The rate at which a substance emits or takes up radiant energy depends upon the nature of its surface. A rough, black surface, such as may be obtained by holding an object in the smoke from burning camphor, radiates and absorbs heat with greater freedom than any other; whilst a polished, metallic surface, which acts as a reflector, is worst of all in these respects. Even a surface of finely divided soot, however, does not completely absorb all the radiations which fall upon it, but exhibits a small degree of reflection. An “absolute black surface,” if such could be found, would be totally devoid of reflecting power, and would absorb all the radiant energy incident upon it; and conversely would radiate all energy reaching it from its under side, without reflecting any back, or allowing any to pass through in the manner that light waves are transmitted through a transparent substance. No such perfect surface is known; but, as Kirchoff showed, it is possible to make a radiating arrangement which will give the same numerical result for the energy radiated as would be obtained by a perfect surface at the same temperature. Such an arrangement is termed a “black body,” and radiations from it are designated “black-body radiations.” [Illustration: FIG. 42.—BLACK-BODY RADIATIONS.] Any enclosure, if opaque to radiant energy, and kept at a constant temperature, constitutes a black body, and radiations received from the interior through a small opening in the side are black-body radiations. Fig. 42 represents such an enclosure; in which, to show the application to pyrometry, a body A is indicated opposite to an opening in the side, through which radiations escape from the surface of A. If this surface were “perfect,” all the waves falling upon it would be completely absorbed and completely radiated; but to prevent change of temperature the energy radiated must balance the energy received. If, on the other hand, the surface of A were a polished metal, the waves falling upon it from the sides of the enclosure would in the main be reflected; but here again the energy leaving the surface must equal the amount received if the temperature be constant. It follows, therefore, that if no alteration in temperature occur, the energy leaving the surface of A is independent of the nature of that surface; and the amount escaping through the opening will therefore be the same, whatever be the character of the surface opposite the opening. With a good radiating surface the rays from the enclosure will first be absorbed and then radiated through the opening; in the case of a poor radiating surface, the rays will be directly reflected through the opening; the total energy escaping being the same in either case. It will be seen later that radiation pyrometers are based upon black-body radiations; and it is important to note that the arrangement under discussion is realised in a furnace at a constant temperature, in which A might represent