74 CURRENT TOPICS. [J. F. I.

I'Iow Old the Earth.-- The Industrial Bulletin of A r t h u r D.Little Inc., No. 181, s ta tes that a planetarium show depicting thevarious ways that the earth might come to its end makes it seemthat the earth cannot long continue to survive the logically andvividly presented hazards. Yet, when a geologist talks in t e rms ofthe millions of years of history already experienced by the earth,the risk of extinction in the next hundred-, thousand- or even tenmillion-year period seems small. Radioactive elements decomposespontaneously at a fixed rate, dependent upon the nature of theparticular element to produce substances that can be measuredwith accuracy. Uranium and thorium both decompose withextreme slowness, bu t at a known rate, into the stable substanceslead and helium. Determinations made today of the relativeproportions of the lead and uranium in a mineral yield, by calcu-lation, the number of years required for uranium to produce theobserved lead, provided there is no other source of lead. In o therminerals, it is convenient and accurate to release and measure thehelium gas imprisoned within and compare that with the contentof uranium or other radioactive substances that give rise to it.Measurements by careful investigators indicate that the oldestrocks to be found on earth are of the order of 1,8OO,OOO,OOO years,and the oldest in meteorites, which strike the earth from outside,some 2,8oo,ooo,ooo years. These ages date not from the origin ofthese bodies but from the crystallization of the minerals that formthe rocks. Previously, the mass from which the minerals crystal-lized must have passed through a long period in a molten condition.Other possibilities exist besides the "radioactive time-clock" forgoing back into the reaches of time. The moon-made tides on theearth sap the energy, causing a slowing-down of the earth and aretreat of the moon from the earth. Astronomers calculate, fromthe present distance of the moon and its rate of slowing, that itmust have revolved around the earth for about 4,ooo,ooo,c~o years.I t is considered conservative to think of the earth as perhaps threeor four billion years old and of the sun as even more venerable.

R. H.O.