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
measure holding a single pint; if we cooled lower still, it will get
smaller and smaller in bulk (though of course not at all at the same
rate) till it arrives at a point when it is just going to freeze; then
suddenly (7 degrees above the freezing point) it again begins to expand.
Ice occupies more space than cold water; its molecules get arranged in a
particular manner by their crystallization.

On the admission of an _intelligent_ Creator providing, by beneficent
design, the laws of matter, it is easy to give a reason for this useful
property. It prevents the inhabitants of northern climates being
deprived of a supply of water. As it is, the solid water or ice
expands, and, becoming lighter, forms at the top of the water, and the
heavier warmer water remains below. But if ice always got denser and
sank, the warmer liquid would be perpetually displaced and so come up to
the surface, where it would freeze and sink in its turn. In a short
time, then, all our water supplies would (whenever the temperature went
down to freezing, which it constantly does in winter) be turned into
solid ice. This would be a source of the gravest inconvenience to the
population of a cold climate. If we deny a designing mind, the
alternative is that this property of water is a mere chance.

But to return to molecules. Molecules are endowed with an inherent
faculty of motion; only under the conditions of what we call the solid,
they are so compressed, that there is no room for any motion appreciable
to the senses. Even if the solid is converted into vapour, the molecules
are still much restrained in their movements by the pressure of the air.
But of late years, great improvements (partly chemical, partly
mechanical) have been made in producing perfect _vacua_; that is to say,
in getting glass or other vessels to be so far empty of air, that the