76 On the Saturation of Salt-Solutions. 



of the constituents forming the double salt takes place. A 

 saturated solution of a double salt is therefore not necessarily 

 saturated to either of its constituents, but may be able to dis- 

 solve more of one or other of the salts. As the amount, however, 

 of this salt increases, there arrives a point at which the solution 

 has become so rich in this salt (B), that any one molecule of 

 the other salt (A) may be regarded as being in contact with 

 a molecule of B. Aggregation or combination to form the 

 double salt (AB) is then possible, and the solution is then satu- 

 rated for the double salt, and crystallization ensues, proceeding 

 pari passu with the addition of B, and resulting in the complete 

 expulsion from the solution of A in cases where the attraction 

 between A and B exceeds the cohesion of either A or B; while, 

 on the other hand, if this be not the case, the expulsion is only 

 partial. This is strongly supported by the definite character 

 and stability of the salts which are completely expelled, and 

 the instability of those which are only partially driven out of 

 solution. The question is, however, complicated by our total 

 ignorance of the nature and amount of the attraction of the 

 various salts for water, which, as I have previously pointed 

 out (loc. cit.), plays an important part in determining the pro- 

 portions in which the various salts dissolve. 



RiidorfY makes the following generalizations from the results 

 of his experiments : — 



Salts divide themselves into three groups according to their 

 behaviour when dissolved in pairs (no double decomposition 

 being possible). 



First. — Salts which form with one another definite (loohl- 

 geformten) double salts. Total expulsion of the double salt 

 by one or other of the constituents. 



Second. — Salts which are isomorphous in pairs, which either 

 form double salts or crystallize together in mixtures. Partial 

 expulsion by excess of either salt. 



Third. — Salts which neither form double salts nor are iso- 

 morphous. The saturated solutions of pairs of these salts are 

 stable and unaffected by excess of either. 



By keeping these three groups of salts fully in view it will 

 be possible to arrive at a satisfactory knowledge of the laws 

 governing the behaviour of salts belonging to the third group, 

 for in this case alone is the problem one relating purely to 

 solution. 



