SACCHOLACTIC ACID. An acid obtained by digesting sugar of milk in nitric acid. It is identical with that ob tained from gum, and termed mucous acid. SAFETY LAMP. A lamp invented by Sir H. Davy, which is so constructed as to burn without any danger in an ex plosive atmosphere. Flame may be con sidered as vapor or aeriform matter in a state of intense ignition ; the tempera ture, 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 pres ence of finely divided matter diffused through them : thus, the intense bril liancy of the flame of ap pears to depend upon the particles of in combustible phosphoric acid diffused through it ; ind the, bright light emitted by a as flame depends upon finely.divi ded charcoal, which is ignited by the gas and the same time burned. The cor rectness of this theory of flame is shown by the circumstance of its being extin guished by cooling ; and this is best ef fected by causing it to pass through a piece of tine wire gauze, which, when held hori zontally in the midst of the flame, extin guishes its upper part : the inflammable vapor orgas, and the soot or carbon, pass through, but in passing are so far cooled as to be extinguished ; they may, how. ever, 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 passing through it when it acquires a white heat, or when its meshes are not fine enough to exert a due cooling power such it is also found that very hot flames, such as that of hydrogen, will pass through tissues which are imper vious to flames of a lower temperature, such as that of a common candle or a gas flame. The application of these princi
ples to the construction of the safety lamp is as follows : The flame of a small oil lamp is surrounded by a cylin der of wire gauze, doubled where likely to become hottest, and protected by the stout wire frame, • and burns within it, the air having free ingress and egress. When it is immersed in an explosive at mosphere, such as that of a coal mine in fested by fire-damp, the inflammable gas enters without and burns in the cage ; but, in consequence of the cooling power of the wire gauze, no flame can pass outwards so as to ignite the surround ing atmosphere : the miner, therefore, is warned of his danger by the appearance of his lamp. As long as the external at mosphere is safe, the lamp burns as usu al ; but upon the approach of the fire damp the flame is more or less enlarged ; and in the 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, how ever, the miner is duly apprised of his danger, and has time to retreat. (SEE LAMP OF NAVY.)