in vacuo and in Gases. 15 



Part W.-^On the Elastic Force of the Vapours emitted by vola- 

 tile Liquids mixed by mutual solution, or superposed. 



It is generally admitted in works on physics, that a mixture 

 of several volatile, substances which do not combine chemi- 

 cally, evolves complex vapours, of which the total elastic force 

 in a state of saturation is equal to the sum of the tensions which 

 would be produced by each of the liquids separately. This pro- 

 position, moreover, is nothing but a particular case of the general 

 law known as Dalton's law, and which would apply to every mix- 

 ture of volatile fluids, permanent gases, or vapours. This law is 

 said to have been established by Dalton, as the consequence of 

 direct experiments; and M. Biot, in his Traite de Physique 

 (vol. i. p. 308), has described the process employed for this pur- 

 pose. This process is very imperfect, and it is difficult to ima- 

 gine how Dalton can have obtained exact results from it, espe- 

 cially in a question which requires exact measurements, like that 

 at present under consideration. 



Gay-Lussac is also said to have verified Dalton's law upon the 

 elastic forces of mixed vapours, but I have nowhere found traces 

 of the experiments of this celebrated physicist. M. Biot {op. 

 cit. vol. i. p. 297) cites the experiments made by Gay-Lussac to 

 determine the density possessed by the complex vapour, furnished 

 by mixtures of variable proportions of alcohol and water, which 

 were completely vaporized in the same space. Gay-Lussac found 

 that the weight of the complex vapour is exactly the same as if 

 the vapours of the liquids were isolated ; and he concluded from 

 this, that the two substances dissolved in the first instance 

 undergo a molecular separation in acquiring the form of vapour, 

 and that they then behave as though each of them were separate, 

 without any mutual influence. But it must be observed, that 

 in Gay-Lussac's experiments the vapours were not in a state of 

 saturation, for they were at a temperature of 212° F., and under 

 a pressure less than that of the atmosphere ; and besides, they 

 were not in contact with an excess of the liquid from which they 

 had been produced. Gay-Lussac's experiments, as transmitted 

 to us by M. Biot, have consequently no bearing upon the present 

 question. 



It may be said that Dalton's law has been accepted, because 

 it appeared to be a natural consequence of the ideas already 

 formed upon the constitution of the aeriform fluids, and which 

 have been developed by Laplace, Poisson, and other mathema- 

 ticians. Physicists have not thought it necessary to verify this 

 law by direct experiment. 



I have made my experiments only upon mixtures of two liquids 

 and in vacuo. The case of three, or a greater number of liquids 



