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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 com- 

 ponent in one direction, compounded with a motion existing in the 

 medium when transmitting no light, gives an equal resultant to that 

 of a greater luminiferous motion in the contrary direction com- 

 pounded with the same non-luminous motion. I think it is not only 

 impossible to conceive any other than this dynamical explanation of 

 the fact that circularly polarized light transmitted through mag- 

 netized glass parallel to the lines of magnetizing 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 reality of Ampere's explanation of the 

 ultimate nature of magnetism ; and gives a definition of magnetiza- 

 tion in the dynamical theory of heat. The introduction of the prin- 

 ciple of moments of momenta (" the conservation of areas ") into 

 the mechanical treatment of Mr. Rankine's hypothesis of " molecular 

 vortices," appears to indicate a line perpendicular to the plane of re- 

 sultant rotatory momentum (" 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 mea- 

 sure of the " magnetic moment." The explanation of all phenomena 

 of electro-magnetic attraction or repulsion, and of electro-magnetic 

 induction, 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 molecu- 

 larly grouped ; or whether all matter is continuous, and molecular 

 heterogeneousness consists in finite vortical or other relative motions 

 of contiguous parts of a body ; it is impossible to decide, and per- 

 haps in vain to speculate, in the present state of science. 



I append the solution of a dynamical problem for the sake of the 

 illustrations it suggests for the two kinds of effect on the plane of 

 polarization referred to above. 



Let the two ends of a cord of any length be attached to two 



