16 M. V. Regnault on the Elastic Forces of Vapours 



in the mixture, would have given too much complication to the 

 question. I have, of course, only operated upon liquids which 

 do not combine chemically by simple mixture. But we must 

 commence by defining exactly what must be understood by sub- 

 stances which react, or do not react chemically upon each other. 

 By substances which react chemically, we must understand those 

 which, when mixed, give rise to a new substance, endowed with 

 properties distinct from those of the primitive substances, and 

 constituted according to the rules of definite proportions. When 

 bodies are mutually soluble, they do not give rise to new com- 

 pounds ; nevertheless, in the phenomenon of solution, particular 

 molecular actions take place, which distinctly modify certain in- 

 dividual physical properties. 



I shall divide the binary mixtures of volatile substances upon 

 which I have operated into three classes : — 



First class. Binary mixtures of volatile substances which are 

 not mutually soluble. 



Second class. Binary mixtures of volatile substances which are 

 mutually soluble in larger or smaller proportions, but which are 

 employed in such quantities, that, after reciprocal saturation, 

 there remain two separate liquids. 



Third class. Binary mixtures of volatile substances which are 

 mutually soluble in all proportions, and therefore always furnish 

 a single fluid. 



The process employed in these experiments is the same as that 

 applied to the determination of the elastic force of the vapours 

 furnished by isolated substances ; the results are therefore per- 

 fectly comparable. 



When the substances exercised little or no dissolving action 

 upon each other, a single capsule was filled with nearly equal 

 volumes of the two liquids, and then hermetically closed by the 

 lamp. To deprive the liquids of air, they were placed separately 

 for a few moments under the receiver of the air-pump, so as to 

 expel the air by ebullition under a low pressure. The capsule 

 was put into the balloon of the apparatus, and the operation was 

 then carried on exactly as in the determination of the elastic 

 force of the vapour of a homogeneous substance. 



In other cases a much larger quantity of the two liquids was 

 introduced into the balloon, and the air was completely expelled 

 from the apparatus by boiling the liquids under a low pressure 

 by the aid of the air-pump. However, these two processes gave 

 the same results, so that they may be employed indifferently. 

 Care was taken also, before making an observation, to keep the 

 bath at a constant temperature for a quarter of an hour, and 

 often longer still. This rendered it certain that a state of equi- 

 librium was obtained. 



