ATMOSPHERE 127 



of cold air and is frozen. Frequently the small hail 

 stone thus formed is carried about by currents of air 

 and more layers of frozen water are added to it. This 

 process may continue for some time, and as a result the 

 large hail stones with which we are all familiar are 

 occasionally formed. Much damage to windows and 

 crops may be caused by the larger hail stones. 



Questions 



1. What is the height to which the atmosphere 

 extends ? 



2. What is meant by ' ' our environment ' ' ? 



3. Of what importance is atmosphere to us? 



4. What changes may take place in the at- 

 mosphere ? 



5. Is air a mechanical mixture or a chemical com- 

 pound? What are the reasons for your answer? 



6. What gases are found in the air? 



7. What per cent of each of these gases is present ? 



8. Are there any advantages in having four- 

 fifths of the air composed of an inactive gas like 

 nitrogen ? 



9. Explain in what way alfalfa or clover makes 

 soil more fertile. 



10. Of what use is the oxygen in the air? 



11. Why does not the supply of oxygen become 

 exhausted? 



12. What are the sources of carbon dioxide in the 

 air? 



13. How is an equilibrium of carbon dioxide 

 maintained? 



