PLATE GLASS. This is made of the same materials as crown glass, and does not contain lead. Vast quantities of it are now used in photography. Plate glass is made by pouring a quantity of the fused " metal ' upon a table or cuvette of cast iron, and then pass ing a roller over the surface. The plate is then tumealed or al lowed to cool slowly in an oven, or carquaise, along with others. When cold the plate is removed, and carried in an upright position to a part of the manufactory where it is to be roughened down and polished. This is accomplished by fixing one side of the plate with plaster of paris to a horizontal stone table, and another plate to a piece of apparatus above it. The apparatus is then put in motion, and the surface of the upper plate rubbed upon that of the under one, with wet sand between them, by a circum-rotatory motion, at the same time that a peculiar lateral motion is given to the table which supports the lower plate. When the plates are in this way
sufficiently worked on one face, the process is repeated on the other. The plates are next smoothed in the same way by substituting moist emery for moist sand, and the polishing is effected by colcothar ',oxide of iron) applied by rubbers of felt. The fmal polishing is given by women, who rub two plates together with a little moistened putty of tin between them.
Manufacturers of plate glass should be particular not to pack it up, when intended for photography, with printed papers between the sheets, for it has been found that permanent impressions are thus communicated to the glass, which are reproduced upon the photo graph. It has been affirmed that these impressions cannot be re moved by the strongest nitric acid.