In ordinary furnace-ventilation, where the expansion of the air may be increased from 53° to 105°, 30,000 cubic feet of air entering the mine per minute, at a density of 1000, will leave it at a density of .8774, or .0706 pound to the cubic foot, and with a volume increased to 36,000 cubic feet. The amount of impurities taken up in the mine in the shape of carbonic acid, nitrogen, &c., from the respiration of 300 men and the burning of their lamps, would require less than 1000 cubic feet per minute, since each man produces about one cubic foot of carbonic acid gas per hour, or 300 men about 5 cubic feet per minute, which requires 25 cubic feet of pure air, while the amount to support the combustion of lamps would be still less: therefore, when we say 1000 cubic feet per minute, we allow a superabundance of air for health and all the purposes of the mine. If, however, the mine produces 1000 feet of gas per minute, it requires not less than 20,000 cubic feet to adulterate it above the explosive point, or 30,000 feet to render it entirely safe and innoxious. It is, therefore, evident that the gas of our mines is the great impediment in the way of ventilation, and requires ten times the volume of pure air required for all other purposes. Consequently, if this gas can be made an active instead of a repellant agent, the great difficulty of ventilation is overcome.
The gases most dangerous to miners, next to carburetted hydrogen, are carbonic acid, carbonic oxide, and nitrogen. Carbonic acid is produced by the respiration of men and
animals, by the combustion of carbon in lamps or candles, and by other causes con stantly operating in deep mines. This gas is fatal to life if mixed with the air to the extent of 12 or 15 per cent. It is a heavy vapor, of a density of 1.548, and can be poured from one bottle to another. It is the lower stratum in all mines, as the carbu retted hydrogen is the highest. Lamps will not burn where it exists in an excess of 15 per cent. It is composed of one part carbon and two parts oxygen.
Carbonic oxide is generally produced by the imperfect combustion of carbon. It is a compound of one part carbon and one part oxygen. Nitrogen is the principal element of the atmosphere, and is produced in mines by the extraction of the oxygen, by combustion, or otherwise. These gases are all fatal to life,—either in their proper equivalents, or when mixed with several equivalents of the atmosphere. They con stitute the "after-damps" of explosions, and often become more destructive to life than the explosion itself, since the effects of explosions generally destroy all air-ways and main avenues, and not only prevent the escape of the men, but prevent the currents of air from supplying the necessary volumes to render the after-damp innoxious.