GLANCE COAL

or anthracite, of which there are two varieties, the slaty and the conchoidal . See Anthracite . GLASS ( Verre , Fr.; Glas , Germ.); is a transparent solid formed by the fusion of siliceous and alkaline matter. It was known to the Phenic i ans, and constituted for a long time an exclusive manufacture of that people, in consequence of its ingredients, natron, sand, and fuel, abounding upon their coasts. It is probable that the more ancient Egyptians were unacquainted with gla , for we find no mention of it in the writings of Moses. But according to Pliny and Strabo, the gla works of Sidon and Alexandria were famous in their times, and produced beautiful articles; which were cut, engraved, gilt, and stained of the most brilliant colours, in imitation of precious stones. The Romans employed gla for various purposes; and have left specimens in Herculaneum of window-gla , which must have been blown by methods analogous to the modern. The Phenic i an proce es seem to have been learned by the Crusaders, and transferred to Venice in the 13th century, where they were long held secret, and formed a lucrative commercial monopoly. Soon after the middle of the 17th century, Colbert enriched France with the blown mirror gla manufacture. 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 and vitrifying heats required for the formation of pottery, and for the extraction of metals from their ores. Pliny ascribes the origin of gla to the following accident. A merchant-ship laden with natron being driven upon the coast at the mouth of 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 ma es 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 supposed to po e a peculiar virtue for making gla , and continued for ages to be sought after and exported to distant countries for this purpose. Agricola, the oldest author who has written technically upon gla , describes furnaces and proce es closely resembling those employed at the present day. Neri, Kunckel, Henckel, Pott, Achard, and some other chemists, have since then composed treatises upon the subject; but Neri, Bosc, Antic, Loysel, and Allut, in the Encyclopédie Méthodique, are the best of the elder authorities. The window-gla manufacture was first begun in England in 1557, in Crutched Friars, London; and fine articles of flint-gla were soon afterwards made in the Savoy House, Strand. In 1635 the art received a great improvement from Sir Robert Mansell, by the use of coal fuel instead of wood. The first sheets of blown gla for looking gla es and coach windows were made in 1673 at Lambeth, by Venetian artisans employed under the patronage of the Duke of Bucking ham. The casting of mirror-plates was commenced in France about the year 1688, by Abraham Thevart; an invention which gave rise soon afterwards to the establishment of the celebrated works of St. Gobin, which continued for nearly a century the sole place where this highly prized object of luxury was well made. In excellence and cheapne , the French mirror-plate has been, however, for some time rivalled by the English. The analysis of modern chemists, which will be detailed in the course of this article, and the light thrown upon the manufacture of gla in general by the accurate means now po e ed of purifying its several ingredients, would have brought the art to the highest state of perfection in this country, but for the vexatious interference and obstructions of our excise laws. The researches of Berzelius having removed all doubts concerning the acid character of silica, the general composition of gla 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 substitute one of these bases for another, provided that one alkaline base be left. Silica in its turn may be replaced by the boracic acid, without causing the gla to lose its principal characters. Under the title gla are therefore comprehended various substances fusible at a high temperature, solid at ordinary temperatures, brilliant, generally more or le transparent, and always brittle. The following chemical distribution of gla es has been proposed. 1. Soluble gla ; a simple silicate of potash or soda; or of both these alkalis. 2. Bohemian or crown gla ; silicate of potash and lime. 3. Common window and mirror gla ; silicate of soda and lime; sometimes also of potash. 4. Bottle gla ; silicate of soda, lime, alumina and iron. 5. Ordinary crystal gla ; silicate of potash and lead. 6. Flint gla ; silicate of potash and lead; richer in lead than the preceding. 7. Stra ; silicate of potash and lead; still richer in lead. 8. Enamel; silicate and stannate or antimoniate of potash or soda, and lead. The gla es 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 proportions, which by crystallizing, separate from each other, so that the general mixture of the ingredients which constituted gla is destroyed. It becomes then very hard, fibrous, opaque, much le fusible, a better conductor of electricity and of heat; forming what Reaumur styled devitrified gla ; and what is called after him, Reaumur’s porcelain. This altered gla can always be produced in a more or le perfect state, by melting the gla and allowing it to cool very slowly; or merely by heating it to the softening pitch, and keeping it at this heat for some time. The proce succeeds best with the most complex vitreous compounds, such as bottle gla ; next with ordinary window gla ; and lastly with gla of potash and lead. This property ought to be kept constantly in view in manufacturing gla . It shows why in making bottles we should fashion them as quickly as po ible with the aid of a mould, and reheat them as seldom as may be absolutely nece ary. If it be often heated and cooled, the gla loses its ductility, becomes refractory, and exhibits a multitude of stony granulations throughout its substance. When coarse gla is worked at the enameller’s lamp, it is apt to change its nature in the same way, if the workman be not quick and expert at his busine . From these facts we perceive the importance of making a careful choice of the gla intended to be worked in considerable ma es, such as the large object gla es of telescopes; as their annealing requires a very slow proce of refrigeration, which is apt to cause devitrified specks and clouds. For such purposes, therefore, no other species of gla is well adapted except that with basis of potash and lead; or that with basis of potash and lime. These two form the best flint gla , and crown gla ; and they should be exclusively employed for the construction of the object gla es of achromatic telescopes.