Marcu 28, 1907 | 
NA TORE 
521 
the absolute value of Faraday’s dielectric constant «, for 
free space, is 10-** cubic centimetre per erg. 
In other words, the intrinsic energy of constitutional 
motion of the ether, to which its rigidity is due, is of the 
order 10° ergs or 107° Joules per cubic centimetre—about 
a hundred foot Ibs. per atomic volume; which is equi- 
valent to the output of a million horse-power working for 
forty million years, in every cubic millimetre of space. It 
can otherwise be expressed as the energy of a thousand 
tons per cubic millimetre, moving with the velocity of 
light; but of course the motion really contemplated is all 
internal and circulatory. 
Transmission of Waves. 
Wherever electrons and atoms exist, they modify the 
ether in their immediate neighbourhood, so that waves 
passing through a portion of space containing them are 
affected by their presence, as if the ether were more or 
less loaded by them; because the electric displacements 
' which go on in the unseparated and still perfectly united 
constituents of free ether are also shared to some extent 
by the separated peculiarities, especially by those of the 
electrons which are not too embedded in or surrounded by 
a positive charge—for instance, like a nucleus in a shell. 
These might be inert, and without influence on the light, 
except as small fixed mechanical obstructions ; but all those 
charges which possess externally-reaching lines of force 
must share in the motion of the waves, without having the 
requisite amount of resilience to compensate for their 
inertia; consequently they, to that extent, constitute a 
retarding, and either an absorbing or a reflecting, agency. 
Furthermore, their motions of vibration and _ rotation 
during the epochs of acceleration, however caused, 
encounter the inertia of the medium, and thereby excite 
waves in it—waves of oscillatory electric displacement 
with magnetic concomitant—and this electromagnetic 
radiation is transmitted out into space; but it is insignifi- 
cant in amount unless the acceleration is violent. It is 
proportional to the square of the acceleration. 
The positive and negative constituents, when they com- 
bine or cohere, do not destroy each other and revert into 
plain ether again; on the contrary, they retain their in- 
dividuality and persist, in either a combined or separate 
state. We do not know how to produce or to destroy 
these peculiarities; and though atoms of matter are com- 
posed of them, and though all electrical phenomena and 
the excitation of radiation are due to their presence and 
behaviour, it is no more and perhaps not much less correct 
to say that the main bulk of the ether is composed of them 
than it is to say that actual sodium and chlorine exist in 
undissociated common salt. These elements only make 
their appearance when the original substance is decomposed. 
But certainly matter can be dissociated with extreme ease, 
whereas the dissociation of ether is unknown and hypo- 
thetical, save as represented by its apparent results. 
Nevertheless, it must be the case that the slight, almost 
infinitesimal, shear, which goes on in a light wave, is of 
the nature of incipient and temporary electric separation ; 
and all electromotive force tends to drive one constituent 
in one direction and the other in the other; thus beginning 
that individualisation or separate manifestation of the two 
ingredients, without a knowledge of which the original 
fluid would have appeared to be of a perfectly uniform 
and homogeneous character. 
It is quite possible that the actually double aspect of 
ether is not only manifested, but really generated, by an 
electromotive force applied to it, just as the elastic recoil 
is so generated. It appears possible that a_ sufficiently 
violent E.M.F. applied to the ether, by some method un- 
known to us at present, must be the kind of influence 
necessary to shear it beyond the critical value and leave 
its components permanently distinct; such constituents 
being opposite electric charges, which, when once 
thoroughly separated, only combine to form matter, and 
do not recoil into ordinary ether again. 
Hypothetical Longitudinal Stress. 
Every attempt at separation of this kind, even if no 
stronger than exists in ordinary light, seems to be accom- 
panied by a slight longitudinal force at right angles both 
to the displacement and to the orbital axis of the excursion 
NO. 1952, VOL. 75 | 
; of radiation 
—a force which is known as the normal pressure of light, 
or Maxwell’s pressure, perpendicular to an advancing 
wave-front: the inertia of the constantly encroached upon 
region of free ether having the effect of momentum. 
If the disturbance could be made so extreme as to result 
in permanent dislocation, this pressure might leave behind 
it, as permanent residue, a longitudinal pressure, extending 
throughout space inversely as the distance; whereby all 
the dislocated material would thereafter be urged together 
with a force which we know as gravitation, proportional 
in any piece of matter to the number of dislocated centres 
which go to compose it, and therefore proportional to its 
mass, irrespective of secondary accidents of a physical or 
chemical constitution. 
Amplitude of Light Wave. 
If a is the amplitude of shear during the passage of a 
wave of light, and if w is the maximum velocity of re- 
covery, then 
where wv is the velocity of light and A the wave-length. 
The total energy per unit volume is 3pu*, where p is the 
density of the medium; for this represents twice the 
average kinetic energy, and of this quantity one-half is 
really kinetic, the other half potential. 
Now direct thermal measurements, such as those con- 
ducted by Pouillet, give as the energy of sunlight near 
the earth 4x10-° erg per c.c.; and consequently, in the 
region of intense light near the solar surface, the energy 
must be about 2 ergs per cubic centimetre. 
There may be more intense light than this, but this is the 
most intense we know of, so it is instructive to consider 
the amplitude of the shear corresponding to such violent 
illumination. Let us therefore put 3pu*=2, whence 
Ue AO eae 
It follows, therefore, that 
1 | =a LO 
and accordingly the amplitude, of the most intense visible 
light we are acquainted with, is only 10-'? of a wave- 
length. The maximum strain is 27 times this fraction ; 
and so the tangential stress thus called out in the medium, 
its rigidity being 10°°, may be estimated as comparable 
with 1o’’ dynes per square centimetre, or 10' atmo- 
spheres. 
The ordinary electrostatic unit of charge, on this esti- 
mate of ethereal elasticity, becomes 10-'* square centi- 
metre, or the superficial dimension of an atom. This also 
corresponds with the estimate above, that the electronic 
charge is equal to the superficies of an electron; since one 
should be 10*° times the other. 
The pressure of light has been represented by Prof. 
Poynting as a travelling momentum, like that of a jet of 
water, resulting in a pressure pcv, where c is the velocity of 
longitudinal motion or circulation in the light beam, and 
v is the velocity of light. Taking the pressure of intensest 
light as 2 dynes per square centimetre, this gives 
c=310-~* centimetre per second: excessively small, there- 
fore, even in that extreme case. 
Hypothetical Flow along Lines of Magnetic Induction. 
It has long been a working hypothesis with some mathe- 
matical physicists (see, for instance, the ensuing April 
number of the Philosophical Magazine) that there was 
probably something of the nature of a flow—an ethereal 
flow—along lines of magnetic induction; and the fact 
that these are always closed curves, in all known circum- 
stances, is in favour of such an idea. The energy of the 
field would then be attributable to the energy of this flow ; 
and though it is possible that the flow might be of the 
nature of components moving in opposite directions, the 
movement is hardly likely to be of this nature, since that 
would correspond with merely an electric current. 
Fourteen years ago, in 1893, having rather perfect 
appliances for examining the effect of drift on the velocity 
of light, I carefully looked for some longitudinal flow 
along lines of magnetic force; repeating the experiments 
