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. For
our present purpose it will be convenient to recalculate the numbers
given by Landrin (_vide supra_) so as to make the calcium sulphate and
water add up to 100. This treatment of the numbers gives a mean result
for the six analyses of 7.68 per cent. of water, the amounts not
varying by more than 1 per cent.

It will be seen that the dehydration has never passed the composition
corresponding to 2 CaSO4 + H2O; indeed, the material approximates
more nearly to the composition 3 CaSO4 + H2O. It appears probable,
therefore, that in the successful preparation of plaster the whole, or
nearly the whole, of the gypsum is changed, but that this change does
not result in the production of CaSO4, or of a mixture of CaSO4 and
CaSO4 + 2 H2O, but of a lower hydrate of calcium sulphate.

In the case of the analyses, given by Landrin, of fine plaster for
potteries, the percentages of water (8.14 and 8.08) correspond closely
to that of a hydrate, 3 CaSO4 + 2 H2O, which would contain 8.1 per
cent. of water.

Some surprise may have been excited by the fact that the well known