CHAPTER XIII 



OXYGEN 



(Oxygen 0) 



Introduction. Oxygen is by far the most abundant 

 of all the elements. It is found practically everywhere, 

 either in the free state or in combination with other 

 elements. It is found in the air in the free state and 

 forms one-fifth of its volume. In combination with 

 another element, hydrogen, it forms eight-ninths of 

 the weight of water. 



Nearly one-half of the earth's crust is oxygen in 

 combination with other elements. Even living matter 

 is made up largely of oxygen in combination with other 

 substances. The human body contains 66% oxygen. 



Oxygen is also one of the most important of the 

 elements. If a lighted candle is placed in a glass jar 

 and the jar tightly closed, the flame will soon be extin- 

 guished. Then if the gas in the jar is examined no 

 oxygen will be found. Likewise, if a living being is 

 enclosed with a limited supply of oxygen, as some- 

 times occurs in the caving in of the wall of mines, the 

 life soon goes out. 



In fact, without this important element there could 



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