216 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1925 



well be excused from thinking that new generations may discover 

 now imknown^ elements. Since each new radioactive element is 

 characterized not only by the length of its average life, but also by 

 the coefficient of absorption of its radiation, and finally by the ve- 

 locity .of its a and ji particles, the multiplicity of these character- 

 istics and the manner in which they behave under chemical in- 

 fluences leave no further doubt upon the objective reality of the 

 most elusive of the radioactive elements. 



Their multiplicity^ alone is disconcerting. It is evidently impos- 

 sible to put each one of them into a separate compartment of the 

 too few compartments of Mendeleeff's classification. Must we, 

 therefore, renounce the universal application of that classification 

 and exclude from it the majority of the radioactive elements? You, 

 Brauner, who have identified yourself with Mendeleeff's cause and 

 have struggled so much for the success of the periodical classi- 

 fication, how anxiously you must have considered the matter. When 

 it became evident that certain elements distinct in their radio- 

 active properties, could not be distinguished in other ways, they 

 were naturally grouped in the same compartment — that is, all those 

 which appeared chemically the same. This phenomenon Fr. Soddy 

 called " isotopy." This new notion, the value of which has become 

 considerable, at first seemed to arouse only a limited scientific pub- 

 lic. For most chemists, ionium, which could not be separated chem- 

 ically from thorium, ultimately, could only be thorium particu- 

 larly affected through radioactive qualities. It did not seem of good 

 augury that two elements could not be separated even slightly ; and 

 yet further, their luminous spectra seemed to show no differences. 

 The daring of Soddy did not seem very great to have thus simply 

 settled the matter. If, as we once admitted, an element is suffi- 

 ciently defined through its spectrum and the value of its atomic 

 weight, one could not at all doubt that ionium and thorium were 

 one and the same element. Then in each compartment of Mendeleeff 

 we would put only one element where the radioactive people would 

 group several. 



The ideas of chemical and radioactive elements seemed to be in 

 conflict. The chemists were inclined to concede as little as possible 

 to the physicists. 



It is true that the matter of the affiliation of the radioactive ele- 

 ments was not then \QYy clear. It was difficult to put order into the 

 labyrinth of radioactive bodies. Fr. Soddy finally brought the 

 desired thread of Ariadne in the guise of the law of dis})lacement 

 which gave a definite means of classifying a radioactive element in 

 the periodic scheme when we know the nature of its radiation and 

 its immediate ancestor. The same laws were also stated at about 

 the same time by Fajans. 



