76 Mr. J. E. Trevor on the Mutual 



phenomena of solution. If to a saturated volume of a partially 

 dissociated gas be added one of its gaseous dissociation pro- 

 ducts, as for example NH 3 to dissociated NH 4 C1 vapour, the 

 partial pressures of this constituent are added together and 

 the dissociation is decreased, solid ammonium chloride is pre- 

 cipitated. An indifferent gas, as nitrogen, produces no such 

 effect. In the same way, if one of the dissociated ions be added 

 to the saturated solution of a dissociated salt, the dissociation 

 will decrease and solid salt will be precipitated. For example, 

 in Dr. Nicolas experiment with the chlorides : in KC1 solution 

 a considerable proportion of the salt molecules are dissociated 

 into K and CI ions. CI ions alone cannot be added to the 

 solution ; addition of NaCl, however, adds CI ions, together 

 with an equivalent amount of Na ions, which latter act in- 

 differently, just as the nitrogen in the case of dissociated 

 ammonium chloride. The partial (osmotic) pressure of these 

 CI ions is added to that of those already present and solid 

 salt is precipitated, the solubility of the KC1 is decreased. 



This consideration applies generally to cases of the mutual 

 influence of dissociated bodies with a common ion upon the 

 solubilities of one another, including Dr. Nicol's " Fresh field 

 of investigation " in determining the solubilities of one salt 

 in solutions of various degrees of concentration of another. 

 The requirements of the theory have been shown by Nernst * 

 and Noyes f to hold quantitatively in cases of rather slightly 

 soluble substances, as, for example, a Brom-iso-cinnamic acid 

 with oxanilic acid, AgBr0 3 with AgN0 3 and KBr0 3 , and 

 many more. In the cases of very soluble salts, where the 

 osmotic pressure is measured by hundreds of atmospheres, 

 the question becomes far more complicated, owing to the 

 serious variations from the simple laws of gases, which are 

 manifested at such high pressures in solutions as well as in 

 gases proper. For this reason it is impossible to calculate 

 quantitatively the results in the cases which Dr. Nicol has 

 given, yet without the simple explanation which the above- 

 mentioned recent researches have made clear, the phenomena 

 would be entirely unintelligible, for in the case of any pair 

 of salts it could not be foreseen whether an increase or de- 

 crease of mutual solubility would take place. A continuation 

 of this line cf work has brought a simple explanation of those 

 anomalous cases where the solubility of an electrolyte is in- 

 creased by the addition of a dissociated body with a common 

 ion. A striking example is the strong increase in the solu- 



* Nernst, Zeitschi'.furphys. Chem. iv. p. 372. 

 t Noyes, Zeitschi'.furphys. Chem. vi. p. 241. 



