130 Professor Frederick Soddy [May 18, 



current of hydrogen chloride, in which it was heated at gradually 

 increasing temperature to constant weight. Only single determina- 

 tions have been done, and they gave the values 207*20 for ordinary 

 lead, and 207*694 for the thorite lead, figures that are in the ratio 

 of 100 to 100*24:. This therefore favoured the conclusion that 

 the atomic volume of isotopes is constant. 



At the request of Mr. Lawson, interned in Austria, and continuing 

 his researches at the Radium Institut under Prof. Stefan Meyer, 

 the first fraction of the distilled thorite lead was sent him, so that 

 the work could be checked. He reports that Professor Honigschmid 

 has carried through an atomic weight determination by the silver 

 method, obtaining the value 207*77 ± 0*014, as the mean of eight 

 determinations. Hence, the conclusion that the atomic weight of 

 lead derived from thorite is higher than that of common lead has 

 been put beyond reasonable doubt. 



Practically simultaneously with .the first announcement of these 

 results for thorium lead, a series of investigations were published on 

 the atomic weight of lead from uranium minerals, by T. W. Richards 

 and collaborators at Harvard, Maurice Curie in Paris, and Honigschmid 

 and collaborators in Vienna, which show that the atomic weight is lower 

 than that of ordinary lead. The lowest result hitherto obtained is 

 206 * 046, by Honigschmid and Mile. Horovitz for the lead from the 

 very pure crystallised pitchblende from Morogoro (German East 

 Africa), whilst Richards and Wadsworth obtained 206-085 for a 

 carefully selected specimen of Norwegian clevite. Numerous other 

 results have been obtained, as, for example, 206*405 for lead from 

 Joachimsthal pitchblende, 206*82 for lead from Ceylon thorianite, 

 207*08 for lead from monazite, the two latter being mixed uranium 

 and thorium minerals. But the essential proportion between the 

 two elements has not, unfortunately, been determined. Richards 

 and Wadsworth have also examined the density of their uranium 

 lead. In every case they have been able to confirm the conclusion 

 that the atomic volume of isotopes is constant, the uranium lead 

 being as much lighter as its atomic weight is smaller than common 

 lead. Many careful investigations of the spectra of these varieties 

 of lead show that the spectrum is absolutely the same so far as can 

 be seen. 



Thorium and Ionium. 



A second quite independent case of a difference in atomic weight 

 between isotopes has been established. It concerns the isotopes 

 thorium and ionium, and it is connected in an important way with 

 the researches which, on two previous occasions, I have given an 

 account of here, the researches on the growth of radium from 

 uranium, which have been in progress now for fourteen years. It is 

 the intervention of ionium and its very long period of life which has 



