LAMP, SAFETY. It has long been known that coal-mines, and especially such as are deep, are occasionally infested with carburetted hydrogen, a gaseous product, which, on account of its combustible property is called fire-damp ; the word being the equivalent of the German dampf, a vapour or exhalation.
Several contrivances had been proposed for safely lightink coal-mines subject to the visitations of this gas ; but the safety-lamp of Sir H. Davy was the first one extensively employed. Davy began a chemical examina tion of various specimens of fire-damp, by which he confirmed the previous statement of Dr. Henry, that the pure inflammable part of it is carburetted hydrogen gas. He found that it required an admixture of a large quantity of atmospheric air to render it explosive. Pro ceeding with his experiments Davy ascertained that explosions of inflammable gases are incapable of being passed through long narrow metallic tubes ; and that this principle of security is still obtained by diminishing their length and diameter at the same time, and likewise by diminishing their length and increasing their number ; so that a great number of small apertures would not pass explosion when their depth was equal to their diameter. This fact led to trials upon sieves made of wire gauze. He found that if a piece of wire gauze was held over the flame of a lamp, or of coal gas, it prevented the flame from passing ; and be found also that a flame confined in a oylinder of very fine wire gauze did not explode even a mixture of oxygen and hydrogen, but that the gases burnt in it with great vivacity.
The experiments to which we have now alluded, served as the basis of the s.afety-lamp, which Davy described as follows : " The apertures in the gauze should not be more than , of an inch square. As the fire-damp is not inflamed by ignited wire, the thickness of the wire is not of importance ; but wire from to k of an inch in diameter is the most convenient. Iron-wire and brass-wire gauze of the required degree of fineness are made for sieves by all wire-workers ; and except when a lamp is to be used by a viewer for dialling, iron-wire gauze is to be preferred : when of the proper degree of thickness it can neither melt nor burn ' • and the coat of black rust which soon forms upon it superficially, defends the interior from the action of the air. The cage or cylinder should be made by double joinings, the gauze being folded over so as to leave no apertures. When it is cylindrical it should not
be more than two inches in diameter ; for hi larger cylinders the com bustion of the fire-damp renders the top incon veniently hot ; and a double top is always a proper precaution, fixed at the distance of half or three-quarters of an inch above the first top. The gauze cylinder should he fastened to the lamp by a screw of four or five turns, and fitted to the screw by a tight ring. All joinings in the lamp should be made with hard solder ; and the security depends upon the circum stance that no aperture exists in the apparatus larger than in the wire gauze." The annexed figure of the safety-lamp re quires but little explanation. The cylinder of wire gauze is defended by three upright strong wires, which meet at the top ; and to them a ring is fixed, from which the instrument is sus. pended. The lamp is screwed on to the bottom of the wire gauze, and is supplied with oil by the pipe projecting from it. A wire, bent at the upper end, is passed through the bottom of the lamp for raising, lowering, or trimming the When the Lamp is lighted and introduced into an atmosphere gradually mixed with fire damp, the first effect of the fire-damp is to increase the size and length of the flame. When the inflammable gas forms as much as of the volume of the air, the cylinder becomes filled with a feeble blue flame ; but the flame of the wick appears bumming brightly within the blue flame. The light of the wick continues till the fire-damp increases to one-sixth or one fifth, when it is lost in the flame of the fire damp, which in this case fills the cylinder with a pretty strong light. The operation of the wire gauze in preventing the communication of flame is thus explained :—FLune is gaseous matter so intensely heated as to be luminous. When the flame comes into contact with wire gauze it loses so much heat in consequence of the conducting power of the metal, that It is cooled down below the point at which . . — gaseous matter can remain luminous ; and consequently the flame of the gaseous matter burning within the lamp is incapable of passing through it so as to set fire to and explode the mixture of fire-damp. This cooling power is exerted even though the wire gauze, by effecting it, be rendered red-hot.