RADIATION OF THE NEW RADIO-ACTIVE SUBSTANCES.
_Methods of Investigation of the Radiation._
In order to investigate the radiation emitted by radio-active bodies,
any one of the properties of this radiation can be utilised. Thus the
action of the rays on photographic plates may serve, or their property
of ionisation of the air, which renders it a conductor, or their
capacity for causing fluorescence of certain bodies. Henceforth, in
speaking of these different methods of working, I shall use the
expressions radiographic method, electrical method, fluoroscopic method.
The first two have been used from the beginning in the study of uranium
rays; the fluoroscopic method can only be applied in the case of the new
bodies which are strongly radio-active, for the feebly active bodies
such as uranium and thorium produce no appreciable fluorescence. The
electrical method is the only one which serves for exact determinations
of intensity; the other two are specially adapted for giving qualitative
results, and only furnish rough approximations. The results obtained
with the three methods just considered are not strictly comparable the
one with the other. The sensitive plate, the gas which is ionised, the
fluorescent screen, are in reality receivers, which absorb the energy of
the radiation, and transform it into another kind of energy, chemical
energy, ionic energy, or luminous energy. Each receiver absorbs a
fraction of the radiation, which depends essentially upon its nature.
Later on, we shall see that the radiation is complex, that the fractions
of the radiation absorbed by the different receivers may differ among
themselves both quantitatively and qualitatively. Finally, it is neither
evident, nor even probable, that the energy absorbed is entirely
transformed by the receiver into the form that we wish for observation;
part of this energy may be transformed into heat, into the evolution of
secondary radiations which may or may not assist in the production of
the observed phenomenon, into chemical action which differs from that
under observation, &c., and here also the effective action of the
receiver, with reference to the end we have in view, depends essentially
upon the nature of that receiver.
Let us compare two radio-active substances, one containing radium and
the other polonium, and which show an equal degree of activity in the
condenser of Fig. 1. If each is covered with a thin leaf of aluminium,
the second appears considerably less active than the first, and the same
is the case when they are placed under the same fluorescent screen, if
the latter is of sufficient thickness, or is placed at a certain
distance from the two radio-active bodies.
_Energy of Radiation._
Whatever be the method of research employed, the energy of radiation of
the new radio-active substances is always found to be considerably
greater than that of uranium and thorium. Thus it is that, at a short
distance, they act instantaneously upon a photographic plate, whereas an
exposure of twenty-four hours is necessary when operating with uranium
and thorium. A fluorescent screen is vividly illuminated by contact with
the new radio-active bodies, whilst no trace of luminosity is visible
with uranium and thorium. Finally, the ionising action upon air is
considerably stronger in the ratio of 10^6 approximately. But it is,
strictly speaking, not possible to estimate the _total intensity of the
radiation_, as in the case of uranium, by the electrical method
described at the beginning (Fig. 1). With uranium, for example, the
radiation is almost completely absorbed by the layer of air between the
plates, and the limiting current is reached at a tension of 100 volts.
But the case is different for strongly radio-active bodies. One portion
of the radiation of radium consists of very penetrating rays, which
penetrate the condenser and the metallic plates, and are not utilised in
ionising the air between the plates. Further, the limiting current
cannot always be obtained for the tensions supplied; for example, with
very active polonium the current remains proportional to the tension
between 100 and 500 volts. Therefore the experimental conditions which
give a simple interpretation are not realised, and, consequently, the
numbers obtained cannot be taken as representing the measurement of the
total radiation; they merely point to a rough approximation.
_Complex Nature of the Radiation._
The researches of various physicists (MM. Becquerel, Meyer and von
Schweidler, Giesel, Villard, Rutherford, M. and Mdme. Curie) have proved
the complex nature of the radiation of radio-active bodies. It will be
convenient to specify three kinds of rays, which I shall denote,
according to the notation adopted by Mr. Rutherford, by the letters α,
β, γ.
I. The α-rays are very slightly penetrating, and appear to constitute
the principal part of the radiation. These rays are characterised by the
laws by which they are absorbed by matter. The magnetic field acts very
slightly upon them, and they were formerly thought to be quite
unaffected by the action of this field. However, in a strong magnetic
field, the α-rays are slightly deflected; the deflection is caused in
the same manner as with cathode rays, but the direction of the
deflection is reversed; it is the same as for the canal rays of the
Crookes tubes.
II. The β-rays are less absorbable as a whole than the preceding ones.
They are deflected by a magnetic field in the same manner and direction
as cathode rays.
III. The γ-rays are penetrating rays, unaffected by the magnetic field,
and comparable to Röntgen rays.
[Illustration: FIG. 4.]
Consider the following imaginary experiment:—Some radium, R, is placed
at the bottom of a small deep cavity, hollowed in a block of lead, P
(Fig. 4). A sheaf of rays, rectilinear and slightly expanded, streams
from the receptacle. Let us suppose that a strong uniform magnetic field
is established in the neighbourhood of the receptacle, normal to the
plane of the figure and directed towards the back. The three groups of
rays, α, β, γ, will now be separated. Then rather faint γ-rays continue
in their straight path without a trace of deviation. The β-rays are
deflected in the manner of cathode rays, and describe circular paths in
the plane of the figure. If the receptacle is placed on a photographic
plate, A C, the portion, B C, of the plate which receives the β-rays is
acted upon. Lastly, the α-rays form a very intense shaft which is
slightly deflected, and which is soon absorbed by the air. These rays
describe in the plane of the figure a path of great curvature, the
direction of the deflection being the reverse of that with the β-rays.
If the receptacle is covered with a thin sheet of aluminium (0·1 m.m.
thick), the α-rays are suppressed almost entirely, the β-rays are
lessened, and the γ-rays do not appear to be absorbed to any great
extent.
_Action of the Magnetic Field._
We have seen that the rays emitted by radio-active bodies have many
properties common to cathode rays and to Röntgen rays. Cathode rays, as
well as Röntgen rays, ionise the air, act on photographic plates, cause
fluorescence, undergo no regular deflection. But the cathode rays differ
from Röntgen rays in being deflected from their rectilinear path by the
action of the magnetic field, and in the transportation of charges of
negative electricity.
The fact that the magnetic field acts upon the rays emitted by
radio-active substances was discovered almost simultaneously by MM.
Giesel, Meyer and von Schweidler, and Becquerel. These physicists
observed that the rays of radio-active substances are deflected by the
magnetic field in the same manner and direction as the cathode rays;
their observations were in relation to the β-rays.
M. Curie demonstrated that the radiation of radium comprises two groups
of quite distinct rays, of which one is readily deflected by the
magnetic field (β-rays), whilst the other seems to be unaffected by the
action of this field (α- and γ-rays).
M. Becquerel did not find that the specimens of polonium prepared by us
emitted rays of the cathode kind. On the contrary, he first noticed the
effect of the magnetic field on a specimen of polonium prepared by
himself. None of the polonium prepared by us ever gave rise to rays of
the cathode order.
The polonium of M. Giesel only gives rise to these rays when recently
prepared, and it is probable that the emission is due to the phenomenon
of induced radio-activity of which we shall speak later.
The following are experiments which prove that one portion of the
radiation of radium, and one portion only, consists of easily deflected
rays (β-rays). These experiments were done according to the electrical
method.
The radio-active body A (Fig. 5) sends forth radiations in the direction
A D between the plates P and P′. The plate P is now at a potential of
500 volts, plate P′ is connected to an electrometer and to a quartz
electric piezometer. The intensity of the current passing through the
air under the influence of the radiations is measured. The magnetic
field can be established at will perpendicular to the plane of the
figure over the whole region E E E E. If the rays are deflected, even
slightly, they no longer pass between the plates, and the current is
suppressed. The region of the passage of the rays is surrounded with
masses of lead, B, B′, B″, and by the armatures of the electro-magnet;
when the rays are deflected, they are absorbed by the masses of lead B
and B′.
[Illustration: FIG. 5.]
The results obtained depend essentially on the distance, A D, of the
radiating substance, A, from the condenser at D. If the distance A D is
great enough (greater than 7 c.m.), most of the radium rays (90 to 100
per cent) arriving at the condenser are deflected and suppressed for a
field of 2500 units. These are the β-rays. If the distance A D is less
than 65 m.m., a smaller part of the rays are deflected by the action of
the field; this portion is also entirely deflected by a field of 2500
units, and the proportion of the rays suppressed is not increased by
increasing the field from 2500 to 7000 units.
The proportion of the rays not suppressed by the field increases with
decrease of the distance, A D, between the radiating body and the
condenser. For small distances, the rays which can be easily deflected
form a very small fraction of the total radiation. The penetrating rays
are therefore, for the most part, deviable rays of the cathode order
(β-rays).
Under the experimental conditions just described, the action of the
magnetic field on the α-rays could not be well observed for the fields
employed. The chief radiation, apparently undergoing no deflection,
observed at a short distance from the radiating source, consisted of
α-rays; the undeflected radiation observed at a greater distance
consisted of γ-rays.
If an absorbing lamina (aluminium or black paper) is placed in the path
of the bundle of rays, those which pass through are nearly all deflected
by the field in such a way that, with the aid of the screen and the
magnetic field, almost all the radiation is suppressed in the condenser,
the remainder being due to the γ-rays, the proportion of which is small.
The α-rays are absorbed by the screen.
An aluminium plate of 1/100 m.m. thickness is sufficient for the
suppression of almost all the rays not readily deflected when the
substance is far enough from the condenser; for smaller distances (34
m.m. and 51 m.m.) two pieces of this aluminium foil are necessary to
give the same result.
Similar determinations were made with four substances containing radium
(chlorides or carbonates) of very different activity; analogous results
were obtained.
It may be remarked that, in all cases, the penetrating rays deflected by
the magnet (β-rays) form only a small fraction of the total radiation;
they influence but slightly the determinations in which the whole
radiation is made use of to produce conductivity of the air.
The radiation emitted by polonium may be studied by the electrical
method. When the distance, A D, of the polonium from the condenser is
varied, no current is observed at first while the distance is fairly
great; on nearing the polonium, the radiation suddenly becomes manifest
with great intensity; the current then increases uniformly whilst
approaching the polonium, but the magnetic field produces no appreciable
effect under these conditions. The radiation of polonium is apparently
limited in space, and does not pass into the air beyond a kind of sheath
surrounding the substance to a thickness of several centimetres.
The interpretation of the experiments I have just described must be
accompanied by some important general reservations. In speaking of the
proportion of the rays deflected by the magnet, I refer only to that
portion of the radiation capable of causing a current in the condenser.
In employing the fluorescent action of the Becquerel rays, or their
action on photographic plates, the proportion would probably be
different—a measure of intensity having, as a rule, no meaning except
for the method of measurement adopted.
The rays of polonium are α-rays. In the experiments just described, I
observed no action of the magnetic field upon them, but the experimental
conditions were such that a slight deflection would pass unnoticed.
The experiments made by the radiographic method confirmed the preceding
results. Taking radium as the source of radiation, and receiving the
impression on a plate parallel to the primitive shaft and normal to the
field, a very clear print is obtained of two shafts separated by the
action of the field, the one deflected, the other not deflected. The
β-rays constitute the deflected beam; the α-rays, being very slightly
deflected, are not to be distinguished from the undeflected bundle of
the γ-rays.
_Deflected β-Rays._
The experiments of M. Giesel and MM. Meyer and von Schweidler showed
that the radiation of the radio-active bodies is, in part at least,
deflected by a magnetic field, and that this deflection resembles that
of the cathode rays. M. Becquerel investigated the action of the field
on the rays by the radiographic method. The experimental arrangement was
that of Fig. 4. The radium was placed in the lead receptacle, P, and
this receptacle was placed on the sensitive face of a photographic
plate, A C, covered with black paper. The whole was placed between the
poles of an electro-magnet, the magnetic field being normal to the plane
of the figure.
If the field is directed to the back of this plane, the part B C of the
plate is acted upon by rays which, after having described circular
paths, return to the plate and strike it at a right angle. These rays
are β-rays.
M. Becquerel has demonstrated that the impression consists of a wide
diffused band, a continuous spectrum indeed, showing that the sheaf of
deviable rays emitted by the source is formed of an infinite number of
radiations unequally deflected. If the gelatin of the plate be covered
with different absorbent screens (paper, glass, metals), one portion of
the spectrum is suppressed, and it is found that the rays most deflected
by the magnetic field—otherwise those which have the smallest radius of
curvature—are the most completely absorbed. With each screen, the
impression on the plate begins at a certain distance from the source of
radiation, this distance being proportional to the absorptive power of
the screen.
_Charge of the Deflected Rays._
The cathode rays are, as shown by M. Perrin, charged with negative
electricity. Further, according to the experiments of M. Perrin and M.
Lenard, they are capable of carrying their charge through the metallic
envelopes connected to earth and through isolating screens. At every
point where the cathode rays are absorbed, there is a continuous
evolution of negative electricity. We have proved that the same is the
case for the deflected β-rays of radium. _The deviable β-rays of radium
are charged with negative electricity._
(NOTE.—Let the radio-active substance be placed on one of the plates of
a condenser, this plate being connected to earth; the second plate is
connected to an electrometer, it receives and absorbs the rays emitted
by the substance. If the rays are charged, a continuous flow of
electricity into the electrometer should be observed. In this
experiment, carried out in air, we were not able to detect a charge
accompanying the rays, but such an experiment is not delicate. The air
between the plates being caused by the rays to conduct, the electrometer
is no longer isolated, and can only respond to charges if these be
sufficiently strong. In order that the α-rays may not interfere with the
experiment, they may be suppressed by covering the source of radiation
with a thin metallic screen. We repeated this experiment, without more
success, by causing the rays to pass through the interior of a Faraday
cylinder in connection with the electrometer).
According to the preceding experiments, it was evident that the charge
of the rays of the radiating body employed was a weak one.
In order to fix a feeble evolution of electricity upon the conductor
which absorbs the rays, this conductor should be completely insulated;
this is effected by screening it from the air, either by placing it in a
tube with a very perfect vacuum, or by surrounding it with a good solid
dielectric. We employed the latter arrangement.
[Illustration: FIG. 6.]
A conducting disc, M M (Fig. 6), is connected by the wire, _t_, to the
electrometer; the disc and wire are completely enveloped by the
insulating substance _i i i i_; the whole is again surrounded with the
metallic covering, E E E E, which is in electric connection with the
earth. The insulator, _p p_, and the metallic envelope are very thin
upon one of the faces of the disc. This face is exposed to the radiation
of the barium and radium salt, R, placed outside in a lead receptacle.
The rays emitted by the radium penetrate the metallic envelope and the
insulating lamina, _p p_, and are absorbed by the metallic disc, M M.
The latter then becomes the source of a continuous evolution of negative
electricity, as determined by the electrometer, and is measured by means
of a quartz piezometer.
The current thus created is very weak. With very active barium-radium
chloride, forming a layer of 2·5 sq. c.m. in area, and of 0·2 c.m. in
thickness, a current of magnitude 10^{–11} ampères is obtained, the rays
utilised having traversed, before being absorbed by the disc M M, a
thickness of aluminium of 0·01 m.m., and a thickness of ebonite of 0·3
m.m.
We used successively lead, copper, and zinc for the disc M M, ebonite
and paraffin for the insulator; the results obtained were the same.
The current diminishes with increasing distance from the source of
radiation, R, also when a less active product is used.
We obtained the same results again when the disc M M is replaced by a
Faraday cylinder filled with air, and covered outside with insulating
material. The opening of the cylinder, closed by the thin insulating
plate, _p p_, was opposite the radiating source.
[Illustration: FIG. 7.]
Finally, we made the inverse experiment, which was to place the lead
receptacle with the radium in the centre of the insulating material and
in connection with the electrometer (Fig. 7), the whole being surrounded
with the metallic covering connected to earth.
Under these conditions, it is evident from the electrometer that the
radium has a positive charge equal in magnitude to the negative charge
of the former experiment. The radium rays penetrate the thin dielectric
plate, _p p_, and leave the conductor inside carrying with them negative
electricity.
The α-rays of radium do not interfere in these experiments, being almost
completely absorbed by a very thin layer of matter. The method just
described is not suitable for the study of the charge of the rays of
polonium, these rays very slightly penetrating. We observed no
indication of any charge in the case of polonium, which gives rise to
α-rays only; but, for the reason just given, no conclusion can be drawn
from this.
Thus, in the case of the deflected β-rays of radium, as in the case of
cathode rays, the rays carry a charge of electricity. But, hitherto, the
existence of electric charges uncombined with matter has been unknown.
In the study of the emission of the β-rays of radium, we are therefore
led to make use of the theory which is in vogue for the study of cathode
rays. In this ballistic theory, formulated by Sir William Crookes, since
developed and completed by Prof. J. J. Thomson, the cathode rays consist
of extremely minute particles, which are hurled from the cathode with
great velocity, and which are charged with negative electricity. We
might similarly conceive that radium sends into space negatively
electrified particles.
A specimen of radium, enclosed in a solid thin perfectly insulated
envelope, should become spontaneously charged to a very high potential.
By the ballistic hypothesis the potential would increase until the
potential difference of the surrounding conductors became sufficient to
hinder the ejection of the electrified particles and to cause their
return to the source of radiation.
We have performed an experiment on these lines. A specimen of very
active radium was enclosed for some time in a glass vessel. In order to
open the vessel, we made a trace on the glass with a glass cutter.
Whilst so doing, we clearly heard the report of a spark, and upon
examining the vessel with a magnifying glass, we observed that the glass
had been pierced by a spark at the spot where it had been weakened by
the scratch. The phenomenon produced is comparable to the rupture of the
glass of an overcharged Leyden jar.
The same phenomenon occurred with another glass. Further, at the moment
of the passing of the spark, M. Curie, who was holding the glass, felt
the electric shock of discharge in his fingers.
Certain kinds of glass have good insulating properties. If the radium is
enclosed in a sealed glass vessel, well insulated, it is to be expected
that, at a given moment, the vessel will be spontaneously perforated.
_Radium is the first example of a body which is spontaneously charged
with electricity._
_Action of the Electric Field upon the Deflected β-Rays of Radium._
The β-rays of radium, being analogous to the cathode rays, should be
deflected by an electric field in a manner similar to the latter;
_i.e._, as would a particle of matter negatively charged and hurled into
space with a great velocity. The existence of such a deflection has been
demonstrated both by M. Dorn and M. Becquerel.
Let us consider the case of a ray which traverses the space situated
between the two plates of a condenser. Suppose the direction of the ray
parallel to the plates: when an electric field is established between
the latter, the ray is subjected to the action of this uniform field
along its whole path in the condenser l. By reason of this action the
ray is deflected towards the positive plate and describes the arc of a
parabola; on leaving the field, it continues its path in a straight
line, following the tangent to the arc of the parabola at the point of
exit. The ray can be received on a photographic plate perpendicular to
its original direction. Observations are taken of the impression
produced on the plate when the field is zero, and when it has a known
value, and from that is deduced the value of the deflection, δ, which is
the distance of the points in which the new direction of the ray and its
original direction meet a common plane perpendicular to the original
direction. If h is the distance of this plane from the condenser,
_i.e._, at the edge of the field, we have, by a simple calculation,—
_e_F_l_ (_l_/2 + _h_)
δ = —————————————————————;
_mv_^2
_m_ being the mass of the moving particles, _e_ its charge, _v_ its
velocity, and F the strength of the field.
The experiments of M. Becquerel enable him to assign a value approaching
to δ.
_Relation of the Charge to the Mass for a Particle Negatively Charged
Emitted by Radium._
When a material particle having a mass _m_ and a negative charge _e_, is
projected with a velocity _v_ into a uniform magnetic field
perpendicular to its initial velocity, this particle describes, in a
plane normal to the field and passing through its initial velocity, an
arc of a circle of radius ρ, so that—H being the strength of the
field—we have the relation—
_m_
Hρ = ——— _v_.
_e_
If, for the same ray, the deflection, δ, and the radius of curvature, ρ,
be measured in a magnetic field, values could be found from these two
experiments for the ratio _e_/_m_ and for the velocity, _v_.
The experiments of M. Becquerel threw the first light upon this subject.
They gave for the ratio _e_/_m_ a value approximately equal to 10^7
absolute electro-magnetic units, and for v a magnitude of 1·6 × 10^{10}.
These values are of the same order of magnitude as those of the cathode
rays.
Accurate experiments have been made on the same subject by M. Kaufmann.
This physicist subjected a narrow beam of radium rays to the
simultaneous action of an electric field and a magnetic field, the two
fields being uniform and having a similar direction, normal to the
original direction of the beam. The impression produced on a plate
normal to the primitive beam and placed beyond the limits of the field
with reference to the source, has the form of a curve, each point of
which corresponds to one of the original beam. The most penetrating and
least deflected rays are at the same time those with the greatest
velocity.
It follows from the experiments of M. Kaufmann, that for the radium
rays, of which the velocity is considerably greater than that of the
cathode rays, the ratio _e_/_m_ decreases, while the velocity increases.
According to the researches of J. J. Thomson and Townsend, we may assume
that the moving particle, which constitutes the ray, possesses a charge,
_e_, equal to that carried by an atom of hydrogen during electrolysis,
this charge being the same for all the rays. We are therefore led to the
conclusion that the mass of the particle, _m_, increases with increase
of velocity.
These theoretical considerations lead to the idea that the inertia of
the particle is due to its state of charge during motion, the velocity
of an electric charge in motion being incapable of modification without
expenditure of energy. To state it otherwise, the inertia of the
particle is of electro-magnetic origin, and the mass of the particle
is—in part at least—a virtual mass or an electro-magnetic mass. M.
Abraham goes further, and assumes that the mass of the particle is
entirely an electro-magnetic mass. If, according to this hypothesis, the
value of this mass, _m_, be calculated for a known velocity, _v_, we
find that _m_ approaches infinity when _v_ approaches the velocity of
light, and that _m_ approaches a constant value when the velocity, _v_,
is much less than that of light. The experiments of M. Kaufmann are in
agreement with the results of this theory, the importance of which is
great because it foreshadows the possibility of establishing mechanical
bases upon the dynamical of little particles of matter charged in a
state of motion.
These are the figures obtained by M. Kaufmann for _e_/_m_ and _v_.
_e_/_m_ _v_c.m./sec. │
Electro-magnetic units. │
1·865 × 10^7 0·7 × 10^{10}│For cathode rays (Simon).
──────────────────────────────────────┼───────────────────────────
1·31 × 10^7 2·36 × 10^{10}│For radium rays (Kaufmann).
1·17 × 10^7 2·48 × 10^{10}│ 〃
0·97 × 10^7 2·59 × 10^{10}│ 〃
0·77 × 10^7 2·72 × 10^{10}│ 〃
0·63 × 10^7 2·83 × 10^{10}│ 〃
M. Kaufmann concludes, from comparison of his experiments with the
theory, that the limiting value of the ratio _e_/_m_ for radium rays of
relatively small velocity would be the same as the value _e_/_m_ for
cathode rays.
The most complete experiments of M. Kaufmann were made with a minute
quantity of pure radium chloride, with which we provided him.
According to M. Kaufmann’s experiments, certain β-rays of radium possess
a velocity very near to that of light. These rapid rays seem to possess
great penetrating capacity towards matter.
_Action of the Magnetic Field upon the α-Rays._
In a recent work, Mr. Rutherford announced that, in a powerful electric
or magnetic field, the α-rays of radium are slightly deflected, in the
manner of particles positively electrified and possessing great
velocity. Mr. Rutherford concludes from his experiments that the
velocity of the α-rays is of the order of magnitude 2·5 × 10^9c.m./sec.
and that the ratio _e_/_m_ for these rays is of the order of magnitude 6
× 10^3, which is 10^4 times as great as for the deflected β-rays. We
shall see later that these conclusions of Mr. Rutherford are in
agreement with the properties already known of the α-radiation, and that
they account, in part at least, for the law of absorption of this
radiation.
The experiments of Mr. Rutherford have been confirmed by M. Becquerel.
M. Becquerel has further demonstrated that polonium rays behave in a
magnetic field like the α-rays of radium, and that, for the same field,
they seem to have the same curvature as the latter.
It also appears from M. Becquerel’s experiments that the α-rays do not
form a magnetic spectrum, but act rather like a homogeneous radiation,
all the rays being equally deflected.
_Action of the Magnetic Field on the Rays of other Radio-active
Substances._
We have just seen that radium gives off α-rays comparable to the tube
rays, β-rays comparable to cathode rays, and γ-rays which are
penetrating and not deflected. Polonium gives off α-rays only. Amongst
the other radio-active substances, actinium seems to behave like radium,
but the study of its radiation has not yet advanced so far as in the
case of radium. As regards the faintly radio-active bodies, we know
to-day that uranium and thorium give rise to α-rays as well as β-rays
(Becquerel, Rutherford).
_Proportion of β-Rays in the Radiation of Radium._
As I have already mentioned, the proportion of β-rays increases with
increase of distance from the source of radiation. These rays never
occur alone, and for great distances the presence of γ-rays is always
discernible. The presence of very penetrating, undeflected rays in the
radiation of radium was first observed by M. Villard. These rays
constitute only a small portion of the radiation measured by the
electrical method, and their presence escaped our notice in our first
experiments, so that we believed falsely that the radiation at great
distances contained only rays capable of deflection.
The following are the numerical results obtained with experiments made
by the electrical method with an apparatus similar to that of Fig. 5.
The radium was only separated from the condenser by the surrounding air.
I shall indicate by the letter _d_ the distance from the source of
radiation to the condenser. The numbers of the second line represent the
current subsisting when the magnetic field is acting, supposing the
current obtained with no field equal to 100 for each distance. These
numbers may be considered as giving the percentage of the total α- and
γ-rays, the deflection of the α-rays having been scarcely observable
with the conditions employed.
At great distances there are no α-rays, and the undeflected radiation is
therefore of the γ kind only.
Experiments made at short distances:—
_d_, in centimetres 3·4 5·1 6·0 6·5
Percentage of undeflected rays 74 56 33 11
Experiments made at long distances with a product considerably more
active than that which was used for the preceding series:—
_d_, in centimetres 14 30 53 80 98
Percentage of undeflected rays 12 14 17 14 16
───────────────────────────────────────────────
_d_ 124 157
Percentage of undeflected rays 14 11
It is thus evident that after a certain distance the proportion of
undeflected rays in the radiation is approximately constant. These rays
probably all belong to the γ species.
The following is another series of experiments in which the radium was
enclosed in a very narrow glass tube, placed below the condenser and
parallel to the plates. The rays emitted traversed a certain thickness
of glass and air before entering the condenser:—
_d_, in centimetres 2·5 3·3 4·1 5·9 7·5 9·6 11·3
Percentage rays not deflected 33 33 21 16 14 10 9
────────────────────────────────────────────────────────────────
_d_ 13·9 17·2
Percentage rays not deflected 9 10
As in the preceding experiments, the number of the second line
approximate to a constant value, when the distance _d_ increases, but
the limit is reached for smaller distances than in the preceding series,
because the α-rays have been more completely absorbed by the glass than
the β- and γ-rays.
The following experiment shows that a thin sheet of aluminium (0·01 m.m.
thick) absorbs principally α-rays. The product being placed 5 c.m. from
the condenser, the proportion of rays other than β, when the magnetic
field is acting, is about 71 per cent. When the same substance is
covered with the sheet of aluminium, the distance remaining the same,
the radiation transmitted is found to be almost totally deflected by the
magnetic field, the α-rays having been absorbed by the aluminium. The
same result is obtained when paper is used as the absorbing screen.
The greatest part of the radiation of radium consists of α-rays, which
are probably emitted principally by the superficial layer of the
radiating matter. When the thickness of the layer of radiating matter is
varied, the intensity of the current increases with this thickness; the
increase is not proportional to the thickness for the whole of the
radiation; it is, moreover, more considerable for the β-rays than for
the α-rays, so that the proportion of β-rays increases with the
thickness of the active layer. The source of radiation being placed at a
distance of 5 c.m. from the condenser, it is found that for a thickness
equal to 0·4 m.m. of the active layer, the total radiation is given by
the number 28, and the proportion of the β-rays is 29 per cent. By
making the layer 2 m.m. thick, _i.e._, five times as thick, a total
radiation equal to 102, and a proportion of β-rays equal to 45 per cent
are obtained. The total radiation which exists at this distance has
therefore been increased in the ratio of 3·6, and the β-radiation has
become five times as strong.
The preceding experiments were made by the electrical method. When the
radiographic method is used, certain results seem to be in contradiction
with what precedes. In the experiments of M. Villard, a beam of radium
rays, subjected to the action of the magnetic field, was received on to
a pile of photographic plates. The undeflected and penetrating γ-beam
passed through all the plates, leaving its trace on each. The deflected
β-beam produced an impression on the first plate only. This beam
appeared therefore to contain no rays of great penetration.
On the contrary, in our experiments a beam which is propagated in the
air contains at the greatest distances accessible to observation about
9/10 of β-rays, and the same is the case when the source of radiation is
enclosed in a little sealed glass vessel. In M. Villard’s experiments,
these deflected and penetrating β-rays did not affect the photographic
plates beyond the first, because they are to a great extent diffused in
all directions by the first solid obstacle encountered, and no longer
form a beam. In our experiments the rays given off by radium and
transmitted through the glass of the vessel were also probably scattered
by the glass, but the vessel being very small would itself act as a
source of β-rays at its surface, and we were able to follow the course
of the latter to a great distance from the vessel.
The cathode rays of Crookes tubes can only traverse very thin screens
(aluminium screens of 0·01 m.m. thickness). A beam of rays striking the
screen normally is scattered in all directions; but the diffusion
becomes less with diminishing thickness of the screen, and for very thin
screens the emerging beam is practically the prolongation of the
incident beam.
The deflected β-rays of radium behave in a similar manner, but the
transmitted beam experiences, for the same thickness of screen, a much
slighter modification. According to the experiments of M. Becquerel, the
very readily deflected β-rays of radium (those with a relatively small
velocity) are powerfully scattered by an aluminium screen of thickness
0·1 m.m.; but the penetrating and less deflected rays (rays of the
cathode kind of great velocity) pass through this screen without being
sensibly diffused, whatever be the inclination of the screen to the
direction of the beam. The β-rays of great velocity penetrate without
diffusion a much greater thickness of paraffin (several centimetres),
and in this the curvature of the beam produced by the magnetic field can
be traced. The thicker the screen, and the more absorbent the material
of which it is composed, the greater is the modification of the
deflected primitive beam, because, with increasing thickness of screen,
diffusion occurs progressively among fresh groups of rays of increasing
penetration.
The β-rays of radium experience a diffusion in passing through the air,
which is very marked for readily deflected rays, but which is much
slighter than that produced by equal thicknesses of solid substances.
For this reason, the β-rays traverse long distances in the air.
_Penetrating Power of the Radiation of Radio-active Bodies._
Since the beginning of the researches on radio-active bodies,
investigations of the absorption produced by different screens upon the
rays given off by these bodies have been carried on. In a previous paper
on this subject I gave figures (quoted at the beginning of this work)
representing the penetrating power of uranium and thorium rays. Mr.
Rutherford has made a special study of the radiation of uranium, and
proved it to be heterogeneous. Mr. Owens has arrived at the same results
for thorium rays. When the discovery of strongly radio-active bodies
immediately followed upon this, the penetrating power of their rays was
also studied by various physicists (Becquerel, Meyer and von Schweidler,
Curie, Rutherford). The first observations brought to light the
complexity of the radiation, which seems to be a general phenomenon, and
common to the radio-active bodies. In them we have sources which give
rise to a variety of radiations, each of which has a power of
penetration proper to itself.
Radio-active bodies emit rays which are propagated both in the air and
_in vacuo_. The propagation is rectilinear; this fact is proved by the
distinctness and shape of the shadows formed by interposing bodies
opaque to the radiation between the source and the sensitive plate or
fluorescent screen which serves as receiver, the source being of small
magnitude in comparison with its distance from the receiver. Various
experiments demonstrating the rectilinear propagation of uranium,
radium, and polonium rays have been made by M. Becquerel.
It is interesting to know the distance that rays can travel in air. We
have found that radium emits rays which can be detected in the air at a
distance of several metres from the source. In certain of our electrical
determinations, the action of the source upon the air of the condenser
made itself felt at a distance of between 2 and 3 metres. We have also
obtained fluorescent effects and radiographic impressions at similar
distances. The experiments are not easily carried out, except with very
intense radio-active sources, because, independently of the absorption
by the air, the action upon a given receiver varies inversely as the
square of the distance from a source of small dimensions. This
radiation, which travels a long distance in the case of radium,
comprises rays of the cathode kind and rays which are undeflected;
however, the deflected rays predominate, according to the results of the
experiments already mentioned. The greater part of the radiation
(α-rays) is, on the contrary, limited in air to a distance of about 7
c.m. from the source.
I made several experiments with radium enclosed in a little glass
vessel. The rays emerging from the vessel, after traversing a certain
space of air, were received in a condenser, which served to measure
their ionising capacity by the usual electrical method. The distance,
_d_, from the source to the condenser was varied, and the current of
saturation, _i_, obtained in the condenser was measured. The following
are the results of one of the series of determinations:—
_d_, c.m. _i._ _i_ × _d_^2 × 10^{–3}.
10 127 13
20 38 15
30 17·4 16
40 10·5 17
50 6·9 17
60 4·7 17
70 3·8 19
100 1·65 17
After a certain distance, the intensity of radiation varies inversely as
the square of the distance from the condenser.
The radiation of polonium is only propagated in air to a distance of a
few centimetres (4 to 6 c.m.) from the source of radiation.
In the case of the absorption of radiations by solid screens, we find
another fundamental difference between radium and polonium. Radium emits
rays capable of penetrating great thicknesses of solid matter, _e.g._,
several centimetres of lead or of glass. The rays which have passed
through a great thickness of a solid body are extremely penetrating, and
it is practically impossible to absorb them entirely by any material
whatever. But these rays form only a small fraction of the total
radiation, the greater part of which is absorbed by a slight thickness
of solid matter.
Polonium emits rays which are readily absorbed, and which can only pass
through extremely thin screens.
The following are figures showing the absorption produced by an
aluminium lamina of thickness 0·01 m.m. This lamina was placed above and
almost in contact with the substance. The direct radiation and that
transmitted by the aluminium were measured by the electrical method
(apparatus of Fig. 1); the current of saturation was practically
obtained in every case. I have represented the activity of the radiating
body by _a_, that of uranium being unity.
_a._ Fraction of
radiation
transmitted.
Chloride of barium and radium 57 0·32
Bromide of barium and radium 43 0·30
Chloride of barium and radium 1200 0·30
Sulphate of barium and radium 5000 0·29
Sulphate of barium and radium 10,000 0·32
Metallic bismuth and polonium 0·22
Compounds of uranium 0·20
Compounds of thorium in a thin layer 0·38
We see that radium compounds of different nature and activity give very
similar results, as I have already pointed out in the case of uranium
and thorium compounds at the beginning of this work. We see also that,
taking into account the whole of the radiation, and with a given
absorbent screen, the different radio-active bodies can be arranged in
the following decreasing order of penetrating power:—Thorium, radium,
polonium, uranium.
These results are similar to those which have been published by Mr.
Rutherford.
Mr. Rutherford also finds that the order is the same when air is the
absorbent substance. But it is probable that this order has no absolute
value, and would not be maintained independently of the nature and
thickness of the screen. Experiment shows, indeed, that the law of
absorption is very different for polonium and radium, and that, for the
latter, the absorption of the rays of each of the three groups must be
considered separately.
Polonium is particularly well adapted to the study of α-rays, because
the specimens which we possess emit no other kind of rays. I made a
preliminary series of experiments with extremely active recently
prepared specimens of polonium. I found the absorbability of the rays to
increase with increase of thickness of the matter traversed. This
singular law of absorption is contrary to that known for other kinds of
radiation.
I employed for this research our apparatus for the determination of
electrical conductivity arranged in the following manner:—
The two plates of a condenser, P P and P′ P′ (Fig 8), are horizontally
disposed in a metallic box, B B B B, connected to earth. The active
body, A, placed in a thick metallic box, C C C C, connected with the
plate P′ P′, acts upon the air of the condenser across a metallic sheet,
T; the rays which pass through the sheet are alone utilised for
producing the current, the electric field being limited by the sheet.
The distance, A T, of the active body from the sheet may be varied. The
field between the plates is established by means of a battery. By
placing in A upon the active body different screens, and by adjusting
the distance A T, the absorption of rays which travel long or short
distances in the air may be determined.
[Illustration: FIG. 8.]
The following are the results obtained with polonium:—
For a certain value of the distance A T (4 c.m. and more), no current
passes; the rays do not penetrate the condenser. When the distance A T
is diminished, the appearance of the rays in the condenser is manifested
somewhat suddenly, a weak current changing to one of considerable
strength for a slight diminution of distance; the current then increases
regularly as the active body continues to approach the sheet T.
When the active body is covered with a sheet of aluminium 1/100 m.m.
thick, the absorption produced by the lamina becomes greater, the
greater the distance A T.
If a second similar lamina of aluminium be placed upon the first, each
absorbs a fraction of the radiation it receives, and this fraction is
greater for the second lamina than for the first.
In the following table I have represented in the first line the
distances in centimetres between the polonium and the sheet T; in the
second line the percentage of the rays transmitted by a sheet of
aluminium; in the third line the percentage of the rays transmitted by
two sheets of the same aluminium:—
Distance A T 3·5 2·5 1·9 1·45 0·5
Percentage of rays transmitted by one lamina 0 0 5 10 25
Percentage of rays transmitted by two laminæ 0 0 0 0 0·7
In these experiments the distance of the plates, P and P′, was 3 c.m. We
see that the interposition of the aluminium screen diminishes the
intensity of the radiation to a greater degree at further distances than
at nearer distances.
This effect is still more marked than the preceding figures seem to
indicate. For a distance of 0·5 c.m. 25 per cent represents the mean
penetration for all the rays which pass beyond this distance. If, for
example, only those rays between 0·5 c.m. and 1 c.m. be comprehended,
the penetration would be greater. And if the plate P be placed at a
distance of 0·5 c.m. from P′ the fraction of the radiation transmitted
by the aluminium lamina (for A T = 0·5 c.m.) is 47 per cent, and through
two laminæ it is 5 per cent of the original radiation.
I have recently performed a second series of experiments with these same
specimens of polonium, the activity of which was considerably
diminished, the interval of time between the two series of experiments
being three years.
In the former experiments, polonium nitrite was used; in the latter, the
polonium was in the state of metallic particles obtained by fusing the
nitrite with potassium cyanide.
I found that the radiation of polonium had preserved its essential
characteristics, and I discovered new results. The following, for
different values of the distance A T, are the fractions of the radiation
transmitted by a screen composed of four superposed very thin leaves of
beaten aluminium.
Distance A T, in centimetres 0 1·5 2·6
Percentage of rays transmitted by the screen 76 66 39
I also found that the fraction of the radiation absorbed by a given
screen increases with the thickness of the material already traversed by
the radiation, but this only occurs after the distance A T has reached a
certain value. When this distance is zero (the polonium being in contact
with the sheet, either outside or inside the condenser), it is observed
that with several similar superposed screens, each absorbs the same
fraction of the radiation it receives; otherwise expressed, the
intensity of the radiation diminishes therefore according to an
exponential law as a function of the thickness of the material
traversed, as in the case of homogeneous radiation transmitted by the
lamina without changing its nature.
The following numerical results are given with reference to these
experiments:—
For a distance A T equal to 1·5 c.m. a thin aluminium screen transmits
the fraction 0·51 of the radiation it receives when acting alone, and
the fraction 0·34 of the radiation it receives when it is preceded by
another similar screen.
On the contrary, for a distance A T equal to zero, the same screen
transmits in both the cases considered the same fraction of the
radiation it receives, and this fraction is equal to 0·71; it is
therefore greater than in the preceding case.
The following numbers indicate for a distance A T equal to 0 and for a
succession of thin superposed screens, the ratio of the radiation
transmitted to the radiation received for each screen:—
Series of nine very thin copper Series of seven very thin aluminium
leaves. leaves.
0·72 0.69
0·78 0.94
0·75 0.95
0·77 0.91
0·70 0.92
0·77 0.93
0·69 0.94
0·79
0·68
Taking into account the difficulties of the manipulation of very thin
screens and of the superposition of screens in contact, the numbers of
each column may be looked upon as constant; the first number only of the
aluminium column indicates a greater absorption than that indicated by
the following numbers.
The α-rays of radium behave similarly to the rays of polonium. These
rays may be investigated almost isolated by deflecting to one side the
β-rays with the magnetic field; the γ-rays seem of slight importance in
comparison with the α-rays. The operation can only be carried on at some
distance from the source of radiation. The following are the results of
an experiment of this kind. The fraction of the radiation transmitted by
a lamina of aluminium 0·01 m.m. thick is measured; this screen was
placed always in the same position, above and at a little distance from
the source of radiation. With the apparatus of Fig. 5, the current
produced in the condenser for different values of the distance A D is
observed, both with and without the screen:—
Distance A D 6·0 5·1 3·4
Percentage of rays transmitted by the aluminium 3 7 24
The rays which travel furthest in the air are those most absorbed by the
aluminium. There is therefore a great similarity between the absorbable
α-rays of radium and the rays of polonium.
The deflected β-rays and the undeflected penetrating γ-rays are, on the
contrary, of a different nature. The experiments, notably of MM. Meyer
and von Schweidler, clearly show that, considering the radiation of
radium as a whole, the penetrating power of this radiation increases
with the thickness of the material traversed, as is the case of Röntgen
rays. In these experiments the α-rays produce scarcely any effect, being
for the most part suppressed by very thin absorbent screens. Those which
penetrate are, on the one hand, β-rays more or less scattered; on the
other hand, γ-rays, which appear similar to Röntgen rays.
The following are the results of some of my experiments on the subject:—
The radium is enclosed in a glass vessel. The rays, which emerge from
the vessel, traverse 30 c.m. of air, and are received upon a series of
glass plates, each of thickness 1·3 m.m.; the first plate transmits 49
per cent of the radiation it receives, the second transmits 84 per cent
of the radiation it receives, the third transmits 85 per cent of the
radiation it receives.
In another series of experiments the radium was enclosed in a glass
vessel placed 10 c.m. from the condenser which received the rays. A
series of similar screens of lead each 0·115 m.m. thick were placed on
the vessel.
The ratio of the radiation transmitted to the radiation received is
given for each of the successive screens by the following numbers:—
0·40 0·60 0·72 0·79 0·89 0·92 0·94 0·94 0·97
For a series of four screens of lead, each of which was 1·5 m.m. thick,
the ratio of the radiation transmitted to the radiation received was
given for the successive screens by the following numbers:—
0·09 0·78 0·84 0·82
The results of these experiments show that when the thickness of the
lead traversed increases from 0·1 m.m. to 6 m.m., the penetrating power
of the radiation increases.
I found that, under the experimental conditions mentioned, a screen of
lead 1·8 c.m. thick transmits 2 per cent of the radiation it receives; a
screen of lead 5·3 c.m. thick transmits 0·4 per cent of the radiation it
receives. I also found that the radiation transmitted by a thickness of
lead of 1·5 m.m. consists largely of rays capable of deflection (cathode
order). The latter are therefore capable of traversing not only great
distances in the air, but also considerable thicknesses of very
absorbent solids, such as lead.
In investigating with the apparatus of Fig. 2 the absorption exercised
by an aluminium screen 0·01 m.m. thick upon the total radiation of
radium, the screen being always placed at the same distance from the
radiating body, and the condenser being placed at a variable distance, A
D, the results obtained are the sum of those due to the three groups of
the radiation. At a long distance the penetrating rays predominate, and
the absorption is slight; at a short distance the α-rays predominate,
and the absorption becomes less with nearer approach to the substance;
for an intermediate distance the absorption passes through a maximum and
the penetration through a minimum.
Distance A D 7·1 6·5 6·0 5·1 3·4
Percentage of rays transmitted by aluminium 91 82 58 41 48
Certain experiments made in connection with absorption always
demonstrate a certain similarity between the α-rays and the β-rays. Thus
it was that M. Becquerel discovered that the absorbent action of a solid
screen upon the β-rays increases with the distance of the screen from
the source, such that if the rays are subjected to a magnetic field, as
in Fig. 4, a screen placed in contact with the source of radiation
allows a larger portion of the magnetic spectrum to be in evidence than
does the same screen placed upon the photographic plate. This variation
of the absorbent effect of the screen with the distance of the screen
from the source is similar to that which occurs with the α-rays; this
has been verified by MM. Meyer and von Schweidler, who operated by means
of the fluoroscopic method; M. Curie and I observed the same fact when
working by the electrical method. However, when the radium is enclosed
in a glass tube and placed at a distance from the condenser, which is
itself enclosed in a thin aluminium box, it becomes a matter of
indifference whether the screen be placed against the source or against
the condenser; the current obtained is the same in both cases.
The investigation of the α-rays led me to the reflection that these rays
behave like projectiles having a certain initial velocity, and which
lose their force on encountering obstacles. These rays, moreover, travel
by rectilinear propagation, as has been shown by M. Becquerel in the
following experiment:—Polonium emitting rays was placed in a very narrow
straight cavity hollowed in a sheet of cardboard. Thus a linear source
of radiation was produced. A copper wire, 1·5 m.m. in diameter, was
placed parallel and opposite to the source at a distance of 4·9 m.m.
Beyond was placed a parallel photographic plate at a distance of 8·65
m.m. After an exposure of ten minutes, the geometric shadow of the wire
was perfectly reproduced, with a narrow penumbra corresponding to the
size of the source. The same experiment succeeded equally well when a
double leaf of beaten aluminium was placed against the wire, through
which the rays must pass.
There are therefore rays capable of giving perfect geometric shadows.
The experiment with the aluminium shows that these rays are not
scattered in traversing the screen, and that this screen does not give
rise to any noticeable extent to secondary rays similar to the secondary
rays of the Röntgen rays.
The experiments of Mr. Rutherford show that the projectiles which
constitute the α-rays are deflected by a magnetic field, as if they were
positively charged. The deflection in a magnetic field becomes less as
the product _mv_/_e_ becomes greater; _m_ being the mass of the
particle, _v_ its velocity, and _e_ its charge. The cathode rays of
radium are but slightly deflected, because their velocity is enormous;
they are, on the other hand, very penetrating, because each particle has
a very small mass together with a great velocity. But particles which,
with an equal charge and a less velocity, have a greater mass, would be
also only slightly influenced by the action of the field, and would give
rise to very absorbable rays. From the results of Mr. Rutherford’s
experiments, this seems to take place in the case of the α-rays.
The penetrating γ-rays appear to be of quite another nature and similar
to Röntgen rays.
We have now seen how complex a phenomenon is the radiation of
radio-active bodies. The difficulties of investigation are increased by
the question as to whether the radiation undergoes a merely selective
absorption on the part of the material, or whether a more or less
radical transformation.
Little is so far known with regard to this question. If the radiation of
radium be regarded as containing rays both of the cathode and Röntgen
species, it might be expected to undergo transformations in traversing
screens. It is known:—Firstly, that cathode rays emerging from a Crookes
tube through an aluminium window are greatly scattered by the aluminium;
and, further, that the passage through the screen entails a diminution
of the velocity of the rays. In this way, cathode rays with a velocity
equal to 1·4 × 10^{10} c.m. lose 10 per cent of their velocity in
passing through 0·01 m.m. of aluminium. Secondly, cathode rays on
striking an obstacle give rise to the production of Röntgen rays.
Thirdly, Röntgen rays, on striking a solid obstacle, give rise to the
production of _secondary rays_, which partly consist of cathode rays.
The existence, by analogy, of all these preceding phenomena may
therefore be predicted for the rays of radio-active substances.
In investigating the transmission of polonium rays through a screen of
aluminium, M. Becquerel observed neither the production of secondary
rays nor any transformation into cathode rays.
I endeavoured to demonstrate a transformation of the rays of polonium by
using the method of interchangeable screens. Two superposed screens,
E_{1} and E_{2}, being traversed by the rays, the order in which they
are traversed should be immaterial if the passage through the screens
does not transform the rays; if, on the contrary, each screen transforms
the rays during transmission, the order of the screens is of moment. If,
for example, the rays are transformed into more absorbable rays in
passing through lead, and no such effect is produced by aluminium, then
the system lead-aluminium will be more opaque than the system
aluminium-lead; this takes place with Röntgen rays.
My experiments show that this phenomenon is produced with the rays of
polonium. The apparatus employed was that of Fig. 8. The polonium was
placed in the box, C C C C, and the absorbing screens, of necessity very
thin, were placed upon the metallic sheet T.
Screens employed. Thickness. │ Current observed.
M.m. │
Aluminium 0·01 │ 17·9
Brass 0·005 │ 〃
───────────────────────────────────────────────┼───────────────────────
Brass 0·005 │ 6·7
Aluminium 0·01 │ 〃
───────────────────────────────────────────────┼───────────────────────
Aluminium 0·01 │150
Tin 0·005 │ 〃
───────────────────────────────────────────────┼───────────────────────
Tin 0·005 │125
Aluminium 0·01 │ 〃
───────────────────────────────────────────────┼───────────────────────
Tin 0·005 │ 13·9
Brass 0·005 │ 〃
───────────────────────────────────────────────┼───────────────────────
Brass 0·005 │ 4·4
Tin 0·005 │ 〃
The results obtained prove that the radiation is modified in passing
through a solid screen. This conclusion accords with the experiments in
which, of two similar superposed metallic screens, the first is less
absorbent than the second. From this it is probable that the
transforming action of a screen increases with the distance of the
screen from the source. This fact has not been verified, and the nature
of the transformation has not been studied in detail.
I repeated the same experiments with a very active salt of radium; the
result was negative. I only observed insignificant variations in the
intensity of the radiation transmitted with interchange of the order of
the screens. The following systems of screens were experimented with:—
Aluminium, thickness 0·55 m.m.
Aluminium, thickness 0·55 m.m.
Aluminium, thickness 0·55 m.m.
Aluminium, thickness 1·07 m.m.
Aluminium, thickness 0·55 m.m.
Aluminium, thickness 1·07 m.m.
Aluminium, thickness 0·15 m.m.
Aluminium, thickness 0·15 m.m.
Aluminium, thickness 0·15 m.m.
Platinum, thickness 0·01 m.m.
Lead, thickness 0·1 m.m.
Tin, thickness 0·005 m.m.
Copper, thickness 0·05 m.m.
Brass, thickness 0·005 m.m.
Brass, thickness 0·005 m.m.
Platinum, thickness 0·01 m.m.
Zinc, thickness 0·05 m.m.
Lead, thickness 0·1 m.m.
The system lead-aluminium was slightly more opaque than the system
aluminium-lead, but the difference was not great.
Thus, I was unable to discover an appreciable transformation of the rays
of radium. However, in various radiographic experiments, M. Becquerel
observed very intense effects due to scattered or secondary rays,
emitted by solid screens which received radium rays. Lead seemed to be
the most active substance in this respect.
_Ionising Action of Radium Rays on Insulating Liquids._
M. Curie has pointed out that radium rays and Röntgen rays act upon
liquid dielectrics as upon air, imparting to them a certain electrical
conductivity. The experiment was carried out in the following manner
(Fig. 9):—
The experimental liquid is placed in a metal vessel, C D E F, into which
a thin copper tube, A B, is plunged; these two pieces of metal serve as
electrodes. The outer vessel is maintained at a known potential, by
means of a battery of small accumulators, one pole of which is connected
to earth. The tube, A B, is connected to the electrometer. When a
current traverses the liquid the electrometer is kept at zero by means
of a quartz electrical piezometer, which gives the strength of the
current. The copper tube, M N M′ N′, connected to earth, serves as a
guard tube, preventing the passage of the current through the air. A
bulb containing the radium-barium salt may be placed at the bottom of
the tube, A B; the rays act on the liquid after having penetrated the
glass of the bulb and the sides of the metal tube. The radium may also
be allowed to act by placing the bulb beneath the side, D E.
In working with Röntgen rays the course of the rays is through side D E.
The increase of conductivity by the action of the radium rays or the
Röntgen rays seems to be produced in the case of all liquid dielectrics;
but in order to determine this increase, the conductivity of the liquid
itself must be so slight as not to mask the effect of the rays.
M. Curie obtained results of the same order of magnitude with both
radium rays and Röntgen rays.
When investigating with the same apparatus the conductivity of air or of
another gas under the action of the Becquerel rays, the intensity of the
current obtained is found to be proportional to the difference of
potential between the electrodes, as long as the latter does not exceed
a few volts; but at higher tensions, the intensity of the current
increases less and less rapidly, and the saturation current is
practically attained for a tension of 100 volts.
Liquids examined with the same apparatus and the same radio-active body
behave differently; the intensity of the current is proportional to the
tension when the latter varies between 0 and 450 volts, and when the
distance between the electrodes does not exceed 6 m.m.
[Illustration: FIG. 9.]
The figures of the following table multiplied by 10^{–11} give the
conductivity in megohms per c.c.:—
Carbon bisulphide 20
Petroleum ether 15
Amylene 14
Benzine 4
Liquid air 1·3
Vaseline oil 1·6
We may, however, assume that liquids and gases behave similarly, but
that, in the case of liquids, the current remains proportional to the
tension up to a much higher limit than in the case of gases. It
therefore seemed probable that the limit of proportionality could be
lowered by using a much more feeble radiation, and this idea was
verified by experiment. The radio-active body employed was 150 times
less active than that which had served for the previous experiments. For
tensions of 50, 100, 200, 400 volts, the intensities of the current were
represented respectively by the numbers 109, 185, 255, 335. The
proportionality was no longer maintained, but the current showed great
variation when the difference of potential was doubled.
Some of the liquids examined are nearly perfect insulators when
maintained at a constant temperature and when screened from the action
of the rays. Such are liquid air, petroleum ether, vaseline oil, and
amylene. It is therefore very easy to study the effect of the rays.
Vaseline oil is much less sensitive to the action of the rays than is
petroleum ether. This fact may have some relation to the difference in
volatility which exists between these two hydrocarbons. Liquid air,
which has boiled for some time in the experimental vessel, is more
sensitive to the action of the rays than that newly poured in; the
conductivity produced by the rays is one-fourth as great again in the
former case. M. Curie has investigated the action of the rays upon
amylene and upon petroleum ether at temperatures of +10° and –17°. The
conductivity due to the radiation diminishes by one-tenth of its value
only, in passing from 10° to –17°.
In the experiments in which the temperature of the liquid is varied, the
temperature of the radium may be either that of the surrounding
atmosphere or that of the liquid; the same result is obtained in both
cases. This leads to the conclusion that the radiation of radium does
not vary with the temperature, and remains unaltered even at the
temperature of liquid air. This fact has been verified directly by
measurements.
_Various Effects and Applications of the Ionising Action of the Rays
Emitted by Radio-active Substances._
The rays of the new radio-active substances have a strongly ionising
action upon air. By the action of radium _the condensation of
supersaturated water vapour_ can be easily induced, just as happens by
the action of cathode rays and Röntgen rays.
Under the influence of the rays emitted by the new radio-active
substances, _the distance of discharge between two metallic conductors
for a given difference of potential is increased_; to put it otherwise,
the passage of the spark is facilitated by these rays.
In causing conductivity, by the action of radio-active bodies, in the
air in the neighbourhood of two metallic conductors, one of which is
connected to earth and the other to a well-insulated electrometer, the
electrometer is seen to be permanently deflected, which gives a measure
of the electromotive force of the battery formed by the air and the two
metals (electromotive force of contact of the two metals, when they are
separated by air). This method of measurement was employed by Lord
Kelvin and his students, the radiating body being uranium; a similar
method had been previously employed by M. Perrin, who was using the
ionising action of Röntgen rays.
Radio-active bodies may be employed in the study of atmospheric
electricity. The active substance is enclosed in a little box of thin
aluminium fixed at the extremity of a metal wire connected with the
electrometer. The air is made to conduct in the neighbourhood of the end
of the wire, and the latter adopts the potential of the surrounding air.
Radium thus replaces, with advantage, the flames or the apparatus of
running water of Lord Kelvin, till now in general use for the
investigation of atmospheric electricity.
_Fluorescent and Luminous Effects._
The rays emitted by the new radio-active bodies cause fluorescence of
certain substances. M. Curie and myself first discovered this phenomenon
when causing polonium to act upon a layer of barium platinocyanide
through aluminium foil. The same experiment succeeds yet more easily
with barium containing radium. When the substance is strongly
radio-active the fluorescence produced is very beautiful.
A large number of bodies are capable of becoming phosphorescent or
fluorescent by the action of the Becquerel rays. M. Becquerel studied
the effect upon the uranium salts, the diamond, &c. M. Bary has
demonstrated that the salts of the metals of the alkalis and alkaline
earths, which are all fluorescent under the action of luminous rays and
Röntgen rays, are also fluorescent under the action of the rays of
radium. Paper, cotton, glass, &c., are all caused to fluoresce in the
neighbourhood of radium. Among the different kinds of glass, Thuringian
glass is specially luminous. Metals do not seem to become luminous.
Barium platinocyanide is most conveniently used when the radiation of
the radio-active bodies is to be investigated by the fluoroscopic
method. The effect of the radium rays may be followed at distances
greater than 2 m. Phosphorescent zinc sulphide is made extremely
luminous, but this body has the inconvenient property of preserving its
luminosity for some time after the action of the rays has ceased.
The fluorescence produced by radium may be observed when the fluorescent
screen is separated from the radium by absorbent screens. We were able
to observe the illumination of a screen of barium platinocyanide across
the human body. However, the action is incomparably greater when the
screen is placed immediately in contact with the radium, being separated
from it by no solid screen at all. All the groups of rays appear capable
of producing fluorescence.
In order to observe the action of polonium, the substance must be placed
close to the fluorescent screen, without the intervention of a solid
screen, unless the latter be extremely thin.
The luminosity of fluorescent substances exposed to the action of
radio-active bodies diminishes with time. At the same time the
fluorescent substance undergoes a transformation. The following are
examples:—
Radium rays transform barium platinocyanide into a brown, less luminous
variety (an action similar to that produced by Röntgen rays, and
described by M. Villard). Uranium sulphate and potassium sulphate are
similarly altered. The changed barium platinocyanide is partially
regenerated by the action of light. If the radium be placed beneath a
layer of barium platinocyanide spread on paper, the platinocyanide
becomes luminous; if the system be kept in the dark, the platinocyanide
becomes changed, and its luminosity diminishes considerably. But if the
whole be exposed to light, the platinocyanide is partially regenerated,
and if the whole is replaced in darkness the luminosity reappears with
vigour. By means of a fluorescent body and a radio-active body, we have
therefore obtained a system which acts as a phosphorescent body capable
of long duration of phosphorescence.
Glass made fluorescent by the action of radium becomes coloured brown or
violet. At the same time its fluorescence diminishes. If the glass thus
changed be warmed, it is decolorised, and when this occurs the glass
becomes luminous. The glass has now regained its fluorescent property in
the same degree as before the transformation.
Zinc sulphide, which has been exposed for a sufficient length of time to
the action of radium, gradually becomes used up, and loses its
phosphorescent property, whether under the action of radium or that of
light.
The diamond becomes phosphorescent under the action of radium, and may
thus be distinguished from paste imitations, which have only a very
faint luminosity.
All the barium-radium compounds _are spontaneously luminous_. The dry
anhydrous halogen salts emit a particularly intense light. This
illumination cannot be seen in broad daylight, but it is easily visible
in the twilight or by gas-light. The light emitted may be strong enough
to read by in the dark. The light emitted emanates from the entire body
of the product, whilst in the case of a common phosphorescent body, the
light emanates specially from the portion of the surface illuminated.
Radium products lose much of their luminosity in damp air, but they
regain it on drying (Giesel). There is apparently conservation of
luminosity. After many years no sensible modification is produced in the
luminosity of feebly active products, kept in the dark in sealed tubes.
In the case of very active and very luminous radium-barium chloride, the
light changes colour after several months; it becomes more violet and
loses in intensity; at the same time the product undergoes
transformations; on re-dissolving the salt in water and drying it
afresh, the original luminosity is restored.
Solutions of barium-radium salts, which contain a large proportion of
radium, are equally luminous; this fact may be observed by placing the
solution in a platinum capsule, which not being itself luminous permits
of the faint luminosity of the solution being seen.
When a solution of a barium-radium salt contains crystals deposited in
it, these crystals are luminous at the bottom of the solution, and much
more so than the solution itself, so that they alone appear luminous.
M. Giesel has made a preparation of barium-radium platinocyanide. When
this salt is newly crystallised, it has the appearance of ordinary
barium platinocyanide and is very luminous. But gradually the salt
becomes spontaneously coloured, taking a brown tint, the crystals at the
same time becoming dichroic. In this state the salt is much less
luminous, although its radio-activity is increased. The radium
platinocyanide, prepared by M. Giesel, changes still more rapidly.
Radium compounds are the first example of self-luminous bodies.
_Evolution of Heat by the Salts of Radium._
MM. Curie and Laborde have recently discovered that _the salts of radium
are the source of a spontaneous and continuous evolution of heat_. This
evolution has the effect of keeping the salts of radium at a temperature
higher than that of their surroundings; an excess of temperature of 1·5°
has been observed. This excess of temperature is dependent upon the
thermal insulation of the body. MM. Curie and Laborde have determined
the amount of heat produced in the case of radium. They found that the
output is of the order of magnitude of 100 calories per grm. of radium
per hour. One grm.-atom (225 grm.) of radium give rise in one hour to
22,500 cal., a quantity of heat comparable to that produced by the
combustion of 1 grm.-atom (1 grm.) of hydrogen. So great an evolution of
heat can be explained by no ordinary chemical reaction, more
particularly as the condition of the radium remains unaffected for
years. The evolution of heat might be attributed to a slow
transformation of the radium atom. If this were the case, we should be
led to conclude that the quantities of energy generated during the
formation and transformation of the atoms are considerable, and that
they exceed all that is so far known.
_Chemical Effects produced by the New Radio-active Bodies._
_Colourations._—The radiations of strongly radio-active bodies are
capable of causing certain chemical reactions. The rays emitted by
radium products exercise colouring actions upon glass and porcelain.
The colouration of glass, generally brown or violet, is very deep; it is
produced in the body of the glass, and remains after removal of the
radium. All glasses become coloured after a longer or a shorter interval
of time, and the presence of lead is not essential. This fact may be
compared to that recently observed of the colouration of the glass of
vacuum tubes, after having been long in use for the production of
Röntgen rays.
M. Giesel has demonstrated that the crystallised halogen salts of the
alkali metals become coloured under the influence of radium, as under
the action of cathode rays. M. Giesel points out that similar
colourations are obtained when the salts of the alkalis are exposed to
sodium vapour.
I investigated the colouration of a collection of glasses of known
composition, kindly lent me for the occasion by M. Le Chatelier. I
observed no great variety in the colouration. It is generally brown,
violet, yellow, or grey. It appears to be associated with the presence
of the alkali metals.
With the pure crystallised alkali salts more varied and more vivid
colours are obtained; the salt, originally white, becomes blue, green,
yellow, brown, &c.
M. Becquerel has discovered that yellow phosphorus is transformed into
the red variety by the action of radium.
Paper is changed and coloured by the action of radium. It becomes
brittle, scorched, and, finally, resembles a colander perforated with
holes.
Under some circumstances there is a production of ozone in the
neighbourhood of very active compounds. Rays emerging from a sealed jar
containing radium do not produce ozone in the air they pass through. On
the contrary, a strong odour of ozone is detected when the jar is
opened. In a general way, ozone is produced in the air when the latter
is in direct contact with the radium. Communication by a channel, even
if extremely narrow, suffices; it appears as if the production of ozone
is associated with the propagation of induced radio-activity, of which
we shall speak later.
Radium compounds appear to change with lapse of time, doubtless under
the action of their own radiation. It was seen above that crystals of
barium-radium chloride, which are colourless when formed, become
gradually coloured first yellow or orange, then pink; this colouration
disappears in solution. Barium-radium chloride generates oxygen
compounds of chlorine; the bromide those of bromine. These slow changes
generally manifest themselves some time after the preparation of the
solid product, which at the same time changes in form and colour,
becoming yellow or violet. The light emitted also becomes more violet.
A solution of a radium salt evolves hydrogen (Giesel).
Pure radium salts seem to undergo the same changes as those containing
barium. However, crystals of the chloride, deposited in acid solution,
do not become sensibly coloured after some time has elapsed, whereas
crystals of barium-radium chloride, rich in radium, become deeply
coloured.
_Production of Thermo-luminosity._—Certain bodies, such as fluorite,
become luminous when heated; they are thermo-luminescent. Their
luminosity disappears after some time, but the capacity of becoming
luminous afresh through heat is restored to them by the action of a
spark, and also by the action of radium. Radium can thus restore to
these bodies their thermo-luminescent property. Fluorite when heated
undergoes a change, which is accompanied by the emission of light. If
the fluorite is afterwards subjected to the action of radium, an inverse
change occurs, which is also accompanied by an emission of light.
An absolutely similar phenomenon occurs when glass is exposed to radium
rays. Here also a change is produced in the glass while luminous from
the effect of the radium rays; this change shows itself in the
colouration which appears and gradually increases. If the glass is
afterwards heated, the inverse change takes place, the colour
disappears, and this phenomenon is accompanied by production of light.
It appears very probable that we have here a change of a chemical
nature, and the production of light is associated with this change. This
phenomenon may be general. It might be that the production of
fluorescence by the action of radium and the luminosity of radium
compounds is of necessity associated with some chemical or physical
change in the substance emitting the light.
_Radiographs._—The radiographic action of the new radio-active bodies is
very marked. However, the method of operating should be very different
with polonium and radium. Polonium acts only at very short distances,
and its action is considerably weakened by solid screens; it is
practically annihilated by means of a screen of slight thickness (1 m.m.
of glass). Radium acts at considerably greater distances. The
radiographic action of radium rays may be observed at more than 2 m.
distance in air, even when the active product is enclosed in a glass
vessel. The rays acting under these conditions belong to the β- and
γ-groups. Owing to the differences in transparency of different
materials to the rays, radiographs of different objects may be obtained,
as in the case of Röntgen rays. Metals are, as a rule, opaque, with the
exception of aluminium, which is very transparent. There is no
noteworthy difference of transparency between flesh and bone. The
operation may be carried on at a great distance and with a source of
very small dimensions; and very delicate radiographs are thus produced.
The beauty of the radiograph is enhanced by deflecting to one side the
β-rays, by means of a magnetic field, and utilising only the γ-rays. The
β-rays, in traversing the object to be radiographed, undergo a certain
amount of diffusion, and thus cause a slight fog. In suppressing them, a
longer time of exposure is necessary, but better results are obtained.
The radiograph of an object, such as a purse, requires one day with a
radiating source composed of several centigrams of a radium salt,
enclosed in a glass vessel, and placed at a distance of 1 m. from the
sensitive plate, in front of which the object is placed. If the source
is at a distance of 20 c.m. from the plate, the same result is obtained
in one hour. In the immediate vicinity of the source of radiation, a
sensitive plate is instantaneously acted upon.
_Physiological Effects._
Radium rays exert an action upon the epidermis. This has been observed
by M. Walkhoff and confirmed by M. Giesel, since also by MM. Becquerel
and Curie.
If a celluloid or thin indiarubber capsule containing a very active salt
of radium be placed upon the skin, and be left thus for some time, a
redness is produced upon the skin, either immediately or at the end of
some time, which is longer in proportion as the action is weaker; this
red spot appears in the place which has been exposed to the action; the
local change in the skin appears and acts like a burn. In certain cases
a blister is formed. If the exposure was of long duration, an ulceration
is produced which is long in healing. In one experiment, M. Curie caused
a relatively weak radio-active product to act upon his arm for ten
hours. The redness appeared immediately, and later a wound was caused
which took four months to heal. The epidermis was locally destroyed, and
formed again slowly and with difficulty, leaving a very marked scar. A
radium burn with half-an-hour’s exposure appeared after fifteen days,
formed a blister and healed in fifteen days. Another burn, caused by an
exposure of only eight minutes, occasioned a red spot which appeared two
months after, its effect being quite insignificant.
The action of radium upon the skin can take place across metal screens,
but with weakened effect.
The action of radium upon the skin has been investigated by Dr. Daulos,
at the Hospital of St. Louis, as a process of treating certain
affections of the skin, similar to the treatment with the Röntgen rays
or the ultra-violet rays. In this respect radium gives encouraging
results; the epidermis partially destroyed by the action of the radium
is renewed in a healthy condition. The action of radium is more
penetrating than that of light, and its use is easier than that of light
or of Röntgen rays. The study of the conditions of application is of
necessity rather lengthy, because the effect of the application does not
at once appear.
M. Giesel has observed the action of radium upon plant leaves. The
leaves thus treated turn yellow and wither away.
M. Giesel has also discovered the action of radium rays upon the eye. If
a radio-active substance be placed in the dark in the vicinity of the
closed eye or of the temple, a sensation of light fills the eye. This
phenomenon has been studied by MM. Himstedt and Nagel. These physicists
have demonstrated that the centre of the eye is rendered fluorescent by
the action of radium, and this explains the sensation of light
experienced. Blind people whose retina is intact are sensitive to the
action of radium, whilst those whose retina is diseased do not
experience any sensation of luminosity.
Radium rays either arrest or hinder the development of colonies of
microbes, but this action is not very intense.
M. Danysz has recently demonstrated the ready action of radium upon the
marrow and brain. After one hour’s exposure paralysis of the animals
experimented upon occurred, and the latter usually died in a few days.
_Influence of Temperature upon Radiation._
There is so far but little information regarding the manner of variation
of the radiation of radio-active bodies with temperature. We know,
however, that radiation subsists at low temperatures. M. Curie placed a
glass tube containing barium-radium chloride in liquid air. The
luminosity of the radio-active body persisted under these conditions. At
the moment, indeed, of removing the tube from the cold bath, it appears
more luminous than at the ordinary temperature. At the temperature of
liquid air radium continues to cause fluorescence in the sulphates of
uranium and potassium. M. Curie has verified, by electrical
determinations, that the radiation, measured at a certain distance from
the source, possesses the same intensity whether the radium be at the
temperature of the atmosphere or of liquid air. In these experiments the
radium was placed at the bottom of a tube closed at one end. The rays
emerged from the tube at the open end, traversed a certain space in the
air, and were received into a condenser. The action of the rays upon the
air of the condenser was determined both on leaving the tube in the air
and on surrounding it to a certain height with liquid air. The same
result was obtained in both cases.
The radio-activity of radium persists at high temperatures.
Barium-radium chloride after being fused (towards 800°) is radio-active
and luminous. However, prolonged heating at a high temperature has the
effect of temporarily lowering the radio-activity of the body. This
decrease is very considerable; it may constitute 75 per cent of the
total radiation. The decrease is less in proportion for the absorbable
rays than for the penetrating rays, which are to some extent suppressed
by heating. In time the radiation of the product regains the intensity
and composition that it possessed before heating; this occurs after the
lapse of about two months from the occasion of heating.