174 JAMES CLERK MAXWELL 



them. When electrical force acts on the medium this 

 displacement of the particles within each molecule 

 takes place until the stresses duo to the elastic re- 

 action of the vortices balance the electrical force ; the 

 medium behaves like an elastic body yielding to 

 pressure until the pressure is balanced by the elastic 

 stress. When the electric force is removed the cells 

 or vortices recover their form, the electricity returns 

 to its former position. 



In a, medium such as this waves of periodic 

 displacement could bo set up, and would travel with 

 a velocity depending on its electric properties. Tho 

 value for this velocity can bo obtained from electrical 

 observations, and Maxwell showed that this velocity, 

 so found, was, within the limits of experimental error, 

 the same as that of light. Moreover, the electrical 

 oscillations take place, like those of light, in the front 

 of tho wave. Hence, ho concludes, " the elasticity of 

 tho magnetic medium in air is tho same as that of 

 tho luminiferous medium, if these two coexistent, 

 coextensive, and equally elastic media are not rather 

 one medium." 



The paper thus contains the first germs of the 

 electro-magnetic theory of light. Moreover, it is 

 shown that the attraction between two small bodies 

 charged with given quantities of electricity depends 

 on the medium in which they are placed, while tho 

 specific inductive capacity is found to be proportional 

 to the square of the refractive index. 



The fourth and final part of the paper investigates 

 the propagation of light in a magnetic field. 



Faraday had shown that the direction of vibration 



