CHOKE-DAMP, also known as "black-damp" and "stythe," is a mixture of carbon dioxide and nitrogen, pure choke-damp according to Dr. J. Haldane, containing about 13% of the former and 87% of the latter; but probably the relative proportions are variable. Choke-damp, which in this connection is more frequently known as "stythe," is found in old, abandoned, or worked portions of the mines—especially wet mines—where the ventilation is stagnant, and on the occasion of a fall in the atmospheric pressure emerges from the "wastes" ("goaves") and makes its appearance in the workings, and, being heavier than air, lies along the floor of the mine gradually ascending as it increases in volume.
Choke-damp is the result of the absorption of the oxygen in the air by the coal substance in the mine and the formation of carbon dioxide in the process; it is also produced by the decay of timber, the breathing of men and animals, the burning of lamps and candles, and the firing of explosives. Spontaneous combustion, or self-heating of coal, which is a characteristic of some coal mines, is merely the rapid absorption of oxygen by the coal, which pro duces carbon dioxide and carbon monoxide in the process.
Carbon monoxide is known to miners as "white-damp" but is present only in minute quantities in the mine. It is the result of incomplete combustion, may be caused by the exploding of gun powder and other explosives, and is always present in the "af ter damp" resulting from an explosion of fire-damp or of coal dust.
Whereas carbon dioxide, if present in sufficient quantity, ex tinguishes light and life mainly by suffocation, carbon monoxide, if present in the air to an extent greater than i%, can be detected by a cap on the ordinary flame of a safety lamp. It is a virulent poison, its destructive effect being due to its action on the haemo globin of the blood with which it chemically combines, robbing the body of its oxygen. But haemoglobin when saturated with carbon monoxide cannot take up oxygen. An atmosphere containing as low as 0.2% of the dangerous gas will cause in time complete helplessness and loss of consciousness. An analysis of after-damp made by Dr. Bedson after the Usworth explosion (County Dur ham) in 1885 showed it to contain— The bodies of persons killed by after-damp in which carbon monoxide was the fatal agent, present a pink and healthy look due entirely to its coloration effect, and rigor mortis is not present.
(R. R.)