COMMUNICATION OF RADIO-ACTIVITY TO SUBSTANCES INITIALLY INACTIVE.
During the course of our researches on radio-active substances M. Curie
and I have observed that every substance which remains for some time in
the vicinity of a radium salt becomes itself radio-active. In our first
publication on this subject, we confined ourselves to proving that the
radio-activity thus acquired by substances initially inactive is not due
to the transference of radio-active particles to the surface of these
substances. This is proved beyond dispute by all the experiments which
will be here described, and by the laws according to which the
radio-activity excited in naturally inactive bodies disappears when the
latter are removed from the influence of radium.
We have given the name of _induced radio-activity_ to the new phenomenon
thus discovered.
In the same publication, we indicated the essential characteristics of
induced radio-activity. We excited screens of different substances by
placing them in the neighbourhood of solid radium salts, and we
investigated the radio-activity of these screens by the electrical
method. We observed the following facts:—
1. The activity of a screen exposed to the action of radium increases
with the time of exposure, approaching to a definite limit according to
an asymptotic law.
2. The activity of a screen which has been excited by the action of
radium, and which is afterwards withdrawn from its action, disappears in
a few days. This induced activity approaches zero as a function of the
time, following an asymptotic law.
3. Other things being equal, the radio-activity induced by the same
radium product upon different screens is independent of the nature of
the screen. Glass, paper, metals, all acquire the same degree of
activity.
4. The radio-activity induced in one screen by differing radium products
has a limiting value which rises with the increased activity of the
product.
Shortly afterwards, Mr. Rutherford published a research, which showed
that compounds of thorium are capable of producing the phenomenon of
induced radio-activity. Mr. Rutherford discovered for this phenomenon
the same laws as those just enunciated, besides this additional
important fact, that bodies charged with negative electricity become
more active than others. Mr. Rutherford also observed that air passed
over thorium oxide preserves a notable conductivity for about ten
minutes. Air in this condition communicates induced radio-activity to
inactive substances, especially to those negatively charged. Mr.
Rutherford explains his experiences by the supposition that compounds of
thorium, particularly the oxide, give rise to a _radio-active emanation_
capable of being carried by air currents and charged with positive
electricity. This emanation would be the origin of induced
radio-activity. M. Dorn has repeated, with salts of barium containing
radium, the experiments of Mr. Rutherford with thorium oxide.
M. Debierne has shown that actinium causes, to a marked degree, induced
activity of bodies placed in its vicinity. As in the case of thorium,
there is a considerable carriage of activity by air currents.
Induced radio-activity has various aspects, and irregular results are
obtained when the activity of a substance in the neighbourhood of radium
is excited in free air. MM. Curie and Debierne have observed, however,
that the phenomenon is quite regular when operating in a closed vessel;
they therefore investigated induced activity in a closed space.
_Activity Induced in an Enclosed Space._
The active material is placed in a little glass jar, _a_, open at O
(Fig. 11), in the centre of a closed space. Several plates, A, B, C, D,
E, placed in the enclosure become radio-active after one day’s exposure.
The activity is the same whatever be the nature of the plate, for equal
dimensions (lead, copper, aluminium, glass, ebonite, wax, cardboard,
paraffin). The activity of one face of one of the plates is greater in
proportion to the amount of free space about this face.
If the preceding experiment be repeated with the jar, _a_, completely
closed, no activity is induced.
The radiation of radium does not directly affect the production of
induced radio-activity. For this reason, in the preceding experiment the
plate D, screened from the radiation by a lead plate of thickness, P P,
is made as active as B and E.
Radio-activity is transmitted by the air by degrees from the radiating
body to the body to be excited. It can even be transmitted to a distance
by very narrow capillary tubes.
Induced radio-activity is both more intense and more regular if the
solid radium salt be replaced by an aqueous solution of the same.
Liquids are capable of acquiring induced radio-activity. For example,
pure water may be rendered active by placing it with a solution of a
radium salt within an enclosure.
[Illustration: FIG. 11]
Certain substances become luminous when placed in an active enclosure
(phosphorescent and fluorescent bodies, glass, paper, cotton, water,
salt solutions). Phosphorescent zinc sulphide is particularly brilliant
under the circumstances. The radio-activity of these luminous bodies is,
however, the same as that of a piece of a metal or other body which is
excited under the same conditions without becoming luminous.
Whatever be the substance made active in a closed vessel, this substance
acquires an activity which increases with length of time until it
attains a _limiting value_, always the same, for the same material and
the same experimental arrangement.
_The limit of induced radio-activity is independent of the nature and
pressure of the gas inside the active enclosure_ (air, hydrogen,
carbonic acid).
_The limit of induced radio-activity for the same enclosure depends only
on the quantity of radium present_ in the state of solution, and is
apparently proportional to it.
_Part played by Gases in the Phenomena of Induced Radio-activity._
_Emanation._—The gases present in an enclosure containing a solid salt
or a solution of a salt of radium are radio-active. This radio-activity
persists when the gas is drawn off with a tube and collected in a
test-tube. The sides of the test-tube become themselves radio-active,
and the glass of the test-tube is luminous in the dark. The activity and
luminosity of the test-tube finally completely disappear, but very
gradually, and a month afterwards radio-activity may still be detected.
Since the beginning of our researches, M. Curie and I have, by heating
pitchblende, extracted a strongly radio-active gas, but, as in the
preceding experiment, the activity of this gas finally completely
vanished.
We could discern no new ray in the spectrum of this gas; this was
therefore not a case of a new radio-active gas, and we understood later
that it was the phenomenon of induced radio-activity.
Thus, for thorium, radium, and actinium induced radio-activity is
progressively propagated through the gases, from the radiating body to
the walls of the enclosure containing it, and the exciting principle is
carried away with the gas itself, when the latter is extracted from the
enclosure.
When the radio-activity of radium compounds is measured by the
electrical method by means of the apparatus of Fig. 1, the air between
the plates is itself radio-active; however, on passing a current of air
between the plates, there is no observable lowering of the intensity of
the current, which proves that the radio-activity distributed in the
space between the plates is of little account in comparison with that of
the radium itself in the solid state.
It is quite otherwise with thorium. The irregularities which I observed
in determining the radio-activity of the thorium compounds arose from
the fact that at this point I was working with a condenser open to the
air; the least air current caused a considerable change in the intensity
of the current, because the radio-activity dispersed in the space in the
vicinity of the thorium is considerable as compared with the
radio-activity of the substance.
This effect is still more marked in the case of actinium. A very active
compound of actinium appears much less active when a current of air is
passed over the substance.
The radio-active energy is therefore contained in the gas in a special
form. Mr. Rutherford suggests that radio-active bodies generate an
_emanation_ or gaseous material which carries the radio-activity. In the
opinion of M. Curie and myself, the generation of a gas by radium is a
supposition which is not so far justified. We consider the emanation as
radio-active energy stored up in the gas in a form hitherto unknown.
_Dissipation in Free Air of the Induced Activity of Solid Bodies._
A solid body, which has been excited by radium in an enclosed space for
a sufficient length of time, and which has then been removed from the
enclosure, parts with its activity in free air according to an
exponential law, which is the same for all bodies represented by the
following formula:—
I = I_{0}(_a__e_^{–_t_/θ_{1}} – (_a_ – 1)_e_^{–_t_/θ_{2}}).
I_{0} being the initial intensity of the radiation at the moment of
withdrawing the plate from the enclosure; I, the intensity after time,
_t_; _a_ is a numerical coefficient, a = 4·20; θ_{1} and θ_{2} are time
constants, θ_{1} = 2420 secs., θ_{2} = 1860 secs. After two or three
hours this law becomes practically a simple exponential, and the effect
of the second exponential upon the value of I is negligible. The law of
dissipation is therefore such that the intensity of radiation becomes
diminished to one-half its value in twenty-eight minutes. This final law
may be considered as characteristic for the dissipation in an unconfined
air space of the activity induced in solid bodies by radium.
Solid bodies excited by actinium lose their activity in the open air,
according to an exponential law similar to the preceding, the
dissipation being, however, rather slower.
Solid bodies, made active by thorium, lose their activity much more
slowly; the intensity of the radiation is reduced to one-half in eleven
hours.
_Dissipation of Activity in a Confined Space. Velocity of Destruction of
the Emanation._
An enclosure, made active by radium and then removed from its influence,
loses its activity by a law which is much less rapid than that of
dissipation in the open air. The experiment may be carried out with a
glass tube, rendered active internally by placing it for some time in
contact with a solution of a salt of radium. The tube is then sealed in
the flame, and the intensity of radiation emitted by the walls of the
tube is measured while the dissipation takes place.
The law of dissipation is an exponential law. It is given very
accurately by the formula.
I = I_{0}_e_^{t/θ}.
I_{0} = initial intensity of radiation.
I = intensity of radiation at time, _t_.
θ = a time constant, θ = 4·970 × 10^5 secs.
The intensity of the radiation is reduced to one-half in four days.
This law of dissipation is absolutely invariable whatever be the
experimental conditions (dimensions of enclosure, nature of the walls,
nature of the gas within the enclosure, duration of action, &c.). The
law of dissipation remains the same for any temperature between –180°
and +450°. The law is therefore altogether characteristic.
In these experiments it is the radio-active energy accumulated in the
gas that maintains the activity of the walls. If the gas be withdrawn
and a vacuum caused in the enclosure, we have found that dissipation of
activity at once occurs in the rapid method; the intensity of radiation
being reduced to one-half in twenty-eight minutes. The same result is
obtained when ordinary air is substituted for the active air in the
enclosure.
The law of dissipation with reduction of intensity of radiation to
one-half in four days, is therefore characteristic of the disappearance
of radio-active energy accumulated in the gas. By making use of the
expression adopted by Mr. Rutherford, the emanation from radium may be
said to disappear spontaneously as a function of the time, with
reduction to one-half in four days.
The emanation from thorium is of another kind, and disappears much more
rapidly. The intensity of radiation diminishes to one-half in about one
minute ten seconds.
The emanation from actinium disappears still more rapidly; reduction to
one-half takes place in a few seconds.
_Variation of Activity of Liquids rendered Active and of Radium
Solutions._
Any liquid whatever becomes radio-active when placed in an active
confined space. On being removed and left freely exposed to the air, the
liquid rapidly loses its activity, imparting it to the gas and solid
bodies surrounding it. If a liquid thus made active be placed in a
closed flask, it loses its activity much more slowly; the latter being
reduced in intensity to one-half in four days, just as would a gas under
similar circumstances. This fact may be explained by assuming that the
radio-active energy is stored in liquids in exactly the same form as in
gases (in the form of an emanation).
A solution of a radium salt behaves in a somewhat similar manner. At
first, it is a remarkable fact that the solution of a radium salt placed
for some time in a confined space is no more active than pure water
placed in a vessel in the same enclosure, when the equilibrium of
activity is established. If the radium solution be removed from the
enclosure and left standing in the air in a wide-necked vessel, the
activity spreads itself into space, and the solution becomes nearly
inactive, though still containing radium. If this solution be now
enclosed in a stoppered flask, it gradually regains, in about a
fortnight, a maximum of activity, which may be considerable. On the
other hand, a liquid made active, but not containing radium, does not
regain its activity in a closed flask after having been exposed to the
atmosphere.
_Theory of Radio-activity._
The following is, according to MM. Curie and Debierne, a very general
theory which allows of the co-ordination of the results of the
investigation of induced radio-activity, which results I have just
stated, and which constitute facts apart from any hypothesis.
It may be said that each atom of radium acts as a constant and
continuous source of energy, without actually defining the origin of
this energy. The radio-active energy which accumulates in the radium
tends to become dissipated in two different ways:—(1) By radiation (rays
both charged and uncharged with electricity); (2) by conduction, _i.e._,
by gradual transmission to surrounding bodies in a medium of gases and
liquids (production of an emanation and transformation into induced
radio-activity).
The loss of radio-active energy, both by radiation and by conduction,
increases with the amount of energy accumulated in the radio-active
body. The system is necessarily in equilibrium when the double loss of
which I have just made mention compensates the constant gain due to the
action of radium. This manner of regarding the subject is similar to
that in use for calorific phenomena. If in the interior of any body
there is, owing to any cause, a continuous and constant evolution of
heat, the heat accumulates in the body and the temperature rises until
the loss of heat by radiation and conduction is in equilibrium with the
constant gain of heat.
In general, except under certain special circumstances, the activity is
not propagated through solid bodies. When a solution is kept in a sealed
tube, the loss by radiation alone takes place, and the radiating
activity of the solution is of a higher degree.
If, on the contrary, the solution stands in an open vessel, the loss of
activity by conduction becomes considerable, and when the state of
equilibrium is attained, the radiating activity of the solution is very
feeble.
The radiating activity of a solid radium salt left exposed to the air
does not sensibly diminish, because the propagation of activity by
conduction not taking place through solid bodies, it is a very thin
superficial layer only that produces induced radio-activity. The
solution, however, of the same salt produces much more intense phenomena
of induced radio-activity. With a solid salt the radio-active energy
accumulates in the salt, and is dissipated chiefly by radiation. On the
other hand, when the salt has been for several days in aqueous solution,
the radio-active energy is divided between the salt and the water, and
if separated by distillation the water carries with it a large portion
of the activity, and the solid salt is much less active (ten or fifteen
times) than before solution. Afterwards the solid salt gradually regains
its original activity.
The preceding theory may be yet further defined by supposing the
radio-activity of radium itself to be produced through the medium of the
radio-active energy emitted in the form of an emanation.
Each atom of radium may be considered as a constant and continuous
source of emanation. At the same moment that this form of energy is
produced, it undergoes a progressive transformation into radio-active
energy of the Becquerel radiation. The velocity of this transformation
is proportional to the quantity of the emanation accumulated.
When a radium solution is placed within an enclosure, the emanation is
able to expand into the enclosure and to spread out over the walls. Here
it is, therefore, that it is transformed into a radiation, the solution
giving off but few Becquerel rays; the radiation is, in some sort,
_externalised_. On the other hand, with solid radium, the emanation not
being able to escape readily, accumulates, and is transformed into the
Becquerel radiation on the spot; this radiation therefore acquires a
higher value.
If this theory of radio-activity were general, we should have to say
that all radio-active bodies give rise to an emanation. Now this
emission has been confirmed in the case of radium, thorium, and
actinium; with the latter in particular the emission is enormous, even
in the solid state. Uranium and polonium do not seem to emit any
emanation, though they generate Becquerel rays. These bodies produce no
induced radio-activity in an enclosed space, as do the radio-active
bodies mentioned before. This fact is not in absolute contradiction to
the preceding theory. If uranium and polonium were to emit emanations
which become destroyed with very great rapidity, it would be very
difficult to observe the carriage of such emanations by the air and the
effects of induced radio-activity produced by them upon neighbouring
bodies. Such a hypothesis is not improbable, since the times required
for certain quantities of the emanations of radium and thorium to
diminish to one-half are in the proportion of 5000 to 1. We shall see,
moreover, that, under certain conditions, uranium can excite induced
activity.
_Another Form of Induced Radio-activity._
According to the law of dissipation in the open air of the activity
induced by radium in solid bodies, the activity after one day is almost
imperceptible.
Certain bodies, however, form exceptions; such are celluloid, paraffin,
caoutchouc, &c. When these bodies have been acted upon to a sufficient
degree, they lose their activity more slowly than the law can account
for, and it is often fifteen or twenty days before the activity becomes
imperceptible. These bodies appear to have the property of becoming
charged with radio-active energy in the form of an emanation; they
afterwards lose it gradually, causing induced radio-activity in the
vicinity.
_Induced Radio-activity with Slow Dissipation._
There is yet another form of induced radio-activity, which appears to be
produced in all bodies which have been kept for months in an active
enclosure. When these bodies are removed from the enclosure their
activity at first diminishes to a very low value according to the
ordinary law (diminution to one-half in half-an-hour); but when the
activity has fallen to about 1/20,000 of the initial value, it
diminishes no further, or at least it is dissipated very slowly,
sometimes even increasing in amount. We have sheets of copper,
aluminium, and glass which still retain a residual activity after six
months.
These phenomena of induced radio-activity appear to be of a different
kind from the ordinary ones, and show a much slower process of
evolution.
A considerable time is also necessary both for the production and
dissipation of this form of induced radio-activity.
_Radio-activity Induced upon Substances in Solution with Radium._
When a radio-active ore containing radium is treated, with the object of
extracting the radium, chemical separations are effected, after which
the radio-activity is confined entirely to one of the products. In this
way active products, which may be several hundred times as active as
uranium, are separated from totally inactive products, such as copper,
antimony, arsenic, &c. Certain other bodies (iron, lead) were never
separated in an entirely inactive state. As these active bodies are
concentrated, the case is no longer the same; each chemical separation
no longer furnishes absolutely inactive products; all the resulting
products of a separation are active in varying degrees.
After the discovery of induced radio-activity, M. Giesel was the first
to attempt to excite activity in ordinary inactive bismuth by keeping it
in solution with very active radium. He thus obtained radio-active
bismuth, and from this he concluded that the polonium extracted from
pitchblende was probably bismuth made active by the vicinity of the
radium contained in the pitchblende.
I have also prepared active bismuth by keeping bismuth in solution with
a very active radium salt.
The difficulties of this experiment consist in the extreme precautions
which must be taken to remove all traces of radium from the solution. If
we realise what an infinitesimal quantity of radium suffices to produce
very considerable radio-activity in 1 grm. of material, it is difficult
to believe in the possibility of sufficiently washing and purifying the
active product. Each purification causes a diminution of activity of the
product, whether this be due to removal of traces of radium or that the
induced radio-activity is, under these circumstances, not proof against
chemical reactions.
The results I obtained appear, however, to establish with certainty the
fact that the activity is produced and persists after the radium is
removed. On fractionating the nitrate of my active bismuth by
precipitation with water from the nitric acid solution, I found that
after careful purifying it fractionated like polonium, the most active
portion being precipitated first.
If the purification is not complete the opposite occurs, showing that
traces of radium still remain. I thus obtained active bismuth which from
the manner of fractionation showed great purity and which was 2000 times
as active as uranium. This bismuth diminishes in activity with lapse of
time. But another portion of the same product, prepared with the same
precautions, and fractionating in the same manner, preserves its
activity without diminution for actually a period of about three years.
This activity is 150 times as great as that of uranium.
I have also prepared active lead and silver by leaving them in solution
with radium. Generally induced radio-activity obtained in this way
scarcely lessens with lapse of time, but it does not as a rule withstand
many successive chemical changes of the active body.
M. Debierne made active barium by placing it in solution with actinium.
This barium remains active after several chemical reactions, its
activity being therefore a somewhat stable atomic property. Active
barium chloride fractionates like barium-radium chloride, the more
active portions being the least soluble in water and dilute hydrochloric
acid. The dry chloride is spontaneously luminous: its Becquerel
radiation is similar to that of barium-radium chloride. M. Debierne has
prepared an active barium chloride 1000 times as active as uranium. This
barium, however, had not acquired all the characteristics of radium, for
it showed none of the strongest radium lines in the spectroscope.
Further, its activity diminished on standing, and after three weeks it
had become one-third of its original value.
There is a wide field for research upon the radio-activity induced in
substances in solution with active bodies. It appears that, according to
the conditions of experiment, more or less stable forms of induced
atomic radio-activity may be obtained. The radio-activity induced under
these circumstances is perhaps identical with that form, which
dissipates slowly, obtained by prolonged exposure at a distance in an
active enclosure. We have reason to enquire to what degree induced
radio-activity affects the chemical nature of the atom, and if it is
able to modify the chemical properties of the latter, either temporarily
or permanently.
The chemical investigation of bodies excited at a distance is rendered
difficult by the fact that the induced activity is limited to a very
thin superficial layer, and that, consequently, only a very small
proportion of the material has been affected.
Induced radio-activity also results from leaving certain substances in
solution with uranium. The experiment succeeded in the case of barium.
If, as was done by M. Debierne, sulphuric acid be added to a solution
containing uranium and barium, the precipitate of barium sulphate
acquires radio-activity, and, at the same time, the uranium salt loses
part of its activity. M. Becquerel found, after repeating this
experiment several times, that almost inactive uranium was obtained.
This might lead to the opinion that a radio-active body differing from
uranium had been separated from the latter, its presence producing
radio-activity in uranium. This, however, is not the case, for after
some months the uranium regains its original activity; the precipitated
barium sulphate, on the contrary, loses what it acquired.
A similar phenomenon is observed with thorium. Mr. Rutherford
precipitated a solution of a salt of thorium with ammonia; he separated
off the solution and evaporated it to dryness. He thus obtained a small
very active residue, and the precipitated thorium was observed to be
less active than before. This active residue, to which Mr. Rutherford
gives the name of _thorium_ X, loses its activity after a time, whilst
the thorium regains its original activity.
It appears, then, that concerning induced radio-activity all bodies do
not behave in a similar manner, and that certain of them are much more
readily excited than others.
_Dissemination of Radio-active Particles and Induced Radio-activity of
the Laboratory._
In making investigations of strongly radio-active bodies, particular
precautions must be observed for obtaining delicate determinations. The
different objects used in the chemical laboratory and those used for
physical experiments soon acquire radio-activity, and act upon
photographic plates through black paper. Dust particles, the air of the
room, clothing, all become radio-active. The air of the room becomes a
conductor. In our laboratory the evil has become acute, and we no longer
have any apparatus properly insulated.
Special precautions must therefore be taken to avoid as much as possible
the dissemination of active dust particles, and to avoid also the
phenomena of induced activity.
The objects employed in chemistry should never be brought into the room
where physical research is carried on, and as far as possible should be
avoided any unnecessary keeping of active substances in this room.
Before beginning our researches we were in the habit, in the case of
electrical experiments, of making a connection between the different
parts of the apparatus by insulated metallic wires, protected by metal
cylinders connected to earth, which screened the wires from all outside
electrical forces. In the investigation of radio-active bodies this
arrangement is quite defective; the air being a conductor there is
incomplete insulation between the thread and the cylinder, and the
inevitable electromotive force of contact between the thread and the
cylinder tends to produce a current through the air, and to cause a
deflection of the electrometer. We now screen all the wires from the air
by placing them inside cylinders filled with paraffin or other
insulating material. It would also be advantageous in these
investigations to make use of carefully enclosed electrometers.
_Activity Induced Outside the Influence of Radio-active Substances._
Attempts were made to produce induced radio-activity outside the action
of radio-active bodies.
M. Villard subjected to the action of the cathode rays a piece of
bismuth placed as anticathode in a Crookes tube; the bismuth was thus
rendered active to a very slight degree, for it required an exposure of
eight days to obtain a photographic impression.
Mr. MacLennan has exposed different salts to the action of cathode rays,
afterwards warming them slightly. The salts then acquired the property
of neutralising bodies positively charged.
Studies of this kind are of great interest. If, by using known physical
agents, it were possible to create a considerable radio-activity in
bodies originally inactive, we might hope thence to discover the cause
of the spontaneous radio-activity of certain substances.
_Variations of Activity of Radio-active Bodies. Effects of Solution._
The activity of polonium, as I have said above, diminishes with time.
This diminution is slow, and does not take place at the same rate with
different specimens. A sample of bismuth-polonium nitrate lost half its
activity in eleven months, and 95 per cent in thirty-three months. Other
specimens have evidenced similar diminution.
A specimen of metallic bismuth containing polonium was prepared from the
nitrite, its activity after preparation being 100,000 times that of
uranium. The metal is now only a body of medium radio-activity (2000
times that of uranium). Its radio-activity is determined at intervals.
In six months it has lost 67 per cent of its activity.
The loss of activity does not seem to be facilitated by chemical action.
In rapid chemical changes no considerable loss of activity has in
general taken place.
In contrast to that which occurs with polonium, radium salts possess a
permanent radio-activity which evidences no appreciable diminution after
many years.
A freshly prepared radium salt in the solid state does not at first
possess an activity of constant strength. Its activity increases from
the time of preparation until it attains a practically constant limiting
value after about one month. The opposite is the case for a solution.
When freshly prepared the solution is very active, but when left exposed
to the air it rapidly loses activity, and finally reaches a limiting
activity which may be considerably less than the original. These
variations of activity were first observed by M. Giesel. They are easily
accounted for by the emanation theory. The diminution of the activity of
the solution corresponds to the loss of the emanation which escapes into
space; this diminution is much less when the solution is contained in a
sealed tube. A solution which has lost its activity in air recovers a
greater activity in a sealed tube. The time of increase of the activity
of the salt which, after solution, has been recently obtained in the
solid state, is that during which the emanation is being newly stored in
the solid radium.
The following are some experiments on this subject:—
A solution of barium-radium chloride left exposed to the air for two
days becomes 300 times less active.
A solution is enclosed in a stoppered vessel; the vessel is opened, the
solution poured into a dish, and the activity determined:—
Activity immediately determined 67
Activity after two hours 20
Activity after two days 0·25
A solution of barium-radium chloride, which has been kept open to the
air, is enclosed in a sealed glass tube, and the radiation of this tube
determined. The following results were observed:—
Activity determined immediately 27
Activity determined after 2 days 61
Activity determined after 3 days 70
Activity determined after 4 days 81
Activity determined after 7 days 100
Activity determined after 11 days 100
The initial activity of a solid salt after preparation is feeble in
proportion as the time of solution was long. A greater proportion of
activity is then transmitted to the solvent. The following figures give
the initial activity with a chloride whose limiting activity is 800, and
which was kept for a given time in solution; the salt was afterwards
dried, and its activity immediately determined:—
Limiting activity 800
Initial activity after solution and immediate evaporation 440
Initial activity after the salt has remained dissolved 5 days 120
Initial activity after the salt has remained dissolved 18 days 130
Initial activity after the salt has remained dissolved 32 days 114
During this experiment the dissolved salt was placed in a vessel merely
covered with a watch-glass.
I made with the same salt two solutions which I kept in sealed tubes for
thirteen months; one of these solutions was eight times the strength of
the other:—
Initial activity of the salt in concentrated solution after
evaporation 200
Initial activity of the salt in dilute solution after evaporation 100
The loss of activity of the salt is therefore greater when the amount of
solvent is greater, the radio-active energy transmitted to the liquid
having a greater volume of liquid to saturate and a greater space to
fill. The two specimens of the same salt, which thus had a different
initial activity, further increased in activity at very different rates
at first; at the end of one day they had the same activity, and the
increase of activity now continued in the same manner for both till the
limit was reached.
When the solution is dilute the loss of activity by the salt is very
rapid, as is shown by the following experiments:—Three equal portions of
the same radium salt are dissolved in equal quantities of water. The
first solution (_a_) is allowed to stand in contact with the air for one
hour, and is then evaporated. The second solution (_b_) has a current of
air passed through it for one hour, and is then evaporated. The third
solution (_c_) is left exposed to the air for thirteen days, and then
evaporated to dryness. The initial activity of each of the three salts
is:—
For portion _a_ 145·2
For portion _b_ 141·6
For portion _c_ 102·6
The limiting activity of the same salt is about 470. We thus see that
the greatest part of the effect was produced at the end of one hour.
Further, the air current bubbling through solution _b_ for one hour
produced little effect. The proportion of salt in solution was about 0·5
per cent.
Radio-active energy in the form of an emanation is propagated with
difficulty from solid radium in air; it experiences the same resistance
to propagation from solid radium in a liquid. When radium sulphate is
shaken with water for a whole day, its activity after the operation is
practically the same as that of a portion of the same sulphate left
exposed to air.
On placing the radium salt in a vacuum, all the emanation capable of
displacement is withdrawn. However, the radio-activity of a radium
chloride kept _in vacuo_ for six days was not sensibly affected by the
operation. This experiment shows that the radio-activity of the salt is
principally due to the radio-active energy generated within the
particles, and which is unaffected by the vacuum.
The loss of activity that radium undergoes when in solution is
relatively greater for the penetrating rays than for the absorbable
rays. The following are examples of this:—
A radium chloride which had reached its limit of activity, 470, was
dissolved and left in solution for one hour; it was then evaporated, and
its initial radio-activity determined by the electrical method. The
total initial radiation was found to be equal to the fraction 0·3 of the
total limiting radiation. If the determination of the intensity of
radiation be made by covering the active body with an aluminium screen
0·01 m.m. thick, the initial radiation which traverses this screen is
found to be only the fraction 0·17 of the limiting radiation traversing
the same screen.
When the salt has been thirteen days in solution, the total initial
radiation is found to be the fraction 0·22 of the total limiting
radiation, and is 0·13 of the limiting radiation after traversing 0·01
m.m. of aluminium.
In the two cases the ratio of the initial radiation after solution to
the limiting radiation is 1·7 times as great for the total radiation as
for the radiation which has traversed 0·01 m.m. of aluminium.
It must further be mentioned that on evaporating the product after
solution, it is impossible to avoid a certain period of time during
which the product is in an intermediate condition, neither entirely
solid nor entirely liquid. Neither can one avoid warming the product to
remove the water quickly.
For these two reasons it is scarcely possible to determine the true
initial activity of the product passing from solution to the solid
state. In the experiments just quoted, equal quantities of the active
bodies were dissolved in the same quantity of water, and the solutions
were then evaporated to dryness under conditions as identical as
possible, and without heating above 120° or 130°.
I investigated the law according to which the activity of a solid radium
salt increases, from the moment in which the salt is obtained dry after
solution to the moment in which it reaches its limit of activity. In the
tables which follow, the intensity of radiation, I, is represented as a
function of the time, the limiting intensity being supposed equal to
100, and the time being reckoned from the moment at which the product
was dried. Table I. (Fig. 12, Curve I.) refers to the total radiation.
Table II. (Fig. 12, Curve II.) refers only to the penetrating rays (rays
which have traversed 3 c.m. of air and 0·01 m.m. of aluminium).
TABLE I. TABLE II.
Time. I. Time. I.
Days. Days.
0 21 0 1·3
1 25 1 19
3 44 3 43
5 60 6 60
10 78 15 70
19 93 23 86
33 100 46 94
67 100
[Illustration: FIG. 12.]
I made several other series of determinations of the same kind, but they
do not absolutely agree with one another, although the general character
of the curves obtained remains the same. It is difficult to obtain very
regular results. It may, however, be remarked that the acquisition of
activity requires more than one month for its production, and that the
most penetrating rays are the most deeply affected by solution.
The initial intensity of the radiation which is able to traverse 3 c.m.
of air and 0·01 m.m. of aluminium is only 1 per cent of the limiting
intensity, whilst the initial intensity of the total radiation is 21 per
cent of the total limiting radiation.
A radium salt which has been dissolved and recently evaporated to
dryness possesses the same power of causing induced activity (and,
consequently, of allowing the escape of an emanation), as a specimen of
the same salt which, after having been prepared in the solid state, has
remained in this condition long enough to have attained its limiting
radio-activity. The radiant activity of these two products is, however,
quite different; the former is, for example, five times less active than
the latter.
_Variations of the Activity of Radium Salts on Heating._
When a radium compound is heated, it gives off an emanation and loses
activity. The more intense and the more prolonged the heating, the
greater is the loss of activity. Thus, on heating a radium salt for one
hour to 130°, it loses 10 per cent of its total radiation; on the other
hand, heating for ten minutes to 400° produces no apparent effect.
Heating to redness for several hours destroys 77 per cent of the total
radiation.
The loss of activity on heating is more considerable for the penetrating
than for the absorbable rays. Thus, heating for several hours destroys
about 77 per cent of the total radiation, but the same amount of heating
destroys nearly the whole (99 per cent) of the radiation that traverses
3 c.m. of air and 0·01 m.m. of aluminium. If barium-radium chloride be
kept fused for several hours (towards 800°) 98 per cent of the radiation
capable of traversing 0·3 m.m. of aluminium is destroyed. The
penetrating rays may be considered as no longer in existence after
intense and prolonged heating.
When a radium salt has lost part of its activity by heating, the
diminution is not lasting; the activity of the salt is spontaneously
regenerated at the ordinary temperature, and approaches a certain
limiting value. I have observed the curious fact that this limit is
higher than the limiting activity of the salt before being heated—this,
at least, is the case with the chloride. I give examples of this:—A
specimen of barium-radium chloride which, after having been prepared in
the solid state, has long since attained its limiting activity,
possesses a total radiation represented by the number 470, and a
radiation capable of traversing 0·01 m.m. of aluminium, represented by
the number 157. This, specimen is heated to redness for several hours.
Two months after the heating it attains its limit of activity with a
total radiation equal to 690, and a radiation through 0·01 m.m. of
aluminium, equal to 227. The total radiation and the radiation
transmitted by aluminium are therefore increased respectively in the
ratios 690/470 and 227/156. These two ratios are practically equal to
one another, and are equivalent to 1·45.
A specimen of radium-barium chloride which, after having been prepared
in the solid state, has attained a limiting activity of 62, is
maintained in a state of fusion for some hours; the fused product is
then powdered. The product regains a new limiting activity equal to 140,
which is twice as great as that to which it was able to attain when
prepared in the solid state without having been sensibly heated during
evaporation.
I have investigated the law of increase of activity of radium compounds
after heating. The following are the results of two series of
determinations:—The figures of Table I. and II. represent the intensity
of the radiation (I) as a function of time, the limiting intensity being
supposed equal to 100, and the time being reckoned from the close of the
heating. Table I. (Fig. 13, Curve I.) refers to the total radiation of a
specimen of barium-radium chloride. Table II. (Fig. 13, Curve II.)
relates to the penetrating radiation of a specimen of barium-radium
sulphate, the intensity of the radiation which traversed 3 c.m. of air
and 0·01 m.m. of aluminium having been determined. The two products were
subjected to a bright red heat for seven hours.
TABLE I. TABLE II.
Time. I. Time. I.
Days. Days.
0 16·2 0 0·8
0·6 25·4 0·7 13
1 27·4 1 18
2 38 1·9 26·4
3 46·3 6 46·2
4 54 10 55·5
6 67·5 14 64
10 84 18 71·8
24 95 27 81
57 100 36 91
50 95·5
57 99
84 100
I made several other series of determinations, but the results did not
agree well.
The effect of heating does not persist when the heated radium compound
is dissolved. Of two specimens of the same radium compound of activity
1800, one was strongly heated and its activity thereby reduced to 670.
The two portions being now dissolved and left in solution for twenty
hours, their initial activities in the solid state were 460 for the not
heated portion and 420 for the heated one; the difference between the
two portions was therefore not considerable. But if the two products do
not remain for a sufficient length of time in solution—if, for example,
they are evaporated to dryness, immediately after solution—the not
heated product is much more active than the heated one; a certain time
is necessary in the dissolved state for the effects of heating to
disappear. A product of activity 3200 was heated, and its activity
thereby reduced to 1030. This product and a similar portion which had
not been heated were simultaneously dissolved, and the two then
immediately evaporated to dryness. The initial activity was 1450 for the
not heated portion, and 760 for the heated one.
[Illustration: FIG. 13.]
In the case of solid radium salts the capacity for exciting induced
radio-activity is largely affected by heating. During heating radium
compounds give off a larger amount of emanation than at the ordinary
temperature; but on being cooled to the ordinary temperature, not only
is their radio-activity much less than before heating, but their
capacity for inducing activity is much diminished. During the time that
follows the heating, the radio-activity of the product increases, and
may even exceed the original value. The induction capacity is also
partially re-established; however, after prolonged heating to redness,
this capacity is almost entirely destroyed without spontaneous
re-appearance afterwards. The induction capacity may be restored to the
radium salt by dissolving it in water, and drying it in the oven at a
temperature of 120°. This seems to have the effect of leaving the salt
in a peculiar physical condition, in which the emanation is given off
with much less facility than is the case with the same solid product not
heated to a high temperature, and it follows naturally that the salt
attains a higher limit of activity than that which it possessed before
heating. To transform the salts into the physical condition proper to it
before heating, it suffices to dissolve it and to evaporate it to
dryness without heating it above 150°.
The following are numerical examples of the above:—
I represent by _a_ the limit of induced activity produced in a closed
vessel upon a plate of copper by a specimen of barium-radium carbonate
of activity 1600.
Suppose _a_ = 100 for the not heated product. We find—
1 day after heating _a_ = 3·3
4 days after heating _a_ = 7·1
10 days after heating _a_ = 15
20 days after heating _a_ = 15
37 days after heating _a_ = 15
The radio-activity of the product had diminished 90 per cent by heating,
but one month afterwards the original value was regained.
The following is an experiment of the same kind made with a
barium-radium chloride of activity 3000. The induction capacity is
determined in the same manner as before.
For the product not heated _a_ = 100.
Induction capacity of the product after being heated to redness for
three hours:—
2 days after heating 2·3
5 days after heating 7·0
11 days after heating 8·2
18 days after heating 8·2
Induction capacity of the unheated substance which has been
dissolved and then dried at 150° 92
Induction capacity of the heated substance which has been
dissolved and then dried at 150° 105
_Theory of Interpretation of the Causes of Variations of Activity of
Radium Salts after Solution and after Heating._
The facts previously indicated may be in part explained by the theory
according to which the energy of radium is produced in the form of an
emanation, which is then transformed into the energy of radiation. When
a radium salt is dissolved, the emanation produced by it spreads beyond
the solution, and causes radio-activity outside the source from which it
proceeds; when the solution is evaporated, the solid salt obtained is
but slightly active, because it contains only a small amount of
emanation. Gradually the emanation is accumulated in the salt, the
activity of which rises to a limiting value, which is reached when the
production of the emanation by the radium compensates the loss by
external emission and by local transformation into Becquerel rays.
When a radium salt is heated, the external emission of the emanation is
greatly increased, and the phenomena of induced radio-activity are more
intense than when the salt is at the ordinary temperature. But when the
salt returns to the ordinary temperature it is exhausted, as is the case
after being dissolved, and contains but a small amount of emanation, its
activity having become greatly reduced. Gradually the emanation
accumulates afresh in the solid salt, and the radiation increases.
It may be said that radium gives rise to a constant generation of any
emanation—part of which escapes to the exterior, the remainder being
transformed in the radium itself into Becquerel rays. When radium is
raised to a red heat, it loses the greater part of its capacity to cause
the induction of activity; otherwise stated, the evolution of the
emanation is lessened. Consequently, the proportion of the emanation
utilised in the radium itself should be greater, and the substance
attains a higher limit of radio-activity.
We will endeavour to establish theoretically the law of rise of activity
of a solid radium salt which has been dissolved or has been heated. We
will assume that the intensity of radiation of radium is, at each
instant, proportional to the quantity of emanation, _q_, present in the
radium. We know that the emanation is spontaneously destroyed according
to a law such that, at each instant—
_q_ = _q__{0}_e_^{–_t_/θ} . . . . . . . . . 1,
_q__{0} being the amount of the emanation at the moment of starting the
observation, and θ the time constant, equal to 4·97 × 10^5 secs.
Now let Δ be the evolution of the emanation by radium, a quantity which
I will assume constant. Let us consider what would occur if no emanation
were escaping to the exterior. The emanation generated would then be
completely utilised by the radium for the production of the radiation.
We have from Formula 1—
_dq_/_dt_ = –(_q__{0}/θ)_e_^{–_t_/θ} = –_q_/θ;
and consequently, in the state of equilibrium, the radium would contain
a certain quantity of emanation, Q, such that—
Δ = Q/θ . . . . . 2,
and the radiation of the radium would then be proportional to Q.
Let us suppose the radium placed in the circumstances under which it
gives off the emanation to the exterior; this is obtained by dissolving
the radium compound or by heating it. The equilibrium will be disturbed,
and the activity of the radium diminished. But as soon as the cause of
the loss of emanation has been abolished (the body being restored to the
solid state or the heating having ceased), the emanation is accumulated
afresh in the radium and we have a period during which the evolution, Δ,
surpasses the velocity of destruction, _q_/θ. We then have—
_dq_/_dt_ = Δ – _q_/θ = (Q – _q_)/θ,
from which—
(_d_/_dt_)(Q – _q_) = –(Q – _q_)/θ,
Q – _q_ = (Q – _q__{0})e^{–_t_/θ} 3,
_q__{0} being the amount of emanation present in the radium at time _t_
= 0.
According to Formula 3, the excess of the quantity of emanation, Q,
contained by the radium in a state of equilibrium above the quantity,
_q_, contained at a given moment, decreases as a function of the time
according to an exponential law, which is also the law of the
spontaneous disappearance of the emanation. The radiation of radium
being proportional to the amount of emanation, the excess of the
intensity of the limiting radiation above the actual intensity should
decrease as a function of the time by the same law; the excess should
thus diminish to one-half in about four days.
The preceding theory is incomplete, since the loss of emanation to the
exterior has been neglected. It is also difficult to determine the
manner in which this acts as a function of the time. In comparing the
results of experiment with those of this incomplete theory, there is
found to be no satisfactory agreement; the conviction is, however,
retained that the theory in question is partially true. The law by which
the excess of limiting activity above the actual activity diminishes to
one-half in four days represents approximately the course of the renewal
of activity after heating for ten days. In the case of the renewal of
activity after solution, the same law appears to hold approximately for
a certain period of time, which begins two or three days after
evaporation to dryness and continues for ten or fifteen days. The
phenomena are otherwise complex; the theory sketched out does not
explain the reason of the suppression of the penetrating rays in greater
proportion than the absorbable rays.
NATURE AND CAUSE OF THE PHENOMENA OF RADIO-ACTIVITY.
From the beginning of research upon the radio-active bodies, and when
the properties of these bodies were yet hardly known, the spontaneity of
their radiation presented itself as a problem having the greatest
interest for physicists. To-day we have advanced considerably in the
understanding of radio-active bodies, and are able to isolate one of
very great power, viz., radium. With the object of making use of the
remarkable properties of radium, a profound investigation of the rays
emitted by radio-active bodies is indispensable; the various groups of
rays under investigation present points of similarity with the groups of
rays existing in Crookes tubes: cathode rays, Röntgen rays, canal rays.
The same groups of rays are found in the secondary radiation produced by
Röntgen rays, and in the radiation of bodies which have acquired
radio-activity by induction.
But if the nature of the radiation is actually better known, the cause
of this spontaneous radiation remains a mystery, and the phenomena
always presents itself to us as a profound and wonderful enigma.
The spontaneously radio-active bodies, and in the first place radium,
are sources of energy. The evolution of energy, to which they give rise,
is manifested by Becquerel radiation, by chemical and luminous effects,
and by the continuous generation of heat.
The question often arises as to whether energy is created within the
radio-active bodies themselves, or whether it is borrowed by them from
external sources. No one of the numerous hypotheses arising from these
two points of view has yet received experimental confirmation.
The radio-active energy may be assumed to have been initially
accumulated and then gradually dissipated, as happens in the case of
long continued phosphorescence. We imagine the evolution of radio-active
energy to correspond to a transformation of the nature of the atom of
the active body; the fact of the continuous generation of heat by radium
speaks in favour of this hypothesis. The transformation may be assumed
to be accompanied by a loss of weight and by an emission of material
particles constituting the radiation. The source of energy may yet be
sought in the energy of gravitation. Finally, we may imagine that space
is constantly traversed by radiations yet unknown, which are arrested in
their course by radio-active bodies and transformed into radio-active
energy.
Many reasons are adduced for and against these different views, and most
often attempts at experimental verifications of the conclusions drawn
from these hypotheses have given negative results. The radio-active
energy of uranium and radium apparently neither becomes exhausted nor
varies appreciably with lapse of time. Demarçay examined
spectroscopically a specimen of pure radium chloride after a five
months’ interval, and observed no change in the spectrum. The principal
barium line, which was visible in the spectrum indicating the presence
of a trace of barium, had not increased in intensity during the
interval, showing therefore that there was no transformation of radium
into barium to an appreciable extent.
The variations of weight announced by M. Heydweiller in radium compounds
cannot yet be looked upon as established facts.
Elster and Geitel found that the radio-activity of uranium is not
affected at the bottom of a mine-shaft 850 m. deep; a layer of earth of
this thickness would therefore not affect the hypothetical primary
radiation which would be excited by the radio-activity of uranium.
We have determined the radio-activity of uranium at midday and at
midnight, thinking that if the hypothetical primary radiation had its
origin in the sun it would be partly absorbed in traversing the earth.
The experiment showed no difference in the two determinations.
_Conclusions._
I will define, in conclusion, the part I have personally taken in the
researches upon radio-active bodies.
I have investigated the radio-activity of uranium compounds. I have
examined other bodies for the existence of radio-activity, and found the
property to be possessed by thorium compounds. I have made clear the
atomic character of the radio-activity of the compounds of uranium and
thorium.
I have conducted a research upon radio-active substances other than
uranium and thorium. To this end I investigated a large number of
substances by an accurate electrometric method, and I discovered that
certain minerals possess activity which is not to be accounted for by
their content of uranium and thorium.
From this I concluded that these minerals must contain a radio-active
body different from uranium and thorium, and more strongly radio-active
than the latter metals.
In conjunction with M. Curie, and subsequently with MM. Curie and
Bémont, I was able to extract from pitchblende two strongly radio-active
bodies—polonium and radium.
I have been continuously engaged upon the chemical examination and
preparation of these substances. I effected the fractionations necessary
to the concentration of radium, and I succeeded in isolating pure radium
chloride. Concurrently with this work, I made several atomic weight
determinations with a very small quantity of material, and was finally
able to determine the atomic weight of radium with a very fair degree of
accuracy. The work has proved _that radium is a new chemical element_.
Thus the new method of investigating new chemical elements, established
by M. Curie and myself, based upon radio-activity, is fully justified.
I have investigated the law of absorption of polonium rays, and of the
absorbable rays of radium, and have demonstrated that this law of
absorption is peculiar and different from the known laws of other
radiations.
I have investigated the variation of activity of radium salts, the
effect of solution and of heating, and the renewal of activity with
time, after solution or after heating.
In conjunction with M. Curie, I have examined different effects produced
by the new radio-active substances (electric, photographic, fluorescent,
luminous colourations, &c.).
In conjunction with M. Curie, I have established the fact that radium
gives rise to rays charged with negative electricity.
Our researches upon the new radio-active bodies have given rise to a
scientific movement, and have been the starting-point of numerous
researches in connection with new radio-active substances, and with the
investigation of the radiation of the known radio-active bodies.
------------------------------------------------------------------------
RADIUM
AND OTHER
RADIO-ACTIVE ELEMENTS.
A Popular account of Radium and other Radio-active Elements treated
experimentally.
Including a number of Radium Photographs and other Illustrations.
By LEONARD A. LEVY and HERBERT G. WILLIS.
PRICE 2s. 6d. net; post free 2s. 9d.
CONTENTS.
INTRODUCTION.—Chap. I. The Discovery of Radium; its Extraction.—Chap.
II. The Radium Emanation.—Chap. III. Physical Effects (Electrical and
Magnetic).—Chap. IV. Physical Effects (continued).—Chap. V. Chemical
Effects.—Chap. VI. Actions on Animal Life and Living Tissues.—Chap. VII.
Other Radio-active Elements.—Appendix.—Experimental Apparatus.
PERCIVAL MARSHALL & CO.,
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RADIUM
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For Demonstrating the Radio-activity of Radium.
KUNZITE.
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* * * * *
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TRANSCRIBER’S NOTES
1. Silently corrected typographical errors and variations in spelling.
2. Anachronistic, non-standard, and uncertain spellings retained as
printed.
3. Enclosed italics font in _underscores_.
4. Enclosed bold font in =equals=.
5. Superscripts are denoted by a caret before a single superscript
character or a series of superscripted characters enclosed in
curly braces, e.g. M^r. or M^{ister}.
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subscripted characters enclosed in curly braces, e.g. H_{2}O.
End of Project Gutenberg's Radio-Active Substances, by Sklodowska Curie