186 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



active group which he called ionium, and described as having chem- 

 ical properties similar to those of thorium. So much was this the 

 case that if, by accident, salts of these two elements were mixed, 

 he found it impossible to separate them again by any of the chemical 

 processes. This chemical identity was confirmed in the most con- 

 vincing manner by the later work of Marckwald, Keetman 3 and 

 Welsbach, 4 although the two elements certainly had different radio- 

 active properties and it was extremely probable that they had dif- 

 ferent atomic weights. More identities of a similar nature among 

 the radioactive elements were discovered by Soddy. 5 Hahn, 6 and 

 others, and the situation in 1910 will be found admirably summed 

 up by Soddy in his report to the Chemical Society for that year. 7 

 In 1912, Eussel and Kossi 8 showed that the spectra of ionium and 

 thorium were indistinguishable and Rutherford's theory of the 

 "Nucleus Atom" supplied a possible explanation. The association 

 of the chemical and spectroscopic properties of an element with 

 something more fundamental than its atomic weight, namely, the 

 charge on the nuclei of its atoms or its "Atomic Number," was 

 proved by the epoch-making work of Moseley in 1913. 9 This idea 

 gave a simple and entirely satisfactory meaning to the chemical laws 

 of the radioactive disintegrations discovered a little earlier and pre- 

 dicted that among the numerous products of these disintegrations 

 there must of necessity be some having identical chemical properties 

 but different atomic weights. 



To the latter the name " Isotopes " was applied by Soddy in the 

 following words : " The same algebraic sum of the positive and nega- 

 tive charges in the nucleus when the arithmetical sum is different 

 gives what I call 'isotopes' or 'isotopic elements,' because they 

 occupy the same place in the periodic table. They are chemically 

 identical, and save only as regards the relatively few physical prop- 

 erties which depend upon atomic mass directly, physically identical 

 also." 



The theory of isotopes received its most triumphant vindication, 

 as far as it concerned the products of radioactivity, from the results 

 of work on the atomic weight of lead. Study of the radioactive 

 disintegrations shows that the final product of every series is lead. 

 If we take the main chain of the uranium-radium transformation, 

 this lead must have an atomic weight 206, for it has lost 5 alpha par- 

 ticles — each of weight 4 — since it was radium, and the atomic weight 

 of radium is 226. On the other hand, if we take the main thorium 



3 Keetman : Jahr. Radioactivitat, 6, 269, 1909. 



* A. von Welsbach : Wien. Ber., iia, 119, 1011, 1910. 



6 Soddy : Trans. Cheni. Soc, 99, 72, 1911. 



8 llahn and Meitner : Physikal. Zeitsch., 11, 493, 1910. 



7 Chem. Soc. Ann. Rep., 285, 1910. 



8 Russel and Rossi : Proc. Roy. Soc, 77A, 478, 1912. 



"Moseley: Phil. Mag., 26, 1031, 1913. 



