These retorts are iron cylinders, 2 or 3 feet in diameter, and 6 feet long. [Figs. 954.] and [955.] show the form of the furnace, and the manner in which the cylinders are arranged; the first being a longitudinal, the second a transverse section of it. A, the ash-pits under the grates; B, the fireplaces, arched over at top; C, the vault or bench of fire-bricks, perforated inside with eight flues for distributing the flame; D, a great arch, with a triple voussoir D, d′, d′′, under which the retorts are set. The first arch D, is perforated with twenty vent-holes; the second, with four vent-holes; through which the flame passes to the third arch, and thence to the common chimney-stalk. The retorts e, are shut by the door e′ ([fig. 955.]), luted, and made fast with screw-bolts. Their other ends e′′ terminate in tubes f, f, f, which all enter the main pipe h. The condensing pipe proceeds slantingly downwards from the further end of h, and dips into a large sloping iron cylinder immersed in cold water. See [Gas-light] and [Stove], for a better plan of furnace.
The filters used in the large sal ammoniac works in France are represented in [fig. 956.] The apparatus consists——1. of a wooden chest a, lined with lead, and which is turned over at the edges; a socket of lead b, soldered into the lowest part of the bottom, serves to discharge the liquid; 2. of a wooden crib or grating formed of rounded rods, as shown in the section c, c, and the plan d; this grating is supported one inch at least above the bottom, and set truly horizontal, by a series of wedges; 3. of an open fabric of canvas or strong calico, laid on the grating, and secured over the edges, so as to keep it tense. A large wooden reservoir f, lined with lead, furnished with a cover, is placed under each of the filters; a pump throws back once or twice upon the filters what has already passed through. A common reservoir g, below the others, may be made to communicate at pleasure with one of them, by means of intermediate stopcocks.
The two boilers for evaporating and decomposing are made of lead, about one quarter of an inch thick, set upon a fire-brick vault, to protect them from the direct action of the flame. Through the whole extent of their bottoms above the vault, horizontal cast-iron plates, supported by ledges and brick compartments, compel the flame and burned air, as they issue from the arch, to percur many sinuosities before they pass up the chimney. This floor of cast iron is intended to support the bottom of the boiler, and to diffuse the heat more equably. The leaden boilers are surrounded with brickwork, and supported at their edges with a wooden frame. They may be emptied at pleasure into lower receivers, called crystallizers, by means of leaden syphons and long-necked funnels.
The crystallizers are wooden chests lined with lead, 15 inches deep, 3 or 4 feet broad, and from 6 to 8 feet long; and may be inclined to one side at pleasure. A round cistern receives the drainings of the mother-waters. The pump is made of lead, hardened with antimony and tin.
The subliming furnace is shown in [figs. 957.] and [958.] by a transverse and longitudinal section. a is the ash-pit; b, the grate and fireplace; c, the arch above them. This arch, destined to protect the bottles from the direct action of the fire, is perforated with vent-holes, to give a passage to the products of combustion between the subliming vessels. d, d, are bars of iron, upon which the bottoms of the bottles rest; e, stoneware bottles, protected by a coating of loam from the flame.