Page:Elementary Text-book of Physics (Anthony, 1897).djvu/465

§ 369] of the solar beam which illuminated the slit of his spectroscope, Kirchhoff found the two dark lines corresponding in position to the two bright lines of sodium to become darker, that is, the flame of the lamp had absorbed from the more brilliant solar beam light of the same color as it would itself emit. The explanation of the dark lines of the solar spectrum is obvious. The light from the body of the sun gives a continuous spectrum like that of an incandescent solid or liquid. Somewhere in its course this light passes through an atmosphere of gases which absorbs from the solar beam such light as these gases would emit if they were self-luminous. Some of this absorption occurs in the earth's atmosphere, but most of it is known to occur in the atmosphere of the sun itself. By comparison of these dark lines with the spectra of various incandescent substances upon which we can experiment, the probable constitution of the sun is inferred.

369. Emission of Radiations.—Not only incandescent bodies, but all bodies at whatever temperature they may be, emit radiations. A warm body continues to grow cool until it arrives at the temperature of surrounding bodies, and then if it be moved to a place of lower temperature, it cools still further. To this process we can ascribe no limit, and it is necessary to admit that the body will radiate heat, and so grow cooler, whatever its own temperature, if only it be warmer than surrounding bodies. But it cannot be supposed that a body ceases to radiate heat when it comes to the temperature of surrounding bodies, and begins again when the temperature of these is lowered. It is necessary, therefore, to assume that all bodies, at whatever temperature they may be, are radiating heat, and that, when any one of them arrives at a stationary temperature. It is, if no change take place within it involving the generation or consumption of heat, receiving heat as rapidly as it parts with it. This is called the principle of movable equilibrium of temperature, or Prevost's law of exchanges. We know that if a number of bodies, none of which are generating or consuming heat otherwise than in change of temperature, be placed in an enclosure, the walls of which are maintained at a constant temperature.