92 pal:\ikr : 



from the sensible to the potential condition in case of simple 

 solution is surprisingly large. 



During the last decade we have become familiar with the 

 analogy between solutions and gases. A body in dilute solu- 

 tion is found to exert an osmotic pressure equal to the vapor 

 pressure it would exert if gasified into the same space at the 

 same temperature. The amount of energy necessaiy to gasify 

 a pound of ammonium chloride, for instance, is ver>' large. 

 An equal amount of energy is necessaiy to bring a pound of 

 ammonium chloride into a solution of similar volume, temper- 

 ature being allowed for. Now this energy is supplied in large 

 part by the heat of the solvent and the immediate surroundings. 

 Consequently a pound of ammonium chloride dissolving in 

 water will create a very considerable lowering of temperature. 



Freezing mixtures are therefore devised on the principle 

 that heat is absorbed in the separation of the molecules of a 

 solid from each other. It is obvious that this principle applies 

 not only to the solution of a solid in a liquid, but also to the 

 liquefaction of a solid or of a mixture of solids, however 

 brought about. 



Certain considerations will occur to almost anyone in con- 

 nection with this line of thought. 



Firstly, it is obvious that solution, with consequent absorp- 

 tion of heat, must be comparatively rapid if the local cooling- 

 is to be pronounced in degree. A slow process of solution 

 means a slow absorption ; and after a certain degree of slow- 

 ness is obtained, this absorption becomes counterbalanced by 

 the flowing in of energy from the surroundings. This being 

 so, freezing mixtures will be efficient for this purpose, other 

 things being equal, strictly in proportion to their rapidity of 

 ■operation. If they are constituted of salts dissolving in liq- 

 uids, they will be efficient just in proportion to the amount of 

 salt dissolved in a unit of time. The amount of salt dissolved in 

 units of time in a given volume of liquid will obviously be in 

 proportion to the total solubility of the salt in the given liquid. 

 Consequently, the most soluble salts are used in these mixtures. 



