Evolution and Devolution of the Elements. 51 
That is to say, this element has been formed with a con- 
siderable fall in atomic weight from its theoretical value. 
We have endeavoured to derive expressions for the atomic 
weights of all the other groups of elements, but we have not 
been entirely successful owing to the want of knowledge 
of the atomic weights of the highest members. We find, 
however, that the same periodic phenomenon of rise and fall 
takes place in all the groups, and moreover that a rare earth 
element always appears at the position of the maximum fall. 
This fact is an experimentally suggestive one in connexion with 
the phenomenon of radioactivity. Now the chemical pro- 
perties of the rare earths are apparently extremely complex. 
The fact that all these substances are trivalent we attribute 
to the fact that they possess an outer trivalent ring. Of the 
internal structure of the atom in each case it is impossible 
to speak with any certainty : in fact there is little doubt 
that these substances are quite abnormal. 
Now inasmuch as these elements are produced at a period 
of maxim am fall in the evolution of each family, it would 
appear that they are the result of the degradation of substances 
of higher atomic weight. It does not seem far from im- 
probable that the final degradation products of all radio- 
active elements should be a rare earth. 
In the last section of our paper we introduced the idea of 
a process of devolution, and the natural result of such an 
idea is an inquiry as to whether the structure which we 
assigned to the atoms to account for their evolution will also 
account for what is known of their devolution. As before 
mentioned, we have undoubted cases of devolution in the 
radioactive bodies, and a large number of most interesting- 
facts have recently been observed in connexion with them 
by many workers. 
Of all these facts, perhaps the most interesting, from our 
point of view, is that the mass of the Alpha particles of radium, 
uranium, thorium, and actinium have been found to lie 
betweeen the masses of the atoms of hydrogen and helium. 
Now in developing our theory of evolution we laid great 
stress upon what we described as the principle of " heredity " 
in normal direct evolution ; that is to say, we consider for 
example that the outer rings of the elements, with the 
exception of thulium, in the second group, are of the same 
shape, viz., that shape corresponding to the valency of two. 
Perhaps our meaning in this connexion might be rendered 
clearer by taking an analogy with crystallization. When a 
solution of a salt is sown with the proper form of crystal, 
the salt proceeds to crystallize out along the lines which 
have been as it w r ere laid down for it. Although perhaps 
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