248 NATURE STUDY AND AGRICULTURE 



doubt some were reflected or thrown back. Some sub- 

 stances allow some rays of heat to pass through them. 

 This is true of glass. The rays of heat from the sun pass 

 through it. What use do florists make of this ? 



How is the soil or ground heated ? The radiant heat 

 of the sun strikes it, and while some of this is reflected back 

 into the air, some of it is absorbed into the top layer of soil. 

 Just as different substances differ in their power to con- 

 duct heat, so some substances absorb heat more rapidly 

 than others. 



Lay a piece of black cloth on the surface of the snow 

 on a sunshiny day. Beside it lay a white cloth. Under 

 which does more snow melt? Which absorbs more heat 

 from the sun ? Which is warmer, black or white clothing ? 

 Which will get warm sooner in the spring, a black soil or a 

 light-colored one? 



How does a body that is heated lose its heat ? Think 

 again of the hot-poker experiment. Did it get cold after 

 you held it awhile in the air? What do you mean by 

 "getting cold"? It gave out, or radiated heat. How 

 does the earth cool ? Just as the poker did. All day long 

 while the sun shines the earth absorbs heat. All night long 

 it radiates heat. What do you think becomes of this heat 

 that the earth gives out? Much of it is absorbed by the air, 

 or rather by the water vapor in the air, and thus it warms 

 the air. In fact, the air is warmed much more by the heat 

 from the earth than it is from the direct rays of the sun 

 that pass through it. 





