GLASS 13 change, commencing in a day or a few days in summer, from greenish or bluish white to a yellowish white or light yellow, a deep and deeper yellow, until it becomes a dark yellow or gold color ; and in some Belgian sheet speci- mens a gradual change, commencing in a few weeks in summer, from brownish yellow to deeper yellow, yellowish pink, pink, dark pink, purple, and deep purple." The following state- ment shows the changes produced in nine differ- ent kinds of window glass by one year's ex- posure to the sun's rays : KIND OF GLASS. Color before exposure. Color after exposure. French white plate . . . German crystal plate . English plate Bluish white. Light green. Yellowish. Bluish tinge. Yellowish green. U It Light purple. Belgian sheet .... Brownish yellow. Deep purple. English sheet American crystal sheet " ordinary sheet. Dark green. Light bluish white. Lighter " " Bluish green. Brownish green. Purplish white. Light yellowish green. No change. The colors named above are given from an ob- servation of the glass edgewise, when a body of color several inches' in depth is seen, where- as the usual thickness of the glass varies from one fourteenth to one quarter of an inch, and shows its color easily only when a white curtain or paper is placed behind it. The partial or en- tire disuse of oxide of manganese in many win- dow-glass manufactories of late years, while it has produced an article not so light in color, has made one more permanent, which the action of sunlight changes but little, if any, in color or shade. Mr. Gaffield's experiments were also extended to showing the comparative power of the different kinds of glass to transmit the ac- tinic rays of the sun. Of colored glasses, blue was found to transmit the most and red and orange the least. The crude materials employ- ed in the manufacture of glass are Selected with more or less care, according to the quality of the articles to be produced. The three princi- pal elements of which crown and sheet glass are composed are silica, soda, and lime. Of these by far the largest element is silica, which is now universally supplied in the form of sand. English crown and sheet glass generally contains about 73 per cent, of silica, and 13 each of soda and lime. On the continent less sulphate is used than in England ; the compo- nent parts of foreign sheet glass may be stated at 74 per cent, of silica, 11 of soda, and 14 of lime. In both cases the remainder consists of alumina and oxide of iron. To the above in- gredients it is generally the custom to add a small quantity of arsenic to assist in oxidizing any carbonaceous impurities and to promote the decomposition of the other materials, and of peroxide of manganese to peroxidize and thus reduce the coloring property of the oxide of iron present. Silica is obtained in the form of quartz sand from sea beaches and from the disintegration of quartzose rocks in the inte- rior. It was in England once procured from flints calcined and ground to powder, whence the name flint glass. The purest and best sand in the world for manufacturing glass is from Lanesborough, Mass., and other portions of Berkshire county. Some of it is exported to Europe, where it is known as the " Berkshire white sand," and there used in making the best qualities of glass. The grains are remark- able for their purity ; in the mass they appear white, but under the microscope each grain is limpid like a clear quartz crystal. Other quali- ties are procured in various parts of the country. Next to the American sand in quality is that obtained from Fontainebleau in France, and much used by the French manufacturers. It is almost entirely free from iron, and is well adapted for the manufacture of white glass. The sand used by the extensive establishment of Chance and co., near Birmingham, England, is from Leighton Buzzard, Bedfordshire. Lime may be used either in the state of quicklime or in limestone of the purest qualities. Common wood ashes have been used to furnish potash, and ashes of sea plants to furnish soda; but these have been replaced by the crude alkalies obtained from them and other sources, and for some purposes refined pearlash is employed. The carbonate of soda is also extensively pre- pared from common salt ; and at Newcastle, Eng- land, black bottles are made from rock salt and sand from the bed of the river, with carbonate of lime of the soap works and the tank waste of the alkali makers. Sulphate of soda, the waste product of many chemical works, is suc- cessfully used, except for plate glass. Although glass can be produced from sand and alkali without any other addition, lime is a very im- portant element, as giving to it hardness and insolubility. In flint glass this ingredient is replaced by lead, which gives greater brilliancy to the glass than lime, but, in consequence of the difference between its specific gravity and that of the other materials, is the cause of in- numerable stria?. Saltpetre and binoxide of manganese and arsenic also are often intro- duced into the mixtures with the view of pro- moting the same object. Alumina and oxide of iron are commonly not intentionally used ; 'they come from the impurities of the other materials. Waste glass, called cullet, forms a considerable proportion of the raw materials in some works; it promotes the fusion and the chemical union of the silica and bases mixed with it, but must be well sorted, so that no qualities be introduced inferior to that intended to be made. In melting glass, the raw mate- rials, thoroughly ground, mixed together, and sifted, are well incorporated with from one quar- ter to one third of their weight of broken glass before being introduced into the melting pots. These are already heated to a white heat in the furnace, and receive only two thirds of a charge at a time, more being added as the first portion melts down. The pot being at last filled with the melted "metal," the heat is raised as rapidly as possible, and the progress
