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4. THE ISOTHERM AT 25°. 
(Investigated with Miss W. C. pr BAAT). 
The equilibria occurring at 25° are represented in fig. 2; these 
are distinguishec from those in fig. 1 by the appearance of a new 
saturation surface, namely that of Na,SO,.10H,O. This surface is 
represented in fig. 2 by abqp; for the sake of brevity Na,SO .10H,O 
is called Z,,. 
First, we must again consider the four ternary equilibria. 
a. The system: water—Na,SO,—NaCl. 
This system is distinguished at 25° from the same system at 35° 
by the fact that the hydrate Na,SO,.10H,O also appears. The 
isotherm, therefore, consists of 3 branches. ap is the saturation line 
of the Na,SO,.10H,O, pk that of the anhydrous Na,SO, and kh 
that of the NaCl. 
The solution saturated with Na,SO,.10H,O + Na,SO, is, therefore, 
represented by p and the solution saturated with Na,SO, + NaCl 
by &. 
b. The quaternary system. 
As will be seen at once from a comparison of figs. 1 and 2, the 
equilibria in the systems water—-NaCl—CuCl,, water—CuSO,—CuCl, 
and water—CuSO,—Na,SO, at 25° and at 35° belong to the same type. 
In the system: water—CuSO,—Na,SO, there is however, one point 
of difference, namely that branch ah indicates, at 35°, the solutions 
saturated with Na,SO, whereas branch ab is, at 25°, the saturation 
line of the Na,SO, . 10 H,O. 
ce. The quaternary system. 
In fig. 2 are found the following saturation surfaces: 
fenmg, the saturation surface of CuCl, . 2 HO 
hymlk, ,, + “ » NaCl 
kiqp, 5 an 5 3» Nais: 
abgp, Re L A » Na,SO,.10H,O 
dene, : D 3 „ CuSO,.5H,O ; 
benmlgb, „ “3 ie » Na,Cu (SO), . 2H,0 
= 
dd 
Proceedings Royal Acad. Amsterdam. Vol. XIII. 
