

WHAT MAKES THE AIR USEFUL TO MAN? 51 



vessels, it was moving currents of air, or winds, that 

 carried him to a new and unknown land. Today the air 

 buoys up airships, 

 and propellers push- 

 ing against the air 

 can make the airships 

 move without the aid 

 of a wind. It is not 

 merely in mechanical 

 ways that air is im- 

 portant. There are 

 many vital chemical 

 processes dependent 

 upon it. Breathing 

 animals take oxygen 

 from the air. If the 

 water were all driven 

 out of plants, by far 

 the greater part of the 

 material left would 

 be carbon taken out 

 of the air by the 

 plant. Since plants 



are essential for ani- what causes this boat to move ? 



mal life, we can truly 



say the air is no less important, for without air there 

 would be no plants ; nor would there be animals. 



SELF-TESTING EXERCISE 



Select from the following list those words which best fill the blank 

 spaces in the sentences below and arrange the words in proper numer- 

 ical order. A word may be used more than once. 



oxidation 



oxygen 



out 



air 



heat 



rust 



extinguish 



combustion 



nitrogen 



fire 



dioxide 



oxide 



