The union takes place, not gradually
and uniformly, but by steps, a definite weight of matter being added at
each step. The larger combining quantities of oxygen are thus multiples
of the smaller ones. It is the same with other combinations.
We remain thus far in the region of fact: why not rest there? It might
as well be asked why we do not, like our poor relations of the woods and
forests, rest content with the facts of the sensible world. In virtue of
our mental idiosyncrasy, we demand _why_ bodies should combine in
multiple proportions, and the outcome and answer of this question is the
atomic theory. The definite weights of matter, above referred to,
represent the weights of atoms, indivisible by any force which chemistry
has hitherto brought to bear upon them. If matter were a _continuum_--if
it were not rounded off, so to say, into these discrete atomic
masses--the impassable breaches of continuity which the law of multiple
proportions reveals, could not be accounted for. These atoms are what
Maxwell finely calls "the foundation stones of the material universe,"
which, amid the wreck of composite matter, "remain unbroken and unworn."
A group of atoms drawn and held together by what chemists term affinity
is called a molecule. The ultimate parts of all compound bodies are
molecules. A molecule of water, for example, consists of two atoms of
hydrogen, which grasp and are grasped by one atom of oxygen.
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