186. What becomes of the heat that is radiated from one body to another?
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.
"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?