﻿Lead and the End Product of Thorium. 681 



(b) Broggerite from Norway. In the first case they obtained 

 •a mean value 206*04, and in the second case 206*06. These 

 results would appear to indicate that the presence of ordinary 

 lead as impurity was responsible for the high results obtained 

 with other minerals. The purity of the first mineral (a), and 

 the fact that the second mineral (b) is an old primary un- 

 altered mineral, lend support to the view that these contained 

 & minimum quantity of ordinary lead. The above results 

 show that the lead extracted from the minerals mentioned 

 was practically pure radium G. 



In the case of thorium lead the evidence is as yet less 

 clear than in the case of uranium lead. If we assume the 

 atomic weight of thorium to be 232*4, then the atomic 

 weight of thorium lead, derived by loss of six helium atoms 

 from thorium,, is to be expected to be 208*4 *. Soddy 

 and Hyman t have extracted lead from thorite (Ceylon), 

 ■and in the two preliminary determinations of the atomic 

 weight they have already published, they arrived at the 

 values 208*3 and 208*5. From the known rates of disinte- 

 gration of uranium and thorium, it can readily be seen from 

 the analysis of the mineral used by the authors cited, that if 

 all the lead were of radioactive origin and thorium lead 

 stable, there should be about thirteen times as much thorium 

 lead present as uranium lead. The theoretical atomic weight 

 to be expected can thus be shown to be 208*24, a result less 

 than that found experimentally. Unfortunately, the evidence 

 in the present case must still be regarded as inconclusive, 

 liichards and Lembert (loc. cit.) have determined the atomic 

 weight of lead separated from thorianite from Ceylon. This 

 material contained 60 per cent, thorium and 20 per cent, 

 uranium, so that if thorium lead be stable, the atomic 

 weight to be expected would be 207*40. The value they 

 actually obtained was 206' 82, suggesting at once the insta- 

 bility of thorium lead. Tnat this result is higher than 

 those found for the other minerals they used, cannot without 

 further evidence be assumed to be due to the presence 

 of a stable thorium lead. The presence of ordinary lead 

 as an original constituent of the mineral, or as an infil- 

 trated secondary product, would very well explain the dif- 

 ference. M. Curie obtained for lead extracted from monazite 

 sand the value 207*08. Here again we are not certain of the 

 role played by ordinary lead, and furthermore, this mineral 

 always contains a certain quantity of uranium which would in- 

 troduce a disturbing factor owing to the presence of radium G. 



* If the atomic weight of thorium is 282*2, as determined by Htinig- 

 sehmid (1914), then that of thorium lead would be 208*2, 

 f Soddy £ Ilyman, Trans. Chem. Soc, cv, 1014, p. 1402. 



