140 CONCEPTION OF THE CHEMICAL ELEMENT 



tinguished from the apparent end in all cases, all the 

 ultimate products are isotopes of lead with atomic 

 weight between 210 and 206. The product of 

 radium- C 2 , in the branch claiming only 0-03 per 

 cent, of the whole ultimate product of radium, with 

 atomic weight 210, may be left out of account as 

 being negligible, and also the product of the actinium 

 branch for which the atomic weight is still uncertain ; 

 but the main products, namely, that of uranium with 

 atomic weight 206, and both the thorium products in 

 the two branches, with atomic weight 208, are 

 different in different directions from that of common 

 lead with atomic weight 207-2. 



The conclusion that the ultimate product of 

 thorium, as well as of uranium, was lead, was quite 

 new and opposed to the opinion of those who had 

 made a special study of the Pb/U and Pb/Th ratios 

 of radioactive minerals of various geological periods. 

 I found, however, that the atomic weight of the 

 lead separated from Ceylon thorite was 207-7, and 

 Honigschmid confirmed this with a specimen of my 

 material and obtained the figure 207-77. J ust 

 recently, from a specimen of lead separated from a 

 Norwegian thorite by Fajans and his co-workers, he 

 has found the value 207-90 (Zeitsch. Elektrochem., 

 1918, 24, 163). Whereas the same investigator, and 

 also T. W. Richards and others, have found values 

 for the atomic weight of lead separated from uranium 

 minerals all lower than that of common lead, and in 

 two cases from carefully selected minerals between 

 206-0 and 206-1. I found my thorite lead was denser 

 than common lead in the same proportion as its 

 atomic weight was greater, and the densities of the 

 various specimens of uranium lead have been found 

 by Richards to be less than that of common lead, the 

 atomic volume for all varieties being constant. The 

 spectra of these various isotopes have been repeatedly 



