Hartiey—Wolecular Constitution of Supersaturated Solutions. 58% 
Anhydrous gypsum, or over-burnt plaster, is quite insoluble in 
water, but the mechanism of the crystallization of plaster of Paris 
(CaSO,).,H.O, during the process of setting, is the following. 
The partially dehydrated gypsum becomes at points in its mass, 
Ist, hydrated ; 2ndly, dissolved to supersaturation ; and 3rdly, 
erystallized CaSO,2H,O. When the hydration of a salt is slow, 
the supersaturation, on the contrary, becomes considerable. Anhy- 
drous sodium sulphate sets in contact with water like dried gypsum, 
though it isa much more soluble salt. Coppet! has shown that 
this anhydrous salt, placed in presence of water, gives strongly 
supersaturated solutions, even when care is taken to prevent any 
rise of temperature; and Le Chatelier has confirmed Coppet’s 
observation of the formation of supersaturated solutions both with 
anhydrous sodium sulphate and carbonate. On p. 18, he describes 
a most ingenious experiment, showing how sodium sulphate when 
anhydrous, first dissolves and afterwards recrystallizes to a hard 
mass in presence of water, but at a considerable distance from the 
solid anhydrous salt which becomes hydrated and dissolved before 
it crystallizes. The whole operation was so conducted that no eleva- 
tion of temperature beyond 0°5°C. could take place, and therefore 
heat did not cause the supersaturation of the solution. The con- 
clusion drawn from all the facts quoted by Le Chatelier was, that 
erystallization, which accompanies the setting of all substances 
which harden in water, results from the previous production of a 
supersaturated solution. The hydration and subsequent solution 
of finely powdered manganous sulphate, MnSO,, was observed by 
me in operations conducted on a manufacturing scale. In this 
case it cannot be alleged that there was no rise of temperature, but 
in operations conducted on a smaller scale, the substances being 
immersed in a large excess of water, there was no sensible 
increase, though undoubtedly, when no great excess of water was 
used, the combination of water with the anhydrous salt caused a 
considerable evolution of heat. The same salt mixed with ferric 
oxide, Fe,O;, when immersed in cold water, set so hard that even 
twenty-four hours afterwards it adhered so firmly to a large porce- 
lain basin, the vessel was broken in endeavouring to detach the 
1 Comptes Rendus, \xxti., p. 1324. 
