Coal Gas

Coal Gas. The ordinary gas employed for illuminating purposes is obtained by the distillation of coal. For its preparation this rock is heated to a bright-red heat in large cylindrical retorts built of firebricks. A mass of coke or gas carbon remains in the retorts, and gases are evolved which require to be purified. They are therefore passed first into a large iron pipe in which tar and a watery distillate condense, the latter containing quantities of ammoniacal salts, and known as ammoniacal liquor. The gas and these liquids pass next into the tar-pit, where the latter are collected, the gas passing further through a system of iron pipes, the refrigerators, in which more of the tar and ammoniacal liquors are condensed and collected. It then passes up the scrubbers, wrought iron towers loosely filled with coke, in which it meets a fine spray of water. After leaving the scrubbers the gas consists chiefly of hydrogen, marsh gas, hydrocarbons known as olefines, carbonic oxide, nitrogen, carbonic acid, and sulphurretted hydrogen. These two latter require to be carefully got rid of, as they exert deleterious effects, to the last being due the blackening of silver and gilding, etc., by gas. It therefore passes, in the purifiers, over layers of a mixture of slaked lime, oxide of iron, and sawdust, which absorb these noxious constituents. From the purifiers the gas is collected in the large gasometers, from which it is distributed to the mains, the supply being regulated by a governor. The best coal for use in the manufacture of coal-gas is that non-flaking coal found largely in Staffordshire, Newcastle, Wigan, etc. The illuminating power of gas is reckoned in terms of standard sperm ceendles, against which it is tested when burning five cubic feet per hour. The Gas Works Act provides that the gas supplied shall not be less than sixteen candle-power. The same Act limits the quantity of sulphur in the gas to a certain small amount, and the quality of gas in London in these and other respects is frequently tested in various places by the "gas referees."