VARIETIES OF SPECTRA Emission Spectra.—Self-luminous sources yield emission spectra, which are of different types according to the nature of the source. The light from an incandescent solid body such as a gas mantle, or an electric glow lamp, is spread out into a band showing all the colours of the rainbow. The colours from red to violet merge into each other by insensible gradations, and the spectrum is said to be continuous. With rare exceptions, all incandescent solids and liquids give a spectrum of this kind, and it follows that such substances cannot be distinguished from one another so long as they remain in the solid or liquid state.
The effects are very different when the substances examined are in the state of luminous gas or vapour. They then emit special kinds of light by which they can be identified, whether they are subjected to experiment in our laboratories or are situated in a celestial body far away in the depths of space. The spectra are discontinuous, consisting of a number of bright lines or bands of different colours on a dark background. No two substances yield the same spectrum and it follows that the chemical nature of substances can be determined by an examination of their spectra. This is the foundation of spectrum analysis. Thus, glowing hydrogen is characterized by a bright line in the red, besides others in the more refrangible parts of the spectrum, and since these are exhibited by nothing but hydrogen, they serve to indicate the presence of hydrogen wherever it occurs in the luminous state. Each of the other elements also has its distinctive family of spectrum lines, some consisting of comparatively few lines, while others give hundreds or thousands of lines.
The distinction between line and band spectra is of great im portance. A line spectrum consists of a succession of images of the slit of varying intensity, which are mostly well separated from each other. In a band spectrum, a great number of fine lines—called band lines—are closely crowded together in each band, especially near the so-called head of the band, so that they can only be separated with comparatively powerful instruments. The difference may be best understood by reference to the photo graphs of portions of the line and band spectra of nitrogen which are reproduced in Plate II., is and 'b. A succession of bands with well-marked heads, as in nitrogen, gives a fluted appearance to the spectrum, and bands of this kind are often called flutings. There are other bands of less regular appearance, and others which do not appear to be resolvable into lines. Line spectra are characteristic of atomic emissions, while band spectra originate in molecules. (See BAND SPECTRA.)
Absorption Spectra.—In contrast to emission spectra there is another class of spectra known as absorption spectra. A piece of glass which appears red by transmitted light owes this appear ance to its property of transmitting red light and of absorbing the light of other colours of which white light is composed. (See COLOUR.) Such a piece of glass gives continuous absorption over a certain range of the spectrum, while a piece of neutral-tinted glass exhibits a partial continuous absorption throughout the whole of the visible spectrum. There are, however, many sub stances which, at ordinary temperatures, give characteristic ab sorption bands by which they can be identified. Among solid substances, glass coloured with salts of neodymium shows a beautiful set of dark bands when white light is passed through it before entering the slit of the spectroscope. (See Plate II., 2a.) Solutions of permanganate of potash, chlorophyll, blood and of many aniline dyes similarly show absorption bands which are characteristic in each case. Several gases and vapours in a non luminous state at ordinary or moderate temperatures also yield absorption spectra peculiar to themselves. Iodine vapour, pro duced by slightly warming crystals of iodine in a test-tube, and nitrogen peroxide are good examples. Oxygen and water vapour similarly show their presence in our atmosphere by the appearance of their characteristic low-temperature absorption bands in the spectrum of sunlight.
Absorption spectra are also given by gases and vapours in a luminous state, when they are placed in the path of the rays from a white source at a higher temperature. Kirchhoff's famous ex periment of 1859 proved that a luminous vapour has the property of absorbing precisely the same kind of light that it emits at the same temperature; so that if such a vapour (a sodium flame, for example) lies in front of a source at a higher temperature, and giving a continuous spectrum, the result is a continuous spectrum crossed by dark lines. An example of absorption is shown in Plate II., 2b, which represents the absorption of the vapour in a heated tube containing potassium. This result is of fundamental importance in Astrophysics because the spectrum of the sun and the spectra of most of the stars show dark lines or bands on a bright continuous background. The experiment of Kirchhoff proves that the substances which produce such dark lines can be identified, just as surely as if the lines were bright, by the process of matching the dark lines by emission spectra produced in the laboratory.