RADIO-ACTIVE SUBSTANCES.
BY
MDME. SKLODOWSKA CURIE.
Thesis presented to the Faculté des Sciences de Paris.
_Reprinted from the CHEMICAL NEWS._
(SECOND EDITION).
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1904.
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------------------------------------------------------------------------
RADIO-ACTIVE SUBSTANCES
BY
MDME. SKLODOWSKA CURIE
Thesis presented to the Faculté des Sciences de Paris.
_Reprinted from the CHEMICAL NEWS._
(SECOND EDITION).
LONDON:
_CHEMICAL NEWS_ OFFICE,
16, NEWCASTLE STREET, FARRINGDON STREET, E.C.
1904.
D. VAN NOSTRAND COMPANY,
NEW YORK.
LONDON:
PRINTED BY EDWIN JOHN DAVEY, 16, NEWCASTLE STREET, FARRINGDON STREET,
E.C.
------------------------------------------------------------------------
[_Reprinted from the_ CHEMICAL NEWS, _1903, Vol. 88, p. 85 et seq._]
RADIO-ACTIVE SUBSTANCES.
INTRODUCTION.
The object of the present work is the publication of researches which I
have been carrying on for more than four years on radio-active bodies. I
began these researches by a study of the phosphorescence of uranium,
discovered by M. Becquerel. The results to which I was led by this work
promised to afford so interesting a field that M. Curie put aside the
work on which he was engaged, and joined me, our object being the
extraction of new radio-active substances and the further study of their
properties.
Since the commencement of our research we thought it well to hand over
specimens of the substances, discovered and prepared by ourselves, to
certain physicists, in the first place to M. Becquerel, to whom is due
the discovery of the uranium rays. In this way we ourselves facilitated
the research by others besides ourselves on the new radio-active bodies.
At the termination of our first publications, M. Giesel, in Germany,
also began to prepare these substances, and passed on specimens of them
to several German scientists. Finally, these substances were placed on
sale in France and Germany, and the subject growing in importance gave
rise to a scientific movement, such that numerous memoirs have appeared,
and are constantly appearing on radio-active bodies, principally abroad.
The results of the various French and foreign researches are necessarily
confused, as is the case with all new subjects in course of
investigation, the aspect of the question becoming modified from day to
day.
From the chemical point of view, however, one point is definitely
established:—_i.e._, the existence of a new element, strongly
radio-active, viz., radium. The preparation of the pure chloride of
radium and the determination of the atomic weight of radium form the
chief part of my own work. Whilst this work adds to the elements
actually known with certainty a new element with very curious
properties, a new method of chemical research is at the same time
established and justified. This method, based on the consideration of
radio-activity as an atomic property of matter, is just that which
enabled M. Curie and myself to discover the existence of radium.
If, from the chemical point of view, the question that we undertook
primarily may be looked upon as solved, the study of the physical
properties of the radio-active bodies is in full evolution. Certain
important points have been established, but a large number of the
conclusions are still of a provisional character. This is not surprising
when we consider the complexity of the phenomena due to radio-activity,
and the differences existing between the various radio-active
substances. The researches of physicists on these substances constantly
meet and overlap. Whilst endeavouring to keep strictly to the limits of
this work and to publish my individual research only, I have been
obliged at the same time to mention results of other researches, the
knowledge of which is indispensable.
I desired, moreover, to make this work an inclusive survey of the actual
position of the question.
I indicate at the end the particular questions with which I am specially
concerned, and those which I investigated in conjunction with M. Curie.
I carried on the work in the laboratories of the School of Physics and
Chemistry in Paris, with the permission of Schützenberger, late Director
of the School, and M. Lauth, actual Director. I take this opportunity of
expressing my gratitude for the kind hospitality received in this
school.
_Historical._
The discovery of the phenomena of radio-activity is connected with
researches followed, since the discovery of the Röntgen rays, upon the
photographic effects of phosphorescent and fluorescent substances.
The first tubes for producing Röntgen rays were without the metallic
anticathode. The source of the Röntgen rays was the glass surface
impinged upon by the cathode rays; this surface was at the same time
actively fluorescent. The question then was whether the emission of
Röntgen rays necessarily accompanied the production of fluorescence,
whatever might be the cause of the latter. This idea was first
enunciated by M. Henri Poincaré.
Shortly afterwards, M. Henry announced that he had obtained photographic
impressions through black paper by means of phosphorescent zinc
sulphide. M. Niewenglowski obtained the same phenomenon with calcium
sulphide exposed to the light. Finally, M. Troost obtained strong
photographic impressions with zinc sulphide artificially phosphorescent
acting across black paper and thick cardboard.
The experiences just cited have not been reproduced, in spite of
numerous attempts to this end. It cannot therefore be considered as
proved that zinc sulphide and calcium sulphide are capable of emitting,
under the action of light, invisible rays which traverse black paper and
act on photographic plates.
M. Becquerel has made similar experiments on the salts of uranium, some
of which are fluorescent.
He obtained photographic impressions through black paper with the double
sulphate of uranium and potassium.
M. Becquerel at first believed that this salt, which is fluorescent,
behaved like the sulphides of zinc and calcium in the experiments of MM.
Henry, Niewenglowski, and Troost. But the conclusion of his experiments
showed that the phenomenon observed was in no way related to the
fluorescence. It is not necessary that the salt should be fluorescent;
further, uranium and all its compounds, fluorescent or not, act in the
same manner, and metallic uranium is the most active. M. Becquerel
finally found that by placing uranium compounds in complete darkness,
they continue acting on photographic plates through black paper for
years. M. Becquerel allows that uranium and its compounds emit peculiar
rays—uranium rays. He proved that these rays can penetrate thin metallic
screens, and that they discharge electrified bodies. He also made
experiments from which he concluded that uranium rays undergo
reflection, refraction, and polarisation.
The work of other physicists (Elster and Geitel, Lord Kelvin, Schmidt,
Rutherford, Beattie, and Smoluchowski) confirms and extends the results
of the researches of M. Becquerel, with the exception of those relating
to the reflection, refraction, and polarisation of uranium rays, which
in this respect behave like Röntgen rays, as has been recognised first
by Mr. Rutherford and then by M. Becquerel himself.
CHAPTER I.
RADIO-ACTIVITY OF URANIUM AND THORIUM. RADIO-ACTIVE MINERALS.
_Becquerel Rays._—The uranium rays discovered by M. Becquerel act upon
photographic plates screened from the light; they can penetrate all
solid, liquid, and gaseous substances, provided that the thickness is
sufficiently reduced; in passing through a gas, they cause it to become
a feeble conductor of electricity.
These properties of the uranium compounds are not due to any known
cause. The radiation seems to be spontaneous; it loses nothing in
intensity, even on keeping the compounds in complete darkness for
several years; hence there is no question of the phosphorescence being
specially produced by light.
The spontaneity and persistence of the uranium radiation appear as a
quite unique physical phenomenon. M. Becquerel kept a piece of uranium
for several years in the dark, and he has affirmed that at the end of
this time the action upon a photographic plate had not sensibly altered.
MM. Elster and Geitel made a similar experiment, and also found the
action to remain constant.
I measured the intensity of radiation of uranium by the effect of this
radiation on the conductivity of air. The method of measurement will be
explained later. I also obtained figures which prove the persistence of
radiation within the limits of accuracy of the experiments.
For these measurements a metallic plate was used covered with a layer of
powdered uranium; this plate was not otherwise kept in the dark; this
precaution, according to the experimenters already quoted, being of no
importance. The number of measurements taken with this plate is very
great, and they actually extend over a period of five years.
Some researches were conducted to discover whether other substances were
capable of acting similarly to the uranium compounds. M. Schmidt was the
first to publish that thorium and its compounds possess exactly the same
property. A similar research, made contemporaneously, gave me the same
result. I published this not knowing at the time of Schmidt’s
publication.
We shall say that uranium, thorium, and their compounds emit _Becquerel
rays_. I have called _radio-active_ those substances which generate
emissions of this nature. This name has since been adopted generally.
In their photographic and electric effects, the Becquerel rays
approximate to the Röntgen rays. They also, like the latter, possess the
faculty of penetrating all matter. But their capacity for penetration is
very different; the rays of uranium and of thorium are arrested by some
millimetres of solid matter, and cannot traverse in air a distance
greater than a few centimetres; this at least is the case for the
greater part of the radiation.
The researches of different physicists, and primarily of Mr. Rutherford,
have shown that the Becquerel rays undergo neither regular reflection,
nor refraction, nor polarisation.
The feeble penetrating power of uranium and thorium rays would point to
their similarity to the secondary rays produced by the Röntgen rays, and
which have been investigated by M. Sagnac, rather than to the Röntgen
rays themselves.
For the rest, the Becquerel rays might be classified as cathode rays
propagated in the air. It is now known that these different analogies
are all legitimate.
_Measurement of the Intensity of Radiation._
[Illustration: FIG. 1.]
The method employed consists in measuring the conductivity acquired by
air under the action of radio-active bodies; this method possesses the
advantage of being rapid and of furnishing figures which are comparable.
The apparatus employed by me for the purpose consists essentially of a
plate condenser, A B (Fig. 1). The active body, finely powered, is
spread over the plate B, making the air between the plates a conductor.
In order to measure the conductivity, the plate B is raised to a high
potential by connecting it with one pole of a battery of small
accumulators, P, of which the other pole is connected to earth. The
plate A being maintained at the potential of the earth by the connection
C D, an electric current is set up between the two plates. The potential
of plate A is recorded by an electrometer, E. If the earth connection be
broken at C, the plate A becomes charged, and this charge causes a
deflection of the electrometer. The velocity of the deflection is
proportional to the intensity of the current, and serves to measure the
latter.
But a preferable method of measurement is that of compensating the
charge on plate A, so as to cause no deflection of the electrometer. The
charges in question are extremely weak; they may be compensated by means
of a quartz electric balance, Q, one sheath of which is connected to
plate A and the other to earth. The quartz lamina is subjected to a
known tension, produced by placing weights in a plate, π; the tension is
produced progressively, and has the effect of generating progressively a
known quantity of electricity during the time observed. The operation
can be so regulated that, at each instant, there is compensation between
the quantity of electricity that traverses the condenser and that of the
opposite kind furnished by the quartz. In this way, the quantity of
electricity passing through the condenser for a given time, _i.e._, the
_intensity of the current_, can be measured _in absolute units_. The
measurement is independent of the sensitiveness of the electrometer.
In carrying out a certain number of measurements of this kind, it is
seen that radio-activity is a phenomenon capable of being measured with
a certain accuracy. It varies little with temperature; it is scarcely
affected by variations in the temperature of the surroundings; it is not
influenced by incandescence of the active substance. The intensity of
the current which traverses the condenser increases with the surface of
the plates. For a given condenser and a given substance the current
increases with the difference of potential between the plates, with the
pressure of the gas which fills the condenser, and with the distance of
the plates (provided this distance be not too great in comparison with
the diameter). In every case, for great differences of potential the
current attains a limiting value, which is practically constant. This is
the _current of saturation_, or _limiting current_. Similarly, for a
certain sufficiently great distance between the plates the current
hardly varies any longer with the distance. It is the current obtained
under these conditions that was taken as the measure of radio-activity
in my researches, the condenser being placed in air at atmospheric
pressure.
I append curves which represent the intensity of the current as a
function of the field established between the plates for two different
plate distances. Plate B was covered with a thin layer of powdered
metallic uranium; plate A, connected with the electrometer, was provided
with a guard-ring.
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
Fig. 2 shows that the intensity of the current becomes constant for high
potential differences between the plates. Fig. 3 represents the same
curves on another scale, and comprehends only relative results for small
differences of potential. At the origin, the curve is rectilinear; the
ratio of the intensity of the current to the difference of potential is
constant for weak forces, and represents the initial conduction between
the plates. Two important characteristic constants of the observed
phenomenon are therefore to be recognised:—(1) The _initial conduction_
for small differences of potential; (2) the _limiting current_ for great
potential differences. The limiting current has been adopted as the
measure of the radio-activity.
Besides the difference of potential established between the two plates,
there exists between them an electromotive force of contact, and these
two sources of current combine their effects; for this reason, the
absolute value of the intensity of the current changes with the sign of
the external difference of potential. In every case, for considerable
potential differences, the effect of the electromotive force of contact
is negligible, and the intensity of the current is therefore the same
whatever be the direction of the field between the plates.
The investigation of the conductivity of air and other gases subjected
to the action of Becquerel rays has been undertaken by several
physicists. A very complete research upon the subject has been published
by Mr. Rutherford.
The laws of the conductivity produced in gases by the Becquerel rays are
the same as those found for the Röntgen rays. The mechanics of the
phenomenon appear to be the same in both cases. The theory of ionisation
of the gases by the action of the Röntgen or Becquerel rays agrees well
with the observed facts. This theory will not be put forward here. I
will merely record the results to which they point:—
Firstly, the number of ions produced per second in the gas is considered
proportional to the energy of radiation absorbed by the gas.
Secondly, in order to obtain the limiting current relatively to a given
radiation, it is necessary, on the one hand, to cause complete
absorption of this radiation by the gas by employing a sufficient mass
of it; on the other hand, it is necessary for the production of the
current to use all the ions generated by establishing an electric field
of such strength that the number of the ions which recombine may be a
negligible fraction of the total number of ions produced in the same
time, most of which are carried by the current to the electrodes. The
strength of the electric field necessary to give this result is
proportional to the amount of ionisation.
According to the recent researches of Mr. Townsend, the phenomenon is
more complex when the pressure of the gas is low. At first the current
appears to approach to a constant limiting value with increasing
difference of potential; but after a certain point has been reached, the
current begins again to increase with the field, and with very great
rapidity. Mr. Townsend ascribes this increase to a new ionisation
produced by the ions themselves when, under the action of the electric
field, they acquire a velocity such that a molecule of gas encountering
one of them becomes broken down into its constituent ions. A strong
electric field and a low pressure are favourable to the production of
this ionisation by ions already present, and, as soon as the action is
set up, the intensity of the current increases uniformly with the field
between the plates. The limiting current could, therefore, only be
obtained under conditions of ionisation of which the intensity does not
exceed a certain value, and in such a manner that saturation corresponds
to fields in which, from multiplicity of ions, ionisation can no longer
take place. This condition has occurred in my experiments.
The order of magnitude of the saturation currents obtained with uranium
compounds is 10^{–11} ampères for a condenser in which the plates have a
diameter of 8 c.m., and are at a distance of 3 c.m. Thorium compounds
give rise to currents of the same order of magnitude, and the activity
of the oxides of uranium and thorium is very similar.
_Radio-activity of the Compounds of Uranium and Thorium._
The following are the figures I obtained with different uranium
compounds. I have represented the intensity of the current in ampères by
the letter _i_:—
_i_ × 10^{11}.
Metallic uranium (containing a little carbon) 2·3
Black oxide of uranium, U_{2}O_{5} 2·6
Green oxide of uranium, U_{3}O_{4} 1·8
Hydrated uranic acid 0·6
Uranate of sodium 1·2
Uranate of potassium 1·2
Uranate of ammonium 1·3
Uranium sulphate 0·7
Sulphate of uranium and potassium 0·7
Nitrate of uranium 0·7
Phosphate of copper and uranium 0·9
Oxysulphide of uranium 1·2
The thickness of the layer of the uranium compound used has little
effect, provided that the layer is uniform. The following illustrate
this point:—
Thickness of layer. _i_ × 10^{11}.
M.m.
Uranium oxide 0·5 2·7
Uranium oxide 3·0 3·0
Ammonium uranate 0·5 1·3
Ammonium uranate 3·0 1·4
It may be concluded from this that the absorption of uranium rays by the
substance which generates them is very great, since the rays proceeding
from deep layers produce no significant effect.
The figures I obtained with thorium compounds enable me to state:—
Firstly, that the thickness of the layer used has considerable effect,
especially in the case of the oxide.
Secondly, that the action is only regular if a sufficiently thin layer
is used (_e.g._, 0·25 m.m.). On the contrary, when a thick layer of the
substance is used (6 m.m.), the figures obtained vary between two
extreme limits, especially in the case of the oxide:—
Thickness of layer. _i_ × 10^{11}.
M.m.
Thorium oxide 0·25 2·2
Thorium oxide 0·5 2·5
Thorium oxide 2·5 4·7
Thorium oxide 3·0 5·5 (mean)
Thorium oxide 6·0 5·5 (mean)
Thorium sulphate 0·25 0·8 (mean)
There is here some cause of irregularities which do not exist in the
case of the uranium compounds. The figures obtained for a layer of oxide
6 m.m. thick varied between 3·7 and 7·3.
The experiments that I made on the absorption of uranium and thorium
rays showed that those of thorium are more penetrating than those of
uranium, and that the rays emitted by the oxide of thorium in a thick
layer are more penetrating than those emitted by a thin layer of the
same. The following figures (p. 13) give the fraction of the radiation
transmitted by a sheet of aluminium 0·01 thick.
With the uranium compounds, the absorption is the same whatever be the
compound used, which leads to the conclusion that the rays emitted by
the different compounds are of the same nature.
Radio-active substance. Fraction of radiation transmitted
by the sheet.
Uranium 0·18
Uranium oxide, U_{2}O_{5} 0·20
Uranate of ammonium 0·20
Phosphate of uranium and copper 0·21
Thorium oxide of thickness 0·25 m.m. 0·38
Thorium oxide of thickness 0·5 m.m. 0·47
Thorium oxide of thickness 3·0 m.m. 0·70
Thorium oxide of thickness 0·60 m.m. 0·70
Thorium sulphate 0·25 m.m. 0·38
The characteristics of the thorium radiation have formed the subject of
very complete publications. Mr. Owens has demonstrated that a uniform
current is only obtained after some time has elapsed, with an enclosed
apparatus, and that the intensity of the current is greatly reduced
under the influence of a current of air (which does not occur with the
compounds of uranium). Mr. Rutherford has made similar experiments, and
has explained them by the proposition that thorium and its compounds
produce, besides the Becquerel rays, another _emanation_, composed of
extremely minute particles, which remain radio-active for some time
after their emission, and are capable of being swept along by a current
of air.
The characteristics of the thorium radiation, which have reference to
the thickness of the layer employed and to the action of air currents,
have an intimate connection with the phenomenon of the _radio-activity
induced, and of its propagation from place to place_. This phenomenon
was observed for the first time with radium, and will be described
later.
The radio-activity of thorium and uranium compounds appears as an
_atomic property_. M. Becquerel has already observed that all uranium
compounds are active, and had concluded that their activity was due to
the presence of the element uranium; he also demonstrated that uranium
was more active than its salts. I have investigated, from this point of
view, the compounds of thorium and uranium, and have taken a great many
measurements of their activity under different conditions. The result of
all these determinations shows the radio-activity of these substances to
be decidedly an atomic property. It seems to depend upon the presence of
atoms of the two elements in question, and is not influenced by any
change of physical state or chemical decomposition. The chemical
combinations and mixtures containing uranium or thorium are active in
proportion to the amount of the metal contained, all inactive material
acting as inert bodies and absorbing the radiation.
_Is Atomic Radio-activity a general Phenomenon?_
As I have said above, I made experiments to discover whether substances
other than compounds of uranium and thorium were radio-active. I
undertook this research with the idea that it was scarcely probable that
radio-activity, considered as an atomic property, should belong to a
certain kind of matter to the exclusion of all other. The determinations
I made permit me to say that, for chemical elements actually considered
as such, including the rarest and most hypothetical, the compounds I
investigated were always at least 100 times less active in my apparatus
than metallic uranium.
The following is a summary of the substances experimented upon, either
as the element or in combination:—
1. All the metals or non-metals easily procurable, and some, more rare,
pure products obtained from the collection of M. Etard, at the Ecole de
Physique et de Chimie Industrielles de la Ville de Paris.
2. The following rare bodies:—Gallium, germanium, neodymium,
praseodymium, niobium, scandium, gadolinium, erbium, samarium, and
rubidium (specimens lent by M. Demarçay), yttrium, ytterbium (lent by M.
Urbain).
3. A large number of rocks and minerals.
Within the limits of sensitiveness of any apparatus, I found no simple
substance, other than uranium and thorium, possessing atomic
radio-activity. It will be suitable to add a few words here concerning
phosphorus. White moist phosphorus, placed between the plates of the
condenser, causes the air between the plates to conduct. However, I do
not consider this body radio-active in the same manner as thorium and
uranium. For, under these conditions, phosphorus becomes oxidised and
emits luminous rays, whilst uranium and thorium compounds are
radio-active without showing any chemical change which can be detected
by any known means. Further, phosphorus is not active in the red
variety, nor in a state of combination.
In a recent work, M. Bloch has demonstrated that phosphorus, undergoing
oxidation in air, gives rise to slightly motile ions, which make the air
conduct, and cause condensation of aqueous vapour.
Uranium and thorium are elements which possess the highest atomic
weights (240 and 232); they occur frequently in the same minerals.
_Radio-active Minerals._
I have examined many minerals in my apparatus; certain of them gave
evidence of radio-activity, _e.g._, pitchblende, thorite, orangite,
fergusonite, cleveite, chalcolite, autunite, monazite, &c. The following
is a table giving in ampères the intensity, _i_, of the current obtained
with metallic uranium and with different minerals:—
│_i_ × 10^{11}.
Uranium │2·3
Pitchblende from Johanngeorgenstadt│8·3
Pitchblende from Joachimsthal │7·0
Pitchblende from Pzibran │6·5
Pitchblende from Cornwallis │1·6
Cleveite │1·4
Chalcolite │5·2
Autunite │2·7
───────────────────────────────────┼──────────────
Various thorites │0·1
〃 │0·3
〃 │0·7
〃 │1·3
〃 │1·4
───────────────────────────────────┼──────────────
Orangite │2·0
Monazite │0·5
Xenotime │0·03
Æschynite │0·7
───────────────────────────────────┼──────────────
Fergusonite (two samples) │0·4
〃 │0·1
───────────────────────────────────┼──────────────
Samarskite │1·1
───────────────────────────────────┼──────────────
Niobite (two samples) │0·1
〃 │0·3
───────────────────────────────────┼──────────────
Tantalite │0·02
Carnotite │6·2
The current obtained with orangite (native oxide of thorium) varied
greatly with the thickness of the layer. By increasing this thickness
from 0·25 m.m. to 6 m.m. the current increased from 1·8 to 2·3.
All the minerals which showed radio-activity contained uranium or
thorium: their activity is therefore not surprising, but the intensity
of the action in certain cases is unexpected. Thus pitchblendes (ores of
uranium oxide) are found which are four times as active as metallic
uranium. Chalcolite (double phosphate of copper and uranium) is twice as
active as uranium. Autunite (phosphate of uranium and calcium) is as
active as uranium. These facts do not accord with previous conclusions,
according to which no mineral should be so active as thorium or uranium.
To throw light on this point, I prepared artificial chalcolite by the
process of Debray, starting with the pure products. The process consists
in mixing a solution of uranium nitrate with a solution of copper
phosphate in phosphoric acid and warming to 50° or 60°. After some time,
crystals of chalcolite appear in the liquid.
Chalcolite thus obtained possesses a perfectly normal activity, given by
its composition; it is two and a-half times less active than uranium.
It therefore appeared probable that if pitchblende, chalcolite, and
autunite possess so great a degree of activity, these substances contain
a small quantity of a strongly radio-active body, differing from uranium
and thorium and the simple bodies actually known. I thought that if this
were indeed the case, I might hope to extract this substance from the
ore by the ordinary methods of chemical analysis.