SAFETY LAMP. It has been long known that when methane, marsh gas, or light carbureted hydrogen, which is fre quently disengaged in large quantities from coal seams, is mixed with 10 times its volume of atmospheric air, it becomes highly explosive. Moreover, this gas— the fire damp of miners—in exploding ren ders 10 times its bulk of atmospheric air unfit for respiration, and the choke damp thus produced is often as fatal to miners as the primary explosion. With the view of discovering some means of preventing these dangerous results, Davy instituted those important observations on flame which led him to the invention of the safety lamp. He found that when two vessels filled with a gaseous explosive mix ture are connected by a narrow tube, and the contents of one fired, the flame is not communicated to the other, provided the diameter of the tube, its length, and the conducting power for heat of its ma terial bear certain proportions to each other; the flame being extinguished by cooling, and its transmission rendered im possible. In this experiment high con ducting power and diminished diameter compensate for diminution in length; and to such an extent may this shortening of length be carried that metallic gauze, which may be looked on as a series of very short square tubes arranged side by side, completely arrests the passage of flame in explosive mixtures.
The first lamp which would safely burn in an explosive mixture of gas and air was contrived in 1813 by Dr. W. Reid Clanny of Sunderland. Into this lamp fresh air was blown through water, and heated air escaped through water by means of a recurved tube. Such a lamp was unfit for ordinary use. George Ste phenson invented a safety lamp which was tried at the Killingworth pits in 1815. Both Clanny and Stephenson ap plied wire gauze cylinders to their lamps after Davy's came into use, or at least after a communication about it had been made to the Royal Society in 1815. Por table electric lamps are now in general favor, but they give no warning of gas, and in mines much affected by gases the oil-safety lamp is in use.