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components; this, however, is not the case because between four of 
these substances occurs the reaction: 
Na,SO, + CuCl, 2 CuSO, + Na,Cl,, 
so we are only dealing with a quaternary system in which double 
decomposition occurs. 
In order to represent the equilibria in this system we take a 
quadrangle; its apexes indicate the four substances CuSO,, CuCl,, Na,SO,, 
and Na,Cl,, and in such a manner that the sulphate of the one 
metal is connected by a diagonal with the chloride of the other 
metal. In the intersecting point of the diagonals we take, perpendicularly 
on the quadrangle, the axis on which the water content of the phases 
is shown. 
2. THE ISOTHERM AT 35°. 
The equilibria occurring at 35° are represented schematically in 
fig.1. The sides of the quadrangle have been left out; only a portion 
of the diagonals with their intersecting point W is drawn. Also, 
the representation in space of the equilibria is not drawn but their 
projection on the quadrangle is shown. 
We will first consider the four ternary equilibria. 
a. The system: water — Na,SO, — NaC. 
At 35°, Na,SO, and NaCl only occur as solid phases. The isotherm 
therefore consists only of two branches namely of the saturation line 
of Na,SO, and of that of the NaCl. The first is represented by ak, 
the second by hk. Point a therefore, represents the solubility of the 
Na,SO,, that of the NaCl in water. The intersecting point 4 is 
the solution which is saturated with the two salts simultaneously. 
6. The system: water — NaCl — Cull. 
At 35° occur as solid phases: NaCl and CuCl, .2H,O. The isotherm 
therefore, consists of two branches: hg is the saturation line of the 
NaCl, jg that of the CuCl, .2H,O. Point A, therefore, represents the 
solubility of the NaCl, f that of the CuCl, .2H,O in water: gis the 
solution which is in equilibrium with both salts at the same time. 
c. The system: water — CuSO, — Cull. 
[In this system also, only two solid phases occur at 35° namely: 
CuSO, .5H,O and CuCl, .2H,O. The saturation lines of these salts 
are indicated by de and fe. Solution e is saturated with : 
CuSO, .5H,0 + CuCl, . 2H,O 
