RESPIRATION. 245 



A. Animals require oxygen to keep 

 them alive, and draw it from the air 

 by inspiration; the upper surface of leaves 

 (all day long) gives out oxygen, and thus 

 supplies the air with the very gas required 

 by man and other animals. 



Q. Do not animals EXHALE the VERY GAS 

 needed by VEGETABLES ? 



A. Yes ; animals reject the nitrogen 

 of the air (as not suited to the use of 

 animal life), but vegetables absorb it, 

 as it is the food they live on ; and 

 thus the vegetable world restores the 

 equilibrium of the air, disturbed by man 

 and other animals. 



Q. Is AIR a good CONDUCTOR ? 



A. No j air is a very bad conductor. 



Q. HOW is AIR HEATED ? 



A. By "convective currents. 1 ' 



Q. What are meant by "CONVECTIVE CUR- 

 RENTS?" 



A. When a portion of air is heated, 

 it rises upward in a current, carrying the 

 heat with it: other colder air succeeds, 

 and (being heated in a similar way) 



Y 3 



