EXCITED RADIO-ACTIVITY.
=175. Excited radio-activity.= One of the most interesting and
remarkable properties of thorium, radium, and actinium, is their power
of “exciting” or “inducing” temporary activity on all bodies in their
neighbourhood. A substance which has been exposed for some time in the
presence of radium or thorium behaves as if its surface were covered
with an invisible deposit of intensely radio-active material. The
“excited” body emits radiations capable of affecting a photographic
plate and of ionizing a gas. Unlike the radio-elements themselves,
however, the activity of the body does not remain constant after it has
been removed from the influence of the exciting active material, but
decays with the time. The activity lasts for several hours when due to
radium and several days when due to thorium.
This property was first observed by M. and Mme. Curie[269] for radium,
and independently by the writer[270] for thorium[271].
If any solid body is placed inside a closed vessel containing an
emanating compound of thorium or radium, its surface becomes
radio-active. For thorium compounds the amount of excited activity on a
body is in general greater the nearer it is to the active material. In
the case of radium, however, provided the body has been exposed for
several hours, the amount of excited activity is to a large extent
independent of the position of the body in the vessel containing the
active material. Bodies are made active whether exposed directly to the
action of the radio-active substance or screened from the action of the
direct rays. This has been clearly shown in some experiments of P.
Curie. A small open vessel _a_ (Fig. 62) containing a solution of radium
is placed inside a larger closed vessel _V_.
[Illustration: Fig. 62.]
Plates _A_, _B_, _C_, _D_, _E_ are placed in various positions in the
enclosure. After exposure for a day, the plates after removal are found
to be radio-active even in positions completely shielded from the action
of the direct rays. For example, the plate _D_ shielded from the direct
radiation by the lead plate _P_ is as active as the plate _E_, exposed
to the direct radiation. The amount of activity produced in a given time
on a plate of given area in a definite position is independent of the
material of the plate. Plates of mica, copper, cardboard, ebonite, all
show equal amounts of activity. The amount of activity depends on the
area of the plate and on the amount of free space in its neighbourhood.
Excited radio-activity is also produced in water if exposed to the
action of an emanating compound.
=176. Concentration of excited radio-activity on the negative
electrode.= When thorium or radium is placed in a closed vessel, the
whole interior surface becomes strongly active. In a strong electric
field, on the other hand, the writer found that the activity was
confined entirely to the negative electrode. By suitable arrangements,
the whole of the excited activity, which was previously distributed over
the surface of the vessel, can be concentrated on a small negative
electrode placed inside the vessel. An experimental arrangement for this
purpose is shown in Fig. 63.
[Illustration: Fig. 63.]
The metal vessel _V_ containing a large amount of thoria is connected
with the positive pole of a battery of about 300 volts. The wire _AB_ to
be made active is fastened to a stouter rod _BC_, passing through an
ebonite cork inside a short cylinder _D_, fixed in the side of the
vessel. This rod is connected with the negative pole of the battery. In
this way the wire _AB_ is the only conductor exposed in the field with a
negative charge, and it is found that the whole of the excited activity
is concentrated upon it.
In this way it is possible to make a short thin metal wire over 10,000
times as active per unit surface as the thoria from which the excited
activity is derived. In the same way, the excited activity due to radium
can be concentrated mainly on the negative electrode. In the case of
thorium, if the central wire be charged positively, it shows no
appreciable activity. With radium, however, a positively charged body
becomes slightly active. In most cases, the amount of activity produced
on the positive electrode is not more than 5% of the corresponding
amount when the body is negatively charged. For both thorium and radium,
the amount of excited activity on electrodes of the same size is
independent of their material.
All metals are made active to equal extents for equal times of exposure.
When no electric field is acting, the same amount of activity is
produced on insulators like mica and glass as on conductors of equal
dimensions.
=177. Connection between the emanations and excited activity.= An
examination of the conditions under which excited activity is produced
shows that there is a very close connection between the emanation and
the excited activity. If a thorium compound is covered with several
sheets of paper, which cut off the α rays but allow the emanation to
pass through, excited activity is still produced in the space above it.
If a thin sheet of mica is waxed down over the active material, thus
preventing the escape of the emanation, no excited activity is produced
outside it. Uranium and polonium which do not give off an emanation are
not able to produce excited activity on bodies. Not only is the presence
of the emanation necessary to cause excited activity, but the amount of
excited activity is always proportional to the amount of emanation
present. For example, de-emanated thoria produces very little excited
activity compared with ordinary thoria. In all cases the amount of
excited activity produced is proportional to the emanating power. When
passing through an electric field the emanation loses its property of
exciting activity at the same rate as the radiating power diminishes.
This was shown by the following experiment.
A slow constant current of air from a gasometer, freed from dust by its
passage through cotton-wool, passed through a rectangular wooden tube 70
cms. long. Four equal insulated metal plates _A_, _B_, _C_, _D_, were
placed at regular intervals along the tube. The positive pole of a
battery of 300 volts was connected with a metal plate placed in the
bottom of the tube, while the negative pole was connected with the four
plates. A mass of thoria was placed in the bottom of the tube under the
plate _A_, and the current due to the emanation determined at each of
the four plates. After passing a current of air of 0·2 cm. per second
for 7 hours along the tube, the plates were removed and the amount of
excited activity produced on them was tested by the electric method. The
following results were obtained.
Relative Relative
current due excited
to emanation activity
Plate _A_ 1 1
„ _B_ ·55 ·43
„ _C_ ·18 ·16
„ _D_ ·072 ·061
Within the errors of measurement, the amount of excited activity is thus
proportional to the radiation from the emanation, _i.e._ to the amount
of emanation present. The same considerations hold for the radium
emanation. The emanation in this case, on account of the slow loss of
its activity, can be stored mixed with air for long periods in a
gasometer, and its effects tested quite independently of the active
matter from which it is produced. The ionization current due to the
excited activity produced by the emanation is always proportional to the
current due to the emanation for the period of one month or more that
its activity is large enough to be measured conveniently by an
electrometer.
If, at any time during the interval, some of the emanation is removed
and introduced into a new testing vessel, the ionization current will
immediately commence to increase, rising in the course of four or five
hours to about twice its original value. This increase of the current is
due to the excited activity produced on the walls of the containing
vessel. On blowing out the emanation, the excited activity is left
behind, and at once begins to decay. Whatever its age, the emanation
still possesses the property of causing excited activity, and in amount
always proportional to its activity, _i.e._ to the amount of emanation
present.
These results show that the power of exciting activity on inactive
substances is a property of the radio-active emanations, and is
proportional to the amount of emanation present.
The phenomenon of excited activity cannot be ascribed to a type of
phosphorescence produced by the rays from the emanation on bodies; for
it has been shown that the activity can be concentrated on the negative
electrode in a strong electric field, even if the electrode is shielded
from the direct radiation from the active substance which gives off the
emanation. The amount of excited activity does not seem in any way
connected with the ionization produced by the emanation in the gas with
which it is mixed. For example, if a closed vessel is constructed with
two large parallel insulated metal plates on the lower of which a layer
of thoria is spread, the amount of the excited activity on the upper
plate when charged negatively, is independent of the distance between
the plates when that distance is varied from 1 millimetre to 2
centimetres. This experiment shows that the amount of excited activity
depends only on the amount of emanation emitted from the thoria; for the
ionization produced with a distance of 2 centimetres between the plates
is about ten times as great as with a distance of 1 millimetre.
=178.= If a platinum wire be made active by exposure to the emanation of
thoria, its activity can be removed by treating the wire with certain
acids[272]. For example, the activity is not much altered by immersing
the wire in hot or cold water or nitric acid, but more than 80% of it is
removed by dilute or concentrated solutions of sulphuric or hydrochloric
acid. The activity has not been destroyed by this treatment but is
manifested in the solution. If the solution be evaporated, the activity
remains behind on the dish.
These results show that the excited activity is due to a deposit on the
surface of bodies of _radio-active matter_ which has definite properties
as regards solution in acids. This active matter is dissolved in some
acids, but, when the solvent is evaporated, the active matter is left
behind. This active matter is deposited on the surface of bodies, for it
can be partly removed by rubbing the body with a cloth, and almost
completely by scouring the plate with sand or emery paper. If a
negatively charged wire is placed in the presence of a large quantity of
radium emanation, it becomes intensely active. If the wire, after
removal, is drawn across a screen of zinc sulphide, or willemite, a
portion of the active matter is rubbed off, and a luminous trail is left
behind on the screen. The amount of active matter deposited is extremely
small, for no difference of weight has been detected in a platinum wire
when made extremely active. On examining the wire under a microscope, no
trace of foreign matter is observed. It follows from these results that
the matter which causes excited activity is many thousand times more
active, weight for weight, than radium itself.
It is convenient to have a definite name for this radio-active matter,
for the term “excited activity” only refers to the radiation from the
active matter and not to the matter itself. The term “active deposit”
will be generally applied to this matter. The active deposit from the
three substances thorium, radium, and actinium is, in each case, derived
from its respective emanation, and possesses the same general property
of concentration on the negative electrode in an electric field and of
acting as a non-volatile type of matter which is deposited from the gas
on to the surface of bodies. These active deposits, while all soluble in
strong acids, are chemically distinct from each other.
The term “active deposit” can, however, only be used when the matter is
spoken of as a whole; for it will be shown later that the matter, under
ordinary conditions, is complex and contains several constituents which
have distinctive physical and chemical properties and also a distinctive
rate of change. According to the theory advanced in section 136, we may
suppose that the emanation of thorium, radium, and actinium is unstable
and breaks up with the expulsion of an α particle. The residue of the
atom of the emanation diffuses to the sides of the vessel or is removed
to the negative electrode in an electric field. This active deposit is
in turn unstable and breaks up in several successive stages.
The “excited activity” proper is the radiation set up by the active
deposit in consequence of the changes occurring in it. On this view, the
emanation is the parent of the active deposit in the same way that Th X
is the parent of the emanation. The proportionality which always exists
between the activity of the emanation and the excited activity to which
it gives rise, is at once explained, if one substance be the parent of
the other.
=179. Decay of the excited activity produced by thorium.= The excited
activity produced in a body after a _long_ exposure to the emanations of
thorium, decays in an exponential law with the time, falling to half
value in about 11 hours. The following table shows the rate of decay of
the excited activity produced on a brass rod.
Time in Current
hours
0 100
7·9 64
11·8 47·4
23·4 19·6
29·2 13·8
32·6 10·3
49·2 3·7
62·1 1·86
71·4 0·86
The results are shown graphically in Fig. 64, Curve _A_.
[Illustration: Fig. 64.]
The intensity of the radiation _I_ after any time _t_ is given by
$$ \frac {I} {I₀} = e^{–λt} $$,
where λ is the radio-active constant.
The rate of decay of excited activity, like that of the activity of
other radio-active products, is not appreciably affected by change of
conditions. The rate of decay is independent of the concentration of the
excited activity, and of the material of the body on which it is
produced. It is independent also of the nature and pressure of the gas
in which it decays. The rate of decay is unchanged whether the excited
activity is produced on the body with or without an electric field.
The amount of excited activity produced on a body increases at first
with the time, but reaches a maximum after an exposure of several days.
An example of the results is given in the following table. In this
experiment a rod was made the cathode in a closed vessel containing
thoria. It was removed at intervals for the short time necessary to test
its activity and then replaced.
Time in Current
hours
1·58 6·3
3·25 10·5
5·83 29
9·83 40
14·00 59
23·41 77
29·83 83
47·00 90
72·50 95
96·00 100
These results are shown graphically in Curve _B_, Fig. 64. It is seen
that the decay and recovery curves may be represented approximately by
the following equations.
For the decay curve _A_,
$$ \frac {I} {I₀} = e^{–λt} $$ .
For the recovery curve _B_,
$$ \frac {I} {I₀} = 1 − e^{–λt} $$ .
The two curves are thus complementary to one another; they are connected
in the same way as the decay and recovery curves of Ur X, and are
susceptible of a similar explanation.
The amount of excited radio-activity reaches a maximum value when the
rate of supply of fresh radio-active particles balances the rate of
change of those already deposited.
=180. Excited radio-activity produced by a short exposure.= The initial
portion of the recovery curve _B_, Fig. 64, is not accurately
represented by the above equation. The activity for the first few hours
increases more slowly than would be expected from the equation. This
result, however, is completely explained in the light of later results.
The writer[273] found that, for a _short exposure_ of a body to the
thorium emanation, the excited activity upon it after removal, instead
of at once decaying at the normal rate, _increased_ for several hours.
In some cases the activity of the body increased to three or four times
its original value in the course of a few hours and then decayed with
the time at the normal rate.
For an exposure of 41 minutes to the emanation the excited activity
after removal rose to three times its initial value in about 3 hours and
then fell again at about the normal rate to half value in 11 hours.
With a longer time of exposure to the emanation, the ratio of the
increase after removal is much less marked. For a day’s exposure, the
activity after removal begins at once to diminish. In this case, the
increase of activity of the matter deposited in the last few hours does
not compensate for the decrease of activity of the active matter as a
whole, and consequently the activity at once commences to decay. This
increase of activity with time explains the initial irregularity in the
recovery curve, for the active matter deposited during the first few
hours takes some time to reach its maximum activity, and the initial
activity is, in consequence, smaller than would be expected from the
equation.
The increase of activity on a rod exposed for a short interval in the
presence of the thorium emanation has been further investigated by Miss
Brooks. The curve _C_ in Fig. 65 shows the variation with time of the
activity of a brass rod exposed for 10 minutes in the emanation vessel
filled with dust-free air. The excited activity after removal increased
in the course of 3·7 hours to five times its initial value, and
afterwards decayed at the normal rate. The dotted line curve _D_
represents the variation of activity to be expected if the activity
decayed exponentially with the time. The explanation of this remarkable
action is considered in detail in section 207.
[Illustration: Fig. 65.]
=181. Effect of dust on the distribution of excited activity.= Miss
Brooks[274], working in the Cavendish Laboratory, observed that the
excited activity due to the thorium emanation appeared in some cases on
the anode in an electric field, and that the distribution of excited
activity varied in an apparently capricious manner. This effect was
finally traced to the presence of dust in the air of the emanation
vessel. For example, with an exposure of 5 minutes the amount of excited
activity to be observed on a rod depended on the time that the air had
been allowed to remain undisturbed in the emanation vessel beforehand.
The effect increased with the time of standing, and was a maximum after
about 18 hours. The amount of excited activity obtained on the rod was
then about 20 times as great as the amount observed for air freshly
introduced. The activity of this rod did not increase after removal, but
with fresh air, the excited activity, for an exposure of 5 minutes,
increased to five or six times its initial value.
This anomalous behaviour was found to be due to the presence of dust
particles in the air of the vessel, in which the bodies were made
radio-active. These particles of dust, when shut up in the presence of
the emanation, become radio-active. When a negatively charged rod is
introduced into the vessel, a part of the radio-active dust is
concentrated on the rod and its activity is added to the normal activity
produced on the wire. After the air in the vessel has been left
undisturbed for an interval sufficiently long to allow each of the
particles of dust to reach a state of radio-active equilibrium, on the
application of an electric field, all the positively charged dust
particles will at once be carried to the negative electrode. The
activity of the electrode at once commences to decay, since the decay of
the activity of the dust particles on the wire quite masks the initial
rise of the normal activity produced on the wire.
Part of the radio-active dust is also carried to the anode, and the
proportion increases with the length of time during which the air has
been undisturbed. The greatest amount obtained on the anode was about
60% of that on the cathode.
These anomalous effects were found to disappear if the air was made
dust-free by passing through a plug of glass wool, or by application for
some time of a strong electric field.
=182. Decay of excited activity from radium.= The excited activity
produced on bodies by exposure to the radium emanation decays much more
rapidly than the thorium excited activity. For short times of
exposure[275] to the emanation the decay curve is very irregular. This
is shown in Fig. 66.
It was found that the intensity of the radiation measured by the α rays
decreased rapidly for the first 10 minutes after removal, but about 15
minutes after removal reached a value which remained nearly constant for
an interval of about 20 minutes. It then decayed to zero, finally
following an exponential law, the intensity falling to half value in
about 28 minutes. With longer times of exposure, the irregularities in
the curve are not so marked.
[Illustration: Fig. 66.]
Miss Brooks has recently determined the decay curves of the excited
activity of radium for different times of exposure, measured by the α
rays. The results are shown in Fig. 67, where the initial ordinates
represent the activity communicated to the body from different times of
exposure to a constant supply of emanation. It will be observed that in
all cases there is a sudden initial drop of activity, which becomes less
marked with increasing time of exposure. The activity, several hours
after removal, decreases exponentially in all cases, falling to half
value in about 28 minutes.
Not only do the curves of variation of the excited activity after
removal depend upon the time of exposure to the emanation, but they also
depend upon whether the α or β and γ rays are used as a means of
measurement. The curves obtained for the γ rays are identical with those
from the β rays, showing that these two types of rays always occur
together and in the same proportion. The curves measured by the β rays
are very different, especially for the case of a short exposure to the
emanation. This is clearly shown in Fig. 68, which gives the β and γ ray
curves for exposures of 10 minutes, 40 minutes, and 1 hour, and also the
limiting case of an exposure of 24 hours.
[Illustration: Fig. 67.]
[Illustration: Fig. 68.]
About 25 minutes after removal, the activity decays approximately at the
same rate in each case. For convenience of representation, the ordinates
of the curves were adjusted so that they all passed through a common
point. We shall see later (chapter XI) that the rates of decay are not
identically the same until several hours after removal; but, in the
above figure, it is difficult to represent the slight variations. It
will be observed that for the short exposure of 10 minutes the activity
measured by the β rays is small at first but rises to a maximum in about
22 minutes, and then dies away with the time. The curve of decay of
activity, measured by the β rays for a long exposure, does not show the
rapid initial drop which occurs in all the α ray curves. Curie and
Danne[276] made an investigation of the curves of decay of excited
activity for different times of exposure to the radium emanation, but
apparently did not take into account the fact that measurements made by
the α and β rays give quite different curves of decay. Some of the
family of curves, given in their paper, refer to the α rays and others
to the β rays. They showed, however, the important fact that the curve
of decay obtained by them for a long exposure (which is identical with
the β ray curve) could be empirically expressed by an equation of the
form
$$ \frac {I_t} {I₀} = ae^{–λ_1 t} − (a − 1) e^{–λ_2 t} $$,
where _I₀_ is the initial intensity and _I__{_t_} the intensity after
any time _t_; λ₁ = ¹⁄₂₄₂₀, λ₂ = ¹⁄₁₈₆₀. The numerical constant _a_ =
4·20. After an interval of 2·5 hours, the logarithmic decay curve is
nearly a straight line, that is, the activity falls off according to an
exponential law with the time, decreasing to half value in about 28
minutes.
The full explanation of this equation, and of the peculiarities of the
various decay curves of the excited activity of radium, will be
discussed in detail in chapter XI.
As in the case of the excited activity from thorium, the rate of decay
of the excited activity from radium is for the most part independent of
the nature of the body made active. Curie and Danne (_loc. cit._)
observed that the active bodies gave off an emanation itself capable of
exciting activity in neighbouring bodies. This property rapidly
disappeared, and was inappreciable 2 hours after removal. In certain
substances like celluloid and caoutchouc, the decay of activity is very
much slower than for the metals. This effect becomes more marked with
increase of time of exposure to the emanation. A similar effect is
exhibited by lead, but to a less marked degree. During the time the
activity lasts, these substances continue to give off an emanation.
It is probable that these divergencies from the general law are not due
to an actual change in the rate of decay of the true excited activity
but to an occlusion of the emanation by these substances during the
interval of exposure. After exposure the emanation gradually diffuses
out, and thus the activity due to this occluded emanation and the
excited activity produced by it decays very slowly with the time.
=183. Active deposit of very slow decay.= M. and Mme Curie[277] have
observed that bodies which have been exposed for a long interval in the
presence of the radium emanation do not lose all their activity. The
excited activity at first decays rapidly at the normal rate, falling to
half value in about 28 minutes, but a residual activity, which they
state is of the order of ½0,000 of the initial activity, always
remains. A similar effect was observed by Giesel. The writer has
examined the variation of this residual activity, and has found that it
increases for several years. The results are discussed in detail in
chapter XI. It will there be shown that this active deposit of slow
transformation contains the radio-active constituents present in
polonium, radio-tellurium and radio-lead.
[Illustration: Fig. 69.]
=184. The excited activity from actinium.= The emanation of actinium,
like that of thorium and radium, produces excited activity on bodies,
which is concentrated on the negative electrode in an electric field.
Debierne[278] found that the excited activity decays approximately
according to an exponential law, falling to half value in 41 minutes.
Giesel[279] examined the rate of decay of the excited activity of
“emanium”—which, we have seen, probably contains the same radio-active
constituents as actinium—and found that it decayed to half value in 34
minutes. Miss Brooks[280] found that the curves of decay of the excited
activity from Giesel’s emanium varied with the time of exposure to the
emanation. The results are shown graphically in Fig. 69, for time
exposures of 1, 2, 5, 10 and 30 minutes, and also for a long exposure of
21 hours. After 10 minutes the curves have approximately the same rate
of decay. For convenience, the ordinates of the curves are adjusted to
pass through a common point. For a very short exposure, the activity is
small at first, but reaches a maximum about 9 minutes later and finally
decays exponentially to zero.
The curve of variation of activity for a very short exposure has been
determined accurately by Bronson; it is shown later in Fig. 83. He found
that the decay of activity is finally exponential, falling to half value
in 36 minutes.
The explanation of these curves is discussed in detail in chapter X,
section 212.
=185. Physical and chemical properties of the active deposit.= On
account of the slow decay of the activity of the active deposit from the
thorium emanation, its physical and chemical properties have been more
closely examined than the corresponding deposit from radium. It has
already been mentioned that the active deposit of thorium is soluble in
some acids. The writer[281] found that the active matter was dissolved
off the wire by strong or dilute solutions of sulphuric, hydrochloric
and hydrofluoric acids, but was only slightly soluble in water or nitric
acid. The active matter was left behind when the solvent was evaporated.
The rate of decay of activity was unaltered by dissolving the active
matter in sulphuric acid, and allowing it to decay in the solution. In
the experiment, the active matter was dissolved off an active platinum
wire; then equal portions of the solutions were taken at definite
intervals, evaporated down in a platinum dish, and the activity of the
residue tested by the electric method. The rate of decay was found to be
exactly the same as if the active matter had been left on the wire. In
another experiment, an active platinum wire was made the cathode in a
copper sulphate solution, and a thin film of copper deposited on it. The
rate of decay of the activity was unchanged by the process.
A detailed examination of the physical and chemical properties of the
active deposit of thorium has been made by F. von Lerch[282] and some
important and interesting results have been obtained. A solution of the
active deposit was prepared by dissolving the metal which had been
exposed for some time in the presence of the thorium emanation. In most
cases the active matter was precipitated with the metal. For example, an
active copper wire was dissolved in nitric acid and then precipitated by
caustic potash. The precipitate was strongly active. An active magnesium
wire, dissolved in hydrochloric acid and then precipitated as phosphate,
also gave an active precipitate. The activity of the precipitates
decayed at the normal rate, _i.e._ the activity fell to half value in
about 11 hours.
Experiments were also made on the solubility of the active deposit in
different substances. A platinum plate was made active and then placed
in different solutions, and the decrease of the activity observed. In
addition to the acids already mentioned, a large number of substances
were found to dissolve the active deposit to some extent. The active
matter was however not dissolved to an appreciable extent in ether or
alcohol. Many substances became active if added to the active solution
and then precipitated. For example, an active solution of hydrochloric
acid was obtained by dissolving the deposit on an active platinum wire.
Barium chloride was then added and precipitated as sulphate. The
precipitate was strongly active, thus suggesting that the active matter
was carried down by the barium.
=186. Electrolysis of solutions.= Dorn showed that, if solutions of
radiferous barium chloride were electrolysed, both electrodes became
temporarily active, but the anode to a greater degree than the cathode.
F. von Lerch has made a detailed examination of the action of
electrolysis on a solution of the active deposit of thorium. The matter
was dissolved off an active platinum plate by hydrochloric acid, and
then electrolysed between platinum electrodes. The cathode was very
active, but there was no trace of activity on the anode. The cathode
lost its activity at a rate much _faster_ than the normal. With an
amalgamated zinc cathode on the other hand, the rate of decay was
normal. When an active solution of hydrochloric acid was electrolysed
with an electromotive force smaller than that required to decompose
water, the platinum became active. The activity decayed to half value in
4·75 hours while the normal fall is to half value in 11 hours. These
results point to the conclusion that the active matter is complex and
consists of two parts which have different rates of decay of activity,
and can be separated by electrolysis.
Under special conditions it was found possible to make the anode active.
This was the case if the anion attached itself to the anode. For
example, if an active hydrochloric solution was electrolysed with a
silver anode, the chloride of silver formed was strongly active and its
activity decayed at a normal rate. The amount of activity obtained by
placing different metals in active solutions for equal times varied
greatly with the metal. For example, it was found that if a zinc plate
and an amalgamated zinc plate, which show equal potential differences
with regard to hydrochloric acid, were dipped for equal times in two
solutions of equal activity, the zinc plate was seven times as active as
the other. The activity was almost removed from the solution in a few
minutes by dipping a zinc plate into it. Some metals became active when
dipped into an active solution while others did not. Platinum,
palladium, and silver remained inactive, while copper, tin, lead,
nickel, iron, zinc, cadmium, magnesium, and aluminium became active.
These results strongly confirm the view that excited activity is due to
a deposit of active matter which has distinctive chemical behaviour.
G. B. Pegram[283] has made a detailed study of the active deposits
obtained by electrolysis of pure and commercial thorium salts. The
commercial thorium nitrate obtained from P. de Haen gave, when
electrolysed, a deposit of lead peroxide on the anode. This deposit was
radio-active, and its activity decayed at the normal rate of the excited
activity due to thorium. From solutions of pure thorium nitrate, no
visible deposit was obtained on the anode, but it was, however, found to
be radio-active. The activity decayed rapidly, falling to half value in
about one hour. Some experiments were also made on the effect of adding
metallic salts to thorium solutions and then electrolysing them. Anode
and cathode deposits of the oxides or metals obtained in this way were
found to be radio-active, but the activity fell to half value in a few
minutes. The gases produced by electrolysis were radio-active, but this
was due to the presence of the thorium emanation. The explanation of the
results obtained by Pegram and von Lerch will be considered later in
section 207. It will be shown that the active deposit of thorium
contains two distinct substances which have different rates of
transformation.
=187. Effect of temperature.= The activity of a platinum wire which has
been exposed in the presence of the thorium emanation is almost
completely lost by heating the wire to a white heat. Miss F. Gates[284]
found that the activity was not destroyed by the intense heat, but
manifested itself on neighbouring bodies. When the active wire was
heated electrically in a closed cylinder, the activity was transferred
from the wire to the interior surface of the cylinder in unaltered
amount. The rate of decay of the activity was not altered by the
process. By blowing a current of air through the cylinder during the
heating, a part of the active matter was removed from the cylinder.
Similar results were found for the excited activity due to radium.
F. von Lerch (_loc. cit._) determined the amount of activity removed at
different temperatures. The results are shown in the following table for
a platinum wire excited by the thorium emanation[285].
Temperature Percentage of
activity
removed
Heated 2 minutes 800° C. 0
then „ ½ minute 1020° C. 16
more
„ „ ½ „ 1260° C. 52
„
„ „ ½ „ 1460° C. 99
„
The effect of heat on the volatilization of the active deposit of radium
has been examined in detail by Curie and Danne. The interesting and
important results obtained by them will be discussed in chapter XI,
section 226.
=188. Effect of variation of E.M.F. on amount of excited activity from
thorium.= It has been shown that the excited activity is confined to
the cathode in a strong electric field. In weaker fields the activity
is divided between the cathode and the walls of the vessel. This was
tested in an apparatus[286] shown in Fig. 70.
[Illustration: Fig. 70.]
_A_ is a cylindrical vessel of 5·5 cms. diameter, _B_ the negative
electrode passing through insulating ends _C_, _D_. For a potential
difference of 50 volts, most of the excited activity was deposited on
the electrode _B_. For about 3 volts, half of the total excited activity
was produced on the rod _B_, and half on the walls of the vessel.
Whatever the voltage applied, the sum of the activities on the central
rod and the walls of the cylinder was found to be a constant when a
steady state was reached.
When no voltage was applied, diffusion alone was operative, and in that
case about 13 per cent. of the total activity was on the rod _B_. The
application of an electric field has thus no influence on the sum total
of excited activity, but merely controls the proportion concentrated on
the negative electrode.
A more detailed examination of the variation with strength of field of
the amount on the negative electrode was made in a similar manner by F.
Henning[287]. He found that in a strong electric field the amount of
excited activity was practically independent of the diameter of the rod
_B_, although the diameter varied between ·59 mm. and 6·0 mms. With a
small voltage, the amount on the negative electrode varied with its
diameter. The curves showing the relation between the amount of excited
activity and voltage are very similar in character to those obtained for
the variation of the current through an ionized gas with the voltage
applied.
The amount of excited activity reaches a maximum when all the active
matter is removed from the gas as rapidly as it is formed. With weaker
fields, a portion diffuses to the sides of the vessel, and produces
excited activity on the positive electrode.
=189. Effect of pressure on distribution of excited activity.= In a
strong electric field, the amount of excited activity produced on the
cathode is independent of the pressure down to a pressure of about 10
mms. of mercury. In some experiments made by the writer[288], the
emanating thorium compound was placed inside a closed cylinder about 4
cms. in diameter, through which passed an insulated central rod. The
central rod was connected to the negative pole of a battery of 50 volts.
When the pressure was reduced below 10 mms. of mercury, the amount of
excited activity produced on the negative electrode diminished, and was
a very small fraction of its original value at a pressure of ⅒ mm.
Some excited activity was in this case found to be distributed over the
interior surface of the cylinder. It may thus be concluded that at low
pressures the excited activity appears on both anode and cathode, even
in a strong electric field. The probable explanation of this effect is
given in the next section.
Curie and Debierne[289] observed that when a vessel containing an
emanating radium compound was kept pumped down to a low pressure, the
amount of excited activity produced on the vessel was much reduced. In
this case the emanation given off by the radium was removed by the pump
with the other gases continuously evolved from the radium compound. On
account of the very slow decay of activity of the emanation, the amount
of excited activity produced on the walls of the vessel, in the passage
of the emanation through it, was only a minute fraction of the amount
produced when none of the emanation given off was allowed to escape.
=190. Transmission of excited activity.= The characteristic property of
excited radio-activity is that it can be confined to the cathode in a
strong electric field. Since the activity is due to a deposit of
radio-active matter on the electrified surface, the matter must be
transported by positively charged carriers. The experiments of
Fehrle[290] showed that the carriers of excited activity travel along
the lines of force in an electric field. For example, when a small
negatively charged metal plate was placed in the centre of a metal
vessel containing an emanating thorium compound, more excited activity
was produced on the sides and corners of the plate than at the central
part.
A difficulty however arises in connection with the positive charge of
the carrier. According to the view developed in section 136 and later
in chapters X and XI, the active matter which is deposited on bodies
and gives rise to excited activity, is itself derived from the
emanation. The emanations of thorium and radium emit only α rays, _i.e._
positively charged particles. After the expulsion of an α particle, the
residue, which is supposed to constitute the primary matter of the
active deposit, should retain a negative charge, and be carried to the
anode in an electric field. The exact opposite however is observed to be
the case. The experimental evidence does not support the view that the
positively charged α particles, expelled from the emanation, are
directly responsible for the phenomena of excited activity; for no
excited activity is produced in a body exposed to the α rays of the
emanation, provided the emanation itself does not come in contact with
it.
There has been a tendency to attach undue importance to this apparent
discrepancy between theory and experiment. The difficulty is not so much
to offer a probable explanation of the results as to select from a
number of possible causes. While there can be little doubt that the main
factor in the disintegration of the atom consists in the expulsion of an
α particle carrying a positive charge, a complicated series of processes
probably occurs before the residue of the atom is carried to the
negative electrode. The experimental evidence suggests that one or more
negative electrons of slow velocity escape from the atom at the same
time as the particle. This is borne out by the recent discovery that the
particle expelled from radium, freed from the ordinary β rays, and also
from polonium, is accompanied by a number of slowly moving and
consequently easily absorbed electrons. If two negative electrons
escaped at the same time as the α particle, the residue would be left
with a positive charge and would be carried to the negative electrode.
There is also another experimental point which is of importance in this
connection. In the absence of an electric field, the carriers remain in
the gas for a considerable time and undergo their transformation _in
situ_. There is also some evidence (section 227) that, even in an
electric field, the carriers of the active deposit are not swept to the
electrode immediately after the break up of the emanation, but remain
some time in the gas before they gain a positive charge. It must be
remembered that the atoms of the active deposit do not exist as a gas
and by the process of diffusion would tend to collect together to form
aggregates. These aggregates would act as small metallic particles, and,
if they were electro-positive in regard to the gas, would gain a
positive charge from the gas.
There can be little doubt that the processes occurring between the break
up of the emanation and the deposit of the residue in the cathode in an
electric field are complicated, and further careful experiment is
required to elucidate the sequence of the phenomena.
Whatever view is taken of the process by which these carriers obtain a
positive charge, there can be little doubt that the expulsion of an α
particle with great velocity from the atom of the emanation must set the
residue in motion. On account of the comparatively large mass of this
residue, the velocity acquired will be small compared with that of the
expelled α particle, and the moving mass will rapidly be brought to rest
at atmospheric pressure by collision with the gas molecules in its path.
At low pressures, however, the collisions will be so few that it will
not be brought to rest until it strikes the boundaries of the vessel. A
strong electric field would have very little effect in controlling the
motion of such a heavy mass, unless it has been initially brought to
rest by collision with the gas molecules. This would explain why the
active matter is not deposited on the cathode at low pressures in an
electric field. Some direct evidence of a process of this character,
obtained by Debierne on examination of the excited activity produced by
actinium, is discussed in section 192.
=191.= The following method has been employed by the writer[291] to
determine the velocity of the positive carriers of excited activity of
radium and thorium in an electric field. Suppose _A_ and _B_ (Fig. 71)
are two parallel plates exposed to the influence of the emanation, which
is uniformly distributed between them. If an alternating E.M.F. _E₀_ is
applied between the plates, the same amount of excited activity is
produced on each electrode. If, in series with the source of the
alternating E.M.F., a battery of E.M.F. _E₁_ less than _E₀_ is placed,
the positive carrier moves in a stronger electric field in one half
alternation than in the other. A carrier consequently moves over unequal
distances during the two half alternations, since the velocity of the
carrier is proportional to the strength of the electric field in which
it moves. The excited activity will in consequence be unequally
distributed over the two electrodes. If the frequency of alternation is
sufficiently great, only the positive carriers within a certain small
distance of one plate can be conveyed to it, and the rest, in the course
of several succeeding alternations, are carried to the other plate.
[Illustration: Fig. 71.]
When the plate _B_ is negatively charged, the E.M.F. between the plates
is _E₀_ − _E₁_, when _B_ is positive the E.M.F. is _E₀_ + _E₁_.
Let _d_ = distance between the plates,
_T_ = time of a half alternation,
ρ = ratio of the excited radio-activity on the plate _B_ to the
sum of the radio-activities on the plates _A_ and
_B_,
_K_ = velocity of the positive carriers for a potential-gradient
of 1 volt per centimetre.
On the assumption that the electric field between the plates is uniform,
and that the velocity of the carrier is proportional to the electric
field, the velocity of the positive carrier towards _B_ is
_E₀_ − _E₁_
---------- _K_
_d_
and, in the course of the next half alternation,
_E₀_ + _E₁_
---------- _K_
_d_
towards the plate _A_.
If _x₁_ is less than _d_, the greatest distances _x₁_, _x₂_ passed over
by the positive carrier during two succeeding half alternations is thus
given by
_E₀_ − _E₁_
_x₁_ = ---------- _KT_
_d_
and
_E₀_ + _E₁_
_x₂_ = ---------- _KT_
_d_
Suppose that the positive carriers are produced at a uniform rate of _q_
per second for unit distance between the plates. The number of positive
carriers which reach _B_ during a half alternation consists of two
parts:
(1) One half of those carriers which are produced within the distance
_x₁_ of the plate _B_. This number is equal to
1
--- _x₁_ _qT_
2
(2) All the carriers which are left within the distance _x₁_ from _B_ at
the end of the previous half alternation. The number of these can
readily be shown to be
1 _x₁_
--- _x₁_ ---- _qT_
2 _x₂_
The remainder of the carriers, produced between _A_ and _B_ during a
complete alternation, will reach the other plate _A_ in the course of
succeeding alternations, provided no appreciable recombination takes
place. This must obviously be the case, since the positive carriers
travel further in a half alternation towards _A_ than they return
towards _B_ during the next half alternation. The carriers thus move
backwards and forwards in the changing electric field, but on the whole
move towards the plate _A_.
The total number of positive carriers produced between the plates during
a complete alternation is 2_dqT_. The ratio ρ of the number which reach
_B_ to the total number produced is thus given by
$$ \rho = \frac {\frac {1}{2} x_1qT + \frac {1}{2} x_1 \frac {x_1}{x_2}
qT} {2dqT} = \frac {1}{4} \frac {x_1}{d} \frac {x_1 + x_2} {x_2} $$ .
Substituting the values of _x₁_ and _x₂_, we find that
$$ K = \frac {2 (E₀ + E_1) d^2} {E₀ (E₀ − E_1) T} \rho $$ .
In the experiments, the values of _E₀_, _E₁_, _d_, and _T_ were varied,
and the results obtained were in general agreement with the above
equation.
The following were the results for thorium:
_Plates 1·30 cms. apart._
_E₀_ + _E₀_ − Alternations ρ _K_
_E₁_ _E₁_ per second
152 101 57 ·27 1·25
225 150 57 ·38 1·17
300 200 57 ·44 1·24
_Plates 2 cms. apart._
_E₀_ + _E₀_ − Alternations ρ _K_
_E₁_ _E₁_ per second
273 207 44 ·37 1·47
300 200 53 ·286 1·45
The average mobility _K_ deduced from a large number of experiments was
1·3 cms. per sec. per volt per cm. for atmospheric pressure and
temperature. This velocity is about the same as the velocity of the
positive ion produced by Röntgen rays in air, viz. 1·37 cms. per sec.
The results obtained with the radium emanation were more uncertain than
those for thorium on account of the distribution of some excited
activity on the positive electrode. The values of the velocities of the
carriers were however found to be roughly the same for radium as for
thorium.
These results show that the carriers of the active deposit travel in the
gas with about the same velocity as the positive or negative ions
produced by the radiations in the gas. This indicates either that the
active matter becomes attached to positive ions, or that the active
matter itself, acquiring in some way a positive charge, collects a
cluster of neutral molecules which travel with it.
=192. Carriers of the excited activity from actinium and “emanium.”=
Giesel[292] observed that “emanium” gave off a large quantity of
emanation, and that this emanation gave rise to a type of radiation
which he termed the _E_ rays. A narrow metal cylinder containing the
active substance was placed with the open end downwards, about 5 cms.
above the surface of a zinc sulphide screen. The screen was charged
negatively to a high potential by an electric machine, and the cylinder
connected with earth. A luminous spot of light was observed on the
screen, which was brighter at the edge than at the centre. A conductor,
connected with earth, brought near the luminous spot apparently repelled
it. An insulator did not show such a marked effect. On removal of the
active substance, the luminosity of the screen persisted for some time.
This was probably due to the excited activity produced on the screen.
The results obtained by Giesel support the view that the carriers of
excited activity of “emanium” have a positive charge. In a strong
electric field the carriers travel along the lines of force to the
cathode, and there cause excited activity on the screen. The movement of
the luminous zone on the approach of a conductor is due to the
disturbance of the electric field. Debierne[293] found that actinium
also gave off a large amount of emanation, the activity of which decayed
very rapidly with the time, falling to half value in 3·9 seconds.
This emanation produces excited activity on surrounding objects, and at
diminished pressure the emanation produces a uniform distribution of
excited activity in the enclosure containing the emanation. The excited
activity falls to half value in 41 minutes.
Debierne observed that the distribution of excited activity was altered
by a strong magnetic field. The experimental arrangement is shown in
Fig. 71A. The active matter was placed at _M_, and two plates _A_ and
_B_ were placed symmetrically with regard to the source. On the
application of a strong magnetic field normal to the plane of the paper,
the excited activity was unequally distributed between the plates _A_
and _B_. The results showed that the carriers of excited activity were
deviated by a magnetic field in the opposite sense to the cathode rays,
_i.e._ the carriers were positively charged. In some cases, however, the
opposite effect was obtained. Debierne considers that the excited
activity of actinium is due to “ions activants,” the motion of which is
altered by a magnetic field. Other experiments showed that the magnetic
field acted on the “ions activants” and not on the emanation.
[Illustration: Fig. 71A.]
The results of Debierne thus lead to the conclusion that the carriers of
excited activity are derived from the emanation and are projected with
considerable velocity. This result supports the view, advanced in
section 190, that the expulsion of α particles from the emanation must
set the part of the system left behind in rapid motion. A close
examination of the mode of transference of the excited activity by
actinium and the emanation substance is likely to throw further light on
the processes which give rise to the deposit of active matter on the
electrodes.
Footnote 269:
M. and Mme. Curie, _C. R._ 129, p. 714, 1899.
Footnote 270:
Rutherford, _Phil. Mag._ Jan. and Feb. 1900.
Footnote 271:
As regards date of publication, the priority of the discovery of
“excited activity” belongs to M. and Mme. Curie. A short paper on this
subject, entitled “Sur la radioactivité provoquée par les rayons de
Becquerel,” was communicated by them to the _Comptes Rendus_, Nov. 6,
1899. A short note was added to the paper by Becquerel in which the
phenomena of excited activity were ascribed to a type of
phosphorescence. On my part, I had simultaneously discovered the
emission of an emanation from thorium compounds and the excited
activity produced by it, in July, 1899. I, however, delayed
publication in order to work out in some detail the properties of the
emanation and of the excited activity and the connection between them.
The results were published in two papers in the _Philosophical
Magazine_ (Jan. and Feb. 1900) entitled “A radio-active substance
emitted from thorium compounds,” and “Radio-activity produced in
substances by the action of thorium compounds.”
Footnote 272:
Rutherford, _Phil. Mag._ Feb. 1900.
Footnote 273:
Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902. _Phil. Mag._ Jan.
1903.
Footnote 274:
Miss Brooks, _Phil. Mag._ Sept. 1904.
Footnote 275:
Rutherford and Miss Brooks, _Phil. Mag._ July, 1902.
Footnote 276:
Curie and Danne, _C. R._ 136, p. 364, 1903.
Footnote 277:
Mme Curie, _Thèse_, Paris, 1903, p. 116.
Footnote 278:
Debierne, _C. R._ 138, p. 411, 1904.
Footnote 279:
Giesel, _Ber. d. D. Chem. Ges._ No. 3, p. 775, 1905.
Footnote 280:
Miss Brooks, _Phil. Mag._ Sept. 1904.
Footnote 281:
Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902.
Footnote 282:
F. von Lerch, _Annal. d. Phys._ 12, p. 745, 1903.
Footnote 283:
Pegram, _Phys. Review_, p. 424, Dec. 1903.
Footnote 284:
Miss Gates, _Phys. Review_, p. 300, 1903.
Footnote 285:
A more complete examination of the effect of temperature on the
excited activity of thorium has been made by Miss Slater (section
207).
Footnote 286:
Rutherford, _Phil. Mag._ Feb. 1900.
Footnote 287:
Henning, _Annal. d. Phys._ 7, p. 562, 1902.
Footnote 288:
Rutherford, _Phil. Mag._ Feb. 1900.
Footnote 289:
Curie and Debierne, _C. R._ 132, p. 768, 1901.
Footnote 290:
Fehrle, _Phys. Zeit._ 3, No. 7, p. 130, 1902.
Footnote 291:
Rutherford, _Phil. Mag._ Jan. 1903.
Footnote 292:
Giesel, _Ber. d. D. Chem. Ges._ 36, p. 342, 1903.
Footnote 293:
Debierne, _C. R._ 136, pp. 446 and 671, 1903; 138, p. 411, 1904.