732

THEORY AND PRACTICE OF ENGINEERING.

Book II.

lime, 25 of nitre, and 425 pounds of broken plate glass ; this generally, if well managed,produces 1200 pounds weight of plate glass.

On the continent the best mirrors are formed with 300 pounds of white quartz sand,100 pounds of dry carbonate of soda, 43 pounds of lime slacked in the air, and 300 poundsweight of old glass ; ^ per cent, of the weight of soda is added in manganese. The atomicconstitution of glass consists of five atoms of silicic acid, one of oxide of lead, and one ofpotash.

After the materials are thoroughly prepared and refined, they are put into a cistern,the temperature of which is previously raised to that of the glass, and when the cisternis full, it remains for a considerable time in the furnace, until all the air-bubbles aredispersed ; after this operation it is ready for casting. The table for this purpose is formedof a cast-iron plate, supported on pillars of considerable strength ; the metal is then sufferedto flow readily and equally from the furnace into a cistern, which is carried to the table,it being first heated with hot ashes and carefully cleaned.^ The surface of the metal hasthe scum taken off by a copper instrument, and the cistern that holds it is then hoistedand swung by means of a crane over the end of the casting-table, where it is overset,and the metal immediately flow’s equally over it. A copper roller is passed over the fluid,and the surface is thus rendered comparatively smooth ; by means of this roller thenecessary thickness is given to the plate, as it runs upon a fillet placed on the edges ofthe table: after plates arc cast, they are taken to the annealing chamber, where theyremain for twelve or fifteen days, placed in a horizontal position.