FUNDAMENTAL EQUATIONS OF IDEAL GASES 367 



^H.o = 81, ^H.o = 57 X 10«, 



|3n, = 47.6, ^N: = 1.255 X 10«, 



the units being c.c. per mol and atmospheres. Using the Gibbs- 

 Dalton rule that the total pressure is equal to the sum of the 

 pressures which each of the separate gases would manifest if 

 alone present in the total volume of the mixture we find 



82.06 X 294.3 X 0.02482 82.06 X 294.3 X 0.97516 

 ^ " F + 56.6 "^ 7 + 4.2 



A few trials will be found to give 24144.4 c.c. as the volume 

 for the pressure of one atmosphere. The first term of the right 

 hand side becomes 0.02477 and the second 0.97523. But these 

 terms are the equilibrium pressures according to the Gibbs- 

 Dalton rule and hence the pressure of the water vapor is 18.825 

 mm. The application of the Dalton rule as usually applied 

 (pi = pxi) gives on the other hand 18.866 mm. ; a difference of 

 one part in 460. The actual vapor pressure of the solution is 

 18.820 mm. 



A similar computation may be made using the fugacity 

 function^^'^''''*^'^. In the latter case the equilibrium fugacity, 

 as proposed by Lewis and Randall, is given by the rule /« = fpXi, 

 where fp is the fugacity of the gas of interest at the pressure p of 

 the mixture. 



Finally the equilibrium pressure may be computed using 

 the equation of state constants for the gases of interest and 

 computing the equation of state constants for the mixtures by 

 combination rules for the constants known to hold for mixtures 

 of nitrogen and methane'*^. The latter method has met with 

 success in a number of applications. 



S2. Heat of Evaporation of a Liquid under Constant Pressure. 

 The discussion (Gibbs, I) beginning at the bottom of page 

 161 and continuing to the top of page 163 contains an 

 elegant proof of the impossibility of an uncompensated change in 



ing "gas-current" observations, especially since the procedure has been 

 given in detail recently by H. T. Gerry and L. J. Gillespie (Phys. Rev., 

 40, 269 (1932)) for the case of the vapor pressures of iodine. 



