10 



ART. 10. K. IKEDA : STUDIES ON THE 



hence 



?.^ + PiTlnP, + nTlnC\ = z^' ^RTln P+lîTlnC^' 



or C\P, =C,'P=p, (.5) 



in which 2?i denotes the partial pressure of @i in the gas phase. 

 A similar consideration applies to other components. This im- 

 portant relation can be expressed in the folloAving words : 



At a given temperature the partial pressure of a com- 

 ponent in the gas phase is equal to the vapour tension of 

 the component in the pure state multiplied by the molar 

 fraction of the component in tlie liquid pliase. 

 The total pressure P is of course equal to the sum of the 

 partial pressures. 



In a binary system we have 



and P1+P2 = P- 



If we take the molar fraction 

 C[ as abscissa and the pressure 

 as ordinate we get the diagram 

 shown in Fig. 1. As the curves 

 of the partial pressures pi and 

 2h are straight lines, that of the 

 total pressure is also a straight 

 line. 



The fact that the composi- 

 tion-pressure curve is a straight line for a binary ideal solution 

 was first established by F. Guthrie (Phil. Mag., Y, 18, 517; 

 1884). He found that contraction on mixing of two liquids is 

 generally accompanied by the evolution of heat, and expansion 

 of volume by the absorption of heat. He also observed that in 



