ON PHYSICAL LINES OP FORCE. 505 



in which the absolute direction of rotation is reversed when that of the light 

 is reversed. The rotation in the latter case, whether related to an axis, as in 

 quartz, or not so related, as in fluids, indicates a relation between the direction 

 of the ray and the direction of rotation, which is similar in its formal expression 

 to that between the longitudinal and rotatory motions of a right-handed or a 

 left-handed screw ; and it indicates some property of the substance the mathe- 

 matical form of which exhibits right-handed or left-handed relations, such as are 

 known to appear in the external forms of crystals having these properties. In 

 the magnetic rotation no such relation appears, but the direction of rotation is 

 directly connected with that of the magnetic lines, in a way which seems to 

 indicate that magnetism is really a phenomenon of rotation. 



The transference of electrolytes in fixed directions by the electric current, 

 and the rotation of polarized light in fixed directions by- magnetic force, are 

 the facts the consideration of which has induced me to regard magnetism as a 

 phenomenon of rotation, and electric currents as phenomena of translation, instead 

 of following out the analogy pointed out by Helmholtz, or adopting the theory 

 propounded by Professor Challis. 



The theory that electric currents are linear, and magnetic forces rotatory 

 phenomena, agrees so far with that of Ampere and Weber ; and the hypothesis 

 that the magnetic rotations exist wherever magnetic force extends, that the 

 centrifugal force of these rotations accounts for magnetic attractions, and that 

 the inertia of the vortices accounts for induced currents, is supported by the 

 opinion of Professor W. Thomson*. In fact the whole theory of molecular vor- 

 tices developed in this paper has been suggested to me by observing the 

 direction in which those investigators who study .the action of media are looking 

 for the explanation of electro-magnetic phenomena. 



Professor Thomson has pointed out that the cause of the magnetic action 

 on light must be a real rotation going on in the magnetic field. A right-handed 

 circularly polarized ray of light is found to travel with a different velocity 

 according as it passes from north to south, or from south to north, along a 

 line of magnetic force. Now, whatever theory we adopt about the direction of 

 vibrations in plane-polarized light, the geometrical arrangement of the parts of 

 the medium during the passage of a right-handed circularly polarized ray is 

 exactly the same whether the ray is moving north or south. The only difference 



* See NichoPs Cyclopaedia, art "Magnetism, Dynamical Relations of," edition 1860; Proceedings 

 of Royal Society, June 1856 and June 1861 ; and Phil. Mag. 1857. 



VOL. I. 64 



