344 HELMHOLTZ 



coming latent; that is, some ice in the mixture must melt 

 and be converted into water. In this is found the reason 

 why mechanical pressure can influence the freezing-point. 

 You know that ice occupies more space than the water from 

 which it is formed. When water freezes in closed vessels, 

 it can burst not only glass vessels, but even iron shells. 

 Inasmuch, therefore, as in the compressed mixture of ice 

 and water some of the ice melts and is converted into water, 

 the volume of the mass diminishes, and the mass can yield 

 more to the pressure upon it than it could have done without 

 such an alteration of the freezing-point. Pressure furthers 

 in this case, as is usual in the interaction of various natural 

 forces, the occurrence of a change, that is fusion, which is 

 favourable to the development of its own activity. 



In Sir W. Thomson's experiments water and ice were 

 confined in a closed vessel from which nothing could escape. 

 The case is somewhat different when, as with glaciers, the 

 water disseminated in the compressed ice can escape through 

 fissures. The ice is then compressed, but not the water 

 which escapes. The compressed ice becomes colder in con- 

 formity with the lowering of its freezing-point by pressure; 

 but the freezing-point of water which is not compressed is 

 not lowered. Thus under these circumstances we have ice 

 colder than o in contact with water at o. The consequence 

 is that around the compressed ice water continually freezes 

 and forms new ice, while on the other hand part of the 

 compressed ice melts. 



This occurs, for instance, when only two pieces of ice are 

 pressed against each other. By the water which freezes at 

 their surfaces of contact they are firmly joined into one 

 coherent piece of ice. With powerful pressure, and the chil- 

 ling therefore great, this is quickly effected; but even with a 

 feeble pressure it takes place, if sufficient time be given. 

 Faraday, who discovered this property, called it the regela- 

 tion of ice; the explanation of this phenomenon has been 

 much controverted ; I have detailed to you that which I con- 

 sider most satisfactory. 



This freezing together of two pieces of ice is very readily 

 effected by pieces of any shape, which must not, however, 

 be at a lower temperature than o% and the experiment sue- 



