A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 529 



The propagation of undulations consists in the continual transformation of 

 one of these forms of energy into the other alternately, and at any instant 

 the amount of energy in the whole medium is equally divided, so that half 

 is energy of motion, and half is elastic resilience. 



(7) A medium having such a constitution may be capable of other kinds 

 of motion and displacement than those which produce the phenomena of light 

 and heat, and some of these may be of such a kind that they may be 

 evidenced to our senses by the phenomena they produce. 



(8) Now we know that the luminiferous medium is in certain cases acted 

 on by magnetism ; for Faraday* discovered that when a plane polarized ray 

 traverses a transparent diamagnetic medium in the direction of the lines of 

 magnetic force produced by magnets or currents in the neighbourhood, the plane 

 of polarization is caused to rotate. 



This rotation is always in the direction in which positive electricity must 

 be carried round the diamagnetic body in order to produce the actual mag- 

 netization of the field. 



M. Verdetf has since discovered that if a paramagnetic body, such as 

 solution of perchloride of iron in ether, be substituted for the diamagnetic body, 

 the rotation is in the opposite direction. 



Now Professor W. Thomson J has pointed out that no distribution of forces 

 acting between the parts of a medium whose only motion is that of the lumi- 

 nous vibrations, is sufficient to account for the phenomena, but that we must 

 admit the existence of a motion in the medium depending on the magnetization, 

 in addition to the vibratory motion which constitutes light. 



It is true that the rotation by magnetism of the plane of polarization has 

 been observed only in media of considerable density ; but the properties of the 

 magnetic field are not so much altered by the substitution of one medium for 

 another, or for a vacuum, as to allow us to suppose that the dense medium 

 does anything more than merely modify the motion of the ether. We have 

 therefore warrantable grounds for inquiring whether there may not be a motion 

 of the ethereal medium going on wherever magnetic effects are observed, and 



* Experimental Researclies, Series xix. 



t Comptes Rendus (1856, second half year, p. 529, and 1857, first half year, p. 1209). 



\ Proceedings of the Royal Society, June 1856 and June 1861. 



VOL. i. 67 



