PART IV. 



HEAT. 

 CHAPTER I. 



COXDUCTIXG POWER OF BODIES. 



When any substance or body lias become heated, it 

 loses its heat in two different ways, by conduction and by 

 radiation. When conducted, heat passes off slowly or 

 gradually through bodies, as when a pin is held by the 

 hand in a candle, the heat advances 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, there- 

 fore, 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 without 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 others to be 

 tipped with wax. The different periods of time required 

 to melt the wax indicate the relative conducting powers. 

 It will speedily melt on the copper rod ; soon after, on 

 the rod of iron ; glass will require longer time ; stone or 

 earthenware, stilL longer; while on a rod of wood, it 

 will scarcely melt at all. These rods should be laid hori- 

 zontally, that the hot air rising from the sand may not 

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