278 



HEAT. 



other place the wick of a candle with a small shaving of 

 phosphorus in it. The heat will be reflected, as shown by 



Fig. 295. 



the dotted lines (fig. 295), and, setting fire to the phos- 

 phorus, will light the candle. 



If a thermometer be placed in the focus of one mirror 

 while the hot iron ball is in the other focus, It will rise 

 rapidly ; but if a lump of ice be substituted for the ball, 

 the thermometer will immediately sink, and will continue 

 to do so until several degrees lower than the surrounding 

 air; because the thermometer radiates more heat to the 

 mirrors, and then to the ice, than the ice returns. 



DEW AND FROST. 



All bodies are constantly radiating some heat, and if an 

 equal amount is not returned by others, they grow colder, 

 like the thermometer before the lump of ice. Hence the 

 reason that on clear, frosty nights, objects at the surface 

 of the earth become colder than the air that surrounds 

 them. The heat is radiated into the clear space above 

 without being returned ; plants, stones, and the soil thus 

 become cooled down below freezing, and, coming in con- 

 tact with the moisture of the air, it condenses on them 

 and forms dew^ or freezes into white frost. Clouds return 

 or prevent the passage of the heat that is radiated, which 

 is the reason there are no night-frosts in cloudy weather. 

 A very thin covering, by intercepting the radiated heat, 

 will often prevent serious injury to tender plants. Even 



