APPLICATION OF EQUILIBRIUM LAW TO SEPARATION OF CRYSTALS. 267 



between b and c Potassium chloride alone separates ; at points between 

 A and c Sodium chloride alone. The point c is that at which alone the 

 two salts mutually saturate the solution, and at which, on further evapo- 

 ration, they separate together in constant proportions. 



Case II. — Whereas in the above case the two salts were considered 

 to be incapable of acting on each other, in general the formation of a 

 double salt is possible. It is to be borne in mind, however, that the 

 double salt is not to be regarded as a distinct substance, and an addi- 

 tional equilibrator is therefore not required. As an example, KCl and 

 MgCl.>.6II.,0 may be taken, which give rise to Carnallite, a double salt of 

 great importance in natural deposits. In such a case a stable system is 

 formed when only the one or the other of the two simple salts coexists with 

 the double salt, except at the transition point ; at all other points, when 

 either is present in excess, it acts on the other, forming a fresh quantity 



Fig. 1. 



CO 



CO 



I 



20 40 60 80 



Molecules KCl 



of double salt. The four determinations of solubility to be made in the 

 case in question are (1) that of KCl, (2) that of MgCl.,.6H.,0, (3) that of 

 Carnallite and KCl, (4) that of Carnallite and MgCl,.6H20.' 



On plotting the values as before, the diagram on page 268 is obtained 

 (fig. 2). In this the line ab represents the manner in which the 

 amount of Potassium chloride present in the saturated solution changes 

 as the amount of Magnesium chloride is increased. At the point b the 

 solution is saturated with Potassium chloride and Carnallite. In the 

 region obc Potassium chloride is no longer present as such, but only as 

 Carnallite, and the slope BC represents the gradual depletion of the solu- 

 tions saturated with Carnallite as the amount of Magnesium chloride 

 in solution increases. At the point c the solution is saturated with 

 Carnallite and Magnesium chloride, the line DC showing the decrease in 

 the amount of Magnesium chloride in the saturated solution as the amount 

 of Potassium chloride present increases, Onljr Magnesium chloride apd 



