RADIO-ACTIVE EMANATIONS.
=138. Introduction.= A most important and striking property possessed by
radium, thorium, and actinium, but not by uranium or polonium, is the
power of continuously emitting into the surrounding space a material
emanation, which has all the properties of a radio-active gas. This
emanation is able to diffuse rapidly through gases and through porous
substances, and may be separated from the gas with which it is mixed by
condensation by the action of extreme cold. This emanation forms a
connecting link between the activity of the radio-elements themselves
and their power of exciting activity on surrounding objects, and has
been studied more closely than the other active products on account of
its existence in the gaseous state. The emanations from the three active
bodies all possess similar radio-active properties, but the effects are
more marked in the case of the emanation from radium, on account of the
very great activity of that element.
Thorium Emanation.
=139. Discovery of the emanation.= In the course of examination of the
radiations of thorium, several observers had noted that some of the
thorium compounds, and especially the oxide, were very inconstant
sources of radiation, when examined in open vessels by the electrical
method. Owens[231] found that this inconstancy was due to the presence
of air currents. When a closed vessel was used, the current, immediately
after the introduction of the active matter, increased with the time,
and finally reached a constant value. By drawing a steady stream of air
through the vessel the value of the current was much reduced. It was
also observed that the radiations could apparently pass through large
thicknesses of paper, which completely absorbed the ordinary α
radiation.
In an investigation of these peculiar properties of thorium compounds,
the writer[232] found that the effects were due to an emission of
radio-active particles of some kind from the thorium compounds. This
“emanation,” as it was termed for convenience, possesses the properties
of ionizing the gas and acting on a photographic plate, and is able to
diffuse rapidly through porous substances like paper and thin metal
foil.
The emanation, like a gas, is completely prevented from escaping by
covering the active matter with a thin plate of mica. The emanation can
be carried away by a current of air; it passes through a plug of
cotton-wool and can be bubbled through solutions without any loss of
activity. In these respects, it behaves very differently from the ions
produced in the gas by the rays from active substances, for these give
up their charges completely under the same conditions.
Since the emanation passes readily through large thicknesses of
cardboard, and through filters of tightly packed cotton-wool, it does
not seem likely that the emanation consists of particles of dust given
off by the active matter. This point was tested still further by the
method used by Aitken and Wilson, for detecting the presence of dust
particles in the air. The oxide, enclosed in a paper cylinder, was
placed in a glass vessel, and the dust was removed by repeated small
expansions of the air over a water surface. The dust particles act as
nuclei for the formation of small drops and are then removed from the
air by the action of gravity. After repeated expansions, no cloud was
formed, and the dust was considered to be removed. After waiting for
some time to allow the thorium emanation to collect, further expansions
were made but no cloud resulted, showing that for the small expansions
used, the particles were too small to become centres of condensation.
The emanation then could not be regarded as dust emitted from thorium.
Since the power of diffusing rapidly through porous substances, and
acting on a photographic plate, is also possessed by a chemical
substance like hydrogen peroxide, some experiments were made to see if
the emanation could be an agent of that character. It was found,
however, that hydrogen peroxide is not radio-active, and that its action
on the plate is a purely chemical one, while it is the _radiation_ from
the emanation and not the _emanation_ itself that produces ionizing and
photographic effects.
=140. Experimental arrangements.= The emanation from thorium is given
off in minute quantity. No appreciable lowering of the vacuum is
observed when an emanating compound is placed in a vacuum tube and no
new spectrum lines are observed.
For an examination of the emanation, an apparatus similar in principle
to that shown in Fig. 51 is convenient.
[Illustration: Fig. 51.]
The thorium compound, either bare or enclosed in a paper envelope, was
placed in a glass tube _C_. A current of air from a gasometer, after
passing through a tube containing cotton-wool to remove dust particles,
bubbled through sulphuric acid in the vessel _A_. It then passed through
a bulb containing tightly packed cotton-wool to prevent any spray being
carried over. The emanation, mixed with air, was carried from the vessel
_C_ through a plug of cotton-wool _D_, which removed completely all the
ions carried with the emanation. The latter then passed into a long
brass cylinder, 75 cm. in length and 6 cm. in diameter. The insulated
cylinder was connected with a battery in the usual way. Three insulated
electrodes, _E_, _F_, _H_, of equal lengths, were placed along the axis
of the cylinder, supported by brass rods passing through ebonite corks
in the side of the cylinder. The current through the gas, due to the
presence of the emanation, was measured by means of an electrometer. An
insulating key was arranged so that any one of the electrodes _E_, _F_,
_H_ could be rapidly connected with one pair of quadrants of the
electrometer, the other two being always connected with earth. The
current observed in the testing cylinder vessel was due entirely to the
ions produced by the emanation carried into the vessel by the current of
air. On substituting a uranium compound for the thorium, not the
slightest current was observed. After a constant flow has passed for
about 10 minutes, the current due to the emanation reaches a constant
value.
The variation of the ionization current with the voltage is similar to
that observed for the gas ionized by the radiations from the active
bodies. The current at first increases with the voltage, but finally
reaches a saturation value.
=141. Duration of the activity of the emanation.= The emanation rapidly
loses its activity with time. This is very readily shown with the
apparatus of Fig. 51. The current is found to diminish progressively
along the cylinder, and the variation from electrode to electrode
depends on the velocity of the flow of air.
If the velocity of the air current is known, the decay of activity of
the emanation with time can be deduced. If the flow of air is stopped,
and the openings of the cylinder closed, the current steadily diminishes
with time. The following numbers illustrate the variation with time of
the saturation current, due to the emanation in a closed vessel. The
observations were taken successively, and as rapidly as possible after
the current of air was stopped.
Time in seconds Current
0 100
28 69
62 51
118 25
155 14
210 6·7
272 4·1
360 1·8
Curve _A_, Fig. 52, shows the relation existing between the current
through the gas and the time. The current just before the flow of air
was stopped is taken as unity. The current through the gas, which is a
measure of the activity of the emanation, diminishes according to an
exponential law with the time like the activity of the products Ur X and
Th X. The rate of decay is, however, much more rapid, the activity of
the emanation decreasing to half value in about one minute. According to
the view developed in section 136, this implies that half of the
emanation particles have undergone change in one minute. After an
interval of 10 minutes the current due to the emanation is very small,
showing that practically all the emanation particles present have
undergone change.
[Illustration: Fig. 52.]
The rate of decay has been more accurately determined by Rossignol and
Gimingham[233] who found that the activity fell to half value in about
51 seconds. Bronson[234], using the steady deflection method described
in section 69, found the corresponding time 54 seconds.
The decrease of the current with the time is an actual measure of the
decrease of the activity of the emanation, and is not in any way
influenced by the time that the ions produced take to reach the
electrodes. If the ions had been produced from a uranium compound the
duration of the conductivity for a saturation voltage would only have
been a fraction of a second.
The rate of decay of the activity of the emanation is independent of the
electromotive force acting on the gas. This shows that the radio-active
particles are not destroyed by the electric field. The current through
the gas at any particular instant, after stoppage of the flow of air,
was found to be the same whether the electromotive force had been acting
the whole time or had been just applied for the time of the test.
The emanation itself is unaffected by a strong electric field and so
cannot be charged. By testing its activity after passing it through long
concentric cylinders, charged to a high potential, it was found that the
emanation certainly did not move with a velocity greater than ·00001 cm.
per second, for a gradient of 1 volt per cm., and there was no evidence
to show that it moved at all. This conclusion has been confirmed by the
experiments of McClelland[235].
The rate at which the emanation is produced is independent of the gas
surrounding the active matter. If in the apparatus of Fig. 51 air is
replaced by hydrogen, oxygen, or carbonic acid, similar results are
obtained, though the current observed in the testing vessel varies for
the different gases on account of the unequal absorption by them of the
radiation from the emanation.
If a thorium compound, enclosed in paper to absorb the α radiation, is
placed in a closed vessel, the saturation current due to the emanation
is found to vary directly as the pressure. Since the rate of ionization
is proportional to the pressure for a constant source of radiation, this
experiment shows that the rate of emission of the emanation is
independent of the pressure of the gas. The effect of pressure on the
rate of production of the emanation is discussed in more detail later in
section 157.
=142. Effect of thickness of layer.= The amount of emanation emitted by
a given area of thorium compound depends on the thickness of the layer.
With a very thin layer, the current between two parallel plates, placed
in a closed vessel as in Fig. 17, is due very largely to the α rays.
Since the α radiation is very readily absorbed, the current due to it
practically reaches a maximum when the surface of the plate is
completely covered by a thin layer of the active material. On the other
hand the current produced by the emanation increases until the layer is
several millimetres in thickness, and then is not much altered by adding
fresh active matter. This falling off of the current after a certain
thickness has been reached is to be expected, since the emanation, which
takes several minutes to diffuse through the layer above it, has already
lost a large proportion of its activity.
With a thick layer of thorium oxide in a closed vessel, the current
between the plates is largely due to the radiation from the emanation
lying between the plates. The following tables illustrate the way in
which the current varies with the thickness of paper for both a thin and
a thick layer.
TABLE I. _Thin Layer._
Thickness of sheets of paper ·0027.
No. of Current
layers of
paper
0 1
1 ·37
2 ·16
3 ·08
TABLE II. _Thick Layer._
Thickness of paper ·008 cm.
No. of Current
layers of
paper
0 1
1 ·74
2 ·74
5 ·72
10 ·67
20 ·55
The initial current with the unscreened compound is taken as unity. In
Table I, for a thin layer of thorium oxide, the current diminished
rapidly with additional layers of thin paper. In this case the current
is due almost entirely to the α rays. In Table II the current falls to
·74 for the first layer. In this case about 26% of the current is due to
the α rays, which are practically absorbed by the layer ·008 cm. in
thickness. The slow decrease with additional layers shows that the
emanation diffuses so rapidly through a few layers of paper that there
is little loss of activity during the passage. The time taken to diffuse
through 20 layers is however appreciable, and the current consequently
has decreased. After passing through a layer of cardboard 1·6 mms. in
thickness the current is reduced to about one-fifth of its original
value. In closed vessels the proportion of the total current, due to the
emanation, varies with the distance between the plates as well as with
the thickness of the layer of active material. It also varies greatly
with the compound examined. In the nitrate, which gives off only a small
amount of emanation, the proportion is very much smaller than in the
hydroxide, which gives off a large amount of emanation.
=143. Increase of current with time.= The current due to the emanation
does not reach its final value for some time after the active matter has
been introduced into the closed vessel. The variation with time is shown
in the following table. The saturation current due to thorium oxide,
covered with paper, was observed between concentric cylinders of 5·5
cms. and ·8 cm. diameter.
Immediately before observations on the current were made, a rapid stream
of air was blown through the apparatus. This removed most of the
emanation. However, the current due to the ionization of the gas by the
emanation, as it was carried along by the current of air, was still
appreciable. The current consequently does not start from zero.
Time in Current
seconds
0 9
23 25
53 49
96 67
125 76
194 88
244 98
304 99
484 100
The results are shown graphically in Fig. 52, curve _B_. The decay of
the activity of the emanation with time, and the rate of increase of the
activity due to the emanation in a closed space, are connected in the
same way as the decay and recovery curves of Th X and Ur X.
With the previous notation, the decay curve is given by
$$ \frac {I_T} {I₀} = e^{–λt} $$ .
and the recovery curve by
$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,
where λ is the radio-active constant of the emanation.
This relation is to be expected, since the decay and recovery curves of
the emanation are determined by exactly the same conditions as the decay
and recovery curves of Ur X and Th X. In both cases there is:
(1) A supply of fresh radio-active particles produced at a constant
rate.
(2) A loss of activity of the particles following an exponential law
with the time.
In the case of Ur X and Th X, the active matter produced manifests its
activity in the position in which it is formed; in this new phenomenon,
a proportion of the active matter in the form of the emanation escapes
into the surrounding gas. The activity of the emanation, due to a
thorium compound kept in a closed vessel, thus reaches a maximum when
the rate of supply of fresh emanation particles from the compound is
balanced by the rate of change of those already present. The time for
recovery of half the final activity is about 1 minute, the same as the
time taken for the emanation, when left to itself, to lose half its
activity.
If _q₀_ is the number of emanation particles escaping into the gas per
second, and _N₀_ the final number when radio-active equilibrium is
reached, then (section 133),
_q₀_ = λ_N₀_.
Since the activity of the emanation falls to half value in 1 minute
λ = ¹⁄₈₇,
and _N₀_ = 87_q₀_, or the number of emanation particles present when a
steady state is reached is 87 times the number produced per second.
Radium Emanation.
=144. Discovery of the emanation.= Shortly after the discovery of the
thorium emanation, Dorn[236] repeated the results, and, in addition,
showed that radium compounds also gave off radio-active emanations, and
that the amount given off was much increased by heating the compound.
The radium emanation differs from the thorium emanation in the rate at
which it loses its activity. It decays far more slowly, but in other
respects the emanations of thorium and radium have much the same
properties. Both emanations ionize the gas with which they are mixed,
and affect a photographic plate. Both diffuse readily through porous
substances but are unable to pass through a thin plate of mica; both
behave like a temporarily radio-active gas, mixed in minute quantity
with the air or other gas in which they are conveyed.
=145. Decay of activity of the emanation.= Very little emanation escapes
from radium chloride in the solid state, but the amount is largely
increased by heating, or by dissolving the compound in water. By
bubbling air through a radium chloride solution, or passing air over a
heated radium compound, a large amount of emanation may be obtained
which can be collected, mixed with air, in a suitable vessel.
Experiments to determine accurately the rate of decay of activity of the
emanation have been made by P. Curie[237], and Rutherford and
Soddy[238]. In the experiments of the latter, the emanation mixed with
air was stored over mercury in an ordinary gas-holder. From time to
time, equal quantities of air mixed with the emanation were measured off
by a gas pipette and delivered into a testing vessel. The latter
consisted of an air-tight brass cylinder carrying a central insulated
electrode. A saturation voltage was applied to the cylinder, and the
inner electrode was connected to the electrometer with a suitable
capacity in parallel. The saturation current was observed _immediately_
after the introduction of the active gas into the testing vessel, and
was taken as a measure of the activity of the emanation present. The
current increased rapidly with the time owing to the production of
excited activity on the walls of the containing vessel. This effect is
described in detail in chapter VIII.
The measurements were made at suitable intervals over a period of 33
days. The following table expresses the results, the initial activity
being taken as 100.
Time in Relative
hours Activity
0 100
20·8 85·7
187·6 24·0
354·9 6·9
521·9 1·5
786·9 0·19
The activity falls off according to an exponential law with the time,
and decays to half value in 3·71 days. With the usual notation
$$ \frac {I_T} {I₀} = e^{–λt} $$ .
the mean value of λ deduced from the results is given by
λ = 2·16 × 10⁻⁶ = ¹⁄₄₆₃₀₀₀.
P. Curie determined the rate of decay of activity of the emanation by
another method. The active matter was placed at one end of a sealed
tube. After sufficient time had elapsed the portion of the tube
containing the radium compound was removed. The loss of activity of the
emanation, stored in the other part, was tested at regular intervals by
observing the ionization current due to the rays which passed through
the walls of the glass vessel. The testing apparatus and the connections
are shown clearly in Fig. 53. The ionization current is observed between
the vessels _BB_ and _CC_. The glass tube _A_ contains the emanation.
[Illustration: Fig. 53.]
Now it will be shown later that the emanation itself gives off only α
rays, and these rays are completely absorbed by the glass envelope,
unless it is made extremely thin. The rays producing ionization in the
testing vessel were thus not due to the α rays from the emanation at
all, but to the β and γ rays due to the excited activity produced on the
walls of the glass tube by the emanation inside it. What was actually
measured was thus the decay of the excited activity derived from the
emanation, and not the decay of activity of the emanation itself. Since,
however, when a steady state is reached, the amount of excited activity
is nearly proportional at any time to the activity of the emanation, the
rate of decay of the excited activity on the walls of the vessel
indirectly furnishes a measure of the rate of decay of the emanation
itself. This is only true if the emanation is placed for four or five
hours in the tube before observations begin, in order to allow the
excited activity time to reach a maximum value.
Using this method P. Curie obtained results similar to those obtained by
Rutherford and Soddy by the direct method. The activity decayed
according to an exponential law with the time, falling to half value in
3·99 days.
The experiments were performed under the most varied conditions but the
rate of decay was found to remain unaltered. The rate of decay did not
depend on the material of the vessel containing the emanation or on the
nature or pressure of the gas with which the emanation was mixed. It was
unaffected by the amount of emanation present, or by the time of
exposure to the radium, provided sufficient time had elapsed to allow
the excited activity to reach a maximum value before the observations
were begun. P. Curie[239] found that the rate of decay of activity was
not altered by exposing the vessel containing the emanation to different
temperatures, ranging from +450° to −180° C.
In this respect the emanations of thorium and radium are quite
analogous. The rate of decay seems to be unaffected by any physical or
chemical agency, and the emanations behave in exactly the same way as
the radio-active products Th X and Ur X, already referred to. The
radio-active constant λ is thus a fixed and unalterable quantity for
both emanations, although in one case its value is about 5000 times
greater than in the other.
Emanations from Actinium.
=146.= Debierne[240] found that actinium gives out an emanation similar
to the emanation of thorium and radium. The loss of activity of the
emanation is even more rapid than for the thorium emanation, for its
activity falls to half value in 3·9 seconds. In consequence of the rapid
decay of activity, the emanation is able to diffuse through the air only
a short distance from the active matter before it loses the greater
proportion of its activity. Giesel early observed that the radio-active
substance separated by him, which we have seen (section 18) is identical
in radio-active properties with actinium, gave off a large amount of
emanation. It was in consequence of this property, that he gave it the
name of the “emanating substance” and later “emanium.” The impure
preparations of this substance emit the emanation very freely and in
this respect differ from most of the thorium compounds. The emanation
from actinium like those from thorium and radium possesses the property
of exciting activity on inactive bodies, but it has not yet been studied
so completely as the better known emanations of thorium and radium.
Experiments with large amounts of Radium Emanation.
=147.= With very active specimens of radium a large amount of emanation
can be obtained, and the electrical, photographic, and fluorescent
effects are correspondingly intense. On account of the small activity of
thorium and the rapid decay of its emanation the effects due to it are
weak, and can be studied only for a few minutes after its production.
The emanation from radium, on the other hand, in consequence of the slow
decay of its activity, may be stored mixed with air in an ordinary
gas-holder, and its photographic and electrical actions may be examined
several days or even weeks after, quite apart from those of the radium
from which it was obtained.
It is, in general, difficult to study the radiation due to the emanation
alone, on account of the fact that the emanation is continually
producing a secondary type of activity on the surface of the vessel in
which the emanation is enclosed. This excited activity reaches a maximum
value several hours after the introduction of the emanation, and, as
long as it is kept in the vessel, this excited activity on the walls
decays at the same rate as the emanation itself, _i.e._ it falls to half
its initial value in about 4 days. If, however, the emanation is blown
out, the excited activity remains behind on the surface, but rapidly
loses its activity in the course of a few hours. After several hours the
intensity of the residual radiation is very small.
These effects and their connection with the emanation are discussed more
fully in chapter VIII. Giesel[241] has recorded some interesting
observations of the effect of the radium emanation on a screen of
phosphorescent zinc sulphide. When a few centigrams of moist radium
bromide were placed on a screen any slight motion of the air caused the
luminosity to move to and fro on the screen. The direction of
phosphorescence could be altered at will by a slow current of air. The
effect was still further increased by placing the active material in a
tube and blowing the air through it towards the screen. A screen of
barium platinocyanide or of Balmain’s paint failed to give any visible
light under the same conditions. The luminosity was not altered by a
magnetic field, but it was affected by an electric field. If the screen
were charged the luminosity was more marked when it was negative than
when it was positive.
Giesel states that the luminosity was not equally distributed, but was
concentrated in a peculiar ring-shaped manner over the surface of the
screen. The concentration of luminosity on the negative, rather than on
the positive, electrode is probably due to the excited activity, caused
by the emanation, and not to the emanation itself, for this excited
activity is concentrated chiefly on the negative electrode in an
electric field (see chapter VIII).
An experiment to illustrate the phosphorescence produced in some
substances by the rays from a large amount of emanation is described in
section 165.
=148.= Curie and Debierne[242] have investigated the emanation from
radium, and the excited activity produced by it. Some experiments were
made on the amount of emanation given off from radium under very low
pressures. The tube containing the emanation was exhausted to a good
vacuum by a mercury pump. It was observed that a gas was given off from
the radium which produced excited activity on the glass walls. This gas
was extremely active, and rapidly affected a photographic plate through
the glass. It caused fluorescence on the surface of the glass and
rapidly blackened it, and was still active after standing ten days. When
spectroscopically examined, this gas did not show any new lines, but
generally those of the spectra of carbonic acid, hydrogen, and mercury.
In the light of the results described in section 124 the gas, given off
by the radium, was probably the non-active gases hydrogen and oxygen, in
which the active emanation was mixed in minute quantity. It will be
shown later (section 242) that the energy radiated from the emanation is
enormous compared with the amount of matter involved, and that the
effects observed, in most cases, are produced by an almost infinitesimal
amount of the emanation.
In further experiments, Curie and Debierne[243] found that many
substances were phosphorescent under the action of the emanation and the
excited activity produced by it. In their experiments, two glass bulbs
_A_ and _B_ (Fig. 54) were connected with a glass tube. The active
material was placed in the bulb _A_ and the substance to be examined in
the other.
[Illustration: Fig. 54.]
They found that, in general, substances that were phosphorescent in
ordinary light became luminous. The sulphide of zinc was especially
brilliant and became as luminous as if exposed to a strong light. After
sufficient time had elapsed the luminosity reached a constant value. The
phosphorescence is partly due to the excited activity produced by the
emanation on its surface, and partly to the direct radiation from the
emanation.
Phosphorescence was also produced in glass. Thuringian glass showed the
most marked effects. The luminosity of the glass was found to be about
the same in the two bulbs, but was more marked in the connecting tube.
The effect in the two bulbs was the same even if connected by a very
narrow tube.
Some experiments were also made with a series of phosphorescent plates
placed in the vessel at varying distances apart. With the plates 1 mm.
apart the effect was very feeble, but increased directly as the distance
and was large for a distance of 3 cms.
These effects receive a general explanation on the views already put
forward. When the radium is placed in the closed vessel, the emanation
is given off at a constant rate and gradually diffuses throughout the
enclosure. Since the time taken for diffusion of the emanation through
tubes of ordinary size is small compared with the time required for the
activity to be appreciably reduced, the emanation, and also the excited
activity due to it, will be nearly equally distributed throughout the
vessel.
The luminosity due to it should thus be equal at each end of the tube.
Even with a capillary tube connecting the two bulbs, the gas
continuously given off by the radium will always carry the emanation
with it and cause a practically uniform distribution.
The gradual increase of the amount of emanation throughout the tube will
be given by the equation
$$ \frac {N_t} {N₀} = 1 − e^{–λt} $$ .
where _N_{t}_ is the number of emanation particles present at the time
_t_, _N₀_ the number present when radio-active equilibrium is reached,
and λ is the radio-active constant of the emanation. The phosphorescent
action, which is due partly to the radiations from the emanation and
partly to the excited activity on the walls, should thus reach half the
maximum value in four days and should practically reach its limit after
three weeks’ interval.
The variation of luminosity with different distances between the screens
is to be expected. The amount of excited activity deposited on the
boundaries is proportional to the amount of emanation present. Since the
emanation is equally distributed, the amount of excited activity
deposited on the screens, due to the emanation between them, varies
directly as the distance, provided the distance between the screens is
small compared with their dimensions. Such a result would also follow if
the phosphorescence were due to the radiation from the emanation itself,
provided that the pressure of the gas was low enough to prevent
absorption of the radiation from the emanation in the gas itself between
the screens.
Measurements of Emanating Power.
=149. Emanating power.= The compounds of thorium in the solid state vary
very widely in the amount of emanation they emit under ordinary
conditions. It is convenient to use the term _emanating power_ to
express the amount of emanation given off per second by one gram of the
compound. Since, however, we have no means of determining absolutely the
amount of emanation present, all measurements of emanating power are of
necessity comparative. In most cases, it is convenient to take a given
weight of a thorium compound, kept under conditions as nearly as
possible constant, and to compare the amount of emanation of the
compound to be examined with this standard.
In this way comparisons of the emanating power of thorium compounds have
been made by Rutherford and Soddy[244], using an apparatus similar to
that shown in Fig. 51 on page 240.
A known weight of the substance to be tested was spread on a shallow
dish, placed in the glass tube _C_. A stream of dry dust-free air, kept
constant during all the experiments, was passed over the compound and
carried the emanation into the testing vessel. After ten minutes
interval, the current due to the emanation in the testing vessel reached
a constant value. The compound was then removed, and the standard
comparison sample of equal weight substituted; the saturation current
was observed when a steady state was again reached. The ratio of these
two currents gives the ratio of the emanating power of the two samples.
It was found experimentally that, for the velocities of air current
employed, the saturation current in the testing vessel was directly
proportional to the weight of thorium, for weights up to 20 grams. This
is explained by the supposition that the emanation is removed by the
current of air from the mass of the compound, as fast as it is formed.
Let _i₁_ = saturation current due to a weight ω₁ of the standard,
_i₂_ = „ „ „ „ ω₂ of the sample to be tested.
(emanating power of specimen) _i₂_ ω₁
Then ---------------------------- = ---- ---
(emanating power of standard) _i₁_ ω₂
By means of this relation the emanating power of compounds which are not
of equal weight can be compared.
It was found that thorium compounds varied enormously in emanating
power, although the percentage proportion of thorium present in the
compound was not very different. For example, the emanating power of
thorium hydroxide was generally 3 to 4 times greater than that of
ordinary thoria, obtained from the manufacturer. Thorium nitrate, in the
solid state, had only ¹⁄₂₀₀ of the emanating power of ordinary thoria,
while preparations of the carbonate were found to vary widely among
themselves in emanating power, which depended upon slight variations in
the method of preparation.
=150. Effect of conditions on emanating power.= The emanating power of
different compounds of thorium and radium is much affected by the
alteration of chemical and physical conditions. In this respect the
emanating power, which is a measure of the rate of escape of the
emanation into the surrounding gas, must not be confused with the rate
of decay of the activity of the emanations themselves, which has already
been shown to be unaffected by external conditions.
Dorn (_loc. cit._) first observed that the emanating power of thorium
and radium compounds was much affected by moisture. In a fuller
investigation of this point by Rutherford and Soddy, it was found that
the emanating power of thoria is from two to three times greater in a
moist than in a dry gas. Continued desiccation of the thoria in a glass
tube, containing phosphorus pentoxide, did not reduce the emanating
power much below that observed in ordinary dry air. In the same way
radium chloride in the solid state gives off very little emanation when
in a dry gas, but the amount is much increased in a moist gas.
The rate of escape of emanation is much increased by solution of the
compound. For example, thorium nitrate, which has an emanating power of
only ¹⁄₂₀₀ that of thoria in the solid state, has in solution an
emanating power of 3 to 4 times that of thoria. P. Curie and Debierne
observed that the emanating power of radium was also much increased by
solution.
Temperature has a very marked effect on the emanating power. The
writer[245] showed that the emanating power of ordinary thoria was
increased three to four times by heating the substance to a dull red
heat in a platinum tube. If the temperature was kept constant the
emanation continued to escape at the increased rate, but returned to its
original value on cooling. If, however, the compound was heated to a
white heat, the emanating power was greatly reduced, and it returned on
cooling to about 10% of the original value. Such a compound is said to
be _de-emanated_. The emanating power of radium compounds varies in a
still more striking manner with rise of temperature. The rate of escape
of the emanation is momentarily increased even 10,000 times by heating
to a dull red heat. This effect does not continue, for the large escape
of the emanation by heating is in reality due to the release of the
emanation stored up in the radium compound. Like thoria, when the
compound has once been heated to a very high temperature, it loses its
emanating power and does not regain it. It regains its power of
emanating, however, after solution and re-separation.
A further examination of the effect of temperature was made by
Rutherford and Soddy[246]. The emanating power of thoria decreases very
rapidly with lowering of temperature, and at the temperature of solid
carbonic acid it is only about 10% of its ordinary value. It rapidly
returns to its original value when the cooling agent is removed.
Increase of temperature from 80° C. to a dull red heat of platinum thus
increases the emanating power about 40 times, and the effects can be
repeated again and again, with the same compound, provided the
temperature is not raised to the temperature at which de-emanation
begins. De-emanation sets in above a red heat, and the emanating power
is then permanently diminished, but even long-continued heating at a
white heat never entirely destroys the emanating power.
=151. Regeneration of emanating power.= An interesting question arises
whether the de-emanation of thorium and radium is due to a removal or
alteration of the substance which produces the emanation, or whether
intense ignition merely changes the rate of escape of the emanation from
the solid into the surrounding atmosphere.
It is evident that the physical properties of the thoria are much
altered by intense ignition. The compound changes in colour from white
to pink; it becomes denser and also far less readily soluble in acids.
In order to test if the emanating power could be regenerated by a cyclic
chemical process, the de-emanated thoria was dissolved, precipitated as
hydroxide and again converted into oxide. At the same time a specimen of
the ordinary oxide was subjected to an exactly parallel process. The
emanating power of both these compounds was the same, and was from two
to three times greater than that of ordinary thoria.
Thus de-emanation does not permanently destroy the power of thorium of
giving out an emanation, but merely produces an alteration of the amount
of the emanation which escapes from the compound.
=152. Rate of production of the emanation.= The emanating power of
thorium compounds, then, is a very variable quantity, much affected by
moisture, heat, and solution. Speaking generally, increased temperatures
and solution greatly increase the emanating power of both thorium and
radium.
The wide differences between the emanating powers of these substances in
the solid state and in solution pointed to the conclusion that the
differences were probably due to the rate of escape of the emanation
into the surrounding gas, and not to a variation of the rate of reaction
which gave rise to the emanation. It is obvious that a very slight
retardation in the rate of escape of the thorium emanation from the
compound into the gas, will, on account of the rapid decay of activity
of the emanation, produce great changes in emanating power. The
regeneration of the emanating power of de-emanated thoria and radium by
solution and chemical treatment made it evident that the original power
of thorium and radium of producing the emanation still persisted in an
unaltered degree.
The question whether the emanation was produced at the same rate in
emanating as in non-emanating compounds can be put to a sharp
quantitative test. If the rate of production of emanation goes on at the
same rate in the solid compound where very little escapes, as in the
solution where probably all escapes, the emanation must be _occluded_ in
the compound, and consequently there must be a sudden release of this
emanation on solution of the compound. On account of the very slow decay
of the activity of the emanation of radium, the effects should be far
more marked in that compound than in thorium.
From the point of view developed in section 133, the exponential law of
decay of the emanation expresses the result that _N_{t}_ the number of
particles remaining unchanged at the time _t_ is given by
$$ \frac {N_t} {N₀} = e^{–λt} $$ (1).
where _N₀_ is the initial number of particles present. When a steady
state is reached, the rate of production _q₀_ of fresh emanation
particles is exactly balanced by the rate of change of the particles
_N₀_ already present, _i.e._
_q₀_ = λ_N₀_,
_N₀_ in this case represents the amount of emanation “occluded” in the
compound. Substituting the value of λ found for the radium emanation in
section 145,
_N₀_
----- = 1/λ = 463,000.
_q₀_
The amount of emanation stored in a non-emanating radium compound should
therefore be nearly 500,000 times the amount produced per second by the
compound. This result was tested in the following way[247].
A weight of ·03 gr. of radium chloride of activity 1000 times that of
uranium was placed in a Drechsel bottle and a sufficient amount of water
drawn in to dissolve it. The released emanation was swept out by a
current of air into a small gas holder and then into a testing cylinder.
The initial saturation current was proportional to _N₀_. A rapid current
of air was then passed through the radium solution for some time in
order to remove any slight amount of emanation which had not been
removed initially. The Drechsel bottle was closed air-tight, and allowed
to stand undisturbed for a definite time _t_. The accumulated emanation
was then swept out as before into the testing vessel. The new ionization
current represents the value of _N_{t}_ the amount of emanation formed
in the compound during the interval _t_.
In the experiment _t_ = 105 minutes,
and the observed value
_N__{t}
----- = ·0131.
_N₀_
Assuming that there is no decay during the interval,
_N_{t}_ = 105 × 60 × _q₀_.
_N₀_
Thus --- = 480,000.
_q₀_
Making the small correction for the decay of activity during the
interval,
_N₀_
----- = 477,000.
_q₀_
We have previously shown that from the theory
_N₀_ 1
----- = ---- = 463,000.
_q₀_ λ
The agreement between theory and experiment is thus as close as could be
expected from the nature of the experiments. This experiment proves
conclusively that the rate of production of emanation in the solid
compound is the same as in the solution. In the former case it is
occluded, in the latter it escapes as fast as it is produced.
It is remarkable how little emanation, compared with the amount stored
up in the compound, escapes from solid radium chloride in a dry
atmosphere. One experiment showed that the emanating power in the dry
solid state was less than ½% of the emanating power of the solution.
Since nearly 500,000 times as much emanation is stored up in the solid
compound as is produced per second, this result showed that the amount
of emanation which escaped per second was less than 10⁻⁸ of that
occluded in the compound.
If a solid radium chloride compound is kept in a moist atmosphere, the
emanating power becomes comparable with the amount produced per second
in the solution. In such a case, since the rate of escape is continuous,
the amount occluded will be much less than the amount for the
non-emanating material.
The phenomenon of occlusion of the radium emanation is probably not
connected in any way with its radio-activity, although this property has
here served to measure it. The occlusion of helium by minerals presents
almost a complete analogy to the occlusion of the radium emanation. Part
of the helium is given off by fergusonite, for example, when it is
heated and all of it when the mineral is dissolved.
=153.= Similar results hold for thorium, but, on account of the rapid
loss of activity of the emanation, the amount of emanation occluded in a
non-emanating compound is very small compared with that observed for
radium. If the production of the thorium emanation proceeds at the same
rate under all conditions, the solution of a solid non-emanating
compound should be accompanied by a rush of emanation greater than that
subsequently produced. With the same notation as before we have for the
thorium emanation,
_N₀_ 1
----- = ----- = 87.
_q₀_ λ
This result was tested as follows: a quantity of finely powdered thorium
nitrate, of emanating power ¹⁄₂₀₀ of ordinary thoria, was dropped into a
Drechsel bottle containing hot water and the emanation rapidly swept out
into the testing vessel by a current of air. The ionization current rose
quickly to a maximum, but soon fell again to a steady value; showing
that the amount of emanation released when the nitrate dissolves, is
greater than the subsequent amount produced from the solution.
The rapid loss of the activity of the thorium emanation makes a
quantitative comparison like that for radium very difficult. By slightly
altering the conditions of the experiment, however, a definite proof was
obtained that the rate of production of emanation is the same in the
solid compound as in the solution. After dropping in the nitrate, a
rapid air stream was blown through the solution for 25 seconds into the
testing vessel. The air stream was stopped and the ionization current
immediately measured. The solution was then allowed to stand undisturbed
for 10 minutes. In that time the accumulation of the emanation again
attained a practical maximum and again represented a steady state. The
stream of air was blown through, as before, for 25 seconds, stopped and
the current again measured. In both cases, the electrometer recorded a
movement of 14·6 divisions per second. By blowing the same stream of air
continuously through the solution the final current corresponded to 7·9
divisions per second or about one-half of that observed after the first
rush.
Thus the rate of production of emanation is the same in the solid
nitrate as in the solution, although the emanating power, _i.e._ the
rate of escape of the emanation, is over 600 times greater in the
solution than in the solid.
It seems probable that the rate of production of emanation by thorium,
like the rate of production of Ur X and Th X, is independent of
conditions. The changes of emanating power of the various compounds by
moisture, heat, and solution must therefore be ascribed solely to an
alteration in the rate of escape of the emanation into the surrounding
gas and not to an alteration in the rate of its production in the
compound.
On this view, it is easy to see that slight changes in the mode of
preparation of a thorium compound may produce large changes in emanating
power. Such effects have been often observed, and must be ascribed to
slight physical changes in the precipitate. The fact that the rate of
production of the emanation is independent of the physical or chemical
conditions of the thorium, in which it is produced, is thus in harmony
with what had previously been observed for the radio-active products Ur
X and Th X.
Source of the Thorium Emanation.
=154.= Some experiments of Rutherford and Soddy[248] will now be
considered, which show that the thorium emanation is produced, not
directly by the thorium itself, but by the active product Th X.
When the Th X, by precipitation with ammonia, is removed from a quantity
of thorium nitrate, the precipitated thorium hydroxide does not at first
possess appreciable emanating power. This loss of emanating power is not
due, as in the case of the de-emanated oxide, to a retardation in the
rate of escape of the emanation produced; for the hydroxide, when
dissolved in acid, still gives off no emanation. On the other hand, the
solution, containing the Th X, possesses emanating power to a marked
degree. When the precipitated hydroxide and the Th X is left for some
time, it is found that the Th X decreases in emanating power, while the
hydroxide gradually regains its emanating power. After about a month’s
interval, the emanating power of the hydroxide has nearly reached a
maximum, while the emanating power of the Th X has almost disappeared.
The curves of decay and recovery of emanating power with time are found
to be exactly the same as the curves of decay and recovery of activity
of Th X and the precipitated hydroxide respectively, shown in Fig. 47.
The emanating power of Th X, as well as its activity, falls to half
value in four days, while the hydroxide regains half its final emanating
power as well as half its lost activity in the same interval.
It follows from these results that the emanating power of Th X is
directly proportional to its activity, _i.e._ that the rate of
production of emanating particles is always proportional to the number
of α particles, projected from the Th X per second. _The radiation from
Th X thus accompanies the change of the Th X into the emanation._ Since
the emanation has chemical properties distinct from those of the Th X,
and also a distinctive rate of decay, it cannot be regarded as a vapour
of Th X, but it is a distinct chemical substance, produced by the
changes occurring in Th X. On the view advanced in section 136, the atom
of the emanation consists of the part of the atom of Th X left behind
after the expulsion of one or more α particles. The atoms of the
emanation are unstable, and in turn expel α particles. This projection
of α particles constitutes the radiation from the emanation, which
serves as a measure of the amount of emanation present. Since the
activity of the emanation falls to half value in _one_ minute while that
of Th X falls to half value in four days, the emanation consists of
atoms which disintegrate at intervals nearly 6000 times shorter than
those of the atoms of Th X.
Source of the Radium and Actinium Emanation.
=155.= No intermediate stage—Radium X—between radium and its emanation,
corresponding to the Th X for thorium, has so far been observed. The
emanation from radium is probably produced directly from that element.
In this respect, the radium emanation holds the same position in regard
to radium as Th X does to thorium, and its production from radium can be
explained on exactly similar lines. It will be shown later in chapter X,
that the emanation of actinium, like that of thorium, does not arise
directly from the parent element but from an intermediate product
actinium X, which is very analogous in physical and chemical properties
to Th X.
Radiations from the Emanations.
=156.= Special methods are necessary to examine the nature of the
radiation from the emanations, for the radiations arise from the volume
of the gas in which the emanations are distributed. Some experiments to
examine the radiations from the thorium emanation were made by the
writer in the following way.
[Illustration: Fig. 55.]
A highly emanating thorium compound wrapped in paper was placed inside a
lead box _B_ about 1 cm. deep, shown in Fig. 55. An opening was cut in
the top of the box, over which a very thin sheet of mica was waxed. The
emanation rapidly diffused through the paper into the vessel, and after
ten minutes reached a state of radio-active equilibrium. The penetrating
power of the radiation from the emanation which passed through the thin
mica window was examined by the electrical method in the usual way by
adding screens of thin aluminium foil. The results are expressed in the
following table:
Thickness of mica window ·0015 cm.
Thickness of aluminium foil ·00034 cm.
Layers of Current
foil
0 100
1 59
2 30
3 10
4 3·2
The greater proportion of the conductivity is thus due to α rays, as in
the case of the radio-active elements. The amount of absorption of these
α rays by aluminium foil is about the same as that of the rays from the
active bodies. No direct comparison can be made, for the α rays from the
emanation show the characteristic property of increased rate of
absorption with thickness of matter traversed. Before testing, the rays
have been largely absorbed by the mica window, and the penetrating power
has consequently decreased.
No alteration in the radiation from the emanation was observed on
placing an insulated wire inside the emanation vessel, and charging it
to a high positive or negative potential. When a stream of air through
the vessel carried away the emanation as fast as it was produced, the
intensity of the radiation fell to a small fraction of its former value.
No evidence of any β rays in the radiations was found in these
experiments, although a very small effect would have been detected.
After standing some hours, however, β rays began to appear. These were
due to the excited activity deposited on the walls of the vessel from
the emanation, and not directly to the emanation itself.
The radium emanation, like that of thorium, only gives rise to α rays.
This was tested in the following way[249]:
A large amount of emanation was introduced into a cylinder made of sheet
copper ·005 cm. thick, which absorbed all the α rays but allowed the β
and γ rays, if present, to pass through with but little loss. The
external radiation from the cylinder was determined at intervals,
commencing about two minutes after the introduction of the emanation.
The amount observed at first was extremely small, but increased rapidly
and practically reached a maximum in three or four hours. Thus the
radium emanation only gives out α rays, the β rays appearing as the
excited activity is produced on the walls of the vessel. On sweeping out
the emanation by a current of air, there was no immediately appreciable
decrease of the radiation. This is another proof that the emanation does
not emit any β rays. In a similar way it can be shown that the emanation
does not give out γ rays; these rays always make their appearance at the
same time as the β rays.
The method of examination of the radiations from the emanations has been
given in some detail, as the results are of considerable importance in
the discussion, which will be given later in chapters X and XI, of the
connection between the changes occurring in radio-active products and
the radiations they emit. There is no doubt that the emanations, apart
from the excited activity to which they give rise, only give out α rays,
consisting most probably of positively charged bodies projected with
great velocity.
Effect of pressure on the rate of production of the Emanation.
=157.= It has already been mentioned that the conductivity due to the
thorium emanation is proportional to the pressure of the gas, pointing
to the conclusion that the rate of production of the emanation is
independent of the pressure, as well as of the nature of the surrounding
gas. This result was directly confirmed with the apparatus of Fig. 55.
When the pressure of the gas under the vessel was slowly reduced, the
radiation, tested outside the window, increased to a limit, and then
remained constant over a wide range of pressure. This increase, which
was far more marked in air than in hydrogen, is due to the fact that the
α rays from the emanation were partially absorbed in the gas inside the
vessel when at atmospheric pressure. At pressures of the order of 1
millimetre of mercury the external radiation decreased, but experiment
showed that this must be ascribed to a removal of the emanation by the
pump, and not to a change in the rate of production. The thorium
compounds very readily absorb water-vapour, which is slowly given off at
low pressures, and in consequence some of the emanation is carried out
of the vessel with the water-vapour.
Curie and Debierne[250] found that both the amount of excited activity
produced in a closed vessel containing active samples of radium, and
also the time taken to reach a maximum value, were independent of the
pressure and nature of the gas. This was true in the case of a solution
down to the pressure of the saturated vapour, and in the case of solid
salts to very low pressures. When the pump was kept going at pressures
of the order of ·001 mm. of mercury, the amount of excited activity was
much diminished. This was probably not due to any alteration of the rate
of escape of the emanation, but to the removal of the emanation by the
action of the pump as fast as it was formed.
Since the amount of excited activity, when in a state of radio-active
equilibrium, is a measure of the amount of emanation producing it, these
results show that the amount of emanation present when the rate of
production balances the rate of decay is independent of the pressure and
nature of the gas. It was also found that the time taken to reach the
point of radio-active equilibrium was independent of the size of the
vessel or the amount of active matter present. This proves that the
state of equilibrium cannot in any way be ascribed to the possession by
the emanation of any appreciable vapour pressure; for if such were the
case, the time taken to reach the equilibrium value should depend on the
size of the vessel and the amount of active matter present. The results
are, however, in agreement with the view that the emanation is present
in minute quantity in the tube, and that the equilibrium is governed
purely by the radio-active constant λ, the constant of decay of activity
of the emanation. This has been seen to be the same under all conditions
of concentration, pressure and temperature, and, provided the rate of
supply of the emanation from the active compound is not changed, the
time-rate of increase of activity to the equilibrium value will always
be the same, whatever the size of the vessel or the nature and pressure
of the surrounding gas.
Chemical Nature of the Emanations.
=158.= We shall now consider some experiments on the physical and
chemical properties of the emanations themselves, without reference to
the material producing them, in order to see if they possess any
properties which connect them with any known kind of matter.
It was soon observed that the thorium emanation passed unchanged through
acid solutions, and later the same result was shown to hold true in the
case of both emanations for every reagent that was tried. Preliminary
observations[251] showed that the thorium emanation, obtained in the
usual way by passing air over thoria, passed unchanged in amount through
a platinum tube heated electrically to the highest temperature
obtainable. The tube was then filled with platinum-black, and the
emanation passed through it in the cold, and with gradually increasing
temperatures, until the limit was reached. In another experiment, the
emanation was passed through a layer of red-hot lead-chromate in a glass
tube. The current of air was replaced by a current of hydrogen, and the
emanation was sent through red-hot magnesium-powder and red-hot
palladium-black, and, by using a current of carbon dioxide, through
red-hot zinc-dust. In every case the emanation passed through without
sensible change in the amount. If anything, a slight increase occurred,
owing to the time taken for the gas-current to pass through the tubes
when hot being slightly less than when cold, the decay _en route_ being
consequently less. The only known gases capable of passing in unchanged
amount through all the reagents employed are the recently discovered
members of the argon family.
But another possible interpretation might be put upon the results. If
the emanation were the manifestation of a type of excited radio-activity
on the surrounding atmosphere, then, since from the nature of the
experiments it was necessary to employ in each case as the atmosphere, a
gas not acted on by the reagent employed, the result obtained might be
expected. Red-hot magnesium would not retain an emanation consisting of
radio-active hydrogen, nor red-hot zinc-dust an emanation consisting of
radio-active carbon dioxide. The incorrectness of this explanation was
shown in the following way. Carbon dioxide was passed over thoria, then
through a =T=-tube, where a current of air met and mixed with it, both
passing on to the testing-cylinder. But between this and the =T=-tube a
large soda-lime tube was introduced, and the current of gas was thus
freed from its admixed carbon dioxide, before being tested in the
cylinder for the emanation. The amount of emanation found was quite
unchanged, whether carbon dioxide was sent over thoria in the manner
described, or whether, keeping the other arrangements as before, an
equally rapid current of air was substituted for it. The theory that the
emanation is an effect of the excited activity on the surrounding medium
is thus excluded.
Experiments of a similar kind on the radium emanation were made later. A
steady stream of gas was passed through a radium chloride solution and
then through the reagent to be employed, into a testing-vessel of small
volume, so that any change in the amount of emanation passing through
could readily be detected. The radium emanation, like that of thorium,
passed unchanged in amount through every reagent used.
In later experiments by Sir William Ramsay and Mr Soddy[252], the
emanation from radium was exposed to still more drastic treatment. The
emanation in a glass tube was sparked for several hours with oxygen over
alkali. The oxygen was then removed by ignited phosphorus and no visible
residue was left. When, however, another gas was introduced, mixed with
the minute amount of emanation in the tube and withdrawn, the activity
of emanation was found to be unaltered. In another experiment, the
emanation was introduced into a magnesium lime tube, which was heated
for three hours at a red heat. The emanation was then removed and
tested, but no diminution in its discharging power was observed.
The emanations of thorium and radium thus withstand chemical treatment
in a manner hitherto unobserved except in gases of the argon family.
=159.= Ramsay and Soddy (_loc. cit._) record an interesting experiment
to illustrate the gaseous nature of the emanation. A large amount of the
radium emanation was collected in a small glass tube. This tube
phosphoresced brightly under the influence of the rays from the
emanation. The passage of the emanation from point to point was observed
in a darkened room by the luminosity excited in the glass. On opening
the stop-cock connecting with the Töpler pump, the slow flow through the
capillary tube was noticed, the rapid passage along the wider tubes, the
delay in passing through a plug of phosphorous pentoxide, and the rapid
expansion into the reservoir of the pump. When compressed, the
luminosity of the emanation increased, and became very bright as the
small bubble containing the emanation was expelled through the fine
capillary tube.
Diffusion of the Emanations.
=160.= It has been shown that the emanations of thorium and radium
behave like radio-active gases, distributed in minute amount in the air
or other gas in which they are tested. With the small quantities of
active material so far investigated, the emanations have not yet been
collected in sufficient amount to determine their density. Although the
molecular weight of the emanations cannot yet be obtained by direct
chemical methods, an indirect estimate of it can be made by determining
the rate of their inter-diffusion into air or other gases. The
coefficients of inter-diffusion of various gases have long been known,
and the results show that the coefficient of diffusion of one gas into
another is, for the simpler gases, approximately inversely proportional
to the square root of the product of their molecular weights. If,
therefore, the coefficient of diffusion of the emanation into air is
found to have a value, lying between that of two known gases _A_ and
_B_, it is probable that the molecular weight of the emanation lies
between that of _A_ and _B_.
Although the volume of the emanation given off from radium is very
small, the electrical conductivity produced by the emanation in the gas,
with which it is mixed, is often very large, and offers a ready means of
measuring the emanation present.
Some experiments have been made by Miss Brooks and the writer[253] to
determine the rate of the diffusion of the radium emanation into air, by
a method similar to that employed by Loschmidt[254] in 1871, in his
investigations of the coefficient of inter-diffusion of gases.
[Illustration: Fig. 56.]
Fig. 56 shows the general arrangement. A long brass cylinder _AB_, of
length 73 cms., and diameter 6 cms., was divided into two equal parts by
a movable metal slide _S_. The ends of the cylinder were closed with
ebonite stoppers. Two insulated brass rods, _a_ and _b_, each half the
length of the tube, passed through the ebonite stoppers and were
supported centrally in the tube. The cylinder was insulated and
connected with one pole of a battery of 300 volts, the other pole of
which was earthed. The central rods could be connected with a sensitive
quadrant electrometer. The cylinder was covered with a thick layer of
felt, and placed inside a metal box filled with cotton wool in order to
keep temperature conditions as steady as possible.
In order to convey a sufficient quantity of emanation into the
half-cylinder _A_, it was necessary to heat the radium slightly. The
slide _S_ was closed and the side tubes opened. A slow current of dry
air from a gasometer was passed through a platinum tube, in which a
small quantity of radium compound was placed. The emanation was carried
with the air into the cylinder _A_. When a sufficient quantity had been
introduced, the stream of air was stopped. The side tubes were closed by
fine capillary tubes. These prevented any appreciable loss of gas due to
the diffusion, but served to keep the pressure of the gas inside _A_ at
the pressure of the outside air. The three entrance tubes into the
cylinder, shown in the figure, were for the purpose of initially mixing
the emanation and gas as uniformly as possible.
After standing several hours to make temperature conditions steady, the
slide was opened, and the emanation began to diffuse into the tube _B_.
The current through the tubes _A_ and _B_ was measured at regular
intervals by an electrometer, with a suitable capacity in parallel.
Initially there is no current in _B_, but after the opening of the
slide, the amount in _A_ decreased and the amount in _B_ steadily
increased. After several hours the amount in each half is nearly the
same, showing that the emanation is nearly uniformly diffused throughout
the cylinder.
It can readily be shown[255] that if
_K_ = coefficient of diffusion of the emanation into air,
_t_ = duration of diffusion experiments in secs.,
_a_ = total length of cylinder,
_S₁_ = partial pressure of emanation in tube _A_ at end of
diffusion,
_S₂_ = partial pressure of emanation in tube _B_ at end of
diffusion,
then
$$ \frac {S_1 − S_2} {S_1 + S_2} = \frac {8} {\pi^2} (e^{\frac {\pi^2
Kt} {a^2}} + \frac {1}{9}e^{-\frac {9 \pi^2 Kt} {a^2}} + ...) $$ .
Now the values of _S₁_ and _S₂_ are proportional to the saturation
ionization currents due to the emanations in the two halves of the
cylinder. From this equation _K_ can be determined, if the relative
values of _S₁_ and _S₂_ are observed after diffusion has been in
progress for a definite interval _t_.
The determination of _S₁_ and _S₂_ is complicated by the excited
activity produced on the walls of the vessel. The ionization due to this
must be subtracted from the total ionization observed in each half of
the cylinder, for the excited activity is produced from the material
composing the emanation, and is removed to the electrodes in an electric
field. The ratio of the current due to excited activity to the current
due to the emanation depends on the time of exposure to the emanation,
and is only proportional to it for exposures of several hours.
The method generally adopted in the experiments was to open the slide
for a definite interval, ranging in the experiments from 15 to 120
minutes. The slide was then closed and the currents in each half
determined at once. The central rods, which had been kept negatively
charged during the experiments, had most of the excited activity
concentrated on their surfaces. These were removed, new rods substituted
and the current immediately determined. The ratio of the currents in the
half cylinders under these conditions was proportional to _S₁_ and _S₂_,
the amounts of emanation present in the two halves of the cylinder.
The values of _K_, deduced from different values of _t_, were found to
be in good agreement. In the earlier experiments the values of _K_ were
found to vary between ·08 and ·12. In some later experiments, where
great care was taken to ensure that temperature conditions were very
constant, the values of _K_ were found to vary between ·07 and ·09. The
lower value ·07 is most likely nearer the true value, as temperature
disturbances tend to give too large a value of _K_. No certain
differences were observed in the value of _K_ whether the air was dry or
damp, or whether an electric field was acting or not.
=161.= Some experiments on the rate of diffusion of the radium emanation
into air were made at a later date by P. Curie and Danne[256]. If the
emanation is contained in a closed reservoir, it has been shown that its
activity, which is a measure of the amount of emanation present,
decreases according to an exponential law with the time. If the
reservoir is put in communication with the outside air through a
capillary tube, the emanation slowly diffuses out, and the amount of
emanation in the reservoir is found to decrease according to the same
law as before, but at a faster rate. Using tubes of different lengths
and diameters, the rate of diffusion was found to obey the same laws as
a gas. The value of _K_ was found to be 0·100. This is a slightly
greater value of _K_ than the lowest value 0·07 found by Rutherford and
Miss Brooks. No mention is made by Curie and Danne of having taken any
special precautions against temperature disturbances, and this may
account for the higher value of _K_ obtained by them.
They also found that the emanation, like a gas, always divided itself
between two reservoirs, put in connection with one another, in the
proportion of their volumes. In one experiment one reservoir was kept at
a temperature of 10° C. and the other at 350° C. The emanation divided
itself between the two reservoirs in the same proportion as would a gas
under the same conditions.
=162.= For the purpose of comparison, a few of the coefficients of
inter-diffusion of gases, compiled from Landolt and Bernstein’s tables,
are given below.
Gas or vapour Coefficient of Molecular
diffusion into weight
air
Water vapour 0·198 18
Carbonic acid 0·142 44
gas
Alcohol vapour 0·101 46
Ether vapour 0·077 74
Radium 0·07 ?
emanation
The tables, although not very satisfactory for the purpose of
comparison, show that the coefficient of inter-diffusion follows the
inverse order of the molecular weights. The value of _K_ for the radium
emanation is slightly less than for ether vapour, of which the molecular
weight is 74. We may thus conclude that the emanation is of greater
molecular weight than 74. It seems likely that the emanation has a
molecular weight somewhere in the neighbourhood of 100, and is probably
greater than this, for the vapours of ether and alcohol have higher
diffusion coefficients compared with carbonic acid than the theory would
lead us to anticipate. Comparing the diffusion coefficients of the
emanation and carbonic acid into air, the value of the molecular weight
of the emanation should be about 176 if the result observed for the
simple gases, viz. that the coefficient of diffusion is inversely
proportional to the square root of the molecular weights, holds true in
the present case. Bumstead and Wheeler[257] compared the rates of
diffusion of the radium emanation and of carbon dioxide through a porous
plate, and concluded that the molecular weight of the emanation was
about 180. On the disintegration theory, the atom of the emanation is
derived from the radium atom by the expulsion of one α particle. Thus,
it is to be expected that its molecular weight would be over 200.
It is of interest to compare the value of _K_ = ·07 with the value of
_K_ determined by Townsend (section 37) for the gaseous ions produced in
air at ordinary pressure and temperature, by Röntgen rays or by the
radiations from active substances. Townsend found that the value of _K_
in dry air was ·028 for the positive ions and ·043 for the negative
ions. The radium emanation thus diffuses more rapidly than the ions
produced by its radiation in the gas, and behaves as if its mass were
smaller than that of the ions produced in air, but considerably greater
than that of the air molecules with which it is mixed.
It is not possible to regard the emanation as a temporarily modified
condition of the gas originally in contact with the active body. Under
such conditions a much larger value of _K_ would be expected. The
evidence derived from the experiments on diffusion strongly supports the
view that the emanation is a gas of heavy molecular weight.
Makower[258] has recently attacked the question of the molecular weight
of the radium emanation by another method. The rate of diffusion of the
emanation through a porous plug of plaster-of-Paris was compared with
that of the gases oxygen, carbon dioxide, and sulphur dioxide. It was
found that Graham’s law, viz. that the coefficient of diffusion _K_ is
inversely proportional to the square root of its molecular weight _M_,
was not strictly applicable. The value of _K_ √_M_ was not found to be
constant for these gases, but decreased with increase of molecular
weight of the gas. If, however, a curve was plotted with _K_ √_M_ as
ordinate and _K_ as abscissa, the points corresponding to the values of
O, CO₂ and SO₂ were found to lie on a straight line. By linear
extrapolation, the molecular weight of the emanation was estimated. The
value obtained from experiments on three different porous plugs was
85·5, 97, and 99 respectively. This method indicates that the molecular
weight of the radium emanation is about 100; but in all the experiments
on diffusion, it must be remembered that the emanation, whose rate of
inter-diffusion is being examined, exists in minute quantity mixed with
the gas, and is compared with the rate of inter-diffusion of gases which
are present in large quantity. For this reason, deductions of the
molecular weight of the emanation may be subject to comparatively large
errors, for which it is difficult to make correction.
Diffusion of the Thorium Emanation.
=163.= On account of the rapid decay of the activity of the thorium
emanation, it is not possible to determine the value of _K_ its
coefficient of diffusion into air by the methods employed for the radium
emanation. The value of _K_ has been determined by the writer in the
following way. A plate _C_, Fig. 57, covered with thorium hydroxide, was
placed horizontally near the base of a long vertical brass cylinder _P_.
The emanation released from the thorium compound diffuses upwards in the
cylinder.
[Illustration: Fig. 57.]
Let _p_ be the partial pressure of the emanation at a distance _x_ from
the source _C_. This will be approximately uniform over the cross
section of the cylinder. From the general principles of diffusion we get
the equation
_d²p_ _dp_
_K_ ----- = − ---- .
_dx²_ _dt_
The emanation is continuously breaking up and expelling α particles. The
emanation-residue gains a positive charge, and, in an electric field, is
removed at once from the gas to the negative electrode.
Since the activity of the emanation at any time is always proportional
to the number of particles which have not broken up, and since the
activity decays with the time according to an exponential law,
$$ p = p_1 e^{–λt} $$,
where _p₁_ is the value of _p_ when _t_ = 0 and λ is the _radio-active
constant_ of the emanation.
Then
_dp_
---- = -λ_p_,
_dt_
and
_d²p_
K ----- = λ_p_.
_dx²_
Thus
$$ p = Ae^{-\sqrt {\frac {λ} {K} x}} + Be^{\sqrt {\frac {λ}
{K} x}} $$ .
Since _p_ = 0 when _x_ = infinity, _B_ = 0. If _p_ = _p₀_ when _x_ = 0,
_A_ = _p₀_.
Thus
$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .
It was not found convenient in the experiments to determine the activity
of the emanation along the cylinder, but an equivalent method was used
which depends upon measuring the distribution of “excited activity,”
produced along a central rod _AB_, which is charged negatively.
It will be shown later (section 177) that the amount of excited activity
at any point is always proportional to the amount of emanation at that
point. The distribution of “excited activity” along the central rod from
the plate _C_ upwards thus gives the variation of _p_ for the emanation
along the tube.
In the experiments, the cylinder was filled with dry air at atmospheric
pressure and was kept at a constant temperature. The central rod was
charged negatively and exposed from one to two days in the presence of
the emanation. The rod was then removed, and the distribution of the
excited activity along it determined by the electric method. It was
found that the amount of excited activity fell off with the distance _x_
according to an exponential law, falling to half value in about 1·9 cms.
This is in agreement with the above theory.
Since the activity of the emanation falls to half value in 1 minute, λ =
·0115. The value _K_ = ·09 was deduced from the average of a number of
experiments. This is a slightly greater value than _K_ = ·07, obtained
for the radium emanation, but the results show that the two emanations
do not differ much from one another in molecular weight.
Makower (_loc. cit._) compared the rates of diffusion of the thorium and
radium emanation through a porous plate, and concluded that the two
emanations were of about the same molecular weight, thus confirming the
results obtained by the above method.
Diffusion of the Emanation into Liquids.
=164.= Experiments have been made by Wallstabe[259] on the coefficient
of diffusion of the radium emanation into various liquids. The radium
emanation was allowed to diffuse into a closed reservoir, containing a
cylinder of the liquid under observation. The cylinder was provided with
a tube and a stop-cock extending beyond the closed vessel, so that
different layers of the liquid could be removed. The liquid was then
placed in a closed testing vessel, where the ionization current due to
the escape of the emanation from the liquid was observed to rise to a
maximum after several hours, and then to decay. This maximum value of
the current was taken as a measure of the amount of emanation absorbed
in the liquid.
The coefficient of diffusion _K_ of the emanation into the liquid can be
obtained from the same equation used to determine the diffusion of the
thorium emanation into air,
$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .
where λ is the constant of decay of activity of the radium emanation and
_x_ the depth of the layer of water from the surface.
Putting
$$ a = \sqrt {\frac {λ} {K}} $$,
it was found that
for water α = 1·6,
for toluol α = ·75.
The value of λ expressed in terms of a day as the unit of time is about
·17.
Thus the value of _K_ for the diffusion of the radium emanation into
water = ·066 cm.² / day.
The value of _K_ found by Stefan[260] for the diffusion of carbon
dioxide into water was 1·36 cm.²/day. These results are thus in harmony
with the conclusion drawn from the diffusion of the radium emanation
into air, and show that the radium emanation behaves as a gas of high
molecular weight.
Condensation of the Emanations.
=165. Condensation of the emanations.= During an investigation of the
effect of physical and chemical agencies on the thorium emanation,
Rutherford and Soddy[261] found that the emanation passed unchanged in
amount through a white-hot platinum tube and through a tube cooled to
the temperature of solid carbon dioxide. In later experiments the
effects of still lower temperatures were examined, and it was then found
that at the temperature of liquid air both emanations were
condensed[262].
If either emanation is conveyed by a slow stream of hydrogen, oxygen, or
air through a metal spiral immersed in liquid air, and placed in
connection with a testing vessel as in Fig. 51, no trace of emanation
escapes in the issuing gas. When the liquid air is removed and the
spiral plunged into cotton-wool, several minutes elapse before any
deflection of the electrometer needle is observed, and then the
condensed emanation volatilizes rapidly, and the movement of the
electrometer needle is very sudden, especially in the case of radium.
With a fairly large amount of radium emanation, under the conditions
mentioned, a very few seconds elapse after the first sign of movement
before the electrometer needle indicates a deflection of several hundred
divisions per second. It is not necessary in either case that the
emanating compound should be retained in the gas stream. After the
emanation is condensed in the spiral, the thorium or radium compound may
be removed and the gas stream sent directly into the spiral. But in the
case of thorium, under these conditions, the effects observed are
naturally small owing to the rapid loss of the activity of the emanation
with time, which proceeds at the same rate at the temperature of liquid
air as at ordinary temperatures.
If a large amount of radium emanation is condensed in a glass =U= tube,
the progress of the condensation can be followed by the eye, by means of
the phosphorescence which the radiations excite in the glass. If the
ends of the tube are sealed and the temperature allowed to rise, the
glow diffuses uniformly throughout the tube, and can be concentrated at
any point to some extent by local cooling of the tube with liquid air.
=166. Experimental arrangements.= A simple experimental arrangement to
illustrate the condensation and volatilization of the emanation and some
of its characteristic properties is shown in Fig. 58. The emanation
obtained from a few milligrams of radium bromide by solution or heating
is condensed in the glass =U= tube _T_ immersed in liquid air. This =U=
tube is then put into connection with a larger glass tube _V_, in the
upper part of which is placed a piece of zinc sulphide screen _Z_, and
in the lower part of the tube a piece of the mineral willemite. The
stop-cock _A_ is closed and the =U= tube and the vessel _V_ are
partially exhausted by a pump through the stop-cock _B_. This lowering
of the pressure causes a more rapid diffusion of the emanation when
released. The emanation does not escape if the tube _T_ is kept immersed
in liquid air. The stop-cock _B_ is then closed, and the liquid air
removed. No luminosity of the screen or the willemite in the tube _V_ is
observed for several minutes, until the temperature of _T_ rises above
the point of volatilization of the emanation. The emanation is then
rapidly carried into the vessel _V_, partly by expansion of the gas in
the tube _T_ with rising temperature, and partly by the process of
diffusion. The screen _Z_ and the willemite _W_ are caused to
phosphoresce brilliantly under the influence of the rays from the
emanation surrounding them.
[Illustration: Fig. 58.]
If the end of the vessel _V_ is then plunged into liquid air, the
emanation is again condensed in the lower end of the tube, and the
willemite phosphoresces much more brightly than before. This is not due
to an increase of the phosphorescence of willemite at the temperature of
the liquid air, but to the effect of the rays from the emanation
condensed around it. At the same time the luminosity of the zinc
sulphide gradually diminishes, and practically disappears after several
hours if the end of the tube is kept in the liquid air. If the tube is
removed from the liquid air, the emanation again volatilizes and lights
up the screen _Z_. The luminosity of the willemite returns to its
original value after the lapse of several hours. This slow change of the
luminosity of the zinc sulphide screen and of the willemite is due to
the gradual decay of the “excited activity” produced by the emanation on
the surface of all bodies exposed to its action (chapter VIII). The
luminosity of the screen is thus due partly to the radiation from the
emanation and partly to the excited radiation caused by it. As soon as
the emanation is removed from the upper to the lower part of the tube,
the “excited” radiation gradually diminishes in the upper and increases
in the lower part of the tube.
The luminosity of the screen gradually diminishes with the time as the
enclosed emanation loses its activity, but is still appreciable after an
interval of several weeks.
An apparatus of a similar character to illustrate the condensation of
the radium emanation has been described by P. Curie[263].
[Illustration: Fig. 59.]
=167. Determination of the temperature of condensation.= A detailed
investigation was made by Rutherford and Soddy (_loc. cit._) of the
temperatures at which condensation and volatilization commenced for the
two emanations. The experimental arrangement of the first method is
shown clearly in Fig. 59. A slow constant stream of gas, entering at
_A_, was passed through a copper spiral _S_, over 3 metres in length,
immersed in a bath of liquid ethylene. The copper spiral was made to act
as its own thermometer by determining its electrical resistance. The
resistance temperature curve was obtained by observation of the
resistances at 0°, the boiling point of liquid ethylene −103·5°, the
solidification point of ethylene −169° and in liquid air. The
temperature of the liquid air was deduced from the tables given by Baly
for the boiling point of liquid air for different percentages of oxygen.
The resistance-temperature curve, for the particular spiral employed,
was found to be nearly a straight line between 0° and −192°C., cutting
the temperature axis if produced nearly at the absolute zero. The
resistance of the spiral, deduced from readings on an accurately
calibrated Weston millivoltmeter, with a constant current through the
spiral, was thus very approximately proportional to the absolute
temperature. The liquid ethylene was kept vigorously stirred by an
electric motor, and was cooled to any desired temperature by surrounding
the vessel with liquid air.
The general method employed for the radium emanation was to pass a
suitable amount of emanation, mixed with the gas to be used, from the
gas holder _B_ into the spiral, cooled below the temperature of
condensation. After the emanation was condensed in the spiral, a current
of electrolytic hydrogen or oxygen was passed through the spiral. The
temperature was allowed to rise gradually, and was noted at the instant
when a deflection of the electrometer, due to the presence of emanation
in the testing vessel _T_, was observed. The resistance, subject to a
slight correction due to the time taken for the emanation to be carried
into the testing vessel, gave the temperature at which some of the
emanation commenced to volatilize. The ionization current in the testing
vessel rose rapidly to a maximum value, showing that, for a small
increase of temperature, the whole of the radium emanation was
volatilized. The following table gives an illustration of the results
obtained for a current of hydrogen of 1·38 cubic centimetres per second.
Temperature Divisions per
second of the
electrometer
−160° 0
−156° 0
−154°·3 1
−153°·8 21
−152°·5 24
The following table shows the results obtained for different currents of
hydrogen and oxygen.
Current of Gas _T₁_ _T₂_
Hydrogen ·25 c.c. per sec. −151·3 −150
„ ·32 „ „ −153·7 −151
„ ·92 „ „ −152 −151
„ 1·38 „ „ −154 −153
„ 2·3 „ „ −162·5 −162
Oxygen ·34 „ „ −152·5 −151·5
„ ·58 „ „ −155 −153
The temperature _T₁_ in the above table gives the temperature of initial
volatilization, _T₂_ the temperature for which half of the condensed
emanation had been released. For slow currents of hydrogen and oxygen,
the values of _T₁_ and _T₂_ are in good agreement. For a stream of gas
as rapid as 2·3 cubic centimetres per second the value of _T₁_ is much
lower. Such a result is to be expected; for, in too rapid a stream, the
gas is not cooled to the temperature of the spiral, and, in consequence,
the inside surface of the spiral is above the mean temperature, and some
of the emanation escapes at a temperature apparently much lower. In the
case of oxygen, this effect appears for a gas stream of 0·58 cubic
centimetres per second.
In the experiments on the thorium emanation, on account of the rapid
loss of activity, a slightly different method was necessary. The steady
stream of gas was passed over the thorium compound, and the temperature
was observed at the instant when an appreciable movement of the
electrometer appeared. This gave the temperature at which a small
fraction of the thorium emanation escaped condensation, and not the
value _T₁_ observed for the radium emanation, which gave the temperature
for which a small fraction of the previously condensed emanation was
volatilized.
The following table illustrates the results obtained.
Current of Gas Temperature
Hydrogen ·71 c.c. per −155° C.
sec.
„ 1·38 „ „ −159° C.
Oxygen ·58 „ „ −155° C.
On comparing these results with the values obtained for the radium
emanation, it will be observed that with equal gas streams the
temperatures are nearly the same.
A closer examination of the thorium emanation showed, however, that this
apparent agreement was only accidental, and that there was, in reality,
a very marked difference in the effect of temperature on the two
emanations. It was found experimentally that the radium emanation was
condensed very near the temperature at which volatilization commenced,
and that the points of condensation and volatilization were defined
fairly sharply.
[Illustration: Fig. 60.]
On the other hand, the thorium emanation required a range of over 30° C.
after condensation had started in order to ensure complete condensation.
Fig. 60 is an example of the results obtained with a steady gas stream
of 1·38 c.c. per sec. of oxygen. The ordinates represent the percentage
proportion of the emanation uncondensed at different temperatures. It
will be observed that condensation commences about −120°, and that very
little of the emanation escapes condensation at −155° C.
To investigate this difference of behaviour in the two emanations, a
static method was employed, which allowed an examination of the two
emanations to be made under comparable conditions. The emanation, mixed
with a small amount of the gas to be used, was introduced into the cool
spiral, which had been exhausted previously by means of a mercury pump.
The amount of emanation remaining uncondensed after definite intervals
was rapidly removed by means of the pump, and was carried with a
constant auxiliary stream of gas into the testing vessel.
Tested in this way, it was found that the volatilization point of the
radium emanation was very nearly the same as that obtained by the
blowing method, viz. −150° C. With thorium, on the other hand, the
condensation started at about −120° C., and, as in the blowing method,
continued over a range of about 30° C. The proportion of the emanation
condensed at any temperature was found to depend on a variety of
conditions, although the point at which condensation commenced, viz.
−120° C., was about the same in each case. It depended on the pressure
and nature of the gas, on the concentration of the emanation, and on the
time for which it was left in the spiral. For a given temperature a
greater proportion of the emanation was condensed, the lower the
pressure and the longer the time it was left in the spiral. Under the
same conditions, the emanation was condensed more rapidly in hydrogen
than in oxygen.
=168.= Thus there is no doubt that the thorium emanation begins to
condense at a temperature higher than that at which the radium emanation
condenses. The explanation of the peculiar behaviour of the thorium
emanation is clear when the small number of emanation particles present
in the gas are taken into consideration. It has been shown that both
emanations give out only α rays. It is probable that the α particles
from the two emanations are similar in character and produce about the
same number of ions in their passage through the gas. The number of ions
produced by each α particle before its energy is dissipated is probably
about 70,000. (See section 252.)
Now, in the experiment, the electrometer readily measured a current of
10⁻³ electrostatic units. Taking the charge on an ion as 3·4 × 10⁻¹⁰
electrostatic units, this corresponds to a production in the testing
vessel of about 3 × 10⁶ ions per sec., which would be produced by about
40 expelled α particles per second. Each radiating particle cannot expel
less than one α particle and may expel more, but it is likely that the
number expelled by an atom of the thorium emanation is not greatly
different from that expelled by an atom of the radium emanation.
In section 133 it has been shown that, according to the law of decay,
λ_N_ particles change per second when _N_ are present. Thus, to produce
40 α particles, λ_N_ cannot be greater than 40. Since for the thorium
emanation λ is ¹⁄₈₇, it follows that _N_ cannot be greater than 3500.
The electrometer thus detected the presence of 3500 particles of the
thorium emanation, and since in the static method the volume of the
condensing spiral was about 15 c.c., this corresponded to a
concentration of about 230 particles per c.c. An ordinary gas at
atmospheric pressure and temperature probably contains about 3·6 × 10¹⁹
molecules per c.c. Thus the emanation would have been detected on the
spiral if it had possessed a partial pressure of less than 10⁻¹⁷ of an
atmosphere.
It is not surprising then that the condensation point of the thorium
emanation is not sharply defined. It is rather a matter of remark that
condensation should occur so readily with so sparse a distribution of
emanation particles in the gas; for, in order that condensation may take
place, it is probable that the particles must approach within one
another’s sphere of influence.
Now in the case of the radium emanation, the rate of decay is about 5000
times slower than that of the thorium emanation, and consequently the
actual number of particles that must be present to produce the same
ionization per second in the two cases must be about 5000 times greater
in the case of radium than in the case of thorium. This conclusion
involves only the assumption that the same number of rays is produced by
a particle of emanation in each case, and that the expelled particles
produce in their passage through the gas the same number of ions. The
number of particles present, in order to be detected by the
electrometer, in this experiment, must therefore have been about 5000 ×
3500, _i.e._ about 2 × 10⁷. The difference of behaviour in the two cases
is well explained by the view that, _for equal electrical effects_, the
number of radium emanation particles must be far larger than the number
of thorium emanation particles. The probability of the particles coming
into each other’s sphere of influence will increase very rapidly as the
concentration of the particles increases, and, in the case of the radium
emanation, once the temperature of condensation is attained, all but a
small proportion of the total number of particles present will condense
in a very short time. In the case of the thorium emanation, however, the
temperature might be far below that of condensation, and yet a
considerable portion remain uncondensed for comparatively long
intervals. On this view the experimental results obtained might
reasonably be expected. A greater proportion of emanation condenses the
longer the time allowed for condensation under the same conditions. The
condensation occurs more rapidly in hydrogen than in oxygen, as the
diffusion is greater in the former gas. For the same reason the
condensation occurs faster the lower the pressure of the gas present.
Finally, when the emanation is carried by a steady stream of gas, a
smaller proportion condenses than in the other cases, because the
concentration of emanation particles per unit volume of gas is less
under these conditions.
It is possible that the condensation of the emanations may not occur in
the gas itself but at the surface of the containing vessel. Accurate
observations of the temperature of condensation have so far only been
made in a copper spiral, but condensation certainly occurs in tubes of
lead or glass at about the same temperature as in tubes of copper.
=169.= In experiments that were made by the static method with a very
large quantity of radium emanation, a slight amount of escape of the
condensed emanation was observed several degrees below the temperature
at which most of the emanation was released. This is to be expected,
since, under such conditions, the electrometer is able to detect a very
minute proportion of the whole quantity of the emanation condensed.
Special experiments, with a large quantity of emanation, that were made
with the spiral immersed in a bath of rapidly boiling nitric oxide,
showed this effect very clearly. For example, the condensed emanation
began to volatilize at −155° C. In 4 minutes the temperature had risen
to −153·5°, and the amount volatilized was four times as great as at
−155°. In the next 5½ minutes the temperature had increased to −152·3°
and practically the whole quantity, which was at least fifty times the
amount at the temperature of −153·5°, had volatilized.
It thus seems probable that, if the temperature were kept steady at the
point at which volatilization was first observed, and the released
emanation removed at intervals, the whole of the emanation would in
course of time be liberated at that temperature. Curie and Dewar and
Ramsay have observed that the emanation condensed in a =U= tube,
immersed in liquid air, slowly escapes if the pump is kept steadily
working. These results point to the probability that the condensed
emanation possesses a true vapour pressure, but great refinements in
experimental methods would be necessary before such a conclusion could
be definitely established.
The true temperature of condensation of the thorium emanation is
probably about −120° C., and that of radium about −150° C. Thus there is
no doubt that the two emanations are quite distinct from each other in
this respect, and also with regard to their radio-activity, although
they both possess the property of chemical inertness. These results on
the temperatures of condensation do not allow us to make any comparison
of the condensation points of the emanations with those of known gases,
since the lowering of the condensation points of gases with diminution
of pressure has not been studied at such extremely minute pressures.
=170.= It has been found[264] that the activity of the thorium
emanation, when condensed in the spiral at the temperature of liquid
air, decayed at the same rate as at ordinary temperatures. This is in
accord with results of a similar kind obtained by P. Curie for the
radium emanation (section 145), and shows that the value of the
radio-active constant is unaffected by wide variations of temperature.
Amount of Emanation from Radium and Thorium.
=171.= It has been shown in section 93 from experimental data that 1
gram of radium bromide at its minimum activity emits about 3·6 × 10¹⁰ α
particles per second. Since the activity due to the emanation stored up
in radium, when in a state of radio-active equilibrium, is about one
quarter of the whole and about equal to the minimum activity, the number
of α particles projected per second by the emanation from 1 gram of
radium bromide is about 3·6 × 10¹⁰. It has been shown in section 152
that 463,000 times the amount of emanation produced per second is stored
up in the radium. But, in a state of radio-active equilibrium, the
number of emanation particles breaking up per second is equal to the
number produced per second. Assuming that each emanation particle in
breaking up expels one α particle, it follows that the number of
emanation particles present in 1 gram of radium bromide in radio-active
equilibrium is 463,000 × 3·6 × 10¹⁰, _i.e._ 1·7 × 10¹⁶. Taking the
number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure
and temperature as 3·6 × 10¹⁹ (section 39), the volume of the emanation
from 1 gram of radium bromide is 4·6 × 10⁻⁴ cubic centimetres at
atmospheric pressure and temperature. Assuming the composition of radium
bromide as RaBr₂, the amount from 1 gram of radium in radio-active
equilibrium is 0·82 cubic millimetres. Quite independently of any method
of calculation it was early evident that the volume of the emanation was
very small, for all the earlier attempts made to detect its presence by
its volume were unsuccessful. It will be seen, however, that, when
larger quantities of radium were available for experiment, the emanation
has been collected in volume sufficiently large to measure.
In the case of thorium, the maximum quantity of emanation to be obtained
from 1 gram of the solid is very minute, both on account of the small
activity of thorium and of the rapid break up of the emanation after its
production. Since the amount of emanation, stored in a non-emanating
thorium compound, is only 87 times the rate of production, while in
radium it is 463,000 times, and the rate of production of the emanation
by radium is about 1 million times faster than by thorium, it follows
that the amount of emanation to be obtained from 1 gram of thorium is
not greater than 10⁻¹⁰ of the amount from an equal weight of radium,
_i.e._ its volume is not greater than 10⁻¹³ c.c. at the ordinary
pressure and temperature. Even with large quantities of thorium, the
amount of emanation is too small ever to be detected by its volume.
=172. Volume of the emanation from radium.= The evidence already
considered points very strongly to the conclusion that the emanation
possesses all the properties of a chemically inert gas of high molecular
weight.
Since the emanation continuously breaks up, and is transformed into a
solid type of matter, which is deposited on the surface of bodies, the
volume of the emanation, when separated from radium, should contract at
the same rate as it loses its activity, _i.e._ it should decrease to
half value in about four days. The amount of emanation to be obtained
from a given quantity of radium is a maximum when the rate of production
of new emanation balances its rate of change. This condition is
practically attained when the emanation has been allowed to collect for
an interval of one month. The probable volume of the emanation to be
obtained from 1 gram of radium was early calculated on certain
assumptions, and from data then available the writer[265] deduced that
the volume of the emanation from 1 gram of radium lay between ·06 and ·6
cubic millimetre at atmospheric pressure and temperature, and was
probably nearer the latter value. The volume to be expected on the
latest data has been discussed in the preceding section and shown to be
about ·82 cubic mm. The volume of the emanation is thus very small, but
not too small to be detected if several centigrams of radium are
available. This has been proved to be the case by Ramsay and Soddy[266]
who, by very careful experiment, finally succeeded in isolating a small
quantity of the emanation and in determining its volume. The
experimental method employed by them will now be briefly described.
[Illustration: Fig. 61.]
The emanation from 60 milligrams of radium bromide in solution was
allowed to collect for 8 days and then drawn off through the inverted
siphon _E_ (Fig. 61) into the explosion burette _F_. This gas consisted
for the most part of hydrogen and oxygen, produced by the action of the
radiations on the water of the solution. After explosion, the excess of
hydrogen mixed with emanation was left some time in contact with caustic
soda, placed in the upper part of the burette, in order to remove all
trace of carbon dioxide. In the meantime the upper part of the apparatus
had been completely evacuated. The connection _C_ to the pump was
closed, and the hydrogen and emanation were allowed to enter the
apparatus, passing over a phosphorous pentoxide tube _D_. The emanation
was condensed in the lower part of the capillary tube _A_, by
surrounding it with the tube _B_ filled with liquid air. The process of
condensation was rendered manifest by the brilliant luminosity of the
lower part of the tube. The mercury from the burette was then allowed to
run to _G_, and the apparatus again completely evacuated. The connection
of the pump was again closed, the liquid air was removed and the
volatilized emanation forced into the fine capillary tube _A_.
Observations were then made, from day to day, of the volume of the
emanation. The results are given in the table below.
Time Volume Time Volume
Start 0·124 cub. 7 days 0·0050 cub. mm.
mm.
1 day 0·027 „ 9 „ 0·0041 „
3 „ 0·011 „ 11 „ 0·0020 „
4 „ 0·0095 „ 12 „ 0·0011 „
6 „ 0·0063 „ 28 „ 0·0004 „
The volume contracted with the time, and was very small after a month’s
interval, but the minute bubble of the emanation still retained its
luminosity to the last. The tube became deep purple in colour, which
rendered readings difficult except with a strong light. There was a
sudden decrease in the first day, which may have been due to the mercury
sticking in the capillary tube.
The experiments were repeated with another capillary tube and the volume
of gas observed at normal pressure was 0·0254 c. mm. The gas obtained
was found to obey Boyle’s law within the limit of experimental error
over a considerable range of pressure. But, unlike in the first
experiment, the gas did not contract but expanded rapidly during the
first few hours, and then more slowly, finally reaching a volume after
23 days of 0·262 c. mm. or about 10 times the initial volume. The
measurements were complicated by the appearance of bubbles of gas in the
top of the mercury column. The differences observed in these two
experiments are difficult to account for. We shall see, later, that the
emanation always produces helium, and, in the first experiment, the
decrease of the volume to zero indicates that the helium was buried or
absorbed in the walls of the tube. In the second case, probably owing to
some difference in the glass of the capillary tube, the helium may have
been released. This suggestion is confirmed by the observation that the
volume of gas, after the experiment ended, gave a brilliant spectrum of
helium.
We shall see later that there is considerable evidence that the α
particles expelled from radio-active substances consist of helium atoms.
Since the particles are projected with great velocity, they will first
be buried in the walls of the tube, and then may gradually diffuse out
into the gas again under conditions probably depending on the kind of
glass employed. Since α particles are projected from the emanation and
also from two of the rapidly changing products which arise from it, the
volume of helium should, on this view, be three times the initial volume
of the emanation. If the helium produced escaped from the walls of the
tube into the gas, the apparent volume of the gas in the capillary
should increase to three times the initial volume in a month’s interval,
for during that time the emanation itself has been transformed into a
solid type of matter deposited on the walls of the tube.
Ramsay and Soddy concluded from their experiments that the maximum
volume of emanation to be obtained from 1 gram of radium was about 1
cubic millimetre at standard pressure and temperature, and that the
emanation was produced from 1 gram of radium at the rate of 3 × 10⁻⁶ c.
mm. per second. This amount is in very good agreement with the
calculated value, and is a strong indication of the general correctness
of the theory on which the calculations are based.
=173. Spectrum of the emanation.= After the separation of the emanation
and the determination of its volume, Ramsay and Soddy made numerous
attempts to obtain its spectrum. In some of the earlier experiments
several bright lines were seen for a short time, but these lines were
soon masked by the appearance of the hydrogen lines. In later
experiments Ramsay and Collie[267] succeeded in obtaining a spectrum of
the emanation, which persisted for a short time, during which a rapid
determination of the wave-lengths was made. They state that the spectrum
was very brilliant, consisting of very bright lines, the spaces between
being perfectly dark. The spectrum bore a striking resemblance in
general character to the spectrum of the gases of the argon family.
The spectrum soon faded, and the spectrum of hydrogen began to appear.
The following table shows the wave-length of the lines observed in the
spectrum. The degree of coincidence of the lines of known wave-lengths
shows that the error is probably less than five Ångström units.
Wave-length Remarks
6567 Hydrogen C; true wave-length, 6563; observed each
time.
6307 Observed only at first; evanescent.
5975 „ „ „
5955 „ „ „
5805 Observed each time; persistent.
5790 Mercury; true wave-length, 5790.
5768 „ „ 5769.
5725 Observed only at first; evanescent.
5595 Observed each time; persistent and strong.
5465 Mercury; true wave-length, 5461.
5105 Not observed at first; appeared after some
seconds; persisted and was visible
during the second examination.
4985 Observed each time; persistent and strong.
4865 Hydrogen F; true wave-length, 4861.
4690 Observed only at first.
4650 Not observed when the emanation was examined
again.
4630 „ „ „
4360 Mercury: true wave-length, 4359.
The experiments were repeated with a new supply of emanation, and some
of the stronger lines were observed again, while some new lines made
their appearance. Ramsay and Collie suggest that the strong line 5595
may be identical with a line which was observed by Pickering[268] in the
spectrum of lightning, and was not identified with the spectrum of any
known gas.
Until large quantities of radium are available for the experimenter it
would appear difficult to make sure how many of these lines must be
ascribed to the spectrum of the emanation or to measure the wave-lengths
with accuracy.
The results are of great interest, as showing that the emanation has a
definite and new spectrum of the same general character as the argon
group of gases to which, as we have seen, it is chemically allied.
Summary of Results.
=174.= The investigations into the nature of the radio-active emanations
have thus led to the following conclusions:—The radio-elements thorium,
radium and actinium continuously produce from themselves radio-active
emanations at a rate which is constant under all conditions. In some
cases, the emanations continuously diffuse from the radio-active
compounds into the surrounding gas; in other cases, the emanations are
unable to escape from the material in which they are produced, but are
occluded, and can only be released by solution or by the action of heat.
The emanations possess all the properties of radio-active gases. They
diffuse through gases, liquids, and porous substances, and can be
occluded in some solids. Under varying conditions of pressure, volume,
and temperature, the emanations distribute themselves in the same way
and according to the same laws as does a gas.
The emanations possess the important property of condensation under the
influence of extreme cold, and by that means can be separated from the
gases with which they are mixed. The radiation from the emanation is
material in nature, and consists of a stream of positively charged
particles projected with great velocity.
The emanations possess the property of chemical inertness, and in this
respect resemble the gases of the argon family. The emanations are
produced in minute amount; but a sufficient quantity of the radium
emanation has been obtained to determine its volume and its spectrum.
With regard to their rates of diffusion, the emanations of both thorium
and radium behave like gases of high molecular weight.
These emanations have been detected and their properties investigated by
the property they possess of emitting radiations of a special character.
These radiations consist entirely of α rays, _i.e._ particles, projected
with great velocity, which carry a positive charge and have a mass about
twice that of the hydrogen atom. The emanations do not possess the
property of permanently radiating, but the intensity of the radiations
diminishes according to an exponential law with the time, falling to
half value, from actinium in 4 seconds, from thorium in one minute, and
from radium in about four days. The law of decay of activity does not
seem to be influenced by any physical or chemical agency.
The emanation particles gradually break up, each particle as it breaks
up expelling a charged body. The emanation after it has radiated ceases
to exist as such, but is transformed into a new kind of matter, which is
deposited on the surface of bodies and gives rise to the phenomena of
excited activity. This last property, and the connection of the
emanation with it, are discussed in detail in the next chapter.
Footnote 231:
Owens, _Phil. Mag._ p. 360, Oct. 1899.
Footnote 232:
Rutherford, _Phil. Mag._ p. 1, Jan. 1900.
Footnote 233:
Rossignol and Gimingham, _Phil. Mag._ July, 1904.
Footnote 234:
Bronson, _Amer. Journ. Science_, Feb. 1905.
Footnote 235:
_Phil. Mag._ April, 1904.
Footnote 236:
Dorn, _Abh. der. Naturforsch. Ges. für Halle-a-S._, 1900.
Footnote 237:
P. Curie, _C. R._ 135, p. 857, 1902.
Footnote 238:
Rutherford and Soddy, _Phil. Mag._ April, 1903.
Footnote 239:
P. Curie, _C. R._ 136, p. 223, 1903.
Footnote 240:
Debierne, _C. R._ 136, p. 146, 1903.
Footnote 241:
Giesel, _Ber. D. deutsch. Chem. Ges._ p. 3608, 1902.
Footnote 242:
Curie and Debierne, _C. R._ 132, pp. 548 and 768, 1901.
Footnote 243:
Curie and Debierne, _C. R._ 133, p. 931, 1901.
Footnote 244:
Rutherford and Soddy, _Trans. Chem. Soc._ p. 321, 1902. _Phil. Mag._
Sept. 1902.
Footnote 245:
Rutherford, _Phys. Zeit._ 2, p. 429, 1901.
Footnote 246:
Rutherford and Soddy, _Phil. Mag._ Nov. 1902.
Footnote 247:
Rutherford and Soddy, _Phil. Mag._ April, 1903.
Footnote 248:
Rutherford and Soddy, _Phil. Mag._ Nov. 1902.
Footnote 249:
Rutherford and Soddy, _Phil. Mag._ April, 1903.
Footnote 250:
Curie and Debierne, _C. R._ 133, p. 931, 1901.
Footnote 251:
Rutherford and Soddy, _Phil. Mag._ Nov. 1902.
Footnote 252:
Ramsay and Soddy, _Proc. Roy. Soc._ 72, p. 204, 1903.
Footnote 253:
Rutherford and Miss Brooks, _Trans. Roy. Soc. Canada 1901_, _Chem.
News 1902_.
Footnote 254:
Loschmidt, _Sitzungsber. d. Wien. Akad._ 61, II. p. 367, 1871.
Footnote 255:
See Stefan, _Sitzungsber. d. Wien. Akad._ 63, II. p. 82, 1871.
Footnote 256:
P. Curie and Danne, _C. R._ 136, p. 1314, 1903.
Footnote 257:
Bumstead and Wheeler, _Amer. Jour. Science_, Feb. 1904.
Footnote 258:
Makower, _Phil. Mag._ Jan. 1905.
Footnote 259:
Wallstabe, _Phys. Zeit._ 4, p. 721, 1903.
Footnote 260:
Stefan, _Wien. Ber._ 2, p. 371, 1878.
Footnote 261:
Rutherford and Soddy, _Phil. Mag._ Nov. 1902.
Footnote 262:
_Phil. Mag._ May, 1903.
Footnote 263:
P. Curie, Société de Physique, 1903.
Footnote 264:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 265:
_Nature_, Aug. 20, 1903.
Footnote 266:
_Proc. Roy. Soc._ 73, No. 494, p. 346, 1904.
Footnote 267:
_Proc. Roy. Soc._ 73, No. 495, p. 470, 1904.
Footnote 268:
Pickering, _Astrophys. Journ._ Vol. 14, p. 368, 1901.