Thermodynamical Theory of Ternary Mixtures. 955 



of C 2 , then the addition of C 2 to a mixture in the neighbour- 

 hood of saturation lowers the vapour pressure. If the 

 addition of C x raises the solubility but not the value of the 

 ratio s 2 /s 1? the addition of C 2 lowers the vapour pressure. If 

 the addition of C^ raises the value of the ratio s 2 /s T , the 

 addition of C 2 raises the vapour pressure. Thus in the case 

 of a moderately concentrated aqueous solution of zinc chlo- 

 ride nearly saturated with zinc oxide, the addition of zinc 

 oxide will raise the vapour pressure of the mixture. In the 

 case of a similar mixture containing a very large proportion 

 of zinc chloride, the addition of zinc oxide will lower the 

 vapour pressure *. 



6. Osmotic Equilibrium. 



There are two distinct cases of osmotic equilibrium, one 

 in which the membrane is permeable to two of the compo- 

 nents, the other in which the membrane is permeable to one 

 component only. We will deal with the former case only 

 very briefly. 



Case I. The membrane permeable to two components. 



Suppose that we have a mixture of C and Cj and a 

 mixture of , Ci, and C 2 in equilibrium on opposite sides of 

 a membrane permeable to C and G 1 . For equilibrium it is 

 necessary that the potentials of C and Ci shall have respec- 

 tively the same values on both sides of the membrane. It 

 is evident that a mere increase of pressure will not in general 

 bring about this double equality, and that a further adjust- 

 ment is necessary, viz. a difference in the relative proportions 

 of C and C x on the two sides of the membrane. Thus equi- 

 librium is brought about by an osmotic difference of pressure 

 and an osmotic difference of composition of the binary 

 solvent mixture. 



This case of equilibrium differs from all the other cases 

 discussed in the present paper in being conditioned by two 

 equations. Any adequate treatment of it would necessarily 

 be somewhat lengthy. We will, therefore, not discuss this, 

 case any further. 



Case II. The membrane permeable to one component only. 



To make the discussion as general as possible, we will 

 assume that the membrane is permeable to the component 

 C t - only. Suppose that C\ in the pure state under a pressure p Q 

 is in osmotic equilibrium with the mixture under a pressure/). 



* See Table II. 



