The next stage is the passage of the scrubber, filled with coke over which water perpetually flows. The ammonia gas is here absorbed. There still remain the sulphuretted hydrogen and the carbon bisulphide, both of which are extremely offensive to the nostrils. Slaked lime, laid on trays in an air-tight compartment called the lime purifier, absorbs most of the sulphurous elements of these; and the coal gas is then fit for use. On leaving the purifiers it flows into the gasometer, or gasholder, the huge cake-like form of which is a very familiar object in the environs of towns. The gasometer is a cylindrical box with a domed top, but no bottom, built of riveted steel plates. It stands in a circular tank of water, so that it may rise and fall without any escape of gas. The levity of the gas, in conjunction with weights attached to the ends of chains working over pulleys on the framework surrounding the holder, suffices to raise the holder.
Fig. 196.—The largest gasholder in the world: South Metropolitan Gas Co., Greenwich Gas Works. Capacity, 12,158,600 cubic feet.
Some gasometers have an enormous capacity. The record is at present held by that built for the South Metropolitan Gas Co., London, by Messrs. Clayton & Son of Leeds. This monster (of which we append an illustration, Fig. 196) is 300 feet in diameter and 180 feet high. When fully extended it holds 12,158,600 cubic feet of gas. Owing to its immense size, it is built on the telescopic principle in six "lifts," of 30 feet deep each. The sides of each lift, or ring, except the topmost, have a section shaped somewhat like the letter N. Two of the members form a deep, narrow cup to hold water, in which the "dip" member of the ring above it rises and falls.
Fig. 197.—Drawing retorts. (Photo by F. Marsh.)
AUTOMATIC STOKING.