196. But if (as stated in the Lessons upon Conduction) metal is a better conductor of heat than stone or earthenware, why does not the metal jug conduct away the heat of the water sooner than the earthenware jug?
It would do so, if it were in contact with another conductor; but, being surrounded by air, which is a bad conductor, the heat must pass off by radiation, and as bright metal surfaces are bad radiators, the metal jug would retain the heat of the water longer than the earthenware one.
197. Supposing a red-hot cannon ball to be suspended by a chain from the ceiling of a room, how would its heat escape?
Almost entirely by radiation. But if you were to rest upon the ball a cold bar of iron, a part of the heat would be drawn off by conduction. Warm air would rise from around the ball, and, moving upwards, would distribute some of the heat by convection. And some of its rays, falling upon a mirror, or any other bright surface, might be diffused by reflection.
"I will teach you by the hand of God; that which is with the Almighty will I not conceal."—Job xxvii.
198. Do some substances absorb heat?
Yes; those substances which are the best radiators are also the best absorbers of heat.
199. Why does scratching a bright metal surface increase its power of radiation?