Selenites: an earthy or stony concrete; not dissoluble in acids; calcining in a gentle heat into a soft powder †, which forms a tenacious paste with water: composed of calcareous earth and vitriolic acid.

The vitriolic acid, poured on crude calcareous earths, as chalk, limestone, marble, does not dissolve or unite with them, at least in any considerable degree: but if the earth be previ-oufly dissolved in any other acid, the vitriolic acid, superadded to this solution, absorbs the dissolved earth, and forms with it a concrete no longer soluble, which of course renders the liquor milky, and on standing settles to the bottom, either in a powdery or crystalline, form, according as the liquor was less or more diluted with water. Native mineral concretes of this kind, when pellucid and crystalline, are called selenites; when composed of a number of thin transparent coats or leaves, lapis specularis, Muscovy glass, or ifinglafs; when in large stony masses, of a granulated texture, gyspum; and when the masses are of a fibrous texture, striated gypsum or English talc. All these substances are made to discover their composition, by strongly calcining them in contact with the burning fuel: the inflammable principle of the coals absorbs their vitriolic acid, from which combination is produced common sulphur, greatest part of which exhales; and the remaining calcined earth, thus deprived of the acid, is found to be a perfect quicklime.

(a) Burghart, Medicorum Silesiacorum satyrte, specim. IV. bf. ii. p. II.

† Plaster of Paris.

This concrete, in its different forms, has been recommended as an astringent in fluxes and hemorrhagies; a virtue which agrees but ill with its indissolubility and want of taste. It is often met with in the residua of waters, both of the common and medicinal springs.