142 



On Ice and Brines 



factor is shown by comparing the effects of hydrogen and 

 potassium when united to the common element, chlorine. 

 Hydrochloric acid in solution produces a markedly more 

 powerful lowering effect on the melting-point of ice than the 

 equivalent amount of chloride of potassium. Of all the 

 substances that I have experimented on, hydrochloric acid 

 is the most energetic in reducing the melting-point of ice, 

 and with ordinary strong acid and pounded ice there is no 

 difficulty in producing temperatures as low as the freezing- 

 point of mercury. In the case of hydrochloric acid, sulphuric 

 acid, chloride of sodium, and chloride of calcium, I have 

 carried my experiments to low temperatures and great con- 

 centration. But before passing to them it is well to consider 

 the more dilute solutions with regard to their density. 



That the mere density of the solution in which the ice is 

 melting has no direct connection with the lowering of its 

 melting-point is shown by the following table, in which the 

 specific gravities (at I5C.) are given of the solutions of 

 different salts which gave the same depressions of melting- 

 point : 



There are many similarities in the effects produced by 

 greatly increasing the pressure upon pure water and by 

 dissolving salts in it. First, there is an absolute diminution 

 in the volume of the solution as compared with the sum of 

 the volumes of its components ; second, in virtue of this 

 compression by molecular forces it has become less com- 

 pressible by mechanical means; third, the temperature of 

 maximum density and the freezing temperature are lowered ; 

 and fourth, the former of these two temperatures is lowered 



