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 bad radiator,_ 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 earthenware 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_. And some of its rays, falling upon a mirror, or any other bright surface, might be diffused by _reflection_. [Verse: "I will teach you by the hand of God; that which is with the Almighty will I not conceal."--JOB XXVII.] 198. _Do some substances absorb heat?_ Yes; those substances which are _the best radiators_ are also _the best absorbers_ of heat. 199. _Why does scratching a bright metal surface increase its power of radiation?_ Because every irregularity of the surface acts as a point of radiation, or _an outlet_ by which the heat escapes. 200. _Why does a bright metal tea-pot produce better tea than a brown or black earthenware one?_ Because bright metal _radiates but little heat_, therefore the water is kept hot much longer, _and the strength of the tea is extracted by the heat_. 201. _But if the earthenware tea-pot were set by the fire, why would it then make the best tea?_ Because the dark earthenware tea-pot is a good _absorber of heat_, and the heat it would _absorb_ from the fire would more than counterbalance the loss by _radiation_. 202. _How would the bright metal tea-pot answer if set upon the hob by the fire?_ The bright metal tea-pot would probably _absorb less heat_ than it would radiate. Therefore it would not answer so well, _being set upon the hob_, as the earthenware tea-pot. 203. _Why should dish covers be plain in form, and have bright surfaces?_ Because, being bright and smooth, they will not allow heat to escape _by radiation_.