The Romance of Modern Chemistry

CHEMISTRY OF THE STARS we interpose a Bunsen flame coloured yellow by incan- descent sodium vapour, the effect on the spectrum is rather surprising. The continuity of the spectrum is seen to be broken by a dark line which occupies the exact position of the bright line in the ordinary sodium spectrum, as might easily be shown by momentarily screening off the arc light behind. The arc light is at a much higher temperature than the Bunsen flame, and what has happened is that the sodium vapour in the latter has absorbed or picked out of the light from the hotter source exactly those rays which it itself usually emits. The light which passes on is therefore bereft of those particular rays, and the spectrum shows the deficiency. The reader must remember that light is a species of vibration, and that just as the string of a musical instrument will respond alone to that particular note out of many which has its own pitch, so an incandescent vapour will absorb exactly those rays which it emits. Provided, therefore, that the source of white light behind is hot enough, the passage of the light through various incandescent vapours at a lower temperature will be revealed by a number of dark lines on the spectrum exactly at those positions which bright lines from the vapours themselves would occupy. The solar spectrum, then, consisting as it does of a very large number of dark lines on a coloured back- ground, tells us that the centre of the sun is at a white heat, and that this incandescent core is surrounded by an atmosphere of incandescent vapour at a somewhat lower temperature. By comparing the positions of the dark lines in the solar spectrum with the bright lines in spectra which have already been mapped, we learn 211