Heat, like light, is absorbed in different degrees by different substances. The colour and condition of surface seems to exert a great influence on its absorption; thus a black body absorbs more heat than a white one, and a rough surface more than a smooth one. "Every tree," says Mr. Hunt, "spreading its green leaves to the sunshine, or exposing its brown branches to the air, every flower which lends its beauty to the joyous earth, possesses different absorbing and radiating powers. The chalice-like cup of the pure white lily floating on the lake, the variegated tulip, the brilliant anemone, the delicate rose, and the intensely-coloured peony or dahlia, have each powers peculiar to themselves for drinking in the warming life-stream of the sun, and for radiating it back again to the thirsting atmosphere."
It is impossible to enumerate the wonderful offices performed by heat in the economy of nature. By the influence of heat, water is vaporized and raised into the air, thence to be precipitated in refreshing showers. The atmospheric currents are caused by heat; the trade-wind, that blows from the same quarter throughout the year, the periodical monsoon, the gentle breeze, the boisterous gale, and the devastating hurricane, are alike manifestations of the activity of this mighty principle.
Let us now glance at that mysterious element of the sunbeam which cannot be detected by the senses. To modern science is entirely due the
