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



It will be seen from these tables, that the elastic forces ob- 

 served for the mixtures are very nearly equal to the sum of the 

 elastic forces given by the separate substances. With sulphuret 

 of carbon and benzine the tension is constantly a little weaker 

 than the sum of the tensions. The opposite result is presented 

 by the mixture of water and chloride of carbon. I nevertheless 

 regard this latter fact as an anomaly, which I attribute to 

 the circumstance, that, notwithstanding all the care which had 

 been taken, the chloride of carbon was not absolutely pure. I 

 have already said, that a very small quantity of a foreign sub- 

 stance, inappreciable by our ordinary chemical reagents, exerts 

 a very sensible influence upon the elastic force of the vapour. 



From these experiments I think we may conclude, as a theo- 

 retical law, that two volatile substances, which are not capable of 

 mutual solution, give a tension of vajjuur equal to the sum of the 

 tensions presented by these substances separately, although it is 

 probable that this will never be verified absolutely upon natural 

 substances, because there certainly exist no two substances en- 

 tirely deprived of all reciprocal solvent action. 



This is consequently a verification of Dalton's law ; but we 

 shall see from what follows, that this is the only case to which 

 Dalton's law applies. 



