CONCLUSIONS 147 



element became synonyms, related as the singular 

 to the plural, and implicit throughout this century 

 was the assumption that all the atoms of any one 

 element are identical with one another in every 

 respect. The only exception is in Sir William 

 Crookes's conception of " meta-elements " as applied 

 to the rare earths. Here the idea was rather that of 

 a gradual and continuous difference among the 

 different atoms of the same element, the properties 

 of the latter being the mean of those of its individual 

 atoms. Modern developments have tended definitely 

 away from rather than towards this view. 



The second phase in the development of radio- 

 active change has now negatived each and every 

 one of the conceptions of last century that associated 

 the chemical element with the atom. The atoms of 

 the same chemical element are only chemically alike. 

 Unique chemical and spectroscopic character is the 

 criterion, not of a single kind of atom, but rather of 

 a single type of external atomic shell. Different 

 chemical elements may have the same atomic mass, 

 the same chemical element may have different atomic 

 masses, and, most upsetting of all, the atoms of the 

 same element may be of the same mass and yet be 

 an unresolvable mixture of fundamentally distinct 

 things. Present-day identity may conceal differences 

 for the future of paramount importance when trans- 

 mutation is practically realised. Then it may be 

 found that the same element, homogeneous in every 

 other respect, may change in definite proportion into 

 two elements as different as lead and gold. The 

 goal that inspires the search for the homogeneous 

 constituents of matter is now known to be, like 

 infinity, approachable rather than attainable. The 

 word homogeneity can in future only be applied, 

 qualified by reference to the experimental methods 

 available for testing it. 



