The Reason Why. Robert Kemp Philp

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.

      "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.

      "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.

      "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!

CHAPTER XII.

       Table of Contents

      226. How do we know that plants operate upon the solar and atmospheric heat?

      A delicate thermometer, placed among the leaves and petals of flowers, will at once establish the fact, not only that flowers and plants have a temperature differing from that of the external air, but that the temperature varies in different plants according to the hypothetical, or supposed requirements, of their existences and conditions.

227. What is the chief cause of variation in the temperature of flowers?

      It is generally supposed that their temperature is affected by their colours.

      228. Why is it supposed that the colour of a flower influences its temperature?

      Because it is found by experiment that the colours of bodies bear an important relation to their properties respecting heat, and hold some analogy to the relation of colours to light.

      If when the ground is covered with snow, pieces of woollen cloth, of equal size and thickness, and differing only in colour, are laid upon the surface of the snow, near to each other, it will be found that the relation of colour to temperature will be as follows:—In a few hours the black cloth will have dissolved so much of the snow beneath it, as to sink deep below the surface; the blue will have proved nearly as warm as the black; the brown will have dissolved less of the snow; the red less than the brown; and the white the least, or none at all. Similar experiments may be tried with reference to the condensation of dew, &c. And it will be uniformly found that the colour of a body materially affects its powers of absorption and of radiation.