Oxygen Hydrogen 15 



bonic acid, This compound the plant appropriates, re- 

 taining the carbon for its tissues and giving back the 

 uncombined oxygen to the atmosphere to be again 

 used by animals. All decay and many other chemical 

 changes require the presence of this element. What 

 we speak of as fire is due to its union with the ele- 

 ments of the fuel. It bears an indispensable rela- 

 tion to the mechanical forces that man now employs, 

 for it is the agent which maintains combustion in the 

 furnaces of our industries. All the activities of life 

 are intimately related to it. When a plant grows, oxy- 

 gen is torn from its union with other elements by the 

 dominating power of the sun's rays, and energy is 

 stored in vegetable tissue. When this tissue is used as 

 food the oxygen returns to its former combinations 

 through the opportunities offered by the vital pro- 

 cesses of the animal, and the hidden forces of the plant 

 compounds are thus manifested in a variety of ways. 

 The animal labors and man toils and thinks because of 

 the energy thus stored and liberated. 



Hydrogen. This element, which, in a free state, 

 is the lightest known gas, is found abundantly in na- 

 ture only in combination with other elements. The 

 minute quantities which exist in the air are due to 

 volcanic action and possibly to decay under certain 

 conditions. As a manufactured product, it has an im- 

 portant use in producing intense heat and in filling bal- 

 loons. Hydrogen constitutes about one -ninth of water 

 by weight, and is found in a large number of soil com- 

 pounds. It is an essential constituent of vegetable 

 and animal tissues, although it exists in the compounds 



