Page:A Treatise on Electricity and Magnetism - Volume 2.djvu/447

 831.] ARGUMENT OF THOMSON. 415

constitution of bodies, that it is not probable that any satisfactory theory can be formed relating- to a particular phenomenon, such as that of the magnetic action on light, until, by an induction founded on a number of different cases in which visible phenomena are found to depend upon actions in which the molecules are concerned, we learn something more definite about the properties which must be attributed to a molecule in order to satisfy the conditions of ob served facts.

The theory proposed in the preceding pages is evidently of a provisional kind, resting as it does on unproved hypotheses relating to the nature of molecular vortices, and the mode in which they are affected by the displacement of the medium. We must therefore regard any coincidence with observed facts as of much less scientific value in the theory of the magnetic rotation of the plane of polari zation than in the electromagnetic theory of light, which, though it involves hypotheses about the electric properties of media, does not speculate as to the constitution of their molecules.

831.] NOTE. The whole of this chapter may be regarded as an expansion of the exceedingly important remark of Sir William Thomson in the Proceedings of the Royal Society, June 1856 : The magnetic influence on light discovered by Faraday depends on the direction of motion of moving particles. For instance, in a medium possessing it, particles in a straight line parallel to the lines of magnetic force, displaced to a helix round this line as axis, and then projected tangentially with such velocities as to describe circles, will have different velocities according as their motions are round in one direction (the same as the nominal direction of the galvanic current in the magnetizing coil), or in the contrary direction. But the elastic reaction of the medium must be the same for the same displacements, whatever be the velocities and directions of the par ticles ; that is to say, the forces which are balanced by centrifugal force of the circular motions are equal, while the lumiriiferous motions are unequal. The absolute circular motions being there fore either equal or such as to transmit equal centrifugal forces to the particles initially considered, it follows that the luminiferous motions are only components of the whole motion ; and that a less luminiferous component in one direction, compounded with a mo tion existing in the medium when transmitting no light, gives an equal resultant to that of a greater luminiferous motion in the con trary direction compounded with the same non -luminous motion. I think it is not only impossible to conceive any other than this

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