An incandescent solid or liquid gives a continuous spectrum, i.e. all the different wave-lengths are represented, but the part of the spectrum which has the greatest energy is different for different substances and for different temperatures: cf. arc and gas flame in Fig. 16. This is quite in keeping with the idea already suggested that in solids and liquids there are electrons of almost every period of vibration. When they are agitated by being heated, a mixture of simple waves of all periods will be sent out giving a very irregular wave.
Gases may also become incandescent. Thus when any compound of sodium is put into a colourless flame the flame becomes coloured an intense yellow. This is due to the vapour of sodium, and the agitation of the electrons in it is probably due to the chemical action in which the compound is split up into sodium and some other parts.
We may also make the gas incandescent by enclosing it at low pressure in a vacuum tube and passing an electrical discharge through it. The glow in the tube gives the spectrum of the gas. Incandescent gases give a very characteristic kind of spectrum. It consists usually of a limited number of narrow lines, the rest of the spectrum being almost perfectly dark. The light therefore consists of a few simple waves of perfectly definite period. This would suggest that in the atom of a gas there are only a few electrons which are concerned in the emission of the light waves.
Thus the spectra of gases and of incandescent solids are represented in character by the curves in Fig. 18.
FIG. 18.
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 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.