But though all bodies radiate heat, they have not all the same radiating power. Some substances possess this power in a far greater degree than others. The metals, though they are the best conductors, are the worst radiators. This is particularly the case when they are polished. Dull, dark substances, and especially those which have a rough or scratched surface, are good radiators; light-colored and smooth substances, on the other hand, are bad radiators; and this explains why, as every good housewife knows, a polished metal tea-pot keeps tea warmer than a black earthen one--it does not part with its heat so readily.
Bodies which radiate freely have the power in an equal degree of _absorbing_ heat; that is, they are as ready to take it in, as they are to throw it out again. Dark substances, therefore, must be good absorbers, as they are good radiators of heat. A very simple experiment will illustrate this fact very clearly. If you spread upon snow, in a place exposed to the sunbeams, two pieces of cloth of the same texture, one black and the other white, you will find, after some time, that under the black cloth the snow has been melted, but under the white cloth it remains as it was at first. The black material has been heated quickly and intensely; the white has not been heated at all: the former has _absorbed_ the sun’s rays; the latter has _reflected_ them.
The laws of absorption and radiation enable us to explain many curious facts. Thus, dark-colored clothes are cold in the shade, because they are then radiating heat from our bodies; but they are warm in the sunshine, because they are then absorbing the heat that falls upon them from the sun. On the other hand, light colored clothes are warm in the shade, and cool in the sunshine. Again, a dish-cover or metal tea-pot is kept as brightly polished as possible, in order to prevent the escape of the heat by radiation; a black earthenware tea-pot, on the contrary, has a dull and dark surface, so that it may be placed on the hob and absorb the heat. So, too, if a kettle is to heat quickly, the bottom and sides should be covered with soot, to absorb the heat; while the upper part should be bright, to prevent radiation.