Flint glass derives its name from the fact that in England the silica, which is the main constituent of all glass, was procured from flints which were calcined and pulverised. Being highly refractive it is extensively employed in the manufacture of optical instruments--telescopes, microscopes, etc. Quartz and fine sand are now used in the place of flints. The glass is soft, and hence easily scratched and dulled. It is essential that only the purest materials be employed, and special furnaces and pots are needed. Flint glass was known in quite early times. It was probably discovered by accident that certain stones were fusible, for fossil glass is found in many places where great fires have been. Volcanic glass--obsidian--is a well-known substance, while there exist in Scotland ancient forts, the stones of which have been fused together by the action of heat. The Venetians used quartz in preference to sand, since the latter was liable to contain impurities, and the Venetian craftsmen who settled in England were accustomed to ensure the purity of their silica by calcining flints. Crown glass is the finest sort of ordinary window glass. Plate glass is the superior kind of thick glass used for mirrors, shop windows, etc. It will be noted that it is the only kind of glass which contains soda. The process of glass manufacture comprises three stages, mixing, melting, and blowing. The various ingredients are first finely ground and then thoroughly mixed by the aid of a mixer, forming what is known as the “batch.” This is placed in melting pots. These are crucibles of fire-clay, _i.e._ clay capable of withstanding the action of heat. The clay must be of the finest quality, and be carefully freed from extraneous matters which might affect the quality of the glass. Hence the manufacture of the “pots” is itself an industry of some importance, and as each costs some £10, they form an important item in the expense of manufacture, especially as the pots are short-lived, some eight to ten weeks being the average life of one of them. The ordinary pot is an inverted section of a cone, the apex being closed. For flint glass a covered pot is essential, the form ordinarily adopted being a bell-jar closed at the bottom and with an arched opening at the top. Each pot holds from ten to fifteen cwt. of the “batch.” When full, the pots are placed in specially constructed furnaces, holding from five to fifteen pots, and capable of producing a temperature of from 10,000° to 12,000° F. The details of the firing are intricate and interesting but have no direct bearing on our purpose; their object is to produce complete fusion, to allow for the removal of all impurities, and to ensure the homogeneity of the product.