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_.