The New Student's Reference Work/Safety-Lamp

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Safe′ty-Lamp. When marsh-gas, which is often set free in large quantities from coal-seams, is mixed with ten times its volume of air, it becomes highly explosive. Moreover, this gas, the fire-damp of mines, in exploding, renders ten times its bulk of air unfit for breathing, and the choke-damp thus produced is often as fatal to miners as the first explosion. To do away with these dangers, Sir Humphry Davy began his experiments on flame, which resulted in his invention of the safety-lamp. He found that when two vessels filled with a gaseous explosive mixture are joined by a narrow tube and the contents of one fired, the flame does not reach the other, provided the thickness of the tube, its length and the conducting power for heat of its material bear certain proportions to each other; the flame being put out by cooling and its transmission made impossible. He found also that high conducting power and less thickness make up for less length; and to such an extent may this shortening of length be carried that metallic gauze, which may be looked upon as a series of very short, square tubes arranged side by side, wholly stops the passage of flame in explosive mixtures. His lamp consists of a burner inclosed in a wire-gauze. When such a lighted lamp is brought into an explosive mixture of air and fire-damp, the flame is seen slowly to enlarge as the amount of fire-damp increases, until it fills the entire gauze-cylinder. Whenever this pale, enlarged flame is seen, the miners should at once get to a place of safety; for although no explosion can take place while the gauze is sound, yet at that high temperature the metal becomes rapidly burnt and might easily break, and a single opening, if large enough, would then cause a destructive explosion. Many improvements have since been made in Davy's lamp.