ON THE SOLUBILITY OP SALTS. 293 



elude, both from Cagniard de la Tour's experiments and the theoretical results 

 of Frankenheim and Brunner, that at a red heat water would be completely 

 gasified. If instead of pure water we subject a saline solution to a high tem- 

 perature, what would be the result ? Long before the solution would attain the 

 temperature at which water would pass into gas of the same volume, it seems 

 probable that a large number of salts would become insoluble, owing to the 

 gradual diminution of cohesion between the molecules of water. If this sup- 

 position be correct, the point of maximum solubility of a great many salts 

 cannot be higher than 200° Cent. At very high temperatures water is capable 

 of decomposing a large number of salts, even otherwise very stable double 

 silicates ; but as this action appears to depend in many instances upon the 

 mass of the water, saturated solutions of salts heated under pressure, are not 

 so liable to be decomposed as when salts are exposed to a current of hot 

 steam. If the salt did not become insoluble before the water reached the 

 point of gasification, and that it was capable of resisting that temperature 

 without decomposition, and was not per se volatile at a red heat, we may 

 conclude from the slight affinity between gases and solids, as well as from 

 many other considerations, that the water and salt molecules would completely 

 separate. The singular anomaly which boracic acid offers of being volatile 

 in the vapour of water, a property which the experiments of Larocque* 

 show belongs to many other fixed substances, also indicates that possibly 

 several salts may not precipitate on the passage of the water into gas, but 

 remain attached to the gaseous molecules. Under such conditions of tempera- 

 ture and pressure the most unforeseen phenomena may be presented to us. 



The study of the laws of solubility of salts at very high temperatures is 

 obviously, then, of very considerable importance. In undertaking their 

 investigation I did not underrate the difficulty of the subject, though perhaps 

 I did its extent. The most superficial consideration will at once convince 

 any one that a mere table of the quantities of salt held in solution at different 

 temperatures would be of very little value ; and that, to be of any use, the 

 investigation should include that of the action of salts in solution upon one 

 another at those high temperatures. Further, as the study of the solubility 

 of salts at any given temperature is but a particular case of the general 

 question of solution, every such investigation must necessarily deal more or 

 less with the whole of the phenomena of solution. The problem I proposed to 

 investigate, while apparently limited enough, involves in fact the study of a 

 very considerable branch of the physics of molecules. In so extensive a field 

 of inquiry, and especially where we have to deal with very complicate 

 phenomena, the study of which is beset with practical difficulties, and even 

 danger, the individual investigator cannot hope to reap a very large return 

 for his labour, however successful he may be. Every one who has worked 

 at such subjects will understand that considerable progress must be made in 

 an investigation of this kind before the results admit of being coordinated. 

 Notwithstanding many unexpected interruptions, I have devoted a good deal 

 of time in preliminary experiments upon the best methods of conducting my 

 researches, and in endeavouring to devise apparatus for the purpose. Even 

 though my progress were very rapid, instead of being very slow, as it has 

 been, I should not be in a position to bring before you on this occasion a 

 report of any numerical results which I may have obtained. It is due, how- 

 ever, to the Association to explain in a short preliminary report the point of 

 view from which I am proceeding, and the extent and character of the field 

 of research. As in so extended a subject any results obtained during the 

 inquiry must be communicated piecemeal, such a preliminary report may 

 * Jouin. de Pharm. vol. xiv. p. 315. 



