of different Vapours. 485 



experimental researches of Avogrado and Faraday to be preferable 

 to the law of Dalton, still the manner in which M. Groshans 

 deduces his equations leaves much to be desired. He premises 

 the deduction by two equations which can only be regarded as 

 approximately correct, inasmuch as they contain the expression 

 of the law of Mariotte and Gay-Lussac for vapours at their 

 maximum density. For the further development, however, he 

 makes use of the following proposition : — If in the case of any 

 two vapours the temperatures are so chosen that the tensions 

 of both are equal, then, if the density of each vapour at the 

 temperature in question be measured by its density at the boiling- 

 point, these densities are equal. This proposition is introduced 

 by the author in the memoir alluded to without any proof what- 

 ever. In a later memoir*, however, he says that he was led to 

 the above conclusion by observing that in the case of seven dif- 

 ferent bodies composed oipQ + qR + rO the density of the va- 

 pour at the boiling-point compared with the density of steam at 

 100° could be expressed by the formula 



^~" 3 ' 



and immediately afterwards he states, that "there are several 

 bodies to which the formula 



3 



is inapplicable.^' From this it appears that the foundation on 

 which the proposition rests cannot be regarded as established. 

 It seems to me, that although the law mentioned above has ob- 

 tained from M. Groshans a more definite form than in Faraday's 

 expression, its probable validity is in no way augmented thereby. 



In this state of uncertainty every new point of view from 

 which a more extended insight as to the deportment of fluids 

 during evaporation may be obtained is deserving of attention ; 

 and hence it will not perhaps be without interest, to establish 

 such a connexion between the above la^ as regards the tension 

 and another law regarding the latent heat, — the latter being also 

 empirically established in a manner totally independent of the 

 former — that the one shall appear to be a necessary consequence 

 of the other. 



I refer to the law, that the latent heat of a unit of volume of 

 vapour developed at the boiling-point is for all fluids the same. 

 Although this has not been completely corroborated by the ex- 

 periments hitherto made, and even if it were perfectly true could 

 not be so corroborated, our knowledge of the volumes of vapours 

 at their maximum density being too scanty, still, an approxima- 



* Pogg, Ann. vol. Ixxviii. p. 292. 



