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



reached, and (2) the preservation of that proportion exhibited 

 by the identity of composition of any crop of the solid and the 

 mother- liquor. It is clear that if ice and the hydrated or anhy- 

 drous salt separated out in constant proportion and not com- 

 bined, but merely mixed with one another, the mass would 

 have a constant solidifying- and melting-point ; and this would 

 be below zero to the same amount as would be reached on mix- 

 ing artificially the anhydrous or hydrated salt with ice in the 

 same proportion. But when we have distinct and unchange- 

 able relation by weight demanded by the constancy of the soli- 

 difying- and melting-point, we have undoubtedly a numerical 

 physical relation as fixed and no less important than the points 

 of fusion or degrees of solubilities. And if, as I shall show, all 

 the hydrates formed under these conditions have distinct crys- 

 talline forms, we have all the conditions of chemical associa- 

 tion ; at least I know of none other. It is an essential ele- 

 ment in the existence of these compounds that they can only 

 exist in the solid state below 0° C. Hence I propose to call 

 them for the present " cryohydrates/' At the ordinary tempe- 

 ratures they melt in their own water of crystallization, and 

 appear as ordinary solutions never saturated. And when once 

 the proportion between the salt and the water in a cryohydrate 

 has been found, the cryohydrate can be formed in any quantity 

 by dissolving the salt in water in the required proportion. Such 

 a solution shows no sign of yielding up ice or anhydrous salt (or 

 other hydrate) until its temperature, on being lowered, reaches 

 a certain temperature peculiar to the salt (unless under supersa- 

 turation) ; it then solidifies as a whole, maintaining throughout 

 that constant temperature. Above this temperature (that is, in 

 the melted state) it is precisely in the same predicament as a 

 salt melted in its own water of crystallization. 



§ 20. The same cryohydrates are (and indeed must inevi- 

 tably be) formed if we cool such a solution of the respective salt 

 in water as contains a greater proportion of water than the cryo- 

 hydrate. As we have seen in §§ 10, 13, where the experiments 

 dealt with brine of NaCl, ice is then formed, and the liquid gets 

 richer and richer in salt, falling in temperature till the ratio 

 proper to the cryohydrate is reached ; whereupon, as before, 

 homogeneous solidification ensues without abasement of tempe- 

 rature. If we approach the cryohydrates from this side (that 

 is, by removing ice from a dilute solution), we are sure not to 

 run the risk of being inconvenienced by the intervention of any 

 intermediate hydrate similar to the bihydrate of chloride of 

 sodium (§ 11). But such intermediate hydrates are of the 

 rarest possible occurrence; so that, on account of the high spe- 

 cific and latent heat of water, it is invariably most convenient to 

 start from a saturated solution. 



