Ill 
1919 - 20 .] Molecular Energy in Gases. 
value of y is accordingly If. When the gas is cold the energy of vibration 
is nearly negligible, except in certain gases such as the vapours of the 
halogen elements, where it forms an appreciable part of the whole energy 
even at ordinary temperatures. When a diatomic gas is heated the to-and- 
fro vibration of the atoms comes increasingly into play and contributes a 
substantial addition to the energy, with the result that the specific heat 
rises and y falls below If. 
4. In all gases at all temperatures there is equipartition of energy 
between each degree of freedom of translation and each effective degree of 
freedom of rotation. 
5. The abnormal behaviour of hydrogen at very low temperatures, 
discovered by Eucken, may conjecturally be accounted for by supposing 
a change of molecular structure to occur which deprives the hydrogen 
molecule of its two normal degrees of freedom of rotation. If such a 
change of structure occurs it may be expected to exhibit hysteresis in 
relation to the temperature. 
6. In triatomic and polyatomic gases there are three effective degrees 
of freedom of rotation which, along with the three degrees of freedom of 
translation, would make C v equal to 3R and y equal to 1^ if there were no 
energy of vibration. But in addition there is in general a considerable 
amount of energy of vibration, resulting from to-and-fro movements of the 
atoms within the molecule, to which the principle of equipartition does not 
apply. Vibrations of relatively long period become important at relatively 
low temperatures. This makes the specific heat actually greater than 3R 
and y less than 1^, especially at high temperatures, when the energy of 
vibration becomes a large part of the whole energy. 
7. It does not appear to be necessary to have recourse to the Quantum 
Theory in dealing with molecular rotations in gases : at the same time, the 
observed facts do not conflict with the theory. 
8. The general effect of to-and-fro vibrations of atoms within the 
molecules of a gas is satisfactorily expressed in terms of the Quantum 
Theory. The resemblance which exists between the type of curve given 
by Planck's theory and the curve of magnetisation of a ferromagnetic 
substance suggests that if the nature of the atoms and their constraints 
were better understood the results might admit of interpretation in terms 
which would not be inconsistent with the older dynamics. 
{Issued separately August 4, 1920.) 
