There are three mechanical mixtures of gases found in mines, and these are (a) air, a mixture of oxygen and nitrogen gases; (b) firedamp, a valuable mixture of marsh gas and air: and (c) afterdamp, a variable mixture of nitrogen, carbonic acid, and carbonic oxide. The compound g: usually found in coal mines are four in number, and these are (a) light carburetted hydrogen or marsh gas I'll:': (6) sulphuretted hydrogen (H2S), sometimes called stinkdamp; (c) carbonic oxide (CO) or white-damp; ami (d) carbonic acid CO2) or blackdamp. Marsh gas K'ilo, the lightest of the hydrocarbons, having a specific gravity of only "559, is a colourless, odourless, and tasteless gas. It burns with a blue flame, but will not support combustion. It diffuses rapidly in the air and forms firedamp. It does not poison the system. and may be breathed with impunity for a long time. Carbonic oxide gas, whitedamp (CO), has a specific gravity of .967, and is a colourless, odourless, and taste-Less gas burning with a pale blue flame. It is very poisonous to the system, acting as a narcotic-, producing stupor and pains in the back and limbs, followed by delirium. Lamps burn brightly in this gas.

It is the most dangerous gas found in coal mines, because its detection is often too late to enable its baneful effects to be avoided. Carbonic acid gas, blackdamp (002), is heavier than air, having a specific gravity of 1.529. It is a colourless and oderless gas, but has a distinctly sweet taste. It is incombustible, and will not support, combustion. Lamps burn dimly in air containing a small percentage of the gas, but are extinguished if the percentage increases sufficiently. Its effect upon the system is to produce headache and nausea, and finally unconsciousness, causing death by suffocation.