41(5 MAGNETIC ACTION ON LIGHT. [831.
dynamical explanation of the fact that circularly-polarized light transmitted through magnetized glass parallel to the lines of mag netizing force, with the same quality, right-handed always, or left- handed always, is propagated at different rates according as its course is in the direction or is contrary to the direction in which a north magnetic pole is drawn ; but I believe it can be demonstrated that no other explanation of that fact is possible. Hence it appears that Faraday s optical discovery affords a demonstration of the re ality of Ampere s explanation of the ultimate nature of magnetism ; and gives a definition of magnetization in the dynamical theory of heat. The introduction of the principle of moments of momenta (" the conservation of areas") into the mechanical treatment of Mr. Rankine s hypothesis of " molecular vortices," appears to indi cate a line perpendicular to the plane of resultant rotatory mo mentum ("the invariable plane") of the thermal motions as the magnetic axis of a magnetized body, and suggests the resultant moment of momenta of these motions as the definite measure of the "magnetic moment." The explanation of all phenomena of electromagnetic attraction or repulsion, and of electromagnetic in duction, is to be looked for simply in the inertia and pressure of the matter of which the motions constitute heat. Whether this matter is or is not electricity, whether it is a continuous fluid inter- permeating the spaces between molecular nuclei, or is itself mole- cularly grouped ; or whether all matter is continuous, and molecular heterogeneousness consists in finite vortical or other relative mo tions of contiguous parts of a body ; it is impossible to decide, and perhaps in vain to speculate, in the present state of science.
A theory of molecular vortices, which I worked out at consider able length, was published in the Phil. Mag. for March, April, and May, 1861, Jan. and Feb. 1862.
I think we have good evidence for the opinion that some pheno menon of rotation is going on in the magnetic field, that this rota tion is performed by a great number of very small portions of matter, each rotating on its own axis, this axis being parallel to the direction of the magnetic force, and that the rotations of these dif ferent vortices are made to depend on one another b}^ means of some kind of mechanism connecting them.
The attempt which I then made to imagine a working model of this mechanism must be taken for no more than it really is, a de monstration that mechanism may be imagined capable of producing a connexion mechanically equivalent to the actual connexion of the
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