﻿Lead and the End Product of Thorium, 677 



of the equations. Our previous assumption regarding the 

 magnitude of k is confirmed. The mean value of Pb o = 0*0009 

 is not far removed from that assumed in the first two methods. 

 The result O0026 obtained from Nos. 6, 7, 12 is certainly 

 too high, the reason most likely being that the analyses 6 

 and 7 are practically the same. 



§ 10. Bismuth as a Possible End Product of the 

 Thorium Series. 



The results obtained in the present section have an im- 

 portant significance in relation to another possible end product 

 of thorium. Were thorium E, with a half-period of about 

 10 6 years, to emit a rays, these should, according to the 

 Geiger-Nuttall law, have a range of about 3 cm. This fact 

 renders it highly improbable that we have here to do with 

 an a-raj product, because a rays with the above range would 

 hardly have escaped detection. This would appear to exclude 

 the possibility of the end product being an isotope of mercury 

 in Group II. b of the Periodic Classification. It seems more 

 likely that the disintegration of thorium E is accompanied 

 by the loss of a /3 ray, which would bring the resulting pro- 

 duct into the position of bismuth in Group V. h. In such a 

 case, we are again faced with the task of deciding whether 

 what we may by analogy call thorium-bismuth is a stable or 

 an unstable product. Here again the method used in the 

 present section might be applied. A systematic examination 

 of thorium minerals for bismuth and thorium would be 

 necessary. If the end product of thorium is a stable bismuth 

 isotope, the ratio Bi/Th for minerals of the same age should 

 he constant, whereas for minerals of different ages it should 

 vary in a similar manner to that found in the case of the 

 Pb/U ratio. On the other hand, if the Group V. b product 

 of thorium is unstable, the suggested analyses would serve 

 to determine its disintegration constant. The following ex- 

 pression would be used in this connexion — Bi^ = Bi 4-?i . Th* . 

 Insertion of the results of two suitable analyses would result 

 in two equations from which the amount of original bismuth 

 (Bi ) and the equilibrium constant (n) between thorium- 

 bismuth (thorium F) and thorium could be found. From 

 the value of n so obtained, the half-period of thorium F could 

 be calculated, and evidence adduced as to whether the suc- 

 ceeding change takes place with loss of an a ray or of a 

 /3 ray. In the former case, the succeeding product would be 

 an isotope of thallium with atomic weight 204'4 (Group ITT. J>\ 

 .and in the latter an isotope of polonium (Group VI. M. 



