GLASS (FR., Terre ; GER., Glas).
Glass is an amorphous mixture of various silicates, formed by the fusion of the ingredients. There are, indeed, certain silicates which are themselves commonly known as glasses, and resemble glass in appearance. Such are the potassic tetra-silicate (Fuch's soluble glass), and the plumbic sesqui-silicate. These, however, are deficient in durability, and are useless for those purposes to which glass is generally applied. Silicates are formed by the combination of silica (silicon dioxide) with metallic oxides, in various proportions. Most silicates are fusible. The fusing-point of a mixture of silicates is generally about the mean of the fusing-points of the constituents. Thus the calcic and aluminic silicates are separately fusible only with great difficulty, but in combination fuse readily. Silica (silicon dioxide, symbol molecular weight 60), which unites with metallic oxides to form silicates, and is therefore the principal ingredient of all glasses, is the only known oxide of silicon. Sand is silica mixed with various impurities.
Silicates may be formed directly or indirectly. When sand is heated with the oxide of lead in proper proportions, plumbic silicate is formed : in this case, the silica combines directly with the metallic oxide. If aodic carbonate be substituted for plumbic oxide, odic silicate will be formed by indirect action, for before the silicate can be formed, it is necessary that the carbonic acid, which is in combination with the oxide of sodium in the carbonate, ahall Brat be expelled. That carbonic acid is expelled from its combinations by silica at a high temperature may be proved thus :—A mixture of sodic and potasaic carbonates is melted in a platinum crucible (the mixed carbonates have a lower fusing-point than either constituent) ; the crucible, whilst still hot, is lowered into a tall glass vessel, and some perfectly dry sand is poured into the molten mixture carbonic acid escapes with effervescence, and, being retained in the tall glass vessel, may be detected by its reaction with lime-water. By replacing the sodic or potassic carbonate by calcic, plumbic, or baric carbonate, calcic, plumbic, or baric silicate, may be formed. Again, if, at an
elevated temperature, silica be added to sodic sulphate, sulphuric anhydride will be expelled, and aodic silicate produced. The decomposition of the sulphate of soda by silica is facilitated by mixing the sulphate with of its weight of charcoal ; the sulphate is thus reduced to a lower state of oxidation, and the sulphur escapes as sulphurous anhydride at a lower temperature than is required to decompose the sulphate.
All glasses are mixtures of silicates. Bohemian glass is a mixture of calcic and potassic silicates; sheet-, plate-, or crown-glass, of calcic and sodic silicates ; bottle-glass, of calcic, &KIM, aluminic, and ferric silicates ; flint-glass, of potasaic and plumbic silicates. For experimental purposes, many silicates can be obtained, in absolutely correct chemical proportions, by precipitation from solutions. If these precipitated silicates be dried, and two or more be mixed in proper combining proportions, and the mixtures be fused, pure glasses will be obtained. The aodic and potasaic silicates, formed by the fusion of silica with excess respectively of aodic and potassic carbonate, can be obtained in solution. If to a solution of aodic or potassic silicate, a solution of calcic chloride be added, calcic silicate will be precipitated; similarly, plumbic silicate will be precipitated by the addition of plumbic nitrate. Bohemian glass may therefore experimentally be produced by the fusion, in combining proportions, of precipitated oalcic silicate with potasaic silicate ; plate-glass, by the fusion of precipitated calcic silicate with sodic silicate ; and flint-glass, by the fusion of plumbic silicate with potassic silicate. The following formulae may be taken as approximately representing the composition of the various glasses which will be considered in this article :—Bohemian, sheet-, plate-, and crown-glass = (or CaO, ; lead- or flint-glass = PbO, ; optical lead-glaaa = PbO, optical crown-glass = CaO, bottle-glass = 270a0, 455i02.