Marci 28, 1907] 
NATURE 
319 
VIEWS OF THE ETHER. 
Introduction. 
MODERN 
[N putting into print once more the book called ‘‘ Modern 
Views of Electricity,’’ my object is to recall atten- 
tion to the ethereal aspect of affairs, and to assist in the 
combination of those ideas with the comparatively recent 
notion of electrons on the strength of which such great 
advances have been made. ‘There are several additions to 
the book, and especially there is a concluding chapter 
which, since the other portions of the original book 
appeared in the columns of Narure, may likewise be 
allowed so to appear. It will be observed that, on the 
basis of a consistent working hypothesis therein, an 
attempt is made to estimate the absolute value of the two 
ethereal constants—a thing which I have for many years 
sought to do. It will also be seen that, from the point 
of view adopted, the density of the ether comes out, not 
merely greater than platinum, as had several times been 
surmised, but very much greater: in fact, something com- 
parable to a billion times the density of water, and its 
intrinsic constitutional energy is correspondingly enormous. 
There is nothing paradoxical, nor, so far as I can see, 
improbable, about these figures. Matter is an excessively 
porous or gossamer-like structure, and the inertia of matter 
must be a mere residual fraction of the inertia of the con- 
tinuous incompressible complex fluid, of which it is hypo- 
thetically composed, and in which it moves. 
The following is the chapter referred to :— 
CONCLUDING CHAPTER OR SUMMARY. 
Structure of the Ether. 
What, then, is the conclusion of the whole matter, so 
iar as a conclusion is possible at present? 
The material universe seems to consist of a_ perfectly 
continuous incompressible and inextensible medium, filling 
all space without interstices or breach of continuity ;—not 
of a molecular or discrete structure, and as a whole com- 
pletely at rest: as frictionless moreover, and unresisting 
to all ordinary motion of what we call matter through it, 
as is the mathematical conception—a perfect fluid. But 
in spite of immobility as a whole, it possesses that pro- 
perty of ‘‘rigidity,’” or elastic resilience to ‘‘ shear,’ 
which is characteristic of what we ordinarily call a solid; 
wherefore it would appear that it must be, throughout, 
in such a state of excessively fine-grained turbulent motion 
as would confer this property upon it. And the resilience 
is so complete and instantaneous, without any delay or 
permanent set, that the elasticity must be described as 
“ perfect."’ It is the gyrostatic kind of elasticity, dis- 
covered dynamically and applied ethereally by Lord Kelvin, 
whereby a perfect fluid can kinetically acquire some of the 
properties of a perfect solid. 
It is well known that every solid possesses two kinds of 
elasticity—elasticity of bullx and elasticity of shape. The 
first or volume elasticity may also’ be called ‘* the in- 
compressibility,"” and is common to all forms of matter— 
fluid as well as solid. In the case of the ether, however, 
the value of this quantity appears to be infinite: it is, at 
any rate, greater than we have as yet been able to 
appreciate by specially directed experiments—meaning 
especially the Cavendish experiment referred to in §§ 4 
and 14a. The elasticity of figure, or shape-elasticity, is 
only possessed by solids, and is technically called 
“rigidity ’’; it is small in the case of india-rubber, great 
in the case of steel or glass; it is the property on which 
spiral springs and torsion-balances depend. The two kinds 
of elasticity are quite independent of each other—quite 
independent also of anything akin to viscosity, which in 
the case of the ether appears to be zero. ‘ 
Now something analogous to shape-elasticity the ether 
possesses. It does not possess ordinary “mechanical 
rigidity, because that is an affair of molecules; but it 
possesses something which may be called an_ electric 
rigidity, or electromotive élasticity. It is identical with 
the electromotive elasticity of a dielectric,—it is the pro- 
perty which causes recoil after. charge; and it has been 
denoted by 47/«x, where « is the absolute Faraday’s 
dielectric constant, or specific inductive capacity, for free 
space. : 
NO. 1952, VOL. 75] 
The property thus analogous to rigidity, or shape- 
elasticity, is accompanied by another property, akin to 
inertia. This is the property to which magnetism is due; 
it is a magnetic inertia, to pair with electric rigidity, and 
it has been denoted throughout by 47m, where p» is the 
absolute magnetic permeability of free space. The seltf- 
induction of quasi-inertia associated with every electric 
current, of which 47m is the non-geometrical and essential 
factor, is explicable, up to a point, as due to the mag- 
netic field excited by electric motion; but it would seem 
as if ultimately it must necessarily be dependent on 
an unexplained and fundamental kind of inertia possessed 
by the ether itself; so that the ether may be said to have 
a certain density, or mass per unit volume,—something 
at least so like ordinary material specific-gravity or 
density that we have to call it by the same name. 
By reason of these two properties—electric elasticity 
and magnetic density—transverse electromagnetic waves 
are transmitted through and by the ether, at a perfectly 
definite and known speed. This speed of wave propagation 
is far greater than any we are accustomed to in connec- 
tion with matter; and if ever the motion of matter can 
be made to approach this speed, it must encounter a re- 
action or impedance or opposition to further acceleration, 
which ultimately, in the limit, amounts to a practically 
infinite obstruction, at the actual critical speed. 
This obstruction is not of the nature of friction,—it is 
not resistance proportional to the velocity, or in any way 
dependent on the velocity: it solely opposes acceleration, 
and is of the nature of impedance or inertia. 
The fact of inertia enables an oscillatory wave-process 
to go on in the ether, and endows those oscillations with 
a particular kind of alternating kinetic, as well as with 
potential, energy. 
The energy of strained or distorted ether is always 
potential energy, and is all the potential energy there is; 
but accessible or convertible kinetic energy is usually only 
possessed by those individualised and discriminated regions, 
or ethereal structures, which possess the power of loco- 
motion, and ‘which in their aggregate appeal to our senses 
as | smatter. }7 
During the passage of waves, the ethereal structure is 
sheared to and fro; not with any movement as a whole, 
but with equal opposite movement of two aspects, or 
elements, or conditions, of its structure: such shear being 
equivalent to what is called an electric displacement, and 
being subject to a restoring force accurately proportional 
to that displacement. 
This elasticity is ‘‘ perfect’ in free space, apart from 
matter, until a critical shear, of unknown value, is 
reached. If strained beyond that, it may be supposed that 
a separation, or dislocation, or decomposition, of the ether 
into two components or constituents would occur ;—con- 
stituents generated, as it were, by means of the shear, 
and probably not existing, as such, in the unperturbed 
ether. One of these components we call positive, and the 
other negative, electricity. Once formed they do not dis- 
appear again: they may combine—or approach each other 
so closely that they neutralise each other's effects at a 
distance; but they are still readily separable by electro- 
motive force. They do not combine in the sense of destroy- 
ing each other,—they do not re-form the original sub- 
stance out of which they were produced. 
The negative electricity, when separated, is freely mobile 
and easily isolated: it is what we experience as an 
electron. The positive constituent does not appear thus in 
an isolated manner, but is only known to exist in a mass, 
as if matted together and associated with an indistinguish- 
able and inseparable aggregate of charges—opposite 
charges apparently in combination, going about as a whole. 
Some of these aggregates may unite into larger ones; 
others, when too large, may split up into smaller ones; 
and so finally a set of sub-permanent stable aggregates are 
formed, which we recognise as the atoms of the so-called 
‘elements’? of matter: each with its appropriate degree 
of stability. 
These masses or aggregates may temporarily acquire, 
or may lose, one or more of the free electrons; and by 
thus becoming amenable. to electrical or chemical attrac- 
tiohs and repulsions, constitute what we call ‘‘ ions,’’ so 
long as the unbalanced or electrified condition lasts. 
