THE ELECTRIC AND LUMINIFEROUS MEDIUM. 
779 
82. The maximum electric force which air can sustain at ordinary temperatures 
and pressures is about 130 c.g.s, ; and on Pouillet’s data the maximum electric 
force involved in the solar radiation, near the Sun’s surface, is about 30 c.g.s., a value 
which would be much increased on more recent estimates. One result of taking a 
high value for the sethereal density would be that in the most intense existing field 
of radiation we are certain of being still far from the limits of perfect elasticity of 
the comparatively free aether. 
The kinetic energy in the free aether is the square of the magnetic intensity 
divided by Stt ; and this must be where p is its density and v its velocity. 
Now from Professor Lodge’s result the velocity corresponding to the c.g.s. unit of 
magnetic force is less than ’2 centimetre per second; hence the inertia of the asther 
must exceed twice that of water. The elasticity must of course be taken large in 
proportion to the density, in order to preserve the proj)er velocity of radiation. In 
view of the very great intensity of the chemical and electrical forces acting between 
the atoms in the molecule, values even much greater than these would not appear 
excessive. But on the other hand such a value of the density requires us to make 
the aether absolutely stationary except in a magnetic field, in order to avoid 
hydrodynamical forcives between moving bodies. The residual forcive betwmen 
bodies at rest in a field of aethereal motion, due to very slight defect of permeability, 
has already been shown, after Lord Kelvin’s illustration, to simulate diamagnetism ; 
and the fact that there exist no powerfully diamagnetic substances is so far a 
confirmation of the present hypothesis. The view that the magnetic field of a 
current involves only slight circulation of the fluid aether is also in keeping with the 
account which has been given (§ 46) of the genesis of such a field. 
On Magneto-Optic Rotation. 
83. The rotation of the plane of polarization of light in a uniform magnetic field 
depends on the interaction of the uniform velocity of the aether, which constitutes 
that field, with the vibrational velocity which belongs to the light-disturbance. The 
uniform flow in the medium we may consider to be connected with a partial orien¬ 
tation of the vortex-molecules; the chemical or hydrodynamic vibrations, in other 
words vibrations of the magnetism, can now be propagated in waves, and it is natural 
to expect that the propagation of the light will be somewhat affected by this regu¬ 
larity. Now for the light-waves the motion that is elastically effective is the rotation 
didt (f /<); and the varying part of the velocity of an element of volume containing 
the rotational motion of the magnetic vortices which is to some extent interlinked 
with the motion of the light-waves, is proportional to 
iL 
Lie 
V, 0, 
where^ = o.„£+/3„|^+y, 
0 G 2 
