It is either _absorbed_ by those bodies, or transmitted through them and passed to other bodies by _conduction_, or diffused by _convection_, or returned by _reflection_. 187. _How do we know that heat is diffused by radiation?_ If we set a metal plate (or any other body, though metal is best for the experiment) before the fire, _rays of heat will fall upon it_. If we turn the plate at a slight angle, and place another object in a line with it, we shall find that the plate will _reflect the rays it has received by radiation_, on to the object so placed; but if we place an object _between the fire and the plate_, we shall find that the rays of heat _will be intercepted_, and that the latter can no longer _reflect heat_. [Verse: "The fear of the Lord is the beginning of wisdom: a good understanding have all they that do his commandments."--PSALM CXI.] 188. _Does the agitation of the air interfere with the direction of rays of heat?_ It has been found that the agitation of the air does _not_ affect the direction of rays of heat. 189. _Why, then, if a current of air passes through a space across which heat is radiating, does the air become warmer?_ Because it takes up _some portion of the heat_, but it does not alter the direction of the rays. This is clearly illustrated by reference to _rays of light_ which are seen under many circumstances. But they are never bent, moved, nor in any way affected by the wind. 190. _Why will not a current of air disturb the rays of heat, just as it would a spider's web, or threads of silk?_ Because heat is an _imponderable_ agent, that is, something which cannot be acted upon by the ordinary physical agencies. It has _no weight_, presents no _substantial body_, and is, in these latter respects, similar to _light and electricity_. 191. _What other sources of radiation of heat are there besides the sun and the fire?_ The _earth_, and all _minor bodies_, are, in some degree, _radiators of heat_. 192. _What substances are the best radiators?_ All _rough_ and _dark_ coloured substances and surfaces are the _best radiators of heat_. 193. _What substances are the worst radiators of heat?_ All _smooth_, _bright_, and _light coloured_ surfaces are _bad radiators of heat_. Dr. Stark, of Edinburgh, has proved, by a series of experiments, the influence which the _colours_ of bodies have upon the _velocity of radiation_. He surrounded the bulb of a thermometer successively with equal weights of _black_, _red_, and _white_ wool, and placed it in a glass tube, which was heated to the temperature of 180 deg. by immersion in hot water. The tube was then cooled down to 50 deg. by immersion in cold water; the _black_ cooled in 21 minutes, the _red_ in 26 minutes, and the _white_ in 27 minutes. [Verse: "Say unto wisdom, Thou art my sister; and call understanding thy kinswoman."--PROVERBS VII.]