32 HEAT. 



solid is slow, because the gas contains comparatively 

 few atoms, and therefore furnishes few points of con- 

 tact. Between a liquid and a solid it is more rapid, be- 

 cause there are more. A cannon ball would grow hot 

 much more rapidly in boiling water than in air of the 

 same temperature. Between solid and solid, again, con- 

 duction is less rapid, because the surfaces cannot so 

 adapt themselves to each other, like liquid and solid, so 

 as to bring all their atoms together. This paragraph 

 refers solely to the passage of heat from the atoms of 

 one surface into those of the other. Th further con- 

 duction of heat depends on the substance into which 

 it has passed. 



61. HEATING WATER. Water is sooner 



Why is water . .. . 



heated sooner heated m an iron pot, or other metallic 

 ianina vessel, than in one of porcelain, glass, or 

 glass vessel? earthen- ware, because the metal conducts 

 the heat through from the fire more rapidly. Cooling, 

 or the passage of heat outward when the vessel is re- 

 moved from the fire, goes on more rapidly in the case 

 of the metallic vessel for the same reason. These 

 statements have reference only to vessels which are not 

 polished. In the case of bright surfaces, another prin- 

 ciple is involved to be considered hereafter. 



62. CLOTHING. Fibrous substances, like 



Exn am the , 



subject of do- wool, cotton, and furs are best adapted for 



l clothi g> be cause they are such poor con- 

 ductors, and beside, because they contain 

 air shut in between their fibres, which is a non-con- 

 ductor. as will be hereafter shown. The object of 

 clothing is not to impart heat, but to prevent its escape 



