208. _Why, if you hold a piece of looking-glass at an angle towards the
sum, will light fall upon an object opposite to the looking-glass?_
Because the rays of the sun are _reflected_ by the looking-glass.
209. _Why, when we stand before a mirror, do we see our features
therein?_
Because the rays of light that fall upon us are _reflected_ upon the
bright surface of the mirror.
210. _Why, if a plate of bright metal were held sideways before a fire,
would heat fall upon an object opposite to the plate?_
Because rays of heat may be _reflected_ in the same manner as the rays
of light.
[Verse: "But the wise answered saying, Not so; lest there be not
enough for us and you: but go ye rather to them that sell, and buy for
yourselves."--MATT. XXV.]
211. _Why would not the same effect arise if the plate were of a black
or dark substance?_
Because black and dark substances are not _good reflectors of heat_.
212. _What are the best reflectors of heat?_
Smooth, light-coloured, and highly polished surfaces, especially those
of _metal_.
213. _Why does meat become cooked more thoroughly and quickly when a
tin screen is placed before the fire?_
Because the bright tin reflects the rays of heat back again to the meat.
214. _Why is reflected heat less intense than the primary heat?_
Because it is impossible to collect all the rays, and also because a
portion of the caloric, imparting heat to the rays, is absorbed by
the air, and by the various other bodies with which the rays come in
contact.
215. _Can heat be reflected in any great degree of intensity?_
Yes; to such a degree that inflammable matters may be ignited by it.
If a cannon ball be made red hot, and then be placed in an iron stand
between two bright reflectors, inflammable materials, placed in a
proper position to catch the reflected rays, _will ignite from the
heat_.
There is a curious and an exceptional fact with reference to
_reflected_ heat, for which we confess that we are unable to
give "_The Reason Why_." It is found that snow, which lies near
the trunks of trees or the base of upright stones, melts before
that which is at a distance from them, though the sun may shine
equally upon both. If a blackened card is placed upon ice or snow
under the sun's rays, the frozen body underneath it will be thawed
before that which surrounds it. But if we _reflect_ the sun's
rays from a metal surface, the result is _directly contrary_--the
exposed snow is the first to melt, leaving the card standing as
upon a pyramid. Snow _melts_ under heat which is _reflected_ from
the trees or stones while it withstands the effect of the _direct
solar rays_. In passing through a cemetery this winter (1857),
when the snow lay deep, we were struck with the circumstance that
the snow in front of the head-stones facing the sun was completely
dissolved, and, in nearly every instance, the space on which the
snow had melted assumed a coffin-like shape. This forced itself so
much upon our attention that we remained some time to endeavour
to analyse the phenomena; and it was not until we remembered the
curious effect of _reflected heat_ that we could account for it.
It is obvious that the rays falling from the upper part of the
head-stone on to the _foot_ of the grave would be less powerful
than those that radiated from the _centre_ of the stone to the
centre of the grave. Hence it was that the heat dissolved at the
foot of the grave only a narrow piece of snow, which widened
towards the centre, and narrowed again as it approached the foot
of the head-stone, where the lines of radiation would naturally
decrease. Such a phenomena would prove sufficient to raise
superstition in untutored minds.
[Verse: "The light of the righteous rejoiceth, but the lamp of the
wicked shall be put out."--PROVERBS XIII.]
216. _Are good reflectors of heat also good absorbers?_
No; for reflectors at once _send back_ the heat which they receive,
while absorbers _retain it_. It is obvious, therefore, that
_reflectors_ cannot be good _absorbers_.
217. _How do fire-screens contribute to keep rooms cool?_
Because they turn away from the persons in the room rays of heat which
would otherwise make the warmth excessive.
218. _Why are white and light articles of clothing cool?_
Because they _reflect_ the rays of heat.
White, as a _colour_, is also a bad _absorber_ and _conductor_.
219. _Why is the air often found excessively hot in chalk districts?_
Because the soil _reflects_ upon objects near to it the heat of the
solar rays.
220. _How does the heat of the sun's rays ultimately become diffused?_
It is first _absorbed_ by the earth. Generally speaking, the earth
_absorbs_ heat by day, and _radiates_ it by night. In this way an
equilibrium of temperature is maintained, which we should not otherwise
have the advantage of.
221. _Does not the air derive its heat directly from the sun's rays?_
Only partially. It is estimated that the air absorbs only _one-third_
of the caloric of the sun's rays--that is to say, that a ray of solar
heat, entering our atmosphere at its most attenuated limit (a height
supposed to be about _fifty miles_), would, in passing through the
atmosphere to the earth, part with only one-third of its calorific
element.
[Verse: "As for the earth, out of it cometh bread; and under it is
turned up as it were fire."--JOB XXVIII.]
222. _What becomes of the remaining two-thirds of the solar heat?_
They are _absorbed_ chiefly by the _earth_, the great medium of
calorific _absorption_; but some portions are taken up by _living
things_, both animal and vegetable. When the _rays of heat_ strike upon
the earth's surface, they are passed from particle to particle into the
interior of the earth's crust. Other portions are distributed through
the air and water by _convection_, and a third portion is thrown
back into space by _radiation_. These latter phenomena will be duly
explained as we proceed.
223. _How do we know that heat is absorbed, and conducted into the
internal earth?_
It is found that there is a given depth beneath the surface of the
globe at which an equal temperature prevails. The depth increases as we
travel south or north from the equator, and corresponds with the shape
of the earth's surface, _sinking under the valleys, and rising under
the hills_.
224. _Why may we not understand that this internal heat of the earth
arises, as has been supposed by many philosophers, from internal
combustion?_
Because recent investigations have thrown considerable and satisfactory
light upon the subject. It has been ascertained that the internal
temperature of the earth _increases_ to a certain depth, _one degree in
every fifty feet_. But that below that depth the temperature _begins to
decline_, and continues to do so with every increase of depth.
225. _Do plants absorb heat?_
Yes. They both _absorb_ and _radiate_ heat, under varying
circumstances. The majestic tree, the meek flower, the unpretending
grass, all perform a part in the grand alchemy of nature.
[Verse: "Consider the lilies of the field, how they grow; they toil
not, neither do they spin."]
When we gaze upon a rose it is not its beauty alone that should
impress us: every moment of that flower's life is devoted to
the fulfilment of its part in the grand scheme of the universe.
It decomposes the rays of solar light, and sends the red rays
only to our eyes. It absorbs or radiates heat, according to the
temperature of the ærial mantle that wraps alike the flower and
the man. It distills the gaseous vapours, and restores to man the
vital air on which he lives. It takes into its own substance, and
incorporates with its own frame, the carbon and the hydrogen of
which man has no immediate need. It drinks the dew-drop or the
rain-drop, and gives forth its sweet odour as a thanksgiving. And
when it dies, it preaches eloquently to beauty, pointing to the
end that is to come!