42 K. D. Kraevitch on an Approximate Law of the 



The French chemist Bertrand * obtained the same equation, 

 but he avows its inapplicability to liquids. Indeed, according 

 to Begnault f, for water 



r = 606-5 -0695*; 



whence 



dr= -0-695 i*. 



On the other hand, taking c=l and ^ = 0*4805 J, we find 



ir= -0-5195 <ft, 



which differs considerably from the above. The same dis- 

 agreement is observed for other liquids. Moreover, for the 

 majority of liquids r is expressed by a trinomial function, and 

 hence the increment of r is not proportional to the increment 

 of temperature, as it should be according to equation (2). 

 But the discordance is completely removed if equation (2) 

 be not referred to vapour at any temperature, but only at 

 that temperature when the vapour is entirely subject to the 

 laws of Boyle and Gay-Lussac. It has been said above that 

 such a temperature does actually exist for aqueous and the 

 majority of investigated vapours, and therefore equation 

 (2) is quite applicable for this and its adjacent temperatures. 

 Let us take the known equation 



A ( v — w )p -~ T = rp . 



Inasmuch as when a vapour nearly approaches the state of 

 a perfect gas (v—w)p> may be taken as equal to DT, therefore 

 it follows from the preceding equation that 



dp 



If it be supposed that c and c x are constants and do not 

 depend upon the temperature, then equation (2) may be 

 integrated : 



r-r = ( c - Cl )(T-T„) (4) 



By T we understand the temperature at which the vapour 

 follows the laws of Boyle and Gay-Lussac ; r the latent 

 heat of vaporization at the temperature T . On sub- 

 stituting r by the quantity equal to it, deduced from the last 



* Thermodynamique par J. Bertrand, 1887, p. 77. 

 t Memoires de V Academie des Sciences^ t. xxi. 

 \ Idem, t. xxvi. 



