CHAPTER XI.
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!