122 
GYPSUM. 
which, however, are remarkably soft, so as to be easily scraped with a 
knife, or even with the nail. They are distinguifhed from alkaline 
earths and stones by not effervescing with acids, and from all other 
ſtony bodies, by making ſulphat of potaſh with the mild vegetable 
fixed alkali, if boiled in water with this salt, or melted with it. When 
this vitriolated tartar is separated by repeated waſhing with hot wa 
ter, the remaining powder is found to be a mild calcareous earth. The 
ſame materials also, with the addition of charcoal dust, form, by fusion, a 
bepar sulpburis, or sulpburat of potasb. Thus, the ingredients of gyp 
sum are made sufficiently evident. (See Note 5. at the end of the Volume.) 
Gypsum is found in some places in great abundance. In the 
neighbourhood of Paris there are hills chiefly composed of it. In 
moſt places where it occurs, it is intermixed with a marly clay, form 
ing separate masses interspersed through the stratum: And it is also 
sometimes found in veins. When pure, it is white and semi-transpa 
rent in the larger masses, and perfectly transparent in its small particles. 
The structure or aggregation of these masses has some variety. Often 
ſmall crystalline grains are compacted together like sugar. This is 
called gypsum, and sometimes alabaster. A second kind is the fibraria, 
or fibrous gypsum, having a somewhat fibrous structure, or being com 
posed of oblong crystallized concretions, closely compacted together, 
which are moſtly parallel among themſelves, but lie acrofs the maſs, 
from the upper to the under furface. This fort has varieties according to 
the fize and regularity of the concretions of which it is composed. In 
some kinds, the concretions are larger, and more irregular in dispo 
sition : In others, more flender and regular. A third species is that 
which is composed of clear transparent plates, like the finest glaſs, ly 
ing parallel to one another, and in close cohesion through their whole 
extent, but easily separable by splitting them asunder with a knife. These 
plates are excessively thin, or can be fubdivided, by splitting into very 
thin ones, and have an apparent flexibility, but no elasticity. This species 
is called Glacies Maria, and sometimes, but improperly, Muscovy glass,