276 Maxwell. 



In the interval since the publication of his previous memoir 

 Maxwell had become convinced by Thomson's arguments that 

 magnetism is in its nature rotatory. "The transference of 

 electrolytes in fixed directions by the electric current, and the 

 rotation of polarized light in fixed directions by magnetic force, 

 are," he wrote, "the facts the consideration of which has 

 induced me to regard magnetism as a phenomenon of rotation, 

 and electric currents as phenomena of translation." This con- 

 ception of magnetism he brought into connexion with Faraday's 

 idea, that tubes of force tend to contract longitudinally and to 

 expand laterally. Such a tendency may be attributed to 

 centrifugal force, if it be assumed that each tube of force 

 contains fluid which is in rotation about the axis of the tube. 

 Accordingly Maxwell supposed that, in any magnetic field, the 

 medium whose vibrations constitute light is in rotation about 

 the lines of magnetic force; each unit tube of force may for the 

 present be pictured as an isolated vortex. 



The energy of the motion per unit volume is proportional 

 to /jH 2 , where /j. denotes the density of the medium, and H 

 denotes the linear velocity at the circumference of each vortex. 

 But, as we have seen,* Thomson had already shown that the 

 energy of any magnetic field, whether produced by magnets or 

 by electric currents, is 



where the integration is taken over all space, and where it 

 denotes the magnetic permeability, and H the magnetic force. 

 It was therefore natural to identify the density of the medium 

 at any place with the magnetic permeability, and the circum- 

 ferential velocity of the vortices with the magnetic force. 



But an objection to the proposed analogy now presents 

 itself. Since two neighbouring vortices rotate in the same 

 direction, the particles in the circumference of one vortex must 

 be moving in the opposite direction to the particles contiguous 



* Cf. pp. 248, 250. 



