﻿€84 Messrs. A. Holmes and R. W. Lawson 



on 



above — and all the minerals hitherto examined are actually 

 very much older — the atomic weights to be expected would 

 be somewhat lower than 207*2. It is interesting to notice 

 that the actual determinations by Richards and Lembert on 

 Ceylon thorianite, and by Curie on monazite, give results in 

 complete harmony with this conclusion. This experimental 

 verification adds still further support to the view that thorium 

 lead is unstable. 



§ 13. The Possible End Product of the Actinium Series. 



When uranium is present in a mineral, actinium lead will 

 always be a disturbing factor. Should actinium lead be a 

 stable product, the minimum value of the atomic w r eight of 

 lead extracted from a thorium-free uranium mineral would 

 be 206'26 if the atomic weight of actinium lead is 207, and 

 206*50 if the atomic weight of actinium lead is 210. On 

 the other hand, if actinium lead is of relatively short life, 

 and if about 8 per cent, of uranium is transformed along the 

 actinium series, then the high degree of stability of uranium 

 lead renders it certain that the atomic weight of the latter 

 would be quite inappreciably affected by the presence of 

 actinium lead, even if this has an atomic weight of 210. 

 The recent determinations of the atomic weight of lead from 

 crystalline pitchblende and broggerite by Honigschmid, 

 indicate either that the atomic weight of actinium lead is 

 207, or that if it is 210 then actinium lead is an unstable 

 product. In the case that it is unstable, evidence with regard 

 to its half-period might be obtained by determining the 

 atomic weight of lead of radioactive origin, obtained on the 

 one hand from geologically young, and on the other hand 

 from geologically old, uranium minerals. It is clear that 

 a higher value should be obtained in the former case than in 

 the latter. 



Regarding the end product of actinium there would appear 

 to be three possibilities, each of which we will now consider 

 in turn. 



As mentioned in a previous paragraph, the atomic weight 

 determinations of radium G by Honigschmid indicates that 

 if actinium lead has an atomic weight of 210, it must be 

 unstable, otherwise instead of obtaining a value 206'04 for 

 lead from Morogoro pitchblende, the value 206*50 should 

 have been found. It would thus appear certain that the end 

 product of actinium cannot be a stable lead isotope unless 

 this has an atomic weight nearly the same as radium Gr. 



The second possibility is that the end product is an isotope 



