacuum maintained

throughout the induction by continuous pumping, as they

stressed, activation did not occur and previously induced

bodies lost their activity. Suppression of activation

failed, however, when the evacuated vessel was simply

sealed and left. Curie and Debierne attributed this

result to the release of highly radioactive gases occluded

within the radium sample; when collected by gently heating

the specimen these produced spectacular effects such as

the luminosity of the entire containing vessel. Evidently

all this fits well with Rutherford's earlier ideas on

thorium emanation and with his as yet unpublished study

of the effects of heat upon the production of emanations.

Yet without again citing that theory the French team

spoke with justifiable reserve concerning the only explan-

ation of their results which they mentioned. It might be

supposed that 'des gaz ordinaires contenus dans lair

slactivent au contact de la matiere radioactive';11 the

activated gas could then excite solid bodies by contact.

But, as they noted, this accounted neither for the maximum

activity's independence of the pressure and nature of the

surrounding gas nor for the rapid transmission of the

activity along a capillary tube, which appeared to proceed

faster than ordinary diffusion would allow. Evidently

the idea of a heavy gas, on which Rutherford was shortly

to publish, was in even greater disagreement with the

apparently rapid transfer through gases than Curies's and

Debiern's own discarded conjecture. The French scientists

were to find their answer to the problems of the inductive

184

transmission of radioactivity by employing a totally

different medium.

Researches on this subject differed between Paris

and Montreal in technique as well as interpretation in

an unfortunate and perhaps unavoidable manner. P.Curie

and A.Debierne were moved to comment openly on the 'stat

deplorable' to which things had come in the laboratory:

the air had become so conducting that only 'des mesures

grossieres' with the electrometer could noftrbe made.12

They attributed this situation to the continuous formation

of activated gases rather than to dust as previously

assumed;13 in another context they reported induced

activities of up to 8,000 times the intensity of uranium

rays.14 The difficulties of repeating any of Rutherford's

work on thorium must have been great. This seems to be

confirmed by information recorded in a laboratory notebook15 of the Curies: the 'mouvement propre' of their weight-balance

piezo-electric electrometer varied from day to day during

some activation studies16 and sometimes made the radiation

from thorium impossible to measure.17 In compensation

however the extreme activity of the Curies' radium samples

soon led them to experimental discoveries which could

otherwise not have been made. Within two months of their complaint, Curie and

Debierne had been able to construct the most comprehensive

theory of radioactive phenomena yet achieved. Its experi-

mental basis developed as they turned from gases to water

as the medium of radioactive transmission. Debierne had

been one of the first to investigate the solution method

of induction in 1900; now with P.Curie in a note 'Sur la

radioactivity des eels de radium' of July 190118 he

explained that the heating of radium salts produced not

only active gases but radioactive water too. Here was

their clue. They followed it by showing that water could

also be activated simply by placing it in a dish within

the same sealed enclosure as a similar dish containing

the solution of a radium compound. Perhaps more revealingly

water could also be activated by immersing in it a sealed

185

celluloid capsule of the radium salt. Their interpretation

was that the celluloid 'joue is role d'une membrane semi-

permeable parfaite' allowing the activity but not the

radium to pass. Transmission did not occur through a

dry celluloid wall. Radioactive water lost its activity

within a few days when in a sealed vessel, much more

rapidly in an open one, and faster if the surface area

was greater. The fact that solutions containing radium

itself appeared to behave similarly, with the difference

that here the activities declined to a minimum but not to

zero, was the final point. The resulting theory was the

first which 'permet de coordonner' the rise, decay,

equilibrium and transmission of radioactivity. Curie's

and Debierne's fundamental assumptions were, firstly, that

'chaque atome de radium fonctionne come une source continue

et constants d'energie radioactive', and secondly that this

energy thence dissipated itself in two different ways:

1. par rayonnement (rayons charges et non charges d'electricit6); 2. par conduction, c'est-a-dire par transmission de proche en proche aux corps environnants par 1'interm6diare des gaz et des liquides (radio-activite induite).19

The authors made clear the analogy which they saw between

this formulation and expressions in use 'dans l'etude des

phonomenes calorifiques'; but other points were not so

plain.

Although they wished to elucidate 'le mecanisme de

la propagation de la radioactivite induite'2° Curie and

Debierne could or would give no details beyond the phrase

'de proche en proche', or as a translator put it 'from

particle to particle' .21 This remained so even after the

end of 1901 when they were exploring the spatial aspect

of radioactive transmission.22 Having excluded convection

and diffusion as major modes of transfer in favour of a

step by step process the impermeability of dry solid

materials would seem hard for them to explain. And for us

the explanation of the apparently rapid transmission along

capillaries is equally obscure. By means of their theory

Curie and Debierne pushed a multitude of observations into

the background. One may note their failure to comment

186

upon the different chemical properties of emanations and

induced activities, a problem which worried F.Giesel who

mentioned it to the Curies.23 Among other details of

which Curie and Debierne knew but gave no account were

the electrical concentration of induced radioactivity

and the characteristics of the complex radioactive rays.

In this respect it seems they placed the phenomena of

radioactivity in a hierarchy of importance which was

almost the reverse of that adopted by the prononents of

disintegration theories.

Evidently the authors had good reason to keep their

options open. This they did with the claim of July 1901

that the radium atom constituted a constant source of

radioactive energy 'sans qu'il soft nbcessaire, d'ailleurs,

de preciser vient cette energie',24 and in a footnote

they placed Marie Curie's speculations of January 1899:

the energy might have been previously stored, or derived

from an external radiation, or taken from the surrounding

medium, or produced 'par une modification du radium lui-

mame'. The last of these conjectures and then the first

were soon to become elevated in Paris and elsewhere to

positions of the highest significance. But P.Curie heaped

harsh criticisms upon those who did this.

With his work on the deviable rays from radium, his

studies of the chemical deactivation of uranium in 1900,

and some points from the above discussions of Curie and

Debierne, Becquerel combined theoretically a new discovery

of his own. The attempt thus to produce a complete

explanation of radioactive phenomena was nowhere ignored.

The researches on secondary radiation which he undertook

during 190125 had not turned Becquerel from his beliefs

that the primary phenomenon of radioactivity was the

emission of the deviable rays, that the consequent

undetectable loss of mass was the origin of the energy,

and that the non-deviable rays (probably meaning the

absorbable alpha rays but possibly gamma rays also) were

a type of X-ray produced by the primary rays. At the end

of that year he clarified his views 'Sur la radio-activite

de l'uranium'26 with a detailed description of the processes

187

involved. He considered that 'en se s6parant' the small

negatively charged corpuscles of Thomson's theory were

matched by large particles, oppositely charged, recoiling

at low velocity. These particles, which would not be

penetrating, formed the gas-like positively eleotrified

material emanation which would deposit upon all surfaces

except those similarly charged. Once deposited 'Ce depot

de matiere serait capable de se diviser A son tour en

particules plus petites qui traverseraient le verre'.

Thus in Becquerel's explanation emanations fade into

radiations in an interesting manner; however, among other

discrepancies, he missed the point that the emanations

themselves are electrically neutral. He appears not to

have anticipated the view which was later to become

important that the alpha rays consisted of rapidly moving

large particles; this had already been suggested in 1900 by R.J.Strutt27 on penetration evidence. But Becquerel's account contains the first published expression of the

conception that molecules suffer a mechanical recoil upon

the release of a corpuscle; such a notion was also adopted

by Rutherford during that same month as a new means of

explaining the electrical properties of thorium emanation.

As revealed in his paper Becquerel's most striking

advance lay on the chemical side of radioactivity. Persuaded

by the evidence of his own and of Crookes' observations he

had come to believe in 1900 that uranium owed its entire

radioactivity to a removable impurity. Since then, as

Becquerel stated, he had realised that such a conclusion

stood in contradiction to the fact that all commercial

samples of uranium salts of whatever purity were equally

active; it is noteworthy that this 'fact' was by no means

so clear to all.28 His observation of June 1900 that old fractionated uranium samples possessed identical activities29

may possibly have been a clue to it. However, Becquerel

did not explain the long delay indicated in his announcement

that now, eighteen months later, he had reexamined both

the deactivated uranium and the barium sulphate specimens

activated by precipitation from the solution of uranium.

188

Though the path to its attainment remains obscure

Becquerel's discovery that his deactivated uranium had

completely regained its activity whilst the activated

barium sulphate had entirely lost this power had a direct

impact. His interpretation of these results extended the

Curie-Debierne induction theory just so far as to make

contact with his own ionic speculations; he was doubtless

surprised when sparks flew. Becquerel's words reflect

his adoption of their distinction between permanent

primary radioactivity and temporary induced effects, but

his influential statements are in need of analysis:

Laperted'activite, qui est le propre des corps activ6s ou induits, montre que le baryum n'a pas entraine la partie essentiellement active et permanente de l'uranium.30

The active barium's decline was thus equivocally explained. Did he mean that the barium had merely become temporarily induced? Or was he suggesting that the barium had in fact

extracted one impurity, temporarily active, from uranium

leaving another, permanently radioactive, behind? Evidently

Crookes' preparation of an inactive Ur would fit neither

of these; but it was not to be Becquerel who proved him

wrong. One might expect that Becquerel's conjectures as to the means by which uranium spontaneously regained its depleted activity would indicate his preference. But this

is not the case; instead he employed what appears to be

an uneasy combination of the two. He compared uranium's

revival with the well-known rise in activity of freshly

precipitated radium-barium salts and to explain both of

these he proposed 'L'hypothese d'une auto-induction'.

Becquerel suggested that this could occur in mixtures of

active and inactive substances 'et mgme a une combinaison chimique de molecules'; this comment may be better under-

stood by replacing 'molecules' with 'atomes'. His final

remark on the point was perhaps intended to cover any

loopholes concerning the attribution of an element's own

activity: 'pour un corps purl elle equivaut a celle d'une transformation molgiculairel. If one similarly substitutes

atomic for molecular then there appears one of the vital points

against which the Curies reacted.

189

Certainly in their note 'Sur les corps radioactifs',31 read to the Academy by Becquerel himself a few weeks later

in January 1902, the Curies attributed to him a theory of

'transformation atomique'. This they attacked in both

specific and general terms but in doing so left some of

their own difficulties exposed. Their first point was

directed against the hypothesis that a process of auto-

induction operated to revive diminished activities.

'Certaines exp6riences, mal interpret-6es', they wrote 'conduirent a admettre une destruction partielle de la puissance du radium'. They insisted that on the contrary

each of the known radioelements had always exhibited the

same unvarying activity when placed in the same physical

and chemical state. It must be pointed out that this

straightforward statement immediately compounded the

problems of polonium which element they therefore committed

to the footnotes as an 'exception', branded 'une espece

de bismuth active'. The Caries concluded the argument

regarding induction with the claim that neither the laws

of dissipation of radioactive energy nor the effects of

physical and chemical state were known; this appears to

be a retreat from the assertions of the previous summer.

The second part of their reasoning was directed both

against the notion of the emission of material rays and

against the related concept of atomic transformation; it

was conducted in mainly energetic terms. If the source

lay within the radioatom in the form of potential energy

then the activity should decline. The Curies considered

that this was contrary to their observations. Alternatively,

the atom could be a transformer of external energy. By

stressing the scientist's ignorance of the medium surrounding

him the Curies in effect defended such a viewpoint

notwithstanding the evident violation of Carnot's principle.

They placed Becquerel's explanation of induced activity

and J.Perrin's theory of the origin of radioactive radiations

into the internal category and labelled both of these,

perhaps questionably, as theories of 'transformation

atomique'. Gone was the Curies' acceptance of the material

190

nature of cathode and radioactive rays on the grounds

that electrical charge had always been associated with

matter.32 Instead they pointed, without calculations,

to the negative results of their experiments designed

to detect a loss in weight from radium. It is remarkable

how completely the Curies' joint statements of 1902

contradict Marie Curie's own apparently favourable dis-

cussions of a disintegration hypothesis of atomic trans-

formation published but eighteen months before.33 The verdict seems now to have been that Becquerel's theory

was at best premature: their final word concerned the

procedures suitable for the attainment of scientific

knowledge and implied that there might be no truth whatsoever

in such hypotheses. The arguments of the Curies may appear

unhelpful but at least these ensured a thorough airing of

the issues. It is doubtful whether they stemmed the advance

of disintegration theories.

Becquerel, for example, repeated his ideas 'On the

radio-activity of matter' in a slightly abbreviated form

a few months later in March 190234 and maintained these

for several years. Before continuing the discussion of

comparable speculations which also appeared at this time

let us examine the prior statements of J.Perrin which

were critically cited by the Curies. In his popular

lecture of 1901 entitled 'Les hypotheses moleculaires'35

Perrin attempted briefly and largely qualitatively to

explain spectroscopic, chemical and radioactive phenomena

in terms of a singular atomic structure. Concentrating

the positive charge into one or more central 'soleils' he

made the arrangement and orbital motion of corpuscles

surrounding each of these account for valencies and

spectral frequencies; radioactivity received a brief

explanation: Si l'atome est tree lourd, c'est-a-dire probablement tree grand, be corpuscule le plus oloigne du centre - le Neptune du systeme - sera mal retenu dans sa course par l'attraction blectrique du reste de l'atome; la moindre cause l'en dftachera; la formation des rayons cathodiques pourra devenir tellement facile que la matiere paraisse spontanement radio-active... 36

191

Should this necessarily be called a theory of atomic

transformation as the Curies said? G.Martin, whose

similar view is described below, might not have done so.

One can ask the same question of Thomson's explanation

of ionisation, involving the temporary separation of a

corpuscle, which he held continuously from 1899. And

our answer may be provided by his own rejection for a

time of a transformation theory of radioactivity in

favour of a mechanism of minimal corpuscular ionisation.

Crookes too published an explanation of radioactive

phenomena which involved corpuscular dissociation without

atomic transformation. Its expression, early in 1902,

requires some clarification which can be obtained by an

examination of the development of Crookes' ideas. We

have seen37 that during 1898-9 he had proposed that the

energy of radioactivity came from the kinetic energy of

the surrounding air molecules. He then discussed with

Stokes experiments, such as the influence of air pressure

on radiation, which might serve to distinguish between his

own idea and the latter's uranium molecule 'wagtail'

hypothesis.38 By the end of 1900 they were communicating

on the deflecting effect of the magnet upon radium rays.

In his recent work on this subject R.J.Strutt39 had rather

confusingly used the term 'emanation' for what was more

often described as 'radiations'. Stokes adopted the

former expression and interpreted Strutt's results in an

individual manner. He considered that the 'emanation'

consisted of two different portions, namely 'rays', or

ether waves, and 'jets', or molecular projectiles.

Crookes replied40 that he did not now accept such a

dichotomy but was 'inclined to think that all the radio-

active actions are to be accounted for by the theory of

"bodies smaller than atoms"'. His conception of radio-

activity in these material terms involved an unusual

interpretation of Thomson's theory in which the corpuscles

were supposed to act in the manner of a gas, diffusing

slowly through certain materials:

It may be urged that Thomson's ultra-atomic particles are only existent theoretically, and

192

no instance is known of such a phenomenon as a gas or projection or emanation passing through matter ... Now however it has been shown by Villard (C.R. June 25, 1900) that hydrogen will pass through fused quartz at a red heat ... Now if a dense body like hydrogen gas will get through quartz, how much more easily will particles much smaller than the ordinary chemical atom get through glass, aluminium and black paper?41

Whilst some physicists had accepted earlier in 1900 that

materials were penetrated by high velocity radium projections

Villard's belief on the other hand was that hydrogen itself

rather than subatomic particles comprised the radiant matter of the vacuum tube.42 Crookes however, after seeing Rutherford's paper on the new gas from radium, in the June 1901 issue of Nature emphasised his point to Stokes: 'I

cannot agree that the chief radio-active body in pitchblende

is a gas, in the ordinary sense of the word'.43 Instead that position was held by the 'Thomsonian ultra-atomic' particles which, after emerging, temporarily behaved like a gas. Crookes' additional suggestion that some bodies

might be capable of 'temporarily fixing additional atoms of electricity - unstable perelectrides' then expelling

these surplus atoms of electricity came with an apology:

'Forgive my crude speculations. I feel as it were groping in an unknown laboratory in the dark'. His laboratory work may have furthered this feeling. Among other experi-

ments some attempts made during 1901-2 to prepare an

inactive thorium by the successive entrainment method are

recorded in his notebooks.44 These show 'EA' and 'PA' (electrical and photo activity) moving irregularly in

opposite directions as well as problems with leaking

electrometers.45 He felt sufficient confidence in some of

his results to publish early in 1902 a paper on 'Radio-

activity and the Electron Theory'.46 The view that 'Electrons emanating from radio-active bodies behave like

material particles and are impeded by the molecules of the

surrounding medium' Crookes now illustrated by the diffuse

photographic effects which he had obtained. In this manner

he explained emanations and radiations by the same means;

other researchers might have considered that he had not.

193

clearly distinguished these entities. Crookes accepted

Strutt's suggestion that the emission of subatomic

corpuscles was matched by the release of large positive

ions and likened this to his own earlier conclusions

concerning the electrical evaporation of metals. It

could be said that his theory included a minimal atomic

dissociation but that he was far from any consideration

of radioactivity in terms of atomic transformation or

irreversible disintegration. He believed until mid-1903

that the molecules of the surrounding air provided the

source of radioactive energy.

The young chemist Geoffrey Martin began his communi-

cations on radioactivity by expressing views akin to those

of Perrin and Crookes but he soon proceeded well beyond

them. The first of his series of letters to the Chemical News47 concerned 'Radio-activity and Atomic Weight'. In

it he broached the subject of a possible connection

between radioactivity and variable valency via a process

of ionic interchange, which he saw as the basis of both

phenomena. He thus arrived 'at the conception of very

heavy metals continually casting out into space light ions,

until finally their supply runs short or diminishes'.

Evidently no permanent atomic change was envisaged here:

if the exhausted metal were chemically 'treated with

another body full of such particles ... the heavy element

will abstract from it a sufficient quantity to replenish

its store, and thus the radio-activity increases again'.

One may possibly discern a movement towards his forth-

coming depiction of a more destructive atomic process in

Martin's letter on 'Prout's Hypothesis and Radio-active

Elements'.48 Though this simply raised the question of

whether heavy metallic impurities might be the cause of

some of the observed deviations of atomic weights from

whole numbers, it tended marginally to strengthen the

tenuous link between the subjects of the title. It may

also be recalled that R.J.Strutt published separate

articles on radioactivity and upon Prout's hypothesis in

1901. Martin's publication of most significance for the

emergence of atomic disintegration theories appeared

194

early in 1902 shortly after Crookes' paper on ultra-atomic

diffusion. Endowed with the title 'The radio-active elements

considered as examples of elements undergoing decomposition

at ordinary temperatures. Together with a discussion of

their relationship TO the other elements',49 its contents

may be described as a collection of speculations, some

well-worn others premature, all loosely linked to the

experimental basis of the hypothesis of subatomic electrons.

Martin considered that the long-sought laboratory evidence

which could combine with the work of Lockyer to support

the notion of a common 'protyle' was now at hand. In his

opinion decomposition of atoms occurred not only at the

high laboratory temperatures which caused any metal to

ionise gases but also spontaneously at room temperature.

He cited as experimental support the view of the Curies,

which they had themselves revoked shortly before, that

'radio-active matter is at ordinary temperatures giving

off electrons (and other particles?)', and put Russell's

studies of photo-active zinc and hydrogen peroxide into

the same supporting bracket. These points together with

the correlation of high atomic weight with both variable

valency and radioactivity took him beyond his former

position regarding radioactive atoms. He now supposed

that these atoms suffered 'incipient decomposition' and

were completely 'shattered' into positive, negative and

'inactive' particles; these latter 'which may be very small

indeed' comprised 'the unelectrified matter which composes

the bulk of the atom'. This statement shows that Martin

probably agreed with W.Crookes, L.Boltzmann, W.Sutherland

and others in his conception of the atom as a spherical

solid mass furnished with electrical or other appendages.50

His idea of atomic decomposition would therefore seem to

be more revolutionary than that deriving from the corpus-

cular theory of matter. Nevertheless Martin's comments

evidently covered but a small proportion of the known

phenomena of radioactivity. He later claimed priority

with the leading question 'Who first suggested that the

radio-active elements are elements undergoing decomposition

at ordinary temperatures?'51 But the appearance of his

195

paper in 1902 is perhaps best looked upon as part of a

general upsurge of discussion concerning the decomposition

of atoms.

During that year a subtly different set of ideas

appeared in a textbook of J.Stark, privatdocent at

G6ttingen University, entitled Die Elektrizitdt in Gasen.52

Stark advocated an electronic view of matter of the kind

then gaining ground among physicists; an approach which,

in part independently of radioactivity, gave hopes for the

transformation of the elements.53 Supposing that the chemical

atom consisted of equal numbers of negative and as yet

undetected positive electrons, each of which comprised a

vortex in the ether, he provided explanations of the

various phenomena of electrical conduction and chemical

valency in terms of electronic dissociation. As for the

emission of electrons by radioactive substances he conject-

ured in the first place that the electrification thus lost

could be regained by the capture of negative electrons from

the surroundings. Stark's second suggestion, that electrical

neutrality might also be maintained by the loss of positive

electricity, indicates his view that the emanations and

induced activity consisted of positively charged material

particles, presumably atoms or molecules.54 However, he

failed to make any theoretical connection between such

phenomena and his suggestive ideas on the pressing question

of the source of radioactive energy. Stark introduced his

answer to the energy problem by calling upon what were

becoming standard arguments, based on spectroscopic and

ionisation studies, against the indivisibility of the

chemists' atom. His conjecture, which was accompanied by

rough numerical estimates, that the high temperature of

many celestial bodies was due partly to the electronic

'Genesis der Atome'55 effectively inverted the view of

Lockyer and Martin that heat was the cause of elemental

dissociation. The naturally radioactive substances with

their continuous release of electrons and energy Stark saw

as remnants of this cosmical process. They were elements

which having been stable at the astronomical temperature

of their formation now possessed the property of slowly

196

dissociating and recombining into stabler forms.56 But whether these were chemically distinct entities he did

not say. As Stark completed his speculative exposition

at Easter 1902 the race towards a successful theory of

radioactivity was almost won. Rutherford and Soddy were

already watching the exothermal creation of new radioactive

elements in their laboratory.

2. A quantitative theory of atomic transmutation (1902)

I am now busy writing up papers for publication and doing fresh work. I have to keep going, as there are always people on my track. I have to publish my present work as rapidly as possible in order to keep in the race. The best sprinters in this road of investigation are Becquerel and the Curies in Paris, who have done a great deal of very important work in the subject of radio-active bodies during the last few years.

Thus wrote Rutherford to his mother in the first week of

190257 soon after seeing the latest paper of Becquerel;

he knew also that Crookes was about to publish.

Just six years earlier, shortly before the original

discovery of uranium rays, he had written of the new X-ray

photographs:

One of a frog is very good ... The Professor of course is trying to find out the real cause and nature of the waves, and the great object is to find out the theory of the matter before anyone else, for nearly every Professor in Europe is now on the warpath...58

197

Though no single scientist emerged to dominate the study

of X-rays E.Rutherford and F.Soddy, physicist and chemist,

not only stayed in the radioactive race but ran out clear

if disputed winners. The strong field comprised similar

mixed teams of P.Curie and A.Debierne, G.G.Stokes and

W.Crookes, also researchers of a single discipline such as

Elster and Geitel, or Becquerel. By late 1903 the leading

pair had forged sufficiently far ahead for one of its

partners privately to pour scorn upon such reputable

competitors.59 As Rutherford's letter shows, the situation

early in 1902 was quite different. The manuscript he had

just mailed to London was but the first of a score of

papers published during those two years, about half of

them jointly with Soddy, which established a position of

superiority only gradually.

During that period at McGill these researchers

developed a theory of atomic transformation which passed

rapidly through several phases. Fortunately Rutherford

and Soddy provided in their joint publications fairly

explicit discussions of experimental studies and theoretical

problems concerning the relationship between thorium and

ThX. This aspect of radioactivity provided them with the

most successful quantitative test of their theory. This

furrow has been ploughed deeper by successive writers who

have thereby followed the steps leading to the attainment

of the 'full' theory of 1903.60 Much is left to be said

even along these lines. However, my intention is to place

those thorium studies in a wider perspective by viewing

them both as a continuation of earlier developments and as

a part of the contemporary network of radioactive investi-

gations. Emphasis will be placed on the origins of the

theory of disintegration rather than upon its experimental

confirmation. We shall see for example that even before

the discovery of ThX a notion of chemical transmutation

in radioactivity had entered the laboratory; this was

itself related to prior discussions. It is notable that

the two known members of the uranium series and four or

five of the thorium sequence featured together in

Rutherford's and Soddy's earliest exposition of the

198

disintegration theory; that all of these contributed to

its origins as well as to its extensions is a possibility

which should not be ignored. The influence and implications

throughout the period of the theory of induction and of

the work of scientists such as Becquerel, Crookes, Curie,

Dorn, Giesel and others are also considered.

Rutherford perhaps felt that to stay the course of

radioactivity he would need more than minor chemical

assistance. By the end of May 1901 some of the most

interesting questions were chemical ones. That these had

been created by Rutherford is an indication of that

physicist's chemical leanings, the origins of which we

have sought in earlier Chapters to unearth. He had

subjected different thorium salts to the action of heat

and had concluded that the production of emanation was a

kind of chemical process highly dependent on temperature.

He was sure that radium emanation was in reality a non-

radium 'heavy radioactive vapour or gas'. And a notable

change in interpretation was that instead of viewing

excited activity as a deposit of thorium emanation

positively charged, Rutherford now considered that the

similar radium emanation 'manufactures from itself a

positively charged substance'.61 Whether or not this

material difference between emanation and active deposit

had come to the fore by virtue of evidence additional to

the chemical pointers of 1899 it is not clear. Certainly

by May 1901 there had been discussions with a chemist at

McGill but these may not yet have been constructive.

In the autumn of 1901 F.Soddy, Demonstrator in

Chemistry at McGill University since the summer of 1900,

joined Rutherford's investigations.62 However, prior to

their experimental union the partners had clashed mightily

at a McGill Physical Society debate in March 1901.

Rutherford commented in a letter to J.J.Thomson, mainly

concerning appointments, that in a forthcoming 'great

discussion' on the latter's physical corpuscular diss-

ociation theory 'we hope to demolish the Chemists'.63

According to Soddy's report to his biographer long after-

wards the physicists were quite unable to do this64 in

199

the face of his own vigorous debating-style attack.65

He spoke against the evidence for atomic dissociation

provided by Lockyer and pointed to the weakness in

Thomson's early statement on the mie ratio for cathode

ray corpuscles; the latter had stated that e might be

large as well as m small. Perhaps Soddy became familiar

with and less antagonistic towards the more recent

studies which provided separate estimates of e. His

later statement that he had always been sceptical of 'the

electrical theory of matter'66 may not exclude this

possibility; a purely electrical view of matter did not

become widely acceptable to physicists, including Thomson,67

until after 1901. Despite these theoretical disagreements

Soddy though not Rutherford's first choice as an

assistant68 no doubt seemed a good one since he was

involved in lecturing on gas analysis at McGill during

190169 and claims to have been familiar with this subject

and with the inert gases before leaving Oxford.70 A series of exciting discoveries and allied interpretations was

soon to make Soddy the champion of transmutation rather

than its challenger.

In a lengthy publication entitled 'The Radioactivity

of Thorium Compounds.I. An Investigation of the Radioactive

Emanation',71 whose abstract72 was read at the Ordinary

Meeting of the Chemical Society of London on 16th January

1902, Rutherford and Soddy described the many fruits of

their first few months of united labour. Prime among these

were two remarkable conclusions. Firstly, thorium emanation

belonged to the family of the recently discovered inert

gases. And secondly, discovered late in the day we are

told, it was not in fact thorium which produced thorium

emanation. These deductions constituted part of the

answers to the five largely chemical questions explicitly

posed; though one cannot apportion the authorship the

200

interested physicist was evidently capable of formulating,

if not of answering, each of these. Did the emanation

come from thorium itself or from a 'foreign substance';

could the almost non-emanating thorium oxide, rendered

thus by excessive heating, recover this property; was

the radioactive gas chemically similar to any known

matter; did its emission cause a loss in weight; was there

anything in the chemistry of thorium to account for its

'almost unique power' of giving an emanation?73 Many of

the experimental techniques involved in these new investi-

gations were modifications of or improvements on those

employed earlier by Rutherford. For example, in measuring

the conductivity produced by thin and thick layers of a

powdered thorium compound in combination respectively with

an air draught or screening, the researchers were satisfied

that their separate estimates of the direct radiation and

emanation emerging from the same specimen were accurate

to within one or two per cent. Their preferred method for

determining the emanation, which avoided waiting for

equilibrium, was to pass it at a known speed down a tube

along which a number of electrodes were spaced. The

successively lower electric currents detected at these

points gave a value for the emanation which agreed well

with the simpler arrangement; each method was comparative

in conception with a 10 gm. sample of thorium oxide taken

as the standard.74 These techniques, which can be traced

back to the time of Rutherford's first studies of thorium

emanation in 1899 or earlier, were vital for Soddy's

answer to the question of the chemical nature of this

entity. Able thus to estimate the emanation's quantity

by means of its radiations, while they lasted, Soddy found

that this gaseous material refused to combine with any

reagent. The conclusion which the investigators boldly

announced was that the emanation belonged to the recently

established75 family of inert gas elements.76

Whilst acquiring this understanding of its nature

Rutherford and Soddy enquired experimentally after the

emanation's source. In response to their own question

201

'Is the Emanating Power a Specific Property of Thorium?'77 they reported that samples of thorium sulphate from

opposite ends of a fractionation each exhibited the same

intensity of direct radiation and identical emanating

powers. The entailed affirmative answer, though soon to

be stifled, was backed by further evidence. For Soddy

was able in effect to smooth out the sharp differences in

emanating power between oxide specimens subjected to

different heat treatments. Such variations had contributed

to Rutherford's description of the phenomenon as a heat-

dependent chemical process. 'The Regeneration of the

Emanating Power by Chemical Processes'78 which they claimed to have achieved was impressive though irregular and

incomplete. The ordinary oxide (adopted as the standard

at 100%), after de-emanation by ignition (to 10% power),

dissolution, then reconversion to the oxide, exhibited a

partial recovery of its emanating property. Following

such a cycle via the sulphate thorium oxide turned out as

high as 40% effective, via the chloride 55%. But a

different chemical conversion tried by Soddy gave quite

different results which, however, tended to make the

situation clearer. Both ordinary and ignited thorium

oxides when dissolved then reprecipitated as the hydroxide

were endowed with enhanced powers of emanation. And in

addition these hydroxides' values rose spontaneously

during the course of a week or so from an initial in-

equality (hydroxide from normal thoria, 108%; from ignited

thoria, 128%) to exact equality at a high level (about

250% of the standard). Furthermore they were able to

clarify the effect of moisture79 thus freeing themselves

from the influence of Dorn's suggestion of a 'physikalisch-

chemischen Prozess'. Rutherford and Soddy were beginning

to realise that these chemical and physical influences

might only be superficial. They noted, for example, 'that

the cause of the emanating power is not removed by ignition,

but only rendered, for the time being, inoperative'.80 The

authors' statement that the evidence 'certainly seemed to

point to the conclusion that the power of giving an

202

emanation is really a specific property of thorium'81 is

doubly significant. Firstly its expression gives an

unusual illustration of conclusions which were already

withdrawn. Now the word 'specific' was practically

synonymous with the term 'atomic' which others such as

the Curies tended to use in this context. Thus, secondly,

the proof of a chemical transmutation may briefly have

become apparent.

It may well have appeared so to Soddy who remarked,

on looking back more than fifty years,82 that he had first realised in 1900(sic) that a genuine chemical transmutation

of thorium to an inert gas was at hand. In the published

paper one reads the cautious statement that though it was

'perhaps early' for theoretical discussion one of two

possible alternatives was:

to look upon the emanation as consisting of a gas emitted by the thorium compound. It is not necessary that such should contain thorium, it might conceivably be an inert gas continuously emitted in the radioactive state.83

But this is very similar to their better known assertions

made in the spring of 1902 which are generally taken to

mark the great innovation of atomic transformation. One

might argue instead that radioactive transmutation had

been conceived by Rutherford and Soddy as an experimental

reality before the end of 1901. Rutherford's recollection

long afterwards was that:

The great contrast in the physical and chemical properties of thorium X and the emanation gave us the first definite clue that radio-activity was a consequence of the successive transformation of elements and led ultimately to the disintegration theory...84

'Definite clue' is an ambiguous phrase; but Rutherford

unlike Soddy evidently refers to the period following the

time in about December 1901 when the idea of the direct

production of the emanation by thorium became untenable

and thorium X appeared.

In a section which forms an appendix to their first

paper Rutherford and Soddy admitted that 'since the

preceding account was written developments have been made

203

in the subject which completely alter the aspect of the

whole question of emanating power and radioactivity'.85

The revelation was twofold; its cause was their discovery

that both emanating power and direct radioactivity, the

latter so far unaltered in all experiments which affected

the former, were properties not of thorium but of a

different substance. This they labelled ThX. The conn-

ection with W.Crookes' earlier conclusions is an interesting

one which should perhaps be brought out. Rutherford and

Soddy stated, rather surprisingly it seems, that their

negative results in fractionating thorium sulphate were

obtained before they knew of his similar work.86 Yet it is true that almost everyone else who wrote on radioactivity

had referred to Crookes' important preparation of inactive

Ur and his discovery of UrX shortly after the announcement

in May 1900. Rutherford, however, left for New Zealand to

get married at just this time and although he may have

read the Curies' and Becquerel's papers to the Paris

Congress of August 190087 each of these referred to Crookes'

discoveries with uranium alone. Only Baskerville's dis-

cussion 'On the existence of a new element associated with

thoriuml88 published shortly before Soddy began these

studies, and Crookes' own publication, described the

latter's failure with thorium sulphate but his partial

success in fractionating the nitrate. However, Rutherford

and Soddy made no mention of this work on the nitrate; they

merely pointed out that the photographic methods used by

the above chemists were qualitative and incapable of dis-

tinguishing between thorium rays and those from the eman-

ation.89 Nevertheless, the McGill scientists may have owed

some debt to these nitrate studies; they were certainly

aware of them. In his paper Crookes had mentioned the

German commercial source of a highly purified thorium

nitrate; Rutherford wrote asking him to forward a request

for this material. Crookes replied welcoming Rutherford's

promised paper to the Chemical Society, stating that he too

was preparing a publication, and reporting the 'curious

circumstance' of Becquerel's finding that an old specimen ,90 of inactive uranium nitrate 'had reassumed its radioactivity.

204

At that time, in December 1901, Rutherford and Soddy were

themselves studying the spontaneous recovery or increase

of the emanating power of thoria. Now since in their

view uranium gave no emanation the question of the revived

activity of this element was an open one. Evidently there

was much to consider even beyond the late additions and

alterations which appeared; here they described to their

readers the successful outcome of a new search for ThX,

the hypothetical emanating and radiating constituent in thorium.

Crookes' attempts to correlate the electrical and photographic activities of thorium with its chemical

treatment, which had yielded only irregular results, were

thus overtaken at the end of 1901. But the passage of

Rutherford and Soddy was not smooth. Their emanation

studies began to take on a coherent form as 'it was beg-

inning to be realized' that emanating power depended on

the 'previous history', or mode of preparation, as well

as upon the chemical nature of a compound.91 But the continuing approach towards the origin of this power was

disturbed by further striking irregularities. As they

noted in their late addition, whilst the powdered crystals

of thorium nitrate were of surprisingly low emanating power

(1.8% of standard thoria), quantitatively prepared solutions

whose study they had newly taken up possessed a very high

power (about 300%) independent of dilution. They inter-

preted the phenomenon as a 'latent emanating power'92 of

thorium nitrate in the solid state. 'Simultaneously with

this observation' they remarked, it was noticed that

preparations of thorium carbonate varied enormously in

emanating power according to their method of preparation .93

They recorded a notable vagary, evidently one of many,94

in which Soddy precipitated thorium carbonate from the

nitrate by means of sodium carbonate, partially redissolved

the carbonate with nitric acid, presumably removed the

remaining solid, then reprecipitated the dissolved portion

as hydroxide using a solution of ammonia. 'The result was

remarkable: the carbonate had an emanating power of only

6 per cent, the hydroxide one of 1225 per cent of that of

205

the ordinary oxide'.95 It was perhaps equally remarkable

that the hydroxide's emanating power then decreased

spontaneously to 1/3 value after 14 days whilst the

carbonate's remained constant. They ascribed this result

to an 'accident' of the conditions but did not let the

matter rest. Repetitions of the procedure gave totally

different results: the carbonate and hydroxide precipitates

were of approximately equal low powers (about 15%) which

in the manner usually anticipated rose spontaneously during

a week or two to values of 100 to 300%. The first carbonate

of very low emanating power displayed no abnormality in its

direct thorium rays and was turned into a normal emanating

sample of the carbonate on redissolution in acid, followed

by reprecipitation. But we are told that 'The production

of preparations of such low emanating power led naturally

to an examination being made of the filtrates and washingsq6

which should contain no metallic substance whatever. This

examination was indeed a fortunate step. Its well-known

results have been seen97 as a turning point. For although

'they should be chemically free from thorium' the ammonium

nitrate/hydroxide filtrates possessed a definite emanating

power and, after evaporation, direct radioactivity too.

Subsequent closer tests of the filtrates yielded a mysterious

white phosphate precipitate, highly active and 'in very

appreciable quantities', but they were able to dismiss this

as an irrelevant impurity.98 'The evidence of long series

of experiments in two directions' afforded them 'little

doubt of the actual existence' though in 'altogether minute

amount' of 'a constituent ThX to which the properties of

radioactivity and emanating power must be ascribed'.99

There seem to have been several implications of this

conclusion. Evidently one of these was that the direct

radiations of thorium as well as its emanating power were

now in question. Another relates to parallel studies

under way in Paris. Curie and Debierne had announced in

July that ordinary water could readily acquire induced

radioactivity,100 or 'emanating power' in Rutherford's

and Soddy's terms. In the light of Curie's experiments

and theories it was surely to be expected that all

206

filtrates from thorium or radium would posses an emanating

power which might in turn be transformed into direct

radiation: Rutherford indeed found that the solid traces

in thorium filtrates emitted such rays. But the fact that

this activity could be made far higher than that of thorium

itself, by factors of up to 1800, appears to confirm ThX

after the fashion of its predecessors Po, Ra, UrX and

others. Rutherford and Soddy noted that its inconsistent

chemical properties, for example solubility in and prec-

ipitation by hydroxide or phosphate, could be explained by

entrainment of the minute amounts of ThX present. And they

made the identification of ThX safer by the technique of

penetration analysis of the radiations, a method which was

becoming increasingly important. The rays from the active

residue were identical with thorium rays and different

from the several other varieties; furthermore this residue

produced an emanation whose activity decayed at a rate

identical to that from thorium. Having thus attributed

the entire radioactive phenomena of thorium to ThX they

considered naturally that the preparation of a totally

inactive thorium would form a desirable confirmation. To

this end they reported their actual attainment, by repeat-

edly washing with water, of a 20% reduction in the activity of thorium.101 However, the path leading from this point,

like that leading to it, was not a straight one.

During the course of their later experiments of 1901

Rutherford and Soddy achieved the beginnings of a unific-

ation of the two major aspects of thorium's, now ThX's,

radioactivity. That the 'straight line' radiation of

thoria remained constant throughout wide fluctuations in

its emanating power had at first served 'to bring out the

fact' that these two powers were independent.102 The apparent difficulty of reconciling this with their con-

clusions that each was connected with the thorium atom or

molecule was soon to be eased. In directing attention to

the 'straight line radioactivity, which is generally un-

affected by these changes of conditions and previous

history' in order to follow 'the progress of the removal

of the active material', Rutherford and Soddy flatly

207

contradicted their initial statement with the comment

that 'The two phenomena are undoubtedly connected'.103

The empirical correlation, whioh had arisen from the use

of solutions, was soon to become of great theoretical significance. From the modern viewpoint one sees a con-

tinuous emission of emanation from ThX which is itself

continuously produced by thorium; when formed within the

various solid compounds of thorium the actual rate of

release of the gas is complicated greatly by a temperature-

dependent process of occlusion. By April 1902 these scientists suspected all this.104 But in December 1901 they thought that 'the surprising uniformity' of the

emanating powers of variously treated thorium compounds,

despite the known loss of a large proportion of the supposed

emanating source ThX made the process appear:

rather as the result of a dynamical change, possibly in the nature of a chemical reaction where the active mass of emanating material is a constant, than as the property of a peculiar kind of matter in the static state, additive with regard to mass.105

Here 'active mass' is a term drawn from chemical reaction

kinetics rather than radioactivity; 'effective mass' may be a clearer substitute.

The vision of a strange chemioal reaction which they

saw here was obscurely mirrored in Rutherford's parallel

and neglected researches on the excited radioactivity

produced by the emanations. During the Christmas vacation

of 1901 at about the time of completion of the joint paper

on thorium emanation and ThX he presented to the American

Physical Society in New York papers on the 'Transmission

of Excited Radioactivity'106 of thorium and radium, and

on 'Excited Radioactivity and Ionization of the Atmosphere,107 To the physicists Rutherford revealed some of the mechanisms

he had in mind. Regarding the question of the positive

charge of excited activity Rutherford noted his own earlier

208

explanation that this might be caused by condensation of

the emanation around the positive ions produced by its

radiation, and Thomson's alternative idea of an average positive charge left by the temporary loss of a corpuscle.108

The version he preferred, however, which could better

account for the suppresion of the charge effect at low

pressure was an extension of Thomson's view: the emission

of a material corpuscle or electron at its high velocity of 1010 cm./sec. would impart an opposite impulse to the

remaining positively charged molecule. This could fling

it contrary to the field against the positive electrode.109 Presumably this molecule would have to pick up another

electron or a negative ion before actually adhering to the 10 electrode to form the active deposit. Thus, like Becquerel,1

Rutherford saw the first step in the production of excited

activity as the violent emission of an electron. But he was as yet prepared openly to discuss the notion of a

minimal disintegration only. Rutherford thought that the

radiation from excited radioactivity was caused by the recoil vibrations within each deposited molecule. 'Da es unwahr-scheinlich ist, days innere Schwingungen von Molekulen

verschiedener chemischer Natur sowohl nach Charakter wie

naoh Dauer dieselben sind' it was to be expected that the

induced activities from radium, thorium and perhaps

atomospheric air would differ both in duration and pene-

tration, as they did.111 However, Rutherford's explanation

of the electrical character of its transmission left open

many questions regarding excited radioactivity.

These can be isolated and grouped into three areas.

Firstly there were problems concerning atmospheric excited

activity. Whilst a chemical audience read or heard that

the existence of an atmospheric emanation was most probable112

to the physicists Rutherford spoke equivocally. Although he

accepted that its electrical properties and the duration

and penetration of its radiation appeared to identify an

atmospheric deposit he queried, on the basis of two

observations, Elster and Geitel's assignment of the cause

of this deposit to an atmospheric emanation. The conduct-

ivity of a sealed mass of air failed to decline, remaining

209

constant for a month; and carbon dioxide exhibited the

same properties as air.113 The contradiction was soon to

be eased114 by C.T.R.Wilson's conclusions, based on

pressure-variation experiments, that the walls of the

vessel contributed to the 'Spontaneous Ionisation of

Gases'.115 But Rutherford's only visible step towards

clarification at the end of 1901 was his statement that

experiments were in progress to see whether any of the components of the atmosphere, prepared chemically, dis-

played sufficient ionisation and excited activity to

account for the observed atmospheric effects.116 Whilst it

is not entirely clear how these might affect the problem

such experiments seem relevant to statements published

with Soddy on thorium emanation. Their joint abstract

boldly denied that the emanation might be a mere sport of

induction:

The possible explanation that the emanation is the manifestation of excited radioactivity on the surrounding atmosphere was shown to be untenable by a crucial experiment.117

Vernon Harcourt who read this at the meeting and who had

been one of Soddy's referees at Oxford seems nonetheless

to have been unimpressed. He pointed in a different

direction to Russell's work on hydrogen peroxide emana-

tions.118 However, in the full paper Rutherford and Soddy

had dismissed the Russell effect as having no connection

with radioactivity and had written with more caution

regarding induction. They had performed certain experiments

which proved the amount of emanation emerging from a source to be independent of the type of gaseous carrier and others

which showed that the emanation differed from the trans-

porting medium in its resistance to any chemical attack.

Bitt they admitted that these facts still left open the

possibility that one of the inert gases known to be present

in the atmosphere might be 'rendered radioactive in the

presence of thoria'.119 It is of interest that the authors referred to experiments in progress which appear comple-

mentary to those mentioned by Rutherford in the context of

a supposed atmospheric emanation; they hoped to measure the

210

emanating power of a specimen placed in a current of gas

as free from air as possible. No results were published;

perhaps this test came to appear unnecessary during the

following months.

A second group of problems relating to excited

radioactivity and its transmission which existed at the

end of 1901 may be loosely described as chemical. Whether

Rutherford's complete avoidance of any description of

thorium emanation as an elemental inert gas was deliberate

one can only speculate. It is notable however that he

expressed doubt as to the 'Ausstramungsanschauung' or

emanation view of atmospheric effects in favour of the

'Elektronhypotheses.120 Yet he did not say what variety

of molecules in the atmosphere might be supposed to emit

the electron. In his account of 'Versuche fiber erregte

Radioaktivitdt' dated mid-January 1902121 Rutherford confirmed that the rate of decline of the radioactivity

excited by thorium suffered no change when this deposit

found itself in acid solution. Having deduced therefrom

that the decay was a process occurring within the active

substance he gave no further indication of what this

substance might consist. And, thirdly, the process of

radioactive decay was itself made to seem more complex by

his discovery122 that the weak excited activity produced

upon a wire by brief exposure to thorium in fact spontan-

eously increased in intensity for an hour or two before

declining at the known rate. The activity excited by

radium showed something similar.123 The curves were unaffec-

ted on heating the wire or plate to redness; nor could any

secondary emanation be detected. Rutherford concluded

without elaboration that this pattern of changing radiation

was caused either by a gradual molecular rearrangement or

chemical reaction, or a second excited activity produced

upon the surface by the first, presumably without an

intermediate emanation. Evidently the phenomenon of a rise

in activity could not be explained so readily as the usual

decay. It is interesting to speculate whether that dis-

covery had an influence upon the emerging theory of atomic

211

transformation which was to bring these diverse facts

rapidly to order. This spontaneous rise in activity was

the first of three which appeared almost simultaneously.

The second occurred in Becquerel's deactivated uranium.

And the third involved the vital extraction of ThX. All

three took their place in the famous joint publication

which followed within months.

Rutherford and Soddy probably read Crookes' letter

upon their return to the laboratory after Christmas 1901.

On checking their partially deactivated thorium, which

they no doubt would have done as a matter of course, they

found that like Becquerel's nearly inactive uranium it too

had regained its full direct radioactivity. That of the

extracted ThX had declined almost to zero. They may have

seen at the same time the paper which Becquerel read on

9th December 1901. Rutherford mentioned this scientist

when he wrote home on 5th January and in the second joint

paper with Soddy124 where credit was assigned for the

discoveries of the respective revival and decay of the

activities of deactivated uranium and activated barium.

Becquerel's paper contained far more than this. He was no

more satisfied than were Rutherford and Soddy merely to

report an empirical discovery; some of the ideas he expressed

may have seemed highly significant at the time. As we have

seen125 Becquerel was first to publish the valuable sugg-

estion that the mechanical recoil from a corpuscular

emission might play a part in radioactivity. He explained

radioactive induction in alternative ways either by direct

contact or by the deposition of the large positive particles

ejected on the emission of electrons, which remnants were

themselves subject to further division. The recovery of

uranium's activity he attributed to self-induction which

might constitute a 'transformation moleculaire'. By

ignoring his assumption of a radioactivity induced by

contact or direct rays one can extract the consequence that

the rise in uranium's radioactivity is entirely due to its

own disintegration into smaller atoms themselves radio-

active. Doubtless it could also have been deduced that

there might exist a linked series of distinct and

212

radioactive chemical products and that the apparently

constant maximum activity of uranium or thorium was a

resultant or equilibrium value of these changing

activities. By the time Becquerel's paper was published

Rutherford and Soddy already had an obscure knowledge

of such a chemical series, thus: ThX---) emanation—>first

active deposit (---? second active deposit); directly after

Christmas they added with their own discovery the vital

initial stage Th---ThX. Upon the basis of the quantit-

ative experimental examination of this step they were to

argue their theoretical case.

Thomson's unhappy news regarding Rutherford's

candidature for the Royal Society126 arrived at McGill early in May some days after Rutherford and Soddy had

sent off their second joint paper to the Chemical Society;

its confident title was 'The Radioactivity of Thorium

Compounds.II. The Cause and Nature of Radioactivity'.127

It is fortunate that they there included a brief history

of the events; (changes with the passage of time were now

of considerable importance. We are told that on reexamin-

ation after three weeks the thorium hydroxide which they

had deactivated to only 36% of the normal value had com-

pletely recovered, that the ThX residues 'had almost completely lost their original activity', and that 'At this

time, M.Becquerel'e paper ... came to hand':

A long series of observations was at once started to determine: (1) The rate of recovery of the activity of thorium rendered less active by removal of ThX, (2) The rate of decay of the activity of the separated ThX. 128

Using chemical methods similar to those employed previously

they obtained 'numerous series of observations made with

different preparations at different times'. Apart from

'the difficult questions' of 'initial irregularities' in

the first few days of each 3 to 4 week series, and an

appreciable unseparated or inseparable 'residual activity'

left in the thorium, these rise and decay curves matched

perfectly. They fitted respectively the related equations

213

ItiIo = e -Xt and It/Io = 1-e- >t with the same X. The

authors noted explicitly129 that these were identical in

form with the pair of equations which Rutherford had dev-

eloped in 1899 to describe the asymptotic rise of current

observed on steadily passing gaseous thorium emanation into

a vessel, and the geometric decline of activity which

ensued once this flow was stopped. The explanation which

they formulated may seem startling perhaps because it

could be 'put to experimental test very simply'. It was

this which made the Th-ThX relationship the main spearhead

of their theoretical claims. Like the arrival of particles

of emanation in the vessel 'the active constituent ThX is

being produced at a constant rate1130 and similarly its

activity then suffered a geometric decay. Thus the 'normal

or constant radioactivity possessed by thorium is an

equilibrium value', a balance of the two processes. One

of these effects, the decay of activity of the separated

ThX, was directly observable and the other, a continuous

growth of ThX itself in the thorium freed from it, almost

so. Having refined the separation techniques they proceeded

as follows. A double precipitation of thorium as hydroxide

from a solution of the nitrate left the ThX in the filtrates.

Evaporation of these gave a measure of the maximum or

equilibrium value of ThX's activity. A third precipitation

of the thorium after redissolution confirmed that it now

contained a negligible amount of ThX. Several thorium

hydroxide precipitates thus initially freed from ThX were

later dissolved and reprecipitated after different periods

of time from a few hours up to one month. The fact that

the new filtrates always possessed activities agreeing

with the normal recovery of deactivated thorium131 provided

a striking confirmation that 'The process of production of

ThX is continuous' and that this transformation was unaffected

by the separation procedure.

One of the major differences between the observed

measurements and those expected for a continuous arrival or

production of active material followed by a geometric decay

concerned the initial portion of each curve. The discrepancy

214

was complementary: the activity of freshly removed ThX

began by rising for a few days (by about 10%) before

decaying geometrically, and the deactivated Th actually

lost activity (about 15%) for the same period before

rising asymptotically. The thorium deactivated by

removal of all its ThX still possessed 46% of the normal

activity - a second discrepancy since according to theory

thorium should be inactive. Perhaps these were welcome

problems, for Rutherford and Soddy were able to use them

to justify their hypotheses at the same time helping to

tie together the entire incipient transformation series.

ThX was known to create excited radioactivity, via an

emanation, 'on surrounding inactive matter' so that the

46% 'residual activity of thorium might consist in whole

or part of a secondary or excited radioactivity produced

on the whole mass of the thorium compound by its association

with the ThX1.132 In brilliant style they tested this

supposition by preventing the ThX from producing any excited

activity. A series of 23 successive dissolutions and

reprecipitatione in 9 days to remove ThX from one tortured

thorium hydroxide specimen allowed the excited activity

initially present to decay completely, as the constant

minimum of their readings showed. This still left the

thorium with 25% of its normal activity - a discrepancy

smaller but harder to accomodate as will be seen. When

left to recover from this treatment the activity of the

specimen rose without the initial fall; to clinch the point

they showed that the 'difference curve' between ideal and

observed curves for ThX, both rising and falling, had a

half-value time of 12 hours which was equal to that for

the known decay of the 'ordinary excited activity'. There

was thus 'no reason to doubt that the effect is the same

as that produced by the thorium emanation, which is itself

a secondary effect of ThX'.133 The production of thorium

emanation from ThX took its place in the new scheme: just

as thorium produced the non-thorium material ThX, a 'further

transformation° of the latter resulted in the continuous

emission of a radiating inert gas; the sometimes highly

215

irregular results could readily be attributed to occlusion

or changes in crystal structure.134 Furthermore an analysis of the radiations showed that specimens from which the

emanation could not escape possessed the expected high

proportion of excited activity. Rutherford and Soddy were

not yet ready to say whether the production of emanation

imitated the primary change of thorium to ThX in proceeding

at a rate independent of the conditions;135 they were

prepared only to state that this applied to the decay of

the emanation's radiation. This aspect of thorium had

given rise to the first notion of its transformation to an

inert gas; but now the emanation took second place both in

sequence and in certainty. Rutherford and Soddy extended

their discussion beyond the confines of thorium: they went

so far as to suggest that the apparently constant activities

of uranium and radium too were the resultants of chemical

changes 'also independent of the conditions'. And describing

the radiated energy as a loss after each such change from a

supposed store within the system they came to the impressive

conclusion that 'All known types of radioactivity can thus

be brought into the same category'.136 It may be noted in

return that considerations of several varieties of radio-activity had contributed to the theoretical development.

It is true that the remarkable quantitative confirm-

ations with thorium and ThX increased the power of this theory far beyond any other. However, in its explanations

there were weaknesses. Some of these led to a broadening

of its success but others to criticism from without. In

a final discussion Rutherford and Soddy tried to justify

the applicability of expressions such as 'the chemical atom

in certain cases spontaneously breaking up with evolution

of energy' and 'sub-atomic chemical change'137 to radio-

activity but their chemical evidence was not strong. ThX,

the most important member of the thorium decay series from

a theoretical point of view, was perhaps also the least

certain. In their section on 'Chemical Properties of

ThX1138 radioactivity measurements correlated with precip-

itations formed the only evidence they could muster to

justify the statement that 'There can thus be no question

216

that both ThX and UrX are distinct types of matter with

definite chemical properties'.139 Though they believed

that 'transmutation' was the origin of these materials

such an expression did not appear in print; nor did they

describe these substances as 'elements' known or new. Yet

claims of the latter kind based on similar evidence had

played a vital part in radioactive studies since their

early days. Radium was the only thoroughly confirmed new

radioelement. It had been preceded on the scene by polonium

and had been followed by actinium and radiolead all of which

were of disputed status. As for UrX, Crookes and the Curies

thought it might be identified as actinium and Debierne

interpreted ThX similarly. As Rutherford and Soddy noted140

there was also the non-radioactive splitting of thorium by

Brauner and Baskerville to consider. Since 1899 each

claimant had to refute the possibility that his supposed

new radioelement was merely an induced activity. We have

seen141 how Hofmann defended his radiolead against

F.Giesel's accusation of induction. At the time of

Rutherford's and Soddy's first publication, in January 1902,

Hofmann reported his own results on the fractionation of

thorium. These led him to suppose that thorium itself was

really inactive and that it possessed only a temporary

radioactivity induced by uranium; he claimed to have success-

fully prepared inactive thorium from minerals which contained

no uranium. This work142 was not of a high electroscopic

standard143 and was not mentioned by Rutherford and Soddy

in their second publication. However, as with the emanation

previously, they were at pains to refute any suggestion

that the invisible ThX was no more than the manifestation

of a temporary activity induced by thorium upon some portion

of the neighbouring materials. They described such a view

as 'quite untenable'; if it were true any precipitation of

thorium should leave active residues in solution whereas

experiments showed that carbonate, oxalic acid, and

phosphate precipitated thorium leaving non-active solutions;

only ammonia was capable of the separation. It should

perhaps be recognised that this point taken in isolation

217

may have seemed less than convincing to others; for they

failed to dismiss explicitly the possibility that ThX was

a trace element in thorium which suffered temporary

activation by induction.

Rutherford and Soddy at this stage may themselves

have entertained the idea of direct induction in order to

explain a second questionable aspect of their theory.

What was the cause of 'The Non-separable Radioactivity of

Thorium',144 the 25% of the radioactivity remaining, when

both ThX and excited activity were absent, which did not

decay appreciably with time? One hypothesis of the two

they described involved the production of 'a second type

of excited activity' with a 'very slow rate of decay' by

ThX. It should be possible to observe this decay if

thorium were continually freed from ThX over a very long

period. Notably, their assumption that 'it will not be

possible to free thorium from this activity by chemical

means' suggests that by 'excited activity' in this instance

they meant a radioactivity directly induced into the

thorium. And since they referred to it as a 'second type

of excited activity ... similar to that known',145

Rutherford and Soddy may also have wavered towards an

induction view of ordinary excited activity. They appear,

at first sight, to have avoided the consequences of such a

belief by instead accepting a second hypothesis: the

initial transformation gave two products rather than one.

The constant residual activity would accordingly itself

be an equilibrium value and the other substance continually

produced by thorium should be chemically separable from it.

Magnetic analysis of the radiations tended to support this

view for the residual activity consisted only of non-

deviable rays whereas both ThX and the excited activity

emitted mixed radiations.146 But the main reason for their

conclusion regarding thorium's residual activity, as they

said,147 was the work of Soddy on uranium where both

emanation and excited activity were absent. His active

investigations of that element had probably been induced

by Becquerel. These were thus pursued throughout the

course of the ThX studies and contributed directly to the

218

early transformation theory generally associated exclusively

with thorium.

Soddy concluded his companion paper on 'The Radio-

activity of Uranium'148 by turning a current criticism

of his own emanation studies against the researches of

another; he asserted that the diffusing emanation from

uranium discussed by Crookes 'is not a radio-active substance

in the accepted sense of the word' but 'an agent similar to

hydrogen peroxide in its photographic action'. And Soddy

began this piece by announcing that the 'inactive Ur'

prepared according to Crookes was actually not inactive

electrically but only photographically; it emitted a non-

deviable and readily absorbed 'alpha' radiation and thus

possessed a 'residual activity' similar in character to

that of thorium. The same questions arose here:

1. Is this residual activity to be regarded as a secondary radiation produced by the presence of UrX? Or,

2. Is it caused by a distinct material substance capable of chemical separation? 149

According to the corresponding joint paper one might expect

definite answers; these were provided but on the basis of

insecure evidence. Soddy considered the first view to be

improbable. He kept uranium free of UrX for three weeks

by barium sulphate precipitations and then compared it

with Ur not so treated. The alpha rays in one sample had

thus been given three weeks to decay but Soddy could detect

no difference between the two. He concluded that 'it takes

at least a year to decay to half value' which was 'not in

accordance with what is known of the nature of excited

radio-activity'.150 But what indeed was known of excited

radioactivity? F.Giesel's comment151 which appeared two or

three months before that of Soddy illustrates the difficulty.

Having found that a radioactive preparation of lead was

still active after a year he yet refuted Hofmann's idea

of a new element; although induced activity was usually

'relatively soon lost' Giesel considered it possible that

the extreme conditions of induction - great excess of

radium and a year-long exposure - might result in an induced

lead of greater activity and longer duration than obtained

219

formerly. The fact that Soddy's argument requires the

existence of directly induced radioactivity seems to

indicate that he too believed in its reality at this time.

However, he had no particular interest in maintaining such a view.

For his second hypothesis regarding uranium's residual

activity involved the attribution of these alpha rays to

'a second distinct type of matter'. Soddy tells us it was

Rutherford's idea that the Curies' polonium 'fulfils in

almost all respects the functions of this hypothetical

constituent'. He expected that the known slow decay of its

activity would be observable 'after, but not before, its

separation from the uranium producing it'.152 One might represent the suggested process as Ur<roUrX . Presumably Soddy saw the constant level of activity, exhibited before

the hypothetical separation, as an equilibrium value. He

thus reawakened the polonium controversy after the Curies

had recently settled with Giosel that polonium was 'une esp6ce de bismuth actif'.153 W.Marckwald's paper of June 1902 'Veber das radioactive Wismuth (Polonium)'154 provided evidence which Soddy regarded as most important for his theoretical views.155 The method of electrolytic displace-ment in solution gave Marckwald his success in separating

from radioactive bismuth a particularly active substance

emitting only absorbable rays which were undiminished in

intensity after several months. According to the assumptions

of the Curies, to whom Marckwald referred, only true radio-

active elements possessed a permanent activity. Soddy's

theory on the other hand required polonium to be such an

element but with an activity which must decline. In his

paper on uranium presented in May 1902156 Soddy, however, reported that his attempts to separate a second constituent

either from uranium or from thorium had so far failed. His

continuing efforts to this end were soon to be overtaken by

the development of a third hypothesis which demanded their

failure as a logical consequence. But first let us consider

further ramifications which underlay the publication of the

original 'disintegration or transmutation' theory of

Rutherford and Soddy.

220

Neither of these crucial terms appeared in the second

joint communication which contained the core of that theory.

At the end of April 1902, when this paper together with

Soddy's on uranium were mailed, Rutherford caused Crookes

to become the first scientist outside McGill to see those

expressions on paper. Rutherford began his letter157 by

thanking Crookes for copies of the latter's two Royal

Society papers, which were probably those on 'Radio-activity

and the electron theory'158 and on 'The Stratifications of

Hydrogen'.159 In his discussion of vacuum-tube phenomena Crookes expressed the views that what he had once called

'Radiant Matter' now passed as 'electrons' or 'atoms of

electricity' which were the same as Kelvin's 'satellites'

or J.J.Thomson's 'corpuscles', that only a few of these were attached to each 'material nucleus or atom of matter'

to constitute a chemical ion, but that there was nevertheless

a'protyle' basis to matter.160 Perhaps these and earlier

conjectures played a part in persuading Rutherford that Crookes was a suitable recipient of the request 'to

facilitate the publication of the paper if difficulties

arise over "atomic views". The fact that publication was

achieved does not imply that Crookes accepted Rutherford's

conclusions, as will be seen. In his letter Rutherford

briefly summarised the equilibrium view of thorium's

activity, noted the similarity of uranium and radium to

thorium in this respect, and explained in a well-known

passage that:

All these processes are independent of chemical & physical conditions & we are driven to the conclusion that the whole process is sub-atomic. Although of course it is not advisable to put the case too bluntly to a Chemical Society, I believe that in the radioactive elements we have a process of disintegration or transmutation steadily going on which is the source of the energy dissipated in radioactivity.161

These words thus supplement the published 'General

Theoretical Considerations' regarding 'sub-atomic chemical

change'162 where the authors were doubtless also constrained

by the attack which the Curies had recently launched upon

Becquerel. Rutherford's usage of the expression

221

'disintegration or transmutation' may perhaps be interpreted

on the physical side by comparing it with the respective

alternatives of emission or rearrangement of the electrons

comprising the corpuscular atom.163 On the chemical side

the phrase seems to fit Rutherford's and Soddy's published

statements that thorium might undergo the subatomic version

either of a chemical 'decomposition' or a idepolymerisations

to produce respectively either two or one non-thorium

radiating substances.164 From the advanced position which

they held in April 1902 Rutherford and Soddy could see

novel problems and fresh ways of solving them.

The difficult question of residual activity, which had

led to the idea of 'decomposition' into two products, was

soon to be given a definitive answer. Attained apparently

by the head and not the hand the revised transformation

theory which they put forward was both a solution and a

synthesis. In November 1902 the second of Rutherford's

and Soddy's joint papers, slightly altered in structure,

appeared for the first time in a leading journal of physics.

To their account of 'The Cause and Nature of Radioactivity.

Part II'165 was appended a brief section which contained a far-reaching theoretical modification. Trenn166 has

argued cogently against Romer that the new view arose within

weeks, rather than months, of the submission of the old.

One may also note that the revision contains an elementary

error; it may therefore have been hurriedly penned. Possibly

one of its several consequences initially led them to it.

Instead of assuming 'as the simplest explanation' that

radiation was 'preceded by chemical change' their new inter-

pretation was that 'Radioactivity may be an accompaniment

of the change'.167 The apparently accidental correlation

between the radiating and the emanating powers of ThX, which

both suffered a geometric decay to half-value in 4 days, now

followed logically from the new theory; two pairs of decay-

rise curves thus became one. Instead of viewing the decay

in Hertzian or vibrational terms, as the loss of excess

energy mainly in the form of soft X-rays from a freshly

formed product, they now saw the phenomenon in a quite

different light. The decline of activity obeyed 'the simple

222

law of chemical change' according to which a single

substance is transformed at a rate directly proportional

to the amount present. The observed 'decay' was thus

essentially a dissipation of the substance itself; a lasting

view. On its basis they tried to explain the seeming

permanence of the residual activities: 'In the primary

change the amount remaining is infinitely great compared with

the amount that alters in a short time, and therefore the velocity of reaction is constant'.168 But this account seems unfortunate since the amount of material present in any

reaction following that law makes no difference to its

proportional rate of disappearance; reasons for the dis-

tinctive slowness of the primary change were hard to find.

Nevertheless the success of the newer theory is further

marked by its consequences that both uranium and thorium

should themselves in fact be truly radioactive. Accordingly,

Soddy's as yet unsuccessful path to an inactive uranium

should not exist, and that to a second 'decomposition'

product need not.

It would also seem to follow as a probable implication

of the accompaniment theory of radioactivity that each

radiating material should be in the course of producing a

substance different from itself. The statement of Rutherford and Soddy169 that the changes in uranium detectable by radio-

activity 'appear to be at an end' with those causing the

radiation of UrX is thus an interesting one. For it appears

to suggest either the production of an inactive UrY or,

since UrX was exceptional in giving for a period of 'many

weeks' only the 'cathode rays', a total corpuscular dis-

integration reminiscent of that proposed by Becquerel. When

applied to thorium the assumption that each active element

was creating another forced open the gateway to disintegration

series of increasing length and complexity. At the same time

the theory of induction began to recede. Rutherford's and

Soddy's first complete and public rejection of the

'Radioactive Induction' by irradiation or contact accepted

by others appeared early in 1903.170 However, the worried note which Soddy wrote to Rutherford in the autumn of that

year gives an indication both of their deep involvement

223

with the question of induction and of how difficult it was

to disprove its existence. Soddy's discovery that a sealed

glass tube of radium emanation produced upon a brass

electroscope temporary radioactivity lasting two or three

days provoked his opening remark:

The new fact, concerning which, or the possibility of which, you were always harping when I was prone to be too positive in the statement of the disinteg-ration theory has I fear arrived.l71

He could evidently see no fault in his own technique and

managed to turn the result against a rival, as was his wont,

wondering 'how the dickens P.Curie managed to measure the

decay of the Ra emanation by this means ... unless the

effect is peculiar to brass'.172 Nevertheless, the theory

of induced or artificial radioactivity died completely

within the next few years to be resurrected decades later in

entirely different circumstances.

In mid-1902, only a few weeks after he had framed with

his partner the new accompaniment version of the transform-

ation theory, Rutherford made an explicit denial of directly

induced radioactivity in so far as it related to 'Excited

Radioactivity and the Method of its Transmission'.173 It

has not been clearly recognised that in this same publication

the near full-grown flower of the final modification of the

disintegration theory was already evident. Rutherford's

investigations of the comparatively long-standing questions

of thorium emanation and its active deposit here provided

a pictorial account of the disintegration process. Such

studies had led during 1899-1902 to a situation in which

three related points required explanation, the almost

complete concentration of the excited activity upon a

negative electrode, the suppression of this effect at low

pressures, and new experiments174 which showed that even

with powerful electric fields some few percent of the

carriers moved in a direction contrary to that of the

majority. To recall and to expand upon the developments

of those years, Rutherford had relinquished his own early

condensation hypothesis for an extended version of an idea

of J.J.Thomson. The latter had privately suggested in

April 1901 that the loss of an electron left a positive

224

charge whose mean magnitude depended on the speed of

neutralisation by negative ions; the pressure-dependent

balance between the number and mobility of these ions in

turn determined this speed. Rutherford's additional

contribution, made by December 1901,175 was to suppose that a recoil at the moment of separation of the electron

gave some carriers a sufficient velocity to reach the

repelling electrode. This mechanism satisfied all of the

above three points. However, at its very time of initiation

one sees the beginnings within the scheme of a serious dis-

crepancy. The problem was the lack of a corpuscular type of

emission from certain radioactive substances in particular

the emanations which theoretically needed it most. Thus in

May 1901 Rutherford had published the remark that radium

emanation emitted absorbable X-rays (alpha rays) without

mentioning any other radiation. Moreover, discussion of

the nature of the rays from the two emanations was con-

spicuously absent from his later account, dated March 1902,

of research performed with Miss Brooks on the 'Comparison

of the Radiations from Radioactive Substances' although both

substances were actually listed for examination.176 And in June, as an explanation of atmospheric excited activity,177

Rutherford again invoked the hypothesis of 'the expulsion

of a negative electron' from some aerial 'carrier' giving

as its only justification the fact 'that all the radioactive

substances, thorium, radium, and uranium, as well as the

excited activity due to thorium and radium, possess the

property of spontaneously expelling electrons'.178 But this

claim rings rather hollow in view of earlier, contemporary

and later events; and it was soon to disintegrate completely.

At the end of July 1902 Rutherford finally confessed 'I was

at first inclined to suppose that the particle expelled from

the emanation was a negative electron' but that 'a close

examination' had detected none.179 The gamma rays might have

answered here, see below, but presumably he could not detect

these either. Still requiring such an emission of negative

particles from the emanation he found it in a new inter-

pretation of the alpha radiation. We are told that he had

been recently led 'by a mass of indirect evidence', largely

225

concerning absorption characteristics, to the conclusion that

alpha rays were not after all soft X-rays. They consisted

instead of streams of rapidly moving and as yet undeflected180

particles of atomic size.181 Rutherford noted the earlier

suggestion of Strutt, taken up by Crookes in his recent publi-

cation, that the alpha rays might be positively charged atoms

and mentioned Wien's work on canal rays which also were pos-

itively electrified.182 However, he used the recent results

of electrolytic studies on radium solutions as support for

the adoption of the negative charge which his theory required.

The alpha rays were thus negatively charged atoms ejected

from radioactive substances. One can observe the considerable

contribution of the chemical transformation theory to inter-

pretations of the phenomena of 'Excited Radioactivity and the

Method of its Transmission'. For Rutherford now understood

that like thorium and ThX the emanation 'consists of matter

in an unstable state' which in producing the material excited

activity suffered a 'chemical change'. Conversely, a crucial

influence of studies of excited activity upon that theory is

illustrated by Rutherford's remark that 'The change consists

in the expulsion of a negative particle from the neutral

molecule'.183 This statement provided a final, permanent and

mechanical link between chemical change and radiation within

the accompaniment theory of atomic disintegration.

How Rutherford succeeded during the autumn of 1902 in

demonstrating the characteristic positive electrification

of the atomic particles constituting alpha rays instead of

the expected negative charge is a well known story.184 His

'Magnetic and Electric Deviation of the Easily Absorbed

Rays from Radium' was announced early in 1903. It was closely

followed by the extension of these results and ideas towards

the important generalisation that in the various series of

changes the alpha rays 'are in all cases the first to be

produced'.185 It appears, however, that other striking

experimental discoveries which arose at this time had a

more profound influence than such developments upon the

views of contemporary scientists regarding the mysteries

of radioactivity.

226

CHAPTER 5

RECEPTIONS, GENERALISATIONS, SPECULATIONS

1. Reception of the disintegration theory (1902-3) The year 1903 was especially important ... Pierre Curie demonstrated the astonishing discharge of heat by this element [radium], which nevertheless remained unaltered in appearance. In England, Ramsay and Soddy announced a great discovery. They proved that radium continually produces helium gas and under conditions that force one to believe in an atomic transformation ... It furnished us the first example of a transformation of atoms.l

This later account of Marie Curie hints at the fact that

she and Pierre were not easily persuaded of the validity

of the disintegration theory of radioactivity and indicates

the evidence which to them appeared crucial. But the story

of that year is more complex than this record shows.

Important and spectacular experimental discoveries were

announced in rapid succession: the condensation of the

emanations in November 1902, described more fully in 1903,

by Rutherford and Soddy; deflection of the alpha rays in

February 1903 by Rutherford; the spontaneous emission of

heat from radium in March 1903 by P.Curie and A.Laborde;

scintillation effect of the radiations in March 1903 by

W.Crookes and others independently; and the production of

helium from radium, in July 1903. Although the last of

these certainly affected the views of the Curies it seems

that the previous discoveries and experiments were not

without influence. In tracing the development of this

period, partly chronologically and in part by taking up

parallel threads, we shall see that scientists with

different interests received these discoveries and the

associated theoretical advances in various ways. The

arguments presented by Rutherford and Soddy during 1902

were intricate, contained late appended revisions, and

could easily be misunderstood. The new discoveries

immediately aroused a tremendous scientific and general

interest in radioactivity. This in turn may have hastened

the publication of the powerful and lasting notions at

227

which Rutherford and Soddy had arrived before the spring

of 1903. Although by that time they had the theoretical

side effectively sewn up it was necessary for them to

campaign vigorously against misinterpretation and opposition

during the rest of that year; this they did in what was more

than a mere defence.

The Curies were acknowledged authorities on radio-

activity. The acceptance of the basis of the disintegration

theory by them seems therefore particularly significant.

In tracing its course we shall see that this acquiescence

involved the almost complete overthrow of their own published

views of 1901-2. This occurred despite a tendency, discern-

able after their attack upon Becquerel's disintegration

theory in January 1902, to keep their theoretical conclusions

as general as possible. In the period following that event

P.Curie attempted to remedy his proclaimed ignorance of the

experimental laws of dissipation of radioactive energy.

Moving away from the study of spatial arrangements, which

may not have provided regular results, he investigated more

closely the effects of time and temperature. Curie began

to eliminate the problematical effect of a cooling jacket

in absorbing the relevant rays by the ingenious method of

determining conductivities produced not in the air but in

the surrounding liquid itself. Thus in February he

announced, in a paper on 'ConductibilitO des dielectriques

sous l'influence des rayons du radium et des rayons de

Rantgen',2 that radium rays remained constant over a small

temperature range. By November he had extended the study

to highly accurate determinations of the rate of dissipation

of activity excited by radium. His researches 'Sur la

constante de temps caractoristique de la disparition de la

radioactivit6 induite par le radium dans une enceinte

fermee'3 showed that the decay of the rays emerging from a

sealed tube followed closely the equation I = I0 e-t/T

with a half-period of 3 days, 23 hrs., 42 mins. He thought

that this law's independence of conditions such as the

nature of the walls of the vessel and the gas within was

sufficiently firm to provide 'La mesure absolue du temps'.4

228

That this also applied whilst the vessel was maintained

at widely different temperatures, from -180° with liquid air to +450° in an oven, was a point of great theoretical

importance to Curie. For it served both to confirm his own

position and to rebut that which he attributed to Rutherford

and Soddy. In a paper delivered in January 1903 Curie

revealed his views 'Sur la radioactivite induite et sur l'emanation du radium'5 which were based upon further

studies of the abnormally rapid dissipation of induced

activity from open vessels; this was apparently the only

means by which the decay law could be altered. He considered

that the radium atom, the unchanging source of radioactive

energy, gave no direct rays at all but merely an 'emanation'.

Only if this were unable to escape such as in a solid radium

compound would it 'se transforme sur place en rayonnement de

Becquerel'. Evidently the emanation occupied a position of

great importance within Curie's theory. It is notable

however that he explicitly rejected Rutherford's usage of

the term. Although he adopted the expression 'emanation'

as 'commode' Curie restricted the meaning to 'Ponergie

radioactive &Ilse par les corps radioactifs sous la forme

speciale sous laquelle elle est emmagasinee dans les gaz

et dans le vide'.6 Citing Rutherford's and Soddy's revised

paper on thorium emanation of November 1902 he remarked

that there was insufficient evidence to establish 'l'existance

dune emanation de matiere sous sa forme atomique ordinaire',

firstly because spectroscopic evidence was lacking, and

secondly since the emanation vanished spontaneously from

sealed tubes containing it. This last comment is interesting

in that it seems to impinge only upon the second version of

Rutherford's theory;7 the first had assumed a loss of

radiation8 rather than the disappearance of emanation to

be the major cause of the observed decay. Whether Curie

distinguished between the two versions, both of which were

contained in the paper he cited, it is not certain. Curie

concluded on experimental grounds that after energy had

been released by the radium atoms to their surroundings:

dans les gaz l'energie transmise de proche en proche est emmagasinee sous une forme speciale

229

qui so dissipe suivant une loi exponentielle en provoquant la radioactivite des corps materiels.9

Thus he provided an alternative explanation of the results

which Rutherford saw in terms of material emanations and

active deposits. The observed invariability of the decay

law over a wide temperature range allowed Curie a final

criticism:

Je considere aussi come peu vraisemblable que les effete qui accompagnent l'existence de l'omanation aient leur origine dans une trans-formation chimique.10

This too was misdirected but again perhaps understandably

since the particular paper to which Curie referred con-

cluded with the late theoretical discussion of whether the

radiation accompanies or precedes 'chemical change' without

mentioning that this was not a normal chemical change.11

Rutherford's reply in an open letter comprising 'Some Remarks

on Radioactivity'12 corrected Curie's criticisms. Rutherford

pointed out that the chemical change conceived was not

'ordinary' but 'sub-atomic', that he had described both the

condensation and the diffusion of the emanation some months

before, and that the rays from the emanation were themselves

material consisting of 'heavy charged bodies'. It is notable

that a condensation of the emanation explains perfectly the

single outstanding exception to his own theory which Curie

reported. After keeping the sealed tube at the temperature

of liquid air he found that the standard rate of decay

occurred only after the recovery of the activity from an

abnormally low value. Curie vaguely attributed this to the

effect of temperature on the walls of the glass vessel.

And his description of the remarkable spontaneous rise (for

some 30 minutes) of the activity induced by very brief

exposure to radium, given in the succeeding publication of

February 1903,13 demonstrates the hopeful flexibility of

Curie's view. He thought that the explanation of the

initial increasing portion of what should be a decay curve

might lie 'dans la presence et dans la transformation d'une

certain quantite d'emanation'. Rutherford's explanation

of this phenomenon had come to be couched in terms which

230

appear similar to those of Curie but which in fact possessed

a much more material meaning. But even before the publi-

cation of Rutherford's informative reply Pierre Curie had

made a dramatic discovery of sufficient force to produce

the beginnings of a shift in his own outlook.

The path to Curie's revelation of 'La chaleur degagee

spontanement par les eels de radium', in a note published

jointly with a younger assistant A.Laborde in March 1903,14

was via the latter's apparently successful attempt to detect

a mechanical pressure produced by radium radiation.15 These results were instead seen by Pierre Curie as explicable in terms of a small temperature difference, known to affect

delicate weighings. This was directly confirmed by means

of a sensitive thermometer. And they announced that a

radium-barium chloride sample (about 4% Ra) remained

permanently at a temperature 1.5 degrees above its surround-

ings; background variations were but 1/100 degree. The rate

of heat production, easily measured both by electrical

comparison and calorimetrically, was incredible yet had to

be believed. 1 gm. of radium gave 100 calories per hour;

or as they significantly put it 1 gm.-atom of radium con-

tinously released in each hour as much heat as the combustion

of 1 gm.-atom of hydrogen in oxygen. Thus Curie could

reason with conviction that 'Le degagement continu d'une

telle quantito de chaleur ne pout s'expliquer par une

transformation chimique ordinaire'.16 If this was intended

to be an additional argument against Rutherford the effect

was perhaps the opposite; the points that Curio made were

somewhat similar to those already aimed at himself by

Rutherford and which were still on their way. If one sought

the origin of the heat in 'une transformation interne' this

must be more profound than a chemical change and might be

due to 'une modification de l'atome de radium lui-mime'.

However, since no change in the spectrum of radium was

visible each atom must change 'avec une extreme lenteur'.

Thus the energy in such a transformation would be

'extraordinairement grande'. His figure was in fact some

60 times greater than Rutherford and Soddy's forthcoming

231

'under-estimate' based upon alpha-ray ionisation measure-

ments.17 Curie failed to make it clear whether he still

preferred the alternative hypothesis, which he again

mentioned, that 'le radium utilise une energie

extOrieure'.18 Within three months he had seen Rutherford's

letter, had himself demonstrated the condensation and

diffusion of the emanation,19 knew of the scintillation

effect of the alpha rays and had accepted their atomic

nature. Some of this experimental evidence served to move

him theoretically one step further. The concluding remark

of his lecture at the Royal Institution2° on 19th June 1903

was that the two competing hypotheses regarding the source

of the energy - 'un "element en voie d'evolution' or an

unknown external radiation - 'ne sont pas du reste incom-

patibles'. Perhaps this was the effect Soddy desired as

he wrote to Rutherford regarding their general attack, to

be published in May, against the theory of induction:

'I feel it would be unwise to get Curie into a position

he was unwilling to go back on, before he has seen all our

evidence'.21 Certainly the Curies were not easily moved.

For in that June lecture Pierre repeated his own ideas on

the transmission of radioactive energy through gases together

with some of his reservations on the material nature of the

emanation. And Marie, in her D.Sc. thesis on 'Radio-active

Substances' which was probably completed in May,22 aimed a

blow at the most vital area of the disintegration theory,

namely 'ThX'. She asserted that this was no more than the

manifestation of an activity induced by thorium upon some

inactive substance whose chemical properties might be

temporarily or permanently altered to give the results

obtained by Soddy; she did not omit these comments from

the 1904 edition of her work despite making other important

changes. However, the balance of Pierre Curie's newly

attained compromise of mid-1903 was soon to be tilted, by

the weight of still more experimental evidence, towards

the idea of atomic change.

F.Giesel, a figure not without influence in the early

field of radioactivity, was a representative of those who,

232

even before the striking discoveries of 1903, accepted

the existence of atomic disintegration as well as radio-

active induction. Perhaps wisely however he offered no

coherent theory. The essential part played by the Curies'

induction theory in Giesel's chemical controversies over

polonium and radiolead during 1901 has been described above.23 The German chemist's concern with the problems

of the theoretical side of radioactivity is illustrated

in his letter to the Curies of March 1902.24 Having mentioned his new radium-barium bromide fractionation, which was

shortly to supersede the chloride method of Marie Curie,

he commented upon Pierre's experiments on the transmission

of radioactivity through water. Giesel suggested an idea

later to be extended by the Curies, that radium might release

energy only in the form of a Rutherford emanation which

would in turn produce direct radiations as a secondary

effect. Regarding induced activities he expressed unease

at the possibility that all such manifestations, of which

he had seen many, might be due to traces of known active

substance; as he admitted, the Curies' demonstration that

the activity induced by soluble radium chloride was itself

insoluble constituted an exception to this.25 In a paper 'Veber Becquerelstrahlen und die radioaktiven Su.bstanzen'26 published some months later Giesel advocated a material

interpretation of thorium emanation; he compared it to the

odour emitted by Musk. It is interesting that he pointed

to the characteristic chemical properties of induced radio-

activity whilst still expressing uncertainty as to whether

this was a deposit of the primary substance. By October

1902 Giesel appears to have combined the Curies' current

view that the emitter of radioactive energy was the atom,

'die Arbeitsmaschine' driven by an unknown power, with the

prevalent physical theory of electronic dissociation:

das Atom dabei nicht bestehen bleiben kann, sondern sich in noch west kleinere Theilchen aufliisen muss, in Ionen (oder Elektronen), welche als Zwillinge mit + und - Elektricitat geladen zur Welt kommen.27

233

He was also inclined favourably toward Rutherford's

opinion that since the emanation came from radium itself

its study might clarify 'die inneren Vorggnge im Radium-atom'.28 On the other hand his classification of radioactive

substances in no way agreed with Rutherford's disintegration

theory. Giesel divided these materials into three groups

according to their radiation characteristics: intensely

active and constant, feeble and constant, and weakly to

intensely active with declining radiations.29 His attribution of the activity of the entire third group to inductions by

the permanently active elements, and his placement of

polonium in this category invoked again the comment of

Rutherford that radioactive induction did not exist.30 After this Giesel quietly dropped the idea to work like others

within the disintegration theory. These developments high-

light the difficulties in understanding induced radioactivity

which also troubled others during 1902-3.

J.J.Thomson too expressed an idea of direct induction

or self-induction by radiation which was also to be dismissed

by Rutherford in this case by straightforward experimental

means.31 However, Thomson's suggestion of April 190332

marked not the beginning but the end of a scientific struggle

between the two. For more than a year they had differed

over the emanations which had played so vital a part in the

development of the disintegration theory. Whilst the Curies

steadfastly reiterated that the emanations were a special

non-material form of energy and spoke against Rutherford's

and Soddy's belief that these were a particular kind of

matter, J.J.Thomson came to adopt a quite distinct position.33

At first he had accepted Rutherford's idea that thorium

emanation was a radioactive material, and had contributed

welcome suggestions regarding its gaseous nature and the

origin of its acquired positive charge. After some three

years of harmony disagreement arose over Elster and Geitel's

234

invocation of a third radioactive emanation as the cause

of the temporary activity produced on negatively charged

wires in the atmosphere. The tension was almost at its

greatest when in May 1902 Thomson wrote to Rutherford of his own experiments on this phenomenon:

These results make me doubt whether Eleter and Geitel's induced radioactivity is really due to some rare substance; it seems to me it is probably made from wind and water! C.T.R.Wilson has discovered that freshly fallen rain is radioactive.34

But worse was to come as the criticism tended to expand.

Let us briefly follow the tale up to this point and on to

its conclusion. Some earlier experiments of Thomson's

research students were directed to show what could be done

with ordinary materials. For example Wilson announced in

1899 that large uncharged (non-radiating) nuclei could be

created in gases by irradiation.35 By the end of 1901 he had demonstrated an apparently 'Spontaneous Ionisation of

Gases'36 and J.C.McLennan had written 'On a kind of Radio-

activity imparted to certain Salts by Cathode Rays'.37

Thomson's experiments 'On Induced Radio-activity'38 which he described without interpretation in March 1902 appear

to continue this trend. Although a negatively charged rod

did not become active in a sealed vessel of air, which one can see might be explained by the limited quantity of

emanation therein, when the air was continuously irradiated

by X-rays with all that implied, then the rod did become

active: it is notable that this was entirely contrary to

Rutherford's statement of 1899.39 Furthermore, as Thomson reported, chemical substances especially hydrogen peroxide

produced large currents when absorbed on paper in a layer

around the rod; this may possibly be interpreted as a

renewed link between radioactivity and Russell's earlier

photographic work. By May 1902 the 'continuation of the

experiments' to a related subject showed that 'The Increase

in the Electrical Conductivity of Air produced by its

passage through Water,40 could be as great as 10 to 12 times

the initial value. Such observations led him to compare

explicitly 'the "emanation" from radio-active substances'

235

and ordinary air 'put in this highly conducting state'

simply by bubbling. Writing to Rutherford on various

matters at this time Thomson gave some details of the

ionic mechanism which he saw operating here:

I think the effect is due to excessively minute drops of water so small that they fall with extreme slowness, & that around each drop there is a layer of ionised gas which is pulled off by the electric field.41

He openly linked this only with 'atmospheric electricity',

but the direction of his reasoning is suggested by questions

regarding Rutherford's emanations in the same letter: Have you tried whether the emanation is longer lived when it is in a solid or liquid than when in the air; if you let it bubble in very small bubbles through water for a minute will it lose half its radio-activity as it would in air; it seems as if the ability of the emanation to get through a great many layers of paper rather pointed to the conclusion that the emanation does not fade away so rapidly in solids as it does when free.42

Despite the compliment that Rutherford's explanation of

radioactivity 'clears up a great deal of obscurity', which

would help Thomson's forthcoming book, the latter evidently

remained suspicious of the emanation aspect. Besides the

above discrepancies and queries it is also possible that

he had noticed Rutherford's and Soddy's easy recognition

of an atmospheric emanation in their first joint paper43

without the promised44 tests. It seems that Thomson found

many of Rutherford's results not unrepeatable but too easily

so. He wrote again some days later45 mentioning Wilson's

radioactive rain and ascribing Elster and Geitel's 'rare

substance' to 'wind and water'. Then into print went

Thomson's 'Experiments on Indueed-Radioactivity in Air,

and on the Electrical Conductivity produced in Gases when

they pass through Water'46 which extended and united the

two main aspects of his previous researches on radioactivity

mentioned in the title of his paper.

Thomson argued squarely both that the existence of a

radioactive component in the atmosphere was 'possible'

but 'not necessary', and that 'negatively electrified

surfaces may become radio-active without the deposition

236

upon them of substances having specific radio-active

properties'.47 Let us consider firstly the emanation side

of the study. The air which Thomson passed through water

in various ways attained an increased electrical conduct-

ivity by factors as great as twenty and retained this

property for some days; it followed that 'In the air

modified by passing through water there must be a continuous

production of ions'.48 And he appears to have cast aspersions not only upon the hypothetical atmospheric emanation but

also at Rutherford's thorium and radium emanations. Though

no-one seems to have known it, Elster and Geitel, the cited

targets, had already moved out of range49 before Thomson launched his attack. It therefore fell entirely upon

Rutherford. Thomson asserted that although certain extremes

of heat and cold destroyed the artificial conductivity an

electric field did not:

Thus, in this respect, the modified gas resembles a gas mixed with the 'emanation' from thorium. Rutherford has shown that in this case the conduct-ivity is not destroyed by a strong electric field.50

Regarding the second major aspect of the discussion, that

of surface induction, he explained the 'induced radio-

activity caused by negative electrification' within the

modified gas in a manner ironically reminisoent of Elster

and Geitel's newly favoured hypothesis. Thomson supposed

that positive ions in the gas adhered to the negatively

charged wire causing the corpuscles there present to be

accelerated into the surrounding gas as 'cathode rays'.

Thus was the wire radioactive. He postulated a similar

mechanism involving minute water drops surrounded by

positive ions to explain the lasting conductivity of the

modified air. Some positive ions, for example those from

flames, did not produce such effects; but an electrode

polarised in solution did so. Evaporation to dryness

satisfied Thomson that no active material was present in

the water which he had used for spraying and bubbling.51

The inference that all of the emanations and induced

activities were not each 'a special kind of matter', as

Rutherford and Soddy claimed, was quite clear. The

237

implications left ThX isolated, exposed and highly

vulnerable. But Thomson had not expressed the criticism

very forcefully in that direction. Perhaps this was

fortunate. For by the end of 1902, only six months after

the completion of his paper, he had adopted a separation

of the atmospheric emanation hypothesis from the trans-

formation theory of radioactivity which he now fully

supported. He had evidently not heeded Rutherford's dig

that 'although the amount of excited activity ... varies

greatly with the weather and amount of wind' its decay law was always the same.53 But perhaps Thomson had come to

appreciate the revised discussions of 'The Cause and Nature

of Radioactivity'54 before writing his piece on 'Becquerel

Rays' for Harpers Monthly Magazine.55 In that article

Thomson publicly recognised in the disintegration theory

both a solution of the energy problem and an explanation

of the chemical manifestations of radioactivity:

what is the nature of this energy, and how is it stored? A satisfactory answer to this question has, I think, been given by some quite recent researches made by Professors Rutherford and Soddy in Montrea1.56

Having described the separation of ThX from Th, the

recovery and decay of activities, the continuous production

of ThX and the equilibrium nature of thorium's constant

radiation he concluded:

We see now the source from which the energy required to sustain the radiation is derived; the radio-active substance is undergoing a continuous transformation into a state in which it has less energy ... Ordinary thorium is thus steadily being transformed into the active thorium X, while this is continually passing into some inactive form.57

That Thomson envisaged some form of chemical transmutation

seems clear. For he suggested that this final inactive

substance might be detected in thorium minerals 'by ordinary

chemical means'. Most significantly, despite the confusion

in the last words of the above extract, Thomson now accepted

that the emanations were inert-gas elements58 produced from

ThX; and he pointed to the presence of radioactive elements

in all helium-bearing minerals. Conversely he also repeated

his recent claim regarding atmospheric excited activity

238

that 'this induced radio-activity'59 could be explained

otherwise - in terms of ionic clusters and layers. The

skepticism of Thomson, thus narrowed, was soon to disappear

completely along with the experimental basis of his own

conclusions.

In a long letter sent to Thomson just after Christmas

1902 Rutherford persuasively described his recent successful

condensation of the emanations using liquefied air.60 'The experiment is an extremely simple one to show and works

like a charm'; the gas leaving a spiral tube at that

temperature 'had not a trace of emanation in it' and, on

warming, the emanation 'comes off in a rush - all at once

apparently or at any rate within a degree'. Delicately

Rutherford phrased his remark that 'Anyone whodoes'nt (sic)

believe it is a gas after such an expt. is difficult to

convince'.61 Furthermore he had 'proved' that much of

Wilson's '"spontaneous" ionization' of air was due to a

penetrating radiation from outside the containing vessel,

from the walls of the room itself. Thomson presented and

may have been influenced by J.C.McLennan's paper on

'Induced Radioactivity Excited in Air at the Foot of

Waterfalls'62 also dated December 1902. The latter had

written to Rutherford in October63 concerning high voltage

experiments at Niagara and others in the laboratory which

involved the dropping of water through thorium oxide.

McLennan confided that these 'would explain the radio-

activity of rain found by Wilson' and 'seem to point

against J.J.'s results'; but on repeating Thomson's own

experiments, McLennan 'found exactly what he found'.

That the conflict was soon resolved for the latter is

indicated by his conclusion, published in April 1903, that.

'the consensus of opinion' appeared to be that atmospheric

excited radioactivity 'is due to the presence in the

atmosphere of some peculiar constituent similar to the

emanation from thorium'. By that time in April 1903

when he replied to Rutherford Thomson too had fitted into

this category of opinion. He announced, or admitted:

I have found a radio-active gas in Cambridge water, or rather in that part of the water

239

which comes from deep wells. Dewar liquefied for me the gas extracted from the water... 64

He made the first open statement to this effect soon

afterwards;65 his ionic condensation theory of radio-activity was not heard of again. However, it is not

true that he gave up the belief that radioactivity was

a property of ordinary matter. Papers on this subject

had already begun to appear and the discussion which ensued

during the next few years similarly involved both Thomson

and Rutherford.66 This provides one reason for saying that

Soddy's claim of June 1903, that 'Professor J.J.Thomson

and Sir William Crookes have both recently abandoned their

former theories in favour of the new hypothesis',67 is

something of an oversimplification at least with regard

to the former. Besides, in 1898 he had been the first to

ascribe uranium's energy source to an internal rearrangement

of its atom. It is hard to decide whether Thomson can be said to have retained or revived this conjecture. But it

is clear that he now attempted to strengthen it with a

rough calculation of the possible magnitude of the available

atomic energy: if the radium atom were totally corpuscular

and each negative charge of 3.4 x 1010 e.s.u. were 10-8 cm.

distant from an equal positive charge then a 1% reduction in

the intrinsic energy would suffice to maintain its heat

production for 30,000 years.68 To Crookes, however, Soddy's

straightforward interpretation fully applies. Crookes too

had proposed a theory of radioactivity in 1898 and indeed

abandoned it in mid-1903. But, as the placement of the

younger chemist's claim in The Times Literary Supplement

hints, not before the skeletal explanations of that public

figure had been well aired.

240

In the spring of 1903 Soddy sent to Rutherford from

London an illuminating letter oontaining his comments on

their forthcoming publication:

wish you immediately to get into thinking array & to consider it & this letter. Events are moving rapidly here. The announcement of the heat given out by Curie has created quite a furore in the Press, & in yesterday's Times Johnstone Stoney had a letter which I have enclosed. Ramsay told me & from his attitude seemed to think it quite possible.69

Soddy confessed that he was unable to convince Ramsay that

the surroundings were not the energy source and in relation

to these matters he continued:

I mention this to show we are on a flood-tide of interest & I do not want to delay (1) If there is a controversy all our papers should be out. They all predate recent developments ... (2) The fewer 'grand-savants' who make asses of themselves the better for our (personal) relations with them afterwards.70

He also feared that Becquerel, who had made certain claims

concerning the deflection of the alpha rays, might come

out with a theory 'of his own'; in a way he was right.

The events which prompted Soddy's urgent message had

begun with the delivery on 16th March of Curie's paper on

the heat of radium and continued with Crookes' announcement

on the 19th of his discovery of a remarkable scintillating

phosphorescence produced by that substance. Crookes

attributed this phenomenon to the individual impacts of a:

bombardment of the screen by the electrons ([footnote] Radiant matter, satellites, corpuscles, nuclei• whatever they are they at like material masses) hurled off by radium with a velocity of the order of that of light.71

The 'furore in the Press' began with The Times' editorial

of 25th March entitled 'The Mystery of Radium' which

summarised both Crookes' very beautiful demonstration' of

the scintillation effect and the discovery of P.Curie:

M.Curie, a French physicist of the highest reputation and attainments, has made a communication to the Academy of Sciences which would have been received with absolute incredulity had it been offered on less unimpeachable authority.72

241

For radium, as they put it, produced heat spontaneously

'without combustion, without chemical change of any kind,

and without any change in its molecular structure'.73 The conclusion that radium could 'gather up and convert into

heat some ambient form of energy with which we are not yet

acquainted' was clarified by Crookes the next day, after

Punch had interpreted his discovery in its own terms:

'On Ions'. - Such was the subject of Sir W. CROOKES' most recent lecture. Were they Spanish? Pickled? Boiled or fried...? They were made 'visible'. This was hardly necessary, as in such a case the evidence to the eyes would be less convincing than that to the nose.74

In a letter to The Times of 26th March Crookes explained

that the source of ambient energy need not be a mystery.

Explicitly reviving his theory of 1898 he again referred

to the large store of kinetic energy in the surrounding

air and suggested that radium might use the faster molecules

in the manner of Maxwell's 'Demons'. Crookes made no mention

of the disintegration theory about which Rutherford had

written to him a year earlier. Despite the success of the

transformation theory in explaining the chemical, radiant

and electrical phenomena of radioactivity the supposition

that the considerable energy involved was stored within

the chemical atoms was the subordinate hypothesis with the

least direct supporting evidence. Rutherford and Soddy had

turned the observed indestructibility of the ordinary chemical

elements to their own theoretical advantage; but this move

implied that the atoms of every element contained such a

reservoir. The notion of an internal store was evidently

at first unacceptable to Crookes but his own answer to 'The

Mystery of Radium' was immediately questioned by others in

the flurry of correspondence which followed the editorial

and his letter in The Times. 'Ignoramus'75 pointedly mention-

ed the known constant intensity of radium rays in vacuo. On

the other hand G.J.Stoney in his opening letter76 not only

stated his agreement with Crookes but claimed to have employed

just such an explanation in 1893. Stoney's 'Suggestion as

to a possible Source of the Energy required for the Life of Bacilli, and as to the Cause of their small Size'77 had been

242

that such organisms might be penetrated by 'swifter moving

molecules' and could thus abstract the energy of formation

of organic compounds from the surroundings, which would

become slightly cooler as a result. The process 'therefore-

belongs to the recognized exceptions to the Second Law of

Thermodynamics'. In Stoney's opinion the restoration by

molecular collisions of radiated energy 'whenever the motion

of the electron has transferred energy from the molecule to

the aether' was another exception.78 This is the point which

seems relevant to radioactive radiation; it indicates how

the new energy problem was seen by some in terms of an old

but still living enigma.

Crookes, Stoney, and 'Ignoramus' were joined by others,

some anonymously, as they continued the correspondence into

mid-April; the names of Rutherford and Soddy received a

mention79 before Crookes withdrew from the argument describing

it as unfruitful.80 But perhaps it was not entirely so, for

Crookes soon changed his view on the source of the energy.

Despite the latter's protest that the fast-moving molecules

in a 'vacuum' could well provide radium's energy81 one might

say that the comments of 'Ignoramus' among others made their

mark. For in his note with Dewar 'On the effect of extreme

cold on the emanations of radium' read to the Royal Society

during the next month82 Crookes described the use of his new

'spinthariscope',83 whose scintillations he now took to be

caused by the impact of 'positive atoms'. His intention

was to test the effect of a vacuum on radium, the source of

these particles. The 'very good vacuum' obtained by using

Dewar's liquid air or hydrogen as condensing agents did not

diminish the scintillations nor did the low temperatures

thus provided; though the screen lost its fluorescing ability

when allowed to become cold. The knowledge that such extreme

experimental conditions ought to affect the availability of

gaseous kinetic energy may have helped to turn Crookes away

from his attribution of radioactive energy to this source.

His stated intention of following up 'the important dis-

covery' by Rutherford and Soddy of a condensable emanation

from radium salts indicates another likely influence.84

243

In expressing his 'Modern Views on Matter: The Realisation of a Dream', 05 some days later, Crookes revealed an important change in his ideas. He now believed that in addition to

the process of ultra-atomic dissociation, which he had

postulated in 1902, radioactivity involved an atomic trans-

formation. To the historic discovery of radium he credited

the coalescence 'into one harmonious whole' of the 'isolated

hypotheses' of 'ultra-gaseous' matter, electrons, subatomic

particles, X-rays, and 'the emanations from uranium'.86 The twin threads of electrical theories of matter and notions

of the complexity of the chemical elements, both of which

ran from Davy and Faraday via himself to contemporary

studies, he saw thus united. But, apart from the miscon-

strued 'emanations from uranium', Crookes' own seemingly

attractive joinery appears in its context less than perfect.

In the previous year Crookes had believed that the protyle

atoms of matter were attended by comparatively few electrons87 But now in mid-1903 he considered that 'the electron would

be the "protyle" of 1886, whose different groupings cause

the Genesis of the Elements'.88 According to what he called 'a Darwinian development of chemical evolution',89 the elements were formed in order of increasing atomic weight

presumably correlated with decreasing thermal stability.

He stated that radium exhibited 'a spontaneous dissociation'

and that its atom 'might be actually suffering a katabolic

transformation'.90 Radium, outshining uranium in this

respect and being thus the least stable element, ought

therefore to have the highest atomic weight. Probably for

this reason Crookes chose to adopt the tentative spectros-

copically extrapolated estimate by C.Runge and J.Precht91

of 258 ignoring Marie Curie's correct gravimetric result

of 225. Crookes' only hint as to the cause of atomic dis-

integration was that since radium held the position 'next

after uranium' in order of original creation and present

instability it would be sensitive to 'our terrestrial

sources of heat'.92 Crookes may have derived this idea from

0.Lodge whose positive and negative electronic atom of

electromagnetic mass ho described.

244

It is one indication of the speed of developments at

this time that Lodge had already changed his mind about

each of these subjects. In his address similarly entitled

'Modern Views on Matter',93 delivered one week after that

of Crookes, Lodge placed reservations both upon the

existence of the positive electron94 and upon external

influences on atomic disintegration.95 And it is a corres-

ponding sign of rapid movement that he attacked the position,

from which Crookes had already shifted, of supposing that air

molecules supplied radium's energy. Lodge followed closely

the theory of Rutherford and Soddy, probably as revealed in their May publication on 'Radioactive Change'.96 In terms bolder than theirs he proclaimed that 'The transmutation

of elements' through 'temporary transitional forms' was

a process 'going on before our eyes';97 the loss from an

atom of radium with atomic weight 225 of a projected portion

with atomic weight 2, which caused it to become the unstable

emanation, was 'the main fact of radio-activity'. Such

comments mark the decisive end to a period of six months,

following his perusal in November 190298 of Rutherford's

and Soddy's paper 'On the Cause and Nature of Radioactivity',

during which he entertained both alternatives for the energy

source. Lodge may also have been influenced by J.J.Thomson

whose estimation of the internal energy of an electronic

atom he appears to have reproduced.99 These two physicists

together with Crookes comprised an important trio of converts

from a variety of opinions to the disintegration theory of

radioactivity. But the major authority in the field lay

with the Curies who had other ideas.

Marie Curie's criticism of the chemical evidence for

radioactive transformation had appeared in May 1903 and

Pierre Curie, his rival induction theory in difficulties,

had moved to a position of compromise regarding the energy

source by June. Let us again take up their story and its

connections. The announcements made by Soddy and Ramsay

in July created an impression both deep and wide; upon the

Curies the impact was almost conclusive. Soddy now partnered

the well-known discoverer of terrestrial helium100 and the

245

inert gas family of elements.101 A preliminary notice

reported simply that the 'Gases Occluded by Radium Bromide'

contained helium.102 Their subsequent description of

'Experiments in Radio-activity, and the Production of

Helium from Radium'103 evidenced a considerable development.

By means of low-temperature purification techniques they

had succeeded in following the first ever spectroscopically

traceable chemical transmutation. Not only had they

watched the characteristic yellow line of helium appear,

after some days, in a tube of radium emanation but they

had at last seen a glimpse of the emanation's own spectrum.

Writing to Dewar on 22nd July regarding the publication

of joint experiments on the heat of radium Pierre Curie

told him that he was '1'ennemi des publications hatives'

which also was why Rutherford and Dorn 'ont public avant

moi des °hoses quo j'avais faites avant eux'.104 Curie did

not include the production of helium from radium, of which

he was already informed, among those things. But perhaps

he recognised the 'presence simultanee dans certains

mineraux de l'uranium, du radium et de l'helium' as such.

For in a review of 'Recherches recentes sur la Radioactivito'

written during the following months he claimed to have been

impressed with this fact 'des le debut de nos recherches'.105

The proposed transmutation suitably confirmed106 sowed the

seeds of change which can be seen still preserved in Curie's

written words. In the main his review repeated earlier

statements for example that the emanation was not 'un gaz

materiel ordinaire' but one of the forms of radioactive

energy.107 One can see that this could form a gulf between

radium and helium. However, in a final section Curie

credited the new experiments with 'une importance fonda-

mentale'. He accepted that'L'helium pourrait, d'apres

cola, etre l'un des produits de la desagregation du

radium'.108 At the same time his recognition of the work

of Kaufmann seems to have removed the basis of Curie's

objection to Becquerel's ballistic hypothesis of the beta

rays; Curie now seemed happy to grant the possibility of

an electronic-electromagnetic theory of matter.109 His note

246

'Sur la disparition de la radioactivity induite par le

radium stir les corps solidest 110 shows that by the spring

of 1904 he was actively using the transformation aspect

of the theory 'de M.Rutherford' in his quantitative research.

Marie Curie was similarly influenced by the helium

experiments; she reported these and mentioned the dis-

integration theory in the 1904 revised edition of her thesis.111 But her work gives a curiously patchy impression.

She largely retained the induction theory, including her

view of ThI, though now admitting the emanation as a

material gas.112 Whilst she was persuaded that 'trans-

formations atomiques' indeed occurred in radioactivity

her final word took the form of a strange though perhaps

not unique defence of the unchanging radium atom:

Au lieu d'admettre que l'atome de radium se transforme, on pourrait admettre que cet atome est lui-mAme stable, mais qu'il agit sur le milieu qui l'entoure (atones materiels voisins oil other du vide) de mani4re a dormer lieu a des transformations atomiques.113

Marie Curie also appears to have attempted to ensure that

she had the last word by asserting that Rutherford had

'franchement adoptee'114 one of her own hypotheses of

1898-9; this she repeated115 in 1906 when there was no

alternative to the disintegration theory of Rutherford

and Soddy.

The aspects of the radium-emanation-helium trans-

formations which thus impressed the Curies also invoked

a wider interest, a second wave of publicity. Ramsay was

applauded as he made the first public announcement of the

creation of helium in mid-July at the Dinner of the Society

of Chemical Industry.116 In the same week The Times117 noted that Sir W.Huggins had found helium lines in 'The Spectrum

of the Spontaneous Luminous Radiation of Radium at Ordinary

Temperatures1118 - an apparently independent discovery,

though based on hints from Rutherford's recent papers.

Huggins seems now to have speculated in a manner reminiscent

of the dissociation hypothesis of his old rival Norman

Lockyer and privately noted elsewhere that a xenon line

247

was identical with one of the radium spark spectrum lines.119 Unfortunately, however, just as the first of a series of

letters on 'Radium and Helium' appeared in The Times

Huggins had to admit that radium's luminous spectrum con-

tained not helium but nitrogen lines,120 a discovery

notable in itself. 'Verily this is the summit of fame'

wrote Soddy as he sent 'a cutting from the current no. of

Punch' to Rutherford.121 The relevant extract may have

been the interesting disclaimer that:

'RADIUM' wishes it to be distinctly understood that he can throw no light on the present political situation. He adds that there is no affinity between him and TIM HELIUM, M.P. 122

Or perhaps, following Soddy's own popularising article in

The Times Literary Supplement on 'Possible Future Appli-

cations of Radium',123 the cutting may have been the

illustration of a subtle connection between 'SCIENCE AND

MATRIMONY' which appeared in the same issue of Punch:

He (the accepted one, enthusiastically discussing their projects for the future). 'I think it would be a splendid idea, when we marry, to have the Kitchen fitted with a Radium Cooking Range!!'

The Betrothed (who doesn't believe in long engagements, very sweetly). 'Er-ye-es, Darling, but if Radium does not come into use - say, in one month's time from to-day, we won't wait for it, dear, will we?'124

Articles by scientists and others125 concerning the physical,

chemical, technological and medical implications of radio-.

activity proliferated in non-technical magazines during

1903. The extent of popular interest in the Curies, perhaps

not matched in French academic circles, can be judged by

Pierre's pained declaration to Ramsay early in 1904:

Nous avons etc terriblement deranges dans ces derniers temps par les journalistes, lee gene du monde, les excentriques de toutes les especes male vous connaissez vous-meme ces visiteurs encombrants.l26

According to Soddy's complaint, two months earlier, Ramsay

himself enjoyed this kind of action:

Ramsay has again been interviewed by the Daily Mail. I can't quite understand it. Sometimes wonder if he forsees the great commercial advantage in the future of being known as the expert on radium, & has done it from this motive or from pure lack of judge-ment & consideration.127

248

However, the students of radioactivity were evidently most

concerned with the professional side of their reputations.

This is shown for example by Rutherford's remark that 'a

photo of my noble self' in Harper's Magazine gave him 'as

much advertisement as is good for me', but that 'These

things ... don't count scientifically for it is work that

tells'.128 Nevertheless one can argue that events in the

broader arena did in certain respects affect those in the

smaller. Such an effect may possibily help to explain why

feelings about priority ran particularly high during the

second half of 1903.129 And a belief in such an influence

would certainly account for Soddy's further comment to

Rutherford at the end of the year that Ramsay's Press inter-

views 'must prejudice our case with the real scientists'.130

The publicity at least ensured that the case was brought

to the attention of such persons, but their verdicts were

not uniform.

Rutherford enjoyed more success in his campaign among

the physicists than did Soddy with his fellow chemists.

J.Larmor in his capacity as Secretary of the Royal Society

had written to Rutherford back in April when the first

furore arose:

I am glad to hear that you are coming in May: you may be the lion of the season for the newspapers have suddenly become radioactive. I see you again monopolise most of the Phil.Mag. 131

The editors of The Electrician were in favour of the new

theory.132 They had commented after 'The extra meeting of

the Physical Society, convened last Friday [5th June] at

University College to meet Prof.RUTHERFORD', which 'was a

crowded and enthusiastic gathering', that:

the suggestions put forward by Prof.RUTHERFORD in explanation of some of the mysteries of his subject have special value, and must carry great weight.133

At that gathering Rutherford had received the praises of

Lodge and had also answered the doubts repeated by

J.D.Everett134 and expressed by S.P.Thompson concerning

the source of radium's energy.135 It was the chemist

T.M.Lowry who at this physicists' meeting made perhaps the

249

most outspoken objections to the transformation theory.

He attacked directly the weakness of its experimental

foundations. In return Rutherford pointed to the

inadequacy of Lowry's substitute. But the latter's last

words on the matter had not yet been heard. Nor had those

of his senior associate at the Central College South

Kensington, H.E.Armstrong.

As a chemist and a founder of the atomic disintegration

theory of radioactivity Soddy seems to have experienced a

division of his loyalties. Perhaps he still sympathised

with those who believed in the indivisible chemical atom

as he himself had done but two years earlier:

Having failed utterly as I can see to make you realise the width of the gap between our recent work and anything preceding I do not intend to attempt it in this letter ... I must say I sometimes feel however as if I had been a traitor to my own camp and let you ... in by a back door. But for me the chemists' fraternity would have continued to smile hard and long for many a year.136

And as a confrontation with the figure described by his

excellent biographer as 'the foremost British chemist of

the time'137 loomed near Soddy organised the tactics.

Referring to Armstrong's and Lowry's recent publication

on radioactivity he wrote to Rutherford:

I think they, being chemists, are my fair game & I hope to get an opportunity of asking some questions if they get up on their feet at the B.A. Otherwise I think they are best ignored altogether. So perhaps you will leave them to me if they try to interfere. I shall only engage them under provocation.138

Armstrong, a chemist interested in physical aspects of his

subject, was one of the few scientists to set out fully an

alternative to the disintegration theory. In a paper with

Lowry on 'The phenomena of luminosity and their possible

correlation with radio-activity'139 he attempted to explain

radioactivity 'from the chemists point of view' and to

bring it within the boundaries of his own field. Having

outlined the relationships of triboluminescence, floures-

cence and phosphorescence to the dynamic equilibria of

organic compounds he compared Th and ThX, as 'isodynamic

250

forms of thorium', with the forms of nitrocamphor whose

rate of interconversion followed a 'simple logarithmic

law'. He asserted that this explanation was 'at least

as rational as one which assumes that nature has endowed

radium alone of all the elements with incurable suicidal monomania'.140 This bark, which Soddy found noteworthy,141

may derive its bite from Crookes' well-phrased suggestion

of a 'fatal quality of atomic dissociation'.142

Armstrong's opinions on radioactivity may be understood,

in part at least, by viewing them as an extension or con-

tinuation of earlier convictions. Though once a supporter

of Lockyer's dissociation hypothesis in its early days,143

Armstrong later took every opportunity to criticise various hypotheses of dissociation. He attacked the electrochemical

molecular or ionic dissociation theories of the 1880'9,144

and poured scorn upon the corpuscular atomic dissociation

theory of the late 1890' s.145 Similarly, in discussing 'The

Conditions determinative of Chemical Change and of Electrical

Conduction in Gases, and on the Phenomena of Luminosity' in

1902146 he argued that the occurrence of ordinary reversible

oxidation effects made Crookes' explanation of vacuum tube phenomena in terms of radiant matter or electrons unnecessary.

On the traditional them© of 'The Classification of the

Elements' Armstrong argued along two familiar lines. He believed in the genetic relationship and complexity of the

elements but asserted that 'no direct evidence acceptable

to chemists has been adduced which in any way justifies

the belief that the elements are decomposable'.147 Though

Armstrong's expressions might appear very similar to

Crookes' current and earlier views the meaning of the

term 'decomposable' constituted a point of distinction

which radioactivity soon brought to the fore. In addition

Armstrong appears to have been the author of a series of

anonymous personal cum scientific attacks in Crookes' journal

upon Ramsay and his researches on the inert gases.148 One

• would expect this to have a bearing upon Armstrong's picture

of the radioactive emanations which had been placed within

what was for him an ill-conceived family.149 It seems

251

probable also that his view of Ramsay's helium transformation

was a jaundiced one, which may account for his continuing

resistance to the disintegration theory even after that

most impressive demonstration.

It was indeed necessary for Soddy to speak against

Armstrong at the meeting of the British Association in

September 1903 as the former had anticipated. The unusually

lengthy discussions which followed Rutherford's paper150

have been seen as marking a turning point. An account of

'How the "Newer Alchemy" Was Received' describes the way in which 'The opposition, brought into the open, was all but

demolished by the strength of the demonstrated support for

the theory'.151 It is to be noted that Armstrong's subsequent

three year absence from public discussions of the sabject152

fits with this interpretation but that Soddy's reference

several months later to 'the I expect numerous class of

unconvinced chemists'153 apparently does not. Perhaps the

members of such a chemical class were able to ignore the

electrical results as foreign subject matter, were left

unconvinced by the chemical evidence for transformation, and

accounted for the most recent demonstrations by R.Meldola's suggestion that radium was in fact an unusual chemical com-

pound containing helium.154 Though F.Richarz, disciple of

H.Helmholtz155 and similarly interested in the borders

between physics and chemistry, stressed the 'Analogien

zwischen Radioactivitgt and dem Verhalten des Ozons'156

such physical interpretations were largely extinct by the

end of 1903.157 As for the chemists, The Electrician seems

to have portrayed them as a single group and pointed with

relish to the disparity between the hypotheses of Meldola

and Armstrong.158 There was an air of editorial disappoint-

ment that W.C.D.Whetham (at that time reading the proofs

for Rutherford's book on Radio-activity) in his reply -Go

Meldola merely summarised:

the case for the transmutation hypothesis, from the point of view of the physicists. We should have been glad if a physical chemist so well known as Mr. Whetham had given us, rather, a glimpse of the arguments pro and con which arise in the chemical mind... 159

252

Thus physicist baited chemist across the gulf which divided

them. Radioactivity which might in theory have formed a

bridge of harmony instead served some as a route of attack.

But the response grew faint.

A few dissenting chemists such as W.Ackroyd of the

Halifax Borough Laboratory, who continued to advocate an

external source of radioactive energy,160 and the famous

Marcelin Berthelot who turned from his hopeful 'Etudes sur

to radium' of 1901161 to reservations concerning 'Emanations et radiations' in 1904162 made their voices heard. But

they were outnumbered by those such as F.Giesel, W.Marckwald,163 G.Martin, A.Debierne,164 Ramsay, Crookes and Soddy who

favoured or employed the disintegration theory. If Soddy's

'unconvinced chemists' formed a majority it was largely a

silent one; it may nevertheless have influenced future

recruitment and prospects.

2. The mechanism of radioactivity (1903-4)

Although Rutherford and Soddy were credited with the

discovery of the first chemical evidence of the subatomic

nature of radioactive change they held no monopoly of dis

cussions concerning the mechanism of the process. This

applied both when disintegration was supposed to involve

the emission only of corpuscles or electrons and after

Rutherford had in 1902 recognised as its most striking

253

feature the high-velocity alpha 'projection of a heavy

charged mass from the atom'.165 One can well understand

the closing comment of the latter's earliest exposition

of the disintegration theory: 'Nothing can yet be stated

of the mechanism of the changes involved'.166 We recall that two years earlier he believed that an all-electronic

atom contained insufficient energy to support the observed

radiation; the newly advocated reduction of mass to elec-

tricity167 may not have eased the difficulty. In his first

announcement of the magnetic and electric deviation of the

alpha rays Rutherford made clear his modified view:

There seems to be no doubt that the emission of 3 rays by active substances is a secondary phenomenon, and that the 0( rays play the most prominent part in the changes occurring in radioactive matter.168

His increasing hope of discovering the mechanism of these

changes is indicated by the further comment that:

The power possessed by the radioactive bodies of apparently spontaneously projecting large masses with enormous velocities supports the view that the atoms of these substances are made up, in part at least, of rapidly rotating or oscillating systems of heavy charged bodies large compared with the electron. The sudden escape of these masses from their orbit may be due either to the action of internal forces or external forces of which we have at present no knowledge.169

From the time of publication of these statements early in

1903 various physical scientists sought to grasp by moans

of hypothetical atomic structures the underlying mechanisms

and causes of disintegration; this phenomenon was by some

emphasised less strongly than other areas of chemical

physics. In the case of radioactivity the design of suitably

unstable model atoms which would produce the successive

elements, appropriate rays and stable end point remained as

a recognised problem for decades. The questions of the

origins of these atoms and of the causes of their peculiar

law and rates of decay similarly stood unanswered. Before

discussing the contributions of physicists, who indeed

made most of the running with this approach, I shall briefly

examine relevant views held by some members of the chemists'

fraternity.

254

In his report on 'Inorganic Chemistry' for the year

1904 to the Chemical Society of London P.P.Bedson17° gave pride of place to the latest publication of D.Mendeleef,

Professor of Chemistry at St.Petersburg. The propounder

of one of the earliest periodic tables had now, some thirty

years later, extended the system in An attempt towards A

Chemical Conception of the Ether.171 He had added a zero

group which included Ramsay's five new inert gases172 and which was headed by the 'ether' as the lightest and most

inert element. The noble gases were thus doubly linked

with radioactivity, though Mendeleef made no mention of the

emanations. For the concluding passages of his little

book173 dealt with explanations of radioactivity in terms

of the ether. This substance in his opinion possessed an

'individualised attractive capacity, a mean between gravity

and chemical affinity'174 which caused its condensation

upon the heaviest atoms, those which exhibited the 'photo-

radiant' and ether 'emission' phenomena of radioactivity.

Similar attractive forces were assumed to explain the known

solubilities of the other inert gases in water. Mendeleef's

views were not entirely out of the run of current opinion.

The idea of a zero group had commonly attended attempts to

incorporate the inert elements into a periodic scheme. And

he was not alone in proposing the existence of a chemical,

material ether. C.F.Brush, for example, announced in 1898

the detection, by thermal conductivity measurements, of

'Etherion: A New Gas' consisting of one or more entire

groups of new elements all much lighter than hydrogen and

probably filling 'all celestial space'.175 However, its

properties were soon ascribed by Crookes176 and Dorn177 to

nothing more than water vapour. The leading spectroscopist

W.N.Hartley proclaimed in a review of his 'branch of

chemical physics' in 1903178 that 'atoms of definite groups

of chemically related elements are composed of the same kind

of matter in different states of condensation'; and spec-

ulated that the 'molecules' of matter in the state of

greatest attenuation 'may be imagined to constitute the

ether'.179 And Marie Curie's theoretical attempts to

255

preserve the stability of the radium atom in 1904 involved

an action of this element upon the 'other du vide' to

provide 'transformations atomiques';180 possibly some form of condensation was envisaged here. A reviewer in the Chemical News181 described Mendeleef's work as speculation

'pure and simple' yet put forward somewhat similar ideas

himself; and Bedson, looking back to the reception of

Mendeleef's earliest periodic system, warned against too

dogmatic a critique of the newest one.182

Mendeleef made clear his hopes of creating explanations uniting physics and chemistry183 but he rejected equally

strongly the suggestion of any single material or other

basis of chemical matter. He considered that he had now

provided a superior alternative to the protyle or electron

theory having abandoned that development as atrophied more

than a decade earlier.184 Mendeleef did 'not require the

recognition of a peculiar fourth state beyond the human

understanding (Crookes). All mystical, spiritual ideas about ether disappear'.185 He considered that the 'series

of recently discovered physico-chemical phenomena', partic-

ularly radioactivity, which had prompted his publication

and had 'caused many to return to the emission theory of

light, or to accept the, to me, vague hypothesis of electrons'

were best explained in terms of 'the entrance and egress of

ether atoms' and the 'familiar conception of an etherial

medium transmitting luminous vibrations, &c.'186 Such a conception owed much to the condensable ether of low density

of Kelvin187 whose case will be taken up shortly. Each of

these men died in 1907 whilst in opposition to the disin-

tegration theory. The example of Mendeleef may thus seem

to be a negative one, the more so since material ethers

faded from science during the early decades of the twentieth

century. Yet his low opinion of the hypothesis of electrons

was shared not only by chemists who like him rejected the

closely related disintegration theory but by some who

accepted the latter with enthusiasm.

W.Ostwald began his Faraday Lecture in the spring of

1904 with an unusual description of that 'venerated master'

256

as a past leader in the theory of force or energy. He

ended the address with a discussion of radioactivity which

was similarly inclined. Ostwald had fitted the notion of

spontaneous transmutation with his well known energetic conceptions of 'Elements and Compounds1188 in which 'Chemical dynamics' had 'made the atomic hypothesis unnecc-essary' for deducing the laws of chemical combination.189

He therefore depicted the chemical elements in terms of an

energy curve in the form of a series of 'stalactites'.

'The elements with the highest combining weight' Ostwald

represented by the shortest or rudimentary stalactites on

the 'sloping ceiling'; along these a drop of water would

run at varying speeds of its own accord. The heavy elements

were thereby endowed with the temporary existences required

by a theory of stepwise transmutations. He claimed an

independent realisation of the idea that their known stability constituted an argument for, rather than against, the

presence of a large store of energy within the elements.

In his view the attainment of artificial transmutation was

prevented only by the practical impossibility of concen-

trating sufficient power.19° Ostwald's approach gave no clue as to the subtlety of the eventual attainment of this end.

Its limitations are exemplified by his description of the

complex radiations merely as 'intermediate forms' of energy;

and by his attribution of both the radioactive and chemical

stability of helium to the same 'exceptionally long stalactite' when a distinction had already been recognised,

for example in the case of radium. The importance of

energetics for radioactivity cannot be denied on that

account since many problems of radioactivity and atomic

structure from that time onwards were expressed in such terms.191 However, considerations of energy were never

taken as a complete description of any atomic process; not

even by Soddy who pointedly refrained from depicting any detailed atomic structure for almost a decade.192

One of the most interesting features of Soddy's view

of radioactivity is his attitude towards the corpuscular or

electronic theory which became during the period under

257

oonsideration the very foundation of many physicists'

understanding of chemical matter. It appears that his

more or less continuous skepticism came to a form of

fruition in 1904 when it became almost constructive. The

clash between Oxford chemist and Cambridge physicist at

McGill in mid-1901193 illustrates Soddy's early hostility

to the corpuscular theory. It is possible that he had

acquired these views whilst at Oxford; one of his referees

there was soon to question the validity of his electrical-

emanation studies. During the course of the pioneering

work with Rutherford the antagonism was suppressed within

or absent from Soddy's mind; in their joint publications of

1902 on thorium the emission of corpuscles was cited as

evidence in favour of the occurrence of subatomic chemical

change. However, the earlier attitude began to reappear in

Soddy's lectures and writings of 1903-4 after his return to

England. During the course of his dozen lectures on

'Radio-activity'194 Soddy described quite fully both the

current cathode and beta ray researches and the theoretical

reasoning which led to an electrical explanation of mass

or matter. 'How far these calculations possess a real

meaning I am not in a position to say' he remarked with the

qualification that the electronic hypothesis despite its

many 'doubtful points' was yet 'necessary to assist the

mind in forming concrete mental pictures of the various

relations between matter and electricity'.195 In the ampli-

fication of his lectures in book form196 Soddy went a stage

farther as he closed similar discussions with a ohemist's

claim for independence: It may at once be pointed out that the theory of atomic disintegration, to which, in the succeeding chapters, the study of radio-activity will lead, is independent of the electrical or electronic view of atomic constitution. It postulates no view of atomic structure beyond the original conception of Dalton... 197

He commented that the theoretical dependence of the

electrical mass of an atom upon its internal energy served

to show: how useless it is to attempt to find numerical relations between the atomic weights of the

258

elements of the Periodic Table. It is notorious that all such efforts have been fruitless, but it is only recently that the reasons for the failure have been indicated.198

Now the Cavendish laboratory had quite recently continued

the discussions 'On a General Numerical Connexion between

the Atomic Weights'.199 Such acidic remarks200 can therefore

be interpreted either as another assertion of independence

from the Proutian theme or as an attempt to confound the

physicists with their own labours.

Against only one aspect of the physical view of radio-

activity were Soddy's criticisms effective, but here they

were particularly so. From the accepted theory it followed

that prior to its sudden disruption an atom of uranium for

example enjoyed a long period of quiescence. Physicists

had come to realise this via an interesting route, shortly

to be discussed. By 1904 they were generally satisfied

with the assumption that the period of temporary atomic

stability must be explained by a slow progression in the

atom's internal corpuscular pattern. Soddy considered

such a picture to be totally untenable. His attack was

based upon two main considerations. Firstly, a very slow

approach towards the point of disintegration should be

accompanied by similarly gradual alterations in the physical

and chemical properties of an atom. If this were so then

'it should be easy by chemical analysis to separate the

homogeneous elements into groups'. But apart from the con-

fused case of the rare-earths there was no evidence of any

Each success.201 This argument seems to impinge upon

Rutherford's suggestion202 that an outlying electron might

be the continuous cause of a disintegration. Against the

possibility that any such cause could exist Soddy put

forward a startlingly straightforward argument which could

not be disregarded: the asymptotic law of radioactive

decay could accomodate no stage of a definite timespan.

Although 'the average life' of its atoms was a specific

feature of each radioelement the law implied that 'some of

the atoms break up in the first second' yet others survived

almost for ever.203 He himself went beyond 'the original

259

conception of Dalton' to assert that individual differences

exhibited by different atoms of the same element were man-

ifestations of an 'extremely rapid motion' of 'the internal parts of an atom':204

the internal movements of the atom must be highly irregular and cannot follow a definite sequence if the law of radio-active change is to hold good. The unstable position appears to be rather the result of a chance collocation of the parts than to be due to the operation of any simple law. An analogy might be drawn from the kinetic theory of gases, in which certain of the molecules are regarded as possessing momentarily much higher and others much lower temperature than the average, and the acting causes are so complex that, although the proportion of the whole at any temperature may possibly be calculated when the total number of systems is exceedingly great, the individual history of any one molecule is quite indefinite. In a radio-active substance a definite fraction of the total assumes a peculiar orientation and disintegrates in each second, but the life of any single atom is quite indefinite. The causes at work appear to be so complex that the results can only at present be described as 'chance' or 'accidental' happenings, in the sense of being impossible to predict.205

One may also quite readily devise kinetic models to imitate

aspects of radioactivity for example by depicting an atom

as a vessel containing a number of gas molecules; the diff-

usion or chance escape of a molecule through a minute hole

in the vessel would represent one disintegration. A large

number of such atoms constituting one radioactive sample

would exhibit the required geometrical decay law yet would

allow the life of an individual to range unpredictably

between zero and infinity. Soddy gave no such crude pictures.

But he was of the opinion that the comparison of radioactivity

with gas kinetic theory 'is important' in the further respect

that:

it suggests the question whether all atomic properties are not really average properties, the individual atoms continually passing with great rapidity through phases varying widely among themselves in chemical and physical nature.206

This suggestion was in accord with some contemporary dev-

elopments in both of the areas specified in the title of

Soddy's new post of 'Lecturer in Physical Chemistry and

Radio-activity in the University of Glasgow' .207 His

260

conolusions were unaffected by Bragg's notable discovery,

announced in late 1904,208 of the definite and by no means

irregular velocities of the alpha particles emitted by

each element in a decay series.209 As Soddy had stated,210

his hypothesis was at least consistent with the known law

of decay whereas the current theory of the physicists was

not. Though Soddy's criticism of the electrical explanation

of radioactivity was indeed accepted by his former physical

partner the electronic theory in general did not succumb to

his incursions. J.J.Thomson's treatment of thermal and

electrical conduction in metals211 indicates the possible

compatibility between kinetic and electronic theories of

atomio structure. However when Thomson later defended his

notion of radioactive decay it was on quite different

grounds. Since the disintegration theory of Soddy involved

irregular and unpredictable movements it may appear to

provide an uncertain foundation for experimental advance.

The physicist E.von Schweidler firmly grasped this nettle

with the first formulation of a clear statistical-probabilistic

approach. In his paper at the 1905 Congress of Radiology in

Liege he showed that the law of geometric decay was deducible

from the probability equations for random processes. His

major point giber Sohwankungen der radioaktiven Umwandlung'212

was that as it declined the rate of disintegration should

undergo fluctuations according to probability predictions;

these, he estimated should lie within the grasp of actual

electroscopic detection. Thus in Vienna where Boltzmann

(d.1906) had done so much with the statistical theory of

gases213 this old approach opened up and contributed to the

development of a new experimental path which led quickly

back to Rutherford. Soddy's anticipations may have influenced

the latter who, however, credited Schweidler with the innov-

ation.214

On the other hand it could be argued that a probabilistic

interpretation of radioactive decay pre-dated even the

expressions of Soddy. His conclusions may be seen as the

ultimate internalisation of the 'kinetic' explanation of

261

radioactivity which had in 1898 been among the earliest

to be suggested. One of the turning points in this minor thread came in mid•-1903 at a time when the discrete nature of atomic disintegration was becoming appreciated. The

surrounding molecules of gas became, for some, not the

primary source of the energy released in radioactivity but

the detonators by impact of internal atomic explosions.

The statements of Lodge exemplify this stage. At the end

of 1902, after reading the latest publications of Rutherford

and Soddy, his interpretation of radioactivity apparently

included both internal and external factors: in the case of massive molecules their mutual collision or agitation under the influence of ordinary temperature is sufficient to shake away some of the loose electrons, which then fly off tangentially with whatever orbital velocity they may have had: giving rise to phenomena recently discovered under the name of radio-activity..215

He may have been the first to make the valuable suggestion,

linking the recent results of Curie and Rutherford, that the

heat produced by radium was a result of self-bombardment by

its 'massive' alpha-projectiles. Yet in his letter on

'Radium Emission' 216 which contained this point Lodge still put forward the alternatives of an 'assumed necessary

stimulus, or external supply of molecular energy'. Thomson's

article published one month later, at the end of April 1903,217

shows that despite his specification of an internal energy

supply he too favoured the notion of a kinetic stimulus:

Suppose that the atoms of a gas X become unstable when they possess an amount of kinetic energy 100 times, say, the average kinetic energy of the atoms at the temperature of the room. There would, according to the Maxwell-Boltzmann law of distribution, always be a few atoms in the gas possessing this amount of kinetic energy; these would by hypothesis break up; if in doing so they gave out a large amount of energy in the form of Becquerel radiation, the gas would be radio-active, and would continue to be so until all its atoms had passed through the phase in which they possessed enough energy to make them unstable... 218

A similar 'law of distribution' if applicable to the non-

gaseous radium atoms would account for their passage too

'into some other configuration'. A response to these views

262

was provided by Lodge, whose brief 'Note on the probable

occasional instability of all matter'219 directly followed

his acclamation of Rutherford's exposition of the disint-

egration theory at the London Physical Society in June.220

Considerations of radioactivity appear to have changed

Lodge's view concerning Larmor's well-known solution to

the problem of the theoretical loss of energy from any atom

containing orbiting electrons. This development in return

allowed the electron theory to impinge strongly upon radio-

activity at a point where it had been unconsciously held at

bay. In previous months Lodge had supposed that incessant

radiation exchanges made unneccessary Larmor's proposal of a

zero vector sum of the electrons' accelerations;221 the

process of radioactivity was quite distinct and involved the

release of orbiting electrons by molecular collisions.222

But Lodge was now prepared to insist upon the importance for

radioactivity of the 'radiation or loss of energy' which

'must occur from every atom'.223 Calculation showed that an

electron suffering this loss would move inwards at increasing

speed until, as its velocity approached that of light, the

mass 'becomes suddenly infinite or very great'. It was this

effect which in his opinion constituted the likely cause of

the breaking up of an atom. Like Thomson he understood

that it was 'only a question of time how long an atom shall

last before it reaches this stage'. The significant dep-

arture by Lodge is to be found in his concluding comment

directed specifically at Thomson that 'the slight constant

radiation-loss seems competent to bring about instability

and decay irrespective of collisions, and therefore indep-

endently of any Maxwell-Boltzmann law'.224 Lodge's discussion

had an influence not so much for his explanation of dis-

integration by increased electronic mass, for which there

were alternatives, but for two more general proposals.

Apparently following one of these, Thomson indeed dispensed

with the probabilistic analysis of radioactive decay. Perhaps

gladly so; even Soddy's counterblast implied that the stat-

istical method was no more than a temporary and sometimes unavoidable substitute for definite knowledge. Nevertheless,

263

considerations of that kind had formed a continuous theme

in radioactive studios from almost their beginning.

Lodge's most influential proposal was to the effect

that a steady radiation-loss, understandable in terms of standard electromagnetic theory, was the prelude to dis-

integration. For a time it raised hopes of picturing and

predicting changes within the chemical atom, which at that

time were not high. It appears that the approach initially provided by the electron theory towards a complete account of spectral patterns was offset by the growing complications

of magnetic studies. For those who attempted to design

electronic models of the atom there was also the fundamental

difficulty concerning the nature of the positive charge.

J.H.Jeans was one of those who included shells of the un-detectable positive electrons in the hypothetical model he constructed to explain the above phenomena and others,

though not radioactivity. His appeal of 1901 that this theory of 'The Mechanism of Radiation'225 be not even 'judged as an attempt to attain to ultimate truth' has been

described as a typical disclaimer of the period.226 Though radioactivity placed additional demands upon the proponents

of atomic models its exciting new facts were attended by

experimental certainty. In conjunction with the theoretical

points made by Lodge in 1903 these results perhaps contributed

to the optimism of some expressions which appeared in 1904.

H.Nagaoka indeed hoped that 'The rough calculation and rather

unpolished exposition' relating to the 'Kinetics of a System

of Particles illustrating the Line and the Band Spectrum and

the Phenomena of Radioactivity'227 'may serve as a hint to

a more complete solution of atomic structure'.228 The Electrician's editor heaped an unwonted amount of praise upon Thomson's 'Structure of the Atom'.229 The task of 'handling mathematically the swarm of flying electrons'

constituting the material atom was 'obviously a formidable one'. Yet Thomson had 'made a huge stride towards the goal'.230 He had developed 'lucidly and with great perfection,

a wonderful theory of the chemical elements', an account of

the 'main laws of the line spectra of a series of elements'

and finally 'the most suggestive conception yet offered of

264

the mechanism of the radio-active elements'.231 Yet in the same breath Thomson's electronic arrangement was

described as 'an apparently highly artificial conception'.

Rutherford, who was impressed by these models, did not long

maintain his current confidence that he knew the 'probable

... primary cause' of atomic disintegration.232 In fact deficiencies in the various systems were not hard to find.

One may discern a subsequent withdrawal to more reserved

attitudes as some of the contemporary criticisms found their

mark.

Between the time of his death in 1907 and his earliest

published comments on the disintegration theory in late 1903

Kelvin's interpretation of radioactivity fluctuated con-

siderably. One might say that his ideas moved in an ellip-

tical path since they appear similar at each of these dates.

Kelvin's 'Contribution to Discussion on the Nature of the

Emanations from Radium',233 read by Lodge at the British

Association meeting of 1903, has sufficient faults possibly

to have embarrassed the observer. He was the only disting-

uished physicist to reject, with Armstrong and Lowry, the

theory of Rutherford and Soddy.234 Kelvin's individual views

were that the gamma radiation was 'merely vapour of radium',

that the alpha rays were atoms of radium or molecules of

radium bromide which apparently also comprised the emanation,

and that the experiments which purported to show a loss of

weight from active materials235 were acceptable. He attrib-

uted the extreme activity of radium to its possession of an

abnormally large quota of 'electrions' neutralising the

positive atom. 'But' he noted 'this leaves THE mystery of

radium untouched'. Kelvin's atoms provided no obvious energy

source: he considered it 'utterly impossible' that the known

emission of heat 'can come from a store of energy' in the

radium.236 He wrote in similar vein to J.Dewar237 and

W.Ramsay238 describing as 'utterly improbable' the hypothesis

of 'evolution in the atom or transformation of its substance'.

Instead it was 'absolutely certain' that 'energy must somehow

be supplied from without' possibly by means of ether waves.239

A.S.Eve has noted240 that Kelvin courageously 'abandoned

265

his theory publicly at the 1904 British Association' to

'fall in line with Rutherford's ideas'. But though true

this was not the whole, nor the end, of that story.

Kelvin indeed constructed his 'Plan of a Combination

of Atoms having the Properties of Polonium or Radium':241

(1) To store a large finite amount of energy in a combination having very narrow stability. (2) To expend this energy in shooting off with very great velocity, vitreously and resinously electrified particles.242

But when interpreted in the light of the atomic theory

described in his paper 'Aepinus Atomized'243 the conversion

seems marred by a heresy, or contradiction. Kelvin's model

atoms of 1901 had consisted of ponderable but interpenetrable

spheres244 of various sizes, positive electricity distributed

uniformly within. One or more potentially mobile but normally

static negative 'electrions' occupied each sphere. However,

out of line with the unifying ideal of the electron theory,

he felt that one could not assume that electrical forces

alone operated between atoms:

we must keep ourselves free to add a repulsion or attraction according to any law of force, that we may find convenient for the explanation of electric, elastic, and chemical properties of matter.245

Thus Kelvin's atoms were distinct from each other in several

respects namely, size; 'quantum' of positive charge and

whether or not completely neutralisable by electrions; and

finally 'it is possible that the differences of quality are

to be wholly explained in merely Boscovichian fashion by

differences in the laws of force between atoms'.246 In 1901

Kelvin appears to have allotted one distinct atom to each

chemical element, certainly at least for 0,N,H,C1,C,S and

Na;247 and in 1904 he indicated his retention of these

earlier notions.248 But his plan for polonium contained no

less than sixteen atoms, and that for radium two of different

sizes. Kelvin's statement that these substances differed

from 'ordinary matter' only in the high degree of their

'shooting'249 thus calls into question his conception of a

chemical element.

266

This inconsistency may possibly have been a cause of

the reactionary trend to be seen in his succeeding state-

ments. Thus his 'Plan of an Atom to be capable of Storing

an Electrion with Enormous Energy for Radio-activity'250

of 1905 involved considerations of thelwork-curve' within

a single atom only, albeit of a different, onion-skin,

design. And after opening a public dispute in 1906 con-

cerning among other points the manner in which 'radium'

could be said to contain helium251 Kelvin reverted to views similar to those he held in 1903. His final state-

ments that the energy was drawn from external heat and that

heating effects were mainly produced not by alpha particles,

which were charged radium atoms, but by emitted electrions252

isolate Kelvin from current research. Despite the evident

flexibility of his approach Kelvin was never quite able to

countenance the transmutation of atoms.253

Whilst Kelvin directed his theoretical considerations

specifically upon radioactivity two other physicists

incorporated the subject instead as a more or less important

secondary feature of atomic structures which were designed

primarily to explain other phenomena.

I propose to discuss a system whose small oscillations accord qualitatively with the regularity observed in the spectra of different elements and by which the influence of the magnetic field on band- and line-spectra is easily explicable. The system here considered is quasi-stable, and will at the same time serve to illustrate a dynamical analogy of radioactivity, showing that the singular property is markedly inherent in elements with high atomic weights.254

With these words H.Nagaoka introduced what he described as

a new version of an old story. He took a single 'positively

charged particle' surrounded by a revolving circular ring

of equally spaced electrons to be perhaps 'the most easily

conceivable' system for mathematical treatment; actual

chemical atoms would possess a number of concentric rings one

corresponding to each of the different spectral series

exhibited.255 Nagaoka saw a connection between this spectral

hypothesis and radioactivity in the example of radium.

Since its spectrum appeared simpler than those of elements

267

of comparable or oven lesser atomic weights then the radium

electrons must be arranged in fewer and therefore larger

rings. His dynamical analysis indicated that the more

electrons there were in a ring the greater its susceptibility

to the disturbances which might lead to disruption. Further-

more, as he noted, elements of high atomic weight were most

likely to contain 'massive rings' and consequently to exhibit

radioactivity, the manifestation of instability. This reason-

ing which evidently explains the relative activities of

uranium and radium may appear promising. However, the spec-

tral correlations, electrical neutrality, and mechanical

stablility of the structure were all put in doubt by the close questioning of G.A.Schott who claimed to have rejected

such a system as both unstable and 'not worthy of publication'

some five years earlier.256 He had deduced that the theoret-

ical vibrational instability attributed by Nagaoka to large

rings in fact extended much farther and applied to almost

all rings bar a handful of the smallest variety. In any

case it seems that Nagaoka came close to refuting himself

in mentioning Sir Oliver Lodge and the different problem of

the radiation loss from an orbiting electrical charge. For

he neither followed Lodge in considering this as a cause of

instability nor did he attempt to neutralise the difficulty

in the vectorial manner of Larmor and Thomson. He merely

stated that the loss from a 'Saturnian system' should be

'properly compensated' but did not say how this might be

arranged. Other difficulties of Nagaoka's system relating to

radioactivity were less readily avoidable than this. The

'disintegration of the ideal atom', he thought, involved

the breakage of a ring when its electrons 'will disperse in

various directions with great velocities, and the positively

charged particle at the centre will also fly off'.257 His

failure to indicate whether or how changes in the central

particle could explain the release of two heavy positive

particles, for example from radium, may relate to the

current problem of understanding the nature of the positive

charge. On the other hand there appears to be a quite

268

definite conflict with the known fact that alpha and beta

rays were emitted separately; a solution would seem hard

to find. In considering the stages leading to disinteg-

ration Nagaoka assumed that a ring was subject to 'resonance'

which 'in course of time, if the disturbance be persistent,

will acquire such an amplitude as to break the ring'.258

He was prepared to name as initiators of the resonance both

vibrations of other rings within the atom and incident

electromagnetic waves. But his discussion regarding such

an external cause seems only to weaken the case. He argued

indeed that since the destructive higher harmonics could

be excited by light of short wavelength it followed that

'actino-electric action259 may be the result of the des-

truction of atoms' under the combined influence of an electric

field and incident radiation; semiconduction effects, known

also to be produced by these forces, might be similarly

explained. Nagaoka's optimistic attempts to reunite dis-

parate phenomena in this way were short lived, thanks largely

to Schott. Yet they bore some resemblance to the better-

appreciated efforts of Schott's former professor.

J.J.Thomson too placed radioactivity in an important

supporting role in a plan of atomic structure. Issued in

late 1903, his paper on 'The Magnetic Properties of Systems

of Corpuscles describing Circular Orbits'260 contains a partial defence of such systems by examining:

problems ... met with when we attempt to develop the theory that the atoms of the chemical elements are built up of large numbers of negatively elec-trified corpuscles revolving around the centre of a sphere filled with uniform positive electri-fication.261

Regarding this theory he confided to Lodge some months later

that: I have ... always tried to keep the physical conception of the positive electricity in the background because I have always had hopes (not yet realised) of being able to do without positive electrification as a separate entity, and to replace it by some property of the corpuscles.262

This attitude allowed Thomson to go beyond the ponderable

positive atom adopted by Kelvin during 1902-4 which was

superficially similar to his own. Thomson was thus able to

269

explain physical phenomena, including radioactive trans-

mutations, in almost exclusively electronic terms; the

positive charge was a more follower of the encampment of

corpuscles.

The first point which Thomson considered in his paper

on magnetic properties was the problem of radiation loss.

His analysis showed that when corpuscular velocities were

email compared with the speed of light the radiation dimin-

ished very rapidly as the number of particles increased.

For example, 6 corpuscles rotating at 1/100 the velocity of

light emitted elliptically polarized radiation at only 10-16

of the intensity for a single corpuscle.263 Rather than

claim permanence for the arrangement, as he might,264 Thomson

instead used the very small theoretical lose in a remarkable

attempt to solve two further problems of the system which

at the same time united all three. He confirmed W.Voigt's

deduction that the magnetic properties of corpuscles set

out in rings also tended to nullify each other; but in this

case cancellation was complete. The hypothetical system

failed to display the known magnetic properties. The master-

stroke which might have succeeded was Thomson's proof that

this conclusion did not apply if the system were losing

energy:265 such a dissipation he related to radioactivity:

suppose the atoms of a substance, like the atoms of radio-active substances, were continually emitting corpuscles; the velocity of projection ... being, however, insufficient to carry them clear of the atom ... then, if the motion of the corpuscles were not accompanied by dissipation of energy, the corpuscles would not endow the body with either magnetic or diamagnetic properties; if, however, the energy of the corpuscles was dissipated during their motion outside the atom, so that they ultimately fell with but little energy into the atom, a system consisting of such atoms would be paramagnetic.266

He suggested that if the 'energy of projection were derived

from the internal energy of the atom' then experiment should

reveal a higher temperature within iron than brass. Such

results were never reported; corpuscular theory, magnetism

and radioactivity were not to be connected in this way.

Thomson waited for the major sequel 'On the Structure

270

of the Atom: an Investigation of the Stability and Periods

of Oscillation of a number of Corpuscles arranged at equal

intervals around the Circumference of a Circle; with

Application of the results to the Theory of Atomic

Structure'267 to make clear the link which he envisaged

between a tiny continuous radiation loss and the projection

of particles from the atom. In this paper he gave explan-

ations of the periodic table, chemical valency and affinity

including the inert gases, and spectral formulae. He again

compared the basic structure with Mayer's magnets268 as in

1883 and 1897; the rotations imparted to the latest models

tended to stabilise movements from the plane. And this

was the clue to Thomson's view of the 'Constitution of the

Atom of a Radioactive Element'.269 Standard dynamical

analyses showed that the speed of rotation could be critical

for such stability. With 4 corpuscles, for example, the

planar arrangement would be more stable than the tetrahedral

only if the angular velocity exceeded a definite value

depending on corpuscular charge, mass and number, and the

atomic radius; below this value the stabilities would be

reversed. And at this value 'there will be what is equiv-

alent to an explosion of corpuscles'. The increased kinetic

energy 'might be sufficient to carry the system out of the

atom, and we should have, as in the case of radium, a part

of the atom shot off'. The approach of a long-lived atom

to this value was provided by the radiation loss which

allowed velocities 'slowly - very slowly' to diminish.270

'I think a spinning top is a good illustration of the radium atom' he told Rutherford.271 Thomson did not mention Lodge's

more extreme alternative. On the basis of the former's

suggestions the approach of a corpuscle's velocity close to

that of light appears unneccessary. However, the emission

from radium of beta rays travelling at 90% of this speed

was well known.272

Rutherford continued to follow the electronic tradition

and cited the discussions of Lodge and Thomson on several

occasions. The high velocities and independent emission of

the beta rays may have been reasons for his leaning towards

the hypothesis of Lodge during 1904. Of late Larmor's

271 'lion of the season' Rutherford was now leader by far of the field in his incorporation of these and other facts,

many of his own making, into a coherent theory of radio-

activity. His attempts on this basis to depict the con-

stitution of the chemical atom suffered from the same

fundamental problems as those of his fellows. Yet to the

experimental scientist such diseases were fortunately not

fatal.

3. Conclusion

Early in 1904 Rutherford saw the radium series

extending to at least273 seven members. The succession of changes, with its radiations and half-lives, proceeded from

Radium (alpha, 1500 years274) to Radium Emanation (alpha,

4 days), followed by 'Emanation X' (alpha, 3 mine., soon

afterwards called radium A), 'Second change' (no rays,

36 mins.), 'Third change' (alpha, beta and gamma, 28 mins.), 2 'Fourth change' (alpha, beta, 200 years) and 'Final product'.75

Upon a theory framed only two years earlier was this new and

increasing knowledge founded. As might be expected Rutherford was not content to rest upon this base but

attempted to move towards a deeper synthesis. He argued

that the quite different six-membered thorium series and

272

uranium trio bore a resemblance to the above radium

sequence in one important respect:

The and probably also the rays of the three radio-elements thus only appear in the last of the series of radio-active changes. It is remarkable that the last change, which is readily detected by the radio-active property, should in each of the three radio-elements be accompanied by the expulsion of a single electron with great velocity... 276

Rutherford's subsequent Bakerian Lecture of mid-1904 contains

a description of 'The Succession of Changes in Radioactive

Bodies'277 which was the most detailed physical represen-

tation of the process of disintegration yet to appear; its

expression can be seen as a peak of confidence. The 'single

electron' of the 'last change' had become the mechanical key which released the apparently more important 'groups of

electrons' or alpha particles. According to Rutherford

events took the following course:

It may, perhaps, be supposed that occasionally one of the outlying revolving electrons, comprising the radio-atom, lapses into a position which results in a slow loss of energy ... in the form of radiation.278

In the ensuing situation of instability an alpha particle would fly off 'with its great orbital velocity, but the

atom still retains the disturbing cause' so that the required repetition would result. Meanwhile, and here Rutherford

acknowledged his debt to Lodge, the electron's velocity

would be increasing slowly until 'finally in the last stage a sudden lapse into a new state' ejected another alpha particle together with the rogue electron. Radium C

(RaC,half-life 28 mins.) and thorium B (ThB, 55 mina) were the crucial substances concerned here. The residual atom then 'adjusts itself again into a position of more permanent

equilibrium'279 corresponding to the longer lived product

RaD (40 years). That this latter material and UrX emitted

only beta rays seems not to fit in with this scheme.

Nevertheless Rutherford was sure that 'The experimental

evidence as a whole points strongly to the conclusion that

the change in which the /3 rays appear is far more disruptive in character than any of the preceding ones'. For not only

273

wan the accompanying alpha particle from RaC more pene-

trating than its predecessors, but recent electrolytic

results on ThB could be interpreted as a revelation of,

to use an anachronism, fission products. These were 'to

be expected' from the 'violent character' of this particular

change.280 Though branching disintegrations and high alpha

velocities were later confirmed for both RaC and ThB, and

the actinium series seems to have fitted quite well into

the scheme by 1905,281 yet during that year the fairly clear

theoretical picture became much obscured.

As the expanded second edition of his book shows,

Rutherford continued to think it:

probable that thep particle, which is finally expelled, may be regarded as the active agent in promoting the disintegration of the radio-atom through the successive stages. A dis-cussion of this question will be given with more advantage later (section 270) when the general question of the stability of the atom is under consideration. 282

He did not, however, provide the promised discussion; nor

did he again describe this electron as 'outlying'. One

reason for this may possibly have been the chemical

implications; another may be that such a description begged

the question of the cause of the electron's initial or

occasional 'lapse'. Hence Rutherford settled for his earlier

argument that although the law of decay could not itself

make a distinction negative experimental results283 made it

likely that rather than any external detonator it was 'forces

inherent in the atoms themselves' which brought about their

instability. And he repeated his view that:

It seems probable that the primary cause of the disintegration of the atom must be looked for in the loss of energy of the atomic system due to electro-magnetic radiation.284

But, as we have seen, Soddy's criticisms involving the

law of decay struck an area which no physicist, save perhaps

Nagaoka and Kelvin, had covered. To summarise the problem,

if the vector sum of accelerations of a system of electrons

is zero then the arrangement is stable, as are the atoms

of ordinary chemical elements. If the sum is not zero then

decay should occur: but not according to the observed law.

274

For this implied that some atoms disintegrate immediately

after their formation. To this argument Rutherford

responded with an unhappy compromise, outlined in his

published Silliman Lectures of 1906,285 before withdrawing

to a safer position. He still thought it most probable that

radiation loss was the 'primary cause' of disintegration286

but was aware that a steady drain from all radioatoms 'is

contrary to the observed law of transformation'. In attempt-

ing to avoid the difficulty Rutherford employed for the

first time the following argument:

We thus arrive at the conclusion that the configuration of the atom which gives rise to a radiation of energy only occurs in a minute fraction of the atoms present at one time, and is probably governed purely by the laws of probability.287

To this statement he appended a suitably revised version of

the single-electron theory in which 'one of the electrons

may take up a position in the atomic system which leads to

a radiation of energy'.288 Unfortunately such a hypothesis

harboured unmentioned problems and contradictions. For

since some atoms exploded in less than a second this energy

loss could neither be slow nor regular, nor could it con-

sistently be described as the primary cause of atomic dis-

integration.

The fundamental question of the processes leading to

disintegration was to be explicitly raised on numerous

occasions during the succeeding decades, which saw the dev-

elopment of the nuclear atom, the displacement rule and the

discovery of artificial transmutation. Even in the 1920's

when probabilistic interpretations of atomic structure began

to come into their own it remained unanswered.289 By the

end of 1906 despite his announcement of a new and suggestive

correlation between rate of decay and alpha velocity, which

could readily be related to the underlying concept of atomic

stability, Rutherford had reached the final retreat:

In the absence of any definite knowledge of the causes which lead to the successive disintegrations of the atom, it does not seem possible at the present time to give any adequate explanation of the modes of transformation observed in radioactive matter.290

275

The words with which he concluded that discussion of 'The

Velocity and Energy of the."( Particles from Radioactive

Substances' are of great interest:

A study of radioactive phenomena has emphasized the importance of the a particle as one of the units of which the heavier atoms are built up, and it is not improbable that the < particle may play an equally important role in the constitution of other atoms besides those of uranium, thorium, radium and actinium.291

This statement appears particularly significant when com-

pared with the closing comments of his Silliman Lectures

which had appeared shortly before:

It appears by no means improbable that the so-called radioactive bodies may differ from ordinary matter mainly in their power of expelling o< particles above this critical speed. Ordinary matter .., might be emitting a particles at a rate comparable with uranium ... and ... may be undergoing slow atomic transformation of a character similar to radium, which would be difficult to detect by our present methods.292

For these thoughts exemplify two aspects of what may be

seen as one broad theme which at that time seemed much more

than the mere speculation it had earlier been. A third

facet, which can be termed the 'cosmical' completes a trio

each complex member of which was to suffer a different fate.

Each area of this general idea that the phenomenon of radio-

activity was possessed of a universal nature had caught the

scientific imagination towards the end of the period of our

main concern at the time when the disintegration theory was

making its initial impact.

Early in 1903 Rutherford293 and Soddy cryptically

revealed their appreciation of the cosmical relations of

radioactivity. These authors were interested to point out

that Lockyer's views on Inorganic Evolution agreed with

their own on subatomic change. However, they effectively

reversed his dissociation hypothesis by noting that 'he

regards the temperature as the cause rather than the effect

of the process'.294 One striking implication of this was

made clear by W.E.Wilson's estimate, using Curie's figures

for the heat from radium, that the presence of this element

in a proportion of only 3.6 gm. per cubic metre (about

2.5 p.p.m., of the order of that in pitchblende) 'would

276

suffice to supply the entire output' of heat from the sun.

His brief letter to Nature on 'Radium and Solar EnergY'295 was followed by those of others, who hastened to take up

the exciting corollary of vastly increased astronomical

time scales, under headings such as 'Radioactivity and the

Age of the Sun'296 and 'Radium and the Geological Age of the Earth',297 as well as to point out difficulties.298

Rutherford, surveying these discussions shortly afterwards,

expressed the opinion299 that the time scale of the sun

might be 'from 50 to 500 times longer' than Kelvin's

estimates based on the energy of gravitational contraction

from a dispersed state. And he followed Joly's view that

the physicists' assessment of the earth's quiescent life-

period might now be stretched sufficiently to fit the min-

imum of 100 million years required by the biologists and

geologists against whom Kelvin had long argued. To geology,

radioactivity made a positive contribution via the minerals

which had from the first constituted the source of radium.

Their composition was both empirically studied and theoret-

ically explained on the basis of the disintegration theory

in the search for the parents and inactive descendants of

radium, which had begun in earnest by 1904.300 And a success-

ful approach towards the relative and even absolute dating

of minerals was one of the 'Cosmical Aspects of Radioactivity'

most confidently described by Rutherford in his departing

lecture to the Canadian Royal Astronomical Society.301

A.Schuster, introducing his own speculations on

'Cosmical Radio-activity', joined one aspect of the universal

theme with another in a statement typical of the period:

The fact that every physical property hitherto discovered in one element has always been found to be shared by all suggests the possibility that radio-activity may be a common property of all matter.302

By that time, in the autumn of 1903, Crookes, Kelvin, Lodge,

Thomson and others less well known in the field303 had already

assumed as much. However, the attitude of some experimental

students of radioactivity was more cautious. The Curies in

1900304 and Marie Curie again in 1903305 admitted their

predilection for 'the idea that it was scarcely probable

277 that radio-activity, considered as an atomic property,

should belong to a certain kind of matter to the exclusion

of all other', but then made the point that observation

showed any general activity to be less than 1/100 that of

uranium. By the time the later statement was made

R.J.Strutt and others had independently directed their more

sensitive experimental attention to the 'Radio-activity .of Ordinary Materials'.306 This, Strutt claimed, was small

(1/3,000 Ur) and variable but definite. His comment that to give such an effect one part of radium in three hundred million would suffice indicates the drift of his inter-

pretation. Moreover, the emanations and their active dep-

osits were known to be present in the atmosphere so that.

a minute surface activity of all solid materials was nat-

urally to be expected.307 But did any of the observed activity

in fact belong to the materials themselves? This question was

seen to.be of some importance with regard to general support

for the electron theory and, perhaps even more so, in rel-

ation to the theoretical problem of atomic stability which

beoame acute during 1903-4. As Rutherford succinctly

remarked:

According to the modern views of the constitution of the atom, it is not so much a matter of surprise that some atoms disintegrate as that the atoms of the elements are so permanent as they appear to be.308

J.J.Thomson indeed entertained notions which implied that ordinary chemical atoms were not permanent. We have con-

sidered the theoretical analyses of 1903 which induced him

to suppose that even the force of magnetism might derive

from the atoms' internal energy which would be finally dissipated as heat. And in a paper describing experiments

'On the presence of Radio-Active Matter in ordinary sab-

stances',309 read early in the following year, appeared his

conclusion that the ordinary material of the walls of a

closed vessel emitted, in addition to the effects of the

ubiquitous atmospheric emanation, a specific radiation of its

own. This too he thought 'involves a continual transformation . of the internal energy of the atom into heat',310 so also did

the normal dissociation and recombination of the gaseous ions

by which the radiation was detected. In this way Thomson

278

gave the universal radiation drain implied by his corpus-

cular atomic theory some experimental substance before the

appearance of his theoretical discourse 'On the Structure of the Atom'.311 We recall that during 1902 Thomson had

insisted that radioactivity was due to ionic interactions

of ordinary materials in opposition to the view of Rutherford and Soddy that special kinds of matter were

involved. And in some sense the difference of opinion was

maintained. However, N.R.Campbell in whose hands Thomson loft the subject obtained seemingly positive evidence of

'The Radiation from Ordinary Materials',312 though not of

the hoped-for heat emission. This turned Rutherford's strong

reservations of 1904313 gradually314 to complete acceptance.

Thus Rutherford proclaimed in 1906 that Campbell's results afforded:

very strong proof that ordinary matter does possess the property of emitting ionizing radiations, and that each element emits radiations differing both in character and intensity.315

Now this evidence did not stand alone. Rutherford had com-

bined it with two independent observations, which were

becoming clear during 1903-4, under the familiar 'general

principle' that 'every physical property discovered for one

element has been found to be shared by others'. The existence

firstly of quite rapid 'rayless changes' and secondly of a

rather high 'critical velocity' below which any emitted alpha

rays would be undetectable316 each implied that continual

unseen transformations might be universally proceeding. All

this evidently seemed convincing to Rutherford. But the

direct evidence soon faded into irregularity317 and, as one

can see, the latter two points merely allowed of a possib-

ility.

A further side to the universal view of radioactivity,

which may be termed the 'Proutian', has been introduced by

quoting Rutherford's words of 1906. Rutherford had by 1904 moved towards what he saw as a specifically radioactive

development of 'Prout's hypothesis,318 based on the observed successive release of alpha particles or helium from radio-

atoms. There was in his opinion 'no reason to suppose' that

279

radium was 'not an element in the ordinarily accepted sense

of the term' so that 'the radium atom is built up of parts, one of which, at least, is the atom of helium..319 He was able readily to combine this with the current electronic

version of Prout's hypothesis by asserting that the alpha

particles were themselves 'groups of electrons'. The radio-

atom thus consisted of electrons and large groups of elect-rons.320 W.H.Bragg accepted such ideas as the theoretical

basis of his experiments 'On the Absorption of the «Rays'321

and took them farther. In a letter to Rutherford he argued

that the alpha particle 'or some submultiple of it' might be

a 'common constituent' both of radioactive substances and of ordinary ionisable gases:

Then the electrons would be as it were the soldiers of the army, but the o< particles would be the regi-ments. Might not this account for the atomic weights having a leaning to whole numbers?322

Rutherford in turn extended his previous discussion with.

the remark that 'many of the elements differ in their atomic

weight by four - the atomic weight of helium'.323 Yet his continuing caution that the helium atom was but 'one of the

secondary units with which the heavier atoms are built up,-324

is to be noted. So too is his newly circumspect statement that the 'atoms of all bodies are built up, in part at least, of electrons'.325 If he now felt concern about the problem of the positive charge326 then this was soon to be justified.

In J.J.Thomson the persuasive force of Prout's hypo-thesis was manifested most clearly. Its effect on him

dates back to the 1880's when Crookes was proclaiming as

protyle the sun-element helium, then no more than a gleam

in the spectroscopist's eye. And its influence runs through the entire first decade of radioactivity to Thomson's

demolition in 1906 of his own all-electronic atom. 'The

Number of Corpuscles in an Atom'327 turned out to be a mere one thousandth of the number required to account for its

mass. At the opposite end of the discharge tube, whence he had in 1897 first extracted the corpuscular isubstanceq28

280

Thomson believed ho had found an alternative. The deflect-

able 'Rays of Positive Electricity'329 produced from a

variety of different elemental gases were apparently mainly

composed of streams of the alpha particles of radioactivity

together with hydrogen atoms. Thomson's dualist Proutian

interpretation of this particular result was unfortunately

most impermanent.330 Yet the idea provides one thread by

which to unravel the material upon which Lodge based his

judgement: regarding Rutherford's Radio-activity he wrote

'Scarcely anything to be found in this book was known

10 years ago'.331

281

NOTES FOR CHAPTER 1 (pages 8-47)

1 W.C.D.Dampier-Whetham, A History of Science, London, 1929, 382; D.L.Anderson, The Discovery 6f-The Electron, Princeton, 1964, 16; L.Badash, RutherfoW:Tia Boitwood: Letters on Radioactivity, London, 1969, Introduct on, Trig-eTimond Soientific Revolution', p.lf.; M.P.Crosland, The Science of Matter, London, 1971, 32; M.J.Nye, BWIecular Reali15.71Bndon, 1972, p.x.

2 L.Badash, The Early Developments in Radioactivity, with Emphasis on Contributions from the United States, Dies., Yale Univ., 1964, p.xiii, citing Maxwell, Papers, 244. L.Badash, The Completeness of Nineteenth Century Science, Isis, 1972, 63, 48-58, has since expressed a modified 1-7174.

3 Maxwell, ibid. 4 Page v. 5 Page 303. 6 RI Lib.Sci., (1889), 3, 481-92, 492. 7 W.McGucken, Nineteenth:-Century Spectroscopy, Baltimore

and London, 1969; S.G.Brush, The Development of the Kinetic Theory of Gases.VIII.Randomness and Irrevers-ibility, Arch.Hist.Exact Sci., 1974, 12, 1-88.

8 L.P.Williams the of VI7torian Science, Victorian Studies, 1966, 9, 197-204, 198-9.

9 Mme.S.Curie, Les Rayons de Becquerel et le Polonium, Rev.Ggn.des Sal., 1899, 10, 41f., Jan.; Oeuvres, 60-76.

10 Ibid., Oeuvres, 75. 11 RY-Eib.Sci., (1897), 5, 36-49, 30th Apr. 12 This. 787 13 D.M.Knight, Atoms and Elements, London, 1967, ch.3. 14 T.W.RichardsTriaTF-Weights, Chem.N., 1900, 81, 113. 15 W.V.Farrar, Nineteenth-century speculations on the

complexity of the chemical elements, Brit.J.Hist.Sci., 1965, 2, 297-323, 303f. Also relevani7-2.11/4/6731,-THe theory of the elements and nucleosynthesis in 19th century, Ch ia, 1964, 9, 181-200, but this has some anachronistic n erpretations.

16 T.H.Levere, Affinity and Matter. Elements of Chemical Philoso 1600-1665, Lon:Eli77971. R.Pox Tscusses this in re a on to The caloric Theory of Gases from Lavoisier to Regnault, London, 1971, ch.4, 6E767--

17 W.MoGuoken, Nineteenth-Century Spectroscopy, p.xi, lf. 18 Rep.Brit.Ass., 1666, 556-76. 19 IbidT7-561. 20 Ibid. 21 ysia. 22 Genesis of the Elements, RI Lib.Sci., (1887), 3, 403-26;

Presidential Addressed to-MliEical Society of London, J.Chem.Soc., Trans., 1888, 53, 487f.; 1889, 55, 256f.; Maugura-Address as President of Institute c77 Electrical Engineers, delivered 15th Jan.1891.

Notes for Chapter 1, p.8-47) 282

23 See e.g. E.von Meyer, A Histo of Chemistry, London, 1891, 349-50; also R. K. De kiisyk pecTF-oscopy and the Elements in the Late Nineteenth Century: the Work of Sir William Crookes, Brit.J.Hist.Sci., 1973, 6, 400-23, who discusses disagreements with French speotroscopists.

24 See A.E.Woodruff, William Crookes and the Radiometer, Isis, 1966, 58, 188-98; also G.G.Stokes, Mem.& Correa., 2, 8-408; S.G7Srush (ed.), L.Boltzmann, IdEfEiTtsWITTis

Theo , London, 1964, 25. 25 On t e fractionation of yttria, Rep.Brit.Ass., 1886,

586-90; also Genesis of the Elements, loc.cit., 405-16. 26 Genesis of the Elements, loc.cit., 411=27 27 Rep.Brit.Ass., 1886, 586-90. 28 Genesis 63Whe Elements, 421. 29 W.V.Farrar, op.cit., 319. 30 Kelvin, in G.G.Stokes, Papers, 5, p.xxxi; R.T.Glazebrook,

Report on Optical Theories, Rep.Brit.Ass., 1885, 157-261, 211.

31 Hereafter the name Kelvin is used, to avoid confusion with other scientists.

32 G.G.Stokes, Pa ers, 4, 373. 33 A.J.Meadows, Sc ence and Controversy. A biography of

Sir Norman f..6CITre7717c -gon, 1972, 169. 34 Researches in spectrum analysis etc., Bakerian Lecture,

Phil.Trans., 1874, 164, 479-94, 491. 35 =Stokes, Papers,-W7 365-6. 36 See e.g. On a certain reaction of Quinine, J.Chem.Soc.,

May 1869; Papers, 4, 327-33 37 On the Nature of the Röntgen Rays, Wilde Lecture 1897,

Papers, 5, 273. 38 G.G.Stokes, Mem.& Correa., 1, 406-7. 39 W.H.Brock, Lociq'ei-FIETIThe Chemists: the First Dissoci-

ation Hypothesis, Ambix, 1969, 16, 81-99. 40 W.McGucken, Nineteenth-Century Wectroscopy, 83-101. 41 A.J.Meadows, Science and Controversy 49tc.,ch.6; see also

C.L.Maier, ThITRn6f—gpectroscopy iE7The acceptance of an internally structured atom, 1860-1920, Dies., Univ. Wisconsin, 1964, 186-202 on 'Reactions to TBWEyeris dissociation theory'.

42 W.McGucken, 22.oit., 98; G.D.Liveing and J.Dewar, Collected Papers on Spectroscopy, Cambridge, 1915, 79, 139.

43 Proc.Roy.Soc., 187, 61, 148-209; C.L.Maier, Diss., 202-37, Mrs-Fribes Lockyer's later dissociation theory put forward about this time. 'Enhanced lines' indicate dissociation, but not now into products common to different elements.

44 See above p.18. 45 Quoted in M.W.Travers, A Life of Sir William Ramsay,

London, 1956, 100. 46 Ibid., 110, 154. 47 Phil.Mag., 1901, 1, 311-4; C.L.Maier, Dies., 105-6, 192,

men ions the ProuTian expressions of tEi.:7-s-pectroscopist J.R.Rydberg about this time.

48 The Ultra-Violet Spectra of the Elements, RI Lib.Sci., (1883), 3, 257-67, 259.

49 Quoted in D.M.Knight, Atoms and Elements, 130. 50 RI Lib.Sci., (1888), 3, 472-5.

Notes for Chapter 1, p.8-47) 283

51 W.V.Farrar, Nineteenth-century speculations etc., 317. 52 Proo.Camb.Phil.Soo., 1898, 10, 38-40, 28th Nov. 53 17RanTriiri317WaFEurg, Ueber die specifische Wgrme des

Quecksilbergases, Ann.d.PhT, 1876, 157, 353-69. 54 Liveing, 22.cit., E752, 39- O. 55 G.J.Stoney, b?-the Kinetic Theory of Gas, regarded as

illustrating Nature, Phil.Mag., 1895, 40, 362-83. 56 See Section 4 below, p.32-6. 57 Liveing, 22.cit., n.54. 58 Ibid. 59 kelvin, Nineteenth Century Clouds over the Dynamical Theory

of Heat and Light, RI Lib.Sci., (1900-1), 5, 324-58, 335. 60 J.J.Thomson, RecollectIZEs and Reflections, Toronto,

1937, 341. 61 Lord Rayleigh, The Life of Sir J.J.Thomson, Cambridge,

1942, repr. London, 1969, 62 G.G.Stokes, Wilde Lecture 1897, Papers, 5, 257-8. 63 J.N.Lockyer, On the Chemistry of e otTest Stars,

Proc.Roy.Soc., 1897, 61, 148-209. 64 W.H.Brock, Lockyer aria-the Chemists etc., 98-9. 65 A.Schuster, Note on the chemical constitution of the stars,

Proc.Roy.Soc., 1897, 61, 209-13. Appended to Lockyer's paper, loc.cit., n.63.

66 A.J.Meeff&TrE; -ecience and Controversy etc., 152-3. 67 J.J.Thomson, T-stoTirreciTims and Reflecil7ns, 341.

G.Fitzgerald, 0.Lodge, W.Sutherland, thought the effects purely electrical.

68 Electric Discharge through Gases, RI Lib.Sci., (1894), 4, 282-90.

69 R.F.Schaffner, Nineteenth-Century Aether Theories, Oxford, 1972, 76.

70 On these aspects of Faraday's work, see L.P.Williams, Michael Faraday, London, 1965.

71 Preface to the let ed., 3 ed., 1892, p.viii. 72 M.Faraday, Thoughts of Ray Vibrations, Phil.Mag., 1846,

28, 345. 73 Fe-scribed by K.F.Schaffner, 22.cit.; and E.T.Whittaker,

History of the Theories of Aether and Electricity, 2 vols., London, 175111 1, ch.4f. /ST-Eiglyses of the latter are criticised by the former (p.viii).

74 Rep.Brit.Ass., 1885, 157-261. 75 Ibid. 260. 76 Ibid., 261. 77 Dated let Jan. 1885, Correspondence of J.J.Thomson, CUL. 78 Ibid., 1. 79 Phil.Trans.A., 1894, 185, 719-822. 80 Ibid., 719.- 81 Ibid., 806-22, dated 13th Aug. 1894. 82 H.A.Lorentz, La thOorie electromagn6tique de Maxwell et

son application aux corps mouvants, Archives Neerlandaises des Sciences Exactes et Naturelles, 1892, 25, 363f.; Pa ers 164-343-

83 .H rosige, Electrodynamics before the Theory of Rela-tivity. 1890-1905, Jap.Stud.Hist.Sci., 1966, 5, 1-49; id., Origins of Lorentz' Theory °lc-Electrons and the Tincept of the Electromagnetic Field, Hist.Stud.Phys.Sci., 1969, 1, 151-209.

Notes for Chapter 1, p.8-47) 284

84 R.McCormmach, H.A.Lorentz and the Electromagnetic View of Nature, Isis, 1970, 61, 459-97; id., Einstein, Lorentz, and the-EIT)(3tron Theory, Hist.STEd.Phys.Sci., 1970, 2, 41-87.

85 R.McCormmach, ibid., Isis, 1970, 463-4. 86 Versuch einer THI;7rie7d7gFt elektrischen and optischen

Erscheini'ngen in bewegten Kbrpern, Leiden, 1895; Pa errs, 5, 1-137.

87 On the Influence of Magnetism on the Nature of the Light emitted by a Substance, Phil.Mag., 1897, 43, 226-39, section 17.

88 J.J.Thomson, Cathode Rays, RI Lib.Sci., (1897), 5, 36-49, 49.

89 R.Hertz, London, 1893; repr. New York, 1962, 20. 90 Stuttgart. 91 Curie papers, BN, dossier 9, contains her brief notes

on the book. 92 CR, 1897, 125, 1165-9. 93 Mi;ude, op.-CIT., 589-90. 94 G.G.Stoki7-Un the Change of Refrangibility of Light,

Pa ers, (1852), 3, 259-413, 388-97. 95 Drude, op.cit., p.vi. 96 L.Boltzmann, Vorlesungen liber Maxwell's Theorie der

Elektrizitat und des Lichtes, 2 vols., Leipzig, 1891-3. 97 H.Pofncare, ETJEFIFitt, et Optique. Lee theories de

Maxwell et la thoorie electromagnetique de la lumiare, 2 vols., Paris, 1890-1.

98 Stokes, Mem.& Corres., 1, 250-1. 99 RutherfoRT Papers, 25. 100 2 vols., Oxford, 1892. 101 Recent Researches in Electricity and Magnetism,

Oxford, 1893. 102 Rutherford, Papers, 25, 26, 34, etc. 103 J.J.Thomson, Recent Researches etc., ch.1, 3-5. 104 Ibid., ch.2, 44-6. 105 Ibid., 5; see Section 4, p.38-44, below. 106 London, 1889; 2 ed., 1892. 107 Ibid., 184. 108 Later criticised by P.Duhem, see H.R.Post, Atomism

1900, P sics Education, 1968, 3, 1-13, 6. 109 Pa ers, . 110ee ection 4 below, p.32-3. 111 O.Lodge, Modern Views of Electricity, 1889, ch.10. 112 Ibid., 266-7. 113 TM., 267. 114 rua. 115 Ibid., 301-2. 116 Ibid., 250. 117 172TBrit.Ass., 1891, 574; quoted in J.T.Merz, A History

European Scientific Thought in the Nineteenth Ugntu, (1904), repr. New York, 1965, 2, 193.

118 Gam. money, Of the 'Electron' or Atom o? Electricity, Phil.Mag., 1894, 38, 418-20.

119 b.145dge, Modern Views etc., 1889, 250. 120 Trans. Royal Dublin Society, 1891, 4, 585. 121 W.McGucken, Nineteenth-Century Spectroscopy, 110-6. 122 Ibid., 122-6. However, see Section 2 above,

T7M-2, on Liveing.

Notes for Chapter 1, p.8-47) 285

123 Rep.Brit.Ass., 1874, 22; title only. 124 Phil.Mag. 71881, 11, 381-90; read 16th Feb.1881

Ta-Rokir Dublin Society. 125 Ibid., 385. 126 TurI., 387. 127 W.McGucken, Nineteenth-Century Spectroscopy, 188-9,

202-3. 128 I.e. slowly exchanging energy on collision. 129 Ibid., 376. 130 Ibid., 377. 131 Ibid., 378-9 132 17Tehuster, The Kinetic Theory of Gases, Nature,

1895, 51, 293. 133 H.EberT7 Phil.Mag., 1894, 38, 332-6. 134 G.J.Stoney, Of the 'Electron' etc., Phil.Mag.,

1894, 38, 418-20. 135 H.Eberi7 Electrische Schwingungen molecular Gebilde,

Ann.d.Phys., 1893, 49, 651-72. 136 U77..Utoney, op.cit.-- 137 Id., Of the KfnliFfc Theory etc., 1895, 378-9 138 He Modern Development of Faraday's Conception of

Electricity, J.Chem.Soc., 1881, 39, 277-304. 139 C.A.Russell, he HisiOTY of Valency, Leicester, 1971,

ch.l3, 265; see J.R.Partington, Histo of Chemistry, London, 1964, 4, ch.21, for stud es of electrolysis during those years.

140 See above, p.34-6. 141 J.C.Maxwell, Treatise on Electricity and Magnetism, 3 ed.,

Oxford, 1892, 1, 380. 142 Ibid., 383. 143 Teliholtz, 22.cit., 1881, 302-3. 144 W.C.D.Whetham,17eatise on the Theo of Solution,

Cambridge, 1902, provides many re erece ns. 145 Rep.Brit.Ass., 1885, 723-72.

146 Ibid., 723. 147 }ep.Brit.Ass., 1894, 482-93. 148 Ibid.-7TO: 149 Hereafter referred to as Kelvin. 150 See R.H.Silliman, William Thomson: Smoke Rings and

Nineteenth-Century Atomism, Isis, 1963, 54, 461-73. 151 Atom, Encyclopaedia Britannica, 1875; Papers, 2, 445-84,

473-6. 152 Elasticity viewed as possibly a mode of motion, RI Lib.

Sci., (1881), 3, 136-7. 153 Kelvin, On the molecular dynamics of hydrogen gas etc.,

Papers, (1896), 5, 350-3. 154 London, 1883; repr., London, 1968. 155 Ibid., 1. 'Strength' = mean velocity of rotation x section

area. 156 Ibid., 2. 157 Ibid., 109-14- 158 Ibid., 119. 159 Ibid., 108. 160 Cathode Rays, Phil.Mag., 1897, 44, 293-316, 313-4. 161 Shown by W.Mc6EFFen, Mineteenth7Uentury Spectroscopy,

174; and A.Romer, Experimental History etc., Isis, 1942, 34, 150-61, 151.

Notes for Chapter 1, p.8-47) 286

162 J.J.Thomson, Treatise on the Motion of Vortex Rings, 1883, 120.

163 J.J.Thomson, On the Chemical Combination of Gases, Phil.Mag., 1884, 18, 231-67.

164 W7073twald, Lehrbuch der Allgemeinen Chemie, 2, 745; see J.J.Thomson, Reply to Prof. Wilhelm Ostwald's criticism on my paper etc., Phil.Mag., 1887, 23, 379-80.

165 J.J.Thomson, Applications of-DYFismics to P iras and Chemistry, London, 1888; from leo res-ae were the Cavendish. D.R.Topper, Commitment to mechanism: J.J.Thomson, the early years, Arch.Hist.Exact Sci., 1971, 7, 393-410, has discussed tharggig6TUf Thomson's work.

166 Phil.Mag., 1883, 15, 427-34. 167 Tura., 428. 168 Ibid., 432. 169 R-MSilliman, William Thomson: Smoke Rings etc., 472;

also see above, p.39. 170 J.J.Thomson, On the illustration of the properties of

the electric field by means of tubes of electrostatic induction, Phil.Mag., 1891, 31, 149-71.

171 Oxford. 172 Ibid., 3. 173 Ibid., 43. Different ether-motion theories were used

5T-Fthers, e.g. see Heaviside in Schaffner, 208-9; also Section 3 above.

174 Recent Researches etc., 5. 175 Ibid. 176 Ibid., 44. 177 Ibid., 45. 178 777Thomson, Phil.Mag., 1895, 40, 511-44. 179 Ibid., 513. ---- 180 37,(7i-A.Romer, Experimental History etc., 157. 181 See T.Hirosige, Electrodynamics etc.1890-1905, p.18-20;

also A.Romer, op.cit., 156-7. 182 On the other hand, as early as May 1897 the physical

chemist W.Nernst cited Wiechert's dicovery of subatomic charged particles and discussed its possible application to electrochemistry. G.V.Bykov, Historical Sketch of the Electron Theories of Organic Chemistry, Chymia, 1965, 10, 199-253, 200-1, considers that the app cation of ele-aron theories to chemistry began in 1897; we have seen that there were earlier attempts on these lines.

183 See W.McGucken, Nineteenth-Century Spectroscopy, 211-2; G.E.Owen, The discovery of the electron, Ann.Sci., 1955, 11, 173-82, 177-9.

184 7.J.Thomson, On the cathode rays, Proc.Camb.Phil.Soc., 1897, 9, 243-4.

185 J.J.Thomson, The Röntgen Rays, Nature, 1896, 53, 581-3. 186 W.C.R8ntgen, Ueber eine neue ArT-VaRStrahlen,

Sitzungsberichte der physikal.-medicin. Gesellschaft, WUrzburg, 1895, 177=41, 134.

187 J.J.Thomson, op.cit., 1896, 581. 188 J.J.Thomson, TheOntgen Rays, Nature, 1896, 54,

302-6, lecture delivered 10th Jun. 189 Ibid., 304-5.

Notes for Chapter 1, p.8-47) 287

190 J.J.Thomson, Longitudinal Electric Waves, and Rtintgen's X Rays, Proc.Camb.Phil.Soc., 1896, 9, 49-61; also E.Rutherford, On tEeTTlectriTICation of Cases Exposed to Röntgen Rays, Phil.Mag., 1897, 43, 241-55, Note by J.J.Thomson, 255.

191 J.J.Thomson, Cathode Rays, Phil.Mag., 1897, 44, 293-316. 192 Ibid., 310. The work of Lenard referred to was probably

Te-Be'r die Absorption der Kathodenstrahien, Ann.d.Phys., 1895, 56, 255-75.

193 Thomson, 22.cit., n.191, 312. 194 Ibid., 313=47-- 195 Ibid., 312-3; on evidence of electric strength

UrRases. 196 J.J.Thomson, A Treatise on the Motion of Vortex Rings,

1883, 1. 197 W.Kaufmann, Methode zur exacten Bestimmung von Ladung

and Geschwindigkeit von Becquerelstrahlen, Phys.Z., 1901, 2, 602-3. J.J.Thomson was himself investigating whetheY. corpuscles 'have masses other than electrical' in 1901: letter to Rutherford dated 15th Feb.1901, A.S.Eve, Rutherford.Etc., 76.

198 A.S.Eve, Rutherford.7T6., Cambridge, 1939, 39; letter dated 30tEM=76.

199 In CUL, Add.mss.7653, and Royal Society Library, London; notebooks for the period 1896-1904, CUL, contain material generally similar to that published.

288

NOTES FOR CHAPTER 2 (pages 48-118)

1 W.C.R6ntgen Ueber eine neue Art von Strahlen, Dec.1895; G.Sarton, The discovery of X-rays, Isis, 1936-7, 26, 349-64.

2 T.41asser, Wilhelm Conrad Rönt en and the Early History of the Röntgen Rqys, Illino s, 377-3M.

3 W7CTRUntgen, op.cit., 139. 4 A.Romer, Acciairif-and Professor Röntgen, Amer.J.Phys., 1959, 27, 275-7.

5 A.H.Becquerel, Recherches sur une propriete nouvelle de la matiere etc., Paris, 1907:-

6 mar, 3. This account is repeated by O.Lodge, Becquerel Memorial Lecture, J.Chem.Soc. 1912, 101, 2005-42, 2032-8; also T.W.CHeIigis, A Liort Hil-517 of Radio- activity, pub. The Engineer, 1951, ; .BeTTianarSur l'origine de la decouverte de la radioactivity, CR, 1946, 223, 698-700, from personal memory support by his oWITTaboratory notes, dates Becquerel's interest in the photographic effects of pitchblende to 1893-4.

7 H.Poincare, Les rayons cathodiques et les rayons Röntgen, Rev.Gen.des.Sci., 1896, 7, 52-9, 56, 30th Jan.

8 S.P.Thompson,-agET7Vrable and Invisible, London, 1897, 260.

9 J.J.Thomson, Longitudinal Electric Waves etc., 1896, 60-1, 27-29th Jan.

10 A.Broca, L'Oeuvre d'Henri Becquerel, Rev.GOn.des Sci., 1908, 19, 803-13.

11 E.N.Harvey, A Histo of Luminescence from the Earliest Times until I9 , fgaelphia, 1957, 390-1.

12 7,7EaTikeWITle and G.F.Kunz, The Action of Radium, R6ntgen Rays and Ultra-violet light on minerals, Chem.N., 1904, 89, 1-6.

13 Tie above Chapter 1, Section 3, n.94. 14 E.N.Harvey, History of Luminescence, 363-4. 15 See above Chapter 1, Sections 1,4. 16 E.N.Harvey, 2.cit., 359. 17 Ibid., 364. 18 TRT-1885, 101, 1252-6. 19 711, 1891, 113, 618-23, 623. 20 VR, 1891, 112, 557-63. 21 TEid., 5637-- 22 17147Harvey, Historyof Luminescence, 364. 23 CR, 1896, 122, -1. A translation of this and three more

"Fac BecquereiTs first papers, and others, with commentary, are provided in A.Romer, The Discovery of Radioactivity and Transmutation, New YoT.E7 1964.

24 .C.R7 1696, 122, 662, 695, 790, 791, Mar. Henry became arector 0-The Laboratory of Physiology of Sensations at the Sorbonne in 1897.

25 CR, 1896, 122, 321-4, 10th Feb. 26 Tad., 27 *are-above, p.48-9, n.6.

Notes for Chapter 2, p.48-118) 289

28 G.H.Niewenglowski, Sur la propriete qu'ont les radiations emises par les corps phosphorescents de traverser certains corps opaques a la lumiere solaire, et sur lee experiences de M.G.Le Bon sur la lumiere noire, CR, 1896, 122, 385-6, 17th Feb.

29 CR, 1877 122, 420-1, 24th Feb. 30 E7Becquere17Sur les radiations invisibles emises par

les corps phosphorescents, CR, 1896, 122, 501-3, 2nd Mar. 31 L.Badash, Becquerel's 'Unexposed' Photographic Plates,

Isis, 1966, 57, 267-9, and A.Romer, Discovery of Tia-TOactivity, 9, have stressed the unusual aspects of developing unexposed plates, and the former, of work-ing in the laboratory on a Sunday. It is to be noted however that Becquerel indicated that he worked on Sunday 29th Mar.1896, CR, 1896, 122, 762-7, 30th Mar.

32 H.Becquerel, Seances Soc.Fr.Phys., 1896, 88, 6th Mar., comment by M.de ChardonneT7

33 See e.g. C.Raveau, Les faits nouvellement acquis stir lee rayons de Roentgen, Rev.Gen.des Sci., 1896, 7, 251, 15th Mar.

34 H.Becquerel, Sur quelques proprietes nouvelles des radiations invisibles emises par divers corps phos-phorescents, CR, 1896, 122, 559-64, 9th Mar.

35 I.e. Hertzian waves. 36 G.C.Schmidt, Ueber die von den Thorverbindungen and

einigen anderen Substanzen ausgehende Strahlung, Ann. d.Phys., 1898, 65, 141-51; Schmidt concluded that Ehorium rays were reflected and refracted.

37 E.Rutherford, Uranium Radiation and the Electrical Conduction Produced by It, Phil.Mag., 1899, 47, 109-63, Jan.; Papers, 170-1; H.Becquerel, Note sur quelques proprietes du rayonnement de l'uranium et des corps radio-actifs, CR, 1899, 128, 771-7, 773, Mar.

38 L.Troost, Sur ITemploi blonde hexagonale artificielle pour remplacer lee ampoules de Crookes, CR, 1896, 122, 564-6, 9th Mar.; id., 694, 23rd Mar.

39 n7BecquereTTSur lee radiations Tvisibles emises par lee sels d'uranium, CR, 1896, 122, 689-94, 23rd Mar.

40 H.Becquerel, Seancesoc.Fr.Phys. 1896, 105, 20th Mar., Ch.-Ed. Guillaume, TEid., On Stokes' Law.

41 Op.cit., 23rd Mar.1896. 42 CR, 1596, 122, 762-7. 43 117Moissan,PF6paration et proprietes de l'uranium,

CR, 1896, 122, 1088-93, 18th May; id., Le Four rlectrique, Paris, 1897.

44 CR, 1596, 122, 1086-8, 18th May. 45 Missan, Description d'un nouveau four electrique,

CR, 1892, 115, 1031-3. 46 Mir la preparation de l'uranium a haute temperature,

CR, 1893, 116, 347. 47 Etude du carbure de l'uranium, CR, 1896, 122,

274-80, 10th Feb.

48 Becquerel, Sur diverses proprietes des rayons uraniques, CR, 1896, 123, 855-8, 23rd Nov.

49 TEld., 8567-- 50 amour la loi de la decharge dans l'air de l'uranium

electrise, CR, 1897, 124, 800-3.

Notes for Chapter 2, p.48-118) 290

51 CR, 1896, 122, 689-94. 52 CR, 1896, I77, 762-7, 30th Mar. 53 Z7M.StewarT7Experiments on Beoquerel Rays, Physical

Review, 1898, 6, 239-51. 54 J.J.Thomson, Tile Rantgen Rays, Nature, 1896, 22, 581-3,

23rd Apr. 55 Rayons cathodiques, rayons X et radiations analogues,

Seances Soc.Fr.Phys., 1896, 121, 8th Apr. 56 G.G.Stokes, On the Nature of the Rbntgen Rays, Proc.

Camb.Phil.Soc., 1896, 9, 215-6, 9th Nov.; id., Wilde Lecture, 2nd Jul.1897, Papers, 4, 256-77, Mb: the irregular impacts of 'cathode ray' particles produce a series of thin ether pulses, which constitute the X-rays; the irregularity of the sequence of pulses implies that the molecules of the glass of a prism cannot vibrate in harmony, thus the X-rays are not refracted; the thinness of the pulses implies absence of diffraction; both these properties of the pulses imply their penetrating nature.

57 See 0.M.Stewart, Becquerel Rays, A Resume, P sical Review, 1900, 11, 155-75, 175; R.H.Stuewer, W am H. Bragg's Corpuscular Theory of X-Rays and X-Rays, Brit.J.Hist.Sci., 1971, 5, 258-81, on corpuscular X-ray theories.

58 C.Henry, CR, 1896, 122, 312-4, 10th Feb. 59 S.P.Thompson, On Hyperphosphorescence, Phil.Mag., 1896,

42, 103-7, dated 6th Jun. 60 L.Badash, Radioactivity before the Curies, Amer.J.Phys.,

1965, 33, 128-35; A.S.Russell, Madame Curie Memorial Lecture, J.Chem.Soc., 1935, 654-63; J.S. and H.G.Thompson, Silvanus Phillips Thompson his Life and Letters, London, 1920, 185-9.

61 G.G.Stokes, Mem.& Correa., 2, 495-6, letter from Thompson dafgY-2Uth Feb.189.b.

62 Ibid., letters from Stokes dated 29th Feb. and 2nd Mar.1896.

63 Op.cit., Phil.Mag., Jul.1897. 64 G.G7gTokes, Mem.& Correa., 2, 498, letter from Stokes

dated 28th May. 65 Letter from W.Crookes to S.P.Thompson, dated 2nd Jun.

1896, Imperial College Archives. 66 W.Crookes, Rep.Brit.Ass., 1898, 23. 67 The Evolution of Mater, New York, 1907, 22-3. 68 as Johanniskifferlicht, Ann.d.Phys., 1896, 59, 773-81. 69 H.Muraoka and M.Kasuya, 7ohanniskeerliEHt und die

Wirkung der Dampfe von festen und fliissigen Korpern auf photographischen Platten, Ann.d.Phys., 1898, 64, 186-92, received 24th Nov.1897.

70 Ueber Luminescenz, Ann.d.Phys., 1897, 61, 313-29. 71 A.F.McKissick, Becquerel Rays, ElectriETan, 1897,

38, 313. 72 Verh.phyp.Ges.Berlin, 1896, 15, 101. 73 Ann.dka771TeibI., 1897, 21, 366. 74 G.Le Bon, L'uranium, le radium et les emissions

metalliques, Rev.Sc., 1900, 13, 548-52; id., The Evolution of Rater, 19-25, T9-28.

75 La radioaciTvite de la matiere et l'energie susceptible de se dovelopper A la surface des corps, Rev.Sc., 1901, 16, 161-70, 167.

Notes for Chapter 2, p.48-118) 291

76 See E.Picard, Gustave Le Bon et son Oeuvre, Paris, 1909. 77 D.Martindale, The Nature aig '.hypes of Sociological

Theo , London7-1961, 309717; R.A.Nye, The Origins of row Psychology. Gustave Le Bon and thii-Crisis of Mii-Democracy in the ThirZ-Republre,-ranM,97.

78 T.17 Son, La luia-fere n311757 CR, 1896, 122, 188-90, 27th Jan.; the name 'dark rays' had been used earlier by W.de W.Abney as synonymous with infra-red rays, it simply meant rays not visible to the human eye (Spectrum Analysis etc., RI Lib.Sci., (1882), 3, 207-15).

79 CR, 1896, 122, 75767 80 M, 1896, I22, 463-5. 81 M, 1897, T, 857-9. 82 M, 1897, Imo, 984-8. 83 M, 1896, 122, 233, 386, 462, 522, 1057, Feb. to May 1896;

'aimilar notes in Rev.Sc., 1896, Jan. to May, and 1897, Mar. to May; CR, 1697, 124, 755-8, 892-5; Sur lee propriotes de certaines radiations du spectre. Reponse aux objections de M.Becquerel, CR, 1897, 124, 1148-51.

84 CR, 1897, 124, 892-5. 85 Temarques W-firopos d'une Note recente de M.G.Le Bon, CR,

1900, 130, 1072. 86 G.Le Bon, Intra-Atomic Energy, R2p.Smithsonian Inst.,

1904, 263-93; id., L'kvolution a la Me.tiere, par s, 1905; id., La Naissance et Logvanouissement de Ia Matiore, Wiris, 190d. On universal radioactivity see MUT"- Chapter 5, Section 3.

87 P.Curie, 22.cit., 1900; repeated by H.Becquerel, Recherches sur une propriete nouvelle etc., 1903, 5-6.

88 M.RutherforZTRaEro-activity, Cambridge, 1904, section 2; 1905, 4-5.

89 R.Colson, Wile des differentes formes de l'energie dans la photographie au travers de corps opaques, CR, 1896, 122, 598-600; id., Action du zinc sur la plaque photo-graphique, CR1-1896, 123, 49-51; id., La Plaque Photo-graphique, Taris, 1897:-

90 R.Colson, 22.cit., CR, 1896, 123, 49-51. 91 W.J.Russell; Proc.ROT.Soc., 1877, 61, 424-33, received

13th May, he cis Colson. For his work in another area see J.R.Brown and J.L.Thornton, William James Russell (1830-1909) and investigations on London fog, Ann.Sci., 1955, 11, 331-6.

92 E.Rutherford, A Radioactive Substance emitted from Thorium Compounds, Phil.Mag., 1900, 49, 1-14; Papers, 226.

93 Russell, p.cit., 1897, 424. 94 Ibid., 425:- --- 95 Ibid., 427. Note the use of the terms 'active' and

'activity'; Marie Curie's use of the expressions in 1898 was thus not the first as has been supposed (A.Romer, Radiochemistry and the Discovery of Isotopes, New York, 1970, 64). C.T.R7WiIain too, Proc7Vamb.Phil. Soc., 1897, 9, 337, wrote of 'active' uranium salTat-

96 W77.Russell, 22.cit., 432-3. 97 Ibid., 433. 98 W.Crookes, Presidential Address, ap.Brit.Ass.,

1898, 3-38, 26. 99 C.T.R.Wilson, On the Action of Uranium rays on the

Condensation of Water Vapour, Proc.Cainb.Phil.Soc., 1897, 9, 333-8, 25th Oct.

Notes for Chapter 2, P.48-118) 292

100 Proc.Camb.Phil.Soc., 1897, 9, 372, 22nd Nov. 101 1777Russell, further experiments on the action exerted

by certain metals and other bodies on a photographic plate, Proc.Roy.Soc., 1898, 63, 102-12.

102 Id., On the Action of Certain Metals and Organic Bodies on a Photographic Plate, 222.Brit.Ass., 1898, 834-5 (Abstract).

103 Id., On hydrogen peroxide as the active agent in prod- ucing pictures on a photographic plate in the dark, Proc.Roy.Soc., 1899, 64, 409-19; id., On the Action of Wood on a Photographic Plate in the Dark, Chem.N., 1904, 90, 104-6.

104 G.L.Keenan, Substances which Affect Photographic Plates in the Dark, Chemical Reviews, 1926, 3, 95-111, 108.

105 G.C.Schmidt, Ueber die vom Thorium and den Thorver- bindungen ausgehende Strahlung, Verh.P:ys.Ges.Berlin 1898, 17, 14-16, 4th Feb., id., Ueber die vonen ThorveiTindungen and einigen anderen Substanzen ausgehende Strahlung, Ann.d.Phys., 1898, 65, 141-51.

106 Ibid., Verh.Phys.Ges.B-Olin, 16. 107 771TioquTigl, Sur diverses proprietes des rayons

uraniques, CR, 1896, 123, 855-8, 23rd Nov.; id., Recherches sur les rayons uraniques, CR, 18977 124, 438-44; id., Sur la loi de la decharge dans l'aIr de l'uranium electrise, CR, 1897, 124, 800-3, 12th Apr.

108 E.Rutherford, The Velocity and Rate of Recombination of the Ions of Gases exposed to Röntgen Radiation, Phil.Mag., 1897, 44, 422-40, 440; Pa ers, 148.

109 H.Becqa5rel, CR, 1896, 55-8, 23rd Nov. 110 Id., CR, 1897, 124, 438-44, 443, 1st Mar. 111 U7Elster, Jahre -7d.Ver.f.Wiss.,Braunschweig, 1897, 10,

149-53, 10th Dec.189TT-J.EnTer and H.Geitel, Ann.d. P s.,Beibl., 1897, 21, 455.

112en .d.R.Acad.d.Scienze fis.e mat., 1897, 36, 178f. 113 kelvin, Papers, 6, 1; most oT'ITT) relevant papers

are collected in this volume. 114 Cited in Kelvin, J.C.Beattie, M.S.de Smolan, On

Apparent and Real Diselectrification of Solid Dielect-rics Produced by Röntgen Rays and by Flame, Edin.Roy. Soc.Proc., 1897, 21, 397-403; Kelvin, Paperi7-67 65.

115 RavIETJ.C.Beatfrg, M.S.de Smolan, Edin.Roy.Soc.Proc., 1897, 22, 131-3, 1st Mar.; Kelvin, Pa ers-,--6, 95-77 -

116 Id., Edin.Roy.Soc.Proc., 1897, 21, 1 - , Wth Apr.; re1viETPa ers7-6, 84-95.

117 J.C.Beattiet PhiT.Mag., 1897, 44, 102-7; Note by Kelvin, ibid., 107-8; read-1-6 Edin.Roy.Soc. 7th Jun.1897.

118 Citing J.J.Thomson and McClelland, Proc.Camb.Phil.Soc., Mar.1896, and E.Rutherford, Phil.Mag., ATi.71897; see Kelvin, Papers 6, 72-3, (1siMia-TTT ibid., 184, (17th Jun.

119 See J.J.Thomson, The Relation between the Atom and the Charge of Electricity carried by it, 1895, 537: 'contact electricity' due to oxide coatings; also Kelvin, Contact Electricity of Metals, Papers, (1898), 6, 110-47, 130, 138 etc.: true metallic contact electricity due to the affinity of differing metals.

Notes for Chapter 2, p.48-118) 293

120 Kelvin and M.Maclean, On the Electrification of Air, Phil Mag., 1894, 38, 225-35; Kelvin, Papers, 6, 6-16, 6.

121 'N(7) -Chapter 1, Section 4, p.43. 122 Kelvin, Contact Electricity of Metals, RI Lib.Sci.,

(1897), 5, 50-83; Papers, 6, 110-47. 123 Id., Pa 4Fs, 6, 144-5. 124 ee apter T, Section 4, p.39: chemical H atom consists

of two Boscovichean atoms. 125 Phil.Mag., 1899, 48, 97-106. 126 Ibid., 97. 127 E.V.Appleton, 'The Young Rutherford' in The Collected

Papers of Lord Rutherford of Nelson, ed. J.Chadwick, 3 vole., London, 1962, 1, 17.T71171 of this work is hereafter referred to as E.RutherfUrd, Papers.

128 See Chapter 1. 129 Trans.New Zealand Institute, 1894, 27, 481-513; Papers,

25-55. 130 Ibid., Pa ere, 34. 131 Weabove, Chapter 1, Sections 3-4, p.31-2. 132 Rutherford, Pa ers, 25. 133 See Chapter , ection 2, p.133-4, 141-4. 134 Rutherford, Papers, 51. 135 Trans.New Zealand Institute, 1895, 28, 182-204; Papers,

55-79. 136 Rutherford, Pa ere, 69-70. 137 See below, C ap er 2, Section 3, p.95-6. 138 Refers most frequently to J.J.Thomson, Recent Researches

in Electricity and Magnetism, 1893.

139 Ibid., 35:

140 Chapter 1, Sections 3-4, p.31-2. 141 Lord Rayleigh, The Life of Sir J.J.Thomson, Cambridge,

1942; repr. London, 1962, 62. 142 A.S.Eve, Rutherford.Being the Life and Letters of the

Rt.Hon.Lord Rutherford,O.M., dgER67, 1939, 151 ITtTE57 to Mary Newton Oct.-1895.

143 N.Feather, Lord Rutherford, Glasgow, 1940; repr. London, 1973, 28-9.

144 R.Sviedrys, The Rise of Physical Science at Victorian Cambridge, Hist.Stud.Phys.Sci., 1970, 2, 127-51, 143; A History oTTHEUirrendish Laboratory,I871-1910, London, 9IU.

145 See the letter from Thomson, Cambridge, to Rutherford, London, dated 24th Sep.1895, in A.S.Eve, Rutherford. Etc., 13; also letter from Rutherford, Cambridge, to Mary Newton, N.Z., Oct.1895, ibid., 16.

146 A.S.Eve, Rutherford.Etc., 22-7. 147 Phil.Trans.A., 1697,19, 1-24; Rutherford, Papers,

80-10q7 -- 148 A.S.Eve, Rutherford.Etc., 34; letter to Mary Newton. 149 J.J.Thomson, Recent Researches etc., 53-207. 150 Ibid., 189. 151 Ibid. 152 Ibid., 119-27. 153 Ibid., 128. 154 TM., 45-7, 189-90. 155 Ibid., 193. 156 Ibid.

Notes for Chapter 2, p.48-118) 294

157 Ibid., 45-6, 195-6. 158 TX Thomson, The Connection between Chemical Combin-

ation and the Discharge of Electricity through Gases, Rep.Brit.Ass., 1894, 482-93.

159 Ibid., 489-92. 160 Ibid., 486. 161 Ibid., 487; also id., On the effect of electrification

an chemical action on a steam jet etc., Phil.Mag., 1893, 36, 313-27.

162 Proc.RZY.Soc., 1895, 58, 244-57, received 17th Jun. 163 IETT.filag.77895, 40, 311-44. 164 Heber-die electrolTtische Leitung verdannter Gase,

Ann.d.Phys., 1897, 61, 737-47, and references therein. 165 See above, Chapter IT Section 4, p.39-44. 166 A.S.Eve, Rutherford.Etc., 27. 167 See N.Feather, X-ra s and the electric conductivity of

gases, Alembic u eTi;InTf2, Edinburgh, 1958, 16,-T1f. 168 Longitudinal Electric Waves, and Pecintgen's X Rays,

Proc.Camb.Phil.Soc., 1896, 9, 49-61, 61; Kelvin, Papers, 6, 6571IsTg-geveral independent announcements of this, at about this date.

169 J.J.Thomson, On the Discharge of Electricity produced by the R8ntgen Rays, and the Effects produced by these Rays on Dielectrics through which they pass, Proc.Roy. Soc., 1896, 59, 274-6, received 7th Feb., assisted by J.A.McClellsEa.

170 Ibid., 275. 171 77.7ic.Camb.Phil.Soc., 1896, 9, 126-40, 9th Mar. 172 Ibid.7-170. 173 ITTa., 131-2. 174 Chapter 1, Section 4, p.42-6. 175 J.J.Thomson and J.A.McClelland, 22.cit., 132. 176 Of the order 0.001 cm./sec. per vofWm. 177 22.cit., Mar.1896, 128. 178 J.J7TEomson and E.Rutherford, On the Passage of Elec-

tricity through Gases Exposed to Montgen Rays, Phil.Mag., 1896, 42, 392-407; read to British ssociation, Sep.I896; Rutherford, Papers, 105-18.

179 See N.Feather, Lord Rutherford, Chapter 2, 'Cambridge, the First Period, 1895-1898', 41. For his brief account N.Feather consulted the correspondence of Rutherford, used by A.S.Eve, also Rutherford's laboratory notebooks.

180 1896, 53, 581-3. 181 Ibid.,-583. 182 J.J.Thomson, The Rantgen Rays, Nature, 1896, 54, 302-6;

305-6; Rede Lecture, delivered 10th Sun. 183 Ibid., 304. 184 Ibid., 305. 185 Letter to Mary Newton, dated 18th Jun.1896, A.S.Eve,

Rutherford.Etc., 36. 186 Read to BriiIih Association, Sep.1896; Rutherford,

Papers, 105-18. 187 Similar to those of Kelvin, who is not cited; Kelvin,

Papers, (1895), 6, 35, 51-2. 188 Thomson and Rutherford, op.cit., Rutherford, Papers, 106. 189 Ibid., 107. 190 TEid., 106. 191 Ibid. 192 Ibid., 117.

Notes for Chapter 2, p.48-118) 295

193 Ibid., 107-8. 194 The-intermittency of the X-ray discharge affected the

results, ibid., 109-10. 195 Rep.Brit.1713737, 1894, 482-93, 491-2. 196 ThomTEEailia-Rutherford, op.cit., 1896, 114. 197 See above Chapter 1, SecTfori, p.47. 198 Phil.Mag., 1897, 43, 241-55, Apr. issue, dated 28th Dec.

1896; Rutherford, Papers, 119-31. 199 Ibid., 128. 200 TM., 127. 201 Ibid., 119-22. 202 UtPirrin, Mecanisme de de-charge des corps olectrises

par lee rayons de Ontgen, Seances Soc.Fr.Phys., 1896, 254-61, 261; id., Rayons cathodiques et rayons de Roentgen, Annales de Chimie et de Physique, 1897, 11, 496-554, Thesis Jun.1897.

203 THil.Mag., 1897, 44, 422-40; Papers, 132-48. 204 MIT.Mag., 1898, 7.L 120-54. 205 E.Rutherford, 22.-61t., Papers, 144-8. 206 Ibid., 148. 207 Nacre, 1896, 53, 581. 208 Nature, 1896, 5T, 304. 209 TeTTETtlow, Chapter 2, Section 4, p.114-5. 210 See above, Chapter 2, Section 1, p.63-5. 211 E.Rutherford, Papers, 148. 212 Proc.Camb.Phil.Soc., 1898, 9, 401-16; E.Rutherford,

Pa ers, 149-62. 213 rbid., 149. 214 This was accepted by J.J.Thomson in 1893, Recent

Researches etc., 54. 215 0.Lodge, Modern Views etc., 1889, 301-2. 216 E.Rutherford, Pa ers, 159. 217 Phil.n.E., 189 „ 109-63; Papers, 167-215. 218 mid., 214-5. -- 219 J.J.Thomson, Proc.Camb.Phil.Soc., 1898, 9, 393-7,

24th Jan. ---- - 220 See below, Chapter 2, Section 3, p.104-5. 221 J.J.Thomson, 22.cit., 1898, 397. 222 Ibid. 223 gig-below, Chapter 2, Section 3, p.100-1. 224 E.Rutherford, Uranium Radiation etc., Papers, 180. 225 Ibid., 214-5. 226 Ibid., 214. 227 TETU., 215. 228 Papers, 167-215; Phil.Mag., Jan.1899, dated 1st Sep.1898. 229 Ibid., 167, 170-1. 230 TWoquerel, CR, 1899, 128, 771-7, 772, 27th Mar. 231 E.Rutherford, Papers, 177=6. 232 Ibid., 185-6. 233 UTI7-1896, 122, 762-7, 765, 30th Mar. 234 Proc.Camb.f.al.Soc., 1896, 9, 126-40, 139-40, 9th Mar. 235 ConductioriOEM7tricity through Gases, Cambridge,

1903, 278. Rutherford's own resulfg-6Y-1898 (Pa ers, 178) showed that the alpha/beta ratio increase w h the thickness of the Ur layer; Thomson noted (loc.cit.) that if alpha rays were produced at the surfaci-Ey-Feta, then their ratio should be constant. New evidence of independence was produced by F.Soddy in 1902.

Notes for Chapter 2, p.48-118) 296

236 Rutherford, Uranium Radiation etc., Papers, 180-1. 237 Ibid., 180. 238 Ibid., 178. 239 Ibis. 240 Ibid. 241 t177-1898, 126, 1101-3. 242 17ge e.g. M=Hesse, Forces and Fields, London,

1961, 2-3, 6. 243 J.C.Maxwell, Electricity and Magnetism, 1892, 2, 470. 244 H.R.Post, Atomism 1900, P sics Education, 196U, 3,

1-13, 5; discusses views of L. I.tzmann, E.Mach, - W.Ostwald.

245 Seven papers in CR, 1880-2; P.Curie, Oeuvres, 6-32. 246 M.Curie, Pierre Curie, trans., New York, 1923;

repr., 1963, 20. 247 C.Friedel, Sur la pyroelectricite dans la topaze, la

blends et le quartz, Neues Jahrb.Mineral., 1879, 585-6. 248 A.-C.Becquerel, De quTTTEFisTorigEomenes electriques

produits par la pression et le clivage des cristaux, Annales de Chimie et de Physique, 1827, 36, 265-71.

249 M.Curie, Pierre Curie, N.Y., 21. 250 P. and J.Cur e, Dilatation electrique du quartz,

Journal de P si ue, 1889; P.Curie, Oeuvres, 35-55. 251 P. and J.Tur e, R, 1881; P.Curie, Oeuvres, 18-21. 252 Ibid., 19. 253 157iahem crossed swords with P.Curie (Oeuvres, 33-4) in

1887 over the origin of piezo-electricity. In 1893 Kelvin corresponded with P.Curie (7 letters, Curie papers, dossier 32, BN) who provided him with a piezo-electric electroscope.

254 P.Curie, Sur les questions d'ordre: Repetitions, Oeuvres, (1884), 56-77; id., Sur la symetrie, Oeuvres, (1b84), 78-113.

255 E.g. Kelvin, Papers? 1 281, stated that 'Hall's recent great discovery' (186U) of the e.m.f. produced by a steady current in a constant magnetic field 'proves the rotatory quality to exist for electrical conduction through metals in the magnetic field'; but P.Curie was first to consider the symmetry of the Hall effect, Oeuvres, (1894), 137.

256 M.Curie, Pierre Curie, N.Y., 24-8, gives an account of this.

257 Oeuvres, (1894), 118-41. 258 Ibid., 141. 259 E.g. J.J.Thomson, Applications of Dynamics etc., 1888,

ch.4, 32. 260 L.Rougier, En Marge de Curie de Carnot et d'Einstein,

Paris, 1920, discusses 'Le prEiciTT-E sym6triet, ch.l. The principle is much used in modern electron theories of the chemical atom.

261 Oeuvres, (1895), 232-334. 262 Determined over a more limited temperature range by

others, P.Curie, Oeuvres, 280-1; of present importance in electron theories of magnetism, this is now known as the 'Curie law'.

263 A and R are constants, different for each substance; T = temperature, H = magnetic field, I = magnetic intensity; D = gas density, P = gas pressure.

Notes for Chapter 2, p.48-118) 297

264 P.Curie, Oeuvres, 331-2. 265 The I = f(H) and D = f(P) curves at constant T were

dissimilar, ibid., 334. 266 Ibid., 333. ---- 267 157-Curie, Madame Curie, trans., London, 1938; repr.

The Reprint Societ77EUndon, 1942, is a biography valuable for the non-scientific aspects of Marie Curie's life, much personal correspondence is published here. R.Reid, Marie Curie, London, 1974, provides a much improved account on similar lines. See also M.Curie, Pierre Curie, N.Y., 'Autobiographical Notes', 77-118.

268 'MentioElEtTes bien' and 'AB' respectively, Curie papers, dossier 29, BN. The Licence was of about the present masters or first degree standard.

269 Ibid 270 E.g., M.Curie, Proprietes magnetiques des aciers

trempes, CR, 1897, 125, 1165-9. 271 P.Curie, Oeuvres, 277Y. 272 J.Hurwic,-MER-gSklodowska-Curie en tant que chimiste,

Etudes d'Histoire de la Science et de la Technologie, Warsaw, 1966, 197-0'2.

273 See E.Curie, Madame Curie, Appendix: lists of Marie Curie's prizeT37als, decorations, honorary titles.

274 M. Curie, Pierre Curie, N.Y., 96-7. 275 Discussed in letti7F-fiom E.Rutherford, Manchester, to

W.H.Bragg, dated 20th Dec.1911, CUL. Also R.Reid, Marie Curie, ch.17.

276 See M.Curie, La Radiologie et la Guerre, Paris, 1921; E.Curie, Madame Curie, ch.21, N:1779.

277 Irene Curie, lat1171Fene Joliot-Curie. 278 The film 'Madame Curie', M.G.M., U.S.A., 1943, re-shown

occasionally to the present time by the British Broadcasting Company exemplifies one popular aspect.

279 I am indebted to L.Badash, who informed me of this claim. 280 M.Curie, Opening Lecture, Cours du physique gen6rale

professe a la Sorbonne, Oeuvres, 322-35, 334-5. 281 Rev.Gen.des Sci., 1899, 10, 41i.; Oeuvres, 60-76. 282 See mow, Chapter 5, SeTiion 1, p.227-31, 245-6. 283 M.Curie, Pierre Curie, N.Y., 44-5, 89; her daughter

Irene waiUOTE-in Sep.1897. 284 CR, 1897, 124, 800-3. 285 A.Romer, Radiochemistry, 6. 286 M.Curie, Pierre Curie, N.Y., 45, 89. 287 Ibid., 34. 288 77Terrin, Rayons cathodiques etc., Seances Soc.Fr.Phys.,

1896, 121-9; G.Sagnac, Journal de Physique, 5, 193f.

289 M.Curie, Pierre Curie, N.Y., 40. 290 Seances Soc.Fr.Phys., 1896, 105, 20th Mar. 291 Laboratory notebooks of the Curies, 1897-9, comprise

dossier 1, Curie papers, BN,but unfortunately these are radioactive, are undergoing treatment with some other items in the collection and are not available. They are described as: Uranium I, mainly Marie Curie's hand, pp.159, 1897-8; Uranium II, P. and M.Curie's hands, pp.143, 1898; Uranium III-Polonium, pp.126, P. and M.Curie's hands, 1898-9. Fortunately their

Notes for Chapter 2, p.48-118) 298

291 contd.) previous owner , I.Joliot-Curie, has given a brief account of their contents in M.Curie, Pierre Curie, Paris, 1955, 103-20. See above also A.Romer, REITTOchemistry, 6-8, 64-75, who uses this source and gives translations of published papers. A fourth laboratory notebook of M.Curie, 1899-1902, is at the Wellcome Historical Institute and was made available to me.

292 J.Curie, Recherches sur le pouvoir inducteur specifique et la conductibilite des corps cristallises, Annales de Chimie et de P si ue, 1889, 17, 385-434.

293 M.Cur e, Oeuvres, 60-76,-71, Jan. 294 I.Joliot-Curiel in M.Curie, Pierre Curie, Paris, 103-7. 295 M.Curie, CR, 1898, 126, 1101-3, 12th. 296 G.C.Schmidt, CR, lags, 126, 1264. 297 E.Wiedemann aFa G.C.Schiaat, Ueber Lichtemission

organischer Substanzen etc., Ann.d.Phys., 1895, 56, 18-26; id., Ueber Luminescenz von festen Wirpern und festen TUsungen, ibid., 201-54, 241-8; see above, Chapter 1, SectioriT p.30; on Wiedemann's earlier ether-envelope theory in spectroscopy see McGuoken, Nineteenth-Century Spectroscopy, 179-81.

298 G.C.Schmidt, Ueber die vom Thorium und den Thor-verbindungen ausgehende Strahlung, Verh.Phys.Ges. Berlin, 1898, 17, 14-16, 4th Feb.; see also Cat, Ueber die Jeziehung zwischen Fluorescenz und Actinoelek-tricitat, Ann.d.Phys., 1898, 64, 708-24.

299 J.Elster and H.Geitel, Ann.d.Phys.,Beibl., 1897, 21, 455, reviewed by G.C.Schmidt.

300 Ann.d.Phys., 1889, 60, 507f. 301 G75:chmidt, Ueber Tire von den Thorverbindungen etc.,

Ann.d.Phys., 1898, 65, 141-51. 302 G.C.Schmidt, op.cit., Verh.Phys.Ges.Berlin, 1898, 17, 16. 303 See above, p.aU. 304 CR, 1898, 126, 1101-3, 12th Apr.; Oeuvres, 43-5. 305 E.N.Harvey,Ei!toryof Luminescence, 284; J.Elster and

H.Geitel, Ani.T.aPhSis., 1890, 39, 321-31; S.Bidwell, Diselectri

b

ffe-aTion by Phosphorus, Nature, 1896, 55, 6; G.C.Schmidt, Ueber die Emanation des Phosphors, 7Hys.Z., 1902, 3, 475-81; F.Harms, phys.Z., 1902, 4, 111-3; J.J.Thomson, Conduction of Electricity thrau h Gases, Cambridge, 1903, 324; E. Rutherford, Rado-ac vity, Cambridge, 1905, 529-30.

306 These elements gave 1 to 10% of the uranium reading, which was 24 x 10' amps. All other substances, except phosphorus, gave less than 1% of this current: M.Curie, Oeuvres, 44.

307 See Section 2 above, p.90-1. 308 This consists in mixing a solution of uranium nitrate

with one of copper phosphate in phosphoric acid then warming gently; crystals of chalcolite, copper uranyl phosphate, slowly separate. Although not mentioned in the note of 12th Apr., preliminary success in chemically concentrating the active ingredient may also have con-tributed to the evidence by this time.

309 E.Rutherford, Papers, 178. 310 P. and Mme.S.Curie, CR, 1898, 127, 175-8, 18th Jul.

Notes for Chapter 2, p.48-118) 299

311 W.Crookes, Genesis of the Elements, 1887, 410-11; also W.N.Hartley, Opening Address to Brit.Ass. Chemistry Section, on spectroscopy, Nature, 1903, 68, 472-81, 481.

312 M.Curie, Oeuvres, (1898), 45, 12th Apr. 313 3 new elements were proposed in 1897, no less than 9 in

1898 (3 radioactive), and 2 in 1899, C.Baskerville, The Elements: Verified and Unverified, Chem.N., 1904, 89, 109-10, 121-3, 135-7, 150-1, 162-3, 170-1, 186-7, 194-5, 210.

314 G.Sagnac, Sur is mecanisme de la decharge des conducteurs frappes par lee rayons X, CR, 1898, 126, 36-40, 3rd Jan.; id., Transformation des rayons X par-Transmission, ibid., 4T7-70; id., Emission de rayons secondaires par l's17- sous l'iiTluenoe des rayons X, ibid., 521-3; id., Caracteres de la transformation des rayons X par la matiere, ibid., 887-90, 21st Mar.

315 J.Perrin,-13-6harge par les rayons de Röntgen. Role des surfaces frappoes, CR, 1897, 121, 455-8; L.Benoist and D.Hurmuzescu, CR, 1U76, 122, 779f., had expressed a similar view. --

316 G.Sagnac, Sur la transformation des rayons X par les differents corps simples,Seances Soc.Fr.Phys., 1899 1*, 6th Jan.

317 See Section 2 above, p.89. 318 M.Curie, Oeuvres, (1898), 45, 12th Apr. 319 Ibid. 320 P. and Mme.S.Curie, CR, 1898, 127, 175-8, 18th Jul.;

P.Curie, Oeuvres, 335-8. Active substances are here called 'radioactive' for the first time.

321 A.Romer, Radiochemistry, 80-105, gives a brief account of controversies concerning active bismuth, polonium and radiotellurium during 1899-1906 and provides trans-lations of some papers.

322 P.Curie, M.Curie, G.Bemont, Sur une nouvelle substance fortement radio-active, contenue dans la pechblende, CR, 1898, 127, 1215-7, 26th Dec.; P.Curie, Oeuvres, 779-42.

323 Ibid., 340. 324 The authors thanked M.Suess, correspondent de l'Institut

de France, Professeur a l'Universite de Vienne, for his request to the Austrian government, who donated the waste material freely, ibid., 342. All of the increasing amounts of material subsequently used by the Curies came from Joachimsthal, M.Curie, Pierre Curie, N.Y., 91.

325 Sur le spectre d'une substance radio-actiViTCR, 1898, 127, 1218; appended to the paper of Curies ana-Bemont.

326 P.Curie (with M.Curie and Bemont), Oeuvres, 341. 327 Atomic weights determined by Marie for increas-

ingly concentrated radium were approximately as follows: M.Curie, Sur le poids atomique du metal dans le chlor-ure de baryum radifere, CR, 1899, 129, 760-2, atomic weight of Ba = 138, Ba-Ra = 140 to-175; id., Sur le poids atomique du baryum radifere, CR, 1700, 131, 382-4, Ba-Ra = 174; id., Sur le poids atomique du radium, CR, 1902, 135, 161-3, Ra = 225, the modern value.

328 P.Curie7Twith M.Curie and G.Bemont), Oeuvres, 340. 329 Rev.Gen.des Sci., 1899, 10, 41f., Jan.; M.Curie,

Oeuvres,-0-75.

Notes for Chapter 2, p.48-118) 300

330 Ibid., Oeuvres, 73. 331 Ibid., 71. 332 Ibid., 75. 333 Ibid. 334 Ibid., 72. 335 Ibid., 75. 336 Ibid., 76. 337 M.Curie remarked on this in a later footnote added

during or after Dec.1898, Oeuvres, 76. 338 Re .Brit.Ass., 1898, 3-38. 339d., 26. 340 Ibid., 27. 341 J.Elster and H.Geitel, Versuche aber Hyperphosphor-

escenz, Ann.d.Phys.,Beibl., 1897, 21, 455; J.Elster, Jahresb.17Ver.f.Wiss.,Braunschweig7-1897, 10, 149-53, 10th Dec.187'.

342 Verh.lys.Ges.Berlin, 1898, 17, 14-16. 343 1898, 8, W7U. 344 J.Elsf.E. and H.Geitel, Ann.d.Phys., 1898, 66, 735-40. 345 Ibid., 736. 346 77RUtherford, Pa ers, 169-215, dated 1st Sep.1898. 347 Sur la source e nergie dans lee corps radio-actifs,

CR, 1899, 128, 176-8, 16th Jan., presented by H.Moissan. 348 1898, 739. 349 Tbia77 740. 350 L.Badash, Radioactivity before the Curies, Amer.J.Phys.,

1965, 33, 128-35, 130, 134. 351 Nature, 1896, 53, 581; ibid., 54, 304. 352 See below, p.1.7-5. 353 Proc.Camb.Phil.Soc., 1897, 9, 372. 354 G.G.S=E62-45s, Mem.& Corres., "ff, 471. 355 W.Crookes, La7ratory Notebooks, vol.16, p.54-6,

3rd to 10th Aug.1897, RI. 356 Verh.Phys.Ges.Berlin, 1898, 17, 14-16. 357 Proc.Cam

o . 5HIf7-87F7T 1898, 9, 393-7, 397.

358 LetteTWoET7P.Thompson to G.G.Stokes, dated 28th Feb. 1896; Stokes, Mem.& Corres., 2, 495.

359 Stokes to Thompson, dated 29th Feb.1896; ibid., 495-6. 360 Thompson had already been anticipated by Becquerel,

see this Chapter, Section 1, p.59-60. • 361 Typescript letter from Stokes, correspondence of

S.P.Thompson, Imperial College Archives; the inverted commas are omitted from Stokes, Mem.& Corres., 2, 495-6.

362 G.G.Stokes, Papers, 4, 256-77. 363 Ibid., 273-4. 364 ST61ies, Mem.& Corres., 1, 299. 365 Ibid., 294-7. 366 Saes, Mem.& Corres., 2, 478-83. 367 See Section above, p.'69. 368 E.Rutherford, Papers, 215; pub. Jan.1899. 369 R.B.Owens, Thorium Radiation, Phil.Mag., 1899, 48,

360-87, 361, pub. Oct. 370 J.Elster and H.Geitel, Weitere Versuche an Becquerel-

strahlen, Ann.d.Phys., 1899, 69, 83-90, received 5th Aug.; p.83-7-g1so pub. as Ueber Becquerelstrahlen, Jahresb.d.Ver.f.Wiss.,Braunschweig, 1899, 11, 183, 271-6, IgtliTan.KGiesel, known as a cheast,

Notes for Chapter 2, p.48-118) 301

370 contd.) joined in the discussion of the paper in Brunswick on 19th Jan., ibid., 183; his later public-ations on active substances and their rays are of importance.

371 Letter from J.Elster to E.Rutherford, dated 10th Feb. 1899, CUL.

372 Elster and Geitel, op.cit., Ann.d.Phys., 1899, 88. 373 E.Rutherford, Uranium TgaiatTUE etc., Papers, 215. 374 Having corresponded with Elster and Geitel n 1899

(letters from J.Elster to E.Rutherford, dated 10th Feb., 27th Jun.1899, CUL) and mentioned radioactivity, one would expect him to look out for their publications. Also, Elster in his letter of 10th Feb. promised to send Rutherford their paper on the subject.

302 NOTES FOR CHAPTER 3 (pages 119-178)

1 Jahrosb.d.Ver.f.Wiss.,Braunschweig, 1896, 10, 68, 73-7. 2 Ibid., 1899, 1T, 163; see above, Chapter 2, Section 4, n.370.

3 F.Giesel, Einiges fiber das Verhalten des radioactiven Baryts und tiller Polonium, Ann.d.Phys., 1899, 69, 91-4-

4 J.Elster and H.Geitel, Ann.d.Phys., 1899, 69,-8'3-90, 87. 5 F.Giesel, 6 See E.de Hadn, Ueber eine radioactive Substanz, Ann.d. Phys., 1899, 68, 902.

7 Letter from J.Elster and H.Geitel, in Elster's hand, to E.Rutherford, dated 27th Jun.1899, CUL.

8 J.Elster and H.Geitel, Ann.d.Phys., 1899, 69, 83-90; p.88-90, Ueber den EinflaTiis eines magnetie-CEen Feldes auf die durch die Becquerelstrahlen Bewirkte Leitfahig-keit der Luft, communicated to Deut. Phys. Gee., 5th May 1899.

9 Id., Ann.d.Phys., 1889, 38, 27-39; ibid., 1899, 69, F7-907-U8.

10 I.Joliot-Curie, in M.Curie, Pierre Curie, Paris, 1955, 110.

11 H.Becquerel, Influence d'un champ magnotique sur le rayonnement des corps radio-actifs, CR, 1899, 129, 996-1001, 11th Dec.

12 F.Giesel, Ueber die Ablenkbarkeit der Becquerelstrahlen im magnetische Felde, Ann.d.Phys., 1899, 69, 834-6, received 31st Oct.

13 L.Badash, An Elster and Geitel Failure: Magnetic Deflection of Beta Rays, Centaurus, 1966, 11, 236-40, has calculated the field required to defleFf the rays from radium and concludes that their magnet was too weak to give a noticeable effect in the phosphorescence experiment. A.Romer, Radiochemistry, 11, seems to con-sider that their positive air-conduction results were in fact due to deviation of the rays; the comment made here, that Elster and Geitel did not think in ionic terms, is debatable.

14 S.Meyer and E.von Schweidler, Tiber das Verhalten von Radium und Polonium im magnetischen Felde, Phys.Z., 1899, 1, 90-1, received 10th Nov., from Boltzmann's lab.

15 Ann.d.Phya., 1899, 69, 83-90, 90. 16 S.Meyer and E.von Schweidler, 22.cit., 91. 17 S.Meyer and E.von Schweidler, Ube-Faas Verhalten etc.,

P s.Z., 1899, 1, 113-4, received 18th Nov. Elster and e eT, ibid., D399, 1, 153, made it clear that this

was Giese 'sdiscovery not their own; Meyer's paper did not.

18 See above, Chapter 1, Section 4, p.45-6. 19 W.Sutherland, Cathode, Lenard and Röntgen Rays, Phil.Mag.,

1899, 47, 269-84; J.J.Thomson, Note on Sutherland's paper,-Ibid., 415-6.

20 J.J.Thomson, Phil.Mag., 1899, 48, 547-67, Dec. issue. 21 Ibid., 566-7. 22 J.J.Thomson, The Magnetic Properties of Systems of

Corpuscles etc., Phil.Mag., 1903, 6, 673-93, 689.

Notes for Chapter 3, p.119-178) 303

23 J.J.Thomeon, 22.cit., 1899, 565. 24 Rayleigh, J.J.Thbmson, 132-3. 25 Letter from 3.J.Thomson to E.Rutherford, dated 21st Dec.

1899, CUL. 26 Letter from Rutherford to Thomson, dated 9th Jan.1900,

CUL. See Section 2 below, p.131f. for discussion of 'emanation'.

27 E.Rutherford, Pa ers, (1898), 180. 28 Electrisation negative des rayons secondaires produits

au moyen des rayons Röntgen, CR, 1900, 130, 1013-6, 9th Apr.; P.Curie, Oeuvres, 37-62„ 362.

29 E.Rutherford, Energy of Rbntgen and Becquerel Rays etc., Pa ers, 260-95, 293, received Jun.1900.

30 295. 31 Ibid., 292-3; my stress. 32 H.Becquerel, Seances Soc.Fr.Phys., 1899, 71*-72*,

15th Dec. Mainly repeating 'Influence d'un champ magnetique etc.', CR, 1899, 129, 996-1001, 11th Dec.

33 H.Becquerel, Note our quelques proprietes du rayonnement de Puranium et des corps radio-actifs, CR, 1899, 128, 771-7, 27th Mar.

34 Id., Sur le rayonnement des corps radio-actifs, CR, 1899, 179, 1205-7, 26th Dec.

35 td., Contribution a l'etude du rayonnement du radium, Uff, 1900, 130, 206-11, 29th Jan.

36 PTCurie, Ac lion du champ magnetique sur lea rayons de Becquerel, CR, 1900, 130, 73-6, 8th Jan.; Oeuvres, 349-52. M.Curie, Sur la p6netration des rayons de Becquerel non deviables par le champ magnetique, CR, 1900, 130, 76-9; Oeuvres, 85-8.

37 :aTETT:: CR, 29th Jan.1900 38 R = radius of curvature of path of a particle, produced

by magnetic field H; v = velocity of particle, m = its mass, e = its charge.

39 9.2.cit., n.35, 209. 40 H.Becquerel, CR, 1900, 130, 372-6, 12th Feb. 41 Letter from 147Urookes foiff.G.Stokes, dated 16th Dec.1900;

Stokes, Mem.& Corres., 2, 484. 42 J.J.Thomson, Nature, 1876, 54, 302. 43 Id., Cathode Rays, Phil.Ma., 1897, 44, 293-316, 310. 44 -nem.N., 1900, 81, L45-6, 30th Mar.;-Trans. from

Rev.Gn.des Sci.. 15th Mar.1900. 45 70707-617-539-40, 5th Apr. 46 P. and V.Curie, Sur la charge electrique des rayons

doviables du radium, CR, 1900, 130, 647-50, 5th Mar.; P.Curie, Oeuvres, 353-7.

47 Ibid., 356. 48 tql7-1900, 130, 809-15, 26th Mar.; E.Dorn, CR, 1900, 130,

TT26, 23rd Apr. wrote to claim priority for the qualit-ative electrostatic deviation, in February, of the rays from Giesel's active barium compound.

49 P. and M.Curie, CR, 1899, 129, 714-6, 6th Nov.; prior to the experimental deflection of the rays.

50 E.Dorn, Ueber die von den radioactiven Substanzen aus-gesandte Emanation, Abh.der Naturf.Ges.zu Halle, 1901, 23, 1-15, Jun.1900.

51 PT and M.Curie, op.cit., 716. 52 CR, 1899, 129, 7]- 6,-----6-11 Nov., appended to paper of Curies.

Notes for Chapter 3, p.119-178) 304

53 CR, 1899, 128, 771-7, 773. Refraction and polarisation were also now rejected, for different reasons.

54 1.1RR2ER, 169-215, 180-1, dated 1st Sep.1898; he cited only Schmidt on thorium.

55 Ibid, 56 E.Rutherford and R.B.Owens, Thorium and Uranium Radiation,

Trans.Em.Soo.Canada, 1899, 2, 9-12, read 26th May; 1utherfor47-Pa ers, 216-9, The authors state that thorium nitrate gave a -a_rly constant radiation; we note that this contradicts the stated results of 1898. E.Rutherford, Notebook 3, CUL, contains experimental results of Owens and Rutherford.

57 Ibid., 218; E.Rutherford, Radio-activity, 1905, 238. TaTeter and H.Geitel, Phy2.2., 1899, 1, 11-14, (received 19th Aug.) in examining the conductivity of the ordinary air in the laboratory noted that this was markedly increased by a draught from the room containing Ra and Po samples, without at this stage stating any conclusions; they were soon to take up Rutherford's view.

58 E.Rutherford, Le2212, 218, May 1899. 59 R,B.Owens, Phil.Mag., 1899, 48, 360-87, Oct. issue,

probably wrT117m—Vifore Jul. 60 E.Rutherford, Phil.ns.. 1900, 49, 1-14, Jan. issue,

dated 13th Sep.1899; Papers, 2231. 61 Ibid., 220. 62 E.Rutherford, Some Remarks on Radioactivity, Phil.Mag.,

1903, 5, 481-5; Viers, 578. 63 N.Feather, Lord Rutherford, 69-73, and A.Romor,

The Restless Atom, 43-52, give brief accounts of the researches ofTaherford and others at about this time.

64 Letter from J.J.Thomson to E.Rutherford, dated 23rd Jul. 1899, CUL. The thorium oxide layer is designated AB.

65 Ibid.; question-mark omitted sic. 66 TT :Owens, Thorium Radiation, 1599, 366, 67 Ibid., 372-3. 68 Letter to Mary Newton, dated 2nd Dec.1899, A.S.Eve,

Rutherford.Etc., 69. 69 E.Rutherford, A Radioactive Substance emitted from

Thorium Compounds, Papers, 221. 70 Ibid., 222. 71 Ibid., 225. 72 min, Papers, (1894-7), 6, 17f., had found that gases

could retain conductivity when bubbled through water; Rutherford made no mention of this; his experiments went much further.

73 E.Rutherford, Rp.cit., 224. 74 Id., Papers, (1902), 432. 75 17RutheaER, A Radioactive Substance etc., Papers,

227-9. 76 E.Rutherford, Uranium Radiation etc., Papers, (1898-9),

214-5. 77 Id., A Radioactive Substance etc., Papers, (1899-1900),

728. 78 1900, 62, 31-2, 10th May. Described more fully, and

quantfT5.tively in 'Indications relatives ift la constitution do la matiere etc.', Rapports , Cong.Int.de Physique, 1900, 3, 138-51, Aug.

Notes for Chapter 3, p.119-178) 305

79 Letter from Thomson to Rutherford, dated 23rd Jul. 1899, CUL.

80 J.J.Thomson, 22.cit., Nature, 10th May 1900. 81 A.S.Eve, RutherfOW.Et67767, letter dated Sep.1899. 82 Pa ere, 230. 83 Rutherford in 1905, Radio-activity, 239, mentioned

only the method he used in 1t399. 84 E.Rutherford, Papers, (1899-1900), 230. 85 Ibid., 226. 86 Taa., 230. See below, Section 4, p.176. 87 Ibid., 231. 88 5' above Chapter 2, Section 2, p.81; E.Rutherford,

Papleirs, 106. 89 Ruterford, Pa ere, 231. 90 Id., Phil.Mee., 1900, 49, 161-92; Papers, 232-59;

Tited 22nd Nov.1899. -- 91 Papers, 255-7, Nov.1899. 92 .11, d., 256. 93 6W-above, Chapter 2, Section 2, p.71; Papers, 27-31. 94 E.Rutherford, Papers, (1897), 132f. 95 J.J.Thomson, Conduction of Electricity through Gases,

1903, 296. 96 E.Rutherford, Radioactivity Produced in Substances

etc., Pa ere, 258. 97 Ibid., 98 17-Rutherford (and R.B.Owens), Pa ers (1899), 219. 99 P. and M. Curie, CR, 1899, 129, ; see above, p.129-30. 100 In fact it was Becquerel who had used the term phos-

phorescence, in his appended remarks; the Curies wrote 'rayons secondaires'.

101 E.Rutherford, Pa ere, 238. 102 Letter from J. ."homson to E.Rutherford, dated 21st Dec.

1899, CUL. 103 Letter, id., dated 22nd Nov.1898. 104 J.Zeleny, On the ratio of the velocities of the two ions

produced in gases by Röntgen radiation etc., Phil.Mag., 1898, 46, 120-54.

105 Ibid., 134-5. 106 Letter from J.Zelony, Univ. Minnesota, to E.Rutherford,

dated 25th Mar.1900, CUL; quoted in part, with a different interpretation, in N.Feather, Lord Rutherford, 73.

107 See above, Chapter 2, Section 4, p.117. 108 Ueber Luminescenz von festen Korpern and festen

Losungen, Ann.d.PhT, 1895, 56, 201-54, 241-50. 109 Ann.d.Phys., 69, 220-35, Sep. issue; from Giittingen

where Behrendsen (b.1850) was Professor at the Gymnasium. 110 Behrendsen, ibid., 234, refers to their paper read at

Braunschweig-Tri-Jan., not the reprint in Ann.d.Phys., 1899, 69, 83-90, Sep. issue.

111 BehrenTgen, 22.cit., 233. 112 Only the German scientists had in fact said this;

Marie Curie had speculated on the evolution of the elements, but to this Behrendsen made no reference.

113 Ibid., 235. 114 TEM. 115 T1F71.Z., 1900, 1, 476-8, Aug. issue.

Notes for Chapter 3, p.119-178) 306

116 E.Rutherford, Uranium Radiation etc., Papers, (1898-9), 167-215, 215.

117 M.Curie, Les Rayons de Becquerel etc., Oeuvres, (1899), 60-76, 71.

118 G.G.Stokeo, Mem.& Corres., 1, 293-4, letter to Becquerel dated 16th Aug.1399; see above, Chapter 2, Section 4, P.115.

119 Stokes, Ibid., 294-7, letter dated 25th Aug.1899. 120 See e.g. E.Rutherford, Radio-activity, 1905, 210, 249,

391, and his references. 121 E.Rutherford, Pa era, (1900), 230, Jan. 122 Ann.d.Phys., 1900, 2, 335-7, dated May, pub. Jun. 123 T.-Elster and H.Geitel, Uber Becquerelstrahlen, Verh.

Deut.Phys.Ges., 1900, 5-8, 5th Jan. meeting. 124 Read Jun. 1-970, see below, Section 4, p.172-3. 125 CR, 1899, 128, 771-7. 126 Md., 777. 127 Ibid. 128 G.G.Stokes, Mem.& Corres., 1, 297-9, letter from Stokes

to Becquerel Tdied 4th Sep.1899. 129 H.Becquerel, CR, 1899, 129, 716, appended to the

Curies' paper. 130 H.Becquerel, Deviation du rayonnement du radium dans

un champ electrique, CR, 1900, 130, 809-15, 815; Curie Ra sample of unspecified T6Tivity.

131 Id., Sur le rayonnement de l'uranium etc., Rapports, anig.Int.de Plysique, 1900, 3, 47-78, 78.

132 ma. 133 7:75cquerell 1901, 63, 396-8. 134 Ibid., 398. 135 Tee-below, Chapter 4, Section 1, p.186-8. 136 Nature, 1900, 61, 443. 137 See e.g. 'S.W.', The Principle of Radium, Nature, 1903,

68, 496-7, who makes a similar point. 138 See above, Chapter 2, Section 4, p.113, n.355. 139 See Chapter 5, Section 1, p.230, on the emission of

heat from radium. 140 CR, 1899, 129, 823-5. 141 a.cit.„ Nature, Mar.1900, 142 P.CaTIe, Oeuvres, 353-7; CR, 1900, 130, 647-50; see

above, Section 1, p.127. 143 See above, Section 1, p.126-8. 144 Rev.Sc., 1900, 14, 65-71; M.Curie, Oeuvres, 95-105. 145 P7 aid M.Curie, Les nouvelles substances radioactives

et lee rayons qu'elles omettent, EakvortE,Cona.Int.de P si ue, 1900, 3, 79f., Aug.; P.Curie, Oeuvres, 33

3

s409

146 Ibid., 409. 147 FTUarie and G.Sagnac, kectrisation negative des rayons

secondaires produits au moyen des rayons Röntgen, CR, 1900, 130, 1013-6, 9th Apr.; P.Curie, Oeuvres, 358-62; E.Dorn, Abh.der Naturf.Ges.zu Halle, 1900, 22, 40-2.

148 See above, Chapter 2, sTaTion 4, p.112. 149 22.cit.; M.Curie, Oeuvres, 95-105. 150 mx7Fre, ibid., 104. 151 Ibid. 152 Nature, 1900, 62, 31-2, 3rd May issue.

Notes for Chapter 3, p.119-178) 307

153 See above, Chapter 2, Section 4, p.117. 154 Mt. Curie, 92.cit., Oeuvres, 104-5. 155 See below, Chapter 4, Section 1, 156 See H.Becquerel, Sur le rayonnement des corps radio-

actifs, CR, 1899, 129, 1205-7. 157 See below, Chaptei27 Section 1, p.189. 158 Phil.Trans.A., 1901, 196, 25-59, received 15th Jun.1900;

E.RutherforU, Papers, 260-95. 159 Ibid., 292. He mentioned Becquerel and Giesel, not

7:754yer and Schweidler, Phys.Z., 1899, 1, 90-1, Nov., who first pointed out this di?ference.

160 H.Beoquerel, Note our le rayonnement de l'uranium, CR, 1900, 130, 1583-5, in fact announced a magnetic deflection, also in June; he was unsure of the uranium's purity.

161 Rutherford (and McClung), op.cit., Papers, 292. 162 ibid., 260. 163 Letter from E.Rutherford to J.J.Thomson, dated 9th Jan.

1900, CUL. 164 E, Rutherford, 22.cit., 268-70. 165 Ibid., 273-4; assumptions soon rejected - see below,

this Section, n.I80. 166 ibid., 285. 167 See above, Chapter 1, Section 4, p.36-7. 168 E.Rutherford, 22.eit., Pte, 287. 169 Kelvin, RI Lib.8617, (1 J), 3, 227-56, 227; atomic or

molecular diEneters ranged from 10-' to 10-6 cm. 170 E.Rutherford, op.cit., Papers, 294. 171 A.S.Eve, Rutherfo7Z-Etc., 172 E.Rutherford, Papers, 295. 173 Ibid., 294. 174 (7:7:Thomson, Radium, Nature, 1903, 67, 601-2;

E.Rutherford, 22.cit., Papers, 287, 294; id., Radio- _._.qI121-±Y, 1905, 457.

175 -Elhirford, 22.oit., 2222E2, 294. 176 Ibid. 177 Ibid., 295; he had used the term 'atom' in 1898

rfaTers, 214-5). 178 For example, J.P.Cooke, The New Chemis_tEz, London,

11th ed., 1903, 72-7, wiTHreTirence fd-Keivin, states that 1 litre of any gas under standard conditions contains 61 x 10' molecules, and 1 litre of hydrogen weighs 0.09 gm. One can from this deduce the weight of a hydrogen atom to be 1046 gm., hence Ur = 2 x 10-/4' gm.

179 E.Rutherford, op.cit., 0.._.apers, 293. 180 Ibid., 295. RaTrierford, Papers, (1903), 607, had to

accept Townsend's commenin a letter from Oxford mis-dated 14th Jan.1900 (written in 1901), that his value for the ionisation energy was 'far too large' by a factor of at least twelve. Any difference this might have made to Rutherford's arguments soon vanished as the Curies reported radium specimens,of activities sufficiently high to compensate. The discrepancy was in the early assumption that all the radiated energy produced conductivity in the gas; it was later'impossible to estimate' how much was dissipated as heat(RUtherford, Radio-activity., 1905, 58-9).

Notes for Chapter 3, p.119-178) 308

181 ibid., 294. 182 Ibid., 295. 183 X7g:Eve, Rutherford.Eto., 73. 184 Ibid. 185 reigon, 1900. 186 62, 525-6. Also, see the Proutian interpretations of

7Illard's work, below p.168, n.213. W.Kaufmann, Electrician, 1901, 48, 95-7, was to express ideas similar to those of Fitzgerald.

187 Chem.N., 1900, 81, 304-5; trans. from Ber.deut.chem.Ges. 188 7717tTica, Chem N., 1900, 82, 166-7; from Miliiik711"-

Zeitung; he hadFeld such Views for twelve years or more.

189 E.E.Pournier d'Albe, The Life of Sir William Crookes, London, 1923, 372.

190 F.P.Venable, The Nature of Valence, J.Amer.Chem.Soc., 1899, 21, 192-200, 220-31, p.197. -

191 CR, 1899, 129, 593-5. 192 A.Debierne, CSR, 1900, 130, 906-8. 193 H.W.Kirby, T Discovery of Actinium, Isis, 1971, 62,

290-308, questions the identity of theEETerials described in Debierne's two papers and credits F.Giesel with the discovery in 1902 of 'emanium', the element now known as actinium.

194 P.Curie, Oeuvres, (1899), 345, Nov. 195 A.Debierne, p.cit., n.192, Apr.1900. 196 E.Rutherford (and ,.Soddy), The Radioactivity of Thorium

Compounde.I, Papers, (1902), 376-402, 378, Jan. 197 Ann.d.Pplys., 1899, 69, 91-4; briefly in Phy.2.Z., 1899,

. 198 The Curies appear to have observed this by July 1899,

before Giesel's publication, see I.Joliot-Curie in M.Curie, Pierre Curie, Paris, p.120; but they accord priority to GiesCi1 TCongr6s 1900, P.Curie, Oeuvres, 388. The effect was later explained by an accumulation of 'emanation'.

199 F.Giesel, Einiges abor Radium-Baryum etc., Verh.Dout. Phvs.Gos., 1900, 2, 9-10, dated Dec.1899.

200 A.DebTeTne, CR, 1700, 131, 333-5, 30th Jul.; trans. Chem.N., 1907 82, 85.

201 751777 Chem.N. 202 Ibid., TN:- 203 FiTT:deuTTchem.Ges., 1900, 33, 1237-40; Chem.N., 1900, 82,

2576, dat:TiTiffay. 204 Ibid., 25. 205 J76T:deut,chem.Ges., 1900, 33, 1665-8, received 28th May. 206 F.Giesel, ibid., 1668; he sIso pointed out that his own

and the Curies' polonium differed both in radiation type and rate of decay, to which the age of the specimens might be relevant. Marie Curie herself wavered towards the belief that polonium was merely induced bismuth, in 1902.

207 K.A.Hofmann and E.Strauae, Radioactives Bloi and radio-active seltene Erden, Ber.deut i chem.Ges., 1900, 33, 3126-31. Of interest are modern transformation series in which no less than four natural, active, true lead isotopes feature; their half-lives are about 27 min., 36 min., 11 hr., and 22 yr. It seems that Hofmann may have had any or all of these, followed by their active

Notes for Chapter 3, p.119-178) 309

207 oontd.) decay products, Bi, Po, Tl, in his preparations from different minerals; even if he had avoided traces of other active elements.

208 P.Giesel l Ueber radioactive Stoffe, Ber.deut,chem.Ges., 1900, 33, 3569-71; Chem.N., 1901, 837-122-3.

209 Hofmann and StrausiTriiia., 1901, 7, 8-11, received 28th Dec.1900.

210 F.Giesel, ibid., 3772, thought this work unreliable. 211 Hofmann and A.Korn, ibid., 1901, 34, 407-9. 212 SOances Soc.Fr.TIcET.;7700, 59*, bth Jul. 213 In relatICTI T45 hIS G.Sagnac had written to J.Larmor

(letter dated llth May, 1899, Royal Society Library) concerning types of vacuum tube. He suggested that Villard's idea that Whydrog6ne est indispensable a la formation dee ions cathodiques' would be unnecessary if all bodies were 'forme dune meme matiero simple qui serait la matiare radiante de Crookes'. Villard himself believed in the unity of radiant matter but explained this by the universal presence of the penetrating and chemically reducing element hydrogen, presumably as an impurity; see e.g. P.Villard, La formation des rayons cathodiques, Rev.Gon.des Sci., 1899, 10, 301-8; id., Les Rayons CathodUiaes, Rapports,Cong.int.de Physique, 1900, 3, 115-37 / 136-7.

214 See Stokes, Memac Correa., 2, 484. 215 J.C.McLennan, PhIl.Mag., 19u2, 3, 195-203. 216 Hofmann and Strauss, Leber das radioactive Blei.2.

Mitteilung, Ber.deut.chem.Ges., 1901, 34, 907-13, 913. 217 Another feature was the dffiriirence between salts, id.,

3.Mitteilung, Ibid., 3033-9. 218 F.Gieeel, Uebe7-717dleactive Stoffe, ibid., 3772-6; the

modern theory Ewes only admixtures and does not admit induction; active lead is a 'transformation product' of radium.

219 K.A.Hofmann and Strauss, Ueber radioactive Stoffe, ibid., 3970-3, received 27th Nov.1901.

220 P,Giesel, On radio-active lead, Chem.N., 1902, 85, 89-90; from Ber.deut.chem.Ges., 7517, 35, 102f., Jan.

221 K.A.Hofmann 770, 1f1, radioacTIve Stoffe.l. Ueber radioactive° Blei, ibid., 1902, 35, 1453-7, Apr.; self-recovery shows the activity is not the induced kind.

222 Id., Chem.N., 1903, 87, 241-3; from Ber.deut.chem.Gos., 703,77; 7040f.

223 Proc.22y.Soc., 1900, 66, 409-22, read 10th May; reprinted WITFoai-iETTIrpretation in A.Romer, Discovery of Radio-activity, 70-84.

224 Crookes in Romer, 22.cit., 82. 225 G.G.Stokes, Memac.-Co7i-e-s., 2, 490. 226 Ibid., 490-2. 227 7/7nFookes, Notebook 16, pp.270, 305f., RI. 228 Ibid., Notebook 17, 102f. 229 Ibid., 106. 230 Crookes in Romer, opecit., 74. 231 Ibid., 71. 232 MU., 77. Becquerel was soon to deny this, with important

riequences, see below, p.171; and Chapter 4, Section 1, p.187, n.28.

Notes for Chapter 3, p.119-178) 310

233 Letter from W.Crookes to P.Curie, dated 13th Jul.1900, BR.

234 References were provided, namely Ber.deut.chem.Ges., 1900, 33, 1237-40 and ibid., 16657U7 Sae move, p.166-7.

235 TypescHpt translation-GT-letter from P.Curie to W.Crookes, dated 17th Jul.1900, appended at end of Crookes' Notebook 17, RI.

236 Letter from Crookes to P.Curie, dated 19th Jul.1900, BN. 237 W.Crookes, Radio-activity and the Electron Theory,

Chem.N., 1902, 85, 109-12, 109. 238 Chem.7.1., 1901, UT, 179-81, 187-9; he referred to

737Riaunerts similar work; the latter, Chem.N., 1901, 84, 219, claimed priority; these researches did not extend beyond the field of inorganic chemistry.

239 Baskerville, ibid., 179. 240 CR, 1900, 130, 1583-5. 241 H.Becquerel, Sur is rayonnement de l'uranium, ibid.,

1900, 131, 137-8, 16th Jul. 242 BecqueI, Sur le rayonnement de l'uranium etc.,

11122ports,241264.Int.do Pbrsi ue, 1900, 3, 47-78, 74. 243 P. aT7M.(:urieTres nouvo lea substances radioactives

etc., ibid., 79f.; P.Curie, Oeuvres, 374-409. 244 ibid.,qm 245 YETa., 407. 246 See above, Section 3, p.155-6. 247 P.Curie, 22.cit., Oeuvres, 384. 248 Ibid., 379-80, 404-b. 249 Abh.der Naturf.Ges.zu Halle, 1901, 23, 1-15, read

7aii.1900. 250 Ibid., 1. 251 YETU., 11-12. 252 Ibid., 13. 253 TUTU., 15. 254 77gnning, Ueber radioactive Substanzen, Ann.d.Phys.,

1902, 7, 562-75. 255 Ibid., 569. 256 Letter from J.J.Thomson to E.Rutherford, dated 15th

Feb.1901; A.S.Eve, Rutherford.Etc., 76. 257 E.Rutherford, Pa ers, 261. 258 E,Dorn, Phys.Z., 1 1, 2, 218, received 24th Dec.1900. 259 Rutherford, PEyq.Z., 19a, 2, 429-31; Papers, 296-300. 260 Ibid., Papers, 230. 261 Rutherfor op.cit., (1901), 300. 262 A.S.Eve, Rutherford.Etc., 77. 263 Ibid., 78. Letter dET-Jd 12th Apr.1901. 264 Above, Section 2, p.145. 265 Letter from J.J.Thomson to E.Rutherford, dated 25th

Apr., CUL. 266 E.Rutherford, Papers, (1901), 325, 358-9, Dec.; see

below, Chapter 4, Section 2, p.208, 223-4. 267 Id., Papers, 230. 268 TTss Brooks' first published research, performed with

Rutherford's help and on one of his subjects, concerned 'Damping of the Oscillation in the Discharge of a Leyden-jar', Phil.Mag., 1901, 2, 92-108; she was B.A. Tutor in MathWffialics, Royal Victoria College for Women, Montreal, at the time.

Notes for Chapter 3, p.119-178) 311

269 Trans.jya.Soc.Canada, 1901, 7, 21-5; Rutherford, 15656713, 301-5,

270 Nature, 1901, 64, 157-8; Papers, 306-8. 271 Ibid., 305, 3087 272 5-675-e.g. S.Glasstone, Sourcebook on Atomic Energy,

London, 1950, 125. F.Soddy, Radio-activity, Electrician, 1904, 52, 681, deduced from the same results a doubled aTFinic weight of 160, at a time when theory demanded a value oloee to that of radium.

273 E.Rutherford, Papers, (100, 545. 274 Rutherford,227-etT77 Papers, (1901), 305, 308, May. 275 Rutherford, op.-617., Nature, Papers, 308. 276 See below, CH-gpter 5, Section 1, p.233-6. 277 H.Geitel, Ueber die Elektrizitatszerstreuung in abge-

schlossenen Luftmengen, 1900, 2, 116-9. 278 J.Elster and H.Geitel, Weiiire Versuche etc., ibid.,

1901, 2, 560-3. 279 Id., Leber eine fernere Analogie in dem elektrischen

Wrhalten der naturlichen and der durch Becquerel-strahlen abnormleitend gemachten Luft, ibid., 590-3.

280 H.Geitel, Ueber die durch atmospharischirrift induzierte Radioaktivitat, ibid., 1901, 3, 76-9.

281 Id., Archives dee Sciences, S02, 13, 113-28, 122; 271-Fie the followIE7g Section.

282 Ibid., 124.

312 NOTES FOR CHAPTER 4

(pages 179-225)

1 H.Geitel, Archives des Sciences, 1902, 13, 113-28, dated Dec.190i.

2 Ibid., 127. 3 Ibid., 126-7. 4 Ibid., 117. 5 Mister and H.Geitel, Vereuche fiber induzierte Radio-

activitAt der atmosphilrischen Luft durch positive Potentiale, Phys.Z., 1902, 4, 97.

6 Id., On the radio-active emanation in the atmospheric air, Chem.N., 1903, 88, 29-32, 52-4, received by P s:277-6-Th Jun.

7 er .Deut.Ges.Natf., 1902, 73, 83-99, 98; read ME 76571901.--

8 CR, 1901, 132, 548-51; P.Curie, Oeuvres, 410-13. 9 UR, 1901, 137, 768-9; P.Curie, Oeuvres, 414-6;

read 25th /5.7.1901. 10 Paers, 301-5; with Miss H.T.Brooke. See above,

apter 3, Section 4, p.176. 11 P.Curie, 2p.cit., Papers, 414-6. 12 Ibid., 41 13 T.77 at the time of the Congress in Aug.1900, P.Curie,

Oeuvres, 407-8. 14 1'. (,curie, Oeuvres, 412, 5th Mar.1901. 15 Notebook, pp.125 plus pp.18 in reverse direction, written

by P. and M.Curie; records experiments on induced activity from 5th Dec.1900 to mid-1902; also various radiation and chemical studies from May 1899 to late 1902; held at Wellcome Historical Institute.

16 E.g., ibid., p.72; mouvement propre: 10th Jan., 90 gm. in 40 sec.;11th Jan., 90 in 36; 12th Jan. worsening to 90 in 11.

17 Ibid., p.101, 10th Jul.1901, action of the extreme cold of liquid oxygen upon radium and thorium: radium's activity fell from 2000 gm. in 20 sec. to 200 in 12; thorium gave initial readings of 50 gm. in 30, then 23, then 37 sec. But mouvement propre was 50 in 23, consequently 'c'est mt. propre pas action'.

18 CR, 1901, 133, 276-9, 29th Jul.; P.Curie, Oeuvres, 420-3. 19 Tad., 421. 20 CR, 25th Mar.1901; P.Curie, Oeuvres, 416. 21 Mem.N., 1901, 84, 88-9. Perhaps P.de Heen's publication

TO-Tu-Li.1901 on 'La radioactivit4 de la matiere et l'energie susceptible de se developper e. la surface des corps' (Rev.Sc., 1901, 16, 161-70) should be mentioned here, forme gave something of a mechanism for radio-active induction through gases: molecules irradiated by an active source themselves emitted rays or 'jets d'ether', which excited radiations in other molecules, and so on. No doubt P.Curie thought this work as weak as that of G.Le Bon which de Heen cited and which Curie had already criticised. The latter published no further comment on the researches of either of these obscure scientists, reserving his considerations for others now better known.

Notes for Chapter 4, p.179-225) 313

22 P.Curie and A.Debierne, Sur la radioactivito induite provoquee par los sells de radium, CR, 1901, 133, 931-4, 2nd Dec.; P.Curie, Oeuvres, 424-7.

23 Letter from F.Giosei to P. and M.Curie dated 23rd Mar. 1902, BN; see below, Chapter 5, Section 1, p.232.

24 P.Curie, Oeuvres, 421. 25 E.g. Sur la radio-activite secondaire, CR, 1901, 132,

734-9, 25th Mar. 26 CR, 1901, 133, 977-80, 9th Dec. 27 On the Conductivity of Gases under the Becquerel Rays,

Phil.Trans.&., 1901, 196, 507-27, 525. M. Curie, Sur la pbnetration dos rayons etc., CR, 1900, 130, 76-9, 8th Jan., had earlier likened these rays to 'projectiles'.

28 The statement flatly contradicted the comments published by Crookes on Ur nitrate in 1900; see above, Chapter 3, Section 4, n.232.

29 CR, 1900, 130, 1583-5, 1585. 30 -H.Becquerei;-Sur la radio-activitO de l'uranium, 978. 31 P. and M.Curie, CR, 1902, 134, 85-7, 13th Jan; P.Curie,

Oeuvres, 428-30; A.Romer, Discovery of Radioactivity, 117-23, gives translations of this paper an of Becquerel's.

32 P.Curie, Oeuvres, (1900), 356. 33 M.Curie, Oeuvres, 104-5; see above, Chapter 3, Section 3,

p.156-7. 34 Chem.N., 1902, 85, 169-72, read at RI on 7th Mar. 35 Rev.Sc., 1901, 15, 449-61, read Feb., pub. Apr. 36 Ibid., 460-1. 37 767above, Chapter 2, Section 4, p.110-1. 38 G.G.Stokes, Mem.& Corres., 2, 478-81; both hypotheses

required an external supply. 39 See o.g. Phil.Trans.A., 1901, 196, 507. 40 Letter dated 16th Dec.1900 in reply to Stokes' of 15th;

Stokes, Mem.& Correa., 2, 481-5. 41 Crookes, loc.cit. in Stokes, Mem.& Correa. The cited

paper is W-P7VIllard, Sur la perm6EFI1ITe de la silice fondue pour l'hydroOne, CR, 1900, 130, 1752-3.

42 P.Villard, Les Rayons CaliRidiques, tiapportili,22E,E.Int.de Ph ai ue, 1900, 3, 115-37, 136-7, Aug.

43 Crook©s in Stokes, loc.cit., 489-90, letter dated 15th Jun,1901.

44 W.Crookes, Notebooks, 17, and 18, RI. 45 Ibid., e.g. 17, 308-69; 18, 149-85. 46 -076R.IT., 1907 85, 109-12, read to the Royal Society

MeE. 47 1901, 83, 130, from Bristol; Martin gained his B.Sc. in

that year. He studied at University College Bristol and several German universities before becoming Lecturer at University College Nottingham in 1907 and at Birkbeek College, London in 1910. He later held a variety of industrial posts and published prolifically on chemistry -pure, industrial, and popular.

48 Ibid., 141, 22nd Mar.; E.Booth, ibid., 262-3, discussed Ts further.

49 Chem.N., 1902, 85, 205-6, dated 26th Mar., Berlin University.

Notes for Chapter 4, p.179-225) 314

50 L.Boltzmann, Lectures on Gas TheoriL, (1898), 377, dopioted the chemical-. YOna"-Tis an overlap of supposed sensitive regions of blank material atoms. VI.Sutherland, The Cause of the Structure of Spectra, Phil.Mm., 1901, 2, 245-74, 269, illustrated his spherical material atom as furnished with a few electrons, some of whose orbits collided with the atomic surface. See Chapter 1 above and Chapter 5, Section 2 below for further discussions of theories of atomic structure.

51 Chem.N., 1911, 103, 169. 52 Leipzig, 1902; pref. dated Easter. 53 E.g. see W.Kaufmann, The Development of the Electron

Idea, Electrician, 1901, 48, 95-7; see the preceding Section, p.163T-YOr Fitzgerald's similar speculations in 1900.

54 J.Stark, 22.cit., 93-4. 55 Ibid., 34. 56 Tail., 35. 57 Letter dated 5th Jan.1902, A.S.Eve, Rutherford.Eto., 80-1. 58 Letter to Mary Newton, dated 25th Jan.1896, ibid., 23-6,26. 59 Letter from F.Soddy to E.Rutherford, dated 17TE-Dec.1903,

CUL, concerning among other items Becquerel's book of 1903. 60 N.Foather, Lord Rutherford, 1940, 78-90, describes some

of the poin1 A.Romer, The Transformation Theory of Radioactivity, Isis, 1958, 49, 3-12; id., The Restless Atom. The Awakening of Nuclear Physics, NeW-York, 1960, 591, outlines ithe stages in clear and simplified form. T.J.Trenn, Rutherford and Soddy: from a search for radioactive constituents to the disintegration theory of radioactivity, Rote, 1971, 1, 51-70; id., The rise and early development of the disintegration theory of radio-activity, Dies., Univ. Wisconsin, 1972, gives more det-ailed but sometimes less clear descriptions specifically limited to the Rutherford-Soddy experimental collaboration of Sep.1901 to May 1903.

61 E.Rutherford, Pa ere, 305, 308; see above, Chapter 3, Section 4, P-1

62 T.J.Trenn, Dips., 14-15, 60, states that there is no evidence for the assumption that the collaboration began before Sep.1901.

63 A.S.Eve, Rutherford.Etc., 77. 64 M.HoworthTPIWEIFFResearch on the Atom ... The Life

St2ry of Frederick Aaddz, London, 1958, 79-81; the Yaw mss. are in th-e-nddleian Library, Oxford, where they were placed by M.Howorth. Trenn, Dies., 60, has found Rutherford's brief notes on the 465Tings of the Physical Society from 1898 to 1907 in the McGill University Archives, and confirms that both parties refer to the same meeting.

65 'The Indivisibility of the Atom', pp. 23, typescript, Soddy-Howorth Collection, Bodleian Library.

66 M.Howorth, Pioneer Research.Soddy, 81; id., Atomic Transmutation. The Greatest Iiiadvery Ever Made, London, 17577617----

67 J.J.Thomson, On Bodies Smaller than Atoms, pop.Sci. Monthly, 1901, 59, 323-35, Aug.; similar to R1Lecture o 19th Apr.19017

68 M.Howorth, Pioneer Research.Soddy, 65; Soddy's own comment.

Notes for Chapter 4, p.179-225) 315

69 Ibid., 64; six lectures are preserved. 70 Tura., 85. 71 E.Rutherford and F.Soddy, J.Chem.Soc., 1902, 81, 321-50;

Rutherford, aTers, 376-407. 72 E.Rutherford an .Soddy, An Investigation of the Radio-

active Emanation produced by Thorium Compounds.I , Proc. Chem.Soc., 1902, 18, 2-5; Chom.N., 1902, 85, 55-6.

73 E70treFford (and-P.Soddy)77a.at., Papers, 381. 74 Ibid., 385-7. 75 1:.g. at that time G.Martin asked 'Is Argon an Elementary

Substance?', Chom.N., 1902, 85, 9, 3rd Jan., but only to suggest it m3.ghh be a mixare of several inert gases. H.E.Armetrong, who attacked W.Ramsay's conclusions regarding these gases, was in this case an exception.

76 E.Rutherford (and F.Soddy), 22.eit., Papers, 395-6. 77 Ibid., 395-6. 78 YTT3., 388-9. 79 170., 392-4. 80 nu., 390. 81 TEM., 391. 82 TE,frOworth, Atomic Transmutation, 44; id., Pioneer

Research.Soddy, 82-3. 83 E.RutherfordCand F.Soddy), ap.eit., Papers, 396. The

other possibility was that thiiiraR induced activity upon one of the atmospheric inert gases; concerning which see below.

84 E.Rutherford, Early days in radio-activity, J.Franklin Inotitute, 1924, 198, 281-9, 285.

85 T-Fullord, Papers_, 396. 86 Ibid., 391. 87 Tailierford had seen J.J.Thomson's paper to that Congress

by March 1901; letter from Rutherford to Thomson, dated 26th Mar.1901, A.S.Eve, Rutherford.Etc., 77.

88 J.Amer.Chem.Soc., 1901, 2:3, 761f., presented 27th Aug.; aem.N., 1961784, 179-817 187-9, p.181. 11th Oct.

89 TETEerford, 2p.at., Pa ers, 379-80. 90 Letter from Ur-o-61-gs to z erford, dated 18th Dec.1901;

A.S.EVe, Rutherford.Etc., 79. 91 E.Rutherford, op.eit., The Radioactivity of Thorium

Compounds.I.Etc., Papers, 389. 92 Ibid., 396-7. 93 .11-5ra. 94 ibid., 397. 95 mu. 96 ibid., 398. 97 1.RUmer, Restless Atom, 61; T.J.Trenn, Dies., 94. 98 E.RuthernTITTEHff IF:Soddy), 22.cit., 398, 402; they

confirmed this dismissal with the German nitrate from which the impurity was absent, as noted in their 2nd publication of May 1902, Pa ers, 435.

99 Ibid., 398; the 'two directions' are probably the precipitation and washing methods of removing ThI from Th, see below.

100 See the preceding Section, p.184-5. 101 Op.cit., Rutherford, faaE2, 402. 102 ma 390. 103 Ibid., 399-400. 104 R.Rutherford (and F.Soddy), Papers, 447.

Niates for Chapter 4, p.179-225) 316

105 Id., The Radioactivity of Thorium Compounds.I.Etc., Pa ern, 399.

106 Barramer.Ehys.Sac., 1901, 2, 37-43; E.Rutherford, Ti-ir-67525-c30",-7351-9, &I-tea 15th Dec.

107 / .J.Allen, rhys.Z.. 1902, 3, 225-30, dated 20th Dec.1901; RaTher?ord, pppprs, 360-9.

108 E.Rutherford, Transmission off-Excited Radioactivity, DIT2TP, 329.

109 Td:,-Excited Radioactivity etc., Pa ers, 367. 110 LIR, 1901, 133, 977-80, 9th Dec. Ruvher±ord, whose paper

was dated I5Th Dec., received by Ehys.Z., 22nd Jan.1902 (Pa ors, 359) may just possibly have derived the recoil idea rom Becquerel; the Abstract of Rutherford's similar paper to the American Physical Society, dated 14th Dec.(Papere

' 330) does not mention it. Becquerel,

however, u-sedTHE) notion in a slightly different way. 111 E.Rutherford, ap..cit., Papers, 368. 112 E.Rutherford (and F.Soddy), the Radioactivity of

Thorium Compounds.I.Etc., IlusEE, 378; perhaps Soddy influenced the brief statement given there.

113 E.Rutherford, op.cit., Papers, 368. 114 Id., (with S.J.Afien), Papers? 509, dated Jun.1902. 115 13Toc.Rox.Soc., 1902, 6971-77:82, Dec.1901. 116 E.Rutherford, Excited -Radioactivity etc., Pa ers, 369. 117 E.Rutherford and F.Soddy, Chem.N., 1902, 8,-6. -6. 118 Ibid., 56. 119 771athorford (and F.Soddy), Radioactivity of Thorium

Compounds.I.Etc., Pates, 396. 120 E.Rutherford, Exciid Radioactivity etc.. Papers, 368. 121 Id., Phys.Z., 1902, 3, 254-7, Papers, 376-5. There are

apparently no surviving English versions of several of Rutherford's publications.

122 Made by early December, 1901; Rutherford, Papers, 327, 371.

123 ibid., 372-3. 124 E.Rutherford, Papers, 436. 125 CR, 1901, 133.-0778-0; see the previous Section, p.187. 126 rffiomson's letter of 2nd May 1902 to Rutherford (CUL) may

modify the view (A.Romer, Isis, 1958, 3) that the election of this candidate was not to be expected at the first attempt. The former regarded the election as certain, thought the result 'a great scandal', and accused the new Secretary of bias in favour of his 'fellow townsmen' of Belfast. Rutherford at 31 was not particularly young for those days; C.T.R.Wilson at that same age had been one of the fifteen out of sixty candidates to be selected in May 1900. Rutherford, however, along with J.S.Townsend had only a year to wait for the honour, whilst Pierre Curie was experiencing worse problems with the Acadomie des Sciences in Paris.

127 J.Chem.Soc., 1902, 81, 837-60, 15th May meeting; .Rutherford, Papers, 435-56.

128 Ibid., 436. 129 Ma., 438-40. 130 UAW., 440. 131 Mid., 441-2; the emanation was as usual prevented from

interfering by means of a draught of air, ibid., 436.

Notes for Chapter 4, p.179-225) 317

132 Ibid., 449. They did not make it clear that this was E3T—a case of direct induced activity, and in one place referred to excited activity as a 'secondary radiation' (ibid., 450). A year earlier Rutherford (Pa ers, 305, 308) wrote of the production from radium emanation of 'a positively charged substance which ... becomes a source of secondary radio-activity'. The terminology was sometimes ambiguous but was explicitly clarified shortly afterwards as discussed below.

133 Ibid., 451. 134 7737., 447. 135 151a., 442-4, 448. 136 mod., 444-5. 137 Ibid., 455. 138 Ibid., 440-1. 139 TM. 140 !Era., 379. 141 See above, Chapter 3, Section 4, p.167-9. 142 K.A.Hofmann and F.Zerban, Ueber radioactive° Thor,

Ber.deut.chem.Gos., 1902, 35, 531-3, received 23rd Jan. 143 Ylj.561f-discliTa7ge in aboUT 4 mins. 144 Rutherford, 22.cit., 452. 145 Ibid. 146 mod., 454. 147 Ibid., 452. 148 =em.Soc., 1902, 81, 860-5, presented (not read)

15TE-May; also Chem.N., 1902, 86, 199-200. 149 Ibid., 863. Within weeks he hathe modern answer: the

7TEdual activity belongs neither to 1. nor 2. but to Ur itself; it declines immeasurably slowly. The path to that interpretation was not simple; see below.

150 Ibid., 864. 151 F.Giesel, On Radio-active Lead, Chem.N., 1902, 85,

89-90; from Ber.deut.chem.Ges., 18th 7-an. 152 F.Soddy, RadioactivitTU? Uranium, 864. 153 P.and M.Curio, Sur lea corps radioactifs, Jan.1902;

P.Curie, Oeuvres, 429. Rutherfores cautious description at this time was 'polonium (radioactive bismuth)', Deviable Rays etc., Pa ere, 470, dated 7th May 1902.

154 Ber.deut.chem.Ges., 02, 35, 2285-8, presented 9th Jun.; Chem.N., 1g57,-166, 52-3.

155 P. Soddy wrote from Montreal to E.Rutherford, who was on vacation, explaining the paper's contents and stressing its importance; letter dated 12th Jul.1902, CUL.

156 F.Soddy, Radioactivity of Uranium, 864- 157 Dated 29th Apr.1902, CUL. 158 Proc.Roy.Soc., 1902, 69, 413-22, discussed above, p.191-3. 159 ma., pr(567Roy.soc.,-799-413. 160 ibid., 410-3. 161 N.Feather, Lord Rutherford, 88; L.Badash, How the 'Newer

Alchemy' Was Received, Sci.Amer., 1966, 215, 88-95. 162 E.Rutherford (and F.SodiTYT, Radioactivity of Thorium

Compounds.II, Papers, 454-6. 163 E.Rutherford, The Existence of Bodies Smaller than

Atoms, Papers, 403-9, 409; read to the Royal Society of Canada, 2/th May 1902.

Notes for Chapter 4, p.179-225) 318

164 E.Rutherford, Radioaotivity of Thorium Compaunds.II, loo.cit., 452, 455.

165 PEI17E., 1902, 4, 569-85; Rutherford, Papers, 494-508. 166 WY:Trann, Diss.. 316-22, suggests a mid-June submission

since they did not cite Marckwald on Po, and the delay for some other papers at this time was around 5 to 6 months.

167 E.Rutherford, op.cit., Pa ers, 508; the phrase 'accomp-animent of a chemical change- used in the previous paper written in April (id., Papers, 455) evidently had no specific temporal meaning.

168 Ibid., 508. 169 THU. 170 Radioactive Change, Phil.Mag., 1903, 5, 576-91;

Rutherford, Pa ere, 3W608, 603;•proNlably submitted in about Mar. 1 .

171 Letter from P.Soddy to E.Rutherford, dated 26th Sep. 1903, CUL; any reply appears to be lost.

172 Ibid. 173 E.Rutherford, Phil.Mag., 1903, 5, 95-117, dated 29th Jul.

1902; Papers, 377:0, 529. 174 Ibid., 539-41. 175 IFItE, 358-9. 176 IcRatherford, Pa ors, 415-34, 421. 177 E.Rutherford (an .J.Allen), Papers, 509-27. 178 Ibid., 517. 179 ITRUtherford, Excited Radioactivity etc., Papers, 545-6. 180 Ibid., 546. 181 It is interesting that Rutherford at this point had

adopted a completely particulate or non-vibrational view of all radioactive radiations including, temporarily, the penetrating gamma rays which he thought were high velocity electrons; E.Rutherford, Penetrating Rays from Radio-active Substances, Nature, 1902, 66, 318-9, 6th Jul.; faa.2, 410-4, 413.

182 Rutherford, 22.cit., Pa ere, 546. 183 Ibid., 544-7; atom and molecule of an inert gas were

Fin to bo identical. 184 E.Rutherford, Phil.Mag., 1903, 5, 177-87, dated Nov.1902;

id., Papers, 1.ggl-57; T.J.Trenn, Dies., 209-31; A. Romer, Tistless Atom, 71-84.

185 TTUTEriford and F.Soddy, The Radioactivity of Uranium, Phil.Ma .1 1903, 5, 441-5; id., A Comparative Study etc. THU., 45-57; Rutherford, Papers, 561-75, 564, 575.

319 NOTES FOR CHAPTER 5

(pages 226-280)

1 M.Curie, Pierre Curie, N.Y., 57. 2 P.Curie, URT-Tg6277154, 420-3; Oeuvres, 431-4. 3 P.Curie, MI, 1902, 175, 857-9, I7TEYE.; Oeuvres, 435-8. 4 Seances Soc.Fr.phyp., 1902, 60*, 19th Dec. 5 Pieuriel7174-7903-,-- 136, 223-6; Oeuvres, 440-3. 6 Ibid., 4477 7 77RUtherford, Papers, 508. 8 Ibid., 498. 9 TTOTirie, 22.cit., Oeuvres, 442. 10 Id., 443. 11 E.Rutherford, 2E12E2, 507-8, Nov.1902; the previous

publication in Phil.M2a., Sep.1902, had made the point fairly clear however; and an abstract of the earlier paper of May 1902 in Rev.Gen.des Sci., 1902, 592, 30th Jun. ended 'La radioactivitb serai7Th manifestation d'un changement chimiqus sous-atomique'; whether Curie saw or understood this one cannot say.

12 E.Rutherford, Phil.Mea., 1903, 5, 481-5, dated 28th Feb., Apr. issue; Pa ere, 576-9.

13 P.Curie, Sur la r eparition de la radioactivite induite par le radium eur les corps solides, CR, 1903, 136, 364-6; Oeuvres, 444-7.

14 CR, 19077 T77 673-5. 16th Mar.; P.Curie, Oeuvres, 448-51. 15 A.Laborde,-fferre Curie dans son Laboratoire, Univ. do

Paris, 1956, 5773. 16 P.Curie, 22.cit., Oeuvres, 450. 17 E.Rutherford, Papers, 607, probably written Mar.; pub.

May 1903. 18 H.Becquerel, Recherches sur une propriet6 nouvelle etc.,

pref. dated Aug.1903. 333, seems to have thought that Curie had actually adopted a slow atomic transformation theory.

19 P.Curie (and J.Danne), Sur l' emanation du radium et son coefficient de diffusion dane lair, CR, 1903, 136, 1314-6, 2nd Jun.; Oeuvres, 452-5.

20 Proo.RI, 1903, 17, 389-402. 21 Letter from Soddy to Rutherford, dated 31st Mar.1903,CUL. 22 Trans. in Chem.N., 1903, 88, 85f., in several instalments;

contains no menTion of the condensation and diffusion experiments of P.Curie on the emanation.

23 See Chapter 3, Section 4, p.169. 24 Letter from F.Giesel to P. and M.Curie, dated 23rd Mar.

1902, from Braunschweig, pp.4, BN. 25 Ibid. 26 V.Giesel, Zeit.f.Elektrochemie, 1902, 8, 579-85, pub. Aug. 27 F.Giesel, Neues fiber Radium and radioaEtive Substanzen,

Jahresb.d.Ver.f.Wiss.,BraunschweiE, (1902), 13, 43-5, 43; 30th Oct. meeting, pub.1904.

28 Ibid., 45. 29 Ti7e7k)1, ok.cit., n.26. 30 E.Rutherford, Pa ere, (1904), 706. Also, early in 1903

Giesel criticise e chemical evidence for the trans- formation of thorium: this element, he thought, owed its activity to the 'permanently' active constituent which he had extracted from pitchblende, Chem.N., 1903, 87, 97-8.

Notes for Chapter 5, p.226-280) 320

31 E.Rutherford, Loos the Radio-aotivity of Radium depend upon its Concentration?,Nature 1904, 69, 222, dated 18th Dec .1904; Id., Papers, 6.18-9.

32 Radium, Nature,-T903.677601-2, 33 F.Giesel snot© equivoCgIly on the matter, as discussed

above; E.Dorn, discoverer of radium emanation, remained silent.

34 Letter from Thomson to Rutherford, dated 13th May 1902; A.S.Eve, Ruthorford.Etc., 82.

35 J.J.Thoms7,7767 755ffEiises of the Ions etc., 1899, 558-9 refers to this result of Wilson; see C.T.R.Wilson, On the condensation nuclei produced in gases by the action of Röntgen rays, uranium rays, ultra-violet light, and other agents, Phil.Trans.A., 1899, 192, 403-53.

36 C.T.R.Wilson, P166.Roy.goc., 1902, & 277-82. 37 Phil.ym., 1902, 3,-I95:2-0-3, dated Mic.1901; presented

by Thomson. 33 Proe.Camb.Phil.Soc., 1902, 11, 504. 39 ITATDDFCTIvity PY-auced by Action of Thorium Compounds,

papers, 259. 40 T:7.Thomson, Proc.Camb.Phil.Soc., 1902, 11, 505. 41 Letter from ThomsoirTiT RutheTTTIrd, dated-7nd May 1902, (JUL. 42 Ibid. 43 17:17-iutherford, 2.E.p.frs. 378, read Jan., pub. Apr.1902. 44 Ibid., 368-9, dated Dec.1901. 45 175TTer from Thomson to Rutherford, dated 13th May 1902;

A.S.Eve, Rutherford.Etc., 82. 46 Phil4ag., 1902, 4, 35Z-671 dated Jun., Sep. issue. 47 TEIL9-353. 48 'o. d. 357. 49 See above. Chapter 4, Section 1, p.179-80. 50 Thomson, op.cit.. 360. 51 Ibid., 364--57-- 52 17Etherford, papp_KR, 455, pub. Jun.1902. 53 E.Rutherford (iiidS-;J.Allen), Excited Radioactivity and

Ionization of the Atmosphere, Phil.Maa., Dec.1902; Papers, 509-27, 513.

54 II:Rutherford (and F.Soddy), Phil.Mag., Sep., Nov., 1902. Paers, 472-508.

55 1903, 106, 289-93. 56 Ibid., 291. 57 I:FIT..., 292. 58 TBIJ., 292-3. 59 Ibid., 293; my emphasis. 60 Described in a Note read on 19th Nov., Proc.Chem.Soc.,

1902, 219-20; E.Rutherford (and F.Soddy77-Papers, 528. First achieved in Oct. when a liquid air plant was installed, A.S.Eve, E.Rutherford.Etc., 89.

61 Letter from E.Rutherford to J.J.Thomson, dated 26th Dec. 1902, CUL; also reporting the as yet unpublished magnetic and electric deviation of alpha rays; Thomson's article published shortly afterwards in Harpers, 22.eit., describes them still as X-rays.

62 Phil.Mag., 1903, 5, 419-28, Apr. issue. 63 L. 64 Letter from Thomson to Rutherford, dated 14th Apr.1903,

A.S.Eve, Rutherford.Etc., 94; followed by Thomson's paper 'On the existence of a radio-active gas in the Cambridge tapwater', Proc.Camb.Phil.Soc., 1903, 12, 172-4, read

Notes for Chapter 5, p.226-280) 321

64 contd.) 4th May; also Nature, 1903, 68, 90-1. 65 J.J.Thomson, Radium, Nature, 1903, 677 601-2, Apr. 66 See below. Section 3, p.276-9. 67 The Disintegration Theory of Radioactivity, Times Lit.

Suna., 1903, p.201, 26th Jun. 68 T.-J.:Thomson, Radium, Nature, 1903, 67, 602. J.Stark

(Nature, 1903, 68, 230, 9th Jul.) then claimed the prior expression in 192 of this idea (see above Chapter 4, Section 1, p.195-6). Thomson's early statement on uranium was not brought up. The latter (Conduction of Electricity through Gases, 552) had accepted the later atomic-expulsion version of the disintegration theory by Aug.1903, citing Rutherford and Soddy's general statement made in Phil.Mag., May 1903.

69 Letter f176E-S'6d4y to Rutherford, dated 31st Mar.1903,CUL. 70 Ibid. 71 W7Ti'ookes, The Emanations of Radium, Chem.N., 1903, 87,

157-8, 158; read to the Royal Society, 19th Mar.; the scintillation effect was noticed independently by Elster and Geitel, and Becquerel, and given different inter-pretations: respectively, release of electrons, Chem.N., 1903, 88, 37; and crystal fracture, CR, 1903, 137762.g-34.

72 Times,-75th Mar.1903, p.10,d. 73 MiTE's suggestion of a possible atomic transformation

was not mentioned, 74 In a Minor Key, Punch, 1903, 124, 214, 25th Mar. 75 Times, 28th Mar.TUUTT p.14,f.--- 76 30th Mar.1903, p.12,f; dated 26th Mar., from

30 Ledbury Rd., Notting Hill, near Crookes' address; referred to by Soddy, see above.

77 Phil.M2c., 1893, 35, 389-92. 78 YETU., 392. 79 TN5otator'. Times, 13th Apr.1903, p.6,d. 80 W.Crookes, The Mystery of Radium, Times, 14th Apr.1903,

P,5,a- 81 Id., 7th Apr..1903, p.10,b: Chem.N, 1903, 87, 184. 82 TTCrookes and J. Dewar, Chem.N., 1903, 88,-75-6, read

28th May. These two chemists also collaborated e.g. in examining the 'London Water Supply', ibid, 40.

83 W.Crookes, Certain properties of the emanations of radium, Chem.N., 1903, 87, 241, 22nd May.

84 Crookes and Dewar, On tF6 effect of extreme cold etc., loo.olt.

85 UEWm7,77, 1903, 87, 277-81, 12th Jun.; delivered to Congress of Applied Chemistry in Berlin on 5th Jun.

86 Ibid., 279. 87 W.Crookes, The Stratifications of Hydrogen, 410-3; see

the previous Section, p.220. 88 Modern Views, 2E.eit., 280. 89 Ibid., 278. 90 7057., 281. 91 The Position of Radium in the Periodic Table as indicated

by its Spectrum, Chem.N., 1903, 87, 145-6; 2102.Z., 1903, 4, 285-7.

92 W.Crookes, Modern Views, loc.cit., 278. 93 0.Lodge, Pop.Sci.Monthly, 1903758, 289-303; delivered

12th Jun. at baroxd. 94 Ibid., 294-5.

Nbtos for Chapter 5, p.226-280) 322

95 Lodge made this second point most plainly in a Note to Nature, 1903, 68, 128-9, 11th Jun.

96 E.RIITE6Word, Tapers, 596-608. 97 0.Lodge, Modern Views etc., loo.cit., 299. 98 0.Lodge, On Electron°, ElectFIFian, 1903, 51, 286. 99 Lodge, Modern Views etc77-7778766tion 2 ViiTow contains

aocounts of the related views of Lodge and Thomson on the mechanism of radioactivity during 1903-4.

100 M.W.Travers, Life of Sir William Ramsay, ch.8, 133-54. 101 Ibid., ch.7, 100f., 170T. 102 77Titimsay and P. Soddy, Nature, 1903, 68, 246, 16th Jul.

First observed 8th Jul., Travers, RE.cit., 212-5. 103 Ramsay and Soddy, Chem.N., 1903, 100-1; communicated

to Royal Society, OTE Jul. 104 Letter from P.Curie to J.Dewar, dated 22nd Jul.1903, RI. 105 P.Curie, J.de Chimie Physique, 1903, 1, 409f.; Oeuvres,

456-90, 4189. 106 Production of helium from salts of radium (not from

its emanation) confirmed by about Nov.1903 by Curie and Dewar, CR, 1904, 138, 190f., Jan.; P.Curie, Oeuvres, 491=3.

107 P,Curie, Recherches rocentes, Oeuvres, 471-2. 108 Ibid., 489. 109 TFIJ., 463. 110 137.-Caric (and J.Danne), CR, 1904, 138, 683f., 14th Mar.;

Oeuvres, 494-7. 111 -(75iii57.Med after mid-Sep.1903, pub.1904, in Oeuvres. 112 M.Curie, ibid., Oeuvres, 219-21. 113 Ibid., 239. 114 75171. r 238; the adjective refers to 'hypothbse'. 115 See above Chapter 2, Section 3, p.98; M.Curie,

Oeuvres, 334-5. 116 Chem.N., 1903, 88, 40; at Bradford on 16th Jul. 117 TEa.V., 1903, 8d, 39-40; from Times, 20th Jul. 118 Proc.Roy,Soc., 1903, 72, 196-9, received 17th Jul. 119 Letter from Soddy to Rutherford, dated 7th Aug.1903,

CUL, concerning comments by a referee on E.Baly's paper on xenon.

120 Sir W. and Lady Huggins, 22.cit., Proc.RF.Soc., 1903, 72, 198-9, addition received-5Th Aug.; discussion continues in 'Further Observations etc.', ibid., 409-13, Oct.

121 Letter from Soddy to Rutherford, dated 28th Aug.1903, CUL; the cutting is now lost.

122 Punch, 1903, 125, 139, 26th Aug. 123 17th Jul.1903, 225-6. 124 Punch, 1903, 125, 133, 26th Aug. 125 ?(he U.S.A. see L.Badaeh, Dias., 174-82, ch.'Popular-

isation for the Public, 1900-1703'. 126 Lotter from P.Curie to W.Ramsay, dated 14th Feb.1904;

Ramsay, Letters and Papers, 13, p.85a, UC. 127 Letter from F. Soddy to E.Rutherford, dated 12th Dec.

1903, CUL. 128 Letter from Rutherford to his mother, dated 10th Aug.

1904; A.S.Eve, Rutherford.Etc., 118.

Notes for Chapter 5, p.226-280) 323

129 Soddy, loc.cit., 12th Dec., thought Ramsay might claim the entire TEWory as his own; J.J.Thomson wrote in similar vein to Rutherford about Ramsay (letter dated 4th Feb,1904, CUL). Further unpleasant priority disputes developed around mid-1903 involving e.g. Becquerel; Lodge thought he should be 'rapped over the knuckles for it' (letter to Rutherford, dated 11th Dec.1903, CUL). P.Curie's comments to Dewar about Rutherford and Dorn have been noted above; so too has Marie Curie's claim. A difference also arose, to be quickly settled, between Rutherford and Soddy concerning the publication of books.

130 P.Soddy, loc.cit., letter of 12th Dec.1903. 131 Letter dated 7fq Apr.1903, CUL. 132 Electrician, 1903, 51, 210-11, 22nd May; contains the

incorrect statement that excited activity could be produced directly by the rays.

133 Ibid., 314, 12th Jun. issue. 134 "TETE1-J.D.Everett, Analogue to the Action of Radium,

Nature, 1903, 67, 535-6, 9th Apr. 135 MTUUTHerford, Radioactive Processes, Proc.Physical

112121y, 1903, 18, 595-7, abstract and discussion; id., Pa ers, 614-7.

136 Letter from F.Soddy to E.Rutherford, dated 7th Aug. 1903, CUL.

137 J.V.Eyre, Henry Edward Armstrong, London, 1958, 125. See also W. rock, 4.1.E,A7EiTiron and the Teaching of Science, 1880-1930. Cam r dge, 173.

138 Letter from Soddy to Rutherford, dated 28th Aug.1903,CUL, 139 H.E.Armetrong and T.M.Lowry, Chem.N., 1903, 88, 89-91,

21st Aug.; read to Royal SociWiT IBth Jun. 140 ibid., 91. 141 TTOddy, loc.cit., letter of 28th Aug. 142 W.Crookes, Modern Views etc., Chem.N., 1903, 87, 281;

12th Jun. issue. 143 W.Brock, Lockyer and the Chemists etc., 93, 95. 144 H.E.Armstrong, Presidential Address, Rep.Brit.Ass.,

1885, 945-64, 961; id., Osmotic Pressure and Ionic Dissociation, Nature 1896, 55, 78-9. For his alter-native IresiduTIEYrinityl view of valency see C,A.Russell, Histo.,

s of Valenc , 205-13.

145 E.g. Report on p ys c a he rit.Ass., Nature, 1900, 62, 564.

146 Proc.E2y.Soc., 1902, 70, 99-109, 102. 147 H.E.Armstrong, Chem.N, 1902, 85, 86-8, 103-6, p.86. 148 In Chem.N.; mentioned by W.H.Brock, H.E.Armstrong etc.,36. 149 This marbe related to his adoption in 1903 of the view

that weak radioactivity might in fact be due to 'a minute amount of chemical change of an ordinary character ... a sort of Russell effect', H.E.Armstrong, The Assumed Radio-activity of Ordinary Materials, Nature, 1903, 67, 414, 5th Mar.

150 Summarised in Electrician, 1903, 51, 880; not contained in Rutherford, 2222E2,

151 L.Badash, Sci.Amer., 1966, 215, 88-95, 93; no sources given. There are valuable reports of the meeting in Electrician, 1903, 51, 880-1, 892-3.

Notes for Chapter 5, p.226-280) 324

152 In 1906 Armstrong again expressed sceptical views; see F.Soddy, The recent controversy on radium, Nature, 1906, 74, 516-8, 516,

153 Letterrom F.Soddy to E.Rutherford, dated 12th Dec. 1903, GUL.

154 Electrician, 1903, 51, 800, 4th Sep.; refers to letter in The Times.

155 L.Koenigeberger, Hermann von Helmholtz, (1906), London, 1965, 438.

156 F.Richarz and R.Schenck, Sitzber.Akad.Wiss.,Berlin, 1903, 1102-6; R.Schenck, ibid., 17547 77F7; mentioned by Rutherford, Radio--activity, 1905, 441.

157 But for Kelvin's views see Section 2 below, p.266. 158 Electrician, 1903, 51, 800. 159 11765ITEletEtrioian, 1903, 51, 835, 11th Sep. 160 W.Ackroyd, Experiments and-6bservations with Radium

Compounds, Chem.N., 1903, 88, 205-6, read at Brit.Ass. Chemistry (TT-Se-6-tion, Sep.1903; id., The Source of the Energy of Radium Compounds, Nature, 1904, 69, 295, 28th Jan.; id., On the Bearing of the Colour Phenomena presented by Radium Compounds, Chem.N., 1904, 90, 157, read at Brit.Ass. Chemistry (B) Section, Sep.1754. See also, C.Winkler, Radio-activity and Matter, Chem.N., 1904, 89, 289-91, who advocated a magnetic analogy for the energy source and accepted radioactive induction; he appears as a standard 'unconvinced chemist', see p.251 above.

161 M.Berthelot, CR, 1901, 133, 973-6; id., Essais etc., ibid., 659-64:-

162 M., CR, 1904, 138, 1553-5; stresses the effects of traoes of vapours of chemical substances.

163 See e.g. F.Giesel, Emanium, Chem.N., 1904, 90, 259-60, who exhibits some confusion; and W.Marckwald, Heber das Radiotellur, Ber.deut.chem.Ges., 1905, 38, 591-4, who does not.

164 Debierne, to radium et la radio-activity, Rev.Gon.des Sol., 1904, 15, 11-22, 60-71, 69-71, adoptiThle com-promise (like M.Curie) of Ra as a catalyst for atomic transformations; then, CR, 1905, 141, 383-5, the atomic disintegration theory.

165 E.Rutherford, Magnetic and Electric Deviation etc., Pa ere, 557=

166 E,Ru herford (and F.Soddy), Thorium II, Papers, 456, read May 1902.

167 Mentioned e.g. by E.Rutherford and A.G.Grier, Deviable Rays of Radioactive Substances, ibid., 457, dated 7th May 1902.

168 E.Rutherford, Magnetic and Electric Deviation etc., Pa ers, 557; Phil.Mag., Feb.1903, dated Nov.1902.

169 o . 170 1711772.22.p7og.Chem., 1904, 1, 30-54, 30-2. 171 Tic.ilgns., London, 1904; pref: dated Oct.1902. 172 See 'Professor Mendereeff on Argon', Nature, 1895, 51,

543, for his initial reaction. 173 Chemical Conception of the Ether, 44-51. 174 Ibid., 45. 175 Tr:F.:Brush, Chem.N., 1898, 78, 197-8; Science, 1898,

8, 485-94.

Notes for Chapter 5, p.226-280) 325

176 Chem.P., 1898, 78, 221-2. 177 Verh.ply2-Ges.B-6-ilin, 1898, 17, 135-7. 178 Opening address by President of Chemistry (B) Section

of Brit-Ass., Nature, 1903, 58, 472-81. 179 Ibid., 479. 180 Te-W-tAe preceding Section, p.246; M.Curie, Thesis, 2 ed.,

1904; id., Oeuvres, 239. 181 Notices of Books, Chem.N., 1904, 90, 326. 182 P.P.Bedson, op-cit., Anii.liep.prog:Uhem., 1904, 1, 32. 183 Chemical Conception o1 the Ethii; 6; D.MendeleeT, An

attempt to apply to the tryone of the principles of Newton's natural philosophy, RI Lib.Sci., (1889), 3, 540-59.

184 D.Mendeleef, The periodic law of the chemioal elements, J.Chem.Soc., 1889, 55, 634-56, 641-7.

185 Td77-MIWiacal Conce15Tion of the Ether, 14. 186 Tad-, 44-5, 47. 187 :e vinretained that notion at this time, Papers, (1905)

6, 223. 188 W.Ostwald, J.Chem.Soc., 1904, 85, 506-22; reprinted in

D.M.Knight, Classical Scientific Papers.Chemistry, London, 1968, 354-70.

189 Ibid., 356-7. 190 Ibid., 369. 191 -SW-e—e.g. S.Glaestone, Sourcebook on Atomic Ener .

London, 1950, 357-61, on liquid-drop models of nuclear fission in the 1930's.

192 He later appreciated the success of the nuclear atom, Ann.Rep.Prog.Chem., 1913, 10, 262-88, 271-2.

193 Sr(79 above, Chapter4, SectrOn 2, p.198-9. 194 In 19 parts in Electrician, 1903-4, 52, 7-10 etc.,

pub. Oct.1903 to Feb.190/f. 195 Ibid., 163. 196 1570-6-ddy, Radio-activity: an Elementary Treatise from

the Stand,oint of the Didategration Theory, London, 774, pref. datWU 5TE May.

197 Ibid., 55. 198 fbid., 164, 199 U7E.Vincent, Phil.Mag., 1902, 4, 103-15. 200 Soon to be moderated ( F.Soddy, Radioactivity, Ann.Rep.

pERg.Chem., 1904, 1, 244-80, 276) by one who a decade a or paced atomic disintegrations within the Periodic

Table by means of the Displacement Law. 201 F.Soddy, Radio-activtT, 178. 202 Pa ere, 73; see thiT011owing Section, p.272-4. 203 ..Soddy, Radio-activity ; 125, 176-8, citing only Lodge;

see belowTT5726I-.37f61; this physicist's suggestions. 204 Ibid., 178. 205 Ibid., 178-80. 206 fbid., 125. 207 "A-i-J,Walker on 'Time Conception of Minute Concentrations'

in 'General and Physical Chemistry', Ann.Rep.Prog.Chem., 1904, 1, 1-29, 25-6. Also C.A.Russell on 'The Oscillation Theory' in History of Valens , 254-6.

208 W.H.Bragg, On the Absorption ofc(Rays, and on the Class-ification of theotRays from Radium, Phil-Mag.1904, 8, 719-25; id. and R.Kleeman, On the Iorirgation Curves of RadiuiliT ibid., 726-38.

Notes for Chapter 5, p.226-280) 326

209 Letter from E, Soddy to W.H.Bragg in Adelaide, dated 12th Jan.1905, RI.

210 F. Soddy, Radio-aotivitz, 1904, 178. 211 See e.g. the FtiView 'by J.A.Fleming, The electronic

theory of electricity, RI Lib.Sci., (1902), 5, 551-69. 212 E.von Sohweldler, Dmt.-riit.poluTET•21-ude de la

Radiologie etc., 1905,- I, dated Jun.175. 213 E.g., L.BoliFiann, Lectures on Gas Theo , (1896, 1898). 214 E.Rutherford, Papers, (1908), 2710 , provides refer-

ences for 1905731-T5th electrical and scintillation methods were employed by him.

215 0.Lodge, On Electrons, Electrician, 1903, 51, 123-5, 125. 216 Nature, 1903, 67, 511, dated 28th Mar. 217 J,J.Thomson, Radium, Nature, 1903, 67, 601-2. 218 Ibid., 601. 219 0.Lodge, Nature, 1903, 68, 128-9, 11th Jun. 220 Reported in Electrician, 1903, 51, 417-9; see also

E.Rutherford, Pa e737616. 221 J.Larmor, Aether anti Matter, 227-32; 0.Lodge, On

Electrons, Electrician, 1903, 51, 286. 222 Lodge, On EITEIi5iiii,Toc.cit.,-125. 223 Lodge, Note on the pro-E11517occasional instability

of all matter, 128-9. 224 Ibid. 225 .I Jeans, Phil.Mag., 1901, 2, 421-55. 226 J.Heilbron,-15170., 137. 227 Phil.Mr3z., 1777 7, 445-55, paper read 5th Dec.1903

in Tokyo. 228 Ibid., 455. 229 Phil.Meo., 1904, 7, 237-65; see below, p.268-70. 230 IT5.--ft.58, Electrician, 1904, 52, 805; see also abstract,

ibid., 823. 231 laa., 805. 232 aaro-activity, 1905, 488; unchanged comment from 1904

ed.; see the following Section, p.272f. 233 Kelvin, Papers, 6, 206-9. 234 See e.g. Editorial note, 'Explanations of Radio-

activity', Electrician, 1903, 51, 892-3. 235 Perhaps referring io A.HeydweiTler, Phys.Z., 1902, 4,

81-2, and/or R.Geigel, Ueber Absorption von Gravitationsenergie lurch radioactive Substanz, Ann.d.Phys., 1903, 10, 429-35. But e.g. C.Forch, lEY:27Z., 1903, 4, 315.-9, 443-5, citing W.Kaufmann, Ann.d7Phy.z., 1903, 10, 894, had by autumn 1903 published experimen al refutations of the apparent weight-loss.

236 Kelvin, Pa ers, 6, 208. 237 Letter da e 23rd Aug.1903, RI. 238 Letter dated 22nd Aug.1903, in M.W.Travers, A Life of

Sir William Ramsay, 252. Travers here states that at a dinner which he attended in June 1903 Marie Curie attempted to 'convert' Kelvin to the disintegration theory. This however was before the announcement of the radium-helium transmutation in July. Also, a postcard from Soddy to Rutherford dated 22nd Jun.1903 (GuL) rep-orting Ramsay's visit to Marie Curie in Paris reads 'According to R., Curie thinks we are very "hardi" to put forward our hypothesis on such slight evidence. R. replied he thought there was a good deal of evidence'. Marie Curie may thus not have been a convert by that date.

Notes for Chapter 5, p.226-280) 327

239 At the Brit.Ass. 1903, Kelvin, papers, 6, 208-9. 240 Rutherford.Etc., 109; see also WI07on-differences

regarding geological-mineralogical time scales. 241 Pa err, 6, 216-22; Phil.Nas., 1904, 8, 528-34, Oct.

- scUee 242 Ibid., 216. 243 Re Vins Phil.Mag., 1902, 3, 257-83, written in 1901. 244 C.f. L.BaTimann, Lectures on Gas Theory, 3-4, 376-9,

who postulated overlapping moms to explain valency; mentioned above, Chapter 4, n.50.

245 Kelvin, Aepinus Atomized, loc.cit., 259; similar forces he conceived to act between atoms and the ether, Papers, 6, 237.

246 Ibid., 259. 247 Ibid.., 262. 248 RTIVin, Plan of a Combination etc., Papers, 6, 216. 249 Ibid. 250 TiiIVin v Pa ere, (1905), 6, 227-30. 251 Summarise by F.Soddy, THe recent controversy on radium,

Nature, 1906, 74, 516-8. Soo Rayleigh, J.J,Thomson, 141-2 for KelvIri's criticisms in 1906 of Thomson's radiation-loss disintegration theory.

252 An attempt to explain the Radioactivity of Radium, Papers, (1907), 6, 231-4.

253 kelvin, On the Motions of Ether etc., Papers, (1907), 6, 235-43, 235-6,

254 54R. Nagaoka, Kinetics of a system etc., 445. 255 Id., 454. 256 G.A.Schott, A dynamical System etc., Nature, 1904, 69,

437, from University College of Wales. J.Heilbron, Dies., 142-6, discusses the arguments of Schott and Nagaoka; those related largely to the above points but not to radioactivity,

257 H. Nagaoka, Kinetics of a system etc., 454. 258 Ibid., 454-5. 259 i.e. ultra-violet photoelectric action, the emission

of electrons from an irradiated metal. 260 Phil.Mag., 1903, 6, 673-93. 261 Y.hia., 673. 262 Rayleigh, J.j,Thomson, 140, letter dated 11th Apr.1904. 263 J.j.Thomson, Magri 611c Properties etc., 678-81. 264 E.g. J.Larmor, On the Theory of the Magnetic Influence

on Spectra; and on the Radiation from moving Ions, Phil.Mag., 1897, 44, 503-12, 512, had done so despite losses of the order 10-6.

265 Thomson, Magnetic Properties etc., 682-5. 266 Ibid., 689. 267 TEIT.Mag., 1904, 7, 237-65, Mar. issue. 268 Ibid.7-255. 269 MU., 265. 270 T517. 271 Letter from Thomson to Rutherford, dated 18th Feb.1904,

CUL, describing the above ideas shortly before their publication.

272 W.Kaufmann's results of 1901-2 were reported e.g. by E.Rutherford, Radio-activity, 1905, 127; 1904, section 76; by7„,7Trtrutt, The Becquerel Rays and the Praperties of Radium, London, 1904, 69; and by

Notes for Chapter 5, p.226-280) 328

272 contd) Thomson himself, Conduction of Electricity throuEh Gases, 1903, 532-5.

273 17.757he0Fia-T,Radio-octivit 1904, 325, section 200. 274 Ibid„ 333, se-aion 'u3. 275 Ibid., 326, section 200. 276 YETU., 305, section 194. The point was noted by Soddy

and Rutherford (Papers, 564, 599) without great emphasis early in 1903, and had possibly been anticipated by mid-1902 (121=2, 508).

277 Phil.Trans.A., 1904, 204, 169-219, ms. received 20th Aug.;-Pa-b-rs, 671-722.

278 ibid., 1'. 279 ibid. 280 TM., 712-3. 281 E.Rutherford, Radio-activity, 1905, 450. 282 Ibid., 456, 283 The apparent effect of temperature on the decay of

RaC was the only exception; E.Rutherford, Bakerian Lecture, Papers, 713; Radio-activity, 1905, 390-1.

284 Ibid., 1903, 487-8; 1904, section 206. 285 2. Rutherford, Radioactive Transformations, (London,

1906), repr. Yale U.P., 1919; lectures delivered Mar.1905. pref. dated Jun.1906.

286 Ibid., 267. 287 17697., 268; there is no reference to Soddy. See

Rutherford, Radio-activit , 1905, 446, for his own previous discussion regar ing the 'average life' of 'metabolons'.

288 E.Rutherford, Radioactive Transformations, 268. 289 Rutherford considered statistical aspects of radio-

activity by 1908 (Ta.p_sE2, 2, 58, 69, 94, 106-8). However, in 1909 he reported results which made it seem 'probable that the atoms of emanation undergo a progressive change in properties before disintegration' (ibid., 168-9); some months later Thomson defended the theory of continuous atomic change by attributing a suitable distribution to the atoms themselves; these he supposed were of differing intrinsic strength when first formed (Rayleigh, J.J.Thomson, 142). But in 1910 Rutherford rejected the results on progressive change in the emanation (Pa ens, 2, 214-20) and intensified his statistical stud. es; e.g. the number of alphas he detected per minute fluctuated wildly and randomly between zero and twenty, in accord with probability laws. But regarding the cause of instability and dis-integration, whether in nuclear or electronic (1912; ibid., 286-7) or quantum terms (1927: Papers, 3, 178-9, 183), the physicist still admitted ignorance.

290 E.Rutherford, Phil.Mag., 1907, 13, 110-17, dated 1st Nov.1906; Papers, 910-16,91-6.

291 Ibid., 916. 292 E.Rutherford, Radioactive Transformations, 276. 293 Radioactive Change, Papers, 596-608. 294 Ibid., 608, See Chapter 4, Section 1, p.195, for

77.7fark's similar suggestion in mid-1902. 295 1903, 68, 222, 9th Jul.

Notes for Chapter 5, p.226-280) 329

296 G.H.Darviin, ibid., 496. 297 J.Joly, ibid, 7526. Rutherford, Pa ere, (1907), 926,

later chid to have made calculations in 1902, un-published, on the gao-thermal effects of active mineraln.

298 The problem that radioactive rays from the sun should be detectable on earth was raised by W.B.Hardy, 'Radium and the Cosmical Time Scale', ibid., 548, and disposed of by R.J.Strutt, 'Radium and the Sun's Heat', ibid., 572, 15th Oct. The lack of radium lines in the TaTir spectrum was partly eased by the abundance of helium, see e.g. E.Rutherford, Radio-activity, 1905, 492; 1904, section 207.

299 Radio-activit , 1905, 491-6; 1904, section 205. 300 !bid., 1 5, 459-66, and references there cited. 301 17,757therford, Pa era, (1907), 917-31, 930; earlier

discussions, cting Ramsay and Soddy, appear in Rutherford, Radio-activit , 1905, 485-6, 554-8; also • Pa ere, 774-5, bop. n 1907 Rutherford left McGill to - ace A.Schuster's Chair at Manchester.

302 A.Sohuster, Rep.Brit.Aps., 1903, 538. 303 E.g. G.Le Bon, P,de Heen, G.Martin, D.Mendoleef,

H. Wilde. During 1903-4 some connection was also seen, e.g. by Jean Becquerel, between radioactivity and the N-rays, which Blondlot imagined to be emitted by various materials.

304 P.Curie, Oeuvres, 378; Congr6s, 1900. 305 Radio-active Substances, Chem.N., 1903, 88, 99. 306 R.J.Strutt, Nature, 1903,-7,769-70, 19T Feb. See

also J.J.Thomson, ibid., 391, who cited McLennan and Burton; and E.Ruther5ird, ibid., 511-2, 2nd Apr. citing Rutherford and H.L.Cooke.

307 Noted by Rutherford, ibid., also in Radio-activity, 1904, section 220.

308 E.Rutherford, Radio-activity, 1905, 487; 1904, section 206.

309 J.J.Thomson, Proc.Camb.Phil.Soc. 7 1904, 12, 391-7. 310 Ibid., 397. ---- 311 TIETI.MaE., Mar.1904 issue; see the preceding Section,

p.270. 312 N.R.Campbell, Phil.Mafc., 1905, 9, 531-44; id., 545-9;

id., 1906, 11, 202-2 . See also J.J.Thomson, On the emission of negative corpuscles by the alkali metals, Phil.Ma., 1905, 10, 584-90.

313 E.Rutherford, Papers, 708, 775; Radio-activity, 1904, section 220.

314 Rutherford, Radio-activity, 1905, 539-42, section 286. 315 Rutherford, Radioactive Transformations, 217-8. 316 Summarised bYTTETIT6Word, Radio-activitq., 1905, 552-3. 317 See e.g. J.C.McLennan, On the Radio-activity of

Potassium and other Alkali Metals, Nature, 1908, 78, 29-30; N.R.Campbell's defence, ibid., 55; then E.Rutherford, Radioactive Substances and their Radia-tions, Cambridge, 1913, 58U1:9, 596.

Notes for Chapter 5, p.226-280) 330

318 E.Rutherford, Radio-aotivkIE, 1905, 483; 1904, section 201.

319 Ibid. 320 L. Rutherford, Baker-Lan Lecture, Pa ers, (1904), 712. 321 W.H.Bragg, Phil,Mag., 1904, 8, 7 -25,, 719-21, cited

Rutherford's Bakerian 322 Letter from Bragg to Rutherford, dated 18th Dec.1904,CUL. 323 E.Rutherford, Radio-activity, 1905, 484, this edn. only. 324 Ibid., my emphases. 325 lbid., 1905, 77; again, my stress. 326 Men- ioned obliquely, ibid., 78. 327 J.J.Thomson, Phi1.n.A777906, 11, 769-81; discussed

e.g. by 0.Lodge, Electrons etc., 1906, 146-51, 162, 192-4,

328 J.J.Thomson, Cathode Rays, 1897, 312; he estimated its rate of production at one three-millionth gm. per year.

329 Thomson, RI Lib.Sci„ (1907), 6, 232-47. 330 The general notion projected into the future. Rutherford

in 1913, Radioactive Substance° etc., 621, speculated that atomic nucleT-Fonsist in pa of H and He atoms.

331 0.Lodge, Radio-Activity, Electrician, 1904, 53, 216-8.

331

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Academie des Sciences, Institut de France, Paris

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364 ABBREVIATIONS

Abh.der Naturf.Ges.zu Halle Abhaiialungen der naturforschenden Gesellschaft zu Halle.

Amer.J.Phys. American Journal of Physics.

Ann.d.Phys. Annalen der Physik (und Chemie), Leipzig. Also known as Wied.Ann., Wiedemann's Annalen. Beibl.= BeiblUtter.

Ann.1122.212E.Chem. Annual Reports on the Progress of Chemistry, Chemical Society of London.

Ann. Sci. Annals of Science.

Arch.Hist.Exact Sci. AFFave for History of Exact Sciences.

Ber.deut.chem.Ges. —Fgrichte der deutschen chemischen Gesellschaft, Berlin.

BN Bibliotheque Nationale, Paris.

Brit.J.Hist.Sci. British Journal for the History of Science.

Bull.Amer.Phys.Soc. Bulletin of the American Physical Society.

Chem. N. The Chemical News.

CR Comptes Rendus Hebdomadaires des Seances de l'Academie des Sciences, Paris.

CUL Cambridge University Library.

Edin.Roy.Soc.Proc. Proceedings of the Royal Society of Edinburgh.

Hist.Stud.Phys.Sci. Tiritorical Studies in the Physical Sciences.

J.Amer.Chem.Soc. Journal of the American Chemical Society.

365

J.Chem.Soc. Journal/Transactions of the Chemical Society of London.

J.Franklin Inst. Journal of the Franklin Institute.

Jahresb.d.Ver.f.Wiss.,Braunschweig riE7isbericht des Vereins fur Naturwissenschaft zu Braunschweig.

Jap.Stud.Hist.Sci. Japanese Studies in the History of Science.

Manchester Memoirs Memoirs of the Manchester Literary and Philosophical Society.

Nature Nature, London.

Phil.Mag. London, Edinburgh and Dublin Philosophical Magazine, and Journal of Science.

Phil.Trans.A.

Phys.Z.

Pop.Sci.Monthly

Philosophical Transactions of the London Royal Society, series A.

Physikalische Zeitschrift, Leipzig.

Popular Science Monthly, New York.

Proc.Camb.Phil.Soc. Proceedings of the Cambridge Philosophical Society.

Proc.Chem.Soc. Proceedings of the London.

Chemical Society of

Proc.RI

Proceedings of the Great Britain.

Royal Institution of

Proc.Roy.Soc. Proceedings of the London Royal Society.

Rapports,Cong.Int.de Physique Rapports presenters au Congres international de Physique, Paris.

Rep.Brit.Ass. Report of the British Association for the Advancement of Science.

366

Rep. Smithsonian In3t. Annual Report of the Smithsonian Institution.

Rev.Gen.des Sci. Revue Generale des Sciences Pures et Appliquees.

Rev. Sc.

RI

RI Lib.Sci.

Revue Scientifique, also Revue Rose.

Royal Institution of Great Britain.

The Royal Institution Library of Science. Physical Sciences, 10 vols., ed. W.L.Bragg and G.Porter, London, 1970. Mainly reprinted from Proceedings of the Royal Institution, 1851-1939.

Sci.Amer. Scientific American.

Seances Soc.Fr.P s. ociete Franpaise de Physique. Seances,

1873-1901.

Sitzber.Akad.Wiss.,Berlin Sitzungsberichte der preussischen Akademie der Wissenschaften, Berlin.

Trans.Camb.Phil.Soc. Transactions of the Cambridge Philosophical Society.

Trans.Roy.Soc.Canada Transactions of the Royal Society of Canada.

UC University College, London.

Verh.Deut.Ges.Natf. Verhandlungen der Gesellschaft Deutscher Naturforscher and Aertze, Leipzig.

Verh.Deut.Phys.Ges. Verhandlungen der Deutschen Physikalischen Gesellschaft, 1899f.

Ver.Phys.Ges.Berlin Verhandlungen der Physikalischen Gesellschaft in Berlin, 1882-98.

Zeit.f.Elektrochemie Zeitschrift fur Elektrochemie, Halle.