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GENERAL SCIENCE 



this vacuum beyond our atmosphere is full of a transparent, intan- 

 gible, and invisible medium called ether. These heat and light waves 

 are known as radiant energy. This radiant energy is changed into 

 heat when it is absorbed by some opaque object. 



If you put your hand on a window-pane through which the " sun 

 is shining," the window will feel cold while the window-sill will 

 feel very warm. Air, glass, and many liquids are transparent; 

 that is, they allow the light to pass through them freely. The radiant 

 energy, which is coming from the sun and traveling at a rate of 

 about 186,377 miles per second, passes through transparent objects 

 without losing any heat. When the radiant energy strikes the 

 window-sill, which is opaque, it is absorbed and transformed into 

 heat. The process of getting heat from the sun is called radiation. 



RAYS OF SUflLlbrtT AND RADIANT ENERGY 

 PASS THROUGH GLASS CHANGED INTO HEAT 



FIG. 66. 



Hotbeds and hothouses are covered with glass to allow the 

 radiant energy to pass through and be absorbed by the earth. It 

 is then changed into heat, which cannot pass out through the glass. 



Dull and Shiny Objects. If we stand near a fire, we feel the fire 

 sending off great quantities of heat by radiation. When radiant 

 energy falls upon any object it may be reflected by the surfaces of 

 the object, absorbed, or transmitted through the substance. 



Stand two pieces of metal (one shiny and the other black) upright about 

 a foot apart. Place between them a bright light. 



In a short time the black metal will be found to be warmer than 

 the shiny metal. The dark metal was heated by the radiant energy 

 while the shiny metal reflected the heat and light. Because it re- 

 flects light, white clothing is cooler for use in summer. 



