SCIENCE OF COMMON THINGS. 161 



Radiation. Good and bad radiators. 



1043 When is heat radiated from one body to another ? 



When the two bodies are separated by a non-con- 

 ducting medium. 



1044 On ivliat does radiation depend f 



On the roughness of the radiating surface : thus, if 

 metal be scratched, its radiating power is increased, 

 because the heat has more points to escape from. 



1045 Does a fire radiate heat f 



Yes ; and because burning fuel emits rays of heat, 

 we feel warm when we stand before a fire. 



1046 Why does our face feel uncomfortably hot when we approach a 

 fire? 



Because the fire radiates heat upon the face, which 

 (not being covered) feels the effect immediately. 



1047* Why does the fire heat the face more than it does the rest of the 

 body ? 



Because the rest of the body is covered with cloth- 

 ing ; which (being a bad conductor of heat) prevents 

 the same sudden and rapid transmission of heat to the 

 skin. 



1O48 Do those substances which radiate heat absorb heat also f 



Yes. Those substances which radiate most also 

 absorb most heat ; and those which radiate least also 

 absorb the least heat. 



1O4Q Does any thing else radiate heat besides the sun and fire f 



Yes ; all things radiate heat in some measure, but 

 not equally well. 



1050 What things radiate heat the next best to the sun and fire f 



All dull and dark substances are good radiators of 

 heat ; but all light and polished substa'nces are lad ra- 

 diators. 



1051 What is meant by being a " bad radiator of heat ?" 



To radiate heat is to throw off heat ~by rays, as the 

 sun ; a polished tin pan does not throw off the heat of 

 boiling water from its surface, but keeps it in. 



1O553 Why does snow (at the foot of a tree or wall) melt sooner than in 

 an open field ? 



