174 CONDUCTION. 



3rdly If the feet were not protected, 

 the conducting surface of the tin would 

 feel painfully hot. 



Q. What harm would it be if the POLISH of the 

 tin were injured by the perspiration of our feet ? 



A. Polished tin throws off its heat 

 very slowly ; but dull, scratched, painted, 

 or dirty tin, throws off its heat very 

 quickly: if, therefore, the tin foot-warmer 

 were to lose its polish, it would get cold 

 in a much shorter time. 



Q. Why are FURNACES and stoves (where much 

 HEAT is required ) built of porous BRICK ? 



A. As bricks are bad conductors, 

 they prevent the escape of heat : and are, 

 therefore, employed where great heat is 

 required. 



Q. Why are FURNACE DOORS, &c. frequently 

 COVERED with a paste of CLAY and SAND ? 



A. Because this pasters a very bad 

 conductor of heat ; and, therefore, pre- 

 vents the escape of heat from the furnace. 



Q. If a stove be placed in the MIDDLE of a 

 room, should it be made of bricks or IRON ? 



A. A stove in the middle of a room 

 should be made of iron; because iron 



