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.