THE ETHER
§ 1. THE LUMINIFEROUS ETHER
It is in the works of Descartes that we find the first idea of
attributing those physical phenomena which the properties of matter
fail to explain to some subtle matter which is the receptacle of the
energy of the universe.
In our times this idea has had extraordinary luck. After having been
eclipsed for two hundred years by the success of the immortal
synthesis of Newton, it gained an entirely new splendour with Fresnel
and his followers. Thanks to their admirable discoveries, the first
stage seemed accomplished, the laws of optics were represented by a
single hypothesis, marvellously fitted to allow us to anticipate
unknown phenomena, and all these anticipations were subsequently fully
verified by experiment. But the researches of Faraday, Maxwell, and
Hertz authorized still greater ambitions; and it really seemed that
this medium, to which it was agreed to give the ancient name of ether,
and which had already explained light and radiant heat, would also be
sufficient to explain electricity. Thus the hope began to take form
that we might succeed in demonstrating the unity of all physical
forces. It was thought that the knowledge of the laws relating to the
inmost movements of this ether might give us the key to all phenomena,
and might make us acquainted with the method in which energy is stored
up, transmitted, and parcelled out in its external manifestations.
We cannot study here all the problems which are connected with the
physics of the ether. To do this a complete treatise on optics would
have to be written and a very lengthy one on electricity. I shall
simply endeavour to show rapidly how in the last few years the ideas
relative to the constitution of this ether have evolved, and we shall
see if it be possible without self-delusion to imagine that a single
medium can really allow us to group all the known facts in one
comprehensive arrangement.
As constructed by Fresnel, the hypothesis of the luminous ether, which
had so great a struggle at the outset to overcome the stubborn
resistance of the partisans of the then classic theory of emission,
seemed, on the contrary, to possess in the sequel an unshakable
strength. Lamé, though a prudent mathematician, wrote: "_The
existence_ of the ethereal fluid is _incontestably demonstrated_ by
the propagation of light through the planetary spaces, and by the
explanation, so simple and so complete, of the phenomena of
diffraction in the wave theory of light"; and he adds: "The laws of
double refraction prove with no less certainty that the _ether exists_
in all diaphanous media." Thus the ether was no longer an hypothesis,
but in some sort a tangible reality. But the ethereal fluid of which
the existence was thus proclaimed has some singular properties.
Were it only a question of explaining rectilinear propagation,
reflexion, refraction, diffraction, and interferences notwithstanding
grave difficulties at the outset and the objections formulated by
Laplace and Poisson (some of which, though treated somewhat lightly at
the present day, have not lost all value), we should be under no
obligation to make any hypothesis other than that of the undulations
of an elastic medium, without deciding in advance anything as to the
nature and direction of the vibrations.
This medium would, naturally--since it exists in what we call the
void--be considered as imponderable. It may be compared to a fluid of
negligible mass--since it offers no appreciable resistance to the
motion of the planets--but is endowed with an enormous elasticity,
because the velocity of the propagation of light is considerable. It
must be capable of penetrating into all transparent bodies, and of
retaining there, so to speak, a constant elasticity, but must there
become condensed, since the speed of propagation in these bodies is
less than in a vacuum. Such properties belong to no material gas, even
the most rarefied, but they admit of no essential contradiction, and
that is the important point.[20]
[Footnote 20: Since this was written, however, men of science have
become less unanimous than they formerly were on this point. The
veteran chemist Professor Mendeléeff has given reasons for thinking
that the ether is an inert gas with an atomic weight a million times
less than that of hydrogen, and a velocity of 2250 kilometres per
second (_Principles of Chemistry_, Eng. ed., 1905, vol. ii. p. 526).
On the other hand, the well-known physicist Dr A.H. Bucherer, speaking
at the Naturforscherversammlung, held at Stuttgart in 1906, declared
his disbelief in the existence of the ether, which he thought could
not be reconciled at once with the Maxwellian theory and the known
facts.--ED.]
It was the study of the phenomena of polarization which led Fresnel to
his bold conception of transverse vibrations, and subsequently induced
him to penetrate further into the constitution of the ether. We know
the experiment of Arago on the noninterference of polarized rays in
rectangular planes. While two systems of waves, proceeding from the
same source of natural light and propagating themselves in nearly
parallel directions, increase or become destroyed according to whether
the nature of the superposed waves are of the same or of contrary
signs, the waves of the rays polarized in perpendicular planes, on the
other hand, can never interfere with each other. Whatever the
difference of their course, the intensity of the light is always the
sum of the intensity of the two rays.
Fresnel perceived that this experiment absolutely compels us to reject
the hypothesis of longitudinal vibrations acting along the line of
propagation in the direction of the rays. To explain it, it must of
necessity be admitted, on the contrary, that the vibrations are
transverse and perpendicular to the ray. Verdet could say, in all
truth, "It is not possible to deny the transverse direction of
luminous vibrations, without at the same time denying that light
consists of an undulatory movement."
Such vibrations do not and cannot exist in any medium resembling a
fluid. The characteristic of a fluid is that its different parts can
displace themselves with regard to one another without any reaction
appearing so long as a variation of volume is not produced. There
certainly may exist, as we have seen, certain traces of rigidity in a
liquid, but we cannot conceive such a thing in a body infinitely more
subtle than rarefied gas. Among material bodies, a solid alone really
possesses the rigidity sufficient for the production within it of
transverse vibrations and for their maintenance during their
propagation.
Since we have to attribute such a property to the ether, we may add
that on this point it resembles a solid, and Lord Kelvin has shown
that this solid, would be much more rigid than steel. This conclusion
produces great surprise in all who hear it for the first time, and it
is not rare to hear it appealed to as an argument against the actual
existence of the ether. It does not seem, however, that such an
argument can be decisive. There is no reason for supposing that the
ether ought to be a sort of extension of the bodies we are accustomed
to handle. Its properties may astonish our ordinary way of thinking,
but this rather unscientific astonishment is not a reason for doubting
its existence. Real difficulties would appear only if we were led to
attribute to the ether, not singular properties which are seldom found
united in the same substance, but properties logically contradictory.
In short, however odd such a medium may appear to us, it cannot be
said that there is any absolute incompatibility between its
attributes.
It would even be possible, if we wished, to suggest images capable of
representing these contrary appearances. Various authors have done so.
Thus, M. Boussinesq assumes that the ether behaves like a very
rarefied gas in respect of the celestial bodies, because these last
move, while bathed in it, in all directions and relatively slowly,
while they permit it to retain, so to speak, its perfect homogeneity.
On the other hand, its own undulations are so rapid that so far as
they are concerned the conditions become very different, and its
fluidity has, one might say, no longer the time to come in. Hence its
rigidity alone appears.
Another consequence, very important in principle, of the fact that
vibrations of light are transverse, has been well put in evidence by
Fresnel. He showed how we have, in order to understand the action
which excites without condensation the sliding of successive layers of
the ether during the propagation of a vibration, to consider the
vibrating medium as being composed of molecules separated by finite
distances. Certain authors, it is true, have proposed theories in
which the action at a distance of these molecules are replaced by
actions of contact between parallelepipeds sliding over one another;
but, at bottom, these two points of view both lead us to conceive the
ether as a discontinuous medium, like matter itself. The ideas
gathered from the most recent experiments also bring us to the same
conclusion.
§ 2. RADIATIONS
In the ether thus constituted there are therefore propagated
transverse vibrations, regarding which all experiments in optics
furnish very precise information. The amplitude of these vibrations is
exceedingly small, even in relation to the wave-length, small as these
last are. If, in fact, the amplitude of the vibrations acquired a
noticeable value in comparison with the wave-length, the speed of
propagation should increase with the amplitude. Yet, in spite of some
curious experiments which seem to establish that the speed of light
does alter a little with its intensity, we have reason to believe
that, as regards light, the amplitude of the oscillations in relation
to the wave-length is incomparably less than in the case of sound.
It has become the custom to characterise each vibration by the path
which the vibratory movement traverses during the space of a
vibration--by the length of wave, in a word--rather than by the
duration of the vibration itself. To measure wave-lengths, the methods
must be employed to which I have already alluded on the subject of
measurements of length. Professor Michelson, on the one hand, and MM.
Perot and Fabry, on the other, have devised exceedingly ingenious
processes, which have led to results of really unhoped-for precision.
The very exact knowledge also of the speed of the propagation of light
allows the duration of a vibration to be calculated when once the
wave-length is known. It is thus found that, in the case of visible
light, the number of the vibrations from the end of the violet to the
infra-red varies from four hundred to two hundred billions per second.
This gamut is not, however, the only one the ether can give. For a
long time we have known ultra-violet radiations still more rapid, and,
on the other hand, infra-red ones more slow, while in the last few
years the field of known radiations has been singularly extended in
both directions.
It is to M. Rubens and his fellow-workers that are due the most
brilliant conquests in the matter of great wave-lengths. He had
remarked that, in their study, the difficulty of research proceeds
from the fact that the extreme waves of the infra-red spectrum only
contain a small part of the total energy emitted by an incandescent
body; so that if, for the purpose of study, they are further dispersed
by a prism or a grating, the intensity at any one point becomes so
slight as to be no longer observable. His original idea was to obtain,
without prism or grating, a homogeneous pencil of great wave-length
sufficiently intense to be examined. For this purpose the radiant
source used was a strip of platinum covered with fluorine or powdered
quartz, which emits numerous radiations close to two bands of linear
absorption in the absorption spectra of fluorine and quartz, one of
which is situated in the infra-red. The radiations thus emitted are
several times reflected on fluorine or on quartz, as the case may be;
and as, in proximity to the bands, the absorption is of the order of
that of metallic bodies for luminous rays, we no longer meet in the
pencil several times reflected or in the rays _remaining_ after this
kind of filtration, with any but radiations of great wave-length.
Thus, for instance, in the case of the quartz, in the neighbourhood of
a radiation corresponding to a wave-length of 8.5 microns, the
absorption is thirty times greater in the region of the band than in
the neighbouring region, and consequently, after three reflexions,
while the corresponding radiations will not have been weakened, the
neighbouring waves will be so, on the contrary, in the proportion of 1
to 27,000.
With mirrors of rock salt and of sylvine[21] there have been obtained,
by taking an incandescent gas light (Auer) as source, radiations
extending as far as 70 microns; and these last are the greatest
wave-lengths observed in optical phenomena. These radiations are
largely absorbed by the vapour of water, and it is no doubt owing to
this absorption that they are not found in the solar spectrum. On the
other hand, they easily pass through gutta-percha, india-rubber, and
insulating substances in general.
[Footnote 21: A natural chlorate of potassium, generally of volcanic
origin.--ED.]
At the opposite end of the spectrum the knowledge of the ultra-violet
regions has been greatly extended by the researches of Lenard. These
extremely rapid radiations have been shown by that eminent physicist
to occur in the light of the electric sparks which flash between two
metal points, and which are produced by a large induction coil with
condenser and a Wehnelt break. Professor Schumann has succeeded in
photographing them by depositing bromide of silver directly on glass
plates without fixing it with gelatine; and he has, by the same
process, photographed in the spectrum of hydrogen a ray with a
wave-length of only 0.1 micron.
The spectroscope was formed entirely of fluor-spar, and a vacuum had
been created in it, for these radiations are extremely absorbable by
the air.
Notwithstanding the extreme smallness of the luminous wave-lengths, it
has been possible, after numerous fruitless trials, to obtain
stationary waves analogous to those which, in the case of sound, are
produced in organ pipes. The marvellous application M. Lippmann has
made of these waves to completely solve the problem of photography in
colours is well known. This discovery, so important in itself and so
instructive, since it shows us how the most delicate anticipations of
theory may be verified in all their consequences, and lead the
physicist to the solution of the problems occurring in practice, has
justly become popular, and there is, therefore, no need to describe it
here in detail.
Professor Wiener obtained stationary waves some little while before M.
Lippmann's discovery, in a layer of a sensitive substance having a
grain sufficiently small in relation to the length of wave. His aim
was to solve a question of great importance to a complete knowledge of
the ether. Fresnel founded his theory of double refraction and
reflexion by transparent surfaces, on the hypothesis that the
vibration of a ray of polarized light is perpendicular to the plane of
polarization. But Neumann has proposed, on the contrary, a theory in
which he recognizes that the luminous vibration is in this very plane.
He rather supposes, in opposition to Fresnel's idea, that the density
of the ether remains the same in all media, while its coefficient of
elasticity is variable.
Very remarkable experiments on dispersion by M. Carvallo prove indeed
that the idea of Fresnel was, if not necessary for us to adopt, at
least the more probable of the two; but apart from this indication,
and contrary to the hypothesis of Neumann, the two theories, from the
point of view of the explanation of all known facts, really appear to
be equivalent. Are we then in presence of two mechanical explanations,
different indeed, but nevertheless both adaptable to all the facts,
and between which it will always be impossible to make a choice? Or,
on the contrary, shall we succeed in realising an _experimentum
crucis_, an experiment at the point where the two theories cross,
which will definitely settle the question?
Professor Wiener thought he could draw from his experiment a firm
conclusion on the point in dispute. He produced stationary waves with
light polarized at an angle of 45°,[22] and established that, when
light is polarized in the plane of incidence, the fringes persist; but
that, on the other hand, they disappear when the light is polarized
perpendicularly to this plane. If it be admitted that a photographic
impression results from the active force of the vibratory movement of
the ether, the question is, in fact, completely elucidated, and the
discrepancy is abolished in Fresnel's favour.
[Footnote 22: That is to say, he reflected the beam of polarized light
by a mirror placed at that angle. See Turpain, _Leçons élementaires de
Physique_, t. ii. p. 311, for details of the experiment.--ED.]
M.H. Poincaré has pointed out, however, that we know nothing as to the
mechanism of the photographic impression. We cannot consider it
evident that it is the kinetic energy of the ether which produces the
decomposition of the sensitive salt; and if, on the contrary, we
suppose it to be due to the potential energy, all the conclusions are
reversed, and Neumann's idea triumphs.
Recently a very clever physicist, M. Cotton, especially known for his
skilful researches in the domain of optics, has taken up anew the
study of stationary waves. He has made very precise quantitative
experiments, and has demonstrated, in his turn, that it is impossible,
even with spherical waves, to succeed in determining on which of the
two vectors which have to be regarded in all theories of light on the
subject of polarization phenomena the luminous intensity and the
chemical action really depend. This question, therefore, no longer
exists for those physicists who admit that luminous vibrations are
electrical oscillations. Whatever, then, the hypothesis formed,
whether it be electric force or, on the contrary, magnetic force which
we place in the plane of polarization, the mode of propagation
foreseen will always be in accord with the facts observed.
§ 3. THE ELECTROMAGNETIC ETHER
The idea of attributing the phenomena of electricity to perturbations
produced in the medium which transmits the light is already of old
standing; and the physicists who witnessed the triumph of Fresnel's
theories could not fail to conceive that this fluid, which fills the
whole of space and penetrates into all bodies, might also play a
preponderant part in electrical actions. Some even formed too hasty
hypotheses on this point; for the hour had not arrived when it was
possible to place them on a sufficiently sound basis, and the known
facts were not numerous enough to give the necessary precision.
The founders of modern electricity also thought it wiser to adopt,
with regard to this science, the attitude taken by Newton in
connection with gravitation: "In the first place to observe facts, to
vary the circumstances of these as much as possible, to accompany this
first work by precise measurements in order to deduce from them
general laws founded solely on experiment, and to deduce from these
laws, independently of all hypotheses on the nature of the forces
producing the phenomena, the mathematical value of these forces--that
is to say, the formula representing them. Such was the system pursued
by Newton. It has, in general, been adopted in France by the scholars
to whom physics owe the great progress made of late years, and it has
served as my guide in all my researches on electrodynamic
phenomena.... It is for this reason that I have avoided speaking of
the ideas I may have on the nature of the cause of the force emanating
from voltaic conductors."
Thus did Ampère express himself. The illustrious physicist rightly
considered the results obtained by him through following this wise
method as worthy of comparison with the laws of attraction; but he
knew that when this first halting-place was reached there was still
further to go, and that the evolution of ideas must necessarily
continue.
"With whatever physical cause," he adds, "we may wish to connect the
phenomena produced by electro-dynamic action, the formula I have
obtained will always remain the expression of the facts," and he
explicitly indicated that if one could succeed in deducing his formula
from the consideration of the vibrations of a fluid distributed
through space, an enormous step would have been taken in this
department of physics. He added, however, that this research appeared
to him premature, and would change nothing in the results of his work,
since, to accord with facts, the hypothesis adopted would always have
to agree with the formula which exactly represents them.
It is not devoid of interest to observe that Ampère himself,
notwithstanding his caution, really formed some hypotheses, and
recognized that electrical phenomena were governed by the laws of
mechanics. Yet the principles of Newton then appeared to be
unshakable.
Faraday was the first to demonstrate, by clear experiment, the
influence of the media in electricity and magnetic phenomena, and he
attributed this influence to certain modifications in the ether which
these media enclose. His fundamental conception was to reject action
at a distance, and to localize in the ether the energy whose evolution
is the cause of the actions manifested, as, for example, in the
discharge of a condenser.
Consider the barrel of a pump placed in a vacuum and closed by a
piston at each end, and let us introduce between these a certain mass
of air. The two pistons, through the elastic force of the gas, repel
each other with a force which, according to the law of Mariotte,
varies in inverse ratio to the distance. The method favoured by Ampère
would first of all allow this law of repulsion between the two pistons
to be discovered, even if the existence of a gas enclosed in the
barrel of the pump were unsuspected; and it would then be natural to
localize the potential energy of the system on the surface of the two
pistons. But if the phenomenon is more carefully examined, we shall
discover the presence of the air, and we shall understand that every
part of the volume of this air could, if it were drawn off into a
recipient of equal volume, carry away with it a fraction of the energy
of the system, and that consequently this energy belongs really to the
air and not to the pistons, which are there solely for the purpose of
enabling this energy to manifest its existence.
Faraday made, in some sort, an equivalent discovery when he perceived
that the electrical energy belongs, not to the coatings of the
condenser, but to the dielectric which separates them. His audacious
views revealed to him a new world, but to explore this world a surer
and more patient method was needed.
Maxwell succeeded in stating with precision certain points of
Faraday's ideas, and he gave them the mathematical form which, often
wrongly, impresses physicists, but which when it exactly encloses a
theory, is a certain proof that this theory is at least coherent and
logical.[23]
[Footnote 23: It will no doubt be a shock to those whom Professor
Henry Armstrong has lately called the "mathematically-minded" to find
a member of the Poincaré family speaking disrespectfully of the
science they have done so much to illustrate. One may perhaps compare
the expression in the text with M. Henri Poincaré's remark in his last
allocution to the Académie des Sciences, that "Mathematics are
sometimes a nuisance, and even a danger, when they induce us to affirm
more than we know" (_Comptes-rendus_, 17th December 1906).]
The work of Maxwell is over-elaborated, complex, difficult to read,
and often ill-understood, even at the present day. Maxwell is more
concerned in discovering whether it is possible to give an explanation
of electrical and magnetic phenomena which shall be founded on the
mechanical properties of a single medium, than in stating this
explanation in precise terms. He is aware that if we could succeed in
constructing such an interpretation, it would be easy to propose an
infinity of others, entirely equivalent from the point of view of the
experimentally verifiable consequences; and his especial ambition is
therefore to extract from the premises a general view, and to place in
evidence something which would remain the common property of all the
theories.
He succeeded in showing that if the electrostatic energy of an
electromagnetic field be considered to represent potential energy, and
its electrodynamic the kinetic energy, it becomes possible to satisfy
both the principle of least action and that of the conservation of
energy; from that moment--if we eliminate a few difficulties which
exist regarding the stability of the solutions--the possibility of
finding mechanical explanations of electromagnetic phenomena must be
considered as demonstrated. He thus succeeded, moreover, in stating
precisely the notion of two electric and magnetic fields which
are produced in all points of space, and which are strictly
inter-connected, since the variation of the one immediately and
compulsorily gives birth to the other.
From this hypothesis he deduced that, in the medium where this energy
is localized, an electromagnetic wave is propagated with a velocity
equal to the relation of the units of electric mass in the
electromagnetic and electrostatic systems. Now, experiments made known
since his time have proved that this relation is numerically equal to
the speed of light, and the more precise experiments made in
consequence--among which should be cited the particularly careful ones
of M. Max Abraham--have only rendered the coincidence still more
complete.
It is natural henceforth to suppose that this medium is identical with
the luminous ether, and that a luminous wave is an electromagnetic
wave--that is to say, a succession of alternating currents, which
exist in the dielectric and even in the void, and possess an enormous
frequency, inasmuch as they change their direction thousands of
billions of times per second, and by reason of this frequency produce
considerable induction effects. Maxwell did not admit the existence of
open currents. To his mind, therefore, an electrical vibration could
not produce condensations of electricity. It was, in consequence,
necessarily transverse, and thus coincided with the vibration of
Fresnel; while the corresponding magnetic vibration was perpendicular
to it, and would coincide with the luminous vibration of Neumann.
Maxwell's theory thus establishes a close correlation between the
phenomena of the luminous and those of the electromagnetic waves, or,
we might even say, the complete identity of the two. But it does not
follow from this that we ought to regard the variation of an electric
field produced at some one point as necessarily consisting of a real
displacement of the ether round that point. The idea of thus bringing
electrical phenomena back to the mechanics of the ether is not, then,
forced upon us, and the contrary idea even seems more probable. It is
not the optics of Fresnel which absorbs the science of electricity, it
is rather the optics which is swallowed up by a more general theory.
The attempts of popularizers who endeavour to represent, in all their
details, the mechanism of the electric phenomena, thus appear vain
enough, and even puerile. It is useless to find out to what material
body the ether may be compared, if we content ourselves with seeing in
it a medium of which, at every point, two vectors define the
properties.
For a long time, therefore, we could remark that the theory of Fresnel
simply supposed a medium in which something periodical was propagated,
without its being necessary to admit this something to be a movement;
but we had to wait not only for Maxwell, but also for Hertz, before
this idea assumed a really scientific shape. Hertz insisted on the
fact that the six equations of the electric field permit all the
phenomena to be anticipated without its being necessary to construct
one hypothesis or another, and he put these equations into a very
symmetrical form, which brings completely in evidence the perfect
reciprocity between electrical and magnetic actions. He did yet more,
for he brought to the ideas of Maxwell the most striking confirmation
by his memorable researches on electric oscillations.
§ 4. ELECTRICAL OSCILLATIONS
The experiments of Hertz are well known. We know how the Bonn
physicist developed, by means of oscillating electric discharges,
displacement currents and induction effects in the whole of the space
round the spark-gap; and how he excited by induction at some point in
a wire a perturbation which afterwards is propagated along the wire,
and how a resonator enabled him to detect the effect produced.
The most important point made evident by the observation of
interference phenomena and subsequently verified directly by M.
Blondlot, is that the electromagnetic perturbation is propagated with
the speed of light, and this result condemns for ever all the
hypotheses which fail to attribute any part to the intervening media
in the propagation of an induction phenomenon.
If the inducing action were, in fact, to operate directly between the
inducing and the induced circuits, the propagation should be
instantaneous; for if an interval were to occur between the moment
when the cause acted and the one when the effect was produced, during
this interval there would no longer be anything anywhere, since the
intervening medium does not come into play, and the phenomenon would
then disappear.
Leaving on one side the manifold but purely electrical consequences of
this and the numerous researches relating to the production or to the
properties of the waves--some of which, those of MM. Sarrazin and de
la Rive, Righi, Turpain, Lebedeff, Decombe, Barbillon, Drude, Gutton,
Lamotte, Lecher, etc., are, however, of the highest order--I shall
only mention here the studies more particularly directed to the
establishment of the identity of the electromagnetic and the luminous
waves.
The only differences which subsist are necessarily those due to the
considerable discrepancy which exists between the durations of the
periods of these two categories of waves. The length of wave
corresponding to the first spark-gap of Hertz was about 6 metres, and
the longest waves perceptible by the retina are 7/10 of a micron.[24]
[Footnote 24: See footnote 3.]
These radiations are so far apart that it is not astonishing that
their properties have not a perfect similitude. Thus phenomena like
those of diffraction, which are negligible in the ordinary conditions
under which light is observed, may here assume a preponderating
importance. To play the part, for example, with the Hertzian waves,
which a mirror 1 millimetre square plays with regard to light, would
require a colossal mirror which would attain the size of a
myriametre[25] square.
[Footnote 25: I.e., 10,000 metres.--ED.]
The efforts of physicists have to-day, however, filled up, in great
part, this interval, and from both banks at once they have laboured to
build a bridge between the two domains. We have seen how Rubens showed
us calorific rays 60 metres long; on the other hand, MM. Lecher, Bose,
and Lampa have succeeded, one after the other, in gradually obtaining
oscillations with shorter and shorter periods. There have been
produced, and are now being studied, electromagnetic waves of four
millimetres; and the gap subsisting in the spectrum between the rays
left undetected by sylvine and the radiations of M. Lampa now hardly
comprise more than five octaves--that is to say, an interval
perceptibly equal to that which separates the rays observed by M.
Rubens from the last which are evident to the eye.
The analogy then becomes quite close, and in the remaining rays the
properties, so to speak, characteristic of the Hertzian waves, begin
to appear. For these waves, as we have seen, the most transparent
bodies are the most perfect electrical insulators; while bodies still
slightly conducting are entirely opaque. The index of refraction of
these substances tends in the case of great wave-lengths to become, as
the theory anticipates, nearly the square root of the dielectric
constant.
MM. Rubens and Nichols have even produced with the waves which remain
phenomena of electric resonance quite similar to those which an
Italian scholar, M. Garbasso, obtained with electric waves. This
physicist showed that, if the electric waves are made to impinge on a
flat wooden stand, on which are a series of resonators parallel to
each other and uniformly arranged, these waves are hardly reflected
save in the case where the resonators have the same period as the
spark-gap. If the remaining rays are allowed to fall on a glass plate
silvered and divided by a diamond fixed on a dividing machine into
small rectangles of equal dimensions, there will be observed
variations in the reflecting power according to the orientation of the
rectangles, under conditions entirely comparable with the experiment
of Garbasso.
In order that the phenomenon be produced it is necessary that the
remaining waves should be previously polarized. This is because, in
fact, the mechanism employed to produce the electric oscillations
evidently gives out vibrations which occur on a single plane and are
subsequently polarized.
We cannot therefore entirely assimilate a radiation proceeding from a
spark-gap to a ray of natural light. For the synthesis of light to be
realized, still other conditions must be complied with. During a
luminous impression, the direction and the phase change millions of
times in the vibration sensible to the retina, yet the damping of this
vibration is very slow. With the Hertzian oscillations all these
conditions are changed--the damping is very rapid but the direction
remains invariable.
Every time, however, that we deal with general phenomena which are
independent of these special conditions, the parallelism is perfect;
and with the waves, we have put in evidence the reflexion, refraction,
total reflexion, double reflexion, rotatory polarization, dispersion,
and the ordinary interferences produced by rays travelling in the same
direction and crossing each other at a very acute angle, or the
interferences analogous to those which Wiener observed with rays of
the contrary direction.
A very important consequence of the electromagnetic theory foreseen by
Maxwell is that the luminous waves which fall on a surface must
exercise on this surface a pressure equal to the radiant energy which
exists in the unit of volume of the surrounding space. M. Lebedeff a
few years ago allowed a sheaf of rays from an arc lamp to fall on a
deflection radiometer,[26] and thus succeeded in revealing the
existence of this pressure. Its value is sufficient, in the case of
matter of little density and finely divided, to reduce and even change
into repulsion the attractive action exercised on bodies by the sun.
This is a fact formerly conjectured by Faye, and must certainly play a
great part in the deformation of the heads of comets.
[Footnote 26: By this M. Poincaré appears to mean a radiometer in
which the vanes are not entirely free to move as in the radiometer of
Crookes but are suspended by one or two threads as in the instrument
devised by Professor Poynting.--ED.]
More recently, MM. Nichols and Hull have undertaken experiments on
this point. They have measured not only the pressure, but also the
energy of the radiation by means of a special bolometer. They have
thus arrived at numerical verifications which are entirely in
conformity with the calculations of Maxwell.
The existence of these pressures may be otherwise foreseen even apart
from the electromagnetic theory, by adding to the theory of
undulations the principles of thermodynamics. Bartoli, and more
recently Dr Larmor, have shown, in fact, that if these pressures did
not exist, it would be possible, without any other phenomenon, to pass
heat from a cold into a warm body, and thus transgress the principle
of Carnot.
§ 5. THE X RAYS
It appears to-day quite probable that the X rays should be classed
among the phenomena which have their seat in the luminous ether.
Doubtless it is not necessary to recall here how, in December 1895,
Röntgen, having wrapped in black paper a Crookes tube in action,
observed that a fluorescent platinocyanide of barium screen placed in
the neighbourhood, had become visible in the dark, and that a
photographic plate had received an impress. The rays which come from
the tube, in conditions now well known, are not deviated by a magnet,
and, as M. Curie and M. Sagnac have conclusively shown, they carry no
electric charge. They are subject to neither reflection nor
refraction, and very precise and very ingenious measurements by M.
Gouy have shown that, in their case, the refraction index of the
various bodies cannot be more than a millionth removed from unity.
We knew from the outset that there existed various X rays differing
from each other as, for instance, the colours of the spectrum, and
these are distinguished from each other by their unequal power of
passing through substances. M. Sagnac, particularly, has shown that
there can be obtained a gradually decreasing scale of more or less
absorbable rays, so that the greater part of their photographic action
is stopped by a simple sheet of black paper. These rays figure among
the secondary rays discovered, as is known, by this ingenious
physicist. The X rays falling on matter are thus subjected to
transformations which may be compared to those which the phenomena of
luminescence produce on the ultra-violet rays.
M. Benoist has founded on the transparency of matter to the rays a
sure and practical method of allowing them to be distinguished, and
has thus been enabled to define a specific character analogous to the
colour of the rays of light. It is probable also that the different
rays do not transport individually the same quantity of energy. We
have not yet obtained on this point precise results, but it is roughly
known, since the experiments of MM. Rutherford and M'Clung, what
quantity of energy corresponds to a pencil of X rays. These physicists
have found that this quantity would be, on an average, five hundred
times larger than that brought by an analogous pencil of solar light
to the surface of the earth. What is the nature of this energy? The
question does not appear to have been yet solved.
It certainly appears, according to Professors Haga and Wind and to
Professor Sommerfeld, that with the X rays curious experiments of
diffraction may be produced. Dr Barkla has shown also that they can
manifest true polarization. The secondary rays emitted by a metallic
surface when struck by X rays vary, in fact, in intensity when the
position of the plane of incidence round the primary pencil is
changed. Various physicists have endeavoured to measure the speed of
propagation, but it seems more and more probable that it is very
nearly that of light.[27]
[Footnote 27: See especially the experiments of Professor E. Marx
(Vienna), _Annalen der Physik_, vol. xx. (No. 9 of 1906), pp. 677 _et
seq._, which seem conclusive on this point.--ED.]
I must here leave out the description of a crowd of other experiments.
Some very interesting researches by M. Brunhes, M. Broca, M.
Colardeau, M. Villard, in France, and by many others abroad, have
permitted the elucidation of several interesting problems relative to
the duration of the emission or to the best disposition to be adopted
for the production of the rays. The only point which will detain us is
the important question as to the nature of the X rays themselves; the
properties which have just been brought to mind are those which appear
essential and which every theory must reckon with.
The most natural hypothesis would be to consider the rays as
ultra-violet radiations of very short wave-length, or radiations which
are in a manner ultra-ultra-violet. This interpretation can still, at
this present moment, be maintained, and the researches of MM. Buisson,
Righi, Lenard, and Merrit Stewart have even established that rays of
very short wave-lengths produce on metallic conductors, from the point
of view of electrical phenomena, effects quite analogous to those of
the X rays. Another resemblance results also from the experiments by
which M. Perreau established that these rays act on the electric
resistance of selenium. New and valuable arguments have thus added
force to those who incline towards a theory which has the merit of
bringing a new phenomenon within the pale of phenomena previously
known.
Nevertheless the shortest ultra-violet radiations, such as those of M.
Schumann, are still capable of refraction by quartz, and this
difference constitutes, in the minds of many physicists, a serious
enough reason to decide them to reject the more simple hypothesis.
Moreover, the rays of Schumann are, as we have seen, extraordinarily
absorbable,--so much so that they have to be observed in a vacuum. The
most striking property of the X rays is, on the contrary, the facility
with which they pass through obstacles, and it is impossible not to
attach considerable importance to such a difference.
Some attribute this marvellous radiation to longitudinal vibrations,
which, as M. Duhem has shown, would be propagated in dielectric media
with a speed equal to that of light. But the most generally accepted
idea is the one formulated from the first by Sir George Stokes and
followed up by Professor Wiechert. According to this theory the X rays
should be due to a succession of independent pulsations of the ether,
starting from the points where the molecules projected by the cathode
of the Crookes tube meet the anticathode. These pulsations are not
continuous vibrations like the radiations of the spectrum; they are
isolated and extremely short; they are, besides, transverse, like the
undulations of light, and the theory shows that they must be
propagated with the speed of light. They should present neither
refraction nor reflection, but, under certain conditions, they may be
subject to the phenomena of diffraction. All these characteristics are
found in the Röntgen rays.
Professor J.J. Thomson adopts an analogous idea, and states the
precise way in which the pulsations may be produced at the moment when
the electrified particles forming the cathode rays suddenly strike the
anticathode wall. The electromagnetic induction behaves in such a way
that the magnetic field is not annihilated when the particle stops,
and the new field produced, which is no longer in equilibrium, is
propagated in the dielectric like an electric pulsation. The electric
and magnetic pulsations excited by this mechanism may give birth to
effects similar to those of light. Their slight amplitude, however, is
the cause of there here being neither refraction nor diffraction
phenomena, save in very special conditions. If the cathode particle is
not stopped in zero time, the pulsation will take a greater amplitude,
and be, in consequence, more easily absorbable; to this is probably to
be attributed the differences which may exist between different tubes
and different rays.
It is right to add that some authors, notwithstanding the proved
impossibility of deviating them in a magnetic field, have not
renounced the idea of comparing them with the cathode rays. They
suppose, for instance, that the rays are formed by electrons animated
with so great a velocity that their inertia, conformably with theories
which I shall examine later, no longer permit them to be stopped in
their course; this is, for instance, the theory upheld by Mr
Sutherland. We know, too, that to M. Gustave Le Bon they represent the
extreme limit of material things, one of the last stages before the
vanishing of matter on its return to the ether.
Everyone has heard of the N rays, whose name recalls the town of
Nancy, where they were discovered. In some of their singular
properties they are akin to the X rays, while in others they are
widely divergent from them.
M. Blondlot, one of the masters of contemporary physics, deeply
respected by all who know him, admired by everyone for the penetration
of his mind, and the author of works remarkable for the originality
and sureness of his method, discovered them in radiations emitted from
various sources, such as the sun, an incandescent light, a Nernst
lamp, and even bodies previously exposed to the sun's rays. The
essential property which allows them to be revealed is their action on
a small induction spark, of which they increase the brilliancy; this
phenomenon is visible to the eye and is rendered objective by
photography.
Various other physicists and numbers of physiologists, following the
path opened by M. Blondlot, published during 1903 and 1904 manifold
but often rather hasty memoirs, in which they related the results of
their researches, which do not appear to have been always conducted
with the accuracy desirable. These results were most strange; they
seemed destined to revolutionise whole regions not only of the domain
of physics, but likewise of the biological sciences. Unfortunately the
method of observation was always founded on the variations in
visibility of the spark or of a phosphorescent substance, and it soon
became manifest that these variations were not perceptible to all
eyes.
No foreign experimenter has succeeded in repeating the experiments,
while in France many physicists have failed; and hence the question
has much agitated public opinion. Are we face to face with a very
singular case of suggestion, or is special training and particular
dispositions required to make the phenomenon apparent? It is not
possible, at the present moment, to declare the problem solved; but
very recent experiments by M. Gutton and a note by M. Mascart have
reanimated the confidence of those who hoped that such a scholar as M.
Blondlot could not have been deluded by appearances. However, these
last proofs in favour of the existence of the rays have themselves
been contested, and have not succeeded in bringing conviction to
everyone.
It seems very probable indeed that certain of the most singular
conclusions arrived at by certain authors on the subject will lapse
into deserved oblivion. But negative experiments prove nothing in a
case like this, and the fact that most experimenters have failed where
M. Blondlot and his pupils have succeeded may constitute a
presumption, but cannot be regarded as a demonstrative argument. Hence
we must still wait; it is exceedingly possible that the illustrious
physicist of Nancy may succeed in discovering objective actions of the
N rays which shall be indisputable, and may thus establish on a firm
basis a discovery worthy of those others which have made his name so
justly celebrated.
According to M. Blondlot the N rays can be polarised, refracted, and
dispersed, while they have wavelengths comprised within .0030 micron,
and .0760 micron--that is to say, between an eighth and a fifth of
that found for the extreme ultra-violet rays. They might be, perhaps,
simply rays of a very short period. Their existence, stripped of the
parasitical and somewhat singular properties sought to be attributed
to them, would thus appear natural enough. It would, moreover, be
extremely important, and lead, no doubt, to most curious applications;
it can be conceived, in fact, that such rays might serve to reveal
what occurs in those portions of matter whose too minute dimensions
escape microscopic examination on account of the phenomena of
diffraction.
From whatever point of view we look at it, and whatever may be the
fate of the discovery, the history of the N rays is particularly
instructive, and must give food for reflection to those interested in
questions of scientific methods.
§ 6. THE ETHER AND GRAVITATION
The striking success of the hypothesis of the ether in optics has, in
our own days, strengthened the hope of being able to explain, by an
analogous representation, the action of gravitation.
For a long time, philosophers who rejected the idea that ponderability
is a primary and essential quality of all bodies have sought to reduce
their weight to pressures exercised in a very subtle fluid. This was
the conception of Descartes, and was perhaps the true idea of Newton
himself. Newton points out, in many passages, that the laws he had
discovered were independent of the hypotheses that could be formed on
the way in which universal attraction was produced, but that with
sufficient experiments the true cause of this attraction might one day
be reached. In the preface to the second edition of the Optics he
writes: "To prove that I have not considered weight as a universal
property of bodies, I have added a question as to its cause,
preferring this form of question because my interpretation does not
entirely satisfy me in the absence of experiment"; and he puts the
question in this shape: "Is not this medium (the ether) more rarefied
in the interior of dense bodies like the sun, the planets, the comets,
than in the empty spaces which separate them? Passing from these
bodies to great distances, does it not become continually denser, and
in that way does it not produce the weight of these great bodies with
regard to each other and of their parts with regard to these bodies,
each body tending to leave the most dense for the most rarefied
parts?"
Evidently this view is incomplete, but we may endeavour to state it
precisely. If we admit that this medium, the properties of which would
explain the attraction, is the same as the luminous ether, we may
first ask ourselves whether the action of gravitation is itself also
due to oscillations. Some authors have endeavoured to found a theory
on this hypothesis, but we are immediately brought face to face with
very serious difficulties. Gravity appears, in fact, to present quite
exceptional characteristics. No agent, not even those which depend
upon the ether, such as light and electricity, has any influence on
its action or its direction. All bodies are, so to speak, absolutely
transparent to universal attraction, and no experiment has succeeded
in demonstrating that its propagation is not instantaneous. From
various astronomical observations, Laplace concluded that its
velocity, in any case, must exceed fifty million times that of light.
It is subject neither to reflection nor to refraction; it is
independent of the structure of bodies; and not only is it
inexhaustible, but also (as is pointed out, according to M. Hannequin,
by an English scholar, James Croll) the distribution of the effects of
the attracting force of a mass over the manifold particles which may
successively enter the field of its action in no way diminishes the
attraction it exercises on each of them respectively, a thing which is
seen nowhere else in nature.
Nevertheless it is possible, by means of certain hypotheses, to
construct interpretations whereby the appropriate movements of an
elastic medium should explain the facts clearly enough. But these
movements are very complex, and it seems almost inconceivable that the
same medium could possess simultaneously the state of movement
corresponding to the transmission of a luminous phenomenon and that
constantly imposed on it by the transmission of gravitation.
Another celebrated hypothesis was devised by Lesage, of Geneva. Lesage
supposed space to be overrun in all directions by currents of
_ultramundane_ corpuscles. This hypothesis, contested by Maxwell, is
interesting. It might perhaps be taken up again in our days, and it is
not impossible that the assimilation of these corpuscles to electrons
might give a satisfactory image.[28]
[Footnote 28: M. Sagnac (_Le Radium_, Jan. 1906, p. 14), following
perhaps Professors Elster and Geitel, has lately taken up this idea
anew.--ED.]
M. Crémieux has recently undertaken experiments directed, as he
thinks, to showing that the divergences between the phenomena of
gravitation and all the other phenomena in nature are more apparent
than real. Thus the evolution in the heart of the ether of a quantity
of gravific energy would not be entirely isolated, and as in the case
of all evolutions of all energy of whatever kind, it should provoke a
partial transformation into energy of a different form. Thus again the
liberated energy of gravitation would vary when passing from one
material to another, as from gases into liquids, or from one liquid to
a different one.
On this last point the researches of M. Crémieux have given
affirmative results: if we immerse in a large mass of some liquid
several drops of another not miscible with the first, but of identical
density, we form a mass representing no doubt a discontinuity in the
ether, and we may ask ourselves whether, in conformity with what
happens in all other phenomena of nature, this discontinuity has not a
tendency to disappear.
If we abide by the ordinary consequences of the Newtonian theory of
potential, the drops should remain motionless, the hydrostatic
impulsion forming an exact equilibrium to their mutual attraction. Now
M. Crémieux remarks that, as a matter of fact, they slowly approach
each other.
Such experiments are very delicate; and with all the precautions taken
by the author, it cannot yet be asserted that he has removed all
possibility of the action of the phenomena of capillarity nor all
possible errors proceeding from extremely slight differences of
temperature. But the attempt is interesting and deserves to be
followed up.
Thus, the hypothesis of the ether does not yet explain all the
phenomena which the considerations relating to matter are of
themselves powerless to interpret. If we wished to represent to
ourselves, by the mechanical properties of a medium filling the whole
of the universe, all luminous, electric, and gravitation phenomena, we
should be led to attribute to this medium very strange and almost
contradictory characteristics; and yet it would be still more
inconceivable that this medium should be double or treble, that there
should be two or three ethers each occupying space as if it were
alone, and interpenetrating it without exercising any action on one
another. We are thus brought, by a close examination of facts, rather
to the idea that the properties of the ether are not wholly reducible
to the rules of ordinary mechanics.
The physicist has therefore not yet succeeded in answering the
question often put to him by the philosopher: "Has the ether really an
objective existence?" However, it is not necessary to know the answer
in order to utilize the ether. In its ideal properties we find the
means of determining the form of equations which are valid, and to the
learned detached from all metaphysical prepossession this is the
essential point.