Ice and its Natural History 



253 



When water, which holds salt in solution, freezes at a 

 temperature below o C., the salt forms no part of the ice 

 produced. But it is the cause of the lowering of the freezing- 

 point, because, if it is removed, the freezing-point reverts to 

 the normal. The depressed melting-point of the ice depends 

 for its persistence on its continued contact with the saline 

 solution from which it sprang. 



When water, which is subjected to any pressure, freezes 



TABLE IV. 



at a lower temperature than it does in a vacuum, the pressure, 

 being immaterial, cannot form part of the ice. But it is the 

 cause of the lowering of the freezing-point; because, on its 

 removal, the freezing-point reverts to the normal. The de- 

 pression of the freezing-point depends for its persistence on 

 the continuance of the pressure to which the ice is exposed. 

 We can explain its action only by describing it. It acts by 

 influence. This influence, however, is subject to quantitative 

 law which has been well investigated. 



