THE ETHER AND MATTER
§ 1. THE RELATIONS BETWEEN THE ETHER AND MATTER
For some time past it has been the more or less avowed ambition of
physicists to construct with the particles of ether all possible forms
of corporeal existence; but our knowledge of the inmost nature of
things has hitherto seemed too limited for us to attempt such an
enterprise with any chance of success. The electronic hypothesis,
however, which has furnished a satisfactory image of the most curious
phenomena produced in the bosom of matter, has also led to a more
complete electromagnetic theory of the ether than that of Maxwell, and
this twofold result has given birth to the hope of arriving by means
of this hypothesis at a complete co-ordination of the physical world.
The phenomena whose study may bring us to the very threshold of the
problem, are those in which the connections between matter and the
ether appear clearly and in a relatively simple manner. Thus in the
phenomena of emission, ponderable matter is seen to give birth to
waves which are transmitted by the ether, and by the phenomena of
absorption it is proved that these waves disappear and excite
modifications in the interior of the material bodies which receive
them. We here catch in operation actual reciprocal actions and
reactions between the ether and matter. If we could thoroughly
comprehend these actions, we should no doubt be in a position to fill
up the gap which separates the two regions separately conquered by
physical science.
In recent years numerous researches have supplied valuable materials
which ought to be utilized by those endeavouring to construct a theory
of radiation. We are, perhaps, still ill informed as to the phenomena
of luminescence in which undulations are produced in a complex manner,
as in the case of a stick of moist phosphorus which is luminescent in
the dark, or in that of a fluorescent screen. But we are very well
acquainted with emission or absorption by incandescence, where the
only transformation is that of calorific into radiating energy, or
_vice versa_. It is in this case alone that can be correctly applied
the celebrated demonstration by which Kirchhoff established, by
considerations borrowed from thermodynamics, the proportional
relations between the power of emission and that of absorption.
In treating of the measurement of temperature, I have already pointed
out the experiments of Professors Lummer and Pringsheim and the
theoretical researches of Stephan and Professor Wien. We may consider
that at the present day the laws of the radiation of dark bodies are
tolerably well known, and, in particular, the manner in which each
elementary radiation increases with the temperature. A few doubts,
however, subsist with respect to the law of the distribution of energy
in the spectrum. In the case of real and solid bodies the results are
naturally less simple than in that of dark bodies. One side of the
question has been specially studied on account of its great practical
interest, that is to say, the fact that the relation of the luminous
energy to the total amount radiated by a body varies with the nature
of this last; and the knowledge of the conditions under which this
relation becomes most considerable led to the discovery of
incandescent lighting by gas in the Auer-Welsbach mantle, and to the
substitution for the carbon thread in the electric light bulb of a
filament of osmium or a small rod of magnesium, as in the Nernst lamp.
Careful measurements effected by M. Fery have furnished, in
particular, important information on the radiation of the white
oxides; but the phenomena noticed have not yet found a satisfactory
interpretation. Moreover, the radiation of calorific origin is here
accompanied by a more or less important luminescence, and the problem
becomes very complex.
In the same way that, for the purpose of knowing the constitution of
matter, it first occurred to us to investigate gases, which appear to
be molecular edifices built on a more simple and uniform plan than
solids, we ought naturally to think that an examination of the
conditions in which emission and absorption are produced by gaseous
bodies might be eminently profitable, and might perhaps reveal the
mechanism by which the relations between the molecule of the ether and
the molecule of matter might be established.
Unfortunately, if a gas is not absolutely incapable of emitting some
sort of rays by simple heat, the radiation thus produced, no doubt by
reason of the slightness of the mass in play, always remains of
moderate intensity. In nearly all the experiments, new energies of
chemical or electrical origin come into force. On incandescence,
luminescence is superposed; and the advantage which might have been
expected from the simplicity of the medium vanishes through the
complication of the circumstances in which the phenomenon is produced.
Professor Pringsheim has succeeded, in certain cases, in finding the
dividing line between the phenomena of luminescence and that of
incandescence. Thus the former takes a predominating importance when
the gas is rendered luminous by electrical discharges, and chemical
transformations, especially, play a preponderant rôle in the emission
of the spectrum of flames which contain a saline vapour. In all the
ordinary experiments of spectrum analysis the laws of Kirchhoff cannot
therefore be considered as established, and yet the relation between
emission and absorption is generally tolerably well verified. No doubt
we are here in presence of a kind of resonance phenomenon, the gaseous
atoms entering into vibration when solicited by the ether by a motion
identical with the one they are capable of communicating to it.
If we are not yet very far advanced in the study of the mechanism of
the production of the spectrum,[47] we are, on the other hand, well
acquainted with its constitution. The extreme confusion which the
spectra of the lines of the gases seemed to present is now, in great
part at least, cleared up. Balmer gave some time since, in the case of
the hydrogen spectrum, an empirical formula which enabled the rays
discovered later by an eminent astronomer, M. Deslandres, to be
represented; but since then, both in the cases of line and band
spectra, the labours of Professor Rydberg, of M. Deslandres, of
Professors Kayzer and Runge, and of M. Thiele, have enabled us to
comprehend, in their smallest details, the laws of the distribution of
lines and bands.
[Footnote 47: Many theories as to the cause of the lines and bands of
the spectrum have been put forward since this was written, among which
that of Professor Stark (for which see _Physikalische Zeitschrift_ for
1906, passim) is perhaps the most advanced. That of M. Jean Becquerel,
which would attribute it to the vibration within the atom of both
negative and positive electrons, also deserves notice. A popular
account of this is given in the _Athenæum_ of 20th April 1907.--ED.]
These laws are simple, but somewhat singular. The radiations emitted
by a gas cannot be compared to the notes to which a sonorous body
gives birth, nor even to the most complicated vibrations of any
elastic body. The number of vibrations of the different rays are not
the successive multiples of one and the same number, and it is not a
question of a fundamental radiation and its harmonics, while--and this
is an essential difference--the number of vibrations of the radiation
tend towards a limit when the period diminishes infinitely instead of
constantly increasing, as would be the case with the vibrations of
sound.
Thus the assimilation of the luminous to the elastic vibration is not
correct. Once again we find that the ether does not behave like matter
which obeys the ordinary laws of mechanics, and every theory must take
full account of these curious peculiarities which experiment reveals.
Another difference, likewise very important, between the luminous and
the sonorous vibrations, which also points out how little analogous
can be the constitutions of the media which transmit the vibrations,
appears in the phenomena of dispersion. The speed of propagation,
which, as we have seen when discussing the measurement of the velocity
of sound, depends very little on the musical note, is not at all the
same in the case of the various radiations which can be propagated in
the same substance. The index of refraction varies with the duration
of the period, or, if you will, with the length of wave _in vacuo_
which is proportioned to this duration, since _in vacuo_ the speed of
propagation is entirely the same for all vibrations.
Cauchy was the first to propose a theory on which other attempts have
been modelled; for example, the very interesting and simple one of
Briot. This last-named supposed that the luminous vibration could not
perceptibly drag with it the molecular material of the medium across
which it is propagated, but that matter, nevertheless, reacts on the
ether with an intensity proportional to the elongation, in such a
manner as tends to bring it back to its position of equilibrium. With
this simple hypothesis we can fairly well interpret the phenomena of
the dispersion of light in the case of transparent substances; but far
from well, as M. Carvallo has noted in some extremely careful
experiments, the dispersion of the infra-red spectrum, and not at all
the peculiarities presented by absorbent substances.
M. Boussinesq arrives at almost similar results, by attributing
dispersion, on the other hand, to the partial dragging along of
ponderable matter and to its action on the ether. By combining, in a
measure, as was subsequently done by M. Boussinesq, the two
hypotheses, formulas can be established far better in accord with all
the known facts.
These facts are somewhat complex. It was at first thought that the
index always varied in inverse ratio to the wave-length, but numerous
substances have been discovered which present the phenomenon of
abnormal dispersion--that is to say, substances in which certain
radiations are propagated, on the contrary, the more quickly the
shorter their period. This is the case with gases themselves, as
demonstrated, for example, by a very elegant experiment of M.
Becquerel on the dispersion of the vapour of sodium. Moreover, it may
happen that yet more complications may be met with, as no substance is
transparent for the whole extent of the spectrum. In the case of
certain radiations the speed of propagation becomes nil, and the index
shows sometimes a maximum and sometimes a minimum. All those phenomena
are in close relation with those of absorption.
It is, perhaps, the formula proposed by Helmholtz which best accounts
for all these peculiarities. Helmholtz came to establish this formula
by supposing that there is a kind of friction between the ether and
matter, which, like that exercised on a pendulum, here produces a
double effect, changing, on the one hand, the duration of this
oscillation, and, on the other, gradually damping it. He further
supposed that ponderable matter is acted on by elastic forces. The
theory of Helmholtz has the great advantage of representing, not only
the phenomena of dispersion, but also, as M. Carvallo has pointed out,
the laws of rotatory polarization, its dispersion and other phenomena,
among them the dichroism of the rotatory media discovered by M.
Cotton.
In the establishment of these theories, the language of ordinary
optics has always been employed. The phenomena are looked upon as due
to mechanical deformations or to movements governed by certain forces.
The electromagnetic theory leads, as we have seen, to the employment
of other images. M.H. Poincaré, and, after him, Helmholtz, have both
proposed electromagnetic theories of dispersion. On examining things
closely, it will be found that there are not, in truth, in the two
ways of regarding the problem, two equivalent translations of exterior
reality. The electrical theory gives us to understand, much better
than the mechanical one, that _in vacuo_ the dispersion ought to be
strictly null, and this absence of dispersion appears to be confirmed
with extraordinary precision by astronomical observations. Thus the
observation, often repeated, and at different times of year, proves
that in the case of the star Algol, the light of which takes at least
four years to reach us, no sensible difference in coloration
accompanies the changes in brilliancy.
§ 2. THE THEORY OF LORENTZ
Purely mechanical considerations have therefore failed to give an
entirely satisfactory interpretation of the phenomena in which even
the simplest relations between matter and the ether appear. They
would, evidently, be still more insufficient if used to explain
certain effects produced on matter by light, which could not, without
grave difficulties, be attributed to movement; for instance, the
phenomena of electrification under the influence of certain
radiations, or, again, chemical reactions such as photographic
impressions.
The problem had to be approached by another road. The electromagnetic
theory was a step in advance, but it comes to a standstill, so to
speak, at the moment when the ether penetrates into matter. If we wish
to go deeper into the inwardness of the phenomena, we must follow, for
example, Professor Lorentz or Dr Larmor, and look with them for a mode
of representation which appears, besides, to be a natural consequence
of the fundamental ideas forming the basis of Hertz's experiments.
The moment we look upon a wave in the ether as an electromagnetic
wave, a molecule which emits light ought to be considered as a kind of
excitant. We are thus led to suppose that in each radiating molecule
there are one or several electrified particles, animated with a
to-and-fro movement round their positions of equilibrium, and these
particles are certainly identical with those electrons the existence
of which we have already admitted for so many other reasons.
In the simplest theory, we will imagine an electron which may be
displaced from its position of equilibrium in all directions, and is,
in this displacement, submitted to attractions which communicate to it
a vibration like a pendulum. These movements are equivalent to tiny
currents, and the mobile electron, when animated with a considerable
velocity, must be sensitive to the action of the magnet which modifies
the form of the trajectory and the value of the period. This almost
direct consequence was perceived by Lorentz, and it led him to the new
idea that radiations emitted by a body ought to be modified by the
action of a strong electromagnet.
An experiment enabled this prevision to be verified. It was made, as
is well known, as early as 1896 by Zeeman; and the discovery produced
a legitimate sensation. When a flame is subjected to the action of a
magnetic field, a brilliant line is decomposed in conditions more or
less complex which an attentive study, however, allows us to define.
According to whether the observation is made in a plane normal to the
magnetic field or in the same direction, the line transforms itself
into a triplet or doublet, and the new lines are polarized
rectilinearly or circularly.
These are the precise phenomena which the calculation foretells: the
analysis of the modifications undergone by the light supplies,
moreover, valuable information on the electron itself. From the
direction of the circular vibrations of the greatest frequency we can
determine the sign of the electric charge in motion and we find it to
be negative. But, further than this, from the variation of the period
we can calculate the relation of the force acting on the electron to
its material mass, and, in addition, the relation of the charge to the
mass. We then find for this relation precisely that value which we
have already met with so many times. Such a coincidence cannot be
fortuitous, and we have the right to believe that the electron
revealed by the luminous wave which emanates from it, is really the
same as the one made known to us by the study of the cathode rays and
of the radioactive substances.
However, the elementary theory does not suffice to interpret the
complications which later experiments have revealed. The physicists
most qualified to effect measurements in these delicate optical
questions--M. Cornu, Mr Preston, M. Cotton, MM. Becquerel and
Deslandres, M. Broca, Professor Michelson, and others--have pointed
out some remarkable peculiarities. Thus in some cases the number of
the component rays dissociated by the magnetic field may be very
considerable.
The great modification brought to a radiation by the Zeeman effect
may, besides, combine itself with other phenomena, and alter the light
in a still more complicated manner. A pencil of polarized light, as
demonstrated by Signori Macaluzo and Corbino, undergoes, in a magnetic
field, modifications with regard to absorption and speed of
propagation.
Some ingenious researches by M. Becquerel and M. Cotton have perfectly
elucidated all these complications from an experimental point of view.
It would not be impossible to link together all these phenomena
without adopting the electronic hypothesis, by preserving the old
optical equations as modified by the terms relating to the action of
the magnetic field. This has actually been done in some very
remarkable work by M. Voigt, but we may also, like Professor Lorentz,
look for more general theories, in which the essential image of the
electrons shall be preserved, and which will allow all the facts
revealed by experiment to be included.
We are thus led to the supposition that there is not in the atom one
vibrating electron only, but that there is to be found in it a
dynamical system comprising several material points which may be
subjected to varied movements. The neutral atom may therefore be
considered as composed of an immovable principal portion positively
charged, round which move, like satellites round a planet, several
negative electrons of very inferior mass. This conclusion leads us to
an interpretation in agreement with that which other phenomena have
already suggested.
These electrons, which thus have a variable velocity, generate around
themselves a transverse electromagnetic wave which is propagated with
the velocity of light; for the charged particle becomes, as soon as it
experiences a change of speed, the centre of a radiation. Thus is
explained the phenomenon of the emission of radiations. In the same
way, the movement of electrons may be excited or modified by the
electrical forces which exist in any pencil of light they receive, and
this pencil may yield up to them a part of the energy it is carrying.
This is the phenomenon of absorption.
Professor Lorentz has not contented himself with thus explaining all
the mechanism of the phenomena of emission and absorption. He has
endeavoured to rediscover, by starting with the fundamental
hypothesis, the quantitative laws discovered by thermodynamics. He
succeeds in showing that, agreeably to the law of Kirchhoff, the
relation between the emitting and the absorbing power must be
independent of the special properties of the body under observation,
and he thus again meets with the laws of Planck and of Wien:
unfortunately the calculation can only be made in the case of great
wave-lengths, and grave difficulties exist. Thus it cannot be very
clearly explained why, by heating a body, the radiation is displaced
towards the side of the short wave-lengths, or, if you will, why a
body becomes luminous from the moment its temperature has reached a
sufficiently high degree. On the other hand, by calculating the energy
of the vibrating particles we are again led to attribute to these
particles the same constitution as that of the electrons.
It is in the same way possible, as Professor Lorentz has shown, to
give a very satisfactory explanation of the thermo-electric phenomena
by supposing that the number of liberated electrons which exist in a
given metal at a given temperature has a determined value varying with
each metal, and is, in the case of each body, a function of the
temperature. The formula obtained, which is based on these hypotheses,
agrees completely with the classic results of Clausius and of Lord
Kelvin. Finally, if we recollect that the phenomena of electric and
calorific conductivity are perfectly interpreted by the hypothesis of
electrons, it will no longer be possible to contest the importance of
a theory which allows us to group together in one synthesis so many
facts of such diverse origins.
If we study the conditions under which a wave excited by an electron's
variations in speed can be transmitted, they again bring us face to
face, and generally, with the results pointed out by the ordinary
electromagnetic theory. Certain peculiarities, however, are not
absolutely the same. Thus the theory of Lorentz, as well as that of
Maxwell, leads us to foresee that if an insulating mass be caused to
move in a magnetic field normally to its lines of force, a
displacement will be produced in this mass analogous to that of which
Faraday and Maxwell admitted the existence in the dielectric of a
charged condenser. But M.H. Poincaré has pointed out that, according
as we adopt one or other of these authors' points of view, so the
value of the displacement differs. This remark is very important, for
it may lead to an experiment which would enable us to make a definite
choice between the two theories.
To obtain the displacement estimated according to Lorentz, we must
multiply the displacement calculated according to Hertz by a factor
representing the relation between the difference of the specific
inductive capacities of the dielectric and of a vacuum, and the first
of these powers. If therefore we take as dielectric the air of which
the specific inductive capacity is perceptibly the same as that of a
vacuum, the displacement, according to the idea of Lorentz, will be
null; while, on the contrary, according to Hertz, it will have a
finite value. M. Blondlot has made the experiment. He sent a current
of air into a condenser placed in a magnetic field, and was never able
to notice the slightest trace of electrification. No displacement,
therefore, is effected in the dielectric. The experiment being a
negative one, is evidently less convincing than one giving a positive
result, but it furnishes a very powerful argument in favour of the
theory of Lorentz.
This theory, therefore, appears very seductive, yet it still raises
objections on the part of those who oppose to it the principles of
ordinary mechanics. If we consider, for instance, a radiation emitted
by an electron belonging to one material body, but absorbed by another
electron in another body, we perceive immediately that, the
propagation not being instantaneous, there can be no compensation
between the action and the reaction, which are not simultaneous; and
the principle of Newton thus seems to be attacked. In order to
preserve its integrity, it has to be admitted that the movements in
the two material substances are compensated by that of the ether which
separates these substances; but this conception, although in tolerable
agreement with the hypothesis that the ether and matter are not of
different essence, involves, on a closer examination, suppositions
hardly satisfactory as to the nature of movements in the ether.
For a long time physicists have admitted that the ether as a whole
must be considered as being immovable and capable of serving, so to
speak, as a support for the axes of Galileo, in relation to which axes
the principle of inertia is applicable,--or better still, as M.
Painlevé has shown, they alone allow us to render obedience to the
principle of causality.
But if it were so, we might apparently hope, by experiments in
electromagnetism, to obtain absolute motion, and to place in evidence
the translation of the earth relatively to the ether. But all the
researches attempted by the most ingenious physicists towards this end
have always failed, and this tends towards the idea held by many
geometricians that these negative results are not due to imperfections
in the experiments, but have a deep and general cause. Now Lorentz has
endeavoured to find the conditions in which the electromagnetic theory
proposed by him might agree with the postulate of the complete
impossibility of determining absolute motion. It is necessary, in
order to realise this concord, to imagine that a mobile system
contracts very slightly in the direction of its translation to a
degree proportioned to the square of the ratio of the velocity of
transport to that of light. The electrons themselves do not escape
this contraction, although the observer, since he participates in the
same motion, naturally cannot notice it. Lorentz supposes, besides,
that all forces, whatever their origin, are affected by a translation
in the same way as electromagnetic forces. M. Langevin and M. H.
Poincaré have studied this same question and have noted with precision
various delicate consequences of it. The singularity of the hypotheses
which we are thus led to construct in no way constitutes an argument
against the theory of Lorentz; but it has, we must acknowledge,
discouraged some of the more timid partisans of this theory.[48]
[Footnote 48: An objection not here noticed has lately been formulated
with much frankness by Professor Lorentz himself. It is one of the
pillars of his theory that only the negative electrons move when an
electric current passes through a metal, and that the positive
electrons (if any such there be) remain motionless. Yet in the
experiment known as Hall's, the current is deflected by the magnetic
field to one side of the strip in certain metals, and to the opposite
side in others. This seems to show that in certain cases the positive
electrons move instead of the negative, and Professor Lorentz
confesses that up to the present he can find no valid argument against
this. See _Archives Néerlandaises_ 1906, parts 1 and 2.--ED.]
§ 3. THE MASS OF ELECTRONS
Other conceptions, bolder still, are suggested by the results of
certain interesting experiments. The electron affords us the
possibility of considering inertia and mass to be no longer a
fundamental notion, but a consequence of the electromagnetic
phenomena.
Professor J.J. Thomson was the first to have the clear idea that a
part, at least, of the inertia of an electrified body is due to its
electric charge. This idea was taken up and precisely stated by
Professor Max Abraham, who, for the first time, was led to regard
seriously the seemingly paradoxical notion of mass as a function of
velocity. Consider a small particle bearing a given electric charge,
and let us suppose that this particle moves through the ether. It is,
as we know, equivalent to a current proportional to its velocity, and
it therefore creates a magnetic field the intensity of which is
likewise proportional to its velocity: to set it in motion, therefore,
there must be communicated to it over and above the expenditure
corresponding to the acquisition of its ordinary kinetic energy, a
quantity of energy proportional to the square of its velocity.
Everything, therefore, takes place as if, by the fact of
electrification, its capacity for kinetic energy and its material mass
had been increased by a certain constant quantity. To the ordinary
mass may be added, if you will, an electromagnetic mass.
This is the state of things so long as the speed of the translation of
the particle is not very great, but they are no longer quite the same
when this particle is animated with a movement whose rapidity becomes
comparable to that with which light is propagated.
The magnetic field created is then no longer a field in repose, but
its energy depends, in a complicated manner, on the velocity, and the
apparent increase in the mass of the particle itself becomes a
function of the velocity. More than this, this increase may not be the
same for the same velocity, but varies according to whether the
acceleration is parallel with or perpendicular to the direction of
this velocity. In other words, there seems to be a longitudinal; and a
transversal mass which need not be the same.
All these results would persist even if the material mass were very
small relatively to the electromagnetic mass; and the electron
possesses some inertia even if its ordinary mass becomes slighter and
slighter. The apparent mass, it can be easily shown, increases
indefinitely when the velocity with which the electrified particle is
animated tends towards the velocity of light, and thus the work
necessary to communicate such a velocity to an electron would be
infinite. It is in consequence impossible that the speed of an
electron, in relation to the ether, can ever exceed, or even
permanently attain to, 300,000 kilometres per second.
All the facts thus predicted by the theory are confirmed by
experiment. There is no known process which permits the direct
measurement of the mass of an electron, but it is possible, as we have
seen, to measure simultaneously its velocity and the relation of the
electric charge to its mass. In the case of the cathode rays emitted
by radium, these measurements are particularly interesting, for the
reason that the rays which compose a pencil of cathode rays are
animated by very different speeds, as is shown by the size of the
stain produced on a photographic plate by a pencil of them at first
very constricted and subsequently dispersed by the action of an
electric or magnetic field. Professor Kaufmann has effected some very
careful experiments by a method he terms the method of crossed
spectra, which consists in superposing the deviations produced by a
magnetic and an electric field respectively acting in directions at
right angles one to another. He has thus been enabled by working _in
vacuo_ to register the very different velocities which, starting in
the case of certain rays from about seven-tenths of the velocity of
light, attain in other cases to ninety-five hundredths of it.
It is thus noted that the ratio of charge to mass--which for ordinary
speeds is constant and equal to that already found by so many
experiments--diminishes slowly at first, and then very rapidly when
the velocity of the ray increases and approaches that of light. If we
represent this variation by a curve, the shape of this curve inclines
us to think that the ratio tends toward zero when the velocity tends
towards that of light.
All the earlier experiments have led us to consider that the electric
charge was the same for all electrons, and it can hardly be conceived
that this charge can vary with the velocity. For in order that the
relation, of which one of the terms remains fixed, should vary, the
other term necessarily cannot remain constant. The experiments of
Professor Kaufmann, therefore, confirm the previsions of Max Abraham's
theory: the mass depends on the velocity, and increases indefinitely
in proportion as this velocity approaches that of light. These
experiments, moreover, allow the numerical results of the calculation
to be compared with the values measured. This very satisfactory
comparison shows that the apparent total mass is sensibly equal to the
electromagnetic mass; the material mass of the electron is therefore
nil, and the whole of its mass is electromagnetic.
Thus the electron must be looked upon as a simple electric charge
devoid of matter. Previous examination has led us to attribute to it a
mass a thousand times less that that of the atom of hydrogen, and a
more attentive study shows that this mass was fictitious. The
electromagnetic phenomena which are produced when the electron is set
in motion or a change effected in its velocity, simply have the
effect, as it were, of simulating inertia, and it is the inertia due
to the charge which has caused us to be thus deluded.
The electron is therefore simply a small volume determined at a point
in the ether, and possessing special properties;[49] this point is
propagated with a velocity which cannot exceed that of light. When
this velocity is constant, the electron creates around it in its
passage an electric and a magnetic field; round this electrified
centre there exists a kind of wake, which follows it through the ether
and does not become modified so long as the velocity remains
invariable. If other electrons follow the first within a wire, their
passage along the wire will be what is called an electric current.
[Footnote 49: This cannot be said to be yet completely proved. _Cf_.
Sir Oliver Lodge, _Electrons_, London, 1906, p. 200.--ED.]
When the electron is subjected to an acceleration, a transverse wave
is produced, and an electromagnetic radiation is generated, of which
the character may naturally change with the manner in which the speed
varies. If the electron has a sufficiently rapid periodical movement,
this wave is a light wave; while if the electron stops suddenly, a
kind of pulsation is transmitted through the ether, and thus we obtain
Röntgen rays.
§ 4. NEW VIEWS ON THE CONSTITUTION OF THE ETHER AND OF MATTER
New and valuable information is thus afforded us regarding the
properties of the ether, but will this enable us to construct a
material representation of this medium which fills the universe, and
so to solve a problem which has baffled, as we have seen, the
prolonged efforts of our predecessors?
Certain scholars seem to have cherished this hope. Dr. Larmor in
particular, as we have seen, has proposed a most ingenious image, but
one which is manifestly insufficient. The present tendency of
physicists rather tends to the opposite view; since they consider
matter as a very complex object, regarding which we wrongly imagine
ourselves to be well informed because we are so much accustomed to it,
and its singular properties end by seeming natural to us. But in all
probability the ether is, in its objective reality, much more simple,
and has a better right to be considered as fundamental.
We cannot therefore, without being very illogical, define the ether by
material properties, and it is useless labour, condemned beforehand to
sterility, to endeavour to determine it by other qualities than those
of which experiment gives us direct and exact knowledge.
The ether is defined when we know, in all its points, and in magnitude
and in direction, the two fields, electric and magnetic, which may
exist in it. These two fields may vary; we speak from habit of a
movement propagated in the ether, but the phenomenon within the reach
of experiment is the propagation of these variations.
Since the electrons, considered as a modification of the ether
symmetrically distributed round a point, perfectly counterfeit that
inertia which is the fundamental property of matter, it becomes very
tempting to suppose that matter itself is composed of a more or less
complex assemblage of electrified centres in motion.
This complexity is, in general, very great, as is demonstrated by the
examination of the luminous spectra produced by the atoms, and it is
precisely because of the compensations produced between the different
movements that the essential properties of matter--the law of the
conservation of inertia, for example--are not contrary to the
hypothesis.
The forces of cohesion thus would be due to the mutual attractions
which occur in the electric and magnetic fields produced in the
interior of bodies; and it is even conceivable that there may be
produced, under the influence of these actions, a tendency to
determine orientation, that is to say, that a reason can be seen why
matter may be crystallised.[50]
[Footnote 50: The reader should, however, be warned that a theory has
lately been put forth which attempts to account for crystallisation on
purely mechanical grounds. See Messrs Barlow and Pope's "Development
of the Atomic Theory" in the _Transactions of the Chemical Society_,
1906.--ED.]
All the experiments effected on the conductivity of gases or metals,
and on the radiations of active bodies, have induced us to regard the
atom as being constituted by a positively charged centre having
practically the same magnitude as the atom itself, round which the
electrons gravitate; and it might evidently be supposed that this
positive centre itself preserves the fundamental characteristics of
matter, and that it is the electrons alone which no longer possess any
but electromagnetic mass.
We have but little information concerning these positive particles,
though they are met with in an isolated condition, as we have seen, in
the canal rays or in the X rays.[51] It has not hitherto been possible
to study them so successfully as the electrons themselves; but that
their magnitude causes them to produce considerable perturbations in
the bodies on which they fall is manifest by the secondary emissions
which complicate and mask the primitive phenomenon. There are,
however, strong reasons for thinking that these positive centres are
not simple. Thus Professor Stark attributes to them, with experiments
in proof of his opinion, the emission of the spectra of the rays in
Geissler tubes, and the complexity of the spectrum discloses the
complexity of the centre. Besides, certain peculiarities in the
conductivity of metals cannot be explained without a supposition of
this kind. So that the atom, deprived of the cathode corpuscle, would
be still liable to decomposition into elements analogous to electrons
and positively charged. Consequently nothing prevents us supposing
that this centre likewise simulates inertia by its electromagnetic
properties, and is but a condition localised in the ether.
[Footnote 51: There is much reason for thinking that the canal rays do
not contain positive particles alone, but are accompanied by negative
electrons of slow velocity. The X rays are thought, as has been said
above, to contain neither negative nor positive particles, but to be
merely pulses in the ether.--ED.]
However this may be, the edifice thus constructed, being composed of
electrons in periodical motion, necessarily grows old. The electrons
become subject to accelerations which produce a radiation towards the
exterior of the atom; and certain of them may leave the body, while
the primitive stability is, in the end, no longer assured, and a new
arrangement tends to be formed. Matter thus seems to us to undergo
those transformations of which the radio-active bodies have given us
such remarkable examples.
We have already had, in fragments, these views on the constitution of
matter; a deeper study of the electron thus enables us to take up a
position from which we obtain a sharp, clear, and comprehensive grasp
of the whole and a glimpse of indefinite horizons.
It would be advantageous, however, in order to strengthen this
position, that a few objections which still menace it should be
removed. The instability of the electron is not yet sufficiently
demonstrated. How is it that its charge does not waste itself away,
and what bonds assure the permanence of its constitution?
On the other hand, the phenomena of gravitation remain a mystery.
Lorentz has endeavoured to build up a theory in which he explains
attraction by supposing that two charges of similar sign repel each
other in a slightly less degree than that in which two charges, equal
but of contrary sign, attract each other, the difference being,
however, according to the calculation, much too small to be directly
observed. He has also sought to explain gravitation by connecting it
with the pressures which may be produced on bodies by the vibratory
movements which form very penetrating rays. Recently M. Sutherland has
imagined that attraction is due to the difference of action in the
convection currents produced by the positive and negative corpuscles
which constitute the atoms of the stars, and are carried along by the
astronomical motions. But these hypotheses remain rather vague, and
many authors think, like M. Langevin, that gravitation must result
from some mode of activity of the ether totally different from the
electromagnetic mode.