2^6 



THEORY OF SOLUTION AND CRYSTALLISATION. 



temperature of the air is shown by the fact that these same salts 

 are all capable of forming crystalline compounds with water when 

 the temperature of the solution is sufficiently reduced. Take for 

 example a solution of common salt, which, at temperatures above 

 o°, always yields anhydrous crystals. If a strong brine is cooled 

 below 0° crystals of the hydrate NaCl 2OH2 are deposited. This 

 continues till the temperature is — 22° is reached, when the whole 

 liquid sets into a crystalline mass of the " cryohydrate/' 2 NaCl 

 .21 OH2 (or NaCl 10 OH2? Guthrie.) This seems to be the 

 maximum quantity of water the salt is capable of combining with. 

 All salts that have been examined behave in a similar way j and 

 some of those which under ordinary circumstances do not unite 

 with water at all are those which form cryohydrates very rich in 

 water. 



The distinctive character of the cryohydrate is that it crystallizes 

 below zero, and that the entire liquid solidifies. The crystals are 

 not deposited leaving an impoverished mother liquor. The liquid 

 just before solidifying behaves, in fact, like a true chemical 

 compound in a state of fusion. Each cryohydrate has a constant 

 solidifying and melting point, and a distinct crystalline form 

 The following are some examples of cryohydrates taken from 

 Dr. Guthrie's papers : — 



Molecular 

 Formula. 



NH4CI+12OH2. 



Zn SO4 + 20 OH2 

 MgS04 + 24 0H2. 

 KNO3 + 44 OH2. 

 CUSO4 + 44 OH2. 

 Na2S04+i66 OH2. 

 KCIO3+222 OH,. 

 K2Cr207 + 292 OH2. 



1 1. Nor do salts combine only with water. Crystallising from 

 alcohol many are capable of combining with a portion of that 



