[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?_
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?_