in vacuo and in Gases, -^^^^u ^.fvr. 279 



it proceeds by jerks and starts, and the thermometer indicates 

 sudden variations, which sometimes exceed 10° C. We know 

 besides that the temperature of ebullition varies according to the 

 nature of the vessel and the form of its walls. 



The ebullition of a liquid is a very complicated phsenomenon, 

 especially when the liquid is not homogeneous. Molecular forces, 

 of which the nature is still but little known, play an important 

 part in it. It is impossible to get rid of these complex actions, 

 and only take into consideration, in the study of this phsenome- 

 non, the pressure of the external atmosphere and the temperature 

 of the boiling liquid. 



But if it be nearly impossible to deduce any certain results 

 from the determination of the temperature of ebullition of saline , 

 solutions, it is very different with the elastic forces of the va- 

 pours given off by these solutions in vacuo. These may be de- 

 termined with great precision, and I have no doubt that the 

 study of them will hereafter furnish a most valuable means of 

 ascertaining the chemical phsenomena which take place in solu- 

 tions. I have convinced myself, in fact, that as soon as a phse- 

 nomenon of this nature takes place with dissolved substances, it 

 betrays itself by a singular point in the curve of the elastic 

 force of the vapour furnished by the solution. 



I shall give a few examples of the phsenomena which may be 

 studied by this method. 



It is well known that certain salts crystallize from their aqueous 

 solutions with different amounts of water, according to the tem- 

 perature at which crystallization takes place. We may inquire 

 whether this water combines with the salt in the midst of the 

 liquid, or whether the combination only takes place at the mo- 

 ment of crystallization. The sulphates of soda, iron, copper, 

 manganese, &c. present very curious examples of these modifica- 

 tions. 



It will be interesting to compare the variations undergone by 

 the elastic force of the vapour furnished by a saline solution at 

 different temperatures, with the variations in the solubility of 

 the salt under the same circumstances. 



Lastly, when we shall have ascertained the law by which we 

 may calculate the elastic force of the vapour furnished by the 

 mixture, in known proportions of two solutions which exert no 

 chemical action upon one another, from the elastic forces of the 

 vapours given off by the separate solutions, we shall be able to 

 ascertain whether double decompositions take place in the solu- 

 tions themselves, or only at the moment of precipitation. 



These examples, which I might readily multiply, will suf- 

 fice to show that the study of the elastic forces of the vapours 

 given off by solutions, will furnish a most valuable mode of in- 



