Spectrum of the Sun.—One of the most interesting examples of the absorption by incandescent gases of their own characteristic lines is provided by the sun. The spectrum of the sun is crossed by a large number of fine dark lines which were mapped out by Fraunhöfer and are therefore called Fraunhöfer lines. These lines are found to be in the position of the characteristic lines of a number of known elements, and therefore we assume that these elements are present in the sun. The interior of the sun is liquid or solid owing to the pressure of the mass round it. It therefore emits a continuous spectrum. But the light has to pass through the outer layers of incandescent vapour, and these layers absorb from the light their characteristic waves and so produce the dark lines in the spectrum.
The spectra of stars show similar characters to those of the sun, and therefore we assume them to be in the same condition as the sun.
The spectra of nebulæ consist only of bright lines, and we therefore assume that nebulæ consist of incandescent masses of gas which have not yet cooled enough to have liquid or solid nuclei.
CHAPTER IV
THE LAWS OF RADIATION
Absorbing Power.—A perfectly dull black surface is simply one which absorbs all the light which is falling on it and reflects or diffuses none of it back. If the surface absorbs the heat as well as the light completely, it is called a perfect or full absorber. Other surfaces merely absorb a fraction of the heat and light falling on them, and this fraction, expressed usually as a percentage, is called the absorbing power of the surface. The absorbing powers of different kinds of surfaces can be measured in a great many ways, but the following may be taken as fairly typical. A perfectly steady beam of heat and light is made to fall on a small metallic disc, and the amount of heat which is absorbed per second is calculated from the mass of the metal and the rate at which its temperature rises. The disc is first coated with lamp-black, and the rate at which it then receives heat is taken as the rate at which a full absorber absorbs heat under these conditions. The disc is then coated with the surface whose absorbing power is to be measured, and the experiment is repeated. Then the rate at which heat is received in the second case divided by the rate at which it is received in the first is the absorbing power of the second surface. Experiments with a large number of surfaces show that the lighter in colour and the more polished is the surface, the smaller is its absorbing power.
Radiating Power.—But the character of the surface affects not only the rate at which heat and light are absorbed, but also the rate at which they are emitted. For example, if we heat a fragment of a willow pattern china plate in a blowpipe flame until it is bright red hot, we shall notice that the dark pattern now stands out brighter than the rest. Thus the dark pattern, which absorbs more of the light which falls on it when it is cold, emits more light than the rest of the plate when it is hot. This is one example of a general rule, for it is found that the most perfect absorbers are the greatest radiators, and vice-versa. The perfectly black surface is therefore taken as a standard in measuring the heat and light emitted by surfaces, in exactly the same way as for heat and light absorbed. Thus the emissive or radiating power of a surface is defined as the quantity of heat radiated per second by the surface divided by the amount radiated per second by a perfectly black surface under the same conditions. As it is somewhat paradoxical to call a surface a perfectly black surface when it may even be white hot, the term "a full radiator" has been suggested as an alternative and will be used in this book.
