APPLICATION or EQUILIBRIUM LAW TO SEPARATION OF CRYSTALS. 279 



of Na.2Cl.> are measured off above this surface on the axis drawn from the 

 origin at right angles to the plane of the paper. The shaded area in 

 fig. 5 gives a picture of the thickness of the salt sheet above the various 

 fields in relation to the number of molecules of other salts present in the 

 solutions. 



The data required for the construction of a diagram and model repre- 

 senting the behaviour of the solutions under consideration are obtained 

 by determining the composition of solutions saturated (a) with Sodium 

 chloride and one other salt ; (6) with Sodium chloride and two other 

 salts ; and (c) with Sodium chloride and three other salts. To ensure 

 uniformity, as the results only express the constitution of the solutions in 

 terms of the salt radicles, the convention followed consists in expressing 

 the whole of the Sodium as chloride, and if there be not sufficient 

 Chlorine for this purpose the excess is reckoned as sulphate ; the Kj, 

 Mg, Clo, SO4 are expressed as K._;Cl2, MgClj, and MgSOj. The expei-i- 

 mental data which have been accumulated are given in the following table, 

 which includes the vapour pressures of the various saturated solutions. 



as 



In constructing a diagram (fig. 5) and model from these data, 

 there is no axis on which Sodium sulphate can be directly represented, 

 to express the amount of this salt present in the solutions C, F, G, H, R, 

 a line of argument is adopted similar to that made use of in equating 

 Magnesium sulphate with Potassium chloride, in the case of the 

 reciprocal salt pair previously considered. It is obvious that we niBy 

 write 



Na.,SO,=MgSOi-l-NaoCl.,-MgCl, ; 



in other words, Sodium sulphate can be expressed in terms of three other 

 salts, 



