Carbon Dioxide.—Carbon dioxide is a product of combustion and respiration, and is generated in many ways during fermentation. It is a constituent of choke damp due to explosions in coal-mines, and is given off from lime-kilns, brick-kilns, and cement-works. It is often met with in dangerous quantities in wells and in brewers' vats. From 10 to 15 per cent. in the atmosphere would prove fatal, but even 2 per cent. inhaled for long would produce serious symptoms. The proportion usually present in air is 0.04 per cent.

Symptoms.—Inhalation of the pure gas causes spasm of the glottis, insensibility, and death from asphyxia, at once; diluted, causes sense of weight in forehead and back of head, giddiness, vomiting, somnolence, loss of muscular power. Insensibility, stertorous breathing, lividity of face and body, and death from asphyxia. Convulsions occasionally.

Post-Mortem Appearances.—Face swollen and livid, or calm and pale; lividity is most marked in eyelids, lips, ears, etc.; limbs usually flaccid, abdomen distended; right side of heart, lungs, and large veins, gorged with dark-coloured blood. Brain and membranes congested.

Treatment.—Pure air, cold affusion, stimulants, artificial respiration, galvanism, inhalation of oxygen, venesection, transfusion.

Carbonic Oxide.—This is one of the most poisonous of gases. It is evolved in the process of burning charcoal and coke in stoves or furnaces. Water-gas, obtained by passing steam over heated coke, contains 40 per cent. of the substance, the remainder being chiefly hydrogen. It forms the chief part of the deadly 'choke damp' after an explosion in a mine. Two per cent. in the atmosphere is immediately fatal.

Symptoms.—When in large amount, insensibility comes on at once; when in very small amounts, headache, giddiness, noises in the ears, nausea, and vomiting, with prostration, insensibility, and coma. There may be convulsions. Even in cases which recover, permanent impairment of the brain may result.

Post-Mortem Appearances.—The blood is bright red in colour, due to the interaction of carbonic oxide with hÆmoglobin. A rosy hue of the skin-surface and viscera is often noticed. Bright red patches of colour are found over the surface of the body. The spectrum of the blood is characteristic.

Treatment.—Ammonia to the nostrils, inhalation of oxygen, cold douche in moderation, artificial respiration, transfusion of blood.

Coal Gas.—Coal gas contains light carburetted hydrogen or marsh gas, olefiant gas, ammonia, sulphuretted hydrogen, carbonic acid, carbonic oxide, free hydrogen, and nitrogen. Coal gas has an offensive odour, burns with a yellowish-white flame, yielding water and carbonic acid. Cases of poisoning often due to escape of gas into the room.

Symptoms.—Headache and giddiness, foaming at mouth, vomiting, convulsions, tetanic spasms, stertorous breathing, dilated pupil. The breath smells of gas; there is profound stupor; the patient, if alive, exhales gas from the lungs when removed into a fresh room or into the air. Smell of gas in the room and in patient's breath.

Post-Mortem Appearances.—Pallor of skin and internal tissues; florid colour of neck, back, and muscles, if much CO present in the coal gas; fluid florid blood; infiltration of lungs.

Treatment.—Fresh air, artificial respiration, cold affusion, diffusible stimulants; inhalation of oxygen freely.

Sulphuretted Hydrogen is characterized by its odour, like that of rotten eggs. It is extremely poisonous.

Symptoms.—Giddiness, pain and oppression in stomach, nausea, loss of power; delirium, tetanus, and convulsions.

Post-Mortem Appearances.—Fluid and black blood (sulph-hÆmoglobin), smell of H₂S on opening the body; loss of contractility of muscles, rapid putrefaction.

Treatment.—Fresh air, stimulants, inhalation of chlorine.

Tests.—Acetate of lead throws down a brown or black precipitate according to the quantity of the gas.

Sewer Gas.—Cesspool emanations usually consist of a mixture of sulphuretted hydrogen, sulphide of ammonium, and nitrogen; but sometimes it is only deoxidized air with an excess of carbonic acid gas.

Symptoms.—If poison concentrated, death may ensue at once; if gas diluted, or exposure only short, insensibility, lividity, hurried respiration, weak pulse, dilated pupils, elevation of temperature to 104°, tonic convulsions not unlike those of tetanus.

Treatment.—Fresh air, oxygen, with artificial respiration. Stimulants, hypodermic of strychnine, and alternate hot and cold douche.

Irritant Gases are—(1) Nitrous acid gas; (2) sulphurous acid gas; (3) hydrochloric acid gas; (4) chlorine; (5) bromine; (6) ammonia. They have the common property of causing irritation and inflammation of the eyes, throat, and air-passages, and may cause spasm of the glottis, bronchitis, and pneumonia.

Sulphurous Acid Gas.—One of the products of combustion of common coal.

Hydrochloric Acid Gas.—Irrespirable when concentrated, and very irritating when diluted. Very destructive to vegetable life.

Chlorine.—Used in bleaching, and as a disinfectant. Greenish-yellow colour, suffocating odour. In poisoning, inhalation of sulphuretted hydrogen gives relief.