CHANGE OF STATE SOLID LIQUID. 203 



received. We must, therefore, reject the supposition of a continuous 

 change from liquid to solid, and regard the change from ice to water as 

 discontinuous, like the change from water to steam. 



Effect of Pressure on the Melting Point. The true explanation 

 of regelation is probably given by a fact first predicted from theory by 

 James Thomson, and since verified by his brother, Lord Kelvin, viz. : 

 that the melting point of ice is lowered by pressure. The following 

 general explanation of this effect is not to be regarded as at all strict, 

 but is given merely as enabling us to think of the main features of the 

 process. A more complete proof will be found in chapter xix. 



Let us take a mixture of ice and water at 0. Any further melting 

 will decrease the volume ; any further freezing will increase it. Now, 

 subject the mixture to great pressure, and let us assume that the mixture 

 will yield to this pressure in the most effective way. This will take 

 place if some of the ice melts. In melting, latent heat will be taken up, 

 and the mixture will consequently cool until there is once more equilibrium 

 between the ice and the water at a lower temperature in other words, 

 the melting point is lowered. If the pressure is still further increased 

 more ice melts to yield to it, more latent heat is taken up, and the 

 mixture will cool still further till a new melting point is reached. 



Similarly, with substances in which there is an expansion on melting, 

 pressure will raise the melting point. For, on subjecting a mixture of 

 solid and liquid to pressure, the mixture will yield most effectively 

 by solidification, latent heat is given up and the temperature is raised. 



To explain regelation by the lowering of melting point by pressure, let 

 us suppose that two blocks of ice at are pressed together. The pressure 

 lowers the melting point below 0. Some of the ice at and near the 

 surface is subjected to this pressure, and is, therefore, above the new 

 melting point. It melts, and taking up latent heat it cools the liquid 

 and the surroundings to the new melting point. The cooled water is 

 squeezed out to the space between the two blocks where there is no 

 pressure, so that the water is now below its freezing point. Therefore, 

 being in contact with ice, so that there is no superfusion, it freezes again 

 and forms a bridge connecting the two blocks. In fact, the effect of the 

 pressure is to transfer the ice from points of greater to points of less 

 pressure, the ice, however, liquefying in order to effect the passage. On 

 the viscous theory of ice, it also passes from points of greater to points 

 of less pressure, but without liquefying. 



The lowering of the melting point of ice by pressure was verified by 

 Lord Kelvin* as follows: He enclosed a mixture of ice and water 

 in an Oersted's piezometer, consisting of a stout glass cylinder provided 

 with a screw. A thermometer with ether in place of mercury, to secure 

 greater sensitiveness was enclosed in a protecting glass tube, and placed 

 in the mixture, the part of the scale to be used being kept free from ice 

 by a lead ring, so that it could be seen. The pressure was indicated by a 

 glass tube closed at one end, and just full of air at the atmospheric 

 pressure. This was put into the piezometer with the open end down- 

 wards. 



When a pressure of several atmospheres was put on, the temperature 

 fell and remained steady, the fall being about 0-0075 per atmosphere, 

 * Mathematical and Physical Papers, vol. i. p. 166. 



