154 ELEMENTARY SCIENCE 



that the total volume of oxygen which they give back 

 to the air is very great. 



But the answer to this question of oxygen-supply im- 

 mediately raises another question, and so it always is in 

 the study of nature; the answer to one question usually 

 raises many others. If the plants use carbon dioxide in 

 maintaining the oxygen-supply, how is the carbon-dioxide 

 supply maintained ? Perhaps you can think of the answer 

 yourself? Think of all the burning and breathing that 

 goes on in the world. From both of these processes carbon 

 dioxide is given off. 



QUESTIONS 



1. What are the limits of the earth's atmosphere, and how have 



they been measured? 



2. What is the composition of air? 



3. Why can you see dust particles in a sunbeam? 



4. Why is there any light in shady places? 



5. What relation has dust to rain? 



6. What maintains the supply of oxygen in the atmosphere? 



How? 



7. What maintains the supply of carbon dioxide? How? 



