CATHODE RAYS AND RADIOACTIVE BODIES
§ 1. THE CATHODE RAYS
A wire traversed by an electric current is, as has just been
explained, the seat of a movement of electrons. If we cut this wire, a
flood of electrons, like a current of water which, at the point where
a pipe bursts, flows out in abundance, will appear to spring out
between the two ends of the break.
If the energy of the electrons is sufficient, these electrons will in
fact rush forth and be propagated in the air or in the insulating
medium interposed; but the phenomena of the discharge will in general
be very complex. We shall here only examine a particularly simple
case, viz., that of the cathode rays; and without entering into
details, we shall only note the results relating to these rays which
furnish valuable arguments in favour of the electronic hypothesis and
supply solid materials for the construction of new theories of
electricity and matter.
For a long time it was noticed that the phenomena in a Geissler tube
changed their aspect considerably, when the gas pressure became very
weak, without, however, a complete vacuum being formed. From the
cathode there is shot forth normally and in a straight line a flood
within the tube, dark but capable of impressing a photographic plate,
of developing the fluorescence of various substances (particularly the
glass walls of the tube), and of producing calorific and mechanical
effects. These are the cathode rays, so named in 1883 by E. Wiedemann,
and their name, which was unknown to a great number of physicists till
barely twelve years ago, has become popular at the present day.
About 1869, Hittorf made an already very complete study of them and
put in evidence their principal properties; but it was the researches
of Sir W. Crookes in especial which drew attention to them. The
celebrated physicist foresaw that the phenomena which were thus
produced in rarefied gases were, in spite of their very great
complication, more simple than those presented by matter under the
conditions in which it is generally met with.
He devised a celebrated theory no longer admissible in its entirety,
because it is not in complete accord with the facts, which was,
however, very interesting, and contained, in germ, certain of our
present ideas. In the opinion of Crookes, in a tube in which the gas
has been rarefied we are in presence of a special state of matter. The
number of the gas molecules has become small enough for their
independence to be almost absolute, and they are able in this
so-called radiant state to traverse long spaces without departing
from a straight line. The cathode rays are due to a kind of molecular
bombardment of the walls of the tubes, and of the screens which can be
introduced into them; and it is the molecules, electrified by their
contact with the cathode and then forcibly repelled by electrostatic
action, which produce, by their movement and their _vis viva_, all the
phenomena observed. Moreover, these electrified molecules animated
with extremely rapid velocities correspond, according to the theory
verified in the celebrated experiment of Rowland on convection
currents, to a true electric current, and can be deviated by a magnet.
Notwithstanding the success of Crookes' experiments, many physicists--
the Germans especially--did not abandon an hypothesis entirely
different from that of radiant matter. They continued to regard the
cathode radiation as due to particular radiations of a nature still
little known but produced in the luminous ether. This interpretation
seemed, indeed, in 1894, destined to triumph definitely through the
remarkable discovery of Lenard, a discovery which, in its turn, was to
provoke so many others and to bring about consequences of which the
importance seems every day more considerable.
Professor Lenard's fundamental idea was to study the cathode rays
under conditions different from those in which they are produced.
These rays are born in a very rarefied space, under conditions
perfectly determined by Sir W. Crookes; but it was a question whether,
when once produced, they would be capable of propagating themselves in
other media, such as a gas at ordinary pressure, or even in an
absolute vacuum. Experiment alone could answer this question, but
there were difficulties in the way of this which seemed almost
insurmountable. The rays are stopped by glass even of slight
thickness, and how then could the almost vacuous space in which they
have to come into existence be separated from the space, absolutely
vacuous or filled with gas, into which it was desired to bring them?
The artifice used was suggested to Professor Lenard by an experiment
of Hertz. The great physicist had, in fact, shortly before his
premature death, taken up this important question of the cathode rays,
and his genius left there, as elsewhere, its powerful impress. He had
shown that metallic plates of very slight thickness were transparent
to the cathode rays; and Professor Lenard succeeded in obtaining
plates impermeable to air, but which yet allowed the pencil of cathode
rays to pass through them.
Now if we take a Crookes tube with the extremity hermetically closed
by a metallic plate with a slit across the diameter of 1 mm. in width,
and stop this slit with a sheet of very thin aluminium, it will be
immediately noticed that the rays pass through the aluminium and pass
outside the tube. They are propagated in air at atmospheric pressure,
and they can also penetrate into an absolute vacuum. They therefore
can no longer be attributed to radiant matter, and we are led to think
that the energy brought into play in this phenomenon must have its
seat in the light-bearing ether itself.
But it is a very strange light which is thus subject to magnetic
action, which does not obey the principle of equal angles, and for
which the most various gases are already disturbed media. According to
Crookes it possesses also the singular property of carrying with it
electric charges.
This convection of negative electricity by the cathode rays seems
quite inexplicable on the hypothesis that the rays are ethereal
radiations. Nothing then remained in order to maintain this
hypothesis, except to deny the convection, which, besides, was only
established by indirect experiments. That the reality of this
transport has been placed beyond dispute by means of an extremely
elegant experiment which is all the more convincing that it is so very
simple, is due to M. Perrin. In the interior of a Crookes tube he
collected a pencil of cathode rays in a metal cylinder. According to
the elementary principles of electricity the cylinder must become
charged with the whole charge, if there be one, brought to it by the
rays, and naturally various precautions had to be taken. But the
result was very precise, and doubt could no longer exist--the rays
were electrified.
It might have been, and indeed was, maintained, some time after this
experiment was published, that while the phenomena were complex inside
the tube, outside, things might perhaps occur differently. Lenard
himself, however, with that absence of even involuntary prejudice
common to all great minds, undertook to demonstrate that the opinion
he at first held could no longer be accepted, and succeeded in
repeating the experiment of M. Perrin on cathode rays in the air and
even _in vacuo_.
On the wrecks of the two contradictory hypotheses thus destroyed, and
out of the materials from which they had been built, a theory has been
constructed which co-ordinates all the known facts. This theory is
furthermore closely allied to the theory of ionisation, and, like this
latter, is based on the concept of the electron. Cathode rays are
electrons in rapid motion.
The phenomena produced both inside and outside a Crookes tube are,
however, generally complex. In Lenard's first experiments, and in many
others effected later when this region of physics was still very
little known, a few confusions may be noticed even at the present day.
At the spot where the cathode rays strike the walls of the tube the
essentially different X rays appear. These differ from the cathode
radiations by being neither electrified nor deviated by a magnet. In
their turn these X rays may give birth to the secondary rays of M.
Sagnac; and often we find ourselves in presence of effects from these
last-named radiations and not from the true cathode rays.
The electrons, when they are propagated in a gas, can ionise the
molecules of this gas and unite with the neutral atoms to form
negative ions, while positive ions also appear. There are likewise
produced, at the expense of the gas still subsisting after
rarefication within the tube, positive ions which, attracted by the
cathode and reaching it, are not all neutralised by the negative
electrons, and can, if the cathode be perforated, pass through it, and
if not, pass round it. We have then what are called the canal rays of
Goldstein, which are deviated by an electric or magnetic field in a
contrary direction to the cathode rays; but, being larger, give weak
deviations or may even remain undeviated through losing their charge
when passing through the cathode.
It may also be the parts of the walls at a distance from the cathode
which send a positive rush to the latter, by a similar mechanism. It
may be, again, that in certain regions of the tube cathode rays are
met with diffused by some solid object, without having thereby changed
their nature. All these complexities have been cleared up by M.
Villard, who has published, on these questions, some remarkably
ingenious and particularly careful experiments.
M. Villard has also studied the phenomena of the coiling of the rays
in a field, as already pointed out by Hittorf and Plücker. When a
magnetic field acts on the cathode particle, the latter follows a
trajectory, generally helicoidal, which is anticipated by the theory.
We here have to do with a question of ballistics, and experiments duly
confirm the anticipations of the calculation. Nevertheless, rather
singular phenomena appear in the case of certain values of the field,
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.