150 



bench of fire-bricks, perforated inside with eight flues for distributing the flame ; D, a 

 great arch, with a triple voissoir D, d", d"', under which the retorts are set. The first 

 arch, D, is perforated with twenty vent-holes, the second with fourvont-holes, through 



which the flame passes to the third arch, and thence to the common chimney-stalks. 

 The retorts are shut by the door ef (fig. 41), luted, and made fast with screw-bolts. 

 Their other ends e", terminate in tubes,/,/,/, which all enter the main pipe h. The 

 condensing pipe proceeds slantingly downwards from the further end of k, and dips 

 into a large sloping iron cylinder immersed in cold water. 



The filters used in the large sal-ammoniac works in Franco are represented in fig. 

 42. 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 scries 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, /, lined with lead, fur- 

 nished 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 take a sinuous course 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 sur- 

 rounded with brickwork, and supported at their edges with a wooden frame. They 

 may be emptied at pleasure into lower receivers, called crystallisers, by moans of 

 leaden syphons and long-necked funnels. 



The crystallisers are wooden chests lined with lead, 15 inches deep, 3 or 4 feet 

 broad, and from 6 to 8 feet long, and may bo inclined to 0110 side at pleasure. A 



