RADIO-ACTIVE PROCESSES.
=254. Theories of radio-activity.= In previous chapters, a detailed
account has been given of the nature and properties of the radiations,
and of the complex processes taking place in the radio-active
substances. The numerous products arising from the radio-elements have
been closely examined, and have been shown to result from a
transformation of the parent element through a number of well-marked
stages. In this chapter, the application of the disintegration theory to
the explanation of radio-active phenomena will be considered still
further, and the logical deductions to be drawn from the theory will be
discussed briefly.
A review will first be given of the working hypotheses which have served
as a guide to the investigators in the field of radio-activity. These
working theories have in many cases been modified or extended with the
growth of experimental knowledge.
The early experiments of Mme Curie had indicated that radio-activity was
an atomic and not a molecular phenomenon. This was still further
substantiated by later work, and the detection and isolation of radium
from pitchblende was a brilliant verification of the truth of this
hypothesis.
The discovery that the β rays of the radio-elements were similar to the
cathode rays produced in a vacuum tube was an important advance, and has
formed the basis of several subsequent theories. J. Perrin[333], in
1901, following the views of J. J. Thomson and others, suggested that
the atoms of bodies consisted of parts and might be likened to a
miniature planetary system. In the atoms of the radio-elements, the
parts composing the atoms more distant from the centre might be able to
escape from the central attraction and thus give rise to the radiation
of energy observed. In December 1901, Becquerel[334] put forward the
following hypothesis, which, he stated, had served him as a guide in his
investigations. According to the view of J. J. Thomson, radio-active
matter consists of negatively and positively charged particles. The
former have a mass about ¹⁄₁₀₀₀ of the mass of the hydrogen atom, while
the latter have a mass about one thousand times greater than that of the
negative particle. The negatively charged particles (the β rays) would
be projected with great velocity, but the larger positive particles with
a much lower velocity forming a sort of gas (the emanation) which
deposits itself on the surface of bodies. This in turn would subdivide,
giving rise to rays (excited activity).
In a paper communicated to the Royal Society in June 1900, Rutherford
and McClung[335] estimated that the energy, radiated in the form of
ionizing rays into the gas, was 3000 gram-calories per year for radium
of activity 100,000 times that of uranium. Taking the latest estimate of
the activity of a pure radium compound as 2,000,000, this would
correspond to an emission of energy into the gas in the form of α rays
of about 66,000 gram-calories per gram per year. The suggestion was made
that this energy might be derived from a re-grouping of the constituents
of the atom of the radio-elements, and it was pointed out that the
possible energy to be derived from a greater concentration of the
components of the atom was large compared with that given out in
molecular reactions.
In the original papers[336] giving an account of the discovery of the
emanation of thorium and the excited radio-activity produced by it, the
view was taken that both of these manifestations were due to
radio-active material. The emanation behaved like a gas, while the
matter which caused excited activity attached itself to solids and could
be dissolved in some acids but not in others. Rutherford and Miss Brooks
showed that the radium emanation diffused through air like a gas of
heavy molecular weight. At a later date Rutherford and Soddy showed that
the radium and thorium emanations behaved like chemically inert gases,
since they were unaffected by the most drastic physical and chemical
treatment.
On the other hand, P. Curie, who, in conjunction with Debierne, had made
a series of researches on the radium emanation, expressed dissent from
this view. P. Curie[337] did not consider that there was sufficient
evidence that the emanation was material in nature, and pointed out that
no spectroscopic evidence of its presence had yet been obtained, and
also that the emanation disappeared when contained in a sealed vessel.
It was pointed out by the writer[338] that the failure to detect
spectroscopic lines was probably a consequence of the minute quantity of
the emanation present, under ordinary conditions, although the
electrical and phosphorescent actions produced by this small quantity
are very marked. This contention is borne out by later work. P. Curie at
first took the view that the emanation was not material, but consisted
of centres of condensation of energy attached to the gas molecules and
moving with them.
M. and Mme Curie have throughout taken a very general view of the
phenomena of radio-activity, and have not put forward any definite
theory. In Jan. 1902, they gave an account of the general working
theory[339] which had guided them in their researches. Radio-activity is
an atomic property, and the recognition of this fact had created their
methods of research. Each atom acts as a constant source of emission of
energy. This energy may either be derived from the potential energy of
the atom itself, or each atom may act as a mechanism which instantly
regains the energy which is lost. They suggested that this energy may be
borrowed from the surrounding air in some way not accounted for by the
principle of Carnot.
In the course of a detailed study of the radio-activity of thorium,
Rutherford and Soddy[340] found that it was necessary to suppose that
thorium was continuously producing from itself new kinds of active
matter, which possess temporary activity and differ in chemical
properties from the thorium itself. The constant radio-activity of
thorium was shown to be the result of equilibrium between the processes
of production of active matter and the change of that already produced.
At the same time, the theory was advanced that the production of active
matter was a consequence of the disintegration of the atom. The work of
the following year was devoted to an examination of the radio-activity
of uranium and radium on similar lines, and it was found that the
conclusions already advanced for thorium held equally for uranium and
radium[341]. The discovery of a condensation of the radio-active
emanations[342] gave additional support to the view that the emanations
were gaseous in character. In the meantime, the writer[343] had found
that the rays consisted of positively charged bodies atomic in size,
projected with great velocity. The discovery of the material nature of
these rays served to strengthen the theory of atomic disintegration, and
at the same time to offer an explanation of the connection between the α
rays and the changes occurring in the radio-elements. In a paper
entitled “Radio-active Change,” Rutherford and Soddy[344] put forward in
some detail the theory of atomic disintegration as an explanation of the
phenomena of radio-activity, and at the same time some of the more
important consequences which follow from the theory were discussed.
In a paper announcing the discovery of the heat emission of radium, P.
Curie and Laborde[345] state that the heat energy may be equally well
supposed to be derived from a breaking up of the radium atom or from
energy absorbed by the radium from some external source.
J. J. Thomson in an article on “Radium,” communicated to _Nature_[346],
put forward the view that the emission of energy from radium is probably
due to some change within the atom, and pointed out that a large store
of energy would be released by a contraction of the atom.
Sir William Crookes[347], in 1899, proposed the theory that the
radio-active elements possess the property of abstracting energy from
the gas. If the moving molecules, impinging more swiftly on the
substance, were released from the active substance at a much lower
velocity, the energy released from the radio-elements might be derived
from the atmosphere. This theory was advanced again later on to account
for the large heat emission of radium, discovered by P. Curie and
Laborde.
F. Re[348] recently advanced a very general theory of matter with a
special application to radio-active bodies. He supposes that the parts
of the atom were originally free, constituting a nebula of extreme
tenuity. These parts have gradually become united round centres of
condensation, and have thus formed the atoms of the elements. On this
view an atom may be likened to an extinct sun. The radio-active atoms
occupy a transitional stage between the original nebula and the more
stable chemical atoms, and in the course of their contraction give rise
to the heat emission observed.
Lord Kelvin in a paper to the British Association meeting, 1903, has
suggested that radium may obtain its energy from external sources. If a
piece of white paper is put into one vessel and a piece of black paper
into an exactly similar vessel, on exposure of both vessels to the light
the vessel containing the black paper is found to be at a higher
temperature. He suggests that radium in a similar manner may keep its
temperature above the surrounding air by its power of absorption of
unknown radiations.
Richarz and Schenck[349] have suggested that radio-activity may be due
to the production and breaking up of ozone which is known to be produced
by radium salts.
=255. Discussion of Theories.= From the survey of the general hypotheses
advanced as possible explanations of radio-activity, it is seen that
they may be divided broadly into two classes, one of which assumes that
the energy emitted from the radio-elements is obtained at the expense of
the internal energy of the atom, and the other that the energy is
derived from external sources, but that the radio-elements act as
mechanisms capable of transforming this borrowed energy into the special
forms manifested in the phenomena of radio-activity. Of these two sets
of hypotheses the first appears to be the more probable, and to be best
supported by the experimental evidence. Up to the present not the
slightest experimental evidence has been adduced to show that the energy
of radium is derived from external sources.
J. J. Thomson (_loc. cit._) has discussed the question in the following
way:—
“It has been suggested that the radium derives its energy from the air
surrounding it, that the atoms of radium possess the faculty of
abstracting the kinetic energy from the more rapidly moving air
molecules while they are able to retain their own energy when in
collision with the slowly moving molecules of air. I cannot see,
however, that even the possession of this property would explain the
behaviour of radium; for imagine a portion of radium placed in a cavity
in a block of ice; the ice around the radium gets melted; where does the
energy for this come from? By the hypothesis there is no change in the
air-radium system in the cavity, for the energy gained by the radium is
lost by the air, while heat cannot flow into the cavity from the
outside, for the melted ice round the cavity is hotter than the ice
surrounding it.”
The writer has recently found that the activity of radium is not altered
by surrounding it with a large mass of lead. A cylinder of lead was cast
10 cms. in diameter and 10 cms. high. A hole was bored in one end of the
cylinder to the centre, and the radium, enclosed in a small glass tube,
was placed in the cavity. The opening was then hermetically closed. The
activity was measured by the rate of discharge of an electroscope by the
γ rays transmitted through the lead, but no appreciable change was
observed during a period of one month.
M. and Mme Curie early made the suggestion that the radiation of energy
from the radio-active bodies might be accounted for by supposing that
space is traversed by a type of Röntgen rays, and that the
radio-elements possess the property of absorbing them. Recent
experiments (section 279) have shown that there is present at the
surface of the earth a very penetrating type of rays, similar to the γ
rays of radium. Even if it were supposed that the radio-elements
possessed the power of absorbing this radiation, the energy of the rays
is far too minute to account even for the energy radiated from an
element of small activity like uranium. In addition, all the evidence so
far obtained points to the conclusion that the radio-active bodies do
not absorb the type of rays they emit to any greater extent than would
be expected from their density. It has been shown (section 86) that this
is true in the case of uranium. Even if it were supposed that the
radio-elements possess the property of absorbing the energy of some
unknown type of radiation, which is able to pass through ordinary matter
with little absorption, there still remains the fundamental difficulty
of accounting for the peculiar radiations from the radio-elements, and
the series of changes that occur in them. It is not sufficient for us to
account for the heat emission only, for it has been shown (chapter XII)
that the emission of heat is directly connected with the radio-activity.
In addition, the distribution of the heat emission of radium amongst the
radio-active products which arise from it is extremely difficult to
explain on the hypothesis that the energy emitted is borrowed from
external sources. It has been shown that more than two-thirds of the
heat emitted by radium is due to the emanation together with the active
deposit which is produced by the emanation. When the emanation is
separated from the radium, its power of emitting heat, after reaching a
maximum, decreases with the time according to an exponential law. It
would thus be necessary on the absorption hypothesis to postulate that
most of the heat emission of radium, observed under ordinary conditions,
is not due to the radium itself but to something produced by the radium,
whose power of absorbing energy from external sources diminishes with
time.
A similar argument also applies to the variation with time of the
heating effect of the active deposit produced from the emanation. It has
been shown in the last chapter that most of the heating effect observed
in radium and its products must be ascribed to the bombardment of the α
particles expelled from these substances. It has already been pointed
out (section 136) that it is difficult to imagine any mechanism, either
internal or external, whereby such enormous velocity can suddenly be
impressed upon the α particles. We are forced to the conclusion that the
α particle did not suddenly acquire this energy of motion, but was
initially in rapid motion in the atom, and for some reason, was suddenly
released with the velocity which it previously possessed in its orbit.
The strongest evidence against the hypothesis of absorption of external
energy is that such a theory ignores the fact, that, whenever
radio-activity is observed, it is always accompanied by some change
which can be detected by the appearance of new products having chemical
properties distinct from those of the original substances. This leads to
some form of “chemical” theory, and other results show that the change
is atomic and not molecular.
=256. Theory of radio-active change.= The processes occurring in the
radio-elements are of a character quite distinct from any previously
observed in chemistry. Although it has been shown that the
radio-activity is due to the spontaneous and continuous production of
new types of active matter, the laws which control this production are
different from the laws of ordinary chemical reactions. It has not been
found possible in any way to alter either the rate at which the matter
is produced or its rate of change when produced. Temperature, which is
such an important factor in altering the rate of chemical reactions, is,
in these cases, almost entirely without influence. In addition, no
ordinary chemical change is known which is accompanied by the expulsion
of charged atoms with great velocity. It has been suggested by Armstrong
and Lowry[350] that radio-activity may be an exaggerated form of
fluorescence or phosphorescence with a very slow rate of decay. But no
form of phosphorescence has yet been shown to be accompanied by
radiations of the character of those emitted by the radio-elements.
Whatever hypothesis is put forward to explain radio-activity must
account not only for the production of a series of active products,
which differ in chemical and physical properties from each other and
from the parent element, but also for the emission of rays of a special
character. Besides this, it is necessary to account for the large amount
of energy continuously radiated from the radio-elements.
The radio-elements, besides their high atomic weights, do not possess in
common any special chemical characteristics which differentiate them
from the other elements, which do not possess the property of
radio-activity to an appreciable degree. Of all the known elements,
uranium, thorium, and radium possess the greatest atomic weights, viz.:
radium 225, thorium 232·5, and uranium 240.
If a high atomic weight is taken as evidence of a complicated structure
of the atom, it might be expected that disintegration would occur more
readily in heavy than in light atoms. At the same time, there is no
reason to suppose that the elements of the highest atomic weight must be
the most radio-active; in fact, radium is far more active than uranium,
although its atomic weight is less. This is seen to be the case also in
the radio-active products; for example, the radium emanation is
enormously more active weight for weight than the radium itself, and
there is every reason to believe that the emanation has an atom lighter
than that of radium.
In order to explain the phenomena of radio-activity, Rutherford and
Soddy have advanced the theory that the atoms of the radio-elements
suffer spontaneous disintegration, and that each disintegrated atom
passes through a succession of well-marked changes, accompanied in most
cases by the emission of α rays.
A preliminary account of this hypothesis has already been given in
section 136, while the mathematical theory of successive changes, which
is based upon it, has been discussed in chapter IX. The general theory
has been utilized in chapters X and XI to account for the numerous
active substances found in uranium, thorium, actinium and radium.
The theory supposes that, on an average, a definite small proportion of
the atoms of each radio-active substance becomes unstable at a given
time. As a result of this instability, the atoms break up. In most
cases, the disintegration is explosive in violence and is accompanied by
the ejection of an α particle with great velocity; in a few cases, α and
β particles are expelled together, while in others a β particle alone
escapes. In a few cases, the change in the atom appears to be less
violent in character, and is not accompanied by the expulsion of either
an α or β particle. The explanation of these rayless changes is
considered in section 259. The expulsion of an α particle, of mass about
twice that of the hydrogen atom, leaves behind it a new system lighter
than the original one, and possessing chemical and physical properties
quite different from those of the original element. This new system
again becomes unstable, and expels another α particle. The process of
disintegration, once started, proceeds from stage to stage at a definite
measurable rate in each case.
At any time after the disintegration has commenced, there exists a
proportion of the original matter, which is unchanged, mixed with the
part which has undergone change. This is in accordance with the observed
fact that the spectrum of radium, for example, does not change
progressively with time. The radium breaks up so slowly that only a
small fraction has been transformed in the course of a few years. The
unchanged part still shows its characteristic spectrum, and will
continue to do so as long as any radium exists. At the same time it is
to be expected that, in old radium, the spectrum of those products which
exist in any quantity should also appear.
The term metabolon has been suggested as a convenient expression for
each of these changing atoms, derived from the successive disintegration
of the atoms of the radio-elements. Each metabolon, on an average,
exists only for a limited time. In a collection of metabolons of the
same kind the number _N_, which are unchanged at a time _t_ after
production, is given by
$$ N = N₀ e^{–λt} $$,
where _N₀_ is the original number. Now _dN_/_dt_ = -λ_N_, or the
fraction of the metabolons present, which change in unit time, is equal
to λ. The value 1/λ may be taken as the _average life_ of each
metabolon.
This may be simply shown as follows:—At any time _t_ after _N₀_
metabolons have been set aside, the number which change in the time _dt_
is equal to λ_Ndt_ or
$$ λ N₀ e^{–λt} dt $$ .
Each metabolon has a life _t_, so that the average life of the whole
number is given by
$$ \int₀^{\infty} λ te^{–λt} dt = \frac {1} {λ} $$
The various metabolons from the radio-elements are distinguished from
ordinary matter by their great instability and consequent rapid rate of
change. Since a body which is radio-active must _ipso facto_ be
undergoing change, it follows that none of the active products, for
example, the emanations and Th X, can consist of any known kind of
matter; for there is no evidence to show that inactive matter can be
made radio-active, or that two forms of the same element can exist, one
radio-active and the other not. For example, half of the matter
constituting the radium emanation has undergone change after an interval
of four days. After the lapse of about one month the emanation as such
has nearly disappeared, having been transformed through several stages
into other and more stable types of matter, which are in consequence
difficult to detect by their radio-activity.
The striking difference in chemical and physical properties which exists
in many cases between the various products themselves, and also between
the primary active substance and its products, has already been drawn
attention to in chapter IX. Some of the products show distinctive
electro-chemical behaviour and can be removed from a solution by
electrolysis. Others show differences in volatility which have been
utilized to effect a partial separation. There can be no doubt that each
of these products is a definite new chemical substance, and if it could
be collected in sufficient quantity to be examined by ordinary chemical
means, would be found to behave like a distinct chemical element. It
would differ, however, from the ordinary chemical element in the
shortness of its life, and the fact that it is continuously changing
into another substance. We shall see later (section 261) that there is
every reason to believe that radium itself is a metabolon in the true
sense of the term, since it is continuously changing, and is itself
produced from another substance. The main point of difference between it
and the other products lies in the comparative slowness of its rate of
change.
It is for this reason that radium exists in pitchblende in greater
quantity than the other more rapidly changing products. By working up a
large amount of the mineral, we have seen that a sufficient quantity of
the pure product has been obtained for chemical examination.
On account of the short life of the emanation, it exists in pitchblende
in much less quantity than radium, but it, too, has been isolated
chemically and its volume measured. The extraordinary properties of this
emanation, or gas, have already been discussed, and there can be no
doubt that, while it exists, it must be considered a new element allied
in chemical properties to the argon-helium group of gases.
There can be no doubt that in the radio-elements we are witnessing the
spontaneous transformation of matter, and that the different products
which arise mark the stages or halting-places in the process of
transformation, where the atoms are able to exist for a short time
before again breaking up into new systems.
=257. Radio-active products.= The following table gives the list of the
active products or metabolons known to result from the disintegration of
the three radio-elements. In the second column is given the value of the
radio-active constant λ for each active product, _i.e._ the proportion
of the active matter undergoing change per second; in the third column
the time _T_ required for the activity to fall to one-half, _i.e._ the
time taken for half the active product to undergo change; in the fourth
column, the nature of the rays from each active product, not including
the rays from the products which result from it; in the fifth column, a
few of the more marked physical and chemical properties of each
metabolon.
Products λ(sec)⁻¹ T Nature of Chemical and
the rays Physical
properties of
the product
────────────────────────────────────────────────────────────────
Uranium — — α Soluble in
excess of
ammonium
carbonate,
soluble in
ether.
Uranium X 3·6 × 10⁻⁷ 22 days β and γ Insoluble in
excess of
ammonium
carbonate,
soluble in ether
and water.
────────────────────────────────────────────────────────────────
Thorium — — α Insoluble in
ammonia.
Thorium X 2·0 × 10⁻⁶ 4 days α Soluble in
ammonia and
water.
Emanation 1·3 × 10⁻² 53 secs. α Chemically inert
gas of heavy
molecular
weight.
Condenses at
−120° C.
Thorium A 1·74 × 10⁻⁵ 11 hours no rays Deposited on
bodies;
concentrated on
the cathode in
an electric
field. Soluble
in some acids;
Th A more
volatile than Th
B; shows
definite
electro-chemical
behaviour.
Thorium B 2·2 × 10⁻⁴ 55 mins. α, β, γ Same
? — — —
────────────────────────────────────────────────────────────────
Actinium — — no rays Insoluble in
ammonia.
Actinium X 7·8 × 10⁻⁷ 10·2 days α (and β?) Soluble in
ammonia.
Emanation ·17 3·9 secs. α Behaves like a
gas.
Actinium A 3·2 × 10⁻⁴ 36 mins. no rays Deposited on
bodies;
concentrated on
the cathode in
an electric
field, soluble
in ammonia and
strong acids;
volatilized at a
temperature of
100° C., A and B
can be separated
by electrolysis.
Actinium B 5·4 × 10⁻³ 2·15 mins. α, β, γ Same
? — — —
────────────────────────────────────────────────────────────────
Radium — 1300 years α Allied
chemically to
barium.
Emanation 2·1 × 10⁻⁶ 3·8 days α Chemically inert
gas of heavy
molecular
weight;
condenses at
−150° C.
Radium A 3·85 × 10⁻³ 3 mins. α Deposited on
(active surface of
deposit of bodies;
rapid concentrated on
change) cathode in
electric field;
soluble in
strong acids; B
volatized at
about 700° C., A
and C at about
1000° C.
Radium B 5·38 × 10⁻⁴ 21 mins. no rays Same
(same)
Radium C 4·13 × 10⁻⁴ 28 mins. α, β, γ Same
(same)
Radium D — about 40 no rays Soluble in
(active acids; volatile
deposit of below 1000° C.
slow change)
Radium E 1·3 × 10⁻⁶ 6 days β and γ Non-volatile at
(same) 1000° C.
Radium F 5·6 × 10⁻⁸ 143 days α Deposited on
(same) bismuth from
solution;
volatile at
about 1000° C.,
same properties
as
radio-tellurium
and polonium.
The products and their radiations are indicated graphically in Fig. 102
on page 450.
[Illustration: Fig. 102.]
One product has been observed in uranium, four in thorium, four in
actinium and seven in radium. It is not improbable that a closer
examination of the radio-elements may reveal still further changes. If
any very rapid transformations exist, they would be very difficult to
detect. The change of thorium X into the emanation, for example, would
probably not have been discovered if the product of the change had not
been gaseous in character. The electrolysis of solutions is, in many
cases, a very powerful method of separating active products from one
another, and its possibilities have not yet been exhausted. The main
family of changes of the radio-elements, as far as they are known, have
been investigated closely, and it is not likely that any product of
comparatively slow rate of change has been overlooked. There is a
possibility, however, that two radio-active products may in some cases
arise from the disintegration of a single substance. This point is
discussed further in section 260.
The remarkable way in which the disintegration theory can be applied to
unravel the intricacies of the succession of radio-active changes is
very well illustrated in the case of radium. Without its aid, it would
not have been possible to disentangle the complicated processes which
occur. We have already seen that this analysis has been instrumental in
showing that the substances polonium, radio-tellurium and radio-lead are
in reality products of radium.
After the radio-active substances have undergone the succession of
changes traced above, a final stage is reached where the atoms are
either permanently stable, or change so slowly that it is difficult to
detect their presence by means of their radio-activity. It is probable,
however, that the process of transformation still continues through
further slow stages.
There is now considerable evidence that the elements uranium, radium and
actinium are intimately connected together. The two latter probably
result from the breaking up of uranium. The evidence in support of this
idea is given in section 262, but there still remains much work to be
done to bridge over the gaps which at present appear to separate these
elements from one another.
After the series of transformations have come to an end, there will
probably remain a product or products which will be inactive, or active
only to a minute extent. In addition, since the α particles, expelled
during the transformation, are material in nature, and are
non-radio-active, they must collect in some quantity in radio-active
matter. The probability that the α particles consist of helium is
considered later in section 268.
The value of _T_, the time for a product to be half-transformed, may be
taken as a comparative measure of the stability of the different
metabolons. The stability of the products varies over a very wide range.
For example, the value of _T_ for radium D is 40 years, and for the
actinium emanation 3·9 secs. This corresponds to a range of stability
measured by 3·8 × 10⁸. The range of stability is still further extended,
when it is remembered that the atoms of the radio-elements themselves
are very slowly changing.
The only two metabolons of about the same stability are thorium X and
the radium emanation. In each case, the transformation is half completed
in about four days. I consider that the approximate agreement of the
numbers is a mere coincidence, and that the two types of matter are
quite distinct from one another; for, if the metabolons were identical,
it would be expected that the changes which follow would take place in
the same way and at the same rate, but such is not the case. Moreover,
Th X and the radium emanation have chemical and physical properties
quite distinct from one another.
It is very remarkable that the three radio-active substances, radium,
thorium and actinium, should exhibit such a close similarity in the
succession of changes which occur in them. Each of them at one stage of
its disintegration emits a radio-active gas, and in each case this gas
is transformed into a solid which is deposited upon the surface of
bodies. It would appear that, after disintegration of an atom of any of
these has once begun, there is a similar succession of changes, in which
the resulting systems have allied chemical and physical properties. Such
a connection is of interest as indicating a possible origin of the
recurrence of properties in the atoms of the elements, as exemplified by
the periodic law. The connection between thorium and actinium is
especially close both as regards the number and nature of the products.
The period of transformation of the successive products, though
differing in magnitude, rises and falls in a very analogous manner. This
indicates that the atoms of these two elements are very similarly
constituted.
=258. Amount of the products.= By application of the theory of
successive changes, the probable amount of each of the products present
in radium and the other radio-elements can readily be estimated.
Since each radio-atom expels one α particle of atomic weight about that
of hydrogen or helium, the atoms of the intermediate products will not
differ much in weight from the parent atom.
The approximate weight of each product present in a gram of radium can
be readily deduced. Let _N_{A}_, _N_{B}_, _N_{C}_ be the number of atoms
of the products A, B, C present per gram in radio-active equilibrium.
Let λ_{_A_}, λ_{_B_}, λ_{_C_} be the corresponding constants of change.
Then if _q_ is the number of the parent atoms breaking up per second,
per gram,
_q_ = λ_{_A_}_N_{A}_ = λ_{_B_}_N_{B}_ = λ_{_C_}_N_{C}_.
Consider the case of the radium products, where the value of _q_ is 6·2
× 10¹⁰ (section 93). Knowing the value of λ and _q_, the value of _N_
can at once be calculated. The corresponding weight can be deduced,
since in one gram of matter of atomic weight about 200, there are about
4 × 10²¹ atoms (section 39). The results are shown in the following
table:—
Product Value of λ Number of Weight of
(sec)⁻¹ atoms, _N_, product gram
present per of radium
gram
Radium 2·0 × 10⁻⁶ 3·2 × 10¹⁶ 8 × 10⁻³
emanation
Radium A 3·8 × 10⁻³ 1·7 × 10¹³ 4 × 10⁻⁶
Radium B 5·4 × 10⁻⁴ 1·3 × 10¹⁴ 3 × 10⁻⁵
Radium C 4·1 × 10⁻⁴ 1·6 × 10¹⁴ 4 × 10⁻⁵
With the small quantities of radium available, the amounts of the
products radium A, B and C are too small to weigh. It may be possible,
however, to detect their presence by means of the spectroscope.
In the case of thorium, the weight of the product Th X, which is present
in greatest quantity, is far too small to be detected. Since the value
of λ for Th X is about the same as for the radium emanation, the maximum
weight present per gram is about 4 × 10⁻¹² of a gram, remembering that
_q_ for radium is about 2 × 10⁶ times the value for thorium. Even with a
kilogram of thorium, the amount of Th X is far too small to be detected
by its weight.
This method can be used generally to calculate the relative amounts of
any successive products in radio-active equilibrium, provided the value
of λ for each product is known. For example, it will be shown later that
uranium is the parent of radium and is half transformed in about 6 × 10⁸
years, while radium and radium D are half transformed in 1300 and 40
years respectively. The weight of radium present in one gram of uranium,
when equilibrium is established, is thus 2 × 10⁻⁶ grams, and the weight
of radium D is 7 × 10⁻⁸ grams. In a mineral containing a ton of uranium
there should be about 1·8 grams of radium and ·063 grams of radium D.
Some recent experiments described in section 262 show that these
theoretical estimates are about twice too great.
=259. Rayless Changes.= The existence of well-marked changes in radium,
thorium, and actinium, which are not accompanied by the expulsion of α
or β particles, is of great interest and importance.
Since the rayless changes are not accompanied by any appreciable
ionization of the gas, their presence cannot be detected by direct
means. The rate of change of the substance can, however, be determined
indirectly, as we have seen, by measurement of the variation with time
of the activity of the succeeding product. The law of change has been
found to be the same as for the changes which give rise to α rays. The
rayless changes are thus analogous, in some respects, to the
monomolecular changes observed in chemistry, with the difference that
the changes are in the atom itself, and are not due to the decomposition
of a molecule into simpler molecules or into its constituent atoms.
It must be supposed that a rayless change is not of so violent a
character as one which gives rise to the expulsion of α or β particles.
The change may be accounted for either by supposing that there is a
rearrangement of the components of the atom, or that the atom breaks up
without the expulsion of its parts with sufficient velocity to produce
ionization by collision with the gas. The latter point of view, if
correct, at once indicates the possibility that undetected changes of a
similar character may be taking place slowly in the non-radio-active
elements; or, in other words, that all matter may be undergoing a slow
process of change. The changes taking place in the radio-elements have
been observed only in consequence of the expulsion with great velocity
of the parts of the disintegrated atom. Some recent experiments
described in Appendix A show that the α particle from radium ceases to
ionize the gas when its velocity falls below about 10⁹ cms. per second.
It is thus seen that α particles may be projected with a great velocity,
and yet fail to produce ionization in the gas. In such a case, it would
be difficult to follow the changes by the electrical method, as the
electrical effects would be very small in comparison with those produced
by the known radio-active bodies.
=260. Radiations from the products.= We have seen that the great
majority of the radio-active products break up with the expulsion of α
particles, and that the β particle with its accompaniment of the γ ray
appears in most cases only in the last rapid change. In the case of
radium, for example, which has been most closely investigated on account
of its great activity, radium itself, the emanation and radium A emit
only α particles; radium B emits no rays at all; while radium C emits
all three kinds of rays. It is difficult to settle with certainty
whether the products thorium X and actinium X emit β particles or not,
but the β and γ rays certainly appear in each case in the last rapid
change in the active deposit, and, in this respect, behave in a similar
manner to radium.
The very slow moving electrons which accompany the particles emitted
from radium (section 93) are not taken into account, for they appear to
be liberated as a result of the impact of α particles on matter, and are
expelled with a speed insignificant compared with that of the β
particles emitted from radium C.
The appearance of β and γ rays only in the last rapid changes of the
radio-elements is very remarkable, and cannot be regarded as a mere
coincidence. The final expulsion of a β particle results in the
appearance of a product of great stability, or, in the case of radium,
of a product (radium D) which has far more stability than the preceding
one. It would appear that the initial changes are accompanied by the
expulsion of an α particle, and that once the β particle is expelled,
the components of the residual atom fall into an arrangement of fairly
stable equilibrium, where the rate of transformation is very slow. It
thus appears probable that the β particle, which is finally expelled,
may be regarded as the active agent in promoting the disintegration of
the radio-atom through the successive stages. A discussion of this
question will be given with more advantage later (section 270), when the
general question of the stability of the atom is under consideration.
It is significant that the change in which the three types of rays
appear is far more violent in character than the preceding changes. Not
only are the α particles expelled with greater velocity than in any
other change, but the β particles are projected with a velocity very
closely approaching that of light.
There is always a possibility that, in such a violent explosion in the
atom, not only may the α and β particles be expelled, but the atom
itself may be disrupted into several fragments. If the greater
proportion of the matter resulting from the disintegration is of one
kind, it would be difficult to detect the presence of a small quantity
of rapidly changing matter from observations of the rate of decay; but,
if the products have distinctive electro-chemical behaviour, a partial
separation should, in some cases, be effected by electrolysis. It has
already been pointed out that the results of Pegram and von Lerch
(section 207) on the electrolysis of thorium solutions may be explained
on the supposition that thorium A and B have distinctive
electro-chemical behaviour. Pegram, however, in addition observed the
presence of a product which decayed to half value in six minutes. This
active product was obtained by electrolysing a solution of pure thorium
salt, to which a small quantity of copper nitrate had been added. The
copper deposit was slightly active and lost half of its activity in
about six minutes.
The presence of such radio-active products, which do not come under the
main scheme of changes, indicates that, at some stage of the
disintegration, more than one substance results. In the violent
disintegration which occurs in radium C and thorium B, such a result is
to be expected, for it is not improbable that there are several
arrangements whereby the constituents of the atom form a system of some
slight stability. The two products resulting from the disintegration
would probably be present in unequal proportion, and, unless they gave
out different kinds of rays, would be difficult to separate from each
other.
=261. Life of radium.= Since the atoms of the radio-elements are
continuously breaking up, they must also be considered to be metabolons,
the only difference between them and metabolons such as the emanations
Th X and others being their comparatively great stability and consequent
very slow rate of change. There is no evidence that the process of
change, traced above, is reversible under present conditions, and in the
course of time a quantity of radium, uranium, or thorium left to itself
must gradually be transformed into other types of matter.
There seems to be no escape from this conclusion. Let us consider, for
example, the case of radium. The radium is continuously producing from
itself the radium emanation, the rate of production being always
proportional to the amount of radium present. All the radium must
ultimately be changed into emanation, which in turn is transformed
through a succession of stages into other kinds of matter. There is no
doubt that the emanation is chemically quite different from radium
itself. The quantity of radium must diminish, to compensate for the
emanation which is formed; otherwise it is necessary to assume that
matter in the form of emanation is created from some unknown source.
An approximate estimate of the rate of change of radium can easily be
made by two different methods depending upon (1) the number of atoms of
radium breaking up per second, and (2) the amount of emanation produced
per second.
It has been shown experimentally (section 93) that 1 gram of radium at
its minimum activity expels 6·2 × 10¹⁰ α particles per second. The
heating effect of radium and also its volume agree closely with
calculation, if it is supposed that each atom of each product in
breaking up emits one α particle. On this supposition it is seen that
6·2 × 10¹⁰ atoms of radium break up per second.
Now it has been shown experimentally (section 39) that one cubic
centimetre of hydrogen at standard pressure and temperature contains 3·6
× 10¹⁹ molecules. Taking the atomic weight of radium as 225, the number
of atoms in 1 gram of radium is equal to 3·6 × 10²¹. The fraction λ of
radium which breaks up is thus 1·95 × 10⁻¹¹ per second, or 5·4 × 10⁻⁴
per year. It follows that in each gram of radium about half a milligram
breaks up per year. The average life of radium is about 1800 years, and
half of the radium is transformed in about 1300 years.
We shall now consider the calculation, based on the observed result of
Ramsay and Soddy, that the volume of emanation to be obtained from one
gram of radium is about 1 cubic millimetre. The experimental evidence
based on diffusion results indicates that the molecular weight of the
emanation is about 100. If the disintegration theory is correct, the
emanation is an atom of radium minus one particle, and therefore must
have a molecular weight of at least 200. This high value is more likely
to be correct than the experimental number, which is based on evidence
that must necessarily be somewhat uncertain. Now the rate of production
of emanation per second is equal to λ_N₀_, where _N₀_ is the equilibrium
amount. Taking the molecular weight as 200, the weight of emanation
produced per second from 1 gram of radium = 8·96 × 10⁻⁶ λ = 1·9 × 10⁻¹¹
gram.
Now the weight of emanation produced per second is very nearly equal to
the weight of radium breaking up per second. Thus the fraction of radium
breaking up per second is about 1·9 × 10⁻¹¹, which is in agreement with
the number previously calculated by the first method.
We may thus conclude that _radium is half transformed in about 1300
years_.
Taking the activity of pure radium as about two million times that of
uranium, and remembering that only one change, which gives rise to α
rays, occurs in uranium and four in radium, it can readily be calculated
that the fraction of uranium changing per year is about 10⁻⁹. From this
it follows that uranium should be half transformed in about 6 × 10⁸
years.
If thorium is a true radio-active element, the time for half
transformation is about 2·4 × 10⁹ years, since thorium has about the
same activity as uranium but contains four products which emit α rays.
If the activity of thorium is due to some radio-active impurity, no
estimate of the length of its life can be made until the primary active
substance has been isolated and its activity measured.
=262. Origin of radium.= The changes in radium are thus fairly rapid,
and a mass of radium if left to itself should in the course of a few
thousand years have lost a large proportion of its radio-activity.
Taking the above estimate of the life of radium, the value of λ is 5·4 ×
10⁻⁴, with a year as the unit of time. A mass of radium left to itself
should be half transformed in 1300 years and only one-millionth part
would remain after 26,000 years. Thus supposing, for illustration, that
the earth was originally composed of pure radium, its activity per gram
26,000 years later would not be greater than the activity observed
to-day in a good specimen of pitchblende. Even supposing this estimate
of the life of radium is too small, the time required for the radium
practically to disappear is short compared with the probable age of the
earth. We are thus forced to the conclusion that radium is being
continuously produced in the earth, unless the very improbable
assumption is made, that radium was in some way suddenly formed at a
date recent in comparison with the age of the earth. It was early
suggested by Rutherford and Soddy[351] that radium might be a
disintegration product of one of the radio-elements found in
pitchblende. Both uranium and thorium fulfil the conditions required in
a possible source of production of radium. Both are present in
pitchblende, have atomic weights greater than that of radium, and have
rates of change which are slow compared with that of radium. In some
respects, uranium fulfils the conditions required better than thorium;
for it has not been observed that minerals rich in thorium contain much
radium, while on the other hand, the pitchblendes containing the most
radium contain a large proportion of uranium.
If radium is not produced from uranium, it is certainly a remarkable
coincidence that the greatest activity of pitchblende yet observed is
about five or six times that of uranium. Since radium has a life short
compared with that of uranium, the amount of radium produced should
reach a maximum value after a few thousand years, when the rate of
production of fresh radium—which is also a measure of the rate of change
of uranium—balances the rate of change of that product. In this respect
the process would be exactly analogous to the production of the
emanation by radium, with the difference that the radium changes much
more slowly than the emanation. But since radium itself in its
disintegration gives rise to at least five changes with the
corresponding production of α rays, the activity due to the radium
(measured by the α rays), when in a state of radio-active equilibrium
with uranium, should be about five times that of the uranium that
produces it; for it has been shown that only one change has so far been
observed in uranium in which α rays are expelled. Taking into account
the presence of actinium in pitchblende, the activity observed in the
best pitchblende is about the same as would be expected if the radium
were a disintegration product of uranium. If this hypothesis is correct,
the amount of radium in any pitchblende should be proportional to the
amount of uranium present, provided the radium is not removed from the
mineral by percolating water.
This question has been experimentally attacked by Boltwood[352],
McCoy[353] and Strutt[354]. McCoy measured the relative activities of
different minerals in the form of powder by means of an electroscope,
and determined the amount of uranium present by chemical analysis. His
results indicated that the activity observed in the minerals was very
approximately proportional to their content of uranium. Since actinium
is present as well as uranium and its products, this would indicate that
the amount of radium and actinium taken together is proportional to the
amount of uranium. This problem has been attacked more directly by
Boltwood and Strutt by measuring the relative amount of the radium
emanation evolved by different minerals. By dissolving the mineral and
then setting it aside in a closed vessel, the amount of emanation
present reaches a maximum value after about a month’s interval. The
emanation is then introduced into a closed vessel containing a gold-leaf
electroscope similar to that shown in Fig. 12. The rate of movement of
the gold-leaf is proportional to the amount of emanation from the
solution, and this in turn is proportional to the amount of radium.
Boltwood has made in this way a very complete and accurate comparison of
the radium content of different varieties of pitchblende and other ores
containing radium. It was found that many of the minerals in the solid
state allowed a considerable fraction of the emanation to escape into
the air. The percentage fraction of the total amount of emanation lost
in this way is shown in Column II of the following table. Column I gives
the maximum amount of emanation present in 1 gram of the mineral in
arbitrary units when none of the emanation escapes; Column III the
weight in grams of uranium contained in 1 gram; and Column IV the ratio
obtained by dividing the quantity of emanation by the quantity of
uranium. The numbers in Column IV should be constant, if the amount of
radium is proportional to the amount of uranium.
Locality I II III IV
Substance
Uraninite North 170·0 11·3 0·7465 228
Carolina
Uraninite Colorado 155·1 5·2 0·6961 223
Gummite North 147·0 13·7 0·6538 225
Carolina
Uraninite Joachimsthal 139·6 5·6 0·6174 226
Uranophane North 117·7 8·2 0·5168 228
Carolina
Uraninite Saxony 115·6 2·7 0·5064 228
Uranophane North 113·5 22·8 0·4984 228
Carolina
Thorogummite North 72·9 16·2 0·3317 220
Carolina
Carnotite Colorado 49·7 16·3 0·2261 220
Uranothorite Norway 25·2 1·3 0·1138 221
Samarskite North 23·4 0·7 0·1044 224
Carolina
Orangite Norway 23·1 1·1 0·1034 223
Euxenite Norway 19·9 0·5 0·0871 228
Thorite Norway 16·6 6·2 0·0754 220
Fergusonite Norway 12·0 0·5 0·0557 215
Aeschynite Norway 10·0 0·2 0·0452 221
Xenotine Norway 1·54 26·0 0·0070 220
Monazite North 0·88 0·0043 205
(sand) Carolina
Monazite Norway 0·84 1·2 0·0041 207
(crys.)
Monazite Brazil 0·76 0·0031 245
(sand)
Monazite Conn. 0·63 0·0030 210
(massive)
With the exception of some of the monazites, the numbers show a
surprisingly good agreement, and, taking into consideration the great
variation of the content of uranium in the different minerals, and the
wide range of locality from which they were obtained, the results afford
a direct and satisfactory proof that the amount of radium in the
minerals is directly proportional to the amount of uranium.
In this connection, it is of interest to note that Boltwood found that a
considerable quantity of radium existed in various varieties of
monazite, although most of the previous analyses agreed in stating that
no uranium was present. A careful examination was in consequence made to
test this point, and it was found by special methods that uranium was
present, and in about the amount to be expected from the theory. The
ordinary methods of analysis failed to give correct results on account
of the presence of phosphates. Results of a similar character have
recently been given by Strutt[355].
The weight of radium in a mineral per gram of uranium is thus a definite
constant of considerable practical importance. Its value was recently
determined by Boltwood by comparison of the emanation, liberated from a
known weight of uraninite, with that liberated from a known quantity of
pure radium bromide, supplied for the purpose by the writer. A measured
weight of radium bromide was taken from a stock which gave out heat at a
rate of slightly over 100 gram calories per hour per gram, and was thus
probably pure. This was dissolved in water, and, by successive
dilutions, a standard solution was made up containing 10⁻⁷ gram of
radium bromide per c.c. Taking the constitution of radium bromide as
RaBr₂, it was deduced that the weight of radium per gram of uranium in
any mineral was 8·0 × 10⁻⁷ gram. The amount of radium in a mineral per
ton of uranium is thus 0·72 gram.
Strutt (_loc. cit._) obtained a value nearly twice as great, but he had
no means of ascertaining the purity of his radium bromide.
This amount of radium per gram of uranium is of the right order of
magnitude to be expected on the disintegration theory, if uranium is the
parent of radium. The activity of pure radium, compared with uranium, is
not known with sufficient accuracy to determine with accuracy the
theoretical proportion of radium to uranium.
The production of radium from uranium, while very strongly supported by
these experiments, cannot be considered definitely established until
direct experimental evidence is obtained of the growth of radium in
uranium. The rate of production of radium to be expected on the
disintegration theory can readily be estimated. The fraction of uranium
breaking up per year has been calculated (section 261) and shown to be
about 10⁻⁹ per year. This number represents the weight of radium
produced per year from 1 gram of uranium. The emanation, released from
the amount of radium produced in one year from 1 gram of uranium, would
cause an ordinary gold-leaf electroscope to be discharged in about
half-an-hour. If a kilogram of uranium is used, the amount of radium
produced in a single day should be easily detectable.
Experiments to detect the growth of radium in uranium have been made by
several observers. Soddy[356] examined the amount of emanation given off
at different times from one kilogram of uranium nitrate in solution,
which was originally freed from the small trace of radium present by a
suitable chemical process. The solution was kept stored in a closed
vessel, and the amount of emanation which collected in the solution was
measured at regular intervals.
Preliminary experiments showed that the actual rate of production of
radium was far less than the amount to be expected theoretically, and at
first very little indication was obtained that radium was produced at
all. After allowing the uranium to stand for eighteen months, Soddy
states that the amount of emanation was distinctly greater than at
first. The solution after this interval contained about 1·5 × 10⁻⁹ gram
of radium. This gives the value of about 2 × 10⁻¹² for the fraction of
uranium changing per year, while the theoretical value is about 10⁻⁹.
Whetham[357] also found that a quantity of uranium nitrate which had
been set aside for a year showed an appreciable increase in the content
of radium, and considers that the rate of production is faster than that
found by Soddy. In his case, the uranium was not originally completely
freed from radium.
Observations extending over years will be required before the question
can be considered settled, for the accurate estimation of small
quantities of radium by the amount of emanation is beset with
difficulties. This is especially the case where observations are made
over wide intervals of time.
The writer has made an examination to see if radium is produced from
actinium or thorium. It was thought possible that actinium might prove
to be an intermediate product between uranium and radium. The solutions,
freed from radium, have been set aside for a year, but no certain
increase in the content of radium has been observed.
There is little doubt that the production of radium by uranium first
proceeds at only a small fraction of the rate to be expected from
theory. This is not surprising when we consider that probably several
changes intervene between the product Ur X and the radium. In the case
of radium, for example, it has been shown that a number of slow changes
follow the rapid changes ordinarily observed. On account of the feeble
activity of uranium, it would not be easy to detect directly the
occurrence of such changes. If, for example, one or more rayless
products occurred between Ur X and radium, which were removed from the
uranium by the same chemical process used to free it from radium, the
rate of production of radium would be very small at first, but would be
expected to increase with time as more of the intermediary products were
stored up in the uranium. The fact that the contents of uranium and
radium in radio-active minerals are always proportional to each other,
coupled with definite experimental evidence that radium is produced from
uranium, affords an almost conclusive proof that uranium is in some way
the parent of radium.
The general evidence which has been advanced to show that radium must be
continuously produced from some other substance applies also to
actinium, which has an activity of the same order of magnitude as that
of radium. The presence of actinium with radium in pitchblende would
indicate that this substance also is in some way derived from uranium.
It is possible that actinium may prove to be produced either from radium
or to be the intermediary substance between uranium and radium. If it
could be shown that the amount of actinium in radio-active minerals is,
like radium, proportional to the amount of uranium, this would afford
indirect proof of such a connection. It is not so simple to settle this
point for actinium as for radium, since actinium gives out a very
short-lived emanation, and the methods adopted to determine the content
of radium in minerals cannot be applied without considerable
modifications to determine the amount of actinium present.
The experimental data, so far obtained, do not throw much light upon the
origin of the primary active matter in thorium. Hofmann and others
(section 23) have shown that thorium separated from minerals containing
uranium is always more active the greater the quantity of uranium
present. This would indicate that the active substance in thorium also
may be derived from uranium.
While much work still remains to be done, a promising beginning has
already been made in determining the origin and relation of the
radio-elements. We have seen that the connection between polonium,
radio-tellurium, and radio-lead with radium has already been
established. Radium itself is now added to the list, and it is probable
that actinium will soon follow.
While the experiments undoubtedly show that there is a definite relation
between the amount of uranium and radium present in the ordinary
radio-active minerals, Danne[358] has recently called attention to a
very interesting apparent exception. Considerable quantities of radium
were found in certain deposits in the neighbourhood of Issy-l’Evêque in
the Saône-Loire district, although no trace of uranium was present. The
active matter is found in pyromorphite (phosphate of lead), in clays
containing lead, and in pegmatite, but the radium is usually present in
greater quantities in the former. The pyromorphite is found in veins of
the quartz and feldspar rocks. The veins are always wet owing to the
presence of a number of springs in the neighbourhood. The content of
uranium in the pyromorphite varies considerably, but Danne considers
that about a centigram of radium is present per ton. It seems probable
that the radium found in this locality has been deposited from water
flowing through it, possibly in past times. The presence of radium is
not surprising, since crystals of autunite have been found about 40
miles distant, and probably there are deposits containing uranium in
that region. This result is of interest, as suggesting that radium may
be removed with water and deposited by physical or chemical action some
distance away.
It will be shown in the next chapter that radium has been found very
widely distributed over the surface of the earth, but generally in very
small quantities.
=263. Does the radio-activity of radium depend upon its concentration?=
We have seen that the radio-active constant λ of any product is
independent of the concentration of the product. This result has been
established over a very wide range for some substances, and especially
for the radium emanation. No certain difference in the rate of decay of
the emanation has been observed, although the amount present in unit
volume of the air has been varied a millionfold.
It has been suggested by J. J. Thomson[359] that the rate of
disintegration of radium may be influenced by its own radiations. This,
at first sight, appears very probable, for a small mass of a pure radium
compound is subjected to an intense bombardment by the radiations
arising from it, and the radiations are of such a character that they
might be expected to produce a breaking up of the atoms of matter which
they traverse. If this be the case, the radio-activity of a given
quantity of radium should be a function of its concentration, and should
be greater in the solid state than when disseminated through a large
mass of matter.
The writer has made an experiment to examine this question. Two glass
tubes were taken, in one of which was placed a few milligrams of pure
radium bromide in a state of radio-active equilibrium, and in the other
a solution of barium chloride. The two tubes were connected near the top
by a short cross tube, and the open ends sealed off. The activity of the
radium in the solid state was tested immediately after its introduction
by placing it in a definite position near an electroscope made of thin
metal of the type shown in Fig. 12. The increased rate of discharge of
the electroscope due to the β and γ rays from the radium was observed.
When a lead plate 6 mms. in thickness was placed between the radium and
the electroscope, the rate of discharge observed was due to the γ rays
alone. By slightly tilting the apparatus, the barium solution flowed
into the radium tube and dissolved the radium. The tube was well shaken,
so as to distribute the radium uniformly throughout the solution. No
appreciable change of the activity measured by the γ rays was observed
over the period of one month. The activity measured by the β and γ rays
was somewhat reduced, but this was not due to a decrease of the
radio-activity, but to an increased absorption of the β rays in their
passage through the solution. The volume of the solution was at least
1000 times greater than that of the solid radium bromide, and, in
consequence, the radium was subjected to the action of a much weaker
radiation. I think we may conclude from this experiment that the
radiations emitted by radium have little if any influence in causing the
disintegration of the radium atoms.
Voller[360] recently published some experiments which appeared to show
that the life of radium varied enormously with its concentration. In his
experiments, solutions of radium bromide of known strengths were
evaporated down in a platinum vessel 1·2 sq. cms. in area, and their
activity tested from time to time. The activity of the radium, so
deposited, at first showed the normal rise to be expected on account of
the production of the emanation, but after reaching a maximum, it
rapidly decayed. For a weight of 10⁻⁶ mgrs. of radium bromide, the
activity for example, practically disappeared in 26 days after reaching
its maximum. The time taken for the activity to disappear increased
rapidly with the amount of radium present. In another set of
experiments, he states that the activity observed on the vessel was not
proportional to the amount of radium present. For example, the activity
only increased 24 times for a millionfold increase of the radium
present, from 10⁻⁹ mgrs. to 10⁻³ mgrs.
These results, however, have not been confirmed by later experiments
made by Eve. He found that, over the range examined, the activity was
directly proportional to the amount of radium present, within the limits
of experimental error. The following table illustrates the results
obtained. The radium was evaporated down in platinum vessels 4·9 sq.
cms. in area.
Weight of Activity in
radium in arbitrary units
milligrams
10⁻⁴ 1000
10⁻⁵ 106
10⁻⁶ 11·8
10⁻⁷ 1·25
For an increase of one-thousandfold of the quantity of radium, the
activity increased 800 times, while Voller states that the activity, in
his experiments, only increased 3 to 4 times.
In the experiments of Eve, the activity was measured by observing the
increased rate of discharge of a gold-leaf electroscope when the
platinum vessel containing the active deposit was placed inside the
electroscope. The activity of 10⁻⁸ mgrs. was too small to be measured
with accuracy in the electroscope employed, while 10⁻³ mgrs. gave too
rapid a rate of discharge. On the other hand, the method of measurement
employed by Voller was unsuitable for the measurement of very weak
radio-activity.
Eve also found that a small quantity of radium _kept in a closed vessel_
did not lose its activity with time. A silvered glass vessel contained a
gold-leaf system, such as is shown in Fig. 12. A solution containing
10⁻⁶ mgrs. of radium bromide was evaporated over the bottom of the
vessel of area 76 sq. cms. The activity, after reaching a maximum, has
remained constant over the 100 days during which observations have so
far been made.
These experiments of Eve, as far as they go, show that the activity of
radium is proportional to the amount of radium present, and that radium,
kept in a closed vessel, shows no signs of decreasing in activity. On
the other hand, I think there is no doubt that a very small quantity of
radium deposited on a plate and _left in the open air_ does lose its
activity fairly rapidly. This loss of activity has nothing whatever to
do with the shortness of life of the radium itself, but is due to the
escape of the radium from the plate into the surrounding gas. Suppose,
for example, that a solution containing 10⁻⁹ mgrs. of radium bromide is
evaporated in a vessel of one sq. cm. in area. This amount of radium is
far too small to form even a layer of molecular thickness. It seems
likely that, during the process of evaporation, the radium would tend to
collect in small masses and be deposited on the surface of the vessel.
These would very readily be removed by slow currents of air and so
escape from the plate. The disappearance of such minute amounts of
radium is to be expected, and would probably occur with all kinds of
matter present in such minute amount. Such an effect has nothing to do
with an alteration of the life of radium and must not be confused with
it.
The result that the total radiation from a given quantity of radium
depends only on the quantity of radium and not on the degree of its
concentration is of great importance, for it allows us to determine with
accuracy the content of radium in minerals and in soils in which the
radium exists in a very diffused state.
=264. Constancy of the radiations.= The early observations on uranium
and thorium had shown that their radio-activity remained constant over
the period of several years during which they were examined. The
possibility of separating from uranium and thorium the active products
Ur X and Th X respectively, the activity of which decayed with the time,
seemed at first sight to contradict this. Further observation, however,
showed that the total radio-activity of these bodies was not altered by
the chemical processes, for it was found that the uranium and thorium
from which the active products were removed, spontaneously regained
their radio-activity. At any time after removal of the active product,
the sum total of the radio-activity of the separated product together
with that of the substance from which it has been separated is always
equal to that of the original compound before separation. In cases where
active products, like Ur X and the radium emanation, decay with time
according to an exponential law, this follows at once from the
experimental results. If _I₁_ is the activity of the product at any time
t after separation, and _I₀_ the initial value, we know that
$$ \frac {I_1} {I₀} = e^{–λt} $$ .
At the same time the activity _I₂_ recovered during the same interval
_t_ is given by
$$ \frac {I_2} {I₀} = 1 − e^{–λt} $$,
where λ is the same constant as before. It thus follows that _I₁_ + _I₂_
= _I₀_, which is an expression of the above result. The same is also
true whatever the law of decay of activity of the separated product (see
section 200). For example, the activity of Th X after separation from
thorium at first increases with the time. At the same time, the activity
of the residual thorium compound at first decreases, and at such a rate
that the sum of the activities of the thorium and its separated product
is always equal to that of the original thorium.