Reason Why — Chapter 10
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_.
204. _Why should the bottoms and back parts of kettles and saucepans be allowed to remain black?
_ Because a _thin_ coating of soot acts as a _good absorber of heat_, and overcomes the _non-absorbing_ quality of the _bright surface_.
[Verse: "And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."
] 205. _But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?
_ Because, although soot is a _good absorber_ of heat, it is a _very bad conductor_; an accumulation of it, therefore, would cause a waste of fuel, by _retarding the effects of heat_.
206. _Why should the lids and fronts of kettles and saucepans be kept bright?
_ Because bright metal _will not radiate heat_; therefore, the heat which is taken up readily through the _absorbing_ and _conducting_ power of the bottom of the vessel, is kept in and economised by the _non-radiating_ property of the bright top and front.
207. _Does cold radiate as well as heat?
_ It was once thought that _cold radiated_ as well as _heat_.
But a mass of ice can only be said to radiate cold, _by its radiating heat in less abundance than that which is emitted from other bodies surrounding it_.
It is, therefore, _incorrect_ to speak of the _radiation of cold_.
