  The kinetic theory of gases explained a range of experimental laws and successfully predicted new ones. However, there were some key difficulties. One of them was the departure of experimentally measured values of the ratio of the two specific heats of a gas, measured at constant pressure and at constant volume, from what the theory predicted. This prediction followed from a central tenet of the theory that energy is distributed equally amongst the degrees of freedom of a molecule. The difficulty could be mitigated by assuming that molecules of monatomic gases were perfectly smooth spheres that could not be set rotating and that diatomic molecules were also smooth to the extent that they could not be set rotating about the axis joining the two atoms in the molecule. But, as Maxwell made clear, (Niven, 1965, Vol. 2, 433) it must be possible for molecules to vibrate in a number of modes in order to give rise to the spectra of radiation that they emit and absorb, and once this is admitted the predictions of the theory clash unavoidably with the measured specific heats.