or 



THERMODYNAMIC AL SYSTEM OF GIBBS 123 



At At 



mi • -^^ dlogpi +nh • j^ d log pa = 0; 



I.e., 



d log pi _ (WMa^ 

 d log P2~ ~ (mi/Mi(«') 



(132) 



This equation was obtained by Duhem,* and may be used to 

 determine the partial pressures of one component of a binary 

 solution when the partial pressures of the other component are 

 known. 



In many cases, when the concentration of a component 

 in the liquid phase is very small, the ratio of its concentrations 

 in the liquid and gaseous phases is constant at a constant 

 temperature (Henry's law), i.e., 



^2(^)/i;(^) = D (m2(«>A(''0, (133) 



where Z) is a function of the temperature. In such cases, 

 substituting this value of W2^°V«^^*'^ in (129), we have 



At rrh^^^ 



At nh^^'> 



= ^^' + i^;^ log -^- (134) 



Henry's law is not, however, a general law of nature. From a 

 consideration of cases in which it fails it has been shown to be 

 probable that it holds when the molecular weight of the solute is 

 the same in the vapour and in the solution. We may therefore 

 substitute M^*^^^ for M^'^^^ in (134). There is no reason to suppose 

 that the equation so obtained, viz., 



At m2^^^ 



M2 = Ca' + ^17717 log -TJ- (135) 



Compt. rend., 102, 1449, (1886). 



