Originally publishod in Phys. Z. 15, 797 (1914) 



6. THE PROBLEM OF THE ISOTOPIC ELEMENTS 



G. IIevesy and F. Paneth 

 From the Institute of Radium Reseaich of the N'^ionna Academy of Science 



1. THE ISOTOPE CONCEPT 



It is well known that the separation methods of analytical chemistry 

 have failed when dealing with some radioelements : Nobody has ever 

 succeeded in separating radium-D from lead, mesothorium from radium, 

 or ionium from thorium, nor has it once been possible in these and nu- 

 merous other cases to achieve even a slight enrichment. As more un- 

 successful experiments became known, the workers in this field adopted 

 the view that they were concerned with an inseparability of quite a 

 different kind from that operative with, for example, the rare earths. 

 F. SoDDY^ was the first to give clear expression to this view by desig- 

 nating such elements as "chemically and physically practically iden- 

 tical" and also to search systematically for new examples of such in- 

 separability among the radioelements^. 



Especially striking in connexion with the inseparable elements was 

 tiie fact that they frequently have considerably different atomic weights 

 which, since the a-particle was known to be identical with the helium 

 atom, could be calculated in many instances with certainty ; for exam- 

 ple, the end product of the uranium series, radium-G, which is generally 

 regarded as lead, must have an atomic weight different from thai of 

 ordinary lead^. Confirmation of the correctness of this conclusion has 

 been obtainerl from the recently performed determinations of atomic 

 weights*, which demonstrated that the lead from pitchblende has in 

 fact an appreciably lower atomic weight than ordinary lead and the 

 lead from thorium minerals. 



1 F. SoDDY, J. Chem. Soc. 99, 72 (1911). 



2 A. Fleck, J. Chem. Soc. 103, 381 (1913). 



3 See, for example, G. Hevesy, Phys. Z. 14, Ul (1913) ; F. Soddy, J. Chem. 

 Soc. 105, 1402 (1914). 



* AI. Lembert, see K. Fajans, Z. Elektrochem., 1 June (1914) who suggested 

 those experiments: O. Honigschmid, Ibid.; M. Curie, C. R. Acad. Sci., Paris 

 •June (1914). 



