328 
NATURE 
[ fed. 1, 1883 
Appended Note.—It was constantly found that in using a non- 
corrodible anode such as platinum, the amount of current passing 
was very much more easily regulated by varying the size of the 
anode than that of the cathode, with a corrodible anode however, 
such as silver, this effect was not observed. 
THE ETHER AND ITS FUNCTIONS + 
Il. 
Consider the effect of wind on sound. Sound is travelling 
through the air at a certain definite rate depending simply on the 
average speed of the atoms in their excursions, and the rate at which 
they therefore pass the knocks on; if there is a wind carrying all 
the atoms bodily in cne direction, naturally the scund will travel 
quicker in that direction than in the opposite. Sound travels 
quicker with the wind than against it. Now is it the same with 
light: does it too travel quicker with the wind? Well that 
altogether depends on whether the ether is blowing along as 
well as the air; if it is, then its motion must help the light on a 
little; but if the ether is at rest no motion of air or matter of 
any kind can make any difference. But according to Fresnel’s 
hypothesis it is not wholly at rest nor wholly in motion ; the free 
is at rest, the bound is in motion ; and therefore the speed of 
light with the wind should be increased by an addition of 
(« - rh of the velocity of the wind. Utterly infinitesimal, 
eB 
of course, in the cace of air, whose u is but a trifle greater than 
1; but for water the fraction is 7-16ths, and Fizeau thought this 
not quite hopeless to look for. Heaccordingly devised a beauti- 
ful experiment, executed it successfully, and proved that when 
light travels with a stream of water, 7-16ths of the velocily of 
the water must be added to the velocity of the light, and when 
it travels against the stream the same quantity must be sub- 
tracted, to get the true resultant velocity. 
Arago suggested another experiment. When light passes 
through a prism, it is bent out of its course by reason of its 
diminished velocity inside the glass, and the refraction is strictly 
dependent on the retardation; now suppose a prism carried 
rapidly forward through space, say at the rate of eighteen miles 
a second by the earth in its orbit, which is the quickest acces- 
sible carriage ; if the ether is streaming freely through the glass, 
light passing through will be less retarded when going with the 
ether than when going against it, and hence the bending will be 
different. 
Maxwell tried the experiment in a very perfect form, but 
found no difference. - If all the ether were free there would 
have been a difference ; if all the ether were bound to the glass 
there would have been a difference the other way; but accord- 
ing to Fresnel’s hypothesis there should be no difference, because 
according to it, the free ether, which is the portion in relative 
motion, has nothing to do with the refraction ; it is the addition 
of the bound ether which causes the refraction, and this part is 
stationary relatively to the glass, and is not streaming through it 
at all. Hence the refraction is the same whether the prism be 
at rest or in motion through space. 
An atom imbedded in ether is vibrating and sending out 
waves in all directions; the length of the wave depends on the 
period of the vibration, and different lengths of wave produce 
the different colour sensations. Now through free ether all 
kinds of waves appear to travel at the same rate ; not so through 
bound ether ; inside matter the short waves are more retarded 
than the long, and hence the different sizes of waves can be 
sorted out by a prism. Now a free atom has its own definite 
period of vibration, like a tuning-fork has, and accordingly sends 
out light of a certain definite colour or of a few definite colours, 
just as a tuning-fork emits sound of a certain definite pitch or 
of a few different pitches called harmonics. By the pitch of the 
sound it is easy to calculate the rate of vibration of the fork ; by 
the colour of the light one can determine the rate of vibration of 
the atom. 
When we speak of the atoms vibrating, we do not mean that 
they are wagging to and fro as a whole, but that they are 
crimping themselves, that they are vibrating as a tuning-fork or 
ora bell vibrates ; we know this because it is easy to make the 
free atoms of a gas vibrate. It is only in the gaseous state, 
indeed, that we can study the rate of vibration of an atom ; 
when they are packed closely together in a solid or liquid, they 
t A lecture by Prof. Oliver Lodge at the London Institution, on December 
28, 1882. Continued from p. 306. 
are cramped, and all manner of secondary vibrations are 
induced. They then, no doubt, wag to and fro also, and in 
fact these constrained vibrations are executed in every variety, 
and the simple periodicity of the free atom is lost. 
To study the free atoms we take a gas—the rarer the better— 
heat it, and then sort out the waves it produces in the ether by 
putting a triangular pric:m of bound ether in their path. 
Why the beund ether retards different waves differently, or 
disperses the light, is quite unknown. It is not easy accurately 
to explain refraction, but it is extremely difficult to explain dis- 
persion. However, the fact is undoubted, and more light will 
doubtless soon fall upon its theory. 
The result of the prismatic analysis is to prove that every atom 
of matter has its own definite rate of vibration, as a bell has ; it 
may emit several colours or only one, and the number it emits 
may depend upon how much it is struck (or heated), but those it 
can emit are a perfectly definite selection, and depend in no 
way on the previous history of the atom, Every free atom of 
sodium, for instance, vibrates in the same way, and has always 
vibrated in the same way, whatever other element it may have 
been at intervals combined with, and whether it exists in the sun 
or in the earth, or in the most distant star. The same is true of 
every Other kind of matter, each has its own mode of vibration 
which nothing changes; and hence has arisen a new chemical 
analysis, wherein substances are detected simply by observing 
the rate of vibration of their free atoms, a branch of physical 
chemistry called spectrum analysis. 
The atoms are small bodies, and accordingly vibrate with in- 
conceivable rapidity. 
An atom of sodium vibrates 5 x 104 times ina second ; that is, 
it executes five hundred million complete vibrations in the 
millionth part of a second. 
This is about a medium pace, and the waves it emits produce 
in the eye the sensation of a deep yellow. 
4x 10! corresponds to red light, 7 x 10! to blue. 
An atom of hydrogen has three different periods, viz. 4°577, 
6°179, and 6°973, each multiplied by the inevitable ro. 
Atoms may indeed vibrate more slowly than this, but the 
retina is not constructed so as to be sensible of slower vibra- 
tions; however, thanks to Capt. Abney, there are ways now of 
photographing the effect of much slower vibrations, ard thus of 
making them indirectly visible ; so we can now hope to observe 
the motion of atoms over a much greater range than the purely 
optical ones and so learn much more about them. 
The distinction between free and bound ether is forced on our 
notice by other phenomena than those of light. When we 
come to electricity, we find that some kind of matter has more 
electricity associated with it than others, so that for a given 
electromotive force we get a greater electric displacement ; that 
the electricity is, as it were, denser in some kinds of matter than 
in others. The density of electricity in space being 1, that 
inside matter is called kK, the specific inductive capacity. In 
optics the density of the <ther inside matter was we. These 
numbers appear to he the same. 
Is the ether electricity then? I do not say so, neither do I 
think that in that coarse statement lies the truth; but that they 
are connected there can be no doubt. 
What I have to suggest is that positive and negative electricity 
together may make up the ether, or that the ether may be 
sheared by electromotive forces into positive and negative elec- 
tricity. Transverse vibrations are carried on by shearing forces 
acting in matter which resists them, or which possesses rigidity. 
The bound ether inside a conductor has no rigidity ; it cannot 
resist shear ; such a body is opaque. Transparent bodies are 
thoce whose bound ether, when sheared, resists and springs back 
again; such bodies are dielectrics. 
We have no direct way of exerting force upon ether at all; 
we can, however, act on it in a very indirect manner, for we 
have learnt how to arrange matter so as to cause it to exert the 
required shearing (or electromotive) force upon the ether 
associated with it, Continuous. shearing force applied to the 
ether in metals produces a continuous and barely resisted stream 
of the two electricities in opposite directions, or a conduction 
current, 5 
Continuous shearing force applied to the ether in transparent 
bodies produces an electric displacement accompanied by elastic 
resilience, and thus all the phenomena of electric induction. 
Some chemical compounds, consisting of binary molecules, 
distribute the bound ether of the molecule, at any rate as scon 
