PEOFESSOE CLEEK MAXWELL OX THE ELECTEOM A GXETIC FIELD. 
461 
depends in some way on the motion of the rest ; and at the same time these connexions 
must be capable of a certain kind of elastic yielding, since the communication of motion 
is not instantaneous, but occupies time. 
The medium is therefore capable of receiving and storing up two kinds of energy, 
namely, the “ actual ” energy depending on the motions of its parts, and “ potential ” 
energy, consisting of the work which the medium will do in recovering from displace- 
ment in virtue of its elasticity. 
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 pheno- 
mena they produce. 
(8) Now we know that the luminiferous medium is in certain cases acted on by 
magnetism ; for Faeaday * discovered that when a plane polarized ray traverses a trans- 
parent 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 magnetization of the field. 
M. VEKDETf 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^ has pointed out that no distribution of forces acting 
between the parts of a medium whose only motion is that of the luminous 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 we have some reason to suppose that this motion is one of rotation, 
having the direction of the magnetic force as its axis. 
(9) We may now consider another phenomenon observed in the electromagnetic 
* Experimental Eesearches, Series 19. 
t Comptes Eendus (1856, second half year, p. 529, and 1857, first half year, p. 1209). 
+ Proceedings of the Eoyal Society, June 1856 and June 1861. 
3 e 2 
