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 the 

 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 to 

 the third law stated in the introduction, approaches asymptotically the 

 hyperbola N N', whose equation is, 



Let us determine the value of \]/ for any point M of the curve. The 

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

 for constant temperature, 



dRT\x\x\, = vdP. 

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



RT 





V d p. 



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



O M 



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



