Marsh-Gas

or LIGHT CARBURETTED HYDROGEN. (Chem.) A gaseous hydro-carbon frequently occurring in nature. It is the fire-damp of miners, and frequently rises from the earth in marshy districts. Sp. gr. 0.557. It has neither taste, smell, nor color, and has no action on test-paper. It is very slightly soluble in water. When ignited, it burns with a pale white flame. Form. CH4. The safety-lamp is an oil lamp, the flame of which is surrounded by a cage of iron wire gauze, having 700 or 800 meshes in the square inch, and made double at the top where the heat of the flame chiefly plays. This cage is protected by stout iron wires attached to a ring for suspending the lamp. A bra tube pa es up through the oil reservoir, and in this there slides, with considerable friction, a wire bent at the top, so that the wick maybe A trimmed without taking off the cage. If this lamp be suspended in a large jar, closed at the top with a perforated wooden cover (A, Fig. 463), and having an aperture (B) below, through which coal-gas may be admitted, the lamp C will burn, of course, in the ordinary way; but if the to pa B gas be allowed slowly into the jar, the flame will be seen to waver, to elongate itself very considerably, and will be ultimately extinguished, when the wire cage will be seen to be filled with a FIG. 463. air mixture burning of tranquilly coal-gas and within the gauze,which prevents the flame from pa ing to ignite the explosive atmosphere surrounding the lamp; that an explosive mixture really fills the jar may be readily ascertained by introducing, through an aperture (C) in the cover, the unprotected flame of a taper, when an explosion will take place. This experiment illustrates the action of the Davy lamp in a mine which contains fire-damp, and makes it evident that this lamp would afford complete protection if carefully used. Its great defect is that it does not afford more than a glimmering light, so that even if the miners were prohibited from employing any candles, they would (and experience has proved that they do) remove the wire cage at all risks. The lamp has been modified so as partially to remove this defect, by substituting gla or talc for some portions of the wire gauze. It is now usual, however, to employ the Davy lamp merely in order to test the state of the Martins burg, air in the different parts of the mine; for this purpose the firemen descend before the commencement of work Martlet, every morning, and examine with their safety-lamps every portion of the mine, giving warning to the miners not to approach those parts in which any accumulation of fire-damp (or, technically, " sulphur ") is perceived. The miners then work with naked candles, and it appears to be not unusual to see a blue flame (or corpse light) playing around the candles, so that the miners may become accustomed to regard with little concern the very indication which shows that the quantity of fire-damp is only a little below that required to form an explosive mixture. [s. 580]