water in the solid state.

It must have been observed a very long time ago that water
expands when it freezes. Otherwise ice would not float on water;
and, what is perhaps more important in your eyes, your water
pipes would not burst in winter when the water freezes therein.
But although the important fact of the expansion of water on
freezing was so long presented to the observation of mankind, it
was not till almost exactly the middle of the last century that
James Thomson, a gifted Irishman, predicted many important
consequences arising from the fact of the expansion of water on
becoming solid. The principles lie enunciated are perfectly
general, and apply in every case of change of volume attending
change of state. We are here only concerned with the case of
water and ice.

James Thomson, following a train of thought which we cannot here
pursue, predicted that owing to the fact of the expansion of
water on becoming solid, pressure will lower the melting point of
ice or the freezing point of water. Normally, as you are aware,
the temperature is 0 deg. C. or 32 deg. F. Thomson said that this would
be found to be the freezing point only at atmospheric pressure.
He calculated how much it would change with change of pressure.
He predicted that the freezing point would fall 0.0075 of a
degree Centigrade for each additional atmosphere of pressure
applied to the water. Suppose,

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for instance, our earth possessed an atmosphere so heavy to as
exert a thousand times the pressure of the existing atmosphere,
then water would not freeze at 0 deg. C., but at -7.5 deg. C. or about
18 deg. F. Again, in vacuo, that is when the pressure has been
reduced to the relatively small vapour pressure of the water, the
freezing point is above 0 deg. C., _i.e._ at 0.0075 deg. C. In parts of
the ocean depths the pressure is much over a thousand
atmospheres. Fresh water would remain liquid there at
temperatures much below 0 deg. C.

It will be evident enough, even to those not possessed of the
scientific insight of James Thomson, that some such fact is to be
anticipated. It is, however, easy to be wise after the event. It
appeals to us in a general way that as water expands on freezing,
pressure will tend to resist the turning of it to ice. The water
will try to remain liquid in obedience to the pressure. It will,
therefore, require a lower temperature to induce it to become
ice.

James Thomson left his thesis as a prediction. But