The selective absorption of ketones

The absorption of light by carbon compounds is either continuous or selective, although not infrequently both kinds of absorption are found to occur together. In the visible and ultra-violet portions of the spectrum the absorption is decidedly a constitutive property of the compound; apparently the only additive relation is the effect of increasing the mass of the molecule, as, for instance, in homologous series, when displacement of the absorption band towards the red accompanies increase in the molecular weights. Hartley’s pioneer work on absorption led him to recognise clearly the dynamic nature of this property, and to arrive at the conclusion that it is caused by the vibrations of sub-molecular particles synchronising with those -of the incident waves of light. Support is lent to this conclusion by the observation that the absorption and emission spectra of simple substances are identical. As regards the nature of the particles to whose vibrations selective absorption is due, Hartley originally expressed the view that these must be atoms, or groups of atoms. More recent investigations confirm this opinion so far as the infra-red region is concerned; but, on the other hand, according to Drude’s work on the electronic theory of dispersion and absorption, the particles whose oscillations cause selective absorption in the visible and ultra-violet regions are sub-atomic and probably correspond to. the valency electrons. If this be so, the relations between the absorption and the constitution of carbon compounds must be sought in the dynamic state of the valencies of the absorbing group of atoms.