38 
NATURE 
genius of Faraday surmised might be possible, in his 
“Thoughts on Ray Vibrations.’’ Indeed, Newton 
guessed something of the same kind, I fancy, when 
he superposed ether-pulses on his corpuscles. 
Whatever be the truth in this matter, a discussion 
on radiation, of extreme weight and interest, though 
likewise of great profundity and technicality, is 
expected on Friday in Section A. We welcome Prof. 
Lorentz, Dr. Arrhenius, Prof. Langevin, Prof. Prings- 
heim, Prof. R. W. Wood, and others, some of whom 
have been specially invited to England because of the 
important contributions which they have made to the 
subject-matter of this discussion. 
Why is so much importance attached to radiation? 
Because it is the best-known and_longest-studied 
link between matter and zther, and the only property 
we are acquainted with that affects the unmodified 
great mass of ether alone. Electricity and magnet- 
ism are associated with the modifications or singu- 
larities called electrons; most phenomena are 
connected still more directly with matter. Radiation, 
however, though excited by an accelerated electron, 
is subsequently let loose in the ether of space, 
and travels as a definite thing at a measurable and 
constant pace—a pace independent of everything so 
long as the zther is free, unmodified and unloaded by 
matter. Hence radiation has much to teach us, and 
we have much to learn concerning its nature. 
How far can the analogy of granular, corpuscular, 
countable, atomic, or discontinuous things be pressed ? 
There are those who think it can be pressed very 
far. But to avoid misunderstanding, let me state, 
for what it may be worth, that I myself am an up- 
holder of ultimate Continuity, and a fervent believer 
in the zther of space. 
We have already learnt something about the zther ; 
and although there may be almost as many varieties 
of opinion as there are people qualified to form one, 
in my view we have learnt as follows: 
The zther is the universal connecting medium which 
binds the universe together, and makes it a coherent 
whole instead of a chaotic collection of independent 
isolated fragments. It is the vehicle of transmission 
of all manner of force, from gravitation down to 
cohesion and chemical affinity; it is therefore the 
storehouse of potential energy. 
Matter moves, but zther is strained. 
What we call elasticity of matter is only the result 
of an alteration of configuration due to movement and 
readjustment of particles, but all the strain and stress 
are in the zther. The zther itself does not move, 
that is to say it does not move in the sense of loco- 
motion, though it is probably in a violent state of 
rotational or turbulent motion in its smallest parts; 
and to that motion its exceeding rigidity is due. 
As to its density, it must be far greater than that 
of any form of matter, millions of times denser than 
lead or platinum. Yet matter moves through it with 
perfect freedom, without any friction or viscosity. 
There is nothing paradoxical in this: viscosity is not 
a function of density; the two are not necessarily 
connected. When a solid moves through an alien 
fluid it is true that it acquires a spurious or apparent 
extra inertia from the fluid it displaces; but, in the 
case of matter and zther, not only is even the densest 
matter excessively porous and discontinuous, with vast 
interspaces in and among the atoms, but the con- 
stitution of matter is such that there appears to be 
no displacement in the ordinary sense at all; the 
zther is itself so modified as to constitute the matter 
in some way. Of course, that portion moves, its 
inertia is what we observe, and its amount depends 
on the potential energy in its associated electric field, 
but the motion is not like that of a foreign body, it 
is that of some inherent and merely individualised 
NO. 2289, VOL. 92] 
[SEPTEMBER II, 1913 
portion of the stuff itself. Certain it is that the ether 
exhibits no trace of viscosity.” 
Matter in motion, zther under strain, constitute the 
fundamental concrete things weshave to do with in 
physics. The first pair represent kinetic energy, the 
second potential energy; and all the activities of the 
material universe are represented by alternations from 
one of these forms to the other. : 
Whenever this transference and transformation of 
energy occur, work is done, and some effect is pro- 
duced, but the energy is never diminished in quan- 
tity : it is merely passed on from one body to another, 
always from ether to matter, or vice versd—except 
in the case of radiation, which simulates matter—and 
from one form to another. 
The forms of energy can be classified as either a 
translation, a rotation, or a vibration of pieces of 
matter of different sizes, from stars and planets down 
to atoms and electrons; or else an ethereal strain 
which in various different ways is manifested by the 
behaviour of such masses of matter as appeal to our 
senses.® 
Some of the facts responsible for the suggestion 
that energy is atomic seem to me to depend on the 
discontinuous nature of the structure of a material 
atom, and on the high velocity of ifs constituent 
particles. The apparently discontinuous emission of 
radiation is, I believe, due to features in the real 
discontinuity of matter. Disturbances inside an atom 
appear to be essentially catastrophic; a portion is 
liable to be ejected with violence. There appears to 
be a critical velocity below which ejection does not 
take place; and, when it does, there also occurs a 
sudden rearrangement of parts which is presumably 
responsible for some perceptible athereal radiation. 
Hence it is, I suppose, that radiation comes off in 
gushes or bursts; and hence it appears to consist 
of indivisible units. The occasional phenomenon of 
new stars, as compared with the steady orbital motion 
of the millions of recognised bodies, may be sug- 
gested as an astronomical analogue. 
The hypothesis of quanta was devised to reconcile 
the law that the energy of a group of colliding mole- 
cules must in the long run be equally shared among 
all their degrees of freedom, with the observed fact 
that the energy is really shared into only a small 
number of equal parts. For if vibration-possibilities 
have to be taken into account, the number of degrees 
of molecular freedom must be very large, and energy 
shared among them ought soon to be all frittered 
away; whereas it is not. Hence the idea is suggested 
that minor degrees of freedom are initially excluded 
from sharing the energy, because they cannot be sup- 
plied with less than one atom of it. 
I should prefer to express the fact by saying that 
the ordinary encounters of molecules are not of a 
kind able to excite atomic vibrations, or in any way 
to disturb the zther. Spectroscopic or luminous vibra- 
tions of an atom are excited only by an exceptionally 
violent kind of collision, which may be spoken of as 
chemical clash; the ordinary molecular orbital en- 
counters, always going on at the rate of millions a 
second, are ineffective in that respect, except in the 
case of phosphorescent or luminescent substances. 
That common molecular deflections are ineffective is 
certain, else all the energy would be dissipated or 
transferred from matter into the zther; and the 
reasonableness of their radiative inefficiency is not far 
to seek, when we consider the comparatively leisurely 
character of molecular movements, at speeds com- 
parable with the velocity of sound. Admittedly, how- 
2 For details of my experiment on this subject see Phil. Trans. Roy. Soc* 
for 1893 and 1897; or a very abbreviated reference to it, and to the other 
matters above-mentioned, in my small book ** The Ether of Space.” 
_3 See, in the Philosophical Magazine for 1879, my article on a classifica- 
tion of the forms of energy. 
