32 Frederick Guthrie on Salt- Solutions 



connected with the ease of thermal separation. It is to be 

 noticed, moreover, that in the region of the lower curve about 

 the subcryohydrate, where the curve is flat because great 

 change of percentage makes little difference in temperature 

 of initial solidification, the curvature of the upper curve, fig. 1, 

 " Triethylamine and Water as liquids," is also nearly nothing. 

 Whether above or below zero, we are dealing apparently at 

 every individual percentage with the same substance, the sub- 

 cryohydrate : it is this region which is so keenly sensitive to 

 changes of temperature. 



§ 239. There is this important and essential difference be- 

 tween the genesis of a cryohydrate and that of a subcryohy- 

 drate. Solutions of nearly the cryohydrate strength approach 

 the composition of that body as they lose heat and sink in 

 temperature — weaker ones by the shedding of water (as ice), 

 stronger ones by the shedding of salt or subcryohydrate. 

 And this is true for all solutions from per cent, up to the 

 strength of the subcryohydrate; that is, on the curve, to the 



point where -— changes sign. 



Not so with the subcryohydrate. A solution weaker than 

 the subcryohydrate, to the left of it on the figure, will " run 

 down " to the cryohydrate by dint of shedding the subcryo- 

 hydrate. A solution stronger than the subcryohydrate will 

 " run down " to the solidifying-point of the anhydrous liquid 

 or to that of some secondary cryohydrate, no example of 

 which is, I think, yet known. In either case, the liquid resi- 

 due gets further and further away from the ratio in the sub- 

 cryohydrate. The whole condition may be illustrated by the 

 motion of a marble rolling upon the curve. It will run down 

 to the bottom of the valley of the cryohydrate wherever it is 

 placed on the slopes of that valley, and there remain in stable 

 equilibrium. But placed upon the top of the great swell of 

 the subcryohydrate, it will be at rest only if undisturbed, and 

 so be in instable equilibrium. It will, if disturbed, either roll 

 down to the bottom of the cryohydrate valley, or roll into the 

 abyss of the anhydrous liquid. The cryohydrate is the level 

 of a lake. The subcryohydrate is the watershed (subcryo- 

 hydrate shed) of a hill-range. 



The cryohydrate being an eutectic alloy, the subcryo- 

 hydrate is a definite chemical combination, existing probably 

 as such in the liquid when above zero, and liberating in 

 its formation the heat observed when the two liquids are 

 mixed; and as, for instance, sulphate of soda crystallizing 

 either from an acid or alkaline solution leaves the liquid 

 more acid or more alkaline than before, so the subcryo- 



