CHEMICAL THEORY OF SOLUTIONS. PART I. 



15 



This expression combined with the equation 



j>h_ _ P-P, 



Hl + llo P1 — P2 



suffices to determine tlie values of ni and 712. In the distillate 

 there are Ni—7ii mois of the first compone-nt, and JYo—no mois 

 of the second. In this way the compositions and quantities of 

 the distillate and residue are determined in terms of pressure, 

 and the problem is solved. 



In the ternary system the composition of the residue suffers 

 a definite course of change during the process of isothermal dis- 

 tillation. This can be represented by a curve in the triangular 

 diagram. The nature of this curve w^as first investigated by 

 ScHREiNEMAKERS (Zcits. physik. Cheui., 36, 422 ; 1901) and is 

 called the distillation curve. So long as the temperature is kept 

 constant the composition of the residue can vary only along this 

 curve. How this comes to pass can be briefly explained in the 

 follow^ing manner. 



Let ^ in Fig. 3 represent the composition of the liquid phase 

 and g that of the gas phase g 



in equilibrium with it. As tlie 

 distillation goes on the composi- 

 tion of the residue will be dis- 

 placed a little in the direction 

 of the line (/'g according to the 

 well known theorem of the ter- 

 nary mixture. Hence gg' is the 

 tangent of the curve, and thus S. 

 the direction of the curve is 

 determined. After a certain amount of liquid has been distilled, 

 the liquid phase will have the composition /, while that of the 



Fi< 



