nary heating. When the time comes, this is done very cautiously in a little furnace specially constructed for the purpose. Here the temperature is gradually raised to that of the melting furnace. The transfer from the one to the other is accomplished as rapidly as possible. The interior of the crucible is then glazed with a little molten glass, and the vessel is ready to lend itself to the transformation of the opaque into the transparent. After a variable term of servitude, whose length is totally unpredictable, the crucible finally succumbs to the combined attacks of heat and chemical action, and must be replaced by a fresh one.
When gas is used as the fuel, the melting furnace is a very simple affair. It consists of a plain rectangular floor or hearth, which supports from eight to ten crucibles, two abreast. On each side of the furnace there is a series of round openings giving access to each pot. Arches at the end permit the admission of the fresh crucibles and the removal of the exhausted ones. The chimney is placed in the center, the gas being admitted at each end. The air necessary for combustion is first heated by passing through chambers in the base of the furnace. It will not be necessary to go into any further details of construction, for if one will simply imagine a white-hot apartment, perhaps forty feet long, eight feet wide, and six feet high, with ten crucibles of molten glass standing two abreast on the floor, and half as many openings on each side, he will have a sufficiently vivid picture of the melting furnace of a glass-factory. The batch is introduced into the crucibles in small quantities at a time, and then patiently coaxed into a proper degree of fluidity. When the last portion is added, a decolorizing agent goes with it, for, however pure the crude materials may be, there is always sufficient iron present to give the glass a greenish cast. Arsenic is a favorite bleaching agent. It acts by converting the iron into a higher oxide. In some establishments the peroxide of manganese is used for this purpose, but the least excess gives the glass a pinkish color, and it is also thought to make its transparency less durable.
Style dominates even so apparently an unmodifiable thing as window glass. Some years ago a slight excess of manganese was employed intentionally. It was thought that the mistress of the house—or her daughters—looked the prettier when seen through rose-colored window-panes. This decidedly pink glass may still be seen in not a few of the older houses in our Eastern cities. Its use is occasionally revived by some emergency.
This completes the chemistry of the process; the remaining operations are purely physical.
After the contents of the crucibles have become thoroughly fused, the temperature of the melting furnace is gradually reduced, so that the molten glass shall become less liquid, and thus