192 G. F. BECKER ISOSTASY AND RADIOACTIVITY 



this origin is likely to be at least twofold. As Mr. Joly points out/'* 

 normal lead, with an atomic weight of 207, might be regarded as a mix- 

 ture of uranium-derived lead, with an atomic weight of 206, and of 

 thorium-derived lead, with an atomic weight of 208. Messrs. T. W. 

 Richards and M. E. Lambert have made comparative atomic weight 

 determinations of lead from five radioactive deposits and of common 

 lead, getting from the lead of a North Carolina uraninite deposit 20G.40^^ 

 and for common lead 207.15. As they remark, "The result is amazing." 

 The various samples were treated exactly alike; protracted purification 

 had no effect on the atomic weight of any sample and no difference could 

 be detected in the spectra.®" 



Isotopism seems to be established in principle. So far as I can see, 

 it precludes age determinations in minerals, even if the radioactive con- 

 stant A has undergone no change since a given mineral crystallized. 

 Isotopism also sufficiently explains the very wide differences in the ages 

 indicated for different crystals. 



Ever since the earliest attempts to determine the age of minerals by 

 radiological means it has been observed that the agreement between dif- 

 ferent specimens of approximately equal geological age, or even between 

 specimens from the same deposit, was extremely poor. In the case of 

 helium this was explicable by the escape of a portion of the gas. Changes 

 in the lead-uranium ratio were less easily explained, since a variety of 

 lead compounds and uranium compounds are very insoluble. I pointed 

 out®^ that Boltwood's method gave ages for the minerals of Barringer 

 Hill varying from 1,671,000,000 years to 11,470,000,000 years, and more 

 recently Mr. F. Zamlionini'''^ has shown that the lead-uranium ratios, of 

 minerals from the neighborhood of Christiania indicate ages varying 

 between 41,000,000 years and 17,302,000,000 years. Because Brogger's 

 observations showed that galena in many cases crystallizes during "the 

 principal phase of pneumatolitic minerals," I inferred that lead minerals 

 were occluded in the uraninite. 



Similar variations occur in the helium-uranium ratio and possibly also 

 for a similar reason. Helium received its name because it is a promi- 

 nent constituent of the sun, and it is difficult to suppose that it was not 

 also one of the original constituents of the earth. Mr. Strutt found that 



^Science Progress, July. 1914, p. 52. 



6»Tlie 1914 atomic weights are U = 238.5. He = 3.99 ; so that U — 8He = 206.6, cor- 

 responding almost precisely to Richards's 206.4. 



*>Jour. Amer. Chem. Soc, vol. 36, 1914, p. 87. It is well to note that similar investi- 

 gations have been made in the Harvard Laboratory ou other elements — notably copper, 

 silver, iron, sodium, and chlorine — which gave constant atomic weights, irrespective of 

 their source. One of the irons was from a meteorite. 



«Bull. Geol. Soc. Am., vol. 19, 1908, p. 134. 



•2AttI. Accad. LIncei, vol. 20, part 2, 1911, p. 131. 



