216 WELLS'S NATURAL PHILOSOPHY. 



SECTION II. 



COMMUNICATION OF HEAT. 



How may heat 433 jjcat mav be commutiicated in three 



be commum- J 



cated? ways: by Conduction, by Convection, and 



by Kadiation. 



489. Heat is communicated by conduction 



How is heat . 1/. • ^ • ^ r> ^ 



communicated wbcn it travols irom particle to particle of the 



byconductioQ? ^1 i r- 1 • 1 



substance, as Irotn the end ot the iron bar 

 placed in the fire to that part of the bar most remote 

 from the fire. 



What is con- 490. Whcu hcat is communicated by being 

 vecuoa? carried by the natural motion of a substance 

 containing it to another substance or i)lace, as when hot 

 water resting upon the bottom of a kettle rises and carries 

 heat to a mass of water through which it ascends, the heat 

 is said to be communicated by convection, 

 whatisradia. 491. Hcat is communicatcd by radiation 

 tioaofheat? "vvhen it leaps, as it were, from a hot to a cold 

 body through an appreciable interval of space ; as when a 

 body is warmed by placing it before a fire removed to a 

 little distance from it. 



How does a 492. A hcatcd body cools itself, first by giv- 

 cooi uscif'?°'^^ ing oif heat from its surface, either by conduc- 

 tion or radiation, or both conjointly ; and sec- 

 ondly, by the heat in its interior passing from particle to 

 particle by conduction, through its substance to the sur- 

 face. A cold body, on the contrary, becomes heated by a 

 process directly the reverse of this. 



Do all bodies 493. Different bodies exhibit a very great 

 equluy'weuf degree of difference in the facility with whic h 

 they conduct heat : some substances oppose 

 very little impediment to its passage, while through others 

 it is transmitted slowly. 



What are con- 494, All bodics are divided into two classes 

 non-conduct"or8 ^^ rcspcct to their conduction of heat, viz., 

 of heat? jj^j-Q conductors and non-conductors. The for- 



