CHEMICAL THEORY OF SOLUTIONS. PART 1. 13 



corresponds tolerably closely to that of an ideal solution. For 

 example, Speraîîski (Zeits. physik. Chem., 51, 55 ; 1905) found 

 that the vapour pressure curve of the solid solution of p-dichloro- 

 benzene and p-dibroraobenzene was very nearly a straight line 

 throughout the whole range of his experiment, which extended up 

 to 45% mol dibromobeuzene. The temperature was varied between 

 40.3°C. and 66.1°C. without any noticeable influence on the result. 

 This mixture was studied by Kïister (Zeits. physik. Chem., 51, 

 235 ; 1905) who employed a dynamical method for the measure- 

 ment of the small vapour pressures. He found that the curve of 

 the partial pressure of p-dichlorobenzene is nearly a straight line, 

 while that of p-dibromobenzene is concave towards the axis of com- 

 position and has a maximum. If the data w'ere exact they would 

 be of considerable interest, because they contradict the relation 

 of Duhe^h-Makgules which ought to be universally applicable. 



{b) Isothermal Distillation. 



Fractional distillation is one of the most important operations 

 employed in separating the various components from mixtures. 

 It is daily practised in laboratories and factories, but the exact 

 quantitative theory of the process remains to be developed. This 

 is no doubt due to the lack of insight into the exact relation 

 between temperrture, pressure, and the compositions of the phases, 

 gaseous and liquid. For ideal solutions such insight is given and 

 a part of the problem can be solved with comparative ease. For 

 example, take tlie subject of isothermal distillation. In this case 

 we not only have to determine how the compositions of the 

 distillate and the residue vary with pressure, but we must also 

 calculate the quantity of the distillate. 



