GLASS is a transparent solid formed by the fusion of officious and alkaline matter. It was known to the Phenicians, and constituted for a long time an exclu sive manufacture of that people, in con sequence of its ingredients, natron, sand, and fuel, abounding upon their coasts. It is probable that the more ancient Egyptians were unacquainted with glass, for we find no mention of it in the writ ings of Moses. But according to Pliny and Strabo, the glass works of Sidon and Alexandria were famous in their times, and produced beautiful articles and which were cut, engraved, gilt, and stained of the most brilliant colors, in imitation of precious stones. The Ro mans employed glass for various pur poses ; and have left specimens in Her culaneum of window-glass, which must have been blown by methods analogous to the modern. The Phenician processes seem to have been learned by the Cru saders, and transferred to Venice in the 13th century, where they were long held secret, and formed a lucrative commer cial monopoly. Soon after the middle of the 17th century, Colbert enriched France with the blown mirror glass manufac ture.
Chance undoubtedly had a principal share in the invention of this curious fabrication, but there were circumstances in the most ancient arts likely to lead to it ; such as the fusing ana vitrifying heats required for the formation of pot tery, and for the extraction of metals from their ores. Pliny ascribes the origin of glass to the following accident. A merchant-ship laden with natron being driven upon the coast at the mouth a the river Belus, in tempestuous weather, the crew were compelled to cook their victuals ashore ; and having placed lumps of the natron upon the sand, as supports to the kettles, found to their surprise masses of transparent stone among the cinders. The sand of this small stream of Galilee, which runs from the foot of Mount Carmel, was in consequence sup posed to possess a peculiar virtue for making glass, and continued for ag.es to be sought after and exported to distant countries for this purpose.
The'researcbes of Berzelius having re moved all doubts concerning the acid character of silica, the general composi tion of glass presents now no difficulty of conception. This substance consists of one or more salts, which are silicates with bases of potash, soda, lime, oxide of iron, alumina, or oxide of lead ; in any of which compounds we can substi tute one of these bases for another, pro vided that one alkaline base be left. Silica in its turn may be replaced by the boracic acid, without causing the glass to lose its principal characters.
Under the title glass arc therefore com prehended various substances fusible at a high temperature, solid at ordinary temperatures, brilliant, generally more or less transparent, and always brittle. The following chemical distribution of glasses has been proposed.
1. Soluble glass ; a simple silicate of potash or soda ; or of both these alka lies.
2. Bohemian or crown glass ; silicate of potash and lime.
3. Common window and mirror glass ; silicate of soda and lime ; sometimes also of potash.
4. Bottle glass ; silicate of soda, lime, alumina, and iron.
5. Ordinary crystal glass ; silicate of potash and lead.
6. Flint glass ; silicate of potash and lead ; richer in lead than the preceding.
7. Strass ; silicate of potash and lead; still richer in lead.
8. Enamel ;. silicate and stannate or antimoniate of potash or soda and lead. The glasses which contain several bases are liable to suffer different changes when they are melted or cooled slowly. The silica is divided among these bases, forming new compounds in definite pro portions, which by crystallizing, separate from each other, so that the general mix ture of the ingredients which constituted glass is destroyed. It becomes then very hard, fibrous, opaque, much less fusible, a better conductor of electricity and of heat ; forming what Reaumur styled de vitrified glass, and what is called after him, Reaumur's porcelain. GLASS-MAKING, GENERAL PRINCI PLES OF. Glass may be defined in tech nical phraseology, to be a transparent homogeneous compound formed by the fusion of silica with oxides of the alka line, earthy, or common metals. It is usually colorless, and then resembles rock crystal, but is occasionally stained by accident or design with colored me tallic oxides. At common temperatures it is hard and brittle, in thick pieces • in thin plates or threads, flexible and elas tic ; sonorous when struck ; fracture conehoidal, and of that peculiar lustre called vitreous ; at a red heat, becoming soft, ductile, and plastic. Besides glass properly so called, other bodies are capa ble of entering into vitreous fusion, as phosphoric acid, boracic acid, arsenic acid ; as also certain metallic oxides, as of lead and antimony, and several chlo rides, some of which are denominated glasses. Impure and opaque vitriform masses are called slags ; such are the productions of blast iron furnaces and many metallurgic operations.