1 G ELEMENTS OF AGRICULTURE" 



prevented from becoming intensely cold. When the* 

 earth warms up the air it is said to radiate its heat. 

 Eadiation means that a warm body is throwing off heat 

 The sun radiates heat; a hot stove or fire also radiates 

 heat; The earth always gives Up its heat when Colder 

 air presses against its surface; Thus radiation may 

 take place at any time, but it is Usually greater at nighty 

 because the air is then colder. 



12. Light Waves*- — The light waves which come to 

 Us from the sun along with the heat waves also have 

 their part to play in aiding the growth of plants and 

 animals. Just what effect they have we do not certainly 

 know, but we do know that light is absolutely necessary 

 to the growth of most plants. In far northern coun- 

 tries, such as Norway and Siberia, farm crops make a 

 more rapid growth than they do farther south. This 

 rapid growth must be due to the abundance of light, for 

 the days are never very warm, and often the ground is 

 frozen a few feet below the surface. But during the 

 season when plants grow the sun shines almost con- 

 tinuously, the nights being only about an hour long, and 

 the plants are thus abundantly supplied with sunshine. 



Questions 



1. What are the three forms in which all substances exist? 

 2. In which of these forms does water naturally exist? 3. When 

 water is heated, what form does it take? 4. When water is 

 frozen, what form does it take? 5. How does heat affect iron? 

 C. How may air be changed into a liquid? 7. What effect has 

 heat on most substances? 8. What effect has cold on liquids 

 and gases? 9. What is a thermometer? 10. How is a ther- 

 mometer made? 11. What causes winds? 12. Tell how the 



