The manufacture of _plate_ glass is altogether different from that of crown and sheet. First of all, much greater care is taken in the selection of the materials, the sand used being of a purer kind than that employed in the manufacture of common window glass; the alkali is of a better quality; and more caution is taken in all the manipulative processes prior to the melting of the mixture. Arsenious acid is more frequently used than manganese for the correction of the iron impurity. It has been noticed that in the plate glass-pots, there are grooves placed around their sides, and these are intended to receive metal claspers, by means of which the pot can be removed bodily from the furnace. In former times the glass was made in large pots, and then ladled out into smaller ones, of a square form called _cuvettes_, and in these it was left exposed to the heat of the furnace for a length of time, in order that it might be refined, by the rising of impurities to the surface and by the escape of air bubbles. The use of these cuvettes is now discontinued, and the pot in which the glass is founded is removed from the furnace and its contents poured upon the tables on which the plate is formed, by the action of rollers. A plate glass table is made of iron; its surface is smooth and of the size required to make a large plate, and it is placed upon wheels and run upon a tramway from one part of the glass-house to another, so as to be opposite to the mouth of the furnace from which the glass-pot has to be removed. Along the sides of this table, taken lengthways, moveable strips of iron are placed, rising above it to a sufficient height to secure the desired thickness for the glass plate, and on these strips runs a roller, so adapted that it can be made to pass pretty readily from one end of the table to the other. The contents of the glass-pot, when placed over the table by means of a crane and tilted up, fall out somewhat as a lump of dough would fall from a kneading trough if it were inverted, for it must be borne in mind that the glass in this process is not in a very fluid state. The roller is made to pass rapidly over the softened glass, and in this way spreads it over the table, until it comes in contact with the strips placed along the edge, which serve as gauges for determining the thickness of the plate. After the plate is formed, it immediately sets, and is removed while hot into an annealing furnace, which is always so placed that the glass can be transferred to it from the table with the least possible delay. In this furnace several plates of fresh-made glass are deposited, and are allowed to cool extremely slowly, in order that the glass may be properly annealed. When this process is completed, the plates are removed, the edges are trimmed off with a diamond, and one plate, bedded in plaster of Paris, is placed upon a flat stone receptacle; another plate, also coated on one of its sides with