Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. In the course of the work, references are made to cases in which the classical wave theory apparently fails to explain the facts relating to molecular scattering in a satisfactory manner, and in the final chapter an attempt is made to consider these cases in terms of the conception introduced by Einstein in 1905 that light is not continuous wave-motion but consists of discrete quanta moving through space.