ABSTRACT
A somewhat closer consideration shows that the relation between the powers of emission and absorption for one temperature is the same for all bodies. Tliis conclusion has been verified in many special cases, both in the last ten years and in former times. The foregoing proposition requires, however, that all the rays of heat under consideration are of one and the same kind; so that these rays are not qualitatively so far different that one part of them are absorbed by the bodies more than another part; for, were this the case, we could not speak of the power of absorption of a body, simply because it would be different for different rays. Now we have long known that there really are different kinds of heating rays, and that in general they are unequally absorbed by bodies. There are both dark-and luminous-heating rays; the former are almost all absorbed by white bodies, whilst the latter rays are thus scarcely absorbed at all. Indeed the variety of the rays of heat is even greater than the variety of the coloured rays of light. The rays of heat, the dark as well as the luminous, are influenced in the same manner as the rays of light, by transmission, refl.exion, refraction, double refraction, polarization, interference, and diffraction. In the case of the luminous rays of heat, it is not possible to separate the light from the heat; when one is diminished in a given relation, the other is diminished in the same ratio. This has led to the conclusion that rays of light and heat are essentially of the same nature; that rays of light are simply a particular class of the heat-giving rays. The dark rays of heat are distinguished from the rays of light, just as the differently coloured rays are distinguished from each other, by their period of vibration, wave-length, and refractive index. They are not visible because the media of our eyes are not transparent to them.
