The fine art of the process seems to be the exact combination of temperature, time, and stirring, suitable for each composition of the glass. There are, too, losses by volatilization during melting, and even afterwards, that must be reckoned with in the proportions of the various materials put into the melting, and in the temperatures reached and maintained. One cannot deduce accurately the percentage mixture of the raw materials from an analysis of the glass, and it is notorious that the product even of the best manufacturers not infrequently fails to run quite true to type. Therefore the optical properties of each melting have carefully to be ascertained, and the product listed either as a very slight variant from its standard type, or as an odd lot, useful, but quite special in properties. Some of these odd meltings in fact have optical peculiarities the regular reproduction of which would be very desirable. The purity of the materials is of the utmost importance in producing high grade glass for optical or other purposes. The silica is usually introduced in the form of the purest of white sand carrying only a few hundredths of one per cent of impurities in the way of iron, alumina and alkali. The ordinary alkalis go in preferably as carbonates, which can be obtained of great purity; although in most commercial glass the soda is used in the form of “salt-cake,” crude sodium sulphate. Calcium, magnesium, and barium generally enter the melt as carbonates, zinc and lead as oxides. Alumina, like iron, is generally an impurity derived from felspar in the sand, but occasionally enters intentionally as pure natural felspar, or as chemically prepared hydrate. A few glasses contain a minute amount of arsenic, generally used in the form of arsenious acid, and still more rarely other elements enter, ordinarily as oxides. Whatever the materials, they are commonly rather fine ground and very thoroughly mixed, preferably by machinery, before going into the furnaces. Glass furnaces are in these days commonly gas fired, and fall into two general classes, those in which the charge is melted in a huge tank above which the gas flames play, and those in which the charge is placed in crucibles or pots open or nearly closed, directly heated by the gas. In the tank furnaces the production is substantially continuous, the active melting taking place at one end, where the materials are introduced, while the clear molten glass flows to the cooler end of the tank or to a cooler compartment, whence it is withdrawn for working. The ordinary method of making optical glass is by a modification of the pot process, each pot being fired separately to permit better regulation of the temperature. The pots themselves are of the purest of fire clay, of moderate capacity, half a ton or so, and arched over to protect the contents from the direct play of the gases, leaving a side opening sufficient for charging and stirring.