﻿Lead and the End Product of Thorium. 679 



include determinations of bismuth, but the latter is clearly 

 independent of the thorium present in each case. More 

 recently Soddy* has examined Ceylon thorite for bismuth 

 and has failed to find an appreciable amount. The evidence 

 is thus fatal to the view that an isotope of bismuth might 

 be the end product of the thorium series. 



§ 11. Further Evidence from Atomic Weight 

 Estimations. 



An initial objection to the view that the end product of 

 the uranium family is lead, was that the theoretical atomic 

 weight of the end product radium G did not agree with the 

 atomic weight of lead (207*1) as closely as could be desired. 

 The recent discovery of isotopic series of elements has de- 

 monstrated that chemical identity does uot necessarily imply 

 equivalence of atomic weight, and the objection has now 

 lost its original force. Radium G ought to have an atomic 

 weight of 206*2, calculated from the atomic weight of 

 uranium (238*2) f, or 206*0 if calculated from the atomic 

 weight of radium (226*0) *. The difference between these 

 two values is very small when we consider the intrinsic 

 difficulties of the atomic weight determinations; and the 

 results bear excellent testimony to the careful and exact 

 atomic weight determinations of Honigschmid. Fortunately, 

 a number of determinations of the atomic weight of lead 

 extracted from radioactive minerals is already to hand, and 

 the results are very gratifying. Thanks to the energy with 

 which investigators are attacking the problem, the amount 

 of evidence of this nature will soon be considerable, so that 

 further conclusions with regard to the lead disintegration 

 products and their stability will be possible. We may now 

 take the theoretical value for the atomic weight of radium G 

 to be 206*2 without appreciable error. Quite recently 

 Honigschmid and Fraulein St. Horovitz§, Richards ami 

 Lembert||, and Maurice CurieU, have published the results 

 of their experiments in this connexion. Honigschmid and 

 Frln. St. Horovitz determined the atomic weight of lead 



* Nature, vol. xciv. Feb. 4th, 1915, p. 615. 



t Honigschmid, Wien. Anz. 22nd January, 1914. 



X Honigschmid, Wien. Sitzunqsler. cxx. p. 1617 (1911) ; cxxi. p. 1973 

 (1912); cxxi. p. 2119 (1912). 



§ Honigschmid and St. Horovitz, Wien. Anz. 12th Juno, 1914 ; 'Ait. 

 fur Eleldroch. xx. p. 319 (1914) ; C. R. clviii. p. 1797 (1914). 



|| Richards and Lembert, Journ. Am. Chem. Soc. vol. xxxvi. 7. 

 p. 1329 (1914) ; Zcit.filr Anon/. Chem. lxxxviii. p. 429 (1914). See also 

 Fajans, Jleid. Ahul. Ber. A. Abh. xi. (1914). 



<fl Maurice Curie, Comptes Rendus, clviii. p. 1676 (1914). 



