applied to the Action of Magnetism on Polarized Light. 87 



are then produced in the substance by the action of a magnet or 

 of an electric current, the plane of polarization of the transmitted 

 light is found to*be changed, and to be turned through an angle 

 depending on the intensity of the magnetizing force within the 

 substance. 



The direction of this rotation in diamagnetic substances is the 

 same as that in which positive electricity must circulate round 

 the substance in order to produce the actual magnetizing force 

 within it; or if we suppose the horizontal part of terrestrial 

 magnetism to be the magnetizing force acting on the substance, 

 the plane of polarization would be turned in the direction of 

 the earth's true rotation, that is, from west upwards to east. 



In paramagnetic substances, M. Verdet* has found that the 

 plane of polarization is turned in the opposite direction, that is, 

 in the direction in which negative electricity would flow if the 

 magnetization were effected by a helix surrounding the substance, 

 ; In both cases the absolute direction of the rotation is the same, 

 whether the light passes from north to south or from south to 

 north, — a fact which distinguishes this phenomenon from the rota- 

 tion produced by quartz, turpentine, &c, 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 mathematical 

 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 mag- 

 netism is really a phenomenon of rotation. 



The transference of electrolytes in fixed directions by the elec- 

 tric current, and the rotation of polarized light in fixed direc- 

 tions by magnetic force, are the facts the consideration of which 

 has induced me to regard magnetism as a phenomenon of rota- 

 tion, and electric currents as phenomena of translation, instead 

 of following out the analogy pointed out by Helmholtz, or adopt- 

 ing 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 rota- 

 tions accounts for magnetic attractions, and that the inertia of 

 * Comptes Rendus, vol. xliii. p. 529 ; vol. xliv. p. 1209. 



