138 Professor Frederick Soddy [May 18, 



further, but then, even if it did, it is in such small quantity that it 

 is doubtful whether the change would have been detected. "^ But, so 

 far as is known, it forms a stable isotope of lead of atomic weight 

 210. formed in the proportion of only 0*03 per cent of the whole. 



Xow the atomic weight evidence merely shows ttiat one of the 

 two isotopes of lead formed from thorium is stable enough to accu- 

 mulate over geological epochs, and it does not necessarily follow that 

 both are. Dr. Arthur Holmes has pointed out to me that the 

 analysis I gave of the Ceylon thorite leads to a curiously anomalous 

 value for the age of the mineral. The quantity of thorium lead per 

 gram of thorium is 0'0062, and this, divided by the rate at which 

 the lead is being produced, 4*72 x 10"^^ gram of lead per gram of 

 thorium per year, gives the age as 131 million years. But a Ceylon 

 pitchblende, with uranium 72-88 percent and lead 4-65 per cent, 

 and ratio of lead to uranium as 0'064, gives the age as 512 million 

 years. Dr. Holmes regards the two minerals as likely to be of the same 

 age, and the pitchblende to be, of all the Ceylon results, the one 

 most trustworthy for age measurement. 



If we suppose that, as in the case of radium-D, the 65 per cent 

 isotope of lead derived from thorium is not stable, and that only the 

 35 per cent isotope accumulates, the age of the mineral would be 

 375 million years, which the geologists are likely to consider much 

 more nearly the truth. But the most interesting point is that, if we 

 take the atomic weight of the lead isotope derived from uranium as 

 206 • 0, and that derived from thorium as 208 • 0, and calculate the 

 atomic weight of the lead in Ceylon thorite, assuming it to consist 

 entirely of uranium lead and of only the 35 per cent isotope from 

 thorium, we get the value 207 '74, which is exactly what I found 

 from the density, and what Prof. Honigschmid determined (207*77). 



The question remains, if this is what occurs, what does this 

 unstable lead change into ? If an a-particle were expelled mercury 

 would result, or if a /5-particle bismuth, two elements of which I 

 could find no trace in the lead group separated from the whole 

 20 kilos, of mineral. But if an a- and a y8-particle were both ex- 

 pelled, the product would l)e thallium, which is present in amount 

 small but sufficient for chemical as well as spectroscopic characterisa- 

 tion. If the process of disintegration does proceed as suggested, it 

 should be possible to trace it, for this particular lead should give a 

 feeble specific a- or y8-radiation, in addition, of course, to that due 

 to other lead isotopes. So far it has not been possible to test this. 

 In the meantime, the explanation offered is put forward provisionally 

 as being consistent with all the known evidence. 



Looking for a moment in conclusion at the broader aspects of 

 the new ideas of atomic structure, it seems that though a sound basis 

 for further development has been roughed out, almost all the detail 

 remains to be supplied. We have got to know the nucleus, but 

 beyond the fact that it is constituted, in heavy atoms, of nuclei of 



