Page:Elementary Text-book of Physics (Anthony, 1897).djvu/466

452 these bodies will in time all come to the temperature of the enclosure. It can be shown that, for this to be true, the ratio of the emissiva to the absorbing power must be the same for all bodies, not only for the sum total of all radiations, but for radiations of each wave length. For example, a body which does not absorb radiations of long wave length cannot emit them, otherwise, if placed in an enclosure where it could only receive such radiations, it would become colder than other bodies in the same enclosure. This is only a general statement of the fact which has been already stated for gases, that bodies absorb radiations of exactly the same kind as those which they emit.

Since radiant energy is energy of vibratory motion, it may be supposed to have its origin in the vibrations of the molecules or atoms of the radiating body. It has been shown that the various phenomena of gases are best explained by assuming a constant motion of their molecules. If the atoms of these molecules should have definite periods of vibration, remaining constant for the same gas through wide ranges of pressure and temperature, this would fully explain the peculiarities of the spectra of gases.

In § 150 it was seen that a vibrating body may communicate its vibrations to another body which can vibrate in the same period, and will lose just as much of its own energy of vibration as it imparts to the other body. Moreover, a body which has a definite period of vibration is undisturbed by bodies vibrating in a period different from its own. This explains fully the selective absorption of a gas. For, if a beam of white light pass through a gas, there are, among the vibrations constitviting such a beam, some which correspond in period to those of the molecules of the gas, and, unless the energy of vibration of these molecules is already too great, it will be increased at the expense of the vibrations of the same period in the beam of light. Hence, at the parts in the spectrum where light of those vibration periods would fall, the light will be enfeebled, and those parts will appear, by contrast, as dark lines.

In solids and liquids, the molecules are so constrained in their