RADIATION OF HEAT, 253 



CHAPTER 11. 



RADIATION OF HEAT. 



The passage of heat through conducting bodies has 

 been aheady explained. There is another way in 

 which it is transmitted, termed radiation, in which it 

 is thrown off instantaneously in straight lines from 

 hot bodies, in the same way that light is thrown off 

 from a candle. A familiar instance is furnished by 

 the common or open fire-place, before which the face 

 may be roasted with the radiated heat, while the back 

 is chilled with cold. A screen held in the hand will 

 intercept this radiated heat, showing that it flies in 

 right lines like the rays of light. 



E-adiated heat is reflected by a polished metallic 

 surface, in the same way that light is reflected by a 

 looking-glass. A plate of bright tin held near the fire 

 will not for a long time become hot, the heat being 

 reflected from it without entering and heating it. But 

 if it be blackened with smoke, it will no longer reflect, 

 but absorb the heat, and consequently will speedily 

 become hot. This experiment may be easily tried by 

 placing a new tin cup containing water over a char- 

 coal fire, which yields no smoke. The heat will be 

 reflected into the fire by the tin, and the water wiU 

 scarcely become warm. But if a few pine shavings 

 be thrown on this fire to smoke the surface of the tin, 

 it will then absorb the heat rapidly, and soon begin to 

 boil. This explains the reason that bread bakes more 



