Mr. F. Guthrie on Salt Solutions and Attached Water. 11 



Cryohydrates. 



§ 18. General. — The discovery of the hydrate of chloride of 

 sodium which contains about ten molecules of water to one of 

 the anhydrous salt, caused me to look for similar combinations of 

 water with other salts. This was of course all the more neces- 

 sary, since if such combinations existed with the salts occurring 

 in the sea, and if such combinations had solidifying-points within 

 the range of the atmospheric temperature, the composition of 

 the solid formed when sea-water freezes would be partly that 

 of ice and partly that of the solid hydrates formed from the 

 brine which had been enriched by the removal of water as ice. 



§ 19. It has long been known that the presence of a soluble 

 salt in water depresses the point at which the liquid solidifies 

 (irrespective of the nature of the body separated by solidifica- 

 tion). Suppose now we take a solution of the salt a b saturated 

 at the ordinary temperature and cool it. We may, above 0° C, 

 either get a separation of the anhydrous a b, or some crystalline 

 combination of the salt with water, a hydrate. To this Na CI is 

 the only exception ; for this body is equally soluble in water at 

 all temperatures above 0° C. Putting Na CI on one side, we 

 may admit, then, that the mother-liquor gets poorer and poorer 

 as the temperature falls. This is obviously the case if the anhy- 

 drous salt falls down ; and experience informs us that in order 

 to dissolve a hydrated salt, water, and not the anhydrous salt, 

 must be added. Experience also teaches that there is no marked 

 discontinuity at 0° C. We never find the whole of the salt se- 

 parated before or even at 0° C. One patent consequence of this 

 is that we can form a freezing-mixture (whereof a portion is 

 liquid) by mixing any soluble salt with ice. We get, there- 

 fore, in all cases a solution of the salt below 0° C. If as the 

 temperature is still lowered anhydrous salt were to separate 

 out, we should at last get pure water unfrozen below 0° C. — an 

 impossible result. The same would ensue if a hydrate richer in 

 salt than is the solution were to separate out ; whereas if ice or 

 a hydrate poorer in salt than is the solution were to separate out, 

 we should then get at a lower and lower temperature a richer 

 and richer solution, and return to the very condition of strength 

 which the lowering of temperature had altered. This is also 

 an impossibility. Accordingly, in all cases some temperature 

 below 0° C. must be reached at which, after separation of the 

 anhydrous salt or a hydrate richer in salt than the solution, the 

 water and salt solidify together, that is, in constant proportion. 

 The effect of such solidification must be (1) the preservation 

 of a constant temperature during solidification from the moment 

 when the proper proportion between the water and the salt is 



