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CHEMISTRY, AS APPLIED TO THE

The constituents of purified coal-gas are hydrogen, light carburetted hydrogen, carbonicoxide, olefiant gas, other illuminating gases having the general formula C n II Dj —that is, con-taining half the number of atoms of carbon as of hydrogen, like olefiant gas; further, thevapours of hydrocarbons having the formula C n IIas benzole and probably other hydro-carbons whose composition is not yet known. In addition to these, there are also presentsmall quantities of nitrogen, oxygen, and bisulphide of carbon vapour, which however, forour present purpose, may be entirely neglected.

It has generally been assumed that hydrogen and carbonic oxide possess no illuminatingpower, and that the light afforded by coal-gas is due to light carburetted hydrogen, olefiantgas, and other hydrocarbons. But late experiments have proved that light carburettedhydrogen possesses, in a practical point of view, no illuminating power whatever; and wemust therefore ascribe all the light of coal-gas to the olefiant gas and hydrocarbons containedin it, whilst the relative proportions of hydrogen, light carburetted hydrogen, and carbonicoxide, with which these luminiferous ingredients are diluted, exercise no beneficial influenceupon the amount of light yielded by a given volume of these illuminating compounds.

I he constituents of coal-gas, and also of other gases used for illuminating purposes, maytherefore be divided into two classes, viz. luminous and non-luminous constituents. To thefirst belong olefiant gas and the rest of the hydrocarbons mentioned above, except perhapsnaphthaline, about the illuminating power of which there is some difference of opinion; tothe second, hydrogen, carbonic oxide, and light carburetted hydrogen. To the first classalone is the illuminating power of every gaseous mixture due, but at least one member ofthe second class is also indispensable, since otherwise the combustion of the hydrocarbons,without the production of much smoke and a consequent loss of light, would be attendedwith great difficulty. The members of the first class are all instantaneously decomposed ata white heat. They all deposit their carbon at that temperature in the form of exceedinglyline particles, which constitute so many centres for the radiation of light in the flame of agas-light. The greater the number of such particles that are at any moment present in aflame, the greater is the amount of light emitted by that flame. These considerations renderit evident that the value of the gaseous and liquid hydrocarbons as light-yielding materials isin direct proportion to the quantity of carbon contained in a given volume ; the densest ofthe gases or vapours of the first class are therefore those which possess the highest illumi-nating power. All the members of this class are however, as just mentioned, more or lessrapidly decomposed at a red heat, and in the usual process of gas-making the inner walls ofthe retorts soon become covered with a layer of carbon derived from this source. This de-struction of luminiferous constituents is dependent, on the one hand, upon the length of timeduring which they are exposed to a high temperature, and on the other, upon the number ofthe particles of such constituents which come in contact with the red-hot sides of the retort.

Two methods therefore suggest themselves for the prevention of this decomposition. Thefirst would consist in the rapid removal of the gases from the retort, and the second in thedilution of the luminous gases by an admixture of non-luminous constituents ; for it is evi-dent that the number of atoms of illuminating gas, in contact with a given surface, wouldonly be half as great if that gas were diluted with an equal volume of hydrogen as it would