THE CHEMICAL ELEMENTS 15 



oxygen. That which enters directly into the uses of animal 

 life is derived chiefly from the atmosphere and water. 



10. Uses of oxygen. — ^AIl life, whether vegetable or 

 animal, is dependent on the use of oxygen, during which 

 use this element passes into fixed combinations and back 

 again into the free form. The free oxygen of the air is 

 used by an animal in breathing and this it retiu-ns to the 

 air in part combined with carbon as carbon dioxid. On 

 the other hand, the plant appropriates the carbon dioxid 

 and, through synthetical processes, the carbon is built 

 into the plant tissues and the oxygen, which is set free, is 

 returned to the atmosphere and may be used to sustain 

 the demands of animal life. Oxygen is a factor in all 

 processes of decay and in many other chemical changes. 

 Fire is due to its union with the elements of the fuel. It 

 is the agent which maintains combustion in the furnaces 

 of our industries. The energy derived from the sun's 

 rays, which is stored in vegetable tissue, when the oxygen 

 is torn from its union with carbon is set free when through 

 combustion the oxygen is returned to its former combina- 

 tion. This circulation of oxygen is effected through the 

 opportunities offered by the vital processes of the plant 

 and animal. (See Par. 207.) 



11. Hydrogen. — In a free state, this element is the 

 lightest known gas and is found abundantly in nature 

 only in combination with other elements. A minute 

 proportion exists in the air which is derived from volcanic 

 action and possibly from decay under certain conditions. 

 When hydrogen and oxygen are combined in the forma- 

 tion of water, intense heat is produced. Hydrogen con- 

 stitutes about one-ninth of water by weight and it is 

 found also in a large number of soil compounds. It is an ' 

 essential constituent of vegetable and animal tissues, but 



