to wave-length, the value 10-2, which is a very safe
limit. It follows that the density of the aether must exceed
10-18, and its elastic modulus must exceed 103, which
is only about 10-8 of the modulus of rigidity of glass.
It thus appears that if the amplitude of vibration could be
as much as 10-2 of the wave-length, the aether would be an
excessively rare medium with very slight elasticity; and yet
it would be capable of transmitting the supply of solar energy
on which all terrestrial activity depends. But on the modern
theory, which includes the play of electrical phenomena as a
function of the aether, there are other considerations which
show that this number 10-2 is really an enormous overestimate;
and it is not impossible that the co-efficient of ultimate
inertia of the aether is greater than the co-efficient of
inertia (of different kind) of any existing material substance.
The question of whether the aether is carried along by the
earth's motion has been considered from the early days of
the undulatory theory of light. In reviving that theory at
the beginning of the 19th century, Thomas Young stated his
conviction that material media offered an open structure
to the substance called aether, which passed through
them without hindrance ``like the wind through a grove of
trees.'' Any convection of that medium could be tested by
the change of effective velocity of light, which would be
revealed by a prism as was suggested by F. J. D. Arago.
Before 1868 Maxwell conducted the experiment by sending light
from the illuminated cross-wires of an observing telescope
forward through the object-glass, and through a train of
prisms, and then reflecting it back along the same path;
any influence of convection would conspire in altering both
refractions, but yet no displacement of the image depending
on the earth's motion was detected. As will be seen later,
modern experiments have confirmed the entire absence of any
effect, such as convection would produce, to very high
precision. It has further been verified by Sir Oliver Lodge
that even in very narrow spaces the aether is not entrained
by its surroundings when they are put into rapid motion.
A train of ideas which strongly impressed itself on Clerk
Maxwell's mind, in the early stages of his theoretical views, was
put forward by Lord Kelvin in 1858; he showed that the special
characteristics of the rotation of the plane of polarization,
discovered by Faraday in light propagated along a magnetic
field, viz. that it is doubled instead of being undone when
the light retraces its path, requires the operation of some
directed agency of a rotational kind, which must be related
to the magnetic field. Lord Kelvin was thereby induced to
identify magnetic force with rotation, involving, therefore,
angular momentum in the aether. Modern theory accepts the
deduction, but ascribes the momentum to the revolving ions
in the molecules of matter traversed by the light; for the
magneto-optic effect is present only in material media.
Long previously Lord Kelvin himself came nearer this view, in
offering the opinion that magnetism consisted, in some way,
in the angular momentum of the material molecules, of which
the energy of irregular translations constitutes heat; but the
essential idea of moving electric ions of both kinds, positive
and negative, in the molecules had still to be introduced.
The question of the transparency of the celestial spaces presents
itself in the presebt connexion. Light from stars at unfathomable
distances reaches us in such quantity as to suggest that space
itself is absolutely transparent, leaving open the question
as to whether there is enough matter scattered through it to
absorb a sensible part of the light in its journey of years
from the luminous body. If the aether were itself constituted
of discrete molecules, on the model of material bodies, such
transparency would not be conceivable. We must be content
to treat the aether as a plenum, which places it in a class
by itself; and we can thus recognize that it may behave very
differently from matter, though in some manner consistent with
itself---a remark which is fundamental in the modern theory.
Action across a Distance contrasted with Transmitted Action.--In
the mechanical processes which we can experimentally modify at
will, and which therefore we learn to apprehend with greatest
fulness, whenever an effect on a body, B, is in causal connexion
with a process instituted in another body, A, it is usually
possible to discover a mechanical connexion between the two
bodies which allows the influence of A to be traced all the
way across the intervening region. The question thus arises
whether, in electric attractions across apparently empty space
and in gravitational attraction across the celestial regions,
we are invited or required to make search for some similar
method of continuous transmission of the physical effect,
or whether we should rest content with an exact knowledge
of the laws according to which one body affects mechanically
another body at a distance. The view that our knowledge in
such cases may be completely represented by means of laws of
action at a distance, expressible in terms of the positions
(and possibly motions) of the interacting bodies without
taking any heed of the intervening space, belongs to modern
times. It could hardly have been thought of before Sir Isaac
Newton's discovery of the actual facts regarding universal
gravitation. Although, however, gravitation has formed the
most perfect instance of an influence completely expressible,
up to the most extreme refinement of accuracy, in terms of
laws of direct action across space, yet, as is well known,
the author of this ideally simple and perfect theory held the
view that it is not possible to conceive of direct mechanical
action independent of means of transmission. In this belief
he differed from his pupil, Roger Cotes, and from most of
the great mathematical astronomers of the 18th century,
who worked out in detail the task sketched by the genius of
Newton. They were content with a knowledge of the truth of
the principle of gravitation; instead of essaying to explain
it further by the properties of a transmitting medium, they
in fact modelled the whole of their natural philosophy on that
principle, and tried to express all kinds of material interaction
in terms of laws of direct mechanical attraction across
space. If material systems are constituted of discrete atoms,
separated from each other by many times the diameter of any of
them, this simple plan of exhibiting their interactions in terms
of direct forces between them would indeed be exact enough to
apply to a wide range of questions, provided we could be certain
that the laws of the forces depended only on the positions
and not also on the motions of the atoms. The most important
example of its successful application has been the theory of
capillary action elaborated by P. S. Laplace; though even here
it appeared, in the hands of Young, and in complete fulness
afterwards in those of C. F. Gauss, that the definite results
attainable by the hypothesis of mutual atomic attractions really
reposed on much wider and less special principles---those,
namely, connected with the modern doctrine of energy.
Idea of an Aether.---The wider view, according to which
the hypothesis of direct transmission of physical influences
expresses only part of the facts, is that all space is
filled with physical activity, and that while an influence
is passing across from a body, A, to another body, B, there
is some dynamical process in action in the intervening
region, though it appears to the senses to be mere empty
space. The problem is whether we can represent the facts more
simply by supposing the intervening space to be occupied by
a medium which transmits physical actions, after the manner
that a continuous material medium, solid or liquid, transmits
mechanical disturbance. Various analogies of this sort are
open to us to follow up: for example, the way in which a
fluid medium transmits pressure from one immersed solid to
another--or from one vortex ring belonging to the fluid to
another, which is a much wider and more suggestive case;
or the way in which an elastic fluid like the atmosphere
transmits sound; or the way in which an elastic solid transmits
waves of transverse as well as longitudinal displacement.
It is on our familiarity with modes of transmission such as
these, and with the exact analyses of them which the science
of mathematical physics has been able to make, that our
predilection for filling space with an aethereal transmitting
medium, constituting a universal connexion between material
bodies, largely depends; perhaps ultimately it depends most
of all, like all our physical conceptions, on the intimate
knowledge that we can ourselves exert mechanical effect on
outside bodies only through the agencies of our limbs and
sinews. The problem thus arises: Can we form a consistent
notion of such a connecting medium? It must be a medium which
can be effective for transmitting all the types of physical
action known to us; it would be worse than no solution to
have one medium to transmit gravitation, another to transmit
electric effects, another to transmit light, and so on. Thus
the attempt to find out a constitution for the aether will
involve a synthesis of intimate correlation of the various
types of physical agencies, which appear so different to us
mainly because we perceive them through different senses.
The evidence for this view, that all these agencies are at
bottom connected together and parts of the same scheme, was
enormously strengthened during the latter half of the 19th
century by the development of a relation of simple quantitative
equivalence between them; it has been found that we can define
quantities relating to them, under the names of mechanical
energy, electric energy, thermal energy, and so on, so that
when one of them disappears, it is replaced by the others to
exactly equal amount. This single principle of energy has
transformed physical science by making possible the construction
of a network of ramifying connexions between its various
departments; it thus stimulates the belief that these constitute
a single whole, and encourages the search for the complete
scheme of interconnexion of which the principle of energy
and the links which it suggests form only a single feature.
In carrying out this scientific procedure false steps will
from time to time be made, which will have to be retraced, or
rather amended; but the combination of experimental science with
theory has elevated our presumption of the rationality of all
natural processes, so far as we can apprehend them at all, into
practical certainty; so that, though the mode of presentation of
the results may vary from age to age, it is hardly conceivable
that the essentials of the method are not of permanent validity.
Atomic Structure of Matter.---The greatest obstacle to such a
search for the fundamental medium is the illimitable complexity
of matter, as contrasted with the theoretical simplicity and
uniformity of the physical agencies which connect together its
different parts. It has been maintained since the times of
the early Greek philosophers, and possibly even more remote
ages, that matter is constituted of independent indestructible
units, which cannot ever become divided by means of any
mutual actions they can exert. Since the period, a century
ago, when Dalton and his contemporaries constructed from this
