Because they would occur, in just the same order, in the absence of light.
230. Why are dark coloured dresses usually worn in winter, and light in summer?
Because black absorbs heat, and therefore becomes warm; while light colours do not absorb heat in the same degree, and therefore they remain cool.
231. Why do iron articles, even when near fire, usually feel cool?
Because they are bad absorbers, and do not take up heat freely, unless they are in contact with a hot body.
232. How is heat diffused through the atmosphere?
By convection. The warmth radiating from the surface of the earth warms the air in contact with it; the air expands, and becoming lighter, flies upwards, bearing with it the caloric which it holds, and diffusing it in its course.
233. How do the waters of the ocean become heated?
Chiefly by convection. Nearly all the heat which the sun sheds upon the ocean is borne away from its surface by evaporation, or is radiated back into the atmosphere. But the ocean gathers its heat by convection from the earth. It girdles the shores of tropical lands where, being warmed to a high degree of temperature, it sets across the Atlantic from the Gulf of Mexico, and exercises an important influence upon the temperature of our latitude.