LEWIS. — THE LAW OF PHYSICO-CHEMICAL CHANGE. 



67 



stance is the same as that of the vapor in equilibrium with it. It is 

 important therefore to know what relation exists in general between thi 

 fugacity of any gas or vapor and its pressure. 



Figure 3. 



If a section of the isothermal of any vapor is plotted on the P V dia- 

 gram (Figure 3) we obtain a curve such as M M', which, according 

 the third law stated in the introduction, approaches asymptotically I 

 hyperbola N N', whose equation is, 



P v = RT. 



Let us determine the value of if/ for any point M of the curve. Tl 

 variation of if/ with P is given by equation (20), which may be written 

 for constant temperature, 



d R T In if/ = v d P. 



Between the two points M and M' we find by integration 



RT 



V J M 



I- d p. 



Now if the lines of constant pressure L M N and L' M' N' are drawn, 

 I v dP is equal to the area M M' L' L, and this is equal to the area 



