Glass, a well known hard, brittle, transparent material. The substances used in making glass vary considerably, but the essentials are three: first, silicon in the shape of pure, clean sand, crushed quartz, or flint; secondly, an alkali in the shape of a soda or a potash; and thirdly, lime in the form of chalk or limestone, or else one of the lead ores. It is not difficult to find materials quite satisfactory for beer bottles, pop bottles, and other articles that may be made of coarse or colored glass, but for clear white glass, it is important to use sand free from iron, lest it give a green or yellow tinge. In the manufacture of window glass, the materials are mixed together in a crucible or furnace, and put over an intense flame, until they are melted together. After a thorough mixing and refining, the liquid is allowed to cool until it becomes pasty. The glassblower then dips one end of a long tube into the pot and gathers a lump of the sticky metal which he proceeds to blow into a huge bubble, just as a boy would blow a soap bubble. By twirling his tube, and by swinging it in a vertical direction like the spoke of a wagon wheel, he draws his bubble out into a large, long, hollow cylinder. If at any time his glass becomes too hard to shape, the workman thrusts it for a moment into a fiery furnace called a "glory hole." In order to have room to swing his pipe the blower stands on a plank laid across a deep pit. When the cylinder has taken the required form it is allowed to cool. The ends are opened and trimmed while yet plastic. The hardened cylinder is then slit along one side with a diamond and placed in an oven, slit uppermost. As the glass heats up the crack widens, and the cylinder opens up, settling finally on the floor of the oven in the shape of a flat sheet of glass. This sheet is then sent through an annealing furnace to harden. It is then laid on a table and cut into window panes of marketable sizes. A glassblower needs to be a man of strong muscles and powerful lungs. He is expected to blow about eighteen pounds of metal into a cylinder about a yard long and a foot in diameter, much larger than an ordinary joint of stovepipe. A skillful blower can form a great variety of glass articles with his pipe and other tools. Heavy tumblers, delicate wine glasses, lamp chimneys, and bottles are entirely within the limits of his art. A great many cheap glass articles are made of glass pressed into molds. An experienced eye can detect the difference readily. In cut glass the general form is given by blowing or pressing. The design is cut by workmen who hold the object against a swiftly revolving wheel of soft steel, copper, or sandstone, the cutting edge of which is fed with emery and water. The polishing is done on a wooden wheel fed with a polishing powder. Plate glass is made in a different way. The liquid glass is poured out on a flat iron casting table as large as the pane is desired to be, and provided with adjustable rims around its edge, which are raised or lowered to give the desired degree of thickness. A long, heavy roller resting on the rim is passed back and forth to prevent any unevenness in the surface. After the glass has cooled and the plate has been annealed in a fiery furnace, a process which seems necessary to prevent glass from snapping apart without apparent excuse, the plate is fastened to a horizontal table and ground off by iron rubbers fed with sand until it has lost nearly half its thickness. The sheet is then polished with leather or felt buffers fed with fine emery powder and putty powder. Plate glass does not have as hard a surface as the blown and annealed window glass; but larger, heavier panes are possible. It is used chiefly for show windows and mirrors. Rough plate, unground, is used for large skylights. The heavy glass used for flooring is also cast in this way. Colored glass is made by the addition of coloring material. The motley dark brown and black glassware lately on the market is made from iron slag. Green is produced by adding copperas or iron filings; yellow, by sulphur, charred horn, or silver; blue, by cobalt or copper; ruby, by copper or gold; milky glass, by tin or zinc. An idle story runs that glass making was discovered by Phoenician sailors who found that the seashore became coated with glass beneath their campfires. Glass articles have been found in the excavations of Nineveh, made perhaps in Tyre and Sidon in the early days of trade by caravan. Glass beads have been found in Egyptian mummy cases, certainly 3,000 years old. Rome acquired the art of glass making from the Egyptians and carried it to Constantinople. Venetian glassblowers were long celebrated, and are noted still, for the beauty and delicacy of their glass work. At the present day Bohemian glass work leads in quality, fineness, and delicacy. Pure glass is not acted upon by ordinary acids and chemicals. For this reason, workers in laboratories use test tubes, retorts, beakers, and thermometers of Bohemian glass almost exclusively. Extensive beds of sand suitable for glass are found near Aix-la-Chapelle, Germany, and Fontainbleau, France. Belgium, England, and Australia have excellent sand for the purpose. At present the glass sands of the United States are found chiefly in Berkshire County, Massachusetts, and in the Juanita Valley of Pennsylvania. Cheap, clean fuel of great heating power, free from smoke and soot, is found in Pennsylvania in the form of gas, oil, and coke, an advantage which has made Pittsburg and vicinity the chief glass-making center of the United States. There are extensive glass works at New Albany and Alexandria, Indiana. Natural gas is used for fuel. The largest gas furnace in the world is said to be in use at the latter city. It accommodates fifty pots or crucibles, and gives employment to 750 men. Mr. Irving Colburn of Franklin, Pennsylvania, has expended $200,000 in perfecting a glass-rolling machine to take the place of the gatherers, blowers, and flatteners. By means of this machine, put into operation in 1908, one man and two boys are able to accomplish the work of thirteen skilled mechanics. The device is 120 feet long. The bystander sees the cooling surface of a molten tank of glass drawn off between rollers. A sheet of glass of the required width and thickness may be seen traveling like a sheet of paper and falling continuously on a table at the rear where it is cut into six-foot lengths on a rotary cutting table. According to the last census the statistics of glass making in the United States are: Glass factories. . . . . . 399 Capital employed. . . . . . . .$89,389,151 Wage earners. . . . . . . .63,969 Annual wages. . . . . . . .$37,288,148 Cost of materials. . . . . . . .$26,145,522 Annual production. . . . . . . .$79,607,998 See MIRRORS; WINDOWS; SAND.