﻿Lead and the End Product of Thorium. 685 



of bismuth. There is ground for believing that if actinium 



lead is unstable, it will disintegrate with the loss of a /3 ray 



and the resulting formation of a bismuth isotope. A3 to 



whether or not this product is stable could readily be 



tested. We require the bismuth and uranium contents of 



a series of uranium minerals. The fact that about 8 per cent. 



of the uranium in a mineral disintegrates along the actinium 



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series gives a direct means of testing whether the amount 

 of bismuth actually found is of the order of magnitude 

 of that to be expected on the assumption that actinium 

 bismuth is stable. Should actinium bismuth be unstable, 

 then clearly the Bi/LT ratio should be about the same 

 for all fresh unaltered minerals of whatever age. If, on 

 the other hand, it is a stable product, the Bi/U ratio 

 should be constant for minerals of the same age, but it 

 should vary directly with the geological age of the mineral. 

 It is worthy of notice that if actinium bismuth is a stable 

 product, so that appreciable quantities of it can be extracted 

 from uranium minerals, it should be found to have an atomic 

 weight of 210 or 206, both of which values differ by two 

 units from the atomic weight of ordinary bismuth. Thi& 

 method would appear to be a hopeful one for the question in 

 hand, since a result lying near 210 or 206 would not only 

 point to bismuth as the end product of actinium, but it would 

 also give a means of deciding between 230 and 226 as the 

 atomic weight of actinium. The occurrence of bismuth in 

 radioactive minerals has already been commented upon, and 

 it is to be hoped that in the near future more definite 

 evidence for or against bismuth as the end product of actinium 

 will be forthcoming. 



The occurrence of thallium in radioactive minerals has 

 also been noted. For instance, Exner and Haschek * found 

 it to be present spectroscopically in appreciable quantities in 

 pitchblende from Cornwall. The fact that if actinium 

 bismuth is unstable, disintegrating with loss of an a. ray, we 

 should have a thallium isotope, renders thallium a possible 

 end product of actinium. The analysis of a series of uranium 

 minerals for uranium and thallium would be necessary to 

 test this suggestion. The method of showing whether this 

 thallium product is stable or unstable is the same as that 

 suggested in the case of bismuth last treated. Further, in 

 this case, the atomic weight of the thallium should be 206 or 

 202, instead of the atomic weight 201 of ordinary thallium. 

 It is doubtful whether wo are justified in laving very much 



* Exner and Haschek, Wien. Sitzunffsber. cxxi. p. 1077 (1912). 



