CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER.
=126.= An account will now be given of some experiments which have
thrown much light, not only on the nature of the processes which serve
to maintain the radio-activity of the radio-active bodies, but also on
the source of the energy continuously emitted by those bodies. In this
chapter, for simplicity, the radio-activity of uranium and thorium will
alone be considered, for it will be seen later that the changes taking
place in these two substances are typical of those which occur in all
radio-active substances.
We have seen (section 23) that there is some doubt whether the
radio-activity of thorium is due to that element itself, or to an
unknown radio-active constituent associated with it. This uncertainty,
however, will present no serious difficulty when we are discussing the
radio-activity of thorium, for the general conclusions are, for the most
part, independent of whether thorium is the primary radio-active
constituent or not. For simplicity, however, it will be assumed for the
present that the radio-activity is due to thorium itself. If future
research should definitely show that the radio-activity, ordinarily
observed in thorium, is due to a new radio-active element mixed with it,
the radio-active processes considered will refer to this new element.
=127. Uranium X.= The experiments of Mme Curie show that the
radio-activity of uranium and radium is an atomic phenomenon. The
activity of any uranium compound depends only on the amount of that
element present, and is unaffected by its chemical combination with
other substances, and is not appreciably affected by wide variations of
temperature. It would thus seem probable, since the activity of uranium
is a specific property of the element, that the activity could not be
separated from it by chemical agencies.
In 1900, however, Sir William Crookes[225] showed that, by a single
chemical operation, uranium could be obtained photographically inactive
while the whole of the activity could be concentrated in a small residue
free from uranium. This residue, to which he gave the name of Ur X, was
many hundred times more active photographically, weight for weight, than
the uranium from which it had been separated. The method employed for
this separation was to precipitate a solution of the uranium with
ammonium carbonate. On dissolving the precipitate in an excess of the
reagent, a light precipitate remained behind. This was filtered, and
constituted the Ur X. The active substance Ur X was probably present in
very small quantity, mixed with impurities derived from the uranium. No
new lines were observed in its spectrum. A partial separation of the
activity of uranium was also effected by another method. Crystallized
uranium nitrate was dissolved in ether, when it was found that the
uranium divided itself between the ether and water present in two
unequal fractions. The small part dissolved in the water layer was found
to contain practically all the activity when examined by the
photographic method, while the other fraction was almost inactive. These
results, taken by themselves, pointed very strongly to the conclusion
that the activity of uranium was not due to the element itself, but to
some other substance, associated with it, which had distinct chemical
properties.
Results of a similar character were observed by Becquerel[226]. It was
found that barium could be made photographically very active by adding
barium chloride to the uranium solution and precipitating the barium as
sulphate. By a succession of precipitations the uranium was rendered
photographically almost inactive, while the barium was strongly active.
The inactive uranium and the active barium were laid aside; but, on
examining them a year later, it was found _that the uranium had
completely regained its activity, while that of the barium had
completely disappeared_. The loss of activity of uranium was thus only
temporary in character.
In the above experiments, the activity of uranium was examined by the
photographic method. The photographic action produced by uranium is due
almost entirely to the β rays. The α rays, in comparison, have little if
any effect. Now the radiation from Ur X consists entirely of β rays, and
is consequently photographically very active. If the activity of uranium
had been measured electrically without any screen over it, the current
observed would have been due very largely to the α rays, and little
change would have been observed after the removal of Ur X, since only
the constituent responsible for the β rays was removed. This important
point is discussed in more detail in section 205.
=128. Thorium X.= Rutherford and Soddy[227], working with thorium
compounds, found that an intensely active constituent could be separated
from thorium by a single chemical operation. If ammonia is added to a
thorium solution, the thorium is precipitated, but a large amount of the
activity is left behind in the filtrate, which is chemically free from
thorium. This filtrate was evaporated to dryness, and the ammonium salts
driven off by ignition. A small residue was obtained which, weight for
weight, was in some cases several thousand times more active than the
thorium from which it was obtained, while the activity of the
precipitated thorium was reduced to less than one half of its original
value. This active constituent was named Th X from analogy to Crookes’
Ur X.
The active residue was found to consist mainly of impurities from the
thorium; the Th X could not be examined chemically, and probably was
present only in minute quantity. It was also found that an active
constituent could be partly separated from thorium oxide by shaking it
with water for some time. On filtering the water, and evaporating down,
a very active residue was obtained which was analogous in all respects
to Th X.
On examining the products a month later, it was found that the _Th X was
no longer active, while the thorium had completely regained its
activity_. A long series of measurements was then undertaken to examine
the time-rate of these processes of decay and recovery of activity.
[Illustration: Fig. 47.]
The results are shown graphically in Fig. 47, where the final activity
of the thorium and the initial activity of the Th X are in each case
taken as 100. The ordinates represent the activities determined by means
of the ionization current, and the abscissae represent the time in days.
It will be observed that both curves are irregular for the first two
days. The activity of the Th X increased at first, while the activity of
the thorium diminished. Disregarding these initial irregularities of the
curves, which will be explained in detail in section 208, it will be
seen that, after the first two days, the time taken for the thorium to
recover half its lost activity is about equal to the time taken by the
Th X to lose half its activity. This time in each case is about four
days. The percentage proportion of the activity regained by the thorium,
over any given interval, is approximately equal to the percentage
proportion of the activity lost by the Th X during the same interval.
[Illustration: Fig. 48.]
If the recovery curve is produced backwards to meet the vertical axis,
it does so at a minimum of 25 per cent., and the above conclusions hold
more accurately, if the recovery is assumed to start from this minimum.
This is clearly shown by Fig. 48, where the percentages of activity
recovered, reckoned from the 25 per cent. minimum, are plotted as
ordinates. In the same figure the decay curve, after the second day, is
shown on the same scale. The activity of the Th X decays with the time
according to an exponential law, falling to half value in about four
days. If _I₀_ is the initial activity and _I{t}_ is the activity after
a time _t_, then
$$ \frac {I₀} {I_t} = e^{–λt} $$, where λ is a constant and _e_
the natural base of logarithms. The experimental curve of the rise of
activity from a minimum to a maximum value is therefore expressed by the
equation
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
where _I₀_ is the amount of activity recovered when the state of
constant activity is reached, _I_{t}_ the activity recovered after a
time _t_, and λ is the _same constant_ as before.
=129. Uranium X.= Similar results were obtained when uranium was
examined. The Ur X was separated by Becquerel’s method of successive
precipitations with barium. The decay of the separated activity and the
recovery of the lost activity are shown graphically in Fig. 49. A more
detailed discussion of this experiment is given in section 205.
[Illustration: Fig. 49.]
The curves of decay and recovery exhibit the same peculiarities and can
be expressed by the same equations as in the case of thorium. The
time-rate of decay and recovery is, however, much slower than for
thorium, the activity of the Ur X falling to half its value in about 22
days.
A large number of results of a similar character have been obtained from
other radio-active products, separated from the radio-elements, but the
cases of thorium and uranium will suffice for the present to form a
basis for the discussion of the processes that are taking place in
radio-active bodies.
=130. Theory of the phenomena.= These processes of decay and recovery go
on at exactly the same rate if the substances are removed from the
neighbourhood of one another, or enclosed in lead, or placed in a vacuum
tube. It is at first sight a remarkable phenomenon that the processes of
decay and recovery should be so intimately connected, although there is
no possibility of mutual interaction between them. These results,
however, receive a complete explanation on the following hypotheses:
(1) That there is a constant rate of production of fresh
radio-active matter by the radio-active body;
(2) That the activity of the matter so formed decreases according to
an exponential law with the time from the moment of its formation.
Suppose that _q₀_ particles of new matter are produced per second from a
given mass of matter. The rate of emission of energy due to the
particles produced in the time _dt_, is, at the moment of their
formation, equal to _Kq₀__dt_, where _K_ is a constant.
It is required to find the activity due to the whole matter produced
after the process has continued for a time _T_.
The activity _dI_, due to the matter produced during the time _dt_ at
the time _t_, decays according to an exponential law during the time _T_ −
_t_ that elapses before its activity is estimated, and in consequence
is given by
$$ dI = Kq₀e^{–λ (T-t)} dt $$,
where λ is the constant of decay of activity of the active matter. The
activity _I_{T}_ due to the whole matter produced in the time _T_ is
thus given by
$$ I_t = \int₀^T Kq₀e^{–λ (T-t)} dt $$
$$ = \frac {Kq₀} {λ} (1 − e^{–λ T}) $$ .
The activity reaches a maximum value _I₀_ when _T_ is very great, and is
then given by
_Kq₀_
_I₀_ = -----
λ
thus
$$ \frac {I_T} {I₀} = 1 − e^{–λ T} $$ .
This equation agrees with the experimental results for the recovery of
lost activity. Another method for obtaining this equation is given later
in section 133.
A state of equilibrium is reached when the rate of loss of activity of
the matter already produced is balanced by the activity supplied by the
production of new active matter. According to this view, the
radio-active bodies are undergoing change, but the activity remains
constant owing to the action of two opposing processes. Now, if this
active matter can at any time be separated from the substance in which
it is produced, the decay of its activity, as a whole, should follow an
exponential law with the time, since each portion of the matter
decreases in activity according to an exponential law with the time,
whatever its age may be. If _I₀_ is the initial activity of the
separated product, the activity _I_{t}_ after an interval _t_ is given
by
$$ \frac {I_T} {I₀} = e^{–λt} $$ .
Thus, the two assumptions—of uniform production of active matter and of
the decay of its activity in an exponential law from the moment of its
formation—satisfactorily explain the relation between the curves of
decay and recovery of activity.
=131. Experimental evidence.= It now remains to consider further
experimental evidence in support of these hypotheses. The primary
conception is that the radio-active bodies are able to produce from
themselves matter of chemical properties different from those of the
parent substance, and that this process goes on at a constant rate. This
new matter initially possesses the property of activity, and loses it
according to a definite law. The fact that a proportion of the activity
of radium and thorium can be concentrated in small amounts of active
matter like Th X or Ur X does not, of itself, prove directly that a
material constituent responsible for the activity has been chemically
separated. For example, in the case of the separation of Th X from
thorium, it might be supposed that the non-thorium part of the solution
is rendered temporarily active by its association with thorium, and that
this property is retained through the processes of precipitation,
evaporation, and ignition, and finally manifests itself in the residue
remaining. According to this view it is to be expected that any
precipitate capable of removing the thorium completely from its solution
should yield active residues similar to those obtained from ammonia. No
such case has, however, been observed. For example, when thorium nitrate
is precipitated by sodium or ammonium carbonate, the residue from the
filtrate after evaporation and ignition is free from activity and the
thorium carbonate obtained has the normal amount of activity. In fact,
ammonia is the only reagent yet found capable of completely separating
Th X from thorium. A partial separation of the Th X can be made by
shaking thorium oxide with water owing to the greater solubility of Th X
in water.
Thorium and uranium behave quite differently with regard to the action
of ammonia and ammonium carbonate. Ur X is completely precipitated with
the uranium in an ammonia solution and the filtrate is inactive. Ur X is
separated by ammonium carbonate, while Th X under the same conditions is
completely precipitated with the thorium. The Ur X and the Th X thus
behave like distinct types of matter with well-marked chemical
properties quite distinct from those of the substances in which they are
produced. The removal of Ur X by the precipitation of barium is probably
not directly connected with the chemical properties of Ur X. The
separation is probably due to the dragging down of the Ur X with the
dense barium precipitate. Sir William Crookes found that the Ur X was
dragged down by precipitates when no question of insolubility was
involved, and such a result is to be expected if the Ur X exists in
extremely minute quantity. It must be borne in mind that the actual
amount of the active constituents Th X and Ur X, separated from thorium
and uranium, is probably infinitesimal, and that the greater proportion
of the residues is due to impurities present in the salt and the
reagents, a very small amount of active matter being mixed with them.
=132. Rate of production of Th X.= If the recovery of the activity of
uranium or thorium is due to the continuous production of new active
matter, it should be possible to obtain experimental evidence of the
process. As the case of thorium has been most fully investigated, a
brief account will be given of some experiments made by Rutherford and
Soddy[228] to show that Th X is produced continuously at a constant
rate. Preliminary experiments showed that three successive
precipitations were sufficient to remove the Th X almost completely from
the thorium. The general method employed was to precipitate a solution
of 5 grams of thorium-nitrate with ammonia. The precipitate was then
redissolved in nitric acid and the thorium again precipitated as before,
as rapidly as possible, so that the Th X produced in the time between
successive precipitations should not appreciably affect the results. The
removal of the Th X was followed by measurements of the activity of the
residues obtained from successive filtrates. In three successive
precipitations the activities of the residues were proportional to 100,
8, 1·6 respectively. Thus two precipitations are nearly sufficient to
free the thorium from Th X.
The thorium freed from Th X was then allowed to stand for a definite
time, and the amount of Th X formed during that time found by
precipitating it, and measuring its radio-activity. According to the
theory, the activity _I_{t}_ of the thorium formed in the time _t_ is
given by
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
where _I₀_ is the total activity of Th X, when there is radio-active
equilibrium.
If λ_t_ is small,
_I_{t}_
------ = λ_t_.
_I₀_
Since the activity of Th X falls to half value in 4 days, the value of λ
expressed in hours = ·0072. After standing a period of 1 hour about
¹⁄₁₄₀, after 1 day ⅙, after 4 days ½ of the maximum should be
obtained. The experimental results obtained showed an agreement, as good
as could be expected, with the equation expressing the result that the
Th X was being produced at a constant rate.
The thorium-nitrate which had been freed from Th X was allowed to stand
for one month, and then it was again subjected to the same process. The
activity of the Th X was found to be the same as that obtained from an
equal amount of the original thorium-nitrate. In one month, therefore,
the Th X had been regenerated, and had reached a maximum value. By
leaving the thorium time to recover fully its activity, this process can
be repeated indefinitely, and equal amounts of Th X are obtained at each
precipitation. Ordinary commercial thorium-nitrate and the purest
nitrate obtainable showed exactly the same action, and equal amounts of
Th X could be obtained from equal weights. These processes thus appear
to be independent of the chemical purity of the substance[229].
The process of the production of Th X is continuous, and no alteration
has been observed in the amount produced in the given time after
repeated separations. After 23 precipitations extending over 9 days, the
amount produced in a given interval was about the same as at the
beginning of the process.
These results are all in agreement with the view that the Th X is being
continuously produced from the thorium compound at a constant rate. The
amount of active matter produced from 1 gram of thorium is probably
extremely minute, but the electrical effects due to its activity are so
large that the process of production can be followed after extremely
short intervals. With a sensitive electrometer the amount of Th X
produced per minute in 10 grams of thorium-nitrate gives a rapid
movement to the electrometer needle. For larger intervals it is
necessary to add additional capacity to the system to bring the effects
within range of the instrument.
=133. Rate of decay of activity.= It has been shown that the activity of
Ur X and Th X decays according to an exponential law with the time.
This, we shall see later, is the general law of decay of activity in any
type of active matter, obtained by itself, and freed from any secondary
active products which it may, itself, produce. In any case, when this
law is not fulfilled, it can be shown that the activity is due to the
superposition of two or more effects, each of which decays in an
exponential law with the time. The physical interpretation of this law
still remains to be discussed.
It has been shown that in uranium and thorium compounds there is a
continuous production of active matter which keeps the compound in
radio-active equilibrium. The changes by which the active matter is
produced must be chemical in nature, since the products of the action
are different in chemical properties from the matter in which the
changes take place. The activity of the products has afforded the means
of following the changes occurring in them. It now remains to consider
the connection between the activity at any time, and the amount of
chemical change taking place at that time.
In the first place, it is found experimentally that the saturation
ionization current _i_{t}_, after the active product has been allowed to
decay for a time _t_, is given by
$$ \frac {i_t} {i₀} = e^{–λt} $$,
where _i₀_ is the initial saturation current and λ the constant of
decay.
Now the saturation current is a measure of the total number of ions
produced per second in the testing vessel. It has already been shown
that the α rays, which produce the greater proportion of ionization in
the gas, consist of positively charged particles projected with great
velocity. Suppose for simplicity that each atom of active matter, in the
course of its change, gives rise to one projected α particle. Each α
particle will produce a certain average number of ions in its path
before it strikes the boundaries or is absorbed in the gas. Since the
number of projected particles per second is equal to the number of atoms
changing per second, the number of atoms _n_{t}_ which change per second
at the time _t_ is given by
$$ \frac {n_t} {n₀} = e^{–λt} $$,
where _n₀_ is the initial number which change per second. On this view,
then, the law of decay expresses the result that the number of atoms
changing in unit time, diminishes according to an exponential law with
the time. The number of atoms _N_{t}_ which remain _unchanged_ after an
interval _t_ is given by
$$ N_t = \int_t^{\infty} n_t dt $$
$$ = \frac {n₀} {λ} e^{–λt} $$ .
If _N₀_ is the number of atoms at the beginning,
$$ N₀ = \frac {n₀} {λ} $$,
Thus
$$ \frac {N_t} {N_₀} = e^{–λt} $$ (1).
or the law of decay expresses the fact that the _activity of a product
at any time is proportional to the number of atoms which remain
unchanged at that time_.
This is the same as the law of monomolecular change in chemistry, and
expresses the fact that there is only one changing system. If the change
depended on the mutual action of two systems, the law of decay would be
different, since the rate of decay in that case would depend on the
relative concentration of the two reacting substances. This is not so,
for not a single case has yet been observed in which the law of decay
was affected by the amount of active matter present.
From the above equation (1)
_dN_{t}_
------- = –λ_N_{t}_,
_dt_
or the number of systems changing in unit time is proportional to the
number unchanged at that time.
In the case of recovery of activity, after an active product has been
removed, the number of systems changing in unit time, when radio-active
equilibrium is produced, is equal to λ_N₀_. This must be equal to the
number _q₀_ of new systems applied in unit time, or
_q₀_ = λ_N₀_,
_q₀_
and λ = ------;
_N₀_
λ has thus a distinct physical meaning, and may be defined as the
proportion of the total number of systems present which change per
second. It has different values for different types of active matter,
but is invariable for any particular type of matter. For this reason, λ
will be termed the “_radio-active constant_” of the product.
We are now in a position to discuss with more physical definiteness the
gradual growth of Th X in thorium, after the Th X has been completely
removed from it. Let _q₀_ particles of Th X be produced per second by
the thorium, and let _N_ be the number of particles of Th X present at
any time _t_ after the original Th X was removed. The number of
particles of Th X which change every second is λ_N_, where λ is the
radio-active constant of Th X. Now, at any time during the process of
recovery, the rate of increase of the number of particles of Th X = the
rate of production − the rate of change; that is
_dN_
------ = _q₀_ − λ_N_.
_dt_
The solution of this equation is of the form
$$ N = ae^{–λt} + b $$,
where _a_ and _b_ are constants.
Now when _t_ is very great, the number of particles of Th X present
reach a maximum value _N₀_.
Thus, since _N_ = _N₀_ when _t_ = infinity,
_b_ = _N₀_;
since _N_ = 0 when _t_ = 0,
_a_ + _b_ = 0;
hence _b_ = -_a_ = _N₀_,
and the equation becomes
$$ \frac {N} {N₀} = 1 − e^{–λt} $$ .
This is equivalent to the equation already obtained in section 130,
since the intensity of the radiation is always proportional to the
number of particles present.
=134. Influence of conditions on the rate of decay.= Since the activity
of any product, at any time, may be taken as a measure of the rate at
which chemical change takes place, it may be used as a means of
determining the effect of conditions on the changes occurring in
radio-active matter. If the rate of change should be accelerated or
retarded, it is to be expected that the value of the radio-active
constant λ will be increased or decreased, _i.e._ that the decay curve
will be different under different conditions.
No such effect, however, has yet been observed in any case of
radio-active change, where none of the active products produced are
allowed to escape from the system. The rate of decay is unaltered by any
chemical or physical agency, and in this respect the changes in
radio-active matter are sharply distinguished from ordinary chemical
changes. For example, the rate of decay of activity from any product
takes place at the same rate when the substance is exposed to light as
when it is kept in the dark, and at the same rate in a vacuum as in air
or any other gas at atmospheric pressure. Its rate of decay is unaltered
by surrounding the active matter by a thick layer of lead under
conditions where no ordinary radiation from outside can affect it. The
activity of the matter is unaffected by ignition or chemical treatment.
The material giving rise to the activity can be dissolved in acid and
re-obtained by evaporation of the solution without altering the
activity. The rate of decay is the same whether the active matter is
retained in the solid state or kept in solution. When a product has lost
its activity, resolution or heat does not regenerate it, and as we shall
see later, the rate of decay of the active products, so far examined, is
the same at a red heat as at the temperature of liquid air. In fact, no
variation of physical or chemical conditions has led to any observable
difference in the decay of activity of any of the numerous types of
active matter which have been examined.
=135. Effect of conditions on the rate of recovery of activity.= The
recovery of the activity of a radio-element with time, when an active
product is separated from it, is governed by the rate of production of
fresh active matter and by the decay of activity of that already
produced. Since the rate of decay of the activity of the separated
product is independent of conditions, the rate of recovery of activity
can be modified only by a change of the rate of production of fresh
active matter. As far as experiments have gone, the rate of production,
like the rate of decay, is independent of chemical or physical
conditions. There are indeed certain cases which are apparent exceptions
to this rule. For example, the escape of the radio-active emanations
from thorium and radium is readily affected by heat, moisture and
solution. A more thorough investigation, however, shows that the
exception is only apparent and not real. These cases will be discussed
more in detail in chapter VII, but it may be stated here that the
differences observed are due to differences in the rate of escape of the
emanations into the surrounding gas, and not to differences in the rate
of production. For this reason it is difficult to test the question at
issue in the case of the thorium compounds, which in most cases readily
allow the emanation produced by them to escape into the air.
In order to show that the rate of production is independent of molecular
state, temperature, etc., it is necessary in such a case to undertake a
long series of measurements extending over the whole time of recovery.
It is impossible to make accurate relative comparisons to see if the
activity is altered by the conversion of one compound into another. The
relative activity in such a case, when measured by spreading a definite
weight of material uniformly on a metal plate, varies greatly with the
physical conditions of the precipitate, although the total activity of
two compounds may be the same.
The following method[230] offers an accurate and simple means of
studying whether the rate of production of active matter is influenced
by molecular state. The substance is chemically converted into any
compound required, care being taken that active products are recovered
during the process. The new compound is then spread on a metal plate and
compared with a standard sample of uranium for several days or weeks as
required. If the rate of production of active matter is altered by the
conversion, there should be an increase or decrease of activity to a new
steady value, where the production of active matter is again balanced by
the rate of decay. This method has the great advantage of being
independent of the physical condition of the precipitate. It can be
applied satisfactorily to a compound of thorium like the nitrate and the
oxide which has been heated to a white heat, after which treatment only
a slight amount of emanation escapes. The nitrate was converted into the
oxide in a platinum crucible by treatment with sulphuric acid and
ignition to a white heat. The oxide so obtained was spread on a plate,
but no change of its activity was observed with time, showing that in
this case the rate of production was independent of molecular state.
This method, which is limited in the case of thorium, may be applied
generally to the uranium compounds where the results are not complicated
by the presence of an emanation.
No differences have yet been observed in the recovery curves of
different thorium compounds after the removal of Th X. For example, the
rate of recovery is the same whether the precipitated hydroxide is
converted into the oxide or into the sulphate.
=136. Disintegration hypothesis.= In the discussion of the changes in
radio-active bodies, only the active products Ur X and Th X have been
considered. It will, however, be shown later that these two products are
only examples of many other types of active matter which are produced by
the radio-elements, and that each of these types of active matter has
definite chemical as well as radio-active properties, which distinguish
it, not only from the other active products, but also from the substance
from which it is produced.
The full investigation of these changes will be shown to verify in every
particular the hypothesis that radio-activity is the accompaniment of
chemical changes of a special kind occurring in matter, and that the
constant activity of the radio-elements is due to an equilibrium
process, in which the rate of production of fresh active matter balances
the rate of change of that already formed.
The nature of the process taking place in the radio-elements, in order
to give rise to the production at a constant rate of new kinds of active
matter, will now be considered. Since in thorium or uranium compounds
there is a continuous production of radio-active matter, which differs
in chemical properties from the parent substance, some kind of change
must be taking place in the radio-element. This change, by which new
matter is produced, is very different in character from the molecular
changes dealt with in chemistry, for no chemical change is known which
proceeds at the same rate at the temperatures corresponding to a red
heat and to liquid air, and is independent of all physical and chemical
actions. If, however, the production of active matter is supposed to be
the result of changes, not in the molecule, but in the _atom itself_, it
is not to be expected that the temperature would exert much influence.
The general experience of chemistry in failing to transform the elements
by the action of temperature is itself strong evidence that wide ranges
of temperature have not much effect in altering the stability of the
chemical atom.
The view that the atoms of the radio-elements are undergoing spontaneous
disintegration was put forward by Rutherford and Soddy as a result of
evidence of this character. The discovery of the _material_ nature of
the α rays added strong confirmation to the hypothesis; for it has been
pointed out (section 95) that the expulsion of α particles must be the
result of a disintegration of the atoms of the radio-element. Taking the
case of thorium as an example, the processes occurring in the atom may
be pictured in the following way. It must be supposed that the thorium
atoms are not permanently stable systems, but, on an average, a constant
small proportion of them—about one atom in every 10¹⁶ will suffice—break
up per second. The disintegration consists in the expulsion from the
atom of one or more α particles with great velocity. For simplicity, it
will be supposed that each atom expels _one_ α particle. It has been
shown that the α particle of radium has a mass about twice that of the
hydrogen atom. From the similarity of the α rays from thorium and
radium, it is probable that the α particle of thorium does not differ
much in mass from that of radium, and may be equal to it. The α
particles expelled from the thorium atoms as they break up constitute
what is known as the “non-separable activity” of thorium. This activity,
measured by the α rays, is about 25 per cent. of the maximum. After the
escape of an α particle, the part of the atom left behind, which has a
mass slightly less than that of the thorium atom, tends to rearrange its
components to form a temporarily stable system. It is to be expected
that it will differ in chemical properties from the thorium atom from
which it was derived. The atom of the substance Th X is, on this view,
the thorium atom minus one α particle. The atoms of Th X are far more
unstable than the atoms of thorium, and one after the other they break
up, each atom expelling one α particle as before. These projected α
particles give rise to the _radiation_ from the Th X. Since the activity
of Th X falls to half its original value in about four days, on an
average half of the atoms of Th X break up in four days, the number
breaking up per second being always proportional to the number present.
After an atom of Th X has expelled an α particle, the mass of the system
is again reduced, and its chemical properties are changed. It will be
shown (section 154) that the Th X produces the thorium emanation, which
exists as a radio-active gas, and that this in turn is transformed into
matter which is deposited on solid bodies and gives rise to the
phenomena of excited activity. The first few successive changes
occurring in thorium are shown diagrammatically below (Fig. 50).
[Illustration: Fig. 50.]
Thus as a result of the disintegration of the thorium atom, a series of
chemical substances is produced, each of which has distinctive chemical
properties. Each of these products is radio-active, and loses its
activity according to a definite law. Since thorium has an atomic weight
of 237, and the weight of the α particle is about 2, it is evident that,
if only _one_ α particle is expelled at each change, the process of
disintegration could pass through a number of successive stages and yet
leave behind, at the end of the process, a mass comparable with that of
the parent atom.
It will be shown later that a process of disintegration, very similar to
that already described for thorium, must be supposed to take place also
in uranium, actinium and radium. The full discussion of this subject
cannot be given with advantage until two of the most important products
of the three substances thorium, radium and actinium, viz. the
radio-active emanations and the matter which causes excited activity,
have been considered in detail.
=137. Magnitude of the changes.= It can be calculated by several
independent methods (see section 246) that, in order to account for the
radio-activity observed in thorium, about 3 × 10⁴ atoms in each gram of
thorium suffer disintegration per second. It is well known (section 39)
that 1 cubic centimetre of hydrogen at atmospheric pressure and
temperature contains about 3·6 × 10¹⁹ molecules. From this it follows
that one gram of thorium contains 3·6 × 10²¹ atoms. The fraction which
breaks up per second is thus about 10¹⁷. This is an extremely small
ratio, and it is evident that the process could continue for long
intervals of time, before the amount of matter changed would be capable
of detection by the spectroscope or by the balance. With the
electroscope it is possible to detect the radiation from 10⁻⁵ gram of
thorium, _i.e._ the electroscope is capable of detecting the ionization
which accompanies the disintegration of a single thorium atom per
second. The electroscope is thus an extraordinarily delicate means for
detection of minute changes in matter, which are accompanied, as in the
case of the radio-elements, by the expulsion of charged particles with
great velocity. It is possible to detect by its radiation the amount of
Th X produced in a second from 1 gram of thorium, although the process
would probably need to continue thousands of years before it could be
detected by the balance or the spectroscope. It is thus evident that the
changes occurring in thorium are of an order of magnitude quite
different from that of ordinary chemical changes, and it is not
surprising that they have never been observed by direct chemical
methods.
Footnote 225:
Crookes, _Proc. Roy. Soc._ 66, p. 409, 1900.
Footnote 226:
Becquerel, _C. R._ 131, p. 137, 1900; 133, p. 977, 1901.
Footnote 227:
Rutherford and Soddy, _Phil. Mag._ Sept. and Nov. 1902. _Trans. Chem.
Soc._ 81, pp. 321 and 837, 1902.
Footnote 228:
Rutherford and Soddy, _Phil. Mag._ Sept. 1902.
Footnote 229:
The general method of regarding the subject would be unchanged, even
if it were proved that the radio-activity of thorium is not due to
thorium at all but to a small constant amount of a radio-active
impurity mixed with it.
Footnote 230:
Rutherford and Soddy, _Phil. Mag._ Sept. 1902.