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."]