PLANT PRODUCTS. FERMENTATION AND FUELS 423 



are concerned mainly with the products obtained by heating it 

 in the absence of air, and with its use as a fuel. 



Bituminous coals give much, and widely varying amounts of 

 volatile matter; anthracite coals give very little. The ash is 

 the mineral matter of the original plants, with additional rock 

 materials in some specimens. The coal is selected according to 

 the purpose for which it is to be used. For coal gas, and even for 

 coke, a variety high in volatile matter is chosen. For water gas 

 (p. 203) anthracite or coke itself is employed. 



Coal Gas. The gas plant (Fig. 105) includes (1) the fire-brick 

 retorts in which the coal is heated (externally) to 1300, (2) the 

 hydraulic main (a wide iron pipe) immediately above them in 



FIG. 105 



which most of the tar collects, (3) the condenser and wash box for 

 cooling, condensing, and removing oils, (4) the scrubbers (ver- 

 tical and rotary) where the ammonia is taken out by water drip- 

 ping over strips of wood and by stirring the gas with water, (5) 

 the purifier where hydrogen sulphide is taken up by hydrated 

 ferric oxide, and (6) the holder in which the gas collects. 



The yield of gas varies considerably with the type of coal used. 

 One ton of good bituminous coal should produce approximately 

 10,000 cubic feet of coal gas, 1300 pounds of coke, 5 pounds of 

 ammonia and 12 gallons of tar. The average composition of coal 

 gas is: Illuminants 4 per cent, carbon monoxide 8 per cent, hydrogen 

 50 per cent, methane 29 per cent, ethane 3 per cent, carbon dioxide 

 2 per cent, oxygen and nitrogen 4 per cent. 



