structure of gypsum employed. Thus, according to him, the fibrous
kinds gives a plaster setting almost instantaneously. The water, he
says, penetrates the material freely, setting takes places almost
simultaneously throughout the mass. The hydration of each particle is
accompanied by an expansion, and under the conditions specified, this
expansion being unresisted takes place to the maximum extent, with the
result of leaving cavities between the crystals, and producing a set
plaster of less coherence and density. On the other hand, where
granular crystalline gypsum has been used, setting begins at the
surface of each group of crystals before the water has penetrated to
the interior; the hydration is in consequence more gradual, and
resistance being offered to the expansion of the inner parts, a harder
and denser material is obtained. That this expansion contains an
element of truth is indicated by the practice of employing the
granular crystalline variety for the preparation of moulding plaster.
The explanation appears, however, to be inadequate in several
respects, especially in view of the fact that plasters for moulding
are reduced to a fine state of division before use. It seems as if
this treatment must, in great part at any rate, break up the
crystalline aggregates.

In order to discover a more satisfactory explanation, let us examine
the results of the chemical analysis of plasters used in commerce. One
is struck by the large percentage of water they usually contain. Thus,
four samples of ordinary plaster analyzed by Landrin have an average
of 90.17 per cent. of CaSO4 and 7.5 per cent. of water, while two
samples of best plaster contained 89.8 per cent. of CaSO4 and 7.93 per
cent. of water. These numbers do not add up to 100, the difference
being due to silica and other impurities of the original gypsum,
amounting altogether to about 3 per cent.