ARTICLES 583 



frequency can only be produced by at least as great an electron 

 energy or its equivalent. It is very likely that a given elec- 

 tron energy produces a frequency of one kind only, viz. its 

 equivalent, and vice versa. In other words a blue electron 

 produces only blue waves and vice versa. Moreover, but this is 

 a separate point, it may well be that the whole of the energy of 

 the electron is converted into energy of radiation of the corre- 

 sponding frequency. Exact knowledge on these points will 

 make it possible to calculate the quantities of X-ray and 

 electron energy which are in equilibrium with each other. 



It is here that the X-ray work has such an interesting 

 bearing on the general problem of radiation. 



But it may be asked whether, when an electron is just able 

 to excite the characteristic rays of say the K series, they must 

 all come out of the atom which is struck. The electron cannot 

 spend any of its energy in exciting one of the rays and then go 

 on to another atom to excite another of the rays, because its 

 energy will have fallen below the critical amount before it gets 

 to the next atom. Either it excites some radiation of each ray 

 in the one atom, dividing up its energy in doing so, or it excites 

 only one ray in the atom in one case, and another in another 

 case, there being a certain probability for each ray. It is a 

 question to be solved. 



A great difficulty has always been met with in attempting 

 to explain the way in which the heat of a body is distributed 

 between the various modes of vibration of which the body and 

 its atoms and the ether in which it lies are capable. Accord- 

 ing to mechanical laws, each mode should have an equal share 

 in the energy. But the number of modes of high-frequency 

 vibration is so great compared to the number of low-frequency 

 modes that the energy ought to be stored mostly on the high- 

 frequency side. As Lorentz said in his opening address to the 

 first Solvay Conference : 



" In a system composed of matter and ether the energy must 

 always finish by accumulating in the ether where it will exist 

 in the form of extremely short waves. This is the inevitable 

 consequence of the theory of equipartition if it is applied to 

 two systems of which one has, thanks to its perfect continuity, 

 an infinite number of degrees of freedom, while for the ponder- 

 able matter the number is finite because of its molecular 

 structure." 



