The Reason Why. Robert Kemp Philp

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.

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      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.

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      208. Why, if you hold a piece of looking-glass at an angle towards the sum, will light fall upon an object opposite to the looking-glass?

      Because the rays of the sun are reflected by the looking-glass.

209. Why, when we stand before a mirror, do we see our features therein?

      Because the rays of light that fall upon us are reflected upon the bright surface of the mirror.

210. Why, if a plate of bright metal were held sideways before a fire, would heat fall upon an object opposite to the plate?

      Because rays of heat may be reflected in the same manner as the rays of light.

      "But the wise answered saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves."—Matt. xxv.

211. Why would not the same effect arise if the plate were of a black or dark substance?

      Because black and dark substances are not good reflectors of heat.

212. What are the best reflectors of heat?

      Smooth, light-coloured, and highly polished surfaces, especially those of metal.

213. Why does meat become cooked more thoroughly and quickly when a tin screen is placed before the fire?

      Because the bright tin reflects the rays of heat back again to the meat.

214. Why is reflected heat less intense than the primary heat?

      Because it is impossible to collect all the rays, and also because a portion of the caloric, imparting heat to the rays, is absorbed by the air, and by the various other bodies with which the rays come in contact.

215. Can heat be reflected in any great degree of intensity?

      Yes; to such a degree that inflammable matters may be ignited by it. If a cannon ball be made red hot, and then be placed in an iron stand between two bright reflectors, inflammable materials, placed in a proper position to catch the reflected rays, will ignite from the heat.

      There is a