LAMP, SAFETY. It has been long known that coal mines, and especially such as are deep, are occasionally infested with carburet ted hydrogen, known to the miners hs fire damp. Several contrivances have been pro posed for safely lighting coal mines subject to the visitations of this gas ; but the safety lamp of Sir H. Davy is the only one which has ever been judged safe, and beeii exten sively employed.
Davy found that this gas requires an ad mixture of a large quantity of atmospheric air to render it explosive ; that explosions of inflammable gases are incapable of being passed through long narrow Metallic tubes ; and that this principle of security is still ob tained by diminishing their length and dia meter at the same time, and likewise dimi nishing their length and increasing their number, so that a great number of small apertures will not pass explosion when their depth is equal to their diameter. This led to trials upon sieves made of wire gauze : and 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 he ascertained that a flame confined in a cylinder 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.
These experinients enabled Davy to deter mine the principles of a new safety-lamp. It consists of a cylinder of wire gauze, formed of iron wire about of an inch in diameter, the meshes being about of an inch. The wire cylinder is strongly jointed, and is de fended by three upright strong wires, which meet at the top, and to them a ring is flied, froni which the instrument is suspended. The lamp is screwed on to the bottom of the wire gauze, and is supplied with oil by the pipe projecting froth it, when the top is unscrewed and removed. A wire, bent at the upper end, is passed throligh the bottom of the lamp for raising, lowering, or trimming the wick.
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 burning brightly within the blue flame, and the light of the wick con tinues till the fire-clan* 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 ; and, when the foul air constitutes one-third of the atmosphere, it is no longer fit for respiration.
The operation of the wire gauze in prevent. ing the communication of flame is thus ex plained : — Flame is gaseous matter to intensely heated as to be luminotiS, and the temperature requisite for producing it exceeds that of the white heat of solids. When the flame comes into contact with wire gauze, it loses so much heat in consequence of the conducting Power of the metal which conveys it to the surrounding air, that it is cooled below the point at which gaseous Matter can remain luminous, and consequently the flame of the gaseous matter burning within tlie lamp is incapable of passing through it so as to set fire to and explode the mixture of fire damp and air by which it is surrounded ; and this cooling power is exerted, even though the wire gauze, by effecting it, is rendered and re mains red-hot.
Much discussion hat arisen concerning the safety of this instrument; it is admitted that there are situations where it would not he dan ger-proof; but it is also admitted that no other invention is yet equal to it in respect to safety. A form of the instrument devised by Dr. Clanny is that which is now most frequently employed.
The original model of the safety lamp, made by Sir Humphrey Davy's own hands, has been lately placed among the collection of scientific instruments belonging to the Royal Society.