MINERALOGY. 85 
with an additional quantity of water, forms mortar. The hydrate loses its 
water in the air, passing slowly and by ,degrees into a carbonate again, 
and thus hardens into a mass resembling the original limestone in 
compactness. 
Sulphate of lime is also used for many purposes. In the form of 
karstenite, or anhydrite, it is used as a building material. Anhydrite 
becomes gradually converted into gypsum by the action of the atmosphere, 
gypsum being a combination of water and sulphate of lime. In the native 
mass gypsum is known as alabaster, and on account of its softness may be 
applied to various purposes. 
The vitriols are used in coloring. Green vitriol, with tannic acid, 
furnishes the black color, ink; blue vitriol is used for similar purposes; a 
third salt of sulphuric acid, used in dyeing, is alum. Its value, like that of 
most salts of alumina, consists in its being a good mordant, that is, it forms 
permanent combinations with the fibre of many fabrics; the color being 
firmly combined with the alum, is indissolubly united to the cloth. The 
fabric intended to be dyed may be first steeped in the alum solution, before 
the application of the coloring matter, or both may be applied together. 
The uses of rock salt, or common cooking salt, are well known. Saltpetre, 
as well that of soda as of potassa, is the single substance yielding nitric acid. 
This is distilled in glass retorts, with sulphuric acid, by which the nitric 
acid is expelled and driven over into large glass reservoirs. Nitric acid is 
indispensable in the separation of metals, as gold from silver; its uses in 
other departments of the arts are manifold. Vast quantities of saltpetre are 
used in the manufacture of gunpowder, this consisting of saltpetre, sulphur, 
and charcoal. It is also used to advantage in pickling and curing meat. 
Borax is valuable as a flux for metals, as also for purifymg the metals from 
oxydes. It also furnishes various borates of importance. Borate of lead 
forms the medium by which gold is applied to porcelain in the operation of 
gilding this ware. 
The alkaline sulphates are used both in the arts and in medicine. 
Sulphate of soda, obtained from salt works and sea water, as also in the 
manufacture of soda, constitutes what is known as Glauber Salts. 
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