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 into vapour, it would occupy
an enormously increased space. I cannot say what it would be for iron
vapour; but if a given volume of water is converted into vapour, it will
occupy about 1,700 times the space it did when liquid, though the weight
would not be altered.

It may here be worth while to mention that it is not invariably true
that a substance gets contracted, and the molecules more and more
pressed together, as it assumes a solid form. There is at least one
exception. If we take 1,700 pints of steam, the water, as I said, on
becoming cool enough to lose the vapourous form, will shrink into a