EADIATION 89 



Why Water does not Heat as Quickly as the Soil. Water does 

 not heat as quickly as the soil for two reasons. The surface of the 

 water reflects more heat when calm than when it is rough. This 

 reflection often causes sunburn, since our skins are opaque and 

 absorb great quantities of radiant energy. The water is also 

 transparent; hence the radiant energy will penetrate several hun- 

 dred feet in the water, while on the soil all the radiant energy is 

 absorbed near the surface, usually not extending much below the 

 roots of plants. 



Why Soil Cools off Faster than Water. Things which absorb 

 radiant energy quickly also radiate heat 

 quickly. 



Fill two cans (one shiny and the other 

 black) with water at the same temperature. 

 Cover each can with a piece of wood. Take 

 the temperature of the water every few 

 minutes. Prove that dull, black objects give p IQ gg 



off heat more quickly than shiny objects. 



A black, sooty cup radiates heat twenty times faster than a 

 bright, shiny cup. I 



The rocks and soil of the earth radiate heat much more quickly 

 than water. In fact, things which absorb heat quickly also radiate 

 heat quickly, and things which absorb heat slowly radiate heat 

 slowly. 



Radiation and the Household. Hot objects, like stoves and 

 steam pipes, lose much of their heat by radiation, and the blacker the 

 objects the more they will lose; hence, stoves and steam pipes 

 should be black, if they are intended to give out heat; but hot-air 

 pipes, cooking utensils, etc., should be bright (tinned or nickeled) 

 in order to lose as little heat as possible. A stove nickel-plated 

 all over will give out only about half as much heat as the same stove 

 at the same temperature, if black. Brightly tinned hot-air furnace 

 pipes often lose less heat when bare than they do when covered with 

 one or two layers of asbestos paper, since the asbestos paper radiates 

 heat so much more readily than the bright tin that it more than 

 balances the insulating effect of the thin asbestos covering. Of 

 course, if the pipes were black to begin with, the covering would be 



