Lead and the End Product of Thorium. 825 



importance to test the validity of the most recent suggestion 

 regarding lead as a possible end product of thorium, and in 

 the present paper all the evidence hitherto available and 

 many new mineralogical data are collected together with 

 that purpose in view. 



Should thorium E (thorium lead) be a stable isotope of 

 lead, the attending consequences are of peculiar significance 

 both us regards the atomic weight of " lead " and the 

 measurement of geological time *. It is clear in such a 

 case that the lead contents of a primary mineral (originally 

 free from lead) would owe its presence not only to the 

 disintegration of uranium, but also to that of thorium. The 

 ages of minerals as hitherto determined would, therefore, be 

 too high, if any appreciable quantity of thorium were pre- 

 sent. However, it was noticed by Boltwood (loc. cit. 1907) 

 that thorium minerals did not appear to contain more lead 

 than could be readily accounted for by the amount of uranium 

 present, and he concluded " with certainty that lead is not a 

 disintegration product of thorium." Similarly, in a series 

 of Devonian minerals examined for lead and uranium by 

 one of us (loc. cit. 1911), the quantity of thorium present 

 varied between wide limits without affecting the lead-uranium 

 ratio. In order to test rigidly the question of the stability 

 of thorium E and its isotopy with lead, it was considered 

 desirable to determine the thorium content of some of the 

 Devonian minerals referred to above, using material iden- 

 tical with that in which lead and uranium were estimated. 



§ 2. Rate of Production of Thoeium E. 

 The atomic equation for the complete disintegration of 

 thorium, as far as it is at present understood, is 



Th ~> 6 He + Thorium E (thorium lead). 

 212*2 21 (208-2) 



Since one gram of thorium produces 3*lxl0 -s c.c. of 

 helium per annum j, it is easily calculated, from the above 

 equation, that the amount (y) of thorium E generated in the 

 same time is 0*48 xlO" 10 gram. On the assumption that 

 thorium E is stable and isotopic with lead, it is possible to 

 calculate its effect in modifying the lead ratio of a mineral, 

 as follows : — 



The equation for the complete disintegration of uranium is 



U -> 8He -i- Radium G- (uranium lead). 

 238-2 32 (206-2) 



* Holmes, ' Nature/ April 2, 1914, p. 109. 



t Rutherford, " Radioactive Substances," London, 1913, p. 560. 



Phil. Maq. S. 6. Vol. 2S. No. 168. Dec. 1914. 3 H 



