It might be suggested that the reason why these plasters set more
slowly than completely dehydrated plaster is owing simply to the fact
that they contain, apparently, some unaltered gypsum, which serves to
_dilute_ the action. Were this so, a similar result, as far as time of
setting is concerned, should be obtained with a plaster containing a
corresponding quantity of dead-burnt material. This, however, is not
found to be the case. The time of setting appears, then, to be
connected in some special and peculiar manner with the retention of
water by the burnt plaster.

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