ATOMIC WEIGHTS — ASTON". 



187 



chain, the lead end product must be 6 alpha particles lighter than 

 thorium (232.15), and so should have an atomic weight about 208. 



Now, ordinary lead from nonradioactive sources has an atomic 

 weight 207.20, so Soddy 10 suggested in 1913 that the lead derived 

 from minerals containing uranium but no thorium might have a 

 smaller atomic weight than ordinary lead, and, on the other hand, 

 the atomic weight of lead from minerals containing thorium but no 

 uranium might be greater. 



The first experiments were made by Soddy and Hyman " with a 

 very small quantity of lead from Ceylon thorite. This gave a per- 

 ceptibly higher atomic weight than ordinary lead. Later a large 

 quantity of the same mineral was available. The lead from this when 

 carefully purified gave a density of 0.26 per cent higher than that 

 of common lead. On the assumption that the atomic volumes of iso- 

 topes are equal, this figure corresponds to an atomic weight of 207.74. 

 A chemical atomic weight determination gave 207.694. A sample of 

 the same lead was sent to Vienna, where Professor Honigschmid, a 

 well-known expert in such matters, obtained from it a value 207.77 

 as a mean of eight determinations. These figures not only showed 

 that thorium lead had a higher atomic weight than ordinary lead 

 but also that their atomic volumes were identical, as expected from 

 theory. 12 



At the same time as this work was in progress the leading Ameri- 

 can authority on atomic weights, T. W. Richards, 13 of Harvard, 

 started a series of investigations on lead derived from various radio- 

 active minerals. The samples of lead from uranium minerals all 

 gave results lower than ordinary lead, as was expected, and one par- 

 ticularly pure specimen of uranio-lead from Norwegian cleveite gave 

 206.08, 14 a very striking agreement with theory. The following table 

 of properties is taken from his presidential address to the American 

 Association at Baltimore, December, 1918 : 



Atomic weight 



Density 



Atomic volume 



Melting point (absolute).. 



Solubility (of nitrate) 



Refractve index (nitrate) . 



Thermoelectric effect 



Spectrum wave-length 



Common 

 lead. 



207. 19 

 11.337 

 18. 277 



600.53 

 37. 2S1 

 1. 7815 



Mixture 



A ustra- 



lian. 



B 



206. 34 

 11. 280 

 18. 278 



600.59 

 37. 130 

 1. 7814 



Uranio- 

 lead. 



Percentage 

 difference. 



206. OS 

 11. 273 



18. 281 



A-B 



0.42 

 .42 

 .01 

 .01 

 .41 

 .01 

 .00 

 .00 



A-C 



0.54 

 .56 

 .02 



.00 



10 Ann. Rep. Chem. Soc, 269, 1913. 



11 Soddy and Hyman : Trans. Chem. Soc, 105, 1402, 1914. 



12 Soddy : Roy. Ins., May 18, 1917. 



13 Richards and Lembert : J. Amer. Chem. Soc, 36, 1329, 1914. 



14 Richards and Wadsworth : J. Amer. Chem. Soc, 38, 2613, 1916. 



