Lead and the End Product of Thorium. 837 



(c) Radioactive Minerals from the Arendal district 



of Norway (Table III.). 

 All that can be said of the geological age of this 

 group o£ minerals is that, like the preceding, it is pre- 

 Jatulian. The constancy of the Pb/U ratios is sufficient, for 

 the purpose of this paper, to justify their independent 

 grouping. The ratio of thorium to uranium in minerals 

 1 and 2 is widely different from that of the others, and as is 

 therefore to be expected, the corresponding Pb/(U + 0*4Th) 

 ratios strongly diverge from those of the other minerals. 



(d) Radioactive Minerals from Ceylon (Table IV.). 



The igneous rocks of Ceylon in which thorite and thorianite 

 occur appear to be of two well-marked ages, both consi- 

 derably older than the oldest sediments of the Indian 

 Peninsula, which are thought to belong to the Pre-Cambrian. 

 The nine analyses here given are all of minerals which 

 belong to the older igneous complex. The younger specimens 

 of thorianite are not cited because in them the thorium- 

 uranium ratio does not vary sufficiently to apply the test 

 ratio. It will be noticed in the analyses quoted that, as in 

 previous examples, the ratio Pb/(U 4-0*4 Th) varies widely 

 when the ratio of thorium to uranium also varies. The 

 evidence clearly indicates the independence of the Pb/U 

 ratio on the thorium content. 



(c) Conclusion. 



The three sories of minerals cited above bear out the 

 conclusion of § 5 that thorium E is an unstable isotope 

 of lead. 



§ 7. Evidence from Atomic Weight Determinations. 



It is possible from the disintegration equations of uranium 

 -and thorium, and the atomic weights of the parent elements, 

 to calculate the atomic weights of the end products. Unfor- 

 tunately the atomic weights of uranium and thorium are 

 not yet definitely agreed upon. Taking uranium as 238*5 

 (International Atomic Weights 1914) we have 



238*5-8x4=206-5 



as the atomic weight of Ra Gr. (uranium lead). 



Honigschmid *, however, in a recent determination of the 

 atomic weight of uranium, obtained a value 238*2. This 

 <nves 20G*2 for Ra G. Again, the International atomic 



* Iluuigschmid, Wie.i. Anzeige)', £2 Jan. 1914. 



