Flint-glass or “crystal” is very heavy, moderately soft (being easily cut with a file), and very bright and white. It is used for all table-glass, as decanters, wine-glasses, &c., and for the drops or lustres of chandeliers. Plate-glass is that kind of glass now in such general use for shop-windows, looking-glasses, &c. It is cast on flat iron tables, and rolled out to the sizes required, then cut and polished by machinery. In the rough state it is called “rough plate,” and is the substance used in the Crystal Palace, and has of late become almost a substitute for ground glass, which is simply ordinary glass ground or roughened on the surface by means of sand, so that it will admit light and yet not allow objects to be seen through it. Crown-glass is the ordinary “window-glass.” It is made in great circular pieces (see “Glass-blowing”), and cut up into the sizes required. Crown or window-glass made some years back, had a disagreeable tinge of green, which has been removed in modern glass by the addition of a minute quantity of oxide of manganese. Illustration: FIG. 1. (‡ OVERHEAD DIAGRAM OF FURNACE.) The ingredients to be made into glass (of whatever kind it may be) are thoroughly mixed together and thrown a little at a time into large crucibles or melting-pots placed in a circle (A A, fig. 1) in a furnace resting on buttresses (B B, fig. 1), and heated to whiteness by means of a fire in the centre, C, blown by a blowing machine, the tube of which is seen at D. This furnace is shown in perspective in fig. 2. The ingredients melt and sink down into a clear fluid, throwing up a scum, which is removed from time to time. This clear glass in the fused state is now kept at a white heat till all air-bubbles have disappeared; the heat is then lowered to a bright redness, when the glass assumes a consistence and ductility suitable to the purposes of the glass blower. Illustration: FIG. 2. (‡ EXTERIOR VIEW OF FURNACE.) Artificial gems are all but varieties of glass. What is called “paste,” “French diamonds,” &c., are glasses of peculiar brilliancy, well cut and polished. Garnets, emeralds, and other precious stones are imitated by coloring the “paste” with various substances, chiefly metallic oxides, as oxide of cobalt, which produces a blue color, oxide of copper a red, and oxide of chromium a green color, &c.