448 



Frederick Guthrie on Salt Solutions 



tions under which there is equilibrium between : — on the one 

 hand, salt ^ salt (or hydrate of salt ^ hydrate of salt) toge- 

 ther with the salt solution ^ salt solution ; on the other, salt 

 solution /"^ salt or hydrate, 



salt /~n salt + ( iv + - J <~^ I w + : ) = ( w + - ) ^ s. 



Looking upon the cryohydrate as a common value, we have 



ice 



ice + 



-f ice 



(s w\ /$ w\ 



n mj \n m/ 



§ 140. Compare now the decomposition of a salt solution by 

 the loss of heat with the decomposition by gain of heat when 

 such a solution boils. And in instituting this comparison, we 

 must bear in mind how much more sensitive to variation in 

 pressure is the boiling- than the solidifying-point. 



(1) A solution poorer than 

 the cryohydrate loses heat ; ice 

 is formed. 



(2) This goes on until the 

 proportion of the cryohydrate is 

 reached, the temperature falling. 



(3J The cryohydrate may be 

 reached by freezing out ice from 

 a weaker solution, or by any 

 other withdrawal of water. 



(4) When ice separates from 

 a liquid, it remains in contact 

 with the liquid, and endeavours 

 to redissolve therein. 



(o) When by the separation 

 of ice the proportion of the 

 cryohydrate is reached (nearly 

 independent of pressure),ice and 

 the salt separate simultaneously. 



(6) The two bodies (ice and 

 the salt) being crystallizable 

 solids, unite to form a crystalli- 

 zable cryohydrate which exhi- 

 bits a constant gravimetric com- 

 position. 



(7) A cryohydrate in the act 

 of solidification shows identity 

 of composition between the solid 

 and liquid portions. The tem- 

 perature of solidification is con- 

 stant. 



(1) A solution poorer than 

 that saturated at a given tem- 

 perature receives heat; vapour 

 is formed. 



(2) This goes on until satura- 

 tion is reached, the temperature 

 rising. 



(3) Saturation may be reached 

 by evaporation, boiling, or any 

 other withdrawal of water. 



(4) Vapour separated from a 

 liquid is removed from the field 

 of contention, unless the liquid 

 be enclosed with the vapour. 



(5) When by the separation 

 of vapour tha proportion of sa- 

 turation is reached (very depen- 

 dent upon pressure), vapour and 

 the salt separate simultaneously. 



(6) One being a solid and the 

 other a vapour, they do not 

 unite, but in their separation 

 preserve a constant gravimetric 

 ratio under like conditions of 

 pressure. 



(7) A saturated solution,when 

 boiling, shows the same ratio be- 

 tween the vapour formed and 

 the salt precipitated as exists be- 

 tween the liquid water present 

 and the salt it holds in solution. 

 The temperature of boiling is 

 (under like pressure) constant. 



