19II.] AGGREGATES OF ELECTRONS. 355 



kinetic energy of the electrons is the same as that of the molecules 

 of a gas at the temperature of the metal which emits them ; and we 

 can calculate the value of the well-known constant R in the gas 

 equation pi'=^R$, where p is the pressure, z' the volume and 6 the 

 absolute temperature of the gas, from purely electrical experiments 

 of the kind indicated. It follows from the results of these experiments 

 together with a simple application of the principles of the dynamical 

 theory of gases that the free electrons inside a metal must have the 

 distribution of velocity which is required by Maxwell's law and in 

 particular must have the same average translational kinetic energy 

 as the molecules of a gas at the temperature of the metal which 

 contains them. 



2. Material Media axd Electromagnetic Radiations. 



The action of light on insulating media is a rather complicated, 

 but extremely important, phenomenon on which the electron theory 

 has thrown a great deal of light. IMaxwell showed, many years ago, 

 that light is an electromagnetic phenomenon. A beam of light is in 

 fact a wave of oscillating electric and magnetic force, the electric 

 and magnetic forces being at right angles to one another and to the 

 direction of propagation. \Mien such a wave falls on an insulating 

 medium the oscillating electric force will set into vibration the com- 

 paratively stable electrons which, as we have seen, are embedded in 

 the medium. The electrons will execute what are appropriately 

 called forced oscillations, about their original equilibrium positions, 

 and these oscillations will have the same periodic time as the light. 

 Thus when it traverses a material insulating medium the light has 

 not only to keep itself going; it has to keep the electrons which make 

 up the medium going as well. Roughly speaking one may say that 

 the electrons in such a medium behave like a load on the luminif- 

 erous ether. We should therefore expect them to diminish the 

 speed of propagation of light through it and this is found to be 

 the case. The exact expression for the velocity cannot be obtained 

 without going more deeply than we have time to into the electro- 

 magnetic theory of light. It w'as first given by Maxwell, who 

 showed that the refractive index, to which the velocity of propaga- 

 tion is inversely proportional, was equal to the square root of the 



