The Gases in Coal

It is not necessary for coal to be heated in order to give off gas. Much gas is occluded or adsorbed in coal, or is pent up in cavities in the seams. This gas is chiefly methane, and, because it is the cause of fires and explosions in mines, it is called fire-damp. Owing to the physical condition of the gas in the coal, its rate of evolution is influenced by variations in atmospheric pressure; hence miners are warned that a sudden fall in the barometer indicates danger from explosion of fire-damp. The occurrence of methane in coal is connected with the vegetable origin of the latter. Just as marsh-gas is generated by vegetable matter decaying under water at the present day, so was it produced from the plants of the carboniferous period. No doubt much of this gas escaped, but some remained adsorbed within the structure of the coal in process of formation. It is particularly in deep mines, and therefore where coal is under great pressure, that firedamp occurs.

The composition of the gas which can be extracted from coal by heating it to 100° c. in vacuo has been investigated by Thomas. The following are representative analyses of the gas from 100 grams of coal:

	Bituminous Coal.	Steam Coal.	Anthracite.
Volume of gas	55.1 c.c.	147.4 c.c.	600.6 c.c.
Percentage of CO2	5.44 c.c.	18.90 c.c.	14.72 c.c.
Percentage of CH4	63.76 c.c.	67.47 c.c.	84.18 c.c.
Percentage of O2	1.05 c.c.	1.02 c.c.	-
Percentage of N2	29.75 c.c.	12.61 c.c.	1.10 c.c.

The greater volume of gas obtained from anthracite is due to the fact that, being less porous than bituminous coal, it retains its gas better than the latter during removal from the mine.