3H THE POPULAR SCIENCE MONTHLY 



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 re- 

 garding 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 sub- 

 ject 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 in- 

 gredient that is necessary to both combustion and respiration, of at- 

 tempts to identify this substance, of the contest between the phlogistic 

 and the antiphlogistic theories, and finally of the rather rapid crystal- 

 lizing 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 recog- 

 nized. Nitrogen was next to appear, the discovery of Daniel Ruther- 

 ford 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 Eayleigh and Sir William Eamsay 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 discov- 

 ered 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 ap- 

 proximate 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- 



