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Hydrogen Sulphide.
A gas popularly known as stinkdamp from its characteristic repulsive odour of rotten eggs. It is slightly heavier than air, and burns with a pale blue flame. It causes silver to become blackened by silver sulphide.
It is a constituent of the gases given off during the spontaneous heating of coal, and is formed by the action of bacteria on decaying organic matter, and is also a constituent of volcanic gases.
Stinkdamp is extremely poisonous, more so than carbon monoxide. As little as 0.005% is dangerous, and the inhalation of an atmosphere containing from 0.0105% to 0.1% causes serious symptoms in a few minutes. An important characteristic is that it destroys the sense of smell.
Exposure over a long period to low concentrations causes conjunctivitis, pains in the chest and bronchitis.
In small percentages it is detected by the sense of smell. Otherwise it can be detected by the use of Lead Acetate papers which turn black in the presence of stinkdamp.

Pit Terminology - Glossary