The gas leaving the scrubbers contains as impurities carbonic acid and sulphureted hydro gen, as well as small quantities of other sulphur compounds and cyanogen. It is necessary to remove the sulphureted hydrogen and reduce the sulphur compounds to an amount not to exceed 25 grains per 100 cubic feet of gas, since these substances produce sulphurous oxide when burned, and thus give rise to disagreeable fumes if present in any quantity. The carbonic acid is sometimes removed also, although, being harm less except as it affects the illuminating value, it is usual in the United States to allow it to remain in the gas. For its removal it is neces sary to employ, in the purifiers, hydrate of lime, which combines with it, forming carbonate of lime. Lime will also combine with sulphureted hydrogen, and was formerly the sole substance employed for its removal, which can, however, be effected much more economically by the use of hydrated sesquioxide of iron, either prepared artificially, or in the shape of a natural bog ore; and this has largely superseded lime. The reac tion between the oxide of iron and the sulphu reted hydrogen results in the formation of sul phide of iron, which is again changed to oxide when the fouled material is exposed to the air. The material can thus be used over and over again until it becomes so charged with the sul phur, deposited at each revivification, as to be rendered inactive. The oxide of iron also ab sorbs some of the cyanogen, and when spent is of value for the manufactute of cyanides and of sulphuric acid. After passing the purifiers the gas is ready for distribution, being measured solely for the convenience of the manufacturer; but as tke demand is not uniform from hour to hour, and it is necessary that the rate of pro duction should be practically so, the gas-holders are provided to store the excess quantity made during the time of small demand for use at the time of large demand.
The scheme of condensation and purification outlined is that usually employed in the United States. In Europe it is becoming customary to scrub the gas with a solution of an iron salt to remove the cyanogen more completely and in a more merchantable form than is done in the purifiers, and also with tar oils to remove naphthalene, a hydrocarbon vapor, which, when the gas is chilled, condenses at once to the solid form in light flakes, and at times causes much trouble by stopping the small pipes of the dis tribution system. When the naphthalene is not removed it is necessary to add to the gas, if trouble is experienced, a hydrocarbon vapor that will condense, at the same, time that the naphtha lene crystallizes, to a liquid capable of dissolving the crystals, and so prevent them from forming obstructions in the pipes.
It will be seen that in addition to the gas there are produced in the manufacture of coal-gas, coke, tar, and ammoniacal liquor, all of which are valuable, the coke as a fuel, the tar as a raw material for the manufacture of paving and roofing pitch, artificial dye-stuffs, various drugs, etc. (see COAL-TAR), and the ammoniacal liquor as a raw material for the manufacture of am monia in various forms. The products from 100 pounds of gas coal will be, about, 65 pounds of coke, 500 cubic feet of gas, .6 gallon of tar, and 1.3 to 1.5 gallons of ammoniacal liquor con taining .23 to .32 pound of pure ammonia.