The Romance of Modern Chemistry

CHEMISTRY OF THE STARS reader might think it sufficient to observe the colour which a substance under examination imparts to the Bunsen flame. So it would be, provided only one of the metals was present; this condition, however, will not always hold good, and when two or more are present the colour of the flame will give no certain indication. But it is just here that the full value of the spectroscope becomes apparent, for each constituent in a mixture contributes to the spectrum its own quota of lines, uninfluenced by the others which are present. This marvellously sensitive spectroscopic method of analysis can be applied not only to metallic salts which are volatile in the Bunsen flame, but also to substances like hydrogen, which are gases at the ordinary tempera- ture, and to refractory metals such as iron. Ingenious devices have been adopted for bringing these into the state of incandescent vapour, from which alone we may expect to obtain a characteristic discontinuous spectrum. Hydrogen, for example, is filled into a glass tube at low pressure, and an electric discharge is passed through the rarefied gas; the spectrum of the glowing hydrogen is then found to be characterised by three main lines, red, green, and blue respectively (see Fig. 11). To obtain the spectrum of iron, on the other hand, the metal or one of its compounds is placed between the poles of an electric arc. At the high temperature of this discharge, the iron is partly converted into incandescent vapour, and its spectrum, containing an enormous number of lines, is visible. When once the characteristic spectra of the elements, obtained by one or other of the methods just described, have been properly mapped out, then each line which we observe in any new spectrum may be referred to the element which is responsible for it 209 o