164 LIGHT AND SOUND 



when the light waves have been changed into heat they 

 cannot escape. 



The water vapor and impurities of the air act like glass 

 in making prisoners of the light waves and thus raising the 

 temperature of the earth. Without this the chill that 

 would follow the rapid radiation of heat from the earth 

 would quickly put an end to all animal and vegetable life. 



184. Light and Life. The energy of the sun is stored 

 up not only by the earth's atmosphere, but also by the 

 action of plants. Plants containing chlorophyll, the green 

 coloring matter of our common vegetation, are able by 

 the aid of sunlight to build up complex organic sub- 

 stances, such as sugar, starch, cellulose, etc. (cf. 310). 

 Plants build up these substances out of carbon dioxide 

 and water. Oxygen is set free (cf. 52). Energy is re- 

 quired, for this is chemical work, and the sun supplies it. 

 When the material produced by the plant is afterwards 

 used as fuel or food, it reunites with the oxygen ; the energy 

 reappears as body heat, muscular force, etc. In a very 

 real sense, therefore, plants store up the sun's energy for 

 their own use and for the use of animals. 



185. Exercises. 



1. Stand before a vertical mirror, and look at the image of your 

 hand while you write your name. What is the result? Explain it. 



2. A mirror on a dresser is 30 inches above the floor. How do you 

 tilt the mirror if you want a full length view of yourself? Why do you 

 step back from the dresser? 



3. Why is the broken top of a wave white (" white cap ")> while the 

 unbroken wave is blue, green, etc.? 



4. If you lay a small piece of thick glass over the middle part of a 



