ISOTOPES AND HETEROTOPES 131 



electricity and the ^-particle one atomic charge of 

 negative electricity, the successive places in the 

 periodic table must thus correspond with unit 

 difference of charge in the atomic structure, a con- 

 clusion reached later for the whole periodic table, 

 as far as aluminium, as the result of Moseley's 

 investigations on the frequency of Barkla's character- 

 istic ^T-radiations of the elements. 



The non-separable elements, with identical chemi- 

 cal character, on this scheme were found all to 

 occupy the same place in the periodic table, and 

 on this account I named them isotopes. Conversely, 

 the different elements recognised by chemical analysis 

 should be termed "heterotopes," that is, substances 

 occupying separate places in the periodic table, but 

 themselves mixtures, actually proved or potential, of 

 different isotopes, not necessarily homogeneous as 

 regards atomic weight and radioactive character, 

 but homogeneous as regards chemical and spectro- 

 scopic character, and also physical character, so far 

 as that is not directly dependent on atomic mass. 



SPECTRA OF ISOTOPES. 



As regards the spectrum, the first indication 

 that chemically non-separable elements probably 

 possessed identical spectra arose out of the failure 

 of Russell and Rossi and of Exner and Haschek 

 in 1912 to detect any lines other than those of 

 thorium in the spectrum of ionium-thorium prepara- 

 tions that might reasonably be supposed to contain 

 an appreciable, if not considerable, percentage of 

 ionium. The work of Honigschmid on the atomic 

 weight of ionium-thorium preparations has fully 

 confirmed this view. The isotopes of lead of 

 different atomic weight separated from uranium 

 and thorium minerals have been found to possess 



