We have hitherto restricted ourselves to the purely atomic processes. But just as in the emission of radiation we meet spectra which owe their characteristics to _molecular processes_ (band spectra, cf. p. 162), we have also absorption spectra with characteristics depending essentially upon motions of the atomic nuclei in the molecules. A particularly interesting and instructive example of this nature is met with in the infra-red region of the spectrum in certain broad absorption lines or absorption bands, which are due to gases having molecules containing several atoms. In hydrogen chloride, for instance, there is found, in the region of the spectrum which corresponds to a wave-length of about 3·5 μ, such an absorption band, which by more accurate investigation has been shown to consist of a great number of absorption lines. The explanation of this collection of lines must be sought in the motions which the hydrogen nucleus and the chlorine nucleus perform, as they in part _vibrate_ with respect to each other and in part _rotate_ about their common centre of gravity. Just as in the case of the motions of the electrons in the atom, there are also certain stationary states for the nuclear motions. When the molecule absorbs radiation energy it will go from one of these states to another, where the energy content is greater. This absorption of energy proceeds according to the quantum rule, _i.e._, the product of the Planck constant _h_ and the frequency ν for the absorbed radiation must be equal to the difference in energy between the two stationary states; only those rays which have frequencies fulfilling this condition are absorbed. [Illustration: FIG. 32.—Schematic representation of possible motions in a molecule of hydrogen chloride. _O_ is the centre of gravity of the molecule. The black circles give the states of equilibrium of the nuclei, the circles _s_ their outer positions in oscillating, and the circles _r_ positions during the rotation of the nuclei.]