84 HEAT AND ENERGY 



4. Name some uses of convection. In what sorts of matter is 

 convection possible V Could a kettle of water be heated if placed 

 beneath a fire? Show how convection is useful in stoves and 

 lamps. 



5 . How does radiation differ from conduction and convection ? 

 Are all substances heated with equal ease by radiation ? Which is 

 warmer in summer, a tar walk or a grass lawn side by side ? Why ? 



6. Carefully explain radiant energy and the ether. 



SECTION IV 

 ARTIFICIAL COLD 



94. How Masses are cooled. Since cold means sim- 

 ply absence of heat, it follows that cold cannot be put 

 into a body, but the only way to cool any mass is to take 

 away some of its heat. This is commonly done by put- 

 ting the thing near some cold substance, when its heat 

 will gradually flow into the other ( 89). In this way, 

 things put into an ice box give up some of their heat 

 to the ice ; thus the ice is slowly melted and the sub- 

 stances become cool. We feel cold on a wintry day, 

 because heat is rapidly taken from our bodies by the 

 cold air about us ; and we wear clothing not to keep 

 out the cold but to keep the heat in. 



95. Artificial Cold. The common method of cooling 

 does not give us very low temperatures, for no sub- 

 stance is naturally colder than the lowest degrees which 

 climate allows. Very low temperatures are obtained by 

 performing some process which requires heat, near the body 

 from which we wish the heat to be taken. The processes 

 generally used are melting and vaporization. 



