1908] 



on Ice and its Natural History. 

 Table IV. 



257 



Table II. The temperature, t, at which Pettersson observed the 

 volume to be a maximum, was the chief guide. The volumes 

 obtained by my method are found in line U. I then took my volume 

 and Pettersson's volume for the temperature at or near the maximum 

 of volume, and obtained a factor by which I reduced my volumes at 

 the other temperatures to the same denomination as those of 

 Pettersson. These are given in line B, those of Pettersson being given 

 in line P. It will be seen that the agreement between calculation and 

 observation is very good. 



Cryoscopic Equivalence hetween Pressure and Salinity. — We have 

 seen that the freezing-point of water and melting-point of ice is 

 lowered by increase of pressure and by addition of salt in solution. 

 The effect produced by these agencies is the same in kind. 



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

 below 0° 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 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 depression 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 



YOL. XIX. (No. 102) S 



