DEW AND FROST. 255 



and, setting fire to the phosphorus, will light the 

 candle. 



If a thermometer be placed in the focus of.one mir- 

 ror 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 thermiometer radi- 

 ates more heat to the mirrors, and then to the ice, than 

 the ice returns. 



DEW AND FROST. 



All bodies are constantly radiatmg 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, ob- 

 jects 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 contact with the moist- 

 ure 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 rea- 

 son 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 a sheet of thin muslin, stretched on pegs over 

 garden vegetables, has afforded sufficient protection, 

 when those around were destroyed. 



