+Spectrum Analysis.+--The lines in a gas spectrum are so sharply defined and are so definitely characteristic of the particular gas that they serve as a delicate method of detecting the presence of some elements. These spectra which are emitted by incandescent bodies are called emission spectra. But not only do different materials emit different kinds of light when raised to incandescence, but they also absorb light differently when it passes through them. When white light is passed through some transparent solids or liquids and then through a prism, it is found that whole regions of the spectrum are absent. Thus a potassium permanganate solution {35} which is not too concentrated absorbs the whole of the middle part of the spectrum, allowing the red and blue rays to pass through. Since with solids and liquids the absorbed regions are large and somewhat ill-defined, the absorption spectra are not of any great use in the detection of substances. The absorption spectra of gases show the same sharply defined characteristics as the emission spectra. Thus if white light from an arc lamp passes through a flame coloured yellow with sodium vapour, the spectrum of the issuing light has two sharply defined narrow dark lines close together in the yellow part of the spectrum in exactly the same position as the two bright yellow lines which incandescent sodium vapour itself gives out. The flame has therefore absorbed just those waves which it gives out. This is perfectly general, and applies to solids and liquids as well as to gases. It is perfectly in keeping with our view of the refraction of light by the resonance of electrons to the Fourier's constituents which have the same period. For if the electrons have a certain period of vibration they will resound to waves of that period and therefore absorb their energy. +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, {36} 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. {37}