APPARATUS. The distillation of the coal is carried on in closed retorts, heated by suitable furnaces. Originally made of east-iron and cir cular in cross-section, these retorts are now made of fire-clay and are oval or fl-shaped. They furnaces by which these groups are heated are of two kinds—direct fire and generator. In the former the carbon of the fuel is burned directly to carbonic acid, while in the latter the com bustion of the carbon is performed in two stages, the first taking place in the furnace proper and forming carbonic oxide, which is burned in the second stage to carbonic acid, this secondary com bustion taking place between the retorts. The use of generator furnaces results in greater economy of fuel and the attaining of a higher temperature in the retorts than is possible with direct-fire furnaces, both of which advantages arc secured to a still greater degree by the use of recuperators, in which the heat of the out going products of combustion is transferred to the incoming air. The retorts are either set horizontally, or, as has been done to a great extent in Europe during the last few years, at an angle of from 29° to 32° to the horizontal. The object of this inclination is to permit the charging of the coal into, and the discharging of the coke from, the retorts to be performed by gravity instead of by manual labor or by ma chinery, as is necessary when they are set horizon tally. The comparative merits and economy of inclined retorts and horizontal retorts charged and drawn by machinery is a matter that is being actively discussed by gas engineers in all countries. In large gas-works coal and coke handling machinery is employed, sometimes to such an extent that the coal is unloaded from the ears or vessels in which it is brought to the gas-works, transported to the retort-house, and charged into the retorts, and the coke drawn, carried to the yard, and stacked or loaded for sale without being touched by hand. Retorts of the size mentioned will take charges of from 250 to 350 pounds of coal, according to the degree to which they are heated.
To the open end of each of the retorts is bolted a cast-iron mouthpiece, of the same cross section as the retort, and from 14 to 16 inches long. On the outer end of the mouthpiece is hinged, so that it can be readily opened and closed, a cast-iron or steel lid, which, when closed, makes a gas-tight joint with the face of the mouthpiece. On the side of the mouth piece a bell is provided, into which is inserted the lower end of the standpipe, or pipe through which the gas passes away from the retort. On the top of the standpipe is a bridge or arch pipe, from which hangs a dip-pipe, which is bolted to the hydraulic main (a large pipe gen erally U-shaped, and made of steel), and passing down into this main dips below the surface of the ammoniacal liquor, with which the hydraulic main is partly filled, and by being thus sealed prevents the return of any gas to the retort when it is open for drawing and charging.
From the hydraulic main the gas passes to the exhauster, a rotary pump driven by a steam engine, employed to relieve the retorts of the pressure thrown by the weight of the gas-holders and the friction encountered b.y the gas as it passes through the apparatus. It then goes into a tar-extractor, in which the gas is subjected to friction and impact for the removal of such of the heavy tar as has not been condensed out in the hydraulic main. After the removal of the tar, which should be effected at a temperature not lower than 100° F., the gas is cooled in the condensers to a temperature of about 50' to 60° F. These condensers may be either at mospheric condensers or water-condensers, the atmospheric condensers employing air, and the concentric steel shells, forming an annular gas space exposed to air on both the inner and outer circumferences, and are used to perform the first part of the cooling, which is completed by the water-condensers, these being somewhat similar in construction to a tubular boiler, the water passing through the tubes in one direction, while the gas passes outside of them in the opposite direction.