GLASS, a substance too well.known to admit of a definition. It is a compound of the fixed alkalies, or alkaline earths, with silica, brought into complete fusion, and then suddenly congealed. Silica, when mixed with the fixed alkalies, and expo sed to a strong heat, readily enters into fusion. In this state the mixture may be moulded into any shape, and if suddenly cooled below the temperature at which it assumes the solid state, it retains the transparency, and those peculiar proper ties that belong to the substance called glass. Metallic oxides are sometimes ad ded, as well to assist in the fusion, as to communicate certain colours to the mass. If the melted glass be suffered to cool very slowly, the different tendency of the constituent parts to assume solid forms, at certain temperatures, will cause them to separate successively in crystals, as salts held in solution in water assume the form of crystals as the liquid is slowly evapo rated. But if the glass be suddenly cool ed down to the point of congelation, the constituents have not time to separate in succession, and the glass remains the same homogenous compound as while in a state of fusion. Hence it should seem that the vitreous quality depends entirely, 1. upon the fusibility of the mixture, and 2. on the suddenness with which it is cool ed down to the point of congela,tion. It was discovered by Sir Jamei IIall, that glass always loses its vitreous state, and assumes that of a stone, if more dian a minute or two elapses while it is cooling down from complete fusion to the point at which it congeals.
There are several kinds of glass adapt ed to different uses. The best and most beautiful are the flint and the plate glass. These, when well made, are perfectly transparent and colourless, heavy and brilliant. They are composed of fixed alkali, pure siliceous sand, calcined flints, and litharge in different proportions. The flint glass contains a large quantity of oxide of lead, which by certain processes is easily separated. The plate glass is poured in the melted state upon a table covered with copper. The plate is cast
half an inch thick, or more, and is ground down to a proper degree of thinness, and then polished.
Crown-glass, that used for windows, is made without lead, chiefly of fixed alkali fused with siliceous sand, to which is add ed some black oxide of manganese, which is apt to give the glass a tinge of purple.
Bottle-glass is the coarsest and cheapest kind ; into this little or no fixed alkali en ters the composition. It consists of an al kaline earth combined with alumina and silica. In England it is composed of sand and the refuse of the soap-boiler, which consists of the lime employed in render ing his alkali caustic, and of the earthy matters with which the alkali was conta minated. The most fusible is flint-glass, and the least fusible is bottle-glass.
Flint-glass melts at the temperature of 10° Wedgewood ; crown glass at 30° ; and bottle-glass at 47°. The specific gra vity varies between 2.48 and 3.33.
Good glass is perfectly transparent, and when cold very brittle, but at a red heat it is one of the most ductile bodies known, and may be drawn into threads so very delicate, as to become almost invisible to. the human eye. It is extremely elastic, and one of the most sonorous of bodies. See ItkintoNicA.
There are but few chemical agents which have any action upon it. Mr. Davy, in one of his lectures delivered in May, 1808, exhibited a method of decompo sing it by means of the Voltaic battery : he, however, first reduced it to powder. Fluoric acid, as we have seen,has a great power over it, and dissolves it very quick ly (see FLuoacc Acm) : so also have the fixed alkalies, when assisted by heat. The continued action of hot water is said to be capable of decom posing gla ss, which it is thought will fully explain how the siliceous earth was obtained by Boyle and others, when they subjected water to very tedious distillations in glass vessels. It has also been supposed, that the defla gration ofthe oxygen and hydrogen gases, in the formation of water, has decomposed the glass, which will account for an acid as part of the result.