58 THE EEASON WHY. 



* Say unto wisdom, Thou art my sister ; and call understanding thy kins- 

 woman." PROTEBBS vii. 



the bulb of a thermometer successively with equal weights of black, red, and 

 white wool, and placed it in a glass tube, which was heated to the temperature 

 of 180 deg. by immersion in hot water. The tube was then cooled down to 50 

 deg. by immersion in cold water ; the black cooled in 21 minutes, the red in 26 

 minutes, and the white in 27 minutes.] 



194. If you wished to keep water hot for a long time, should 

 you put it into a bright metal jug, or into a dark earthenware 

 one? 



You should put it into a bright metal jug, because, being a lad 

 rzdiaior, it would not part readily with the heat of the water. 



195. Why would not the dark earthenware jug keep the 

 water hot as long as the bright metal one ? 



Because the particles of earthenware being rough, and of dark 

 colour, they radiate heat freely, and the water would thereby be 

 quickly cooled. 



CHAPTER X. 



196. But if (as stated in the Lessons upon Conduction) 

 metal is a better conductor of heat than stone or earthenware, 

 why does not the metal jug conduct away the heat of the water 

 sooner than the earthemvare jug ? 



It would do so, if it were in contact with another conductor ; 

 but, being surrounded by air, which is a bad conductor, the heat 

 must pass off by radiation, and as bright metal surfaces are bad 

 radiators, the metal jug would retain the heat of the water longer 

 than the earthenware one. 



197. Supposing a red-hot cannon ball to be suspended by a 

 chain from the ceiling of a room, how would its heat escape ? 



Almost entirely by radiation. But if you were to rest upon the 

 ball a cold bar of iron, a part of the heat would be drawn off by 

 conduction. Warm air would rise from around the ball, and, 

 moving upwards, would distribute some of the heat by convection. 



