The exhauster, when used, is now the next part of the apparatus. It is a species of pump, driven by steam-power, and is made in various forms, both direct-acting and rotary. It serves the purpose • of relieving the retorts of the resistance or pressure, created in the passing of the gas through the apparatus, and in raising the gas-holders. The use of the exhauster greatly lessens the deposit of carbon in the retorts in the form of graphite, and is attended with other important advantages. .
• At this stage of the process, the liquid products have been -separated from the gaseous. A. portion of the ammonia and the sulphureted hydrogen and carbonic acid have still to be removed. As yet, there are uo means practically applicable for the removal of the sulphide of carbon; but the quantity produced is so minute as to be uninjurious. To ammonia from the gas, the washer or scrubber is used. In the washer, the gas is forced to pass through water to a depth of several inches, or through a solution containing an ingredient with which the ammonia will combine. The scrubbei', which may be used instead of the washer,. is an upright vessel, in which the gas is made to pass through hroshwoial, layers of small stones, coke, or suitable shelving of wood or iron, through or over which water may be made to percolate.
There are two kinds of purifiers—the wet hod the dry. Either may be used sepa rately, or they may be used in succession. The wet variety is now rarely to be met with. The dry purifier is a vessel containing a series of perforated trays, on each of which the purifying material is spread. Slaked lime (in the form of dry hydrate) is used in this purifier in layers of from 2f to 3-i in. on each tray. The lime absorbs the sulphureted hydrogen, a portion of the ammonia, and the carbonic acid. When satu rated. it is removed, and the vessel is refilled with fresh material. The ,refuse lime is useful as a manure. When the oxide Of iron is employed as the purifying material, the preparation is spread in the same manner as the lime, but to a much greater thickness. When, by the absorption of sulphureted hydrogen, the oxide of iron has become sul phuret of iron, it is taken out, and, by exposure to the atmosphere, it is reconverted into oxide, and can be used again and again. When oxide of iron is used, a separate lime purifier is necessary for removing the carbonic acid. A narrow chamber, nearly full of water, runs round the upper edge of the dry purifier; into this chamber the sides of the cover, which is of sheet-iron, are let down, and .the gas is thus prevented from
escaping.
After passing the purifier, the gas, which is now fit for is measured by the station-meter, ho instrument similar in principle to the consumers' meter, afterwards described. It is then conveyed to the gas-holder, to be stored and issued as required. • The gas-holder is an inverted cylindrical vessel of sheet-iron, placed in a tank of cast iron, stone, or brick containing water. A pipe ascends from the bottom of the tank through the water, to admit the gas to the space between the surface of the water and the crown of the gas-holder. Another pipe descends through the water and the bottom of the tank, for the issue of the gas to the main-pipe. The water is for the.purpose of retaining the gas within the vessel. The buoyancy of the gas raises the gas-holder; and the weight of the gas-holder, or such part of it as is not taken off by balance-weights, impels the gas through the pipes. When balance-weights are necessary, they are attached to the edge of the crown of the gas-holder by long chains, which pass over pulleys on the top of columns which serve also to guide the motion of the vessel in rising and falling.. Gas-holders are constructed of all sizes, some exceeding 200 ft. in diameter. Ili large establishments, telescopic gas-holders are used, and economy of space and cost are thereby effected—two concentric gas-holders being contained in one tank. The outer vessel of a telescope gas-holder has no crown. The upper edge is turned first inwards and then downwards, forming an inverted hollow chhmber. The under edge of the inner vessel again is turned outwards and upwards, forming a hollow chamber, which, when the vessel rises out of the tank, will be full of water. The inner side of the inverted chamber, round the top of the outer vessel, fits into the inside of the chamber round the bottom of the inner vessel, and enters it when that vessel has nearly ascended to the top of The water in the chamber retains the gas, and the two vessels then rise together. The inner vessel, it will be observed, ascends first; both then ascend and descend till-the outer .vessel has reached the bottom of the tank, on which it rests, and the inner vessel then also descends into the tank. Three or lifts, as they are termed, are occasionally placed in the same tank.