FRESH AIR 315 



mals and largely from man and his works. Its oxygen is often dimin- 

 ished 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 compo- 

 sition 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 compo- 

 nents 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 perma- 

 nently 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 inexperi- 

 enced climber has felt the need of a greater pressure of oxygen. Arch- 

 deacon 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: 



