120 ICE AND GLACIERS. 



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. Pres- 

 sure 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 con- 

 fined in a closed vessel from which nothing could escape. The 

 case is somewhat different when, as with glaciers, the water dis- 

 seminated 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 conformity with the lower- 

 ing 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 in contact with 

 water at 0. 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 chilling 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 regelation of ice; the explanation of 

 this phenomenon has been much controverted ; I have detailed 

 to you that which I consider 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 0, and the experiment succeeds best 



