I have ventured to recall to you these commonplaces of science before considering a mode of melting ice which is less generally known, and which involves no supply of heat on your part.  This method involves for its

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understanding a careful consideration of the thermal properties of 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 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.