As a therapeutical agent, only, is air to be considered here. Nevertheless, various hygienical relations of the subject are also therapeutical, and must therefore be included

Composition

The atmosphere is a mechanical mixture of oxygen, nitrogen, and we should also add carbonic acid, and the vapor of water. In round numbers there are 21 parts of oxygen, 79 parts of nitrogen by volume, and 033 per cent of carbonic acid. The essential constituents—oxygen and nitrogen—are quite uniform in proportion in all parts of the world, but the carbonic acid and vapor of water vary considerably, owing to local causes. Contrary to the generally received opinions, the quantity of carbonic acid is greater at the tops of the highest mountains than on the plains, and the oxygen less. The amount of moisture is determined chiefly by temperature, and when the air can contain no more, it is said to be saturated. A trace of ammonia also is present in the air, and a minute quantity of active oxygen or ozone.

When a beam of electric light is made to traverse the atmosphere, minute particles, previously invisible to the eye, and some even extra-microscopic, come into view. The lowest forms of life—microzymes, bacteria, etc.—are found in great numbers; in cities, particles of carbon, lime, and other earthy matters, fibers of cotton and woolen cloth, leather, ordure of animals, etc.; in the country, pollen of plants, and particles of vegetable tissues; in hospital wards, pus-cells and other matters derived from the persons, wounds and discharges, bedding, and utensils of the sick. We owe our present knowledge of these accidental constituents of the air chiefly to the researches of Prof. Tyndall.

Various gaseous matters are also accidentally present in the air, varying in different localities in consequence of local telluric conditions, and certain trades and manufactures. As respects the first, there are carbonic acid and carbonic oxide, sulphureted and phosphureted compounds of hydrogen, marsh miasm, etc. Under the second may be included sewer-gas, gaseous emanations from decomposing animal matter, from chemical works of various kinds, etc.

Physiological Effects

1. Pure Air.—When air is breathed in the process of respiration, the oxygen passes into the blood, and carbonic acid, the vapor of water, a trace of ammonia, and some organic matter, pass out. The blood which leaves the lungs is changed in color, from the dark, purplish hue of venous, to the scarlet of arterial blood; it has gained oxygen, and lost carbonic acid and water. The cause of the diffusion of gases in the lungs is chiefly the difference in tension of the gases in the blood and in the atmosphere—the tension becoming equal in the process of respiration (Hermann).

In the tissues where oxidation processes are taking place, there is, also, a respiration process going on, consisting in the formation and excretion of carbonic acid.