230 U. S. BUREAU OF CHEMISTRY NOTES. [J. F. I.

process for the manufacture of phthalic anhydride has been studied. Plant investigations on the chlorination of toluene and the manufacture of various alcohols and acetone are now in progress.

As a result of the work done by the Color Laboratory, twelve patents have been dedicated to the people, and applications for a number of new patents are pending.

CERTAIN RELATIONS BETWEEN CRYSTALLINE FORM, CHEMICAL CONSTITUTION AND OPTICAL PROP-

ERTIES IN ORGANIC COMPOUNDS.’

By Edgar T. Wherry.

IN previous work on the refractivity of crystalline substances the mean refractive index has usually been employed. It seemed, however, desirable to inquire into the possibility of correlating the several indices of a given substance with its crystal structure, instead of concealing in mean indices whatever relations may exist. The Lorentz-Lorenz expression, :32 - “G , is most satisfactory, and for short the left-hand term is referred to as the “ refraction ” and denoted by the letter R. If the refractive indices in different directions in a single anisotropic substance be substituted for n, directional values of R, called R,, Rb, and Ro, will be obtained, the first two being identical in uniaxial crystals. The atomic weight (W) being always, and the refractivity (M) usually, the same throughout the several values of R must be proportional to those of the density ( p ), or factors correspond- ing to it, in the several directions. The ratio of the R’s, which may be termed the “ refraction ratio,” should therefore give some information as to the structure of the substance. The phenome- non of dispersion of double refraction renders the refraction ratio variable with the wave-length of the light, but calculation in some typical cases shows that the variation rarely exceeds 3 units in the third decimal place over the visible spectrum, so this may be ignored.

The spacings between the planes passing through the centres

’ Published in Journal Washington Academy Sciences, Vol. 8, pp. 277-c%& and 31g-327, May 4 and IQ, 1918.

Feb., IgIg.1 U. S. BUREAU OF CHEMISTRY KOTES. 231

of gravity of the atoms in crystals are connected with the crys- tallographic axial ratios. The less the spacing of the planes in any direction in an anisotropic substance, the greater should be the refraction in that direction. If the layers are all close-packed, the factor JI may be inversely proportional to the spacing d, and there should be an exact inverse relation between refraction ratio and crystallographic axial ratio, at least in cases where there are no serious disturbing factors. The work reported is limited to the tetragonal system. The crystallographic axial ratio usually adopted is not necessarily the one related to the optical properties, the alternate ratio or even the mean between the usual and the alternate being of significance in some cases.

In urea, CO( NH,),, the axial ratio, c/a, is 0.833, and the refraction ratio, R, R, is 0.834, the inverse proportionality being exact. A possible space-lattice for the substance, worked out on this basis, is found to agree with a variety of data, in particular with the atomic diameters of the elements present. In penta- erythrite a similar relation exists, but the composition is so com- plicated that only a suggestion of the structure can be given. In mellite the facts are similar, provided that the alternate axial ratio is taken. The double propionate group, of the metals Ca, Sr, Ba, and Pb, also shows interesting relationships of the same type. Several exceptions are noted, in which no proportionality between the two kinds of ratios appears to exist, but it can be shown that there are disturbing factors, such as intense colora- tion and asymmetric carbon atoms, present, so that the general rule of inverse proportionality may be regarded as generally applicable. Preliminary results with crystals of other systems are announced.

AMERICAN SUMAC: A VALUABLE TANNING MATERIAL AND DYESTUFF.

By F. P. Veitch and J. S. Rogers.

[ABSTRACT.]

SUMAC grows wild and abundantly on uncultivated lands in certain areas of North America, in quantities more than sufficient to tan all the light leather made in this country, if need be. At

* U. S. Department of Agriculture Bulletin 706.