370 Dr. W. W. J. Nicol on the Mutual 



dissolving M'R and MR', but we cannot tell which pair of the 

 four possible salts is present, nor indeed can we decide 

 definitely that all four salts are not formed in the solution. 

 Again, cases where the salt expelled from solution by the 

 addition of another salt is a well-defined double salt must also 

 be excluded : for example, the separation of alum from a 

 solution of aluminium sulphate by the addition of potassium 

 sulphate ; for here we no longer have to deal with only two 

 salts but with three, the third being formed by the union of 

 the original pair, but whether this union has taken place in 

 the solution or only at the moment of separation we cannot 

 tell. This objection also holds good with regard to the well- 

 defined double salts which are more soluble than one or both 

 of their constituents, such as HgCl 2 2NH 4 Cl ; here too there 

 may be three salts present in the solution. In all these 

 instances, in addition to the objections already stated, there is 

 the further fatal one, that the composition of the resulting 

 solution is dependent on the mass of the undissolved salt. 



The subject is therefore restricted to the case of isomorphous 

 salts which crystallize together in any proportion, and to 

 those salts which, though not isomorphous, do not form double 

 salts. As yet I have examined the behaviour only of those 

 salts which do not form mixed crystals. 



Very numerous and careful experiments have already been 

 made on this subject from one standpoint. Kopp, Karsten, 

 Mulder, and Riklorff have determined the amount dissolved of 

 each member of a pair of salts when water is treated with 

 excess of both salts simultaneously. The work of Kopp*, 

 who, so far as I am aware, was the first systematic investi- 

 gator in this branch of solution, for we may pass over the few 

 experiments of Vauquelint, led him to two main conclusions 

 as to the behaviour of salts, when simultaneously dissolved 

 in water to saturation. Kopp found that the salts examined 

 by him fell into one or other of the following classes : — 



1. Salts with the same metal but with different acid radicals. — 

 In this case neither of the salts ever retains the same solubility 

 as in pure water, each salt affects the solubility of the other ; 

 as a rule the amount dissolved of one of the salts is increased, 

 while that of the other is diminished. 



2. Two salts of different metals but the same acid radical. — 

 " Here the rule holds that, provided there is no chemical 

 action of the salts on one another, the salt with the stronger 

 base retains its own special solubility, exactly as if the other 



* Ann. der Chemie und Pharm. vol. xxxiv. (1840). 

 t Ann. de (Mint. xiii. p. 86 (1792). 



