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: 

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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.

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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 delicate series of experiments, verified in every respect.

Suppose we had nothing but solid ice in the vessel at starting, would the experiment result in the same way?  Yes, it assuredly would.  The ice under the increased pressure would melt a little everywhere throughout its mass, taking the requisite latent heat from itself at the expense of its sensible heat, and the temperature of the ice would fall to the new melting point.

Could we melt the whole of the ice in this manner?  Again the answer is “yes.”  But the pressure must be very great.  If we assume that all the heat is obtained at the expense of the sensible heat of the ice, the cooling must be such as to supply the latent heat of the whole mass of water produced.  However, the latent heat diminishes as the melting point is lowered, and at a rate which would reduce it to nothing at about 18,000 atmospheres.  Mousson, operating on ice enclosed in a conducting cylinder and cooled to -18 deg. at starting