The Safety Lamp was invented by Sir Humphrey Davy, and was constructed so as to burn without any danger in an explosive atmosphere. It is merely a common oil-lamp, the frame of which is enclosed in a cylindrical cage of wire-gauze, sometimes made double at the upper part where the hottest portion of the gas collects, and containing about 400 apertures to the square inch. The wick is trimmed by means of a bent wire, passing tightly through the body of the lamp, so that when the lamp has been supplied with oil, the wick maybe kept burning for any length of time without unscrewing the cage. When this lamp is immersed in an explosive mixture of marsh-gas or coal-gas and common air, the gauze cylinder becomes filled with a blue flame, arising from the combustion of the gas within; but the flame does not communicate to the outside, even though the gauze may be heated to less redness.