116 CONCEPTION OF THE CHEMICAL ELEMENT 



FIRST AND SECOND PHASES OF DEVELOPMENT. 



Looking- backward to the first recognition of the 

 character of radioactive change in 1902, it is possible 

 to distinguish broadly two phases. The first phase, 

 concerned mainly with the disentanglement of the 

 long and complicated series of successive changes, 

 commencing with the two primary radio-elements 

 uranium and thorium, and including ultimately all 

 the known radio-elements, added little to the con- 

 ceptions of chemistry beyond the disturbing fact that 

 the radio-elements, although in every other respect 

 analogous to the ordinary elements, are in process of 

 continuous transmutation. But in the second and 

 more recent phase of radioactive change, the study 

 of the chemical character of the successive products 

 and the law connecting this with the type of ray 

 expelled in the change, the discovery of elements 

 with different radioactive but identical chemical 

 character, the recognition of these as isotopes, or 

 elements occupying the same place in the periodic 

 table, and the interpretation of the significance of 

 the periodic law, conceptions are arrived at which 

 are not merely novel, but upsetting. In this phase, 

 an aspect of the ultimate constitution of matter has 

 been revealed that, although well within the scope of 

 the conceptions of elements and atoms which we owe 

 to the nineteenth century, nevertheless has totally 

 escaped recognition. I am not much concerned with 

 definitions, but I think the Chemical Society might 

 safely offer a prize of a million pounds to any one of 

 its members who will shortly and satisfactorily define 

 the element and the atom for the benefit of and 

 within the understanding of a first-year student of 

 chemistry at the present time. 



