128 Professor Frederick Soddy piay 18, 



characterised chemically, and several lacunifi still existed in the series 

 masking the simplicity of the process. But early in 1913 the whole 

 scheme became clear, and was pointed out first by A. 8. Eussell, in a 

 slightly imperfect form, independently by K. Fajans from electro- 

 chemical evidence, and by myself, in full knowledge of Fleck's 

 results, still for the most part unpublished, all within the same 

 month of February. It was found that, making the assumption 

 that uranium-X was in reality two successive products giving ^-rays, 

 a prediction Fajans and Gohring proved to be correct within a 

 month, and a slight alteration in the order at the beginning of the 

 uranium series, every a-ray change produced a shift of place as 

 described, and every /?-ray change a shift of one place in the opposite 

 direction. Further and most significantly, when the successive 

 members of the three disintegration series were put in the places in 

 the table dictated by these two rules, it was found that all the 

 elements occupying the same place were those which had been found 

 to be non-separable by chemical processes from one another, and 

 from the element already occupying that place, if it was occupied, 

 before the discovery of radioactivity. For this reason the term 

 " isotope " was coined to express an element chemically non-separable 

 from the other, the term signifying " the same place." 



So arranged, the three series extended from uranium to thallium, 

 and the ultimate product of each series occupied the place occupied 

 by the element lead. The ultimate products of thorium should, 

 because six a-particles are expelled in the process, have an atomic 

 weight 24 units less than the parent, or about 208. The main 

 ultimate product of uranium, since eight a-particles are expel ied in 

 this case, should have the atomic weight 206. The atomic weight of 

 ordinary lead is 207 • 2, which made it appear very likely that ordinary 

 lead was a mixture of the two isotopes, derived from ui'anium and 

 thorium. The prediction followed that lead, separated from a 

 thorium mineral, should have an atomic weight about a unit higher, 

 and that separated from uranium minerals about a unit lower, than 

 the atomic weight of common lead, and in each case this has now 

 been satisfactorily established. 



The AT03IIC Weight of Lead FRo:\r Radioactive 

 Minerals. 



It should be said that Boltwood and also Holmes had, from 

 geological evidence, both decided definitely against it being possible 

 that lead was a product of thorium, because thorium minerals contain 

 too little lead, in proportion to the thorium, to accord with their 

 geological ages. AVhereas, the conclusion that lead was the ultimate 

 product of the uranium series had been thoroughly established by 

 geological evidence, and has been the means, in the hands of skilful 

 investigators, of ascertaining geological ages with a degree of pre- 



