In an absolutely pure state, such as could only be secured by the admixture of the two essential constituents, oxygen and nitrogen, in proper proportion, the air does not exist in nature. The purest air contains a minute quantity of carbonic acid and a certain amount of water vapour, with traces of ammonia and ozone, varying in amount according to the situation, as well as organic and mineral particles. Of the constituents of healthy atmosphere, oxygen deserves the chief consideration as a powerful agent in the destruction of various impurities. Roughly, its proportion may be taken as one-fifth, while nitrogen constitutes the greater part of the remaining four-fifths, exercising apparently its chief function, that of diluting the oxygen and modifying its stimulant action. Animal life is not sustained by nitrogen, as it is a non-respirable gas; it is destroyed by oxygen by being carried on too rapidly - the animal under the violently stimulating, exciting influence may be said to live the whole of its life in a short space of time. It is only, therefore, by the combination of the two elements that a respirable atmosphere, capable of supporting life for its normal period, is obtained.

Air is distinguished by its almost unlimited capacity for absorbing impurities of all possible kinds. Wherever animals and plants exist carbonic acid and various organic substances excreted from the system are constantly being thrown into the air. Living organic bodies are also continually being conveyed by the air, sometimes long distances, and in this way certain infective particles are conveyed from diseased to healthy animals. A large number of these, however, cannot in all probability be carried to any great distance, unless under the influence of powerful currents. It is recorded that ships, when several hundred miles from land, sometimes have their sails and yards covered with sand; it can hardly be questioned, however, that such heavy material could only be conveyed such a distance by very high winds.

Examination of air is only possible to the unprofessional observer by the use of the organ of smell, and it may be said of air, as it has already been said of water, that the presence of a smell of any kind is proof of contamination. The taste and the eyesight will assist in certain cases when the air becomes charged with the smoke emanating from chimneys of factories in which trades, which are described as noxious trades, are carried on; but for the purpose of detecting invisible suspended matters, organic bodies, carbonic acid, watery vapour, ammonia, and other solid and gaseous products, the knowledge and skill of the expert are absolutely essential.