are brought to a cherry-red heat. A measured charge of coal is then put in with a long shovel, the mouth of the retort closed with a cover, and the cover sealed with cement. The coal, being heated without access to air, is changed to coke, a hard sponge-like substance, with a metallic luster, which gives forth a ringing sound when struck lightly. The remainder of the coal becomes a mixture of illuminating gas, water vapor (steam), ammonia, sulphur compounds, and tar. As the illuminating gas is the only portion that must be sent through the gas pipe, it becomes necessary to remove the other substances mixed with it. The gas is drawn from the retorts by means of a pump, or exhauster.
61. Composition of Coal Gas. — Coal gas is a mixture of various organic and inorganic compounds, the amounts of which in a given volume of gas vary somewhat, according to the kind of coal used and the temperature at which it is carbonized. The following may be considered the average composition of purified coal gas, from which the carbon dioxide has not been removed:
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Constituents of Gas Per Cent. Hydrocarbon vapors .6 Heavy hydrocarbons 4.4 Carbon dioxide 3.4 Carbon monoxide 10.1 Marsh gas 30.6 Oxygen .3 Hydrogen 45.9 Nitrogen 4.7 Total 100.0
In the unpurified state, coal gas also contains ammonia, hydrogen sulphide (H2S), and a small percentage of other compounds, among which carbon disulphide (CS2) predominates. Of the impurities, ammonia and hydrogen sulphide are carefully removed, but no extra efforts are made to remove the small amount of carbon disulphide that might escape the washer.
62. Specific Gravity, Thermal Value, and Odor of Coal Gas. — Ordinary coal gas has a specific gravity of
