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?

Because every irregularity of the surface acts as a point of radiation, or an outlet by which the heat escapes.200. Why does a bright metal tea-pot produce better tea than a brown or black earthenware one?

Because bright metal radiates but little heat, therefore the water is kept hot much longer, and the strength of the tea is extracted by the heat.201. But if the earthenware tea-pot were set by the fire, why would it then make the best tea?

Because the dark earthenware tea-pot is a good absorber of heat, and the heat it would absorb from the fire would more than counterbalance the loss by radiation.202. How would the bright metal tea-pot answer if set upon the hob by the fire?

The bright metal tea-pot would probably absorb less heat than it would radiate. Therefore it would not answer so well, being set upon the hob, as the earthenware tea-pot.203. Why should dish covers be plain in form, and have bright surfaces?

Because, being bright and smooth, they will not allow heat to escape by radiation.204. Why should the bottoms and back parts of kettles and saucepans be allowed to remain black?

Because a thin coating of soot acts as a good absorber of heat, and overcomes the non-absorbing quality of the bright surface.

"And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."

205. But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?

Because, although soot is a good absorber of heat, it is a very bad conductor; an accumulation of it, therefore, would cause a waste of fuel, by retarding the effects of heat.206. Why should the lids and fronts of kettles and saucepans be kept bright?

Because bright metal will not radiate heat; therefore, the heat which is taken up readily through the absorbing and conducting power of the bottom of the vessel, is kept in and economised by the non-radiating property of the bright top and front.207. Does cold radiate as well as heat?

It was once thought that cold radiated as well as heat. But a mass of ice can only be said to radiate cold, by its radiating heat in less abundance than that which is emitted from other bodies surrounding it. It is, therefore, incorrect to speak of the radiation of cold.