The following explanation of this connection is offered, an
explanation only tentative at present, owing to want of experimental
data.
The following substances are known:
Gypsum, and set plaster, CaSO4 + 2 H2O, containing 20.93
per cent. of water.
Plaster completely burned at moderate temperature, CaSO4,
probably amorphous.
Anhydrite and dead-burned plaster, CaSO4, crystalline.
Selenitic deposit from boilers, 2 CaSO4 + H2O, or CaSO4 +
1/2 H2O, containing 6.2 per cent. of water.
The circumstance that the hot calcium sulphate can crystallize with 1/4
its normal amount of water indicates that for this proportion of water
it has a greater attraction than for the other 3/4. Having a similar
bearing is the fact that when burned at lower temperatures, gypsum
only loses the last portions of water with extreme slowness.
Now, if it be the case that anhydrous calcium sulphate has a greater
attraction for the first half molecule of water, then the operation of
hydration will proceed very rapidly at first, more slowly afterward.
Many such cases are known, e.g., that of copper sulphate. Conversely,
if only 3/4 of the water of hydration be expelled during the baking of
gypsum, the material obtained should hydrate itself more slowly.
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