680 Sir J. J. Thomson on 



energy is equal to the kinetic energy which the mass would 

 possess if it moved with the velocity of light. 



This result suggests that the potential energy in the 

 electrostatic field is really the kinetic energy possessed by 

 the mass which is distributed throughout the field, the mass 

 being regarded as an aggregate of equal particles each one 

 of which moves with the velocity of light. In a stationary 

 electric field we may suppose that these particles revolve 

 with this velocity round the lines of electric force, much as 

 the electrons from a hotwire can be made to revolve, though 

 at a slower speed, round lines of magnetic force. 



It seems natural to generalize this result and to suppose 

 that all mass, that of atoms as well as that of electrons, is 

 distributed through space with a density determined by the 

 electric field at the place where the mass is supposed to exist; 

 and that energy of every kind, kinetic, potential, thermal, 

 chemical or radiant, is of one and the same type, being the 

 kinetic energy possessed by the particles which are supposed 

 to constitute mass, these it is assumed always move with the 

 velocity of light. 



On this view there is no such thing as the transformation 

 of energy, if by that we mean a discontinuous change from 

 something: of one kind into something of another ; on our view 

 the transformation of energy is merely the flow of the mass 

 particles from one place to another. Thus for example, on 

 this view when a body gains kinetic energy, it is not because 

 any of its mass particles are moving faster ; it is because the 

 mass of the body has been increased and the increase in 

 the mass implies a proportional increase in the energy* 



It will perhaps make it clearer if we follow out in detail 

 this process in a special case — we will take that of a moving 

 electron. When an electron is moving relatively to the 

 bodies around if, the lines of electric force which start from 

 it are no longer uniformly distributed in all directions, 

 those running in directions at right angles to the direction 

 of motion of the electron get more concentrated, and those 

 running parallel to this direction more diffuse. The total 

 number of lines starting from the electron is unaltered by 

 the motion and depends only upon the charge on the electron. 

 Sincft the mass per unit volume at any place in the neigh- 

 bourhood of the electron is proportional to the square of the 

 number of lines of force passing through uni 1 area at that 

 place, the amount of mass between two spheres with their 

 centres at the electron and whose radii differ by unity, will 



be proportional to (*N 2 dS, where <^S is an element of the area 



