84:0 Lead and the End Product of Thorium. 



§ 8. Conclusion. 

 Evidence has been given to show that in fresh, primary 

 rock-forming minerals of the same geological age: — 



(a) The ratio Pb/U is remarkably constant and independent 



of the thorium content. 



(b) The ratio Pb/Th is variable. 



(c) The ratio Pb/(U + 0'4Th) is variable. 



From these facts it is deduced that the end product of 

 uranium is stable and isotopic with lead, but that Th E is 

 unstable, and therefore does not accumulate in geological 

 time. A review of atomic weight evidence indicates that 

 the atomic weight of lead prepared from thorium minerals 

 must be definitely decided upon before a conclusive argument 

 can be based on it. The question still remains as to what 

 the end product of thorium really is. A systematic exami- 

 nation of thorium minerals for bismuth is now being made 

 in Vienna, and until much detailed chemical work of this 

 kind is done little further progress can be expected. 



Our thanks are due to Prof. Mache in Vienna and to 

 Prof. Strutt in London for allowing us to use their apparatus 

 for determining thorium and radium respectively. The 

 expenses of part of this investigation have been defrayed 

 from a Royal Society Government Grant made to one of 

 the authors (A. H.) of this paper. 



Note added Aug. Uth, 1914 (A.H.). 



Mr. R. W. Lawson has written two further paragraphs 

 dealing with the period of disintegration of thorium E *, 

 which he intended to include in this paper. Unfortunately he 

 is detained in Vienna, and all correspondence is temporarily 

 at an end owing to the war. I have therefore decided that 

 the main part of the paper ought to be published (as Part I.) 

 instead of holding it back for the two missing sections. As 

 soon as the post from Austria is restored it is probable that 

 a number of valuable atomic weight determinations by 

 Honigschmid and others will become available ; for it is 

 known that several unpublished results have already been 

 obtained, which will have an important bearing on the 

 questions discussed in this paper. At the same time it is 

 hoped that definite experimental evidence will be forth- 

 coming, bearing on the suggestions that the end products of 

 thorium and actinium are respectively bismuth and thallium. 



* For a preliminary statement see Letter to ' Nature/ July 9th, 1914, 

 by E. W. Lawson. 



