﻿Lead and the End Product of Thorium. 687 



<Group V. b product of thorium be unstable, the method here 

 used for thorium E could be applied to find its disintegration 

 ^constant, when the bismuth and thorium contents of suitable 

 minerals are known. Information could thus be obtained as 

 to the next succeeding product, which would be a thallium 

 isotope if the bismuth isotope disintegrates with loss of an 

 a ray, or a polonium isotope if the change takes place with 

 loss of a /3 ray. 



(d) The evidence of atomic weight determinations of lead 

 extracted from radioactive minerals also points to the 

 stability of radium Gr, and the instability of thorium E. It 

 has been shown that the most suitable mineral for the 

 determination of the atomic weight of radium Gr is a geolo- 

 gically old, primary, fresh, uranium mineral. Using such a 

 mineral, broggerite, Honigschmid and St. Horovitz found 

 the value 206*06. Should actinium lead have an atomic 

 weight of 210, the above result for broggerite would indi- 

 cate that it is unstable. Experiments to throw light on the 

 stability of actinium lead are suggested in § 13. If the 

 atomic weight of actinium lead is 206, we are unable to say 

 whether it is unstable or not. Methods of attacking the 

 question of the end product of actinium have been suggested, 

 and it has been shown how one of these might incidentally 

 throw licrht on the atomic weight of actinium. For thorium 

 E it would appear that no very definite conclusions can be 

 drawn from atomic weight determinations, owing to the 

 instability of this element. In the most favourable circum- 

 stances, a value of about 207 would be obtained, instead of 

 the theoretical value 208'4. Determinations of the atomic 

 weights of bismuth and thallium from uranium and thorium 

 minerals, would serve a useful purpose in supplying definite 

 information regarding the end products of thorium and 

 actinium. 



(<?) Atomic weight estimations can now be used to correct 

 the crude determination of the age of a mineral by means of 

 its present lead and uranium contents. Corrections must be 

 applied both for original lead, which may be considerable 

 (e. g. in some specimens of thorianite from Ceylon), and for 

 a small equilibrium amount of thorium lead. These cor- 

 rections appear to make very little difference to the time- 

 scale as at present constituted. That this is true for the 

 Devonian minerals of Norway we have already scon. The 

 pre-Jatulian minerals of the Moss district of Norway require 

 practically no correction, for broggerite from that district 

 contains uranium lead in an almost uncontaminated state, as 

 shown by the atomic weight of lead prepare'! from the 



