Discontinuity in the Pheno')nena of Radiation 65 



to kinetic energy. The verj'^ existence of these intermolecular 

 vibrations of necessity postulates the existence of mutually 

 restraining forces on the various parts of the molecule, and such 

 forces involve the possession by the molecule of a certain amount 

 of internal potential energy. Further, if the molecule constitutes 

 part of a solid body there will be restraining forces of cohesion 

 on the molecule as a whole involving possession of still more 

 potential energy. Under certain rather restrictive conditions, 

 but, nevertheless, conditions which are of considerable import- 

 ance in practice, we can say that each molecule, in addition to 

 the total kinetic energy referred to, possesses on the average an 

 amount 



1 T- 



- a T ergs 



of potential energy for each degree of freedom of motion which 

 involves vibratory movement. I must ask you to bear in mind 

 the fact that these are statements about average values of 

 energy. They are entirely statistical. The behaviour of any 

 individual molecule is much too complex an affair to follow. In 

 any case it probably passes through the most varied conditions in 

 any finite period of time. Further, this equal partitioning of the 

 energy among the various degrees of freedom is a result depend- 

 ent on the complete truth of the dynamical principles as laid 

 down by Newton and extended by Lagrange and Hamilton. 



Now let us turn to the behaviour of the ether which is 

 situated within our enclosure where the walls are at a constant 

 temperature. The trains of radiation passing through it in^'olve 

 periodic changes at every point of this ether. The old view, so 

 skilfully developed by Young Fresnel, Green, MacCullagh, Lord 

 Kelvin was that this periodic change is actually a vibratory 

 movement of the ether, which is pictured as a material comljining 

 properties of rigidity, incompressibility and fluidity in a fashion 

 hard to reconcile with our preconceived notions based as they 

 are on the actual properties of matter as we directly apprehend 

 it, Maxwell, taking up the ideas of Faraday and putting an 



