181. What is Radiation?

The radiation of heat is a motion of the particles, in a series of rays, diverging in every direction from a heated body.182. What is this phenomena of Radiation understood to arise from?

From a strongly repulsive power, possessed by particles of heat, by which they are excited to recede from each other with great velocity.183. What is the greatest source of Radiation?

The sun, which sends forth rays of both light and heat in all directions.184. When does a body radiate heat?

When it is surrounded by a medium which is a bad conductor.185. When we stand before a fire, does the heat reach us by conduction or by radiation?

By radiation.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?

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.

"Say unto wisdom, Thou art my sister; and call understanding thy kinswoman."—Proverbs vii.

194. If you wished to keep water hot for a long time, should you put it into a bright metal jug, or into a dark earthenware one?

You should put it into a bright metal jug, because, being a bad radiator, it would not part readily with the heat of the water.195. Why would not the dark earthenware jug keep the water hot as long as the bright metal one?

Because the particles of earthenware being rough, and of dark colour, they radiate heat freely, and the water would thereby be quickly cooled.