and these phenomena, dimly seen by Plücker and Birkeland, have been
the object of experiments by M. Villard. The two faces of the cathode
seem to emit rays which are deviated in a direction perpendicular to
the lines of force by an electric field, and do not seem to be
electrified. M. Villard calls them magneto-cathode rays, and according
to M. Fortin these rays may be ordinary cathode rays, but of very
slight velocity.

In certain cases the cathode itself may be superficially
disaggregated, and extremely tenuous particles detach themselves,
which, being carried off at right angles to its surface, may deposit
themselves like a very thin film on objects placed in their path.
Various physicists, among them M. Houllevigue, have studied this
phenomenon, and in the case of pressures between 1/20 and 1/100 of a
millimetre, the last-named scholar has obtained mirrors of most
metals, a phenomenon he designates by the name of ionoplasty.

But in spite of all these accessory phenomena, which even sometimes
conceal those first observed, the existence of the electron in the
cathodic flux remains the essential characteristic.

The electron can be apprehended in the cathodic ray by the study of
its essential properties; and J.J. Thomson gave great value to the
hypothesis by his measurements. At first he meant to determine the
speed of the cathode rays by direct experiment, and by observing, in a
revolving mirror, the relative displacement of two bands due to the
excitement of two fluorescent screens placed at different distances
from the cathode. But he soon perceived that the effect of the
fluorescence was not instantaneous, and that the lapse of time might
form a great source of error, and he then had recourse to indirect
methods. It is possible, by a simple calculation, to estimate the
deviations produced on the rays by a magnetic and an electric field
respectively as a function of the speed of propagation and of the
relation of the charge to the material mass of the electron. The
measurement of these deviations will then permit this speed and this
relation to be ascertained.

Other processes may be used which all give the same two quantities by
two suitably chosen measurements. Such are the radius of the curve
taken by the trajectory of the pencil in a perpendicular magnetic
field and the measure of the fall of potential under which the
discharge takes place, or the measure of the total quantity
of electricity carried in one second and the measure of the
calorific energy which may be given, during the same period, to a
thermo-electric junction. The results agree as well as can be expected,
having regard to the difficulty of the experiments; the values of the
speed agree also with those which Professor Wiechert has obtained by
direct measurement.

The speed never depends on the nature of the gas contained in the
Crookes tube, but varies with the value of the fall of potential at
the cathode. It is of the order of one tenth of the speed of light,
and it may rise as high as one third. The cathode particle therefore
goes about three thousand times faster than the earth in its orbit.
The relation is also invariable, even when the substance of which the
cathode is formed is changed or one gas is substituted for another. It
is, on the average, a thousand times greater than the corresponding
relation in electrolysis. As experiment has shown, in all the
circumstances where it has been possible to effect measurements, the
equality of the charges carried by all corpuscules, ions, atoms, etc.,
we ought to consider that the charge of the electron is here, again,
that of a univalent ion in electrolysis, and therefore that its mass
is only a small fraction of that of the atom of hydrogen, viz., of the
order of about a thousandth part. This is the same result as that to
which we were led by the study of flames.

The thorough examination of the cathode radiation, then, confirms us
in the idea that every material atom can be dissociated and will yield
an electron much smaller than itself--and always identical whatever
the matter whence it comes,--the rest of the atom remaining charged
with a positive quantity equal and contrary to that borne by the
electron. In the present case these positive ions are no doubt those
that we again meet with in the canal rays. Professor Wien has shown
that their mass is really, in fact, of the order of the mass of atoms.
Although they are all formed of identical electrons, there may be
various cathode rays, because the velocity is not exactly the same for
all electrons. Thus is explained the fact that we can separate them
and that we can produce a sort of spectrum by the action of the
magnet, or, again, as M. Deslandres has shown in a very interesting
experiment, by that of an electrostatic field. This also probably
explains the phenomena studied by M. Villard, and previously pointed
out.

§ 2. RADIOACTIVE SUBSTANCES

Even in ordinary conditions, certain substances called radioactive
emit, quite outside any particular reaction, radiations complex
indeed, but which pass through fairly thin layers of minerals, impress
photographic plates, excite fluorescence, and ionize gases. In these
radiations we again find electrons which thus escape spontaneously
from radioactive bodies.

It is not necessary to give here a history of the discovery of radium,
for every one knows the admirable researches of M. and Madame Curie.
But subsequent to these first studies, a great number of facts have
accumulated for the last six years, among which some people find
themselves a little lost. It may, perhaps, not be useless to indicate
the essential results actually obtained.

The researches on radioactive substances have their starting-point in
the discovery of the rays of uranium made by M. Becquerel in 1896. As
early as 1867 Niepce de St Victor proved that salts of uranium
impressed photographic plates in the dark; but at that time the
phenomenon could only pass for a singularity attributable to
phosphorescence, and the valuable remarks of Niepce fell into
oblivion. M. Becquerel established, after some hesitations natural in
the face of phenomena which seemed so contrary to accepted ideas, that
the radiating property was absolutely independent of phosphorescence,
that all the salts of uranium, even the uranous salts which are not
phosphorescent, give similar radiant effects, and that these phenomena
correspond to a continuous emission of energy, but do not seem to be
the result of a storage of energy under the influence of some external
radiation. Spontaneous and constant, the radiation is insensible to
variations of temperature and light.

The nature of these radiations was not immediately understood,[32] and
their properties seemed contradictory. This was because we were not
dealing with a single category of rays. But amongst all the effects
there is one which constitutes for the radiations taken as a whole, a
veritable process for the measurement of radioactivity. This is their
ionizing action on gases. A very complete study of the conductivity of
air under the influence of rays of uranium has been made by various
physicists, particularly by Professor Rutherford, and has shown that
the laws of the phenomenon are the same as those of the ionization due
to the action of the Röntgen rays.

[Footnote 32: In his work on _L'Évolution de la Matière_, M. Gustave
Le Bon recalls that in 1897 he published several notes in the Académie
des Sciences, in which he asserted that the properties of uranium were
only a particular case of a very general law, and that the radiations
emitted did not polarize, and were akin by their properties to the X
rays.]

It was natural to ask one's self if the property discovered in salts
of uranium was peculiar to this body, or if it were not, to a more or
less degree, a general property of matter. Madame Curie and M.
Schmidt, independently of each other, made systematic researches in
order to solve the question; various compounds of nearly all the
simple bodies at present known were thus passed in review, and it was
established that radioactivity was particularly perceptible in the
compounds of uranium and thorium, and that it was an atomic property
linked to the matter endowed with it, and following it in all its
combinations. In the course of her researches Madame Curie observed
that certain pitchblendes (oxide of uranium ore, containing also
barium, bismuth, etc.) were four times more active (activity being
measured by the phenomenon of the ionization of the air) than metallic
uranium. Now, no compound containing any other active metal than
uranium or thorium ought to show itself more active than those metals
themselves, since the property belongs to their atoms. It seemed,
therefore, probable that there existed in pitchblendes some substance
yet unknown, in small quantities and more radioactive than uranium.

M. and Madame Curie then commenced those celebrated experiments which
brought them to the discovery of radium. Their method of research has
been justly compared in originality and importance to the process of
spectrum analysis. To isolate a radioactive substance, the first thing
is to measure the activity of a certain compound suspected of
containing this substance, and this compound is chemically separated.
We then again take in hand all the products obtained, and by measuring
their activity anew, it is ascertained whether the substance sought
for has remained in one of these products, or is divided among them,
and if so, in what proportion. The spectroscopic reaction which we may
use in the course of this separation is a thousand times less
sensitive than observation of the activity by means of the
electrometer.

Though the principle on which the operation of the concentration of
the radium rests is admirable in its simplicity, its application is
nevertheless very laborious. Tons of uranium residues have to be
treated in order to obtain a few decigrammes of pure salts of radium.
Radium is characterised by a special spectrum, and its atomic weight,
as determined by Madame Curie, is 225; it is consequently the higher
homologue of barium in one of the groups of Mendeléef. Salts of radium
have in general the same chemical properties as the corresponding
salts of barium, but are distinguished from them by the differences of
solubility which allow of their separation, and by their enormous
activity, which is about a hundred thousand times greater than that of
uranium.

Radium produces various chemical and some very intense physiological
reactions. Its salts are luminous in the dark, but this luminosity, at
first very bright, gradually diminishes as the salts get older. We
have here to do with a secondary reaction correlative to the
production of the emanation, after which radium undergoes the
transformations which will be studied later on.

The method of analysis founded by M. and Madame Curie has enabled
other bodies presenting sensible radioactivity to be discovered. The
alkaline metals appear to possess this property in a slight degree.
Recently fallen snow and mineral waters manifest marked action. The
phenomenon may often be due, however, to a radioactivity induced by
radiations already existing in the atmosphere. But this radioactivity
hardly attains the ten-thousandth part of that presented by uranium,
or the ten-millionth of that appertaining to radium.

Two other bodies, polonium and actinium, the one characterised by the
special nature of the radiations it emits and the other by a
particular spectrum, seem likewise to exist in pitchblende. These
chemical properties have not yet been perfectly defined; thus M.
Debierne, who discovered actinium, has been able to note the active
property which seems to belong to it, sometimes in lanthanum,
sometimes in neodynium.[33] It is proved that all extremely
radioactive bodies are the seat of incessant transformations, and even
now we cannot state the conditions under which they present themselves
in a strictly determined form.

[Footnote 33: Polonium has now been shown to be no new element, but
one of the transformation products of radium. Radium itself is also
thought to be derived in some manner, not yet ascertained, from
uranium. The same is the case with actinium, which is said to come in
the long run from uranium, but not so directly as does radium. All
this is described in Professor Rutherford's _Radioactive
Transformations_ (London, 1906).--ED.]

§ 3. THE RADIATION OF THE RADIOACTIVE BODIES AND THE EMANATION

To acquire exact notions as to the nature of the rays emitted by the
radioactive bodies, it was necessary to try to cause magnetic or
electric forces to act on them so as to see whether they behaved in
the same way as light and the X rays, or whether like the cathode rays
they were deviated by a magnetic field. This work was effected by
Professor Giesel, then by M. Becquerel, Professor Rutherford, and by
many other experimenters after them. All the methods which have
already been mentioned in principle have been employed in order to
discover whether they were electrified, and, if so, by electricity of
what sign, to measure their speed, and to ascertain their degree of
penetration.

The general result has been to distinguish three sorts of radiations,
designated by the letters alpha, beta, gamma.

The alpha rays are positively charged, and are projected at a speed
which may attain the tenth of that of light; M.H. Becquerel has shown
by the aid of photography that they are deviated by a magnet, and
Professor Rutherford has, on his side, studied this deviation by the
electrical method. The relation of the charge to the mass is, in the
case of these rays, of the same order as in that of the ions of
electrolysis. They may therefore be considered as exactly analogous to
the canal rays of Goldstein, and we may attribute them to a material
transport of corpuscles of the magnitude of atoms. The relatively
considerable size of these corpuscles renders them very absorbable. A
flight of a few millimetres in a gas suffices to reduce their number
by one-half. They have great ionizing power.

The beta rays are on all points similar to the cathode rays; they are,
as M. and Madame Curie have shown, negatively charged, and the charge
they carry is always the same. Their size is that of the electrons,
and their velocity is generally greater than that of the cathode rays,
while it may become almost that of light. They have about a hundred
times less ionizing power than the alpha rays.

The gamma rays were discovered by M. Villard.[34] They may be compared
to the X rays; like the latter, they are not deviated by the magnetic
field, and are also extremely penetrating. A strip of aluminium five
millimetres thick will stop the other kinds, but will allow them to
pass. On the other hand, their ionizing power is 10,000 times less
than that of the alpha rays.

[Footnote 34: This is admitted by Professor Rutherford (_Radio-Activity_,
Camb., 1904, p. 141) and Professor Soddy (_Radio-Activity_, London,
1904, p. 66). Neither Mr Whetham, in his Recent _Development of
Physical Science_ (London, 1904) nor the Hon. R.J. Strutt in _The
Becquerel Rays_ (London, same date), both of whom deal with the
historical side of the subject, seem to have noticed the fact.--ED.]

To these radiations there sometimes are added in the course of
experiments secondary radiations analogous to those of M. Sagnac, and
produced when the alpha, beta, or gamma rays meet various substances.
This complication has often led to some errors of observation.

Phosphorescence and fluorescence seem especially to result from the
alpha and beta rays, particularly from the alpha rays, to which
belongs the most important part of the total energy of the radiation.
Sir W. Crookes has invented a curious little apparatus, the
spinthariscope, which enables us to examine the phosphorescence of the
blende excited by these rays. By means of a magnifying glass, a screen
covered with sulphide of zinc is kept under observation, and in front
of it is disposed, at a distance of about half a millimetre, a
fragment of some salt of radium. We then perceive multitudes of
brilliant points on the screen, which appear and at once disappear,
producing a scintillating effect. It seems probable that every
particle falling on the screen produces by its impact a disturbance in
the neighbouring region, and it is this disturbance which the eye
perceives as a luminous point. Thus, says Sir W. Crookes, each drop of
rain falling on the surface of still water is not perceived as a drop
of rain, but by reason of the slight splash which it causes at the
moment of impact, and which is manifested by ridges and waves
spreading themselves in circles.

The various radioactive substances do not all give radiations of
identical constitution. Radium and thorium possess in somewhat large
proportions the three kinds of rays, and it is the same with actinium.
Polonium contains especially alpha rays and a few gamma rays.[35] In
the case of uranium, the alpha rays have extremely slight penetrating
power, and cannot even impress photographic plates. But the widest
difference between the substances proceeds from the emanation. Radium,
in addition to the three groups of rays alpha, beta, and gamma,
disengages continuously an extremely subtle emanation, seemingly
almost imponderable, but which may be, for many reasons, looked upon
as a vapour of which the elastic force is extremely feeble.

[Footnote 35: It has now been shown that polonium when freshly
separated emits beta rays also; see Dr Logeman's paper in _Proceedings
of the Royal Society_, A., 6th September 1906.--ED.]

M. and Madame Curie discovered as early as 1899 that every substance
placed in the neighbourhood of radium, itself acquired a radioactivity
which persisted for several hours after the removal of the radium.
This induced radioactivity seems to be carried to other bodies by the
intermediary of a gas. It goes round obstacles, but there must exist
between the radium and the substance a free and continuous space for
the activation to take place; it cannot, for instance, do so through a
wall of glass.

In the case of compounds of thorium Professor Rutherford discovered a
similar phenomenon; since then, various physicists, Professor Soddy,
Miss Brooks, Miss Gates, M. Danne, and others, have studied the
properties of these emanations.

The substance emanated can neither be weighed nor can its elastic
force be ascertained; but its transformations may be followed, as it
is luminous, and it is even more certainly characterised by its
essential property, i.e. its radioactivity. We also see that it can be
decanted like a gas, that it will divide itself between two tubes of
different capacity in obedience to the law of Mariotte, and will
condense in a refrigerated tube in accordance with the principle of
Watt, while it even complies with the law of Gay-Lussac.

The activity of the emanation vanishes quickly, and at the end of four
days it has diminished by one-half. If a salt of radium is heated, the
emanation becomes more abundant, and the residue, which, however, does
not sensibly diminish in weight, will have lost all its radioactivity,
and will only recover it by degrees. Professor Rutherford,
notwithstanding many different attempts, has been unable to make this
emanation enter into any chemical reaction. If it be a gaseous body,
it must form part of the argon group, and, like its other members, be
perfectly inert.

By studying the spectrum of the gas disengaged by a solution of salt
of radium, Sir William Ramsay and Professor Soddy remarked that when
the gas is radioactive there are first obtained rays of gases
belonging to the argon family, then by degrees, as the activity
disappears, the spectrum slowly changes, and finally presents the
characteristic aspect of helium.

We know that the existence of this gas was first discovered by
spectrum analysis in the sun. Later its presence was noted in our
atmosphere, and in a few minerals which happen to be the very ones
from which radium has been obtained. It might therefore have been the
case that it pre-existed in the gases extracted from radium; but a
remarkable experiment by M. Curie and Sir James Dewar seems to show
convincingly that this cannot be so. The spectrum of helium never
appears at first in the gas proceeding from pure bromide of radium;
but it shows itself, on the other hand, very distinctly, after the
radioactive transformations undergone by the salt.

All these strange phenomena suggest bold hypotheses, but to construct
them with any solidity they must be supported by the greatest possible
number of facts. Before admitting a definite explanation of the
phenomena which have their seat in the curious substances discovered
by them, M. and Madame Curie considered, with a great deal of reason,
that they ought first to enrich our knowledge with the exact and
precise facts relating to these bodies and to the effects produced by
the radiations they emit.

Thus M. Curie particularly set himself to study the manner in which
the radioactivity of the emanation is dissipated, and the
radioactivity that this emanation can induce on all bodies. The
radioactivity of the emanation diminishes in accordance with an
exponential law. The constant of time which characterises this
decrease is easily and exactly determined, and has a fixed value,
independent of the conditions of the experiment as well as of the
nature of the gas which is in contact with the radium and becomes
charged with the emanation. The regularity of the phenomenon is so
great that it can be used to measure time: in 3985 seconds[36] the
activity is always reduced one-half.

[Footnote 36: According to Professor Rutherford, in 3.77 days.--ED]

Radioactivity induced on any body which has been for a long time in
presence of a salt of radium disappears more rapidly. The phenomenon
appears, moreover, more complex, and the formula which expresses the
manner in which the activity diminishes must contain two exponentials.
To find it theoretically we have to imagine that the emanation first
deposits on the body in question a substance which is destroyed in
giving birth to a second, this latter disappearing in its turn by
generating a third. The initial and final substances would be
radioactive, but the intermediary one, not. If, moreover, the bodies
acted on are brought to a temperature of over 700°, they appear to
lose by volatilisation certain substances condensed in them, and at
the same time their activity disappears.

The other radioactive bodies behave in a similar way. Bodies which
contain actinium are particularly rich in emanations. Uranium, on the
contrary, has none.[37] This body, nevertheless, is the seat of
transformations comparable to those which the study of emanations
reveals in radium; Sir W. Crookes has separated from uranium a matter
which is now called uranium X. This matter is at first much more
active than its parent, but its activity diminishes rapidly, while the
ordinary uranium, which at the time of the separation loses its
activity, regains it by degrees. In the same way, Professors
Rutherford and Soddy have discovered a so-called thorium X to be the
stage through which ordinary thorium has to pass in order to produce
its emanation.[38]

[Footnote 37: Professor Rutherford has lately stated that uranium may
possibly produce an emanation, but that its rate of decay must be too
swift for its presence to be verified (see _Radioactive
Transformations_, p. 161).--ED.]

[Footnote 38: An actinium X was also discovered by Professor Giesel
(_Jahrbuch d. Radioaktivitat_, i. p. 358, 1904). Since the above was
written, another product has been found to intervene between the X
substance and the emanation in the case of actinium and thorium. They
have been named radio-actinium and radio-thorium respectively.--ED.]

It is not possible to give a complete table which should, as it were,
represent the genealogical tree of the various radioactive substances.
Several authors have endeavoured to do so, but in a premature manner;
all the affiliations are not at the present time yet perfectly known,
and it will no doubt be acknowledged some day that identical states
have been described under different names.[39]

[Footnote 39: Such a table is given on p. 169 of Rutherford's
_Radioactive Transformations_.--ED.]

§ 4. THE DISAGGREGATION OF MATTER AND ATOMIC ENERGY

In spite of uncertainties which are not yet entirely removed, it
cannot be denied that many experiments render it probable that in
radioactive bodies we find ourselves witnessing veritable
transformations of matter.

Professor Rutherford, Professor Soddy, and several other physicists,
have come to regard these phenomena in the following way. A
radioactive body is composed of atoms which have little stability, and
are able to detach themselves spontaneously from the parent substance,
and at the same time to divide themselves into two essential component
parts, the negative electron and its residue the positive ion. The
first-named constitutes the beta, and the second the alpha rays.

The emanation is certainly composed of alpha ions with a few molecules
agglomerated round them. Professor Rutherford has, in fact,
demonstrated that the emanation is charged with positive electricity;
and this emanation may, in turn, be destroyed by giving birth to new
bodies.

After the loss of the atoms which are carried off by the radiation,
the remainder of the body acquires new properties, but it may still be
radioactive, and again lose atoms. The various stages that we meet
with in the evolution of the radioactive substance or of its
emanation, correspond to the various degrees of atomic disaggregation.
Professors Rutherford and Soddy have described them clearly in the
case of uranium and radium. As regards thorium the results are less
satisfactory. The evolution should continue until a stable atomic
condition is finally reached, which, because of this stability, is no
longer radioactive. Thus, for instance, radium would finally be
transformed into helium.[40]

[Footnote 40: This opinion, no doubt formed when Sir William Ramsay's
discovery of the formation of helium from the radium emanation was
first made known, is now less tenable. The latest theory is that the
alpha particle is in fact an atom of helium, and that the final
transformation product of radium and the other radioactive substances
is lead. Cf. Rutherford, op. cit. passim.--ED.]

It is possible, by considerations analogous to those set forth above
in other cases, to arrive at an idea of the total number of particles
per second expelled by one gramme of radium; Professor Rutherford in
his most recent evaluation finds that this number approaches 2.5 x
10^{11}.[41] By calculating from the atomic weight the number of atoms
probably contained in this gramme of radium, and supposing each
particle liberated to correspond to the destruction of one atom, it is
found that one half of the radium should disappear in 1280 years;[42]
and from this we may conceive that it has not yet been possible to
discover any sensible loss of weight. Sir W. Ramsay and Professor
Soddy attained a like result by endeavouring to estimate the mass of
the emanation by the quantity of helium produced.

[Footnote 41: See _Radioactive Transformations_ (p. 251). Professor
Rutherford says that "each of the alpha ray products present in one
gram of radium product (_sic_) expels 6.2 x 10^{10} alpha particles
per second." He also remarks on "the experimental difficulty of
accurately determining the number of alpha particles expelled from
radium per second."--ED.]

[Footnote 42: See Rutherford, op. cit. p. 150.--ED.]

If radium transforms itself in such a way that its activity does not
persist throughout the ages, it loses little by little the provision
of energy it had in the beginning, and its properties furnish no valid
argument to oppose to the principle of the conservation of energy. To
put everything right, we have only to recognise that radium possessed
in the potential state at its formation a finite quantity of energy
which is consumed little by little. In the same manner, a chemical
system composed, for instance, of zinc and sulphuric acid, also
contains in the potential state energy which, if we retard the
reaction by any suitable arrangement--such as by amalgamating the zinc
and by constituting with its elements a battery which we cause to act
on a resistance--may be made to exhaust itself as slowly as one may
desire.

There can, therefore, be nothing in any way surprising in the fact
that a combination which, like the atomic combination of radium, is
not stable--since it disaggregates itself,--is capable of
spontaneously liberating energy, but what may be a little astonishing,
at first sight, is the considerable amount of this energy.

M. Curie has calculated directly, by the aid of the calorimeter, the
quantity of energy liberated, measuring it entirely in the form of
heat. The disengagement of heat accounted for in a grain of radium is
uniform, and amounts to 100 calories per hour. It must therefore be
admitted that an atom of radium, in disaggregating itself, liberates
30,000 times more energy than a molecule of hydrogen when the latter
combines with an atom of oxygen to form a molecule of water.

We may ask ourselves how the atomic edifice of the active body can be
constructed, to contain so great a provision of energy. We will remark
that such a question might be asked concerning cases known from the
most remote antiquity, like that of the chemical systems, without any
satisfactory answer ever being given. This failure surprises no one,
for we get used to everything--even to defeat.

When we come to deal with a new problem we have really no right to
show ourselves more exacting; yet there are found persons who refuse
to admit the hypothesis of the atomic disaggregation of radium because
they cannot have set before them a detailed plan of that complex whole
known to us as an atom.

The most natural idea is perhaps the one suggested by comparison with
those astronomical phenomena where our observation most readily allows
us to comprehend the laws of motion. It corresponds likewise to the
tendency ever present in the mind of man, to compare the infinitely
small with the infinitely great. The atom may be regarded as a sort of
solar system in which electrons in considerable numbers gravitate
round the sun formed by the positive ion. It may happen that certain
of these electrons are no longer retained in their orbit by the
electric attraction of the rest of the atom, and may be projected from
it like a small planet or comet which escapes towards the stellar
spaces. The phenomena of the emission of light compels us to think
that the corpuscles revolve round the nucleus with extreme velocities,
or at the rate of thousands of billions of evolutions per second. It
is easy to conceive from this that, notwithstanding its lightness, an
atom thus constituted may possess an enormous energy.[43]

[Footnote 43: This view of the case has been made very clear by M.
Gustave le Bon in _L'Évolution de la Matière_ (Paris, 1906). See
especially pp. 36-52, where the amount of the supposed intra-atomic
energy is calculated.--ED.]

Other authors imagine that the energy of the corpuscles is principally
due to the extremely rapid rotations of those elements on their own
axes. Lord Kelvin lately drew up on another model the plan of a
radioactive atom capable of ejecting an electron with a considerable
_vis viva_. He supposes a spherical atom formed of concentric layers
of positive and negative electricity disposed in such a way that its
external action is null, and that, nevertheless, the force emanated
from the centre may be repellent for certain values when the electron
is within it.

The most prudent physicists and those most respectful to established
principles may, without any scruples, admit the explanation of the
radioactivity of radium by a dislocation of its molecular edifice. The
matter of which it is constituted evolves from an admittedly unstable
initial state to another stable one. It is, in a way, a slow
allotropic transformation which takes place by means of a mechanism
regarding which, in short, we have no more information than we have
regarding other analogous transformations. The only astonishment we
can legitimately feel is derived from the thought that we are suddenly
and deeply penetrating to the very heart of things.

But those persons who have a little more hardihood do not easily
resist the temptation of forming daring generalisations. Thus it will
occur to some that this property, already discovered in many
substances where it exists in more or less striking degree, is, with
differences of intensity, common to all bodies, and that we are thus
confronted by a phenomenon derived from an essential quality of
matter. Quite recently, Professor Rutherford has demonstrated in a
fine series of experiments that the alpha particles of radium cease to
ionize gases when they are made to lose their velocity, but that they
do not on that account cease to exist. It may follow that many bodies
emit similar particles without being easily perceived to do so; since
the electric action, by which this phenomenon of radioactivity is
generally manifested, would, in this case, be but very weak.

If we thus believe radioactivity to be an absolutely general
phenomenon, we find ourselves face to face with a new problem. The
transformation of radioactive bodies can no longer be assimilated to
allotropic transformations, since thus no final form could ever be
attained, and the disaggregation would continue indefinitely up to the
complete dislocation of the atom.[44] The phenomenon might, it is
true, have a duration of perhaps thousands of millions of centuries,
but this duration is but a minute in the infinity of time, and matters
little. Our habits of mind, if we adopt such a conception, will be
none the less very deeply disturbed. We shall have to abandon the idea
so instinctively dear to us that matter is the most stable thing in
the universe, and to admit, on the contrary, that all bodies whatever
are a kind of explosive decomposing with extreme slowness. There is in
this, whatever may have been said, nothing contrary to any of the
principles on which the science of energetics rests; but an hypothesis
of this nature carries with it consequences which ought in the highest
degree to interest the philosopher, and we all know with what alluring
boldness M. Gustave Le Bon has developed all these consequences in his
work on the evolution of matter.[45]

[Footnote 44: This is the main contention of M. Gustave Le Bon in
his work last quoted.--ED.]

[Footnote 45: See last note.--ED.]

There is hardly a physicist who does not at the present day adopt in
one shape or another the ballistic hypothesis. All new facts are
co-ordinated so happily by it, that it more and more satisfies our
minds; but it cannot be asserted that it forces itself on our
convictions with irresistible weight. Another point of view appeared
more plausible and simple at the outset, when there seemed reason to
consider the energy radiated by radioactive bodies as inexhaustible.
It was thought that the source of this energy was to be looked for
without the atom, and this idea may perfectly well he maintained at
the present day.

Radium on this hypothesis must be considered as a transformer
borrowing energy from the external medium and returning it in the form
of radiation. It is not impossible, even, to admit that the energy
which the atom of radium withdraws from the surrounding medium may
serve to keep up, not only the heat emitted and its complex radiation,
but also the dissociation, supposed to be endothermic, of this atom.
Such seems to be the idea of M. Debierne and also of M. Sagnac. It
does not seem to accord with the experiments that this borrowed energy
can be a part of the heat of the ambient medium; and, indeed, such a
phenomenon would be contrary to the principle of Carnot if we wished
(though we have seen how disputable is this extension) to extend this
principle to the phenomena which are produced in the very bosom of the
atom.

We may also address ourselves to a more noble form of energy, and ask
ourselves whether we are not, for the first time, in presence of a
transformation of gravitational energy. It may be singular, but it is
not absurd, to suppose that the unit of mass of radium is not attached
to the earth with the same intensity as an inert body. M. Sagnac has
commenced some experiments, as yet unpublished, in order to study the
laws of the fall of a fragment of radium. They are necessarily very
delicate, and the energetic and ingenious physicist has not yet
succeeded in finishing them.[46] Let us suppose that he succeeds in
demonstrating that the intensity of gravity is less for radium than
for the platinum or the copper of which the pendulums used to
illustrate the law of Newton are generally made; it would then be
possible still to think that the laws of universal attraction are
perfectly exact as regards the stars, and that ponderability is really
a particular case of universal attraction, while in the case of
radioactive bodies part of the gravitational energy is transformed in
the course of its evolution and appears in the form of active
radiation.

[Footnote 46: In reality M. Sagnac operated in the converse manner. He
took two equal _weights_ of a salt of radium and a salt of barium,
which he made oscillate one after the other in a torsion balance. Had
the durations of oscillation been different, it might be concluded
that the mechanical mass is not the same for radium as for barium.]

But for this explanation to be admitted, it would evidently need to be
supported by very numerous facts. It might, no doubt, appear still
more probable that the energy borrowed from the external medium by
radium is one of those still unknown to us, but of which a vague
instinct causes us to suspect the existence around us. It is
indisputable, moreover, that the atmosphere in all directions is
furrowed with active radiations; those of radium may be secondary
radiations reflected by a kind of resonance phenomenon.

Certain experiments by Professors Elster and Geitel, however, are not
favourable to this point of view. If an active body be surrounded by a
radioactive envelope, a screen should prevent this body from receiving
any impression from outside, and yet there is no diminution apparent
in the activity presented by a certain quantity of radium when it is
lowered to a depth of 800 metres under ground, in a region containing
a notable quantity of pitchblende. These negative results are, on the
other hand, so many successes for the partisans of the explanation of
radioactivity by atomic energy.

CHAPTER X

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.

CHAPTER XI

THE FUTURE OF PHYSICS

It would doubtless be exceedingly rash, and certainly very
presumptuous, to seek to predict the future which may be reserved for
physics. The rôle of prophet is not a scientific one, and the most
firmly established previsions of to-day may be overthrown by the
reality of to-morrow.

Nevertheless, the physicist does not shun an extrapolation of some
little scope when it is not too far from the realms of experiment; the
knowledge of the evolution accomplished of late years authorises a few
suppositions as to the direction in which progress may continue.

The reader who has deigned to follow me in the rapid excursion we have
just made through the domain of the science of Nature, will doubtless
bring back with him from his short journey the general impression that
the ancient limits to which the classic treatises still delight in
restricting the divers chapters of physics, are trampled down in all
directions.

The fine straight roads traced out by the masters of the last century,
and enlarged and levelled by the labour of such numbers of workmen,
are now joined together by a crowd of small paths which furrow the
field of physics. It is not only because they cover regions as yet
little explored where discoveries are more abundant and more easy,
that these cross-cuts are so frequent, but also because a higher hope
guides the seekers who engage in these new routes.

In spite of the repeated failures which have followed the numerous
attempts of past times, the idea has not been abandoned of one day
conquering the supreme principle which must command the whole of
physics.

Some physicists, no doubt, think such a synthesis to be impossible of
realisation, and that Nature is infinitely complex; but,
notwithstanding all the reserves they may make, from the philosophical
point of view, as to the legitimacy of the process, they do not
hesitate to construct general hypotheses which, in default of complete
mental satisfaction, at least furnish them with a highly convenient
means of grouping an immense number of facts till then scattered
abroad.

Their error, if error there be, is beneficial, for it is one of those
that Kant would have classed among the fruitful illusions which
engender the indefinite progress of science and lead to great and
important co-ordinations.

It is, naturally, by the study of the relations existing between
phenomena apparently of very different orders that there can be any
hope of reaching the goal; and it is this which justifies the peculiar
interest accorded to researches effected in the debatable land between
domains hitherto considered as separate.

Among all the theories lately proposed, that of the ions has taken a
preponderant place; ill understood at first by some, appearing
somewhat singular, and in any case useless, to others, it met at its
inception, in France at least, with only very moderate favour.

To-day things have greatly changed, and those even who ignored it have
been seduced by the curious way in which it adapts itself to the
interpretation of the most recent experiments on very different
subjects. A very natural reaction has set in; and I might almost say
that a question of fashion has led to some exaggerations.

The electron has conquered physics, and many adore the new idol rather
blindly. Certainly we can only bow before an hypothesis which enables
us to group in the same synthesis all the discoveries on electric
discharges and on radioactive substances, and which leads to a
satisfactory theory of optics and of electricity; while by the
intermediary of radiating heat it seems likely to embrace shortly the
principles of thermodynamics also. Certainly one must admire the power
of a creed which penetrates also into the domain of mechanics and
furnishes a simple representation of the essential properties of
matter; but it is right not to lose sight of the fact that an image
may be a well-founded appearance, but may not be capable of being
exactly superposed on the objective reality.

The conception of the atom of electricity, the foundation of the
material atoms, evidently enables us to penetrate further into
Nature's secrets than our predecessors; but we must not be satisfied
with words, and the mystery is not solved when, by a legitimate
artifice, the difficulty has simply been thrust further back. We have
transferred to an element ever smaller and smaller those physical
qualities which in antiquity were attributed to the whole of a
substance; and then we shifted them later to those chemical atoms
which, united together, constitute this whole. To-day we pass them on
to the electrons which compose these atoms. The indivisible is thus
rendered, in a way, smaller and smaller, but we are still unacquainted
with what its substance may be. The notion of an electric charge which
we substitute for that of a material mass will permit phenomena to be
united which we thought separate, but it cannot be considered a
definite explanation, or as the term at which science must stop. It is
probable, however, that for a few years still physics will not travel
beyond it. The present hypothesis suffices for grouping known facts,
and it will doubtless enable many more to be foreseen, while new
successes will further increase its possessions.

Then the day will arrive when, like all those which have shone before
it, this seductive hypothesis will lead to more errors than
discoveries. It will, however, have been improved, and it will have
become a very vast and very complete edifice which some will not
willingly abandon; for those who have made to themselves a comfortable
dwelling-place on the ruins of ancient monuments are often too loth to
leave it.

In that day the searchers who were in the van of the march after truth
will be caught up and even passed by others who will have followed a
longer, but perhaps surer road. We also have seen at work those
prudent physicists who dreaded too daring creeds, and who sought only
to collect all the documentary evidence possible, or only took for
their guide a few principles which were to them a simple
generalisation of facts established by experiments; and we have been
able to prove that they also were effecting good and highly useful
work.

Neither the former nor the latter, however, carry out their work in an
isolated way, and it should be noted that most of the remarkable
results of these last years are due to physicists who have known how
to combine their efforts and to direct their activity towards a common
object, while perhaps it may not be useless to observe also that
progress has been in proportion to the material resources of our
laboratories.

It is probable that in the future, as in the past, the greatest
discoveries, those which will suddenly reveal totally unknown regions,
and open up entirely new horizons, will be made by a few scholars of
genius who will carry on their patient labour in solitary meditation,
and who, in order to verify their boldest conceptions, will no doubt
content themselves with the most simple and least costly experimental
apparatus. Yet for their discoveries to yield their full harvest, for
the domain to be systematically worked and desirable results obtained,
there will be more and more required the association of willing minds,
the solidarity of intelligent scholars, and it will be also necessary
for these last to have at their disposal the most delicate as well as
the most powerful instruments. These are conditions paramount at the
present day for continuous progress in experimental science.

If, as has already happened, unfortunately, in the history of science,
these conditions are not complied with; if the freedoms of the workers
are trammelled, their unity disturbed, and if material facilities are
too parsimoniously afforded them,--evolution, at present so rapid, may
be retarded, and those retrogressions which, by-the-by, have been
known in all evolutions, may occur, although even then hope in the
future would not be abolished for ever.

There are no limits to progress, and the field of our investigations
has no boundaries. Evolution will continue with invincible force. What
we to-day call the unknowable, will retreat further and further before
science, which will never stay her onward march. Thus physics will
give greater and increasing satisfaction to the mind by furnishing new
interpretations of phenomena; but it will accomplish, for the whole of
society, more valuable work still, by rendering, by the improvements
it suggests, life every day more easy and more agreeable, and by
providing mankind with weapons against the hostile forces of Nature.