GYPSUM. 
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or Muscovy talc. A fourth appearance of this ſubſtance is in the ſtate 
of separate crystals, in the forms of which there is some variety. 
They are found, I believe, in thoſe ſtrata of clay which contain gyp 
sum in some of the other ſtates already described; and these separate 
cryftals were especially named felenites, by naturalists. Fifthly and 
laſtly, it is very often met with in waters, in a dissolved state; for it 
is evidently capable of difsolution in water, though only in very small 
quantity, like the artificial gypsum, requiring not less than 500 times 
its weight of hot water to dissolve it. But it can be dissolved more 
plentifully, if it be acidulated with some superfluous sulphuric acid. 
It occurs diffolved in the waters of many springs and wells, and is the 
most general taint of what are called hard waters; but most copiously 
in ſea water. When water containing gypsum is flowly evaporated, 
the gypsum separates, or is deposited in a white ſediment, which, by 
the microſcope, is seen to consist of minute crystals like hairs. 
The composition of gypsum was first clearly explained by Mar 
graaf, in the Berlin Tranfactions. He first ſhewed it to be a com 
pound of calcareous earth with vitriolic acid. And by thus knowing 
that it is a ſaline compound, we can more easily understand some of 
its properties. 
When gypsum is exposed to a moderate heat, it loses its transpa 
rency and glittering appearance, and becomes a white opaque mass 
like chalk, in which the former appearance of its structure is de 
ſtroyed. It also becomes very friable, rather more so than chalk, what 
ever may have been its former firmness. During this change, we 
hear a continual crackling. This must be considered as a sort of decre 
pitation. If powdered first, and then heated, it swells, seems as if 
set afloat, and is agitated like a boiling fluid. This is occasioned by 
the water being extricated from its crystals, which are thus de 
stroyed. But after some time, it subsides into a dry powder. In 
close vessels it emits water, and some kinds of it a little sulphurous 
acid. After being heated in this manner, until the bottom of the 
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