Whenever carbon is burned with an insufficient supply of air, CO in a certain quantity is produced. It is always present in ordinary domestic products of combustion, and must be exhaled from the various chimneys of a large city in considerable volumes. A “smoky” chimney or a defective flue will therefore introduce carbon monoxide into living-rooms. The vapour from burning coke or burning charcoal is rich in carbon monoxide. It is always a constituent of coal gas, in England the carbon monoxide in coal gas amounting to about 8 per cent. Poisoning by coal gas is practically poisoning by carbon monoxide. Carbon monoxide is also the chief constituent in water gas. Carbon monoxide poisoning occurs far more frequently in France and Germany than in England; in those countries the vapour evolved from burning charcoal is a favourite method of suicide, on account of the supposed painlessness of the death. It has also occasionally been used as an instrument of murder. In this country carbon monoxide poisoning mainly takes place accidentally as the effect of breathing coal gas; possibly it is the secret and undetected cause of ill health where chimneys “smoke”; and it may have something to do with the sore throats and debility so often noticed when persons breathe for long periods air contaminated by small leakages of coal gas. The large gas-burners (geysers) emit in burning under certain conditions much carbon monoxide. It has been proved by Gréhant[56] that a bunsen burner “lit below” also evolves large quantities of the same poisonous gas. [56] _Compt. Rend. Soc. de Biol._, ix. 779-780. § 38. =Symptoms.=--Nearly all the experience with regard to the symptoms produced by carbon monoxide is derived from breathing not the pure gas, but the gas diluted by air, by hydrogen or by carburetted hydrogen, as in coal gas, or mixed with large quantities of carbon dioxide. Two assistants of Christison breathed the pure gas: the one took from two to three inhalations; he immediately became giddy, shivered, had headache and then became unconscious. The second took a bigger dose, for, after emptying his lungs as much as possible, he took from three to four inhalations; he fell back paralysed, became unconscious and remained half-an-hour insensible and had the appearance of death, the pulse being almost extinguished. He was treated with inhalations of oxygen, but he remained for the rest of the day extremely ill; he had convulsive muscular movements, stupor, headache, and quick irregular pulse; on this passing away he still suffered from nausea, giddiness, alternate feeling of heat and chilliness, with some fever, and in the night had a restless kind of sleep. The chemist Chenot was accidentally poisoned by the pure gas, and is stated to have fell as if struck by lightning after a single inspiration, and remained for a quarter of an hour unconscious. Other recorded cases have shown very similar symptoms.