miners as "fire-damp." It is colorless, tasteless, odorless, when pure, burning with a yellowish flame. It is formed in swamps and marshy places by the decomposition of vegetable matter, and may be seen bubbling up through the water when the mud is stirred beneath. Marsh-gas is found in such quantities in some places, that it is used for lighting towns, and evaporating brine. In the oil-regions it frequently bursts forth with explosive violence, throwing the oil high in the air when the drill nears it. Coal-gas contains about thirty-eight per cent of marsh-gas. When marsh-gas is evolved in the shape of "blowers," it constitutes about ninetysix per cent of the total volume. Blowers sometimes assume enormous dimensions, and have been conveyed from the workings to the surface by means of pipes, and utilized. Marsh-gas is not poisonous. Sir H. Davy, of safety-lamp fame, was the first to experiment on this gas. He found, that, when mixed with three and a half times its volume of air, it did not explode; with five and a half times its volume, it exploded slightly; and, when mixed with eight or nine volumes of air, the force of explosion was greatest. When there is a deficiency of ventilation, the fire-damp is said to rise to the upper portion or top of a gallery, and there remain, because of its being lighter than air. It is also said that carbonic acid, being heavier than air, lodges on the "floor"

When mixed with oxygen, it will explode if ignited. When inhaled in a pure state, it is a powerful narcotic poison, and produces fainting and asphyxia when present in very small proportions of the atmosphere. It appears to be probable that the gas is generated in small quantities in old worked-out mines. Some claim that it is formed by the decomposition of pyrites in old workings; others, that it is not formed in this manner, but by the decomposition of props and timber standing in water, by breaking up the sulphate of lime, and assimilating its oxygen, while sulphur seizes upon the hydrogen of the wood to form sulphuretted hydrogen. This gas is also known as hydrosulphuric and sulphuric acid gas.

MARSH-GAS.

Symbol, CH4. Equivalent, 8. Specific gravity, 0.55314.

7. One cubic foot of this gas, at 32° F. and barometer of 30′′, weighs 0.044665 of a pound.

It is known by several names,-proto-carburetted hydrogen, light carburetted hydrogen, hydride of methyl, fire-damp. Marsh-gas, however, is better known to

miners as "fire-damp." It is colorless, tasteless, odorless, when pure, burning with a yellowish flame. It is formed in swamps and marshy places by the decomposition of vegetable matter, and may be seen bubbling up through the water when the mud is stirred beneath. Marsh-gas is found in such quantities in some places, that it is used for lighting towns, and evaporating brine. In the oil-regions it frequently bursts forth with explosive violence, throwing the oil high in the air when the drill nears it. Coal-gas contains about thirty-eight per cent of marsh-gas. When marsh-gas is evolved in the shape of "blowers," it constitutes about ninetysix per cent of the total volume. Blowers sometimes assume enormous dimensions, and have been conveyed from the workings to the surface by means of pipes, and utilized. Marsh-gas is not poisonous. Sir H. Davy, of safety-lamp fame, was the first to experiment on this gas. He found, that, when mixed with three and a half times its volume of air, it did not explode; with five and a half times its volume, it exploded slightly; and, when mixed with eight or nine volumes of air, the force of explosion was greatest. When there is a deficiency of ventilation, the fire-damp is said to rise to the upper portion or top of a gallery, and there remain, because of its being lighter than air. It is also said that carbonic acid, being heavier than air, lodges on the "floor"

16

MINE VENTILATION.

§ 7.

or "thill" of a mine. Mr. Thomas, in his book on "Mines, Gases, and Ventilation," says, "This impression is erroneous; and while not denying that fire-damp is often found in larger quantities near the roof, and carbonic acid in larger quantities near the floor, these positions do not prove that they have lodged there, nor is it so on the contrary, marsh-gas is always diffusing in every direction; and it is only in those places where the gas is evolved in greater quantity than will diffuse, or become carried away by the ventilation, that accumulation takes place. These erroneous ideas in reference to marsh-gas arose from the fact that it is found in the crevices and holes, and near the roof, of coalmines." The explanation of this is very simple. The fact of portions of top-rock falling, and the squeezing-in or lowering of the top throughout the whole worked portion above the coal, affords communication with, it may be, some rider or unworked seam of coal above, with the receding working-face, or with crevices which are in communication with stores of fire-damp extending to considerable distances. Now, the pressure of the atmosphere being the same on all sides, the gas in the fissures and cracks in the top are subjected to that pressure externally, so that air finds no outlet through these cracks, and the diffusion which takes place is simply "natural" or mutual diffusion.

The fire-damp issuing into these cracks, etc., encounters the same pressure as if issuing direct into the aircurrent; so that it would find its way downwards by virtue of the extra force of pressure of the imprisoned gas into the top of the holes, goaves, or receptacles in the top-rock, and it would be more likely to escape or find an exit in these places, or come in contact with the ventilating current here, owing to the fact that more easy communication is afforded by the dislocation and partial disintegration of the top caused by a fall, followed by a partial opening-up of the surrounding mass. The fire-damp, therefore, instead of accumulating and lodging at the roof of the gallery by virtue of its lesser density, is forced downwards until it finds its way into the holes and other receptacles, and is continually fed from above.

"When marsh-gas escapes from the floor, and makes its appearance in quantity near the roof, it shows deficiency of ventilation, or a strong outpour of gas; but even then, if there is any ventilation at all, it will be largely mixed with air."

The reason that fire-damp is in the form of an explosive mixture in holes in the top, is because it is fed more quickly from above than it is able to diffuse; the ventilating current not affecting it to a very great

extent.