Saturation of Salt- Solutions. 75 



that each salt possessed the molecular volume it possesses in 

 a saturated solution of itself alone ; and I concluded from this 

 that each salt dissolved in the solution independently of the 

 other. I also showed that the water was not shared between 

 the two salts equally, nor in the ratio of their solubilities 

 when separate, but that, as a general rule, the more soluble salt 

 had its solubility increased to a greater extent than that of 

 the less soluble salt. At that time I was not in a position 

 to enter more fully into this subject, but a recent paper by 

 RudorfT* has furnished data by means of which I hope to be 

 able to advance one step further towards the solution of this 

 difficult question. 



The results obtained by Rudorff are as follows : — 



(«) Double salts (well defined). — When one or other of the 

 constituents of a double salt is added to a saturated solution 

 of such a salt, it is found that precipitation of the double salt 

 ensues, and if excess be added the whole of the double salt is 

 precipitated, the merest trace only remaining in solution. 



The salts experimented with were : — Ammonium alum, cad- 

 mium ammonium sulphate, nickel ammonium sulphate. 



(/3) Double salts (ill defined?). — By this I mean salts which, 

 being isomorphous, are able to crystallize out from solution, 

 forming mixed salts the constituents of which bear no definite 

 relation to one another, or else crystallize together in definite 

 proportions : the distinguishing character between this class 

 and those in class (a) being their isomorphism. 



In this case also either of the salts is able to expel the other 

 from its saturated solution, prepared by treating a mixture of 

 the two salts with water insufficient to dissolve the whole 

 quantity of either salt present ; but this precipitation is only 

 partial, never complete as in class (a). 



Ammonium alum and ferric ammonium alum : cadmium 

 ammonium sulphate and copper ammonium sulphate; ammo- 

 nium and lithium sulphates; ammonium and lithium chlorides; 

 sodium and cadmium sulphates; sodium and zinc sulphates; po- 

 tassium and silver nitrates ; sodium and silver nitrates; nitrate 

 and sulphate of sodium ; sodium and ammonium sulphates. 



This complete or partial driving out of solution of the one 

 salt by the other is readily explained by the theory of solution 

 proposed by me some years ago. It has been concluded from 

 the experiments of Berthelot and othersf that, in the majority 

 of cases, double salts do not exist, as such, in solution ; but 

 that it is only at the moment of crystallization that the union 



* Wiedemann's Annalen, xxv. p. 626. 

 t Xaumann, ThermocJiemie. p. 3-?-5, 



