he predicted
exactly what his distinguished brother, Sir William Thomson--later
Lord Kelvin--found to happen when the matter was put to the test
of experiment. We must consider the experiment made by Lord
Kelvin.

According to Thomson's views, if a quantity of ice and water are
compressed, there must be _a fall of temperature_. The nature of
his argument is as follows:

268

Let the ice and water be exactly at 0 deg. C. to start with. Then
suppose we apply, say, one thousand atmospheres pressure. The
melting point of the ice is lowered to -7.5 deg. C. That is, it will
require a temperature so low as -7.5 deg. C. to keep it solid. It
will therefore at once set about melting, for as we have seen,
its actual temperature is not -7.5 deg. C., but a higher temperature,
_i.e._ 0 deg. C. In other words, it is 7.5 deg. above its melting point.
But as soon as it begins melting it also begins to absorb heat to
supply the 80 thermal units which, as we know, are required to
turn each kilogram of the ice to water. Where can it get this
heat? We assume that we give it none. It has only two sources,
the ice can take heat from itself, and it can take heat from the
water. It does both in this case, and both ice and water drop in
temperature. They fall in temperature till -7.5 deg. is reached. Then
the ice has got to its melting point under the pressure of one
thousand atmospheres, or, as we may put it, the water has reached
its freezing point. There can be no more melting. The whole mass
is down to -7.5 deg. C., and will stay there if we keep heat from
flowing either into or out of the vessel. There is now more water
and less ice in the vessel than when we started, and the
temperature has fallen to -7.5 deg. C. The fall of temperature to the
amount predicted by the theory was verified by Lord Kelvin.

Suppose we now suddenly remove the pressure; what will happen? We
have water and ice at -7.5 deg. C.

269

and at the normal pressure. Water at -7.5 deg. and at the normal
pressure of course turns to ice. The water will, therefore,
instantly freeze in the vessel, and the whole process will be
reversed. In freezing, the water will give up its latent heat,
and this will warm up the whole mass till once again 0 deg. C. is
attained. Then there will be no more freezing, for again the ice
is at its melting point. This is the remarkable series of events
which James Thomson predicted. And these are the events which
Lord Kelvin by a deli