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. When water
is converted into steam, the distances between the molecules are greatly
augmented, but the molecules themselves continue intact. We must not,
however, picture the constituent atoms of any molecule as held so
rigidly together as to render intestine motion impossible. The
interlocked atoms have still liberty of vibration, which may, under
certain circumstances, become so intense as to shake the molecule
asunder. Most molecules--probably all--are wrecked by intense heat, or
in other words by intense vibratory motion; and many are wrecked by a
very moderate heat of the proper quality. Indeed, a weak force, which
bears a suitable relation to the constitution of the molecule, can, by