THE SIGNIFICANCE OF THE LAW. 41 



group NH 4 , known as the ammonium group, will pass 

 from compound to compound maintaining its integrity and 

 behaving marvellously like the element potassium Or sodium. 

 We see, therefore, that groups of atoms may simulate the 

 elements of matter, and we see, moreover, that the scheme 

 of relationship and periodicity of properties, unquestionably 

 evident in the elements, finds a reasonable and natural ex- 

 planation in the assumption that the atoms of the elements 

 are made up of sub-atoms, which act in the atom just as 

 the atoms act in the molecule. It is all natural, reason- 

 able and, indeed, inevitable. One thing is needed, the ex- 

 istence of a body smaller than an atom, and it will be one 

 important business of our book to seek and find this thing. 

 But the bare fact of sub-atomicity is not the only secret of 

 the periodic law. We have spoken not only of gradation 

 of properties, but of similarity and relationship. We have 

 spoken of the elements as sisters and cousins; in other 

 words, of a real relationship, not an analogous one. If we 

 may speak of sisters and cousins, why not of fathers and 

 mothers, and even grandfathers ? Throughout all biological 

 science family relationship is taken indubitably to mean 

 common origin. Evolution is based on this assumption. 

 Whenever the biologist finds a series of organisms, vegetable 

 or animal, betraying in their structure manifold likeness 

 of form, he says at once: " These organisms have had a 

 common ancestry; they have been evolved, all of them, 

 from some one simpler form." Now it is this very form 

 of argument that takes us inevitably to the conclusion that 

 the atomic families, themselves, since they show family re- 

 lationships as sharply defined as any to be found in biology, 

 are of common origin and descent, and that the atoms, as 

 we know them, are the product of an evolution an in- 

 organic evolution. It will be another important part of the 



