Red oxide of Manganese 87A.0 72A.5 Oxygen 11A.6 9A.8 Sesquioxide of Iron 1A.3 4A.2 Alumina 0A.3 Baryta 1A.2 Lime 0A.3 Silica 0A.8 1A.4 Water 5A.8 1A.6 ----- ---- 108A.3 99A.5 The native binoxide often contains both copper and cobalt in addition to iron; frequently to the amount of as much as 1 per cent. of copper and about A.54 per cent. of cobalt. Wad, a native binoxide of manganese, sometimes contains 54A.34 per cent. of iron, while nearly all the manganese ores contain more or less alumina, varying from A.5 per cent. to as much as 20 per cent. From the composition of ordinary commercial black oxide of manganese, as shown by these analyses, it is evident that it is better to use the pure article, and this has been found to be the case by the firm who have adopted it in lieu of commercial black oxide of manganese. I therefore strongly recommend all glass makers to try and experiment with it, for the results obtained will largely counterbalance the extra cost of the pure material; and I also much doubt whether the same injurious effects will be produced on the pots, as is the case where commercial manganese is employed. Arsenious acid also acts as an oxidizing agent, in that it gives up its oxygen to the protoxide of iron, converting it into the peroxide; but the arsenic itself, which has lost its oxygen, is reduced to the metallic state, and being volatile, does not remain with the glass, but passes off by the flues of the furnace. If too much arsenic is used, it sometimes renders the glass milky or cloudy. Before describing in detail the method of mixing and founding glass, it will be necessary to mention the composition of the vessels in which the glass is made. They are called glass-pots, and differ in shape according to the different kinds of glass to be made in them. Glass-pots are made of fire-clay (generally the best Stourbridge), which is a silicate of alumina, and here great care is taken to select that which contains least lime or iron. It is ground, then moistened and well kneaded together, and left to ripen, while a certain quantity of old glass-pot is ground fine and mixed with the fresh fire-clay. Masses about the size of two hands are kneaded separately, the object being to exclude all air bubbles, and to obtain a perfectly homogeneous lump. The bottom of the glass-pot is then laid, the masses of fire-clay being pressed in with the greatest care, so as to avoid all cracks or places where air might enter during the slow process of drying.