﻿Lead and the End Product of Thorium. 683 



§ 12. Remarks on the (Selection of Material for 

 Atomic Weight Determinations. 



If the conclusions reached in the foregoing sections are 

 correct, we are given a reliable means of guiding the selection 

 of material for the determination of the atomic weights of 

 uranium lead and thorium lead respectively. It is obvious 

 in the former case that the most suitable material is a mineral 

 rich in uranium — such as uraninite — and practically free 

 from thorium. In order to eliminate the effect of original 

 lead in controlling the atomic weight of the extracted lead, it 

 is further clear that the older the mineral, the nearer to the 

 theoretical value 206-2 should be the value of the atomic 

 weight actually found. Since thorium lead is relatively 

 unstable, any effect which the presence of thorium in the 

 mineral might exert will also be eliminated by using geolo- 

 gically old minerals. 



The case of thorium lead, however, is not so definite. 

 Here, clearly, rich thorium minerals with as small a per- 

 centage of uranium as possible must be used. Further, the 

 best results will be obtained for geologically young minerals, 

 since once the equilibrium amount of thorium lead has been 

 formed, additional time only results in addition of more and 

 more uranium lead, which tends to give a lower value than 

 the theoretical value 208*4 for the atomic weight of thorium 

 lead. The magnitude of this effect is clearly shown by the 

 following example. Suppose we have a mineral with 60 per 

 cent, thorium and 0*4 per cent, uranium, and that we assume 

 it is of post-Cretaceous age, with a lead- uranium ratio of 

 the order 0*01. On the results of the previous sections 

 it follows that the amount of uranium lead present per 

 100 grams of the mineral will be 0*4x0*01 = 0*0040 gram; 

 the amount of thorium lead present will be 60 X 7*10 -J = 0*0042 

 gram; and the amount of original lead O'OOO.v gram. Thus in 

 such a case — and a thorium mineral more free from uranium 

 will be difficult to find — the atomic weight obtained, instead of 

 being equal to the theoretical value 208'4, would lie at about 

 207*2. Tliis number is practically equal to the atomic weight 

 of ordinary lead, so that in the present case this element 

 would be without appreciable effect. Thus it would appear 

 to be a difficult matter to obtain a thorium mineral which 

 would give a higher value for the atomic weight of its con- 

 tained lead than about 207, and the inevitable presence of 

 ordinary lead in minerals prohibits exact calculation of the 

 atomic weight of thorium lead from that found for the lead 

 mixture. 



In the case of minerals of greater age than that assumed 



