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plaster, white, alabaster, water, compact and near

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GYPSUM, a substance formed by the combination of sulphuric acid with calcareous earth.

Gypsum is found in a compact and crystallized state, as alabaster and selenite, or in the form of a soft chalky stone which in a very moderate heat gives out its water of crystalli zation, and becomes a, very fine white powder, extensively used under the name of plaster-of-paris. This last is the most common, and is found in great masses near Paris, where it forms the hill Alontmartre, near Aix in Provence, and near Burgos in Spain. It is found in smaller portions in various parts of Europe.

Of the different kinds of plaster the coarser sorts are employed, with the admixture of common lime-stone, for cements. The gypsum, which naturally contains carbonate of lime, makes very good cement; but that which has an admixture of clay and sand, affords a cement of an inferior quality.

The kilns in which the plaster-stones are burnt, are gene rally of a very simple construction; often they are built of gypsum itself: The fragments to be calcined are loosely put together, in such a manner as to form a parallelopiped heap, below which are vaulted pipes or flues, for the application of a moderate heat. The calcination must not be carried to excess, since in this case the plaster will be deprived of its quality of forming a solid mass when mixed with a certain portion of water. During the process of calcination, the water of crystallization rises as a white vapour, which, if' the atmosphere be dry, is quickly dissolved in air.

On the river \Volga, in Russia, the burning of gypsum constitutes one of the chief occupations of the peasantry. They calcine all kinds of gypsum promiscuously, on grates made of wood, they then reduce the plaster to powder, pass it through a sieve, and fortn it into small round cakes, which they sell at from one, to one and a half rouble, per thousand.

In order to make use of the plaster, water is added to the powder, which is produced by pounding the calcined f-ag ments; an operation performed either in mills constructed for the purpose, or by the hands of men. This work is exceed

ingly prejudicial to the persons employed in it, whose health is soon impaired by the pernicious effects which the dust of this substance has upon the lungs.

The less the gypsum intended for plaster is mixed with other substances, the better it is qualified for the purpose of making casts, stucco, the si.arry gypsum, or selenite, which of C011 l'Se is the purest of all, is employed for taking impressions from coins and medals; and for those beautiful imitations of marble, granite, and porphyry, that are kno\%n by the name of seagliola, derived from the Italian word, seagli or laminae of selenite ; the latter is vulgarly called talc in Italy and France, and also in England.

The compact gypsum of Kirwan (alabastrite, La Meth. ; albatre gypseux, de Lisle ; (tickler gypstein, Werner) when of a white, or yellowish, or greenish colour, semi-transparent, and capable of receiving a polish, is known among statuaries by the name of alabaster, which term is also retained as a secondary appellation in most books of mineralogy, and is certainly the alabaslrites of Pliny, which is characterized by that author as a stone resembling gypsum. When its colours are disposed in bands or clouds, it is called, in the first case, onyx alabaster, and in the latter, agate alabaster. It not unfrequently contains a sufficient portion of carbonated lime to produce a brisk effervescence with nitrous acid ; and hence has originated the confusion of authors, who make the circum stance of effervescence an essential distinctive character between the gypseous and calcareous alabasters. Its specific gravity seldom exceeds 1.9. Its fracture is compact, splintery, sometimes verging on the fine-grained foliated. In trans parency, it is considerably superior to white wax, allowing light to pass readily through it, but not transmitting the forms of objects.

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