212 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1918. 



that in at least some cases the uranium ore actually had been con- 

 taminated with lead ore. The purest Norwegian specimen contrib- 

 uted by Miss Gleditsch thus acquired especial importance and sig- 

 nificance, because it was only very slightly, if at all, vitiated in 

 this way. As a matter of fact, it gave 206.08 for the atomic weight 

 in question — the lowest of all. Here are typical results, showing the 

 outcome; many more of similar tenor were obtained. 



Atumio Weights. 



{207. 20 1 

 °07 19 I 2 ° 



7.19 



Australian radioactive lead containing probably 25 per cent 

 ordinary lead 



[206. 32 

 206. 36 

 206. 33 

 206. 36 



.206. 34 



f 206. OS } 

 Purest uranio-lead 1206 09 I 206.08 



Honigschmid, from similar pure material, had found figures 

 (206.05) agreeing almost exactly with the last value. One can not 

 help believing that this last specimen of lead is a definite substance, 

 probably in a state almost pure, because of the unmixed quality of 

 the carefully selected mineral from which it was obtained. 



A further question now arises : is it a fermanent substance — really 

 an end-product of the disintegration? Soddy's hypothesis assumes 

 that it is. The only important fact militating against this view is 

 the observation that uranium-lead is always radioactive, and hence 

 might be suspected of being unstable. In various impure specimens, 

 however, the radioactivity is not proportional to the change in the 

 atomic w 7 eight; hence the radioactivity is probably, at least in part, 

 to be referred not to the lead itself, but rather to contamination 

 with minute, unweighable amounts of intensely radioactive impuri- 

 ties — other more transitory products of disintegration. 1 If w T eigh- 

 able, such impurities would almost certainly increase, not diminish, 

 the atomic weight; hence their presence could not account for the 

 low value. 



Let us compare the actual result for the atomic weight of this 

 kind of lead with the theory of Soddy and Fajans. If this theory 

 is sound, the simple subtraction of eight times the atomic weight of 

 helium from that of uranium, or five times the atomic w r eight of 

 helium from that of radium, should give the atomic weight of the 

 lead resulting from the disintegration, as follows: 



1 For this reason the term " radio-active lead " although It describes the fact, is per- 

 haps, from a theoretical point of view, not the best designation of either uranium or 

 thorium lend ; tint the term is convenient, because it distinguishes between these two 

 forms and common lead. 



