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



above, 0*58831, and would be still nearer if a smaller figure 

 were taken instead of 0*4805. 



On making the requisite substitutions in equation (9) we 

 find that 



«/=5106-2, 

 whence 



r =*/.AD = 5106*2. ^=567*36. 



According to Regnault's well-known formula we find that 

 the latent heat of vaporization at 70° ; 



r 70 = 557*6. 



The difference between the values of r and r 70 is less than 

 2 per cent. The accordance between the results of theory 

 and experiment must be regarded as fully satisfactory if we 

 take into consideration the imperfect observation of the 

 requisite conditions and especially the inexactitude of the ex- 

 perimental determinations of the amount of heat. If instead 

 of 70° we take a temperature which is considerably higher or 

 lower, then the results are less accordant ; thus at 125° we find 

 542*8 for the latent heat according to equation (9), while, 

 according to Regnault, r i2 5 = 519*6. The value of c — e 1 will 

 then be 0*3807, which diverges still more from the results of 

 experiment. 



7. We will now consider the second mode of calculating 

 r and c — C\. It follows from equation (3) that 



AD.T 2 ^ 



r= ™. ...... (10) 



P 



This equation gives the possibility of calculating the heat 



of vaporization if it be assumed that the vapour follows 



the laws of Boyle and Gay-Lussac, and that in general the 



above enunciated theory contains no inexactitudes. But in 



order to make this calculation it is necessary to know the 



aY) 

 first differential coefficients of pressure -£, the determination 



of which, however, presents considerable difficulty. If this 

 quantity, at T and p, be taken as equal to the difference be- 

 tween p and the pressure answering to the temperature 

 T — A, divided (i.e. the difference) by A, then we find that 



v^-is too small, while if we take the temperature T + A it is 



too great. Generally the arithmetical mean of these two 



