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e it may be said at once that the crude gas coming from the generating plant is seldom fit for immediate consumption, while if it be required for the illumination of occupied rooms, it must invariably be submitted to a rigorous method of chemical purification.
IMPURITIES OF ACETYLENE.--Combining together what may be termed the carbide impurities and the generator impurities in crude acetylene, the foreign bodies are partly gaseous, partly liquid, and partly solid. They may render the gas dangerous from the point of view of possible explosions; they, or the products derived from them on combustion, may be harmful to health if inspired, injurious to the fittings and decorations of rooms, objectionable at the burner orifices by determining, or assisting in, the formation of solid growths which distort the flame and so reduce its illuminating power; they may give trouble in the pipes by condensing from the state of vapour in bends and dips, or by depositing, if they are already solid, in angles, &c., and so causing stoppages; or they may be merely harmful economically by acting as diluents to the acetylene and, by having little or no illuminating value of themselves, causing the gas to emit less light than it should per unit of volume consumed, more particularly, of course, when the acetylene is not burnt under the mantle. Also, not being acetylene, or isomeric therewith, they require, even if they are combustible, a different proportion of oxygen for their perfect combustion; and a good acetylene jet is only calculated to attract precisely that quantity of air to the flame which a gas having the constitution C_2H_2 demands. It will be apparent without argument that a proper system of purification is one that is competent to remove the carbide impurities from acetylene, so far as that removal is desirable or necessary; it should not be called upon to extract the generator impurities, because the proper way of dealing with them is, to the utmost possible extent, to prevent their formation. The sole exceptio