mals and largely from man and his works. Its oxygen is often diminished in quantity, its carbon dioxide often increased; it always contains the vapor of water in appreciable amounts, traces of nitrous and nitric acids, radio-active constituents, dust and usually bacteria; its composition may be altered by the presence of sea salts, by the respiration of man and other animals and plants, by the combustion of illuminating gas and its products, and by a host of industrial processes. Of these various alterations those produced by the respiration of man is of chief interest to us here. The gases of air as it comes out of man's lungs are present in the following approximate proportions by volume:
| Per Cent. | ||
| Oxygen | 16 | .4 |
| Carbon dioxide | 4 | .1 |
| Nitrogen | 78 | .09 |
| Argon | 0 | .94 |
| Helium, krypton, etc. | traces. | |
Of the air's various gaseous constituents, nitrogen, argon and helium and its companions are what the chemists call inert substances, i. e., they are slow and backward about entering into chemical alliances. However important nitrogen is in the life of living things, neither is it, nor are these other inert gases, known to exert as atmospheric components specific actions on living human beings. They may, therefore, be eliminated from consideration, as may also the minute traces of hydrogen and ammonia, and our attention may be focused at once upon oxygen and carbon dioxide.
With oxygen the case is very different from that of nitrogen. A component of all living tissues and a participant in nearly all vital processes, its one great source is the atmosphere, and its entrance into the human body is by way of the lungs in respiration. Without it man would perish; yet his body is so adapted that to sustain life permanently oxygen must be given to him in a certain percentage and under a certain pressure, both percentage and pressure varying within certain limits. Under the ordinary conditions of life the proportion of oxygen in the air that we breathe varies only slightly. Thus the air of the open country and that of the streets of crowded London differ by less than one tenth of one per cent. At the sea-side and on a mountain top 14,000 feet above, the percentage of atmospheric oxygen is practically the same; on the mountain top, however, the pressure of the gas may be less than two thirds of its pressure at the sea-side. Every inexperienced climber has felt the need of a greater pressure of oxygen. Archdeacon Stuck, when led by the indefatigable Karstens to the top of Mt. McKinley, suffered greatly from the rarefied air. Writing in the third person he says:
