PROCEEDINGS OP THE AMERICAN ACADEMT. 



The agreement is satisfactory and within the limits ( .f experimental 

 error. 



l'. '/'//«■ Vapor Premire Curve. — The change with changing temper- 

 ature of the equilibrium between :i vapor and its liquid or solid phi 

 maybe obtained from equatiou r-' 11 ': for if we consider the vapor to 

 obey with sufficient exactness th<- laws of perfei . the system under 



consideration is one for which equation (20) \\:i- developed, Bince it con- 

 tains only a gaseous phase and a condensed phase of definite composition. 



'/in K V 

 From equation (20), ^ ,, ^ / . 



In K = In v — In ! ■'. 



where v is the molecular volume of the vapor, >•' that <>f the solid or 



,• • , '/In '•' . . , ..... ... '/ In '• . 



liquid. , „ is entirely negligible compared with — --,., , and equa- 



tion (20 1 heroines ,/],, ,. jj 



d T '' ~~ R 7 ,J ' 



RT 



Since v = , where j> is t lie vapor pressure, 



V 



In r = — In p -f- In Ji + hi 1\ 



and dlu v = — d\u j> + T. 



„ , . . dlnp U 1 -U+ RT 

 Substituting, j-^ = — -^yo + -y,= ^-ya — • 



As on page 22, let -U+ R T= L, 



then 



'/In /> _ L 



d T R '/'- ' 



which is the familiar expression for the vapor pressure curve. 



A c pleir expression for the vapor pressure in the case of liquids 



to which equation (36) applies, if the vapor approximates a perfect gas, 

 may be derived as follows : 



l ^_ JL 



<■' -b'~ R T' 



R T 

 Substitute - - for v. and 



V 



R T L R T ( ±\ 



^-fiT^b'rRr or /M/-i'; =e ' 



R T 

 whence p = 



(y' - V) «(&) 



