ever remain as the most drastic demonstration in human history of the bondage of man to the air that surrounds him.
What is this thing upon which the life of the body is so dependent? As history goes, it is only comparatively recently that we have learned what air is. "To tell the story of the development of men's ideas regarding the nature of atmospheric air/' says Sir William Ramsay, "is in great part to write a history of chemistry and physics." Believed at first to be a single substance, by the middle of the seventeenth century men began seriously to try to learn by means of experiment whether air is not compound. It would take us too far from our immediate subject to wander through the mazes of more than a hundred years of those early efforts, of the rise of the belief that air contains some ingredient that is necessary to both combustion and respiration, of attempts to identify this substance, of the contest between the phlogistic and the antiphlogistic theories, and finally of the rather rapid crystallizing out of the air's constituents. The credit of solving the problem belongs almost wholly to Englishmen. In 1755 Joseph Black isolated carbon dioxide, the first constituent of the air to be definitely recognized. Nitrogen was next to appear, the discovery of Daniel Rutherford in 1772; and two years afterward Joseph Priestley published the first description of oxygen. Here the matter rested for more than a century, when in 1895 Lord Rayleigh and Sir William Ramsay aroused the world by the announcement that they had found in air a new gaseous element in minute quantity, which they proposed to christen argon, the inert. To this Ramsay subsequently added the still more rare helium, krypton, neon and zenon, and he says:
It would be rash to predict that no other elements still remain to be discovered among its [the air's] constituents; but if there are they must be present in still more infinitesimal amount than the rarer non-active gases.
We may, therefore, doubtless rest content with the thought that the problem of the air's constituents is practically solved and that, when pure, air is simply a mixture of gases, mostly elementary.
Air of ideal purity never exists outside the chemist's test tubes. The gases of atmospheric air are usually present in the following approximate proportions by volume:
| Per Cent. | ||
| Oxygen | 20.94 | |
| Carbon dioxide | 0.03 | |
| Nitrogen | 78.09 | |
| Argon | 0.94 | |
| Helium, krypton, neon, zenon, hydrogen, hydrogen peroxide, ammonia | traces. | |
As we know it and breathe it, air always contains other constituents derived partly from the inorganic earth, partly from plants and ani
