78 On the Mutual Solubility of Salts in Water, 



possible. The salts chosen were the sulphates of potassium 

 and copper, from whose common solution the well-known 

 double salt K 3 Cu(S0 4 ) 2 .6H 2 is easily obtained. That the 

 components were not, at least to an appreciable extent, com- 

 bined to double molecules in the concentrated solution of 

 the double salt, was shown by the possibility of separating 

 them by diffusion ; and the interesting fact was further 

 brought out, that a saturated solution of the two salts together 

 was possible, in which both were present in the approximate 

 ratio of their separate solubilities; this condition of solution 

 was, however, rendered unstable by the tendency of the more 

 insoluble double salt to crystallize out. This result had been 

 already foreseen and theoretically explained by Ostwald *. 

 It was further shown that either salt, added in solid form, 

 will not drive the other entirely out of its saturated solution, 

 as had been maintained by Riidorff, but that in each case a 

 definite state of equilibrium occurs where the solution is not 

 changed by further additions. These equilibria are the same 

 whether the salts be added each to the saturated solution of 

 the other, or directly to that of the double salt. 



By such investigations as the one just quoted, by means of 

 diffusion, and more particularly by the method of determina- 

 tion of the lowering of the freezing-point of solutions, made 

 use of by Le Blanc and Noyes, are the answers brought to the 

 objection of Dr. Nicol that we cannot tell whether the double 

 molecules be formed in the solution or only at the moment 

 of separation of the solid salt. 



To answer this objection it is not even necessary to assume 

 the electrolytic dissociation; the simple fact shown by Riidorff, 

 whose work Dr. Nicol quotes, that from saturated double salt- 

 solutions the constituent salts may be separated by diffusion, 

 demonstrates that the double molecules cannot to an appreci- 

 able extent be present. 



The main question, however, that of reciprocal solubilities, 

 has, as 1 have endeavoured to point out, already been answered. 

 The extension of the theory quantitatively to concentrated 

 solutions of very soluble salts is seriously complicated by the 

 variations from the simple laws of gases which occur with 

 the high osmotic pressures there found, and this extension 

 must come from a study of these variations and not from 

 the building up of interpolation formube to represent solu- 

 bilities. 



University Laboratory of Physical Chemistry, 

 Leipsic, Germany, May 1891. 



* Ostwald, Allgemeine Chemie, 2 Auf. vol. i. pp. 1076-1078. 



