( 248 ) 
mination of the heat of hydration appeared to be the Only accurate 
one, as a direct measurement of the heat effect was not possible on 
account of the slow reaction. 
The point D is a metastable quadruple point, where in metastable 
state the phases Na 2 S0 4 .7 aq, Na 2 S0 4 , L and G are in equilibrium. 
For this transition point (under a pressure of 1 atmosphere), where 
the transformation. 
Na s S0 4 .7aq*tNa*S0 4 + aq 
takes place, we find graphically from the solubility curves of Loewkl 
and Gay-Lussac the temperature 24,4°. 
As may be derived from the subjoined table, a very pronounced 
break was found at 24°,4 in the determination of the three-phase 
line for Na 2 S0 4 7 aq -f- Na 2 S0 4 + G, from which follows that the 
transition point under the vapour pressure also lies at 24°,4. 
Three phase curve. 
If prolonged to this temperature, the three-phase curve for 
Na 2 S0 4 .7 aq -f- L -f- G might have given a confirmation, but for 
this purpose we ought to have taken exactly the concentration 
corresponding to the transition point, 6,15 mol % Na 2 S0 4 , which 
would be nearly always exceeded on account of particular expe¬ 
rimental difficulties, in consequence of which by the side of the 
hydrate with 7 aq the anhydride would always appear, and so the 
