_. In short, we speak of the smallest
subdivision of which matter is capable without losing its own nature, as
the _molecule_. All matter may be regarded as consisting of a vast mass
of these small molecules.

Now, we know that all known matter is capable of existing either in a
solid, liquid, or gaseous form, its nature not being changed. Water is
very easily so dealt with. Some substances, it is true, require very
great pressure or very great cold, or both, to alter their form; but
even carbonic acid, oxygen, and hydrogen, which under ordinary
conditions are gases, can with proper appliances be made both liquid and
solid. Pure alcohol, has, I believe, never been made solid, but that is
only because it is so difficult to get a sufficient degree of cold:
there is no doubt that it could be done.

It might be supposed that the molecules of which dead matter (whether
solid, liquid, or vapourous) is composed, were equally motionless and
structureless. But it is not so: every molecule in its own kind is
endowed with marvellous properties. In the first place, every molecule
has a double capability of motion. In the solid form the molecules are
so packed together that, of course, the motion is excessively
restricted; in the liquid it is a little easier; in the gaseous state
the molecules are in a comparatively "open order." In most substances
that are solid under ordinary conditions, by applying heat continuously
we first liquefy and ultimately vapourize them. In those substances
which under ordinary conditions are _gas_ (like carbonic acid, for
instance), it is by applying cold, with perhaps great pressure as well,
that we induce them to become liquid and solid; in fact, the process is
just reversed. As we can most easily follow the process of heating, I
will describe that. First, the solid (in most cases) gets larger and
larger as it progresses to liquefaction, and when it gets to vapour, it
suddenly expands enormously. Take a rod of soft iron, and reduce it to
freezing temperature: let us suppose that in that condition it measures
just a thousand inches long. Then raise the temperature to 212 degrees
(boiling point), and it will be found to measure 1,012 inches. Why is
that? Obviously, because the molecules have got a little further apart.
If you heat it till the iron gets liquid, the liquid would also occupy
still more space than the original solid rod; and if we had temperature
high enough to make the melted iron go off