GYPSUM. SuViate of lime, alabaster, plaster of Paris. Plaster. This sub stance is found in three geological situa tions in the crust of the earth : 1st, among the early secondary rocks ; 2d, in the new red sandstone formations, and above the chalk in the tertiary beds. The gypsum of England is found in the new red sandstone, that of France in the ter tiary beds, and that of this country is chiefly in the secondary formation. In the State of New-York are the best de veloped beds of gypsum as yet explored in this country, where they are found in the beds known to correspond with the upper silarian strata of English geolo gists. It extends over the central and western portions of the State in a belt extending from east to west, where it thickens as it advances until it reaches Ontario county, where its greatest purity and development appears to exist. It is quarried extensively in Cayuga, Yates, and Ontario county. The purest varie ties consist of the elements of the crys tallized selenite or alabaster, viz.: Sulphuric acid 40 Lime 28 Water 18 In 86 parts.
But the rock gypsum is never so pure ; it contains aluminous or argillaceous clay, and variable quantities of carbonate of lime and magnesia, soluble salts, and silica. The Editor of this work having occasion to examine the soils and mine rals of Seneca county, N. Y., found the composition of the rock gypsum to be as follows in 100 parts.
Water 6'60 Carbonate of lime Carbonate of magnesia 9.80 Insoluble silicates and sand 39 60 Salts of alkalies soluble in water '40 Sulphate of lime 26-20 100'00 The gypsum of Ontario county is pur er, and is fit for application in the arts, which the above is not. These beds of gypsum occur chiefly in isolated masses, appearing as if they had crystallized out of a plastic moist clay, and are generally surrounded by a gypseous marl made up of the carbonates of lime and magnesia and sulphate of lime. These marls, as well as the above impure gypsum, are admirably adapted for agricultural use as amending manures, and are not used at all to the extent which their value would justify.
The most interesting gypsums in a general point of view are certainly the tertiary, or those of theplains, or hills of comparatively modern formation. They are characterized by the presence of fos sil bones of extinct animals, both mom mifera and birds, by shells, and a large proportion of carbonate of lime, which gives them the property of effervescing with acids, and the title of limestone gypsums. Such are the gypsums of the environs of Paris, as at the heights of Montmartre, which contain crystallized sulphate of lime in many forms, but most commonly the lenticular and lance shaped.
Sulphate of lime occurs either as a dense compound without water, and is called anhydrite from that circumstance, or with combined water, which is its most ordinary state. Of the latter there are six sub-species ; sparry gypsum or selenito in a variety of crystalline forms ; the foliated granular ; the compact • the fibrous ; the scaly foliated ; the earthy. The prevailing color is white, with va rious shades of gray, blue, red, and yel low. More or less translucent. Soft, sectile, yielding to the nail. Specific gravity 2-2. Water dissolves about one five-hundredth part of its weight of gyp stun, when it acquires the quality of hard ness, with the characteristic selenitio taste. When exposed on red hot coals, it decrepitates, becomes white, and splits into a great many brittle plates. At the heat of a bakers oven, or about Fahr., the combined water of gypsum escapes with a species of ebullition. At a higher temperature the particles get indurated. When rightly calcined and pulverized, gypsum is mixed with water to the consistence of cream, and poured into moulds by the manufacturers of stucco ornaments and statues. A spe cies of rapid crystallization ensues, and the thin paste soon acquires a solid con sistence, which is increased by drying the figure in proper stoves. During the consolidation of the plaster, its volume expands into the finest lines of the mould, so as to give a sharp and faithful itnpres8ion.