Chemical and Physical Notes 35 



salt in snow, and it behaves in a saline solution in exactly the 

 same way as the crystals formed by freezing that solution ; 

 therefore the crystals formed by freezing the saline solution 

 must be equally free from salt, and it has thus been proved 

 that the crystals formed in freezing saline solutions of moderate 

 concentration are pure ice, and that the salt from which they 

 cannot be freed does belong to the adJiering brine. 



Analogy between Snow and Sea-Ice. Snow is the result 

 of the crystallisation of water- vapour dissolved in a gaseous 

 mixture of nitrogen and oxygen. The freezing-point of this 

 gaseous mixture is commonly called the " dew-point " of the 

 air. The freezing of a saline solution is analogous. It is 

 a homogeneous liquid, and the water is as much dissolved in 

 the salt as the salt is in the water. When the solution is 

 sufficiently cooled, ice separates in crystals from the liquid 

 salt just as it did from the gaseous air, and the freezing-point 

 of a solution is in reality a dew-point. The snow derived from 

 these different sources has identical properties ; and when 

 real snow of atmospheric origin is mixed with a saline 

 solution, it is as impossible to free it from salt and to get 

 it to melt at o C. as it is to do so with the crystals formed 

 by freezing a saline solution. 



This matter has been dealt with at some length, not only 

 because it is of importance for the Antarctic explorer, but 

 because it is a matter of the highest importance in chemistry. 

 The whole of that great branch of physical chemistry, which 

 has the distinctive title of cryoscopy, depends on the fact 

 that a saline solution, in freezing, yields pure ice ; yet, in 

 treatises on the subject, no adequate proof of this funda- 

 mental fact is offered. 



CryoJiydrates. When a saline solution has been exposed 

 to continued freezing, it finally acquires a concentration at 

 which any further removal of water in the form of ice causes 

 the precipitation of salt, because at the temperature attained 

 the solution is saturated with the salt. If the cooling is 

 continued the temperature remains constant, while ice and 

 crystals of the salt separate out part passu, and finally the 

 whole solution may solidify to a porcelain-like mass, which 



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