SAFETY LAMP

A lamp invented by Sir H. Davy, which is so constructed as to burn without danger in an explosive atmosphere. Flame may he considered as vapour or aeriform matter in a state of intense ignition; the temperature, therefore, of flame is always very high. It is, however, independent of its luminosity; for some of the dimmest flames, those of pure hydrogen gas, and of alcohol, for instance, are those which are hottest; and that this is so may be shown by projecting into them finely powdered substances, such as magnesia or lamp black, or by holding in them fine platinum wire, when the intensity of their temperature is rendered evident by those substances becoming white hot. And whenever flames emit much light they derive that property from the presence of finely divided matter diffused through them: thus, the intense brilliancy of the flame of phosphorus appears to depend upon the particles of incombustible phosphoric acid diffused through it; and the bright light emitted by a gas flame depends upon finely divided charcoal, which is ignited by the gas and at the same time burned. The correctne of this theory of (lame is shown by the circumstance of its being extinguished by cooling; and this is best effected by causing it to pa through a piece of fine wire gauze, which, when lield horizontally in the midst of the flame, extinguishes its upper part: the inflammable vapour or gas, and the soot or carbon, pa through, but in pa ing are so far cooled as to be extinguished; they may, however, be rekindled by applying a flame above the wire gauze. That the wire gauze merely acts by its cooling power, is shown by the flame Ea ing through it when it acquires a white eat, or when its meshes are not fine enough to exert a due cooling power; it is also found that very hot flames, such as that of hydrogen, will pa through ti ues which are impervious to flames of a lower temperature, such as that of a common candle or a gas flame. (Sre Flame.) The application of these principles to the construction of the safety lamp is as follows: — The flame of a small oil lamp C is surrounded by a cylinder of wire gauze A B (doubled at A, where likely to become hottest, and protected by the stout wire frame D), and burns within it, the air having free ingre and egre . When it is immersed in an explosive atmosphere,'such as that of a coal mine infested by fire damp, the inflammable gas enters from without and burns in the cage; but, in consequence of the cooling power of the wire gauze, no flame can pa oiUwards so as to ignite the surrounding atmosphere: the miner, therefore, is warned 1074 SAGO. of his danger by the appearance of the lamp. As long as the external atmosphere is safe, the lamp burns as usual; but upon the approach of the fire damp the flame is more or le enlarged; and in tlje most explosive condition of the surrounding air the cylinder appears filled with a blue lambent flame, which flickers within it, the wick of the lamp appearing for the time extinguished. It is, however, rekindled as the air becomes more pure; or should the fire damp greatly predominate, it may be entirely extinguished. Before this happens, however, the miner is duly apprised of his danger, and has time to retreat. ( See Davy on the Safety Lamp. ) [s. 1087]