GLASS

n. Gla3s. From Sanscr., kelasa through Lat., glades. Gr.,voAos. Lat., vitrum. r.,rerre. Ger., Glas (1st and 2d def's), Britten (3d def.). It., vetro. Sp., vidrio. 1. A hard, brittle, homogeneous, transparent material made by melting sand, etc., and consisting e entially of a metallic silicate with free silicic acid and metallic oxides. 2. A lens, mirror, or other implement having its e ential parts made of that material. 3. In the pi., firVs, spectacles; lenses, prisms, or plane, usually oval sheets of g. mounted to be worn before the eyes for the correction of visual defects or for purposes of protection. Alabaster g. Ger.. Alabasterglas, Opalglas, Reisglas, Rei teiiiglas. An opaque white g., prepared from the common molten compound of crystal g. by suddenly cooling the ma by pouring water over it. [B, 48, 49 (a, 14).] Antimonial g. See Vitrum AN TIM ON I I. Binocular g. An optical instrument for use with both eyes simultaneously. Bohemian g. Ger., bohmisches Glas. A variety of g. consisting e entially of a silicate of pota ium and calcium, made from pure quartz and purified potash. It is very colorle , is fusible with great difficulty, and is little acted upon by chemical reagents. It is used for chemical apparatus, and also combined with flint g. in making achromatic lenses. [B, 3, 270.] Bottle g. A more or le greenish or brownish g. consisting of a silicate of sodium and calcium with varying amounts of alumina and iron oxide, made from common sand, wood-ashes, common salt, etc. It is readily attacked by acids, but is harder and le fusible than crown g. It is used in making bottles. [B, 3, 7.] Burning-g. Ger., Brennglas. A large convex lens used to produce an intense heat by converging the sun's rays to a focus. [L, 56.] Canary g. Ger., Kanarienglas. See Uranium g. Common green g. See Bottle gr. Cover-g. Fr., couvre-objet. Ger., Deckglas. A thin sheet of g. for covering a microscopical specimen on a slide. Crown g. Ger., Kronglas. A g. consisting of a silicate of sodium and calcium, made usually by fusing salt-cake with silicious material. It is harder than Bohemian g., but more fusible and more readily acted upon by acids. It is used in the manufacture of windows and, combined with flint g., in the manufacture of lenses for optical instruments. [B, 3.] Cnpping-g. See CUP (2d def.). Devitrified g. A porcelain-like, opaque substance obtained by heating g. in sand or gypsum. Certain varieties of g. also become clevitrified upon the surface, and hence opaque, by exposure to air and moisture. [B, 3.] Eye-g'es. Fr., lunettes. G'es (3d def.) so mounted as to rest on the nose (usually grasping it by means of a spring) without other support. Cf. SPECTACLES! Field-g. See Field LENS. Flint g. Ger., Flint a las. Syn.: crystal. A silicate of pota ium and lead; more fusible and more readily attacked by chemical agents than other varieties, and also heavier (being of a sp. gr. of about 3-000), more lustrous, and of greater refractive power. It is used in the preparation of optical instruments. [B, 3.] Focusing g. Ger., Einstelllupe. A simple microscope for determining when the image in a photographic or microphotographic camera is sharply focused on the ground-g. The focusing g. is so adjusted that when its mounting is placed against the front or smooth side of the groundg. or focusing screen of the camera an object or real image on the ground side, i. e., in the plane occupied by the film of the sensitive plate, will be in the sharpest focus po ible. fJ-] Franklin g. A bifocal lens: f. e., a lens composed of two portions, an upper and lower, of different focus, such that distant objects can be viewed through the upper portion and near objects through the lower. [B.] Gall g., Gall of g., G. gall. Lat., fel vitri. Ger., Glasgalle. U2, blue; U3, lull; U4, full; U5, urn; U", like U (German). [s. 195]