It may be asked what becomes of the energy which the hydrogen chloride molecule thus absorbs, and whether it necessarily after a longer or shorter time must be re-emitted as radiation. The latter is not the case. In a collision between molecules or atoms, the energy which one molecule (or atom) has absorbed by radiation can undoubtedly be transferred to another molecule, the velocity of which is thereby increased. The theoretical necessity of the occurrence of such collisions was clearly shown for the first time in a very significant investigation by two of Bohr’s students, Klein and Rosseland. Without collisions of this nature the radiation energy absorbed could never be transformed into heat energy. Here we come to a very great and important field, which has a very close connection with the theory of the chemical processes and to a better explanation of which the more recent experiments of Franck and his co-workers have made important contributions. CHAPTER VII THE STRUCTURE OF THE ATOM AND THE CHEMICAL PROPERTIES OF THE ELEMENTS Introduction. We have hitherto restricted ourselves mainly to those applications of the Bohr theory which have a direct connection with the processes of radiation. We have shown how fertile the theory has proved to be, how many problems, previously inexplicable, have been solved, and what exact agreement has been established between experiment and theory in this comprehensive field. We may now ask how the theory accounts for the chemical behaviour of the different elements. As early as 1913, Bohr, in connection with his researches on spectral phenomena, had considered the chemical properties of the elements and had pointed out interesting possibilities. [Illustration: FIG. 33.—Early representation of the formation of a hydrogen molecule (Bohr, 1913).] The Combination of Atoms into Molecules.