COMPONENTS OF THE AIR 387 



all substances. Almost the whole earth, whether it be rich 

 loam, barren clay, or granite bowlder, contains oxygen in 

 some form or other; that is, in combination with other sub- 

 stances. 



A less familiar but more abundant constituent of the atmos- 

 phere is nitrogen. The fact that the oxygen in air is diluted 

 with so large a proportion of nitrogen prevents fires from 

 sweeping over the world and destroying everything in their 

 path. Nitrogen does not support combustion, and a burning 

 match placed in a bottle of air goes out as soon as it has used 

 up the oxygen. The nitrogen in the bottle not only does 

 not assist the burning of the match, but it acts as a damper 

 to the burning. Free nitrogen is not poisonous ; but one 

 would die if surrounded by it alone, just as one would die if 

 surrounded by water. The vast supply of nitrogen in the 

 atmosphere would be useless if the smaller amount of oxygen 

 were not present to keep the body alive. 



Another constituent of the air with which we are familiar 

 is carbon dioxide. Even in pure air, carbon dioxide is present 

 in very small proportions, being continually taken from the 

 air by plants in the manufacture of their food. 



Various other substances are present in the air in minute 

 proportions, but of all the substances in the air, oxygen, 

 nitrogen, and carbon dioxide are the most important. 



CLARK INTRO. TO SC 25 



