﻿6S& Messrs. A. Holmes and R. W. Lawson on 



stress on the occurrence of thallium in the Cornwall pitch- 

 blende, since this is of a very impure nature. Professor 

 Honigschmid informs us that he has been unable to find 

 appreciable traces of thallium in selected pitchblende from 

 Joachimstal. Now the atomic weights of the lead from 

 pitchblende from these two districts are respectively 206*86 

 and 206*40, results v T hich indicate the greater purity of the 

 Joachimstal material. It would thus appear that the sug- 

 gestion regarding thallium as the end product of actinium is 

 highly improbable. 



§ 14. Conclusions. 



(a) In Part I. of this paper it was shown from the uranium, 

 thorium, and lead contents of four scries of radioactive 

 minerals : 



(1) That the lead-uranium ratio is remarkably constant in 



minerals of the same age, and varies sympathetically 

 with the geological age of minerals of different 

 antiquities, so that radium Gr may be regarded as 

 stable or practically so. It appears impossible that 

 any slight instability that may exist could ever be 

 definitely detected. The slight deviations in the 

 values of the Pb/U ratios are such as can be readily 

 accounted for by the presence of traces of original 

 lead, by the presence of the unstable thorium-lead, 

 and by the possible alterations which the minerals 

 may have suffered since their original crystallization. 



(2) That thorium-lead does not tend to accumulate in 



geological time, i. e. thorium E is an unstable 

 product. 



(3) That the Pb/U ratios may be used as before and with 



greater certainty for the determination of geological 

 time and the gradual construction of a complete 

 geological time-scale. 



(b) On the assumption that the total lead present in the 

 minerals is made up of three constituents : (a) Uranium 

 lead (radium G) ; (b) Thorium lead (thorium E) ; and (c) 

 Original lead, it has been showm how the results obtained 

 can be applied to find the half-period of thorium E. This 

 has been found tentatively to be about 10 6 years. 



(c) It has been suggested that the disintegration of 

 thorium E may be accompanied by the loss of a /3 ray, so 

 that the resulting product would be an isotope of bismuth. 

 Evidence has, however, been brought forw T ard which suo-gests 



1 © no 



that the latter cannot be a stable end product. Should this 



