The composition of coal gas as produced for illuminating purposes depends very largely upon the temperature at which the roasting of the coal takes place, and upon the amount of air that gains access to the retorts. It contains not only permanent gases but, in addition, considerable quantities of the vapours of volatile hydrocarbons, to which its luminosity in burring is due. The greater proportion consists of hydrogen, which burns with a practically colourless flame, and marsh gas or methane , which also gives but little light when burned alone. Accompanying these constituents there are proportions of carbon dioxide, carbon monoxide, nitrogen, and oxygen. The illuminating power of coal gas is chiefly attributable to olefiant gas and other associated hydrocarbons, which, in all, amount to about 5 per cent., and the process of roasting the coal is conducted in such a way as to yield about 10,000 cubic feet of gas per ton of coal. In 1913, about twenty million tons of coal were carbonized in the United Kingdom for the manufacture of coal gas. The composition of good coal gas according to one analysis, which is fairly representative, is as follows: The gas, however, now supplied in many places consists of a mixture of ordinary coal gas with socalled water gas, or carburetted water gasthat is, the mixture of carbon monoxide and hydrogen which is produced by passing steam over redhot coke, while at the same time a certain quantity of petroleum is introduced and destructively decomposed or “cracked” in the furnace, to give to the other gases that proportion of hydrocarbons necessary to givelmnin osity to the whole mixture when it is burned.
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