the Structure of the Electric Field. 309 



following the tube of force which is .supposed, until the dis- 

 turbance reaches it, to retain the velocity it had before the 

 stoppage of the corpuscle. The constancy of the energy is 

 due to the length of the kink being proportional to its dis- 

 tance from the corpuscle from which it arose, and this again 

 is due to the uniform motion forward of the part of the tube 

 not yet reached by the disturbance. If the motion of this 

 part of the tube is interfered with after the pulse is started, 

 then the length of the kink may be no longer proportional 

 to the distance from the corpuscle, and then the energy in 

 the pulse will no longer remain constant as it moves away 

 from the corpuscle. If for example we retard by any means 

 the further portions of the tube, the length of the kink when 

 it reaches them will be reduced, and the energy in it less 

 than if the motion had not been retarded. The energy lost 

 will either appear as the energy of a reflected pulse or it will 

 be communicated to the system which retards the motion of 

 the tube of force. 



One way by which the motion might be retarded is by the 

 repulsion exerted on it by a similar tube of force which it is 

 overtaking, or it might be pushed forward by the repulsion 

 of a similar tube from behind which was overtaking it. In 

 the first case the energy in the kink would be reduced, and 

 in the second it would be increased. Wc can in this way 

 picture to ourselves interchanges between the energy in the 

 pulses and that of charged bodies in their path, of a some- 

 what similar character to those which occur between colliding 

 molecules, and which might be expected to lead to statistical 

 equilibrium between the energy in the pul<es and the kinetic 

 energy of the charged bodies. 



We can see, too, in a similar way that there might be 

 interchange of energy between two pulses generated by 

 corpuscles so that one might overtake the other. And thus 

 if we had an assemblage of such pulses travelling backwards 

 and forwards in an enclosure with perfectly reflecting walls 

 we should expect that the interchange of energy between 

 them would produce, when the system was in a steady state, 

 a distribution of energy analogous to that which holds for 

 the distribution of energy among the molecules of a gas in 

 a steady state. 



When the field round the corpuscle is supposed uniformly 

 distributed the Rontgen rays produced by a stoppage of a 

 slowly moving corpuscle are symmetrically distributed with 

 respect to a plane through the corpuscle, at right angles to 

 the direction in which it was moving. When the velocity 

 of the corpuscle approaches that of light there is sensibly 



