Vapours of Mixtures of two Liquids. 45 



large quantity of the latter, liquid iu comparison with the sether, 

 is present, the tension almost corresponds with that of the vapour 

 of alcohol alone at the existing temperature. The same is the 

 case if, instead of alcohol, oil of turpentine, or instead of sether, 

 bisulphuret of carbon be used ; or if alcohol be first introduced 

 into the barometer tube and water added afterwards." 



From these experiments it follows, that the elastic force of the 

 vapours of a mixture of two liquids which mutually dissolve each 

 other is smaller than the sum of the elastic forces of the com- 

 ponent vapours at the same temperature ; and that the elastic 

 force of the vapours of a mixture is dependent on the proportions 

 in which the liquids which produce them are present. 



As regards the deportment of the vapours of a mixture of two 

 liquids which do not mutually dissolve each other, but form 

 separate strata, I also made experiments at the period referred to, 

 and found, in their case, the elastic force of the vapours of 

 the mixture to be equal to the sum of the elastic forces of the 

 vapours of both liquids taken separately at the same temperature. 

 That with such mixtures, therefore, the law of Dalton finds full 

 application. 



In regard to the boiling of such mixtures, I found that two 

 cases are to be distinguished from each other, according as the 

 more volatile liquid constitutes the upper or the under layer. 

 When the more volatile liquid was uppermost, boiling took 

 place within the latter alone, and at the temperature at which 

 the liquid would have boiled under the existing pressure, had it 

 alone been contained in the vessel. If, on the contrary, the 

 more volatile liquid were below, the temperature of the boiling 

 mixture was always somewhat higher than that at which the 

 more volatile liquid would have boiled if alone. The vapours, 

 on the contrary, which developed themselves from this mixture 

 always possessed a temperature, which was not only lower than 

 that of the liquid mixture, but also lower than that at which 

 the more volatile liquid would have boiled under the existing 

 pressure. 



In a mixture, for example, of fresh rectified oil of turpentine 

 and water, which boiled under the pressure of 749-6 millims., 

 the thermometer in the liquid showed a temperature of 102° C, 

 and in the vapour 94 0, 5 C. In like manner, in a mixture of 

 bisulphuret of carbon and water, which boiled under a pressure 

 of 752*2 millims., the thermometer in the liquid stood at 47° C, 

 and that in the vapour at 43°"5 C. 



This difference in the temperatures of the vapour and the liquid 

 is not mentioned by M. Regnault. It is, however, an immediate 

 consequence of the law of l)alton. For the lower liquid, which 

 is the most volatile, must, in order to boil, possess a temperature 



