92 Mr. W. W. J. Nicol on the 



that I have been able to find; but it fails to explain in a satis- 

 factory way many of the phenomena of solution, amongst 

 others the following: — 



The alteration in the solubility of salts when simultaneously 

 dissolved in water. The experiments of Karsten * and Kopp f 

 show that in very many cases the solubility of a salt is in- 

 creased by the presence of a second salt in the solution, 

 while in others, notably in the case of sparingly soluble salts 

 in the presence of more soluble ones, the solubility of the 

 former is diminished while that of the latter is increased. 



Again, no explanation is offered of the almost total insolu- 

 bility of some salts (e. g. barium sulphate) and the great solu- 

 bility of others, nor why the solubility of some salts should 

 increase but slightly with rise of temperature whereas that of 

 others increases rapidly ; nor does the explanation admit of 

 extension to such cases of solution as that of glue, where there 

 is no saturation-point, but the solid is soluble in all propor- 

 tions in water. 



Lastly, if the experiments of Wiedemann^ on the effect of 

 heat on sodium sulphate in the solid state, and my determina- 

 tions § of the coefficient of expansion of solutions of that salt, 

 be admitted to have proved that the crystallized salt parts with 

 its water of crystallization at 34° C, whether in solution or 

 not, and, as a consequence, the salt dissolves in water above 

 that temperature in the anhydrous state, we are brought face 

 to face with the difficulty that we have explained solution by 

 hydration and increased solubility by diminished hydration, 

 and at the same time have an example of a salt dissolving in 

 the anhydrous condition, and being then less soluble than in 

 the hydrated state. 



The hypothesis I have to offer in place of the foregoing 

 is, I believe, applicable to all cases of solution whether of 

 liquids or solids either in water or other liquids; but the 

 proofs I shall adduce in the present paper are confined to the 

 solution of salts in water. In a paper " On the Condition 

 of Ammonium Salts when dissolved in Water," communicated 

 to the Royal Society of Edinburgh last July, I made the fol- 

 lowing statement : — 



"If we wish to observe the results produced solely by the 

 mutual action of the salt and the water, we must take care 

 that our solutions are sufficiently dilute; as the solution 

 becomes more concentrated the observed specific gravity de- 

 pends, not only on the attraction of the salt for the water, but 

 also on that of the salt-molecules for one another." This 



* Ann. der Chem. und Pharm. xl. p. 197. f Ibid, xxxiv. p. 260. 



\ Annalen der Phys. und Che?nie, 1882, p. 561. 

 § Ber. der deut. chem. Ges. xv. p. 1931. 



