The capacity of bodies to absorb heat is in direct proportion to their capacity to emit heat. Light-colored, polished, or smooth surfaces possess this capacity in the least degree, while dark-colored and rough surfaces absorb heat very readily. However, according to the late researches of Melloni, this effect depends less upon the apparent color than upon the nature of the coloring material. He also finds that when the heat-giving body is not luminous, the color is without influence; but when it is luminous, the color has great influence. Melloni has also determined the capacity of absorption of heat by different transparent substances. He found that while transparent rock salt absorbed only 8 per cent, of the heat passing through with the, light, fluor spar absorbed from 25 to 50, Iceland spar and glass 60, alum 90, and ice 94 per cent.; while for heat emitted from a non-luminous body, the latter substances were totally opaque, absorbing all the heat and transmitting none. Recently Tyndall and Magnus have made researches on the absorbent power of gases, and found that under the pressure of one atmosphere, the source of heat being a copper ball heated to 518° F., the absorption by dry air being accepted as the unit, hydrogen was also 1, chlorine 39, carbonic acid 90, nitrous acid 355, marsh gas 403, sulphurous oxide 710, olefiant gas 970, and ammonia 1,195; which means that the latter two gases absorb respectively 970 and 1,195 times more of the heat transmitted through them than is the case with dry air.