THE COMPLEXITY OF ELEMENT 143 



was not the ultimate product of thorium. The point 

 still remains experimentally untested. Isobaric 

 isotopes of the character in question can only at 

 present be distinguished if they are unstable and 

 break up further, but they must be taken into 

 account in any theoretical conception we form of 

 the ultimate structure of matter. The accomplish- 

 ment of artificial transmutation would reveal them 

 if they existed, and the discovery of any new prop- 

 erty, like radioactivity, concerned with the nucleus 

 of the atom rather than its external shell, might also 

 be the means of revealing differences of this character. 



On the other hand, the production of isobaric 

 heterotopes is the ordinary consequence of /3-ray 

 changes, single or successive. Such heterotopes, 

 possessing different chemical and spectroscopic 

 character but the same atomic weight, have been 

 recently termed isobar es by A. W. Stewart (Phil. 

 Mag., 1918, [vi.], 36, 326), who, following Fleck's 

 work on the chemical resemblance, not amounting 

 to non-separability, between quadrivalent uranium 

 and thorium, has drawn a parallel between them 

 and elements existing in more than one state of 

 valency, as, for example, ferrous and ferric iron. 



The extent to which the study of radioactive 

 change has enlarged the conception of the chemical 

 element may be summarised by the statement that 

 now we have to take into account in our analysis of 

 matter, not only the heterobaric heterotopes before 

 recognised, but also heterobaric and isobaric isotopes 

 and isobaric heterotopes or isobares. 



THE NUCLEAR ATOM. 



I have attempted to present the most important 

 facts of radioactive change without introducing any 

 theory or hypothesis at all as to the structure of the 



