PART IV. 



HEAT. 



CHAPTER I. 



CONDUCTION OF HEAT. 

 SECTION I. 



CONDUCTING POWER OF BODIES. 



"When any substance or body has become heated, it 

 loses its heat in two different ways, by conduction and 

 by radiation. When conducted, heat passes off slow- 

 ly or gradually through bodies, as when a pin is held 

 by the hand in a candle, the heat advancing from one 

 end to the other till it burns the fingers ; or, when an 

 iron poker is thrust into the fire, the heat gradually 

 passes through it till the whole becomes hot. Iron 

 and brass are, therefore, said to be good conductors of 

 heat. The end of a pipe-stem may, however, be heated 

 to redness, and a wooden rod may be set on fire, with- 

 out even warming the other extremity, because the 

 heat is very slowly conducted through them. "Wood 

 and burned clay are, therefore, poor conductors. 



The comparative conducting power of different sub- 

 stances may be shown by placing short rods of each 

 with one of their ends in a vessel of hot sand, the oth- 

 ers to be tipped with wax. The different periods of 

 time required to melt the wax indicate the relative 



