GYPSUM, a mineral consisting essentially of sulphate of lime and water, the propor tions of its constituents being lime, 32.56; sulphuric acid, 46.51; water, 20.93. It is very widely diffused, occurs in great abundance in many parts of the world, and is found iu rocks and strata geologically very different, as in transition rocks, in secondary and iu tertiary formations. It often occurs iu nests or kidney-shaped masses in clay or marl. It is found above chalk iu many places, and large quantities of it are quarried in some parts of England from the red marl immediately above the great bed of rock-salt. It sometimes occurs in beds many feet thick. It is transparent or opaque, white, yellowish-white or gray, or even yellow, red, brown, or black, according to its purity of chemical composition or the quantity and nature of impurities present. It is also com pact, fibrous, foliated, or earthy; sometimes crystallized in six-sided prisms or in lenses. Twin crystals are frequent. It is easily broken, scratched, and cut. Before the blow pipe, it becomes opaque, if not already so, and fuses into a white enamel. The water which it contains is driven off by a heat of about 272° F., and it is then easily reduced to powder. in which state it is well known as plaster of Paris. Unburned gypsum is tough, and not easily reduced to powder. Gypsum is soluble in cold water, to the extent of about one part in 461, and is a frequent ingredient in the water of springs; it is scarcely more soluble in boiling water or in acids. To this solubility in water, although so slight, must be ascribed the value of gypsum as a manure; the further chemical explanation of which, however, still remains to be ascertained, although theories have been proposed by sir Humphry Davy and by Liebig, the former supposing the gypsum to act chiefly by itself, becoming the nutriment of the crops to which it is most beneficially applied; the latter supposing it to act chiefly by fixing the ammonia of the atmosphere and conveying it to their roots. As a manure, gypsum is more
extensively used in some parts of the continent of Europe and of North America than of Britain. In North America, it is reduced to a fine powder by mills, in order to be used as a manure, for much of its value depends on the fineness of trituration. To clover crops, the application of gypsum is particularly beneficial, and although it does not produce much benefit in its direct application to grain crops, yet in an alternation of wheat and clover, the crop of wheat is larger because of the liberal supply of this mineral manure to the clover. An excess of gypsum, however, is prejudicial, as has been found in some parts of England; where the subsoil containing it in great quantity has been rashly brought up by the plow.—Gypsum, deprived of its water by burning, and reduced to powder, forms a paste which almost immediately sets, or becomes firm and solid, when mixed with its own bulk of water; hence the great use of plaster of Paris for making casts and cornices. But if the gypsum is burned at too great a heat, it refuses to set, and the powder of the mineral called anhydrite, which is an anhydrous sulphate of lime, has not the property of setting.—One of the finest varieties of uncrystallized and untransparent gypsum is alabaster spar is a beautiful fibrous variety of gypsum, exhibiting a fine play of light, and employed for necklaces, inlaid-work, and other ornamental purposes, but having the disadvantage of being easily scratched.