Page:Elementary Text-book of Physics (Anthony, 1897).djvu/464

450 solid, such as the lime of the oxyhydrogen light or the carbons of the electric lamp, be examined with the spectroscope, a continuous spectrum like that produced by sunlight is seen, but the black lines are absent (§ 361). Solids and liquids give in general only continuous spectra. Gases, however, when incandescent give continuous spectra only very rarely. Their spectra are bright lines which are distinct and separate images of the slit. The number and position of these lines differ with each gas employed. Hence, if a mixture of several gases not in chemical combination be heated to incandescence, the spectral lines belonging to each constituent, provided all be present in sufficient quantity, will be found in the resultant spectrum. Such a spectrum will therefore serve to identify the constituents of a mixture of unknown composition. Many chemical compounds are decomposed into their elements, and the elements are rendered gaseous at the temperature necessary for incandescence. In that case the spectrum given is the combined spectra of the elements. A compound gas that does not suffer dissociation at incandescence gives its own spectrum, which is, in general, totally different from the spectra of its elements.

The appearance of a gaseous spectrum depends in some degree on the density of the gas. When the gas is sufficiently compressed, the lines become broader and lose their sharply defined edges, and if the compression be still further increased the lines may widen suntil they overlap, and form a continuous spectrum. Some of the dark lines of the solar spectrum are found to coincide in position with the bright lines of certain elements. This coincidence is absolute with the most perfect instruments at our command; and not only so, but if the bright lines of the element differ in brilliancy the corresponding dark lines of the solar spectrum differ similarly in darkness.

The close coincidence of some of these lines was noted as early as 1822 by Sir John Herschel, but the absolute coincidence was demonstrated by Kirchhoff, who also pointed out its significance. Placing the flame of a spirit-lamp with a salted wick in the path