Page:Popular Science Monthly Volume 10.djvu/494

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HERE are three kinds of gas, named after the substances from which they are obtained, as coal, petroleum (or naphtha), and water gas. The first two are produced by destructive distillation of coal and petroleum, or naphtha, the last by passing a current of steam over a bed of incandescent anthracite. Coal-gas is the kind in general use, petroleum or naphtha gas being used chiefly as an enricher; and water-gas, used in only a few places and for a comparatively short time, is as yet on its trial. The general principles involved in the manufacture of each kind of gas will first be noticed; then the relative merits of the products will be considered in regard to quality, cost of manufacture, and respective peculiarities; and, finally, a comparative view will be taken of several leading companies; after which the relationship of gas companies to municipalities, and the subject of competition, will be examined.

When coal is subjected to high heat in a closed vessel, certain gases and vapors are evolved, some of which are combustible, and some, like steam, condensible, a residue of charcoal or coke being left behind. This process is termed destructive distillation, and the property displayed by coal is common to all vegetable and animal substances; but only coal and petroleum have been used economically in the production of illuminating gas on an extensive scale. The distillation is the most important operation in the manufacture, but it is necessary to remove from the gas, before it is fit for burning, the condensible vapors, as tar, water, etc.; and those non-condensible gases, as carbonic anhydride (carbonic acid), which either largely diminish the. illuminating power, or which give rise, in the burning of the gas, to injurious products of combustion, such as sulphuretted hydrogen and ammonia. The removing of these very materially affects the cost of production. The distillation is effected in iron or clay