appearance of the rolled surface is hidden or disguised to a greater or lesser extent by the application of a pattern that is impressed upon the glass during the rolling process; thus we have rolled plate having a ribbed or lozenge-patterned surface, or the well-known variety of “figured rolled” plate, sometimes known as “Muranese,” whose elaborate and deeply-imprinted patterns give a very brilliant effect. Rolled plate-glass being practically the roughest and cheapest form of glazing, is principally employed where appearance is not considered, and its chief requirement is, therefore, cheapness, although both the colour and quality of the glass are of importance as affecting the quantity and character of the light which it admits to the building where the glass is used. On the ground of cheapness it will be obvious from what we have said above (Chapter IV.), that such glass can only be produced economically in large tank furnaces, and these are universally used for this purpose. The requirements as regards freedom from enclosed foreign bodies of small size and of enclosed air-bells are not very high in such glass, and, therefore, tanks of very simple form are generally used. No refinements for regulating the temperature of various parts of the furnace in order to ensure perfect fining of the glass are required, and the furnace generally consists simply of an oblong chamber or tank, at one end of which the raw materials are fed in, while the glass is withdrawn by means of ladles from one or two suitable apertures at the other end. For economical working, however, the furnace must be capable of working at a high temperature, because a cheap glass mixture is necessarily somewhat infusible, at all events where colour is considered. This will be obvious if we remember that the fusibility of a glass depends upon its alkali contents, and alkali is the most expensive constituent of such glasses. The actual raw materials used in the production of rolled plate-glass are sand, limestone and salt-cake, with the requisite addition of carbon and of fluxing and purifying materials. The selection of these materials is made with a view to the greatest purity and constancy of composition which is available within the strictly-set limits of price which the low value of the finished product entails. These materials are handled in very large quantities, outputs of from 60 to 150 tons of finished glass per week from a single furnace being by no means uncommon; mechanical means of handling the raw materials and of charging them into the furnace are therefore adopted wherever possible.