( 25S ) 
belongs to the type ether-aiithraqivinone with this peculiarity that in 
this system two transition points occur, and the phenomenon of 
retrograde solidification along the melting-point, line is not found in 
the critical neighbourhood, but between the two points of transition. 
The result of the investigation between 62° and 365° is viz. as 
follows. 
Cone. sat. solution 
in mol. ®/ 0 Na., S0 4 
If we now represent these observations in the T, ^-figure, we get 
the lines ERF and FP with a transition point in F. For clearness’ 
sake the critical point has been drawn at a too great concentration, 
because else we could not show that in P the melting-point line 
Passes continuously into the vapour-line PH, which has been assumed 
as coinciding with the axis for H 2 0 below 320°. 
The retrograde character of the melting-point line of rhombic 
and monoclinic Na 2 S0 4 now brings about very peculiar phenomena 
for concentrations lying between those of the points R and F. 
If we e.g. start from a concentration a — 5.39 mol. % Na 2 S0 4 , 
we are at the ordinary temperature in the region for Na s S0 4 10 aq+L; 
tf we raise the temperature, all the solid substance dissolves at 31° 
in point m, and we get into the region for L. 
62 , however, rhombic anhydrical Na 2 S0 4 deposits in n from 
