So essential to life is the supply of oxygen to warm-blooded animals, that they can not be deprived of it, even for a very short time, without danger to life. As pure oxygen can not be inhaled for any considerable period without injury, admixture with some "indifferent gas" becomes necessary. Thus nitrogen is utilized in the formation of atmospheric air. The other indifferent gases are hydrogen and light carbureted hydrogen, or marsh gas: when mixed with oxygen, they can be breathed with impunity; when inhaled alone, they cause dyspnoea and asphyxia.

Certain other gases are entitled "irrespirable," because, owing to the chemical actions instantaneously set up, they can not be inhaled. Some of these irrespirable gases are acid—as carbonic, muriatic, hypo-nitrous, sulphurous, etc.; others form acids by combination with oxygen, as nitric oxide, etc.; others are alkaline, as ammonia; others establish an ozonizing action, as chlorine, bromine, etc.

Other gases are comprehended in a group as "poisonous," of which there are three classes. The first class contains those which seize and combine with the oxygen of the blood, decomposing haemoglobin. They are hence called "reducing gases," of which sulphu-reted hydrogen, phosphureted hydrogen, etc., are examples. The gases of the second class displace the oxygen and enter into combination with the haemoglobin. Carbonic oxide and nitric oxide are examples of these displacing gases. The third class contains the gases used for the production of anaesthesia.

In the process of respiration, the air loses its oxygen, nitrogen is relatively partly increased, carbonic acid accumulates, and certain ill-defined "organic matters" appear. This vitiated air exerts a most injurious influence, and soon becomes unfit to sustain life. The amount of carbonic acid which can be breathed without injury has not been definitely ascertained, but is greater than is commonly supposed. According to Pettenkofer and Voit, 10 per 1,000 volumes produce no discomfort. When, in a crowded apartment, headache, vertigo, and feeble circulation are apparently caused by the quantity of carbonic acid present, these results are in part due to the lessened supply of oxygen, and to the action of nitrogen and organic matters.

The foul air from cesspools and sewers affects the health of man in two modes: by the direct action of the gases of decomposition, and indirectly by acting as carriers of the germs of disease. As respects the former, its injurious influence has probably been exaggerated; for the original observation of Parent-Duchatelet, that workmen engaged in cleaning the sewers of Paris suffered from no diseases due to their occupation, has been abundantly confirmed in the subsequent experience of the French officials. It is probable, also, that certain persons possess a high degree of susceptibility to the action of sewer-gas, and are affected by comparatively minute quantities. When sewer-gas is admitted to private houses, and is breathed by such susceptible persons, they decline in health, become easily fatigued, lose appetite and suffer with nausea and diarrhoea, waste, and have an earthy, sallow complexion. If the gas is more concentrated, they experience more acute symptoms, and have attacks of a choleriform character. Of course, asphyxia is produced when the gas is so concentrated as to displace oxygen entirely, as is the case sometimes in old drains and privy-vaults, the air of which, as is well known, may extinguish life speedily.

Sewer-gas, examined microscopically, is found to be loaded with minute particles, the lowest forms of life, etc., and hence it is in a high degree probable that the germs of fevers, dysentery, cholera, diphtheria, etc., may in a similar manner be conveyed by it.