FUEL BY-PRODUCTS 49 



generally made of fire-clay with a section approximating that 

 of the letter " D," the front of the " D " being the base, and 

 the section rather more flattened than the ordinary capital 

 " D " of this type. Sometimes the retorts are permanently 

 closed at one end, when they are usually about 8 or 10 ft. 

 long, but more commonly they are of double length, and 

 open at each end. The end is closed by an iron door, from 

 which ascends a wide pipe, conveying the products of 

 distillation. Firing is frequently done in a modern gas 

 house with producer or water gas. By setting the retort 

 at a sharp angle, or even vertically, much economy in 

 machinery is obtained. The retorts are usually heated for 

 gas production to a temperature of from 900-iooo C., 

 (i65o-i8oo F.), the time of distillation being from 

 4-8 hours. With vertical retorts, gravity causes the 

 charge to fall, but additional force is supplied by a 

 screw at the bottom and intermittent poking at the top. 

 The volatile products pass through the ascension pipe into 

 the hydraulic main, which latter consists of a long, horizontal 

 tube, traversing the entire length of the retort house. In 

 its passage through these pipes, the gas undergoes rapid 

 cooling and deposits its tar and water, the latter of which 

 absorbs the ammonia, together with a certain amount of 

 hydrogen sulphide and carbon dioxide. The products after 

 leaving the hydraulic main have fallen in temperature to 

 about 50 C., and thence proceed to a condenser, which 

 usually consists of vertical air-cooled pipes, though in some 

 works these pipes are externally cooled by water sprays. 

 The gas, now freed from much condensable material, passes 

 through the exhaust fan, which draws the gas, as fast as it 

 is made, from the retorts through the condensers, main- 

 taining a slight vacuum in these, and then forces the gas 

 forward under pressure through the purifying plant. The 

 first part of the purifying plant consists generally of scrubbers, 

 which are tall towers filled with wooden boards standing on 

 edge, down which water flows. The L,ivesey scrubber, in 

 which the gas is divided very minutely, and rotary mechanical 

 scrubbers are also used. By these means all the ammonia, 

 v. 4 



