Radio-activity/Chapter 7
CHAPTER VII.
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[1] 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[2] 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.
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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.
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Fig. 52.
The rate of decay has been more accurately determined by Rossignol and Gimingham[3] who found that the activity fell to half value in about 51 seconds. Bronson[4], 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[5].
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 [Greek: alpha] 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 [Greek: alpha] rays. Since the [Greek: alpha] 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 layers | Current |
| of paper | |
+-+-+
| 0 | 1 |
| 1 | ·37 |
| 2 | ·16 |
| 3 | ·08 |
+-+-+
Table II. Thick Layer.
Thickness of paper ·008 cm.
+-++
| No. of layers | Current |
| 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 [Greek: alpha] 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 [Greek: alpha] 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 seconds Current
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
I_{t}/I_{0} = e^{-λt},
and the recovery curve by
I_{t}/I_{0} = 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.
q_{0} = λN_{0}.
λ = 1/87,
144. Discovery of the emanation. Shortly after the
discovery of the thorium emanation, Dorn[6] 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[7], and Rutherford and Soddy[8]. 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 hours Relative Activity
0 100
20·8 85·7
187·6 24·0
354·9 6·9
521·9 1·5
786·9 0·19
time, and decays to half value in 3·71 days. With the usual notation
I_{t}/I_{0} = 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.
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Fig. 53.
Now it will be shown later that the emanation itself gives off only [Greek: alpha] 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 [Greek: alpha] 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[9] 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[10] 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[11] 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 platino-cyanide 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[12] 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[13] 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.
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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
N_{t}/N_{0} = 1 - e^{-λt},
where N_{t} is the number of emanation particles present at the time t, N_{0} 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. 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[14], 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 i_{1} = saturation current due to a weight ω_{1} of the standard,
i_{2} = " " " " ω_{2} of the sample to be tested.
Then (emanating power of specimen)/(emanating power of standard) = (i_{2}/i_{1})(ω_{1}/ω_{2}).
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[15] 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[16]. 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 byN_{t}/N_{0} = e^{-λt},
q_{0} = λN_{0},
N_{0} 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,
N_{0}/q_{0} = 1/λ = 463,000.
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[17].
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 N_{0}. 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}/N_{0} = ·0131.
Assuming that there is no decay during the interval,
N_{t} = 105 × 60 × q_{0}.
Thus N_{0}/q_{0} = 480,000.
Making the small correction for the decay of activity during the interval,
N_{0}/q_{0} = 477,000.
We have previously shown that from the theory
N_{0}/q_{0} = 1/λ = 463,000.
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 1/2% 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,
N_{0}/q_{0} = 1/λ = 87.
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[18] 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.
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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 foil Current
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[19]:
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[20] 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. 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[21] 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[22], 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[23] to determine the rate of the diffusion of the radium emana-*
- tion into air, by a method similar to that employed by Loschmidt[24]
in 1871, in his investigations of the coefficient of inter-diffusion of gases.
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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 moveable 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[25] that if
K = coefficient of diffusion of the emanation into air,
t = duration of diffusion experiments in secs.,
a = total length of cylinder,
S_{1} = partial pressure of emanation in tube A at end of diffusion,
S_{2} = partial pressure of emanation in tube B at end of diffusion,
then
(S_{1} - S_{2})/(S_{1} + S_{2}) = (8/π^2)(e^{-(π^2Kt)/a^2} + (1/9)e^{-(9π^2Kt)/a^2} + . . .).
Now the values of S_{1} and S_{2} 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 S_{1} and S_{2} are observed after diffusion has been in progress for a definite interval t.
The determination of S_{1} and S_{2} 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 S_{1} and S_{2}, 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[26].
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 interdiffusion of gases, compiled from Landolt and Bernstein's
tables, are given below.
+-+++
| Gas or vapour | Coefficient of |Molecular weight|
| |diffusion into air| |
+-+++
|Water vapour | 0·198 | 18 |
|Carbonic acid gas| 0·142 | 44 |
|Alcohol vapour | 0·101 | 46 |
|Ether vapour | 0·077 | 74 |
|Radium emanation | 0·07 | ? |
+-+++
The tables, although not very satisfactory for the purpose of comparison, show that the coefficient of interdiffusion 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[27] 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[28] 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 [sqrt]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 [sqrt]M as ordinate and K as abscissa, the points corresponding to the values of O, CO_{2} and SO_{2} 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 interdiffusion is being examined, exists in minute quantity mixed with the gas, and is compared with the rate of interdiffusion 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. 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.
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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
K(d^2p/dx^2) = - dp/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 p_{1} is the value of p when t = 0 and λ is the radio-active constant of the emanation.
Then dp/dt = -λp,
and K(d^2p/dx^2) = λp.
Thus p = Ae^{-[sqrt](λ/K)x} + Be^{[sqrt](λ/K)x}.
Since p = 0 when x = [infinity], B = 0.
If p = p_{0} when x = 0, A = p_{0}.
Thus p = p_{0}e^{-[sqrt](λ/K)x}.
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[29] 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.
p = p_{0}e^{-[sqrt](λ/K)x},
for water α = 1·6, for toluol α = ·75.
165. Condensation of the emanations. During an investigation
of the effect of physical and chemical agencies on
the thorium emanation, Rutherford and Soddy[31] 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[32].
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.
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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[33].
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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 |
+-++
currents of hydrogen and oxygen.
+++-+-+
| | Current of Gas |T_{1}|T_{2}|
+++-+-+
|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 T_{1} in the above table gives the temperature of initial volatilization, T_{2} the temperature for which half of the condensed emanation had been released. For slow currents of hydrogen and oxygen, the values of T_{1} and T_{2} are in good agreement. For a stream of gas as rapid as 2·3 cubic centimetres per second the value of T_{1} 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 T_{1} 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 sec.| -155° C. |
| " |1·38 " " | -159° C. |
|Oxygen | ·58 " " | -155° C. |
+++-+
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.
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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 × 10^6 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, λN particles change per second when N are present. Thus, to produce 40 α particles, λN 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 × 10^{19} 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 × 10^7. 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 5-1/2 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[34] 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
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 × 10^{10} 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 × 10^{10}. 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 × 10^{10}, i.e. 1·7 × 10^{16}. Taking the number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure and temperature as 3·6 × 10^{19} (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 RaBr_{2}, 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[35] 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[36] 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
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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. mm. 7 days 0·0050 cub. 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[37]
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.
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[38] 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.
- ↑ Owens, Phil. Mag. p. 360, Oct. 1899.
- ↑ Rutherford, Phil. Mag. p. 1, Jan. 1900.
- ↑ Rossignol and Gimingham, Phil. Mag. July, 1904.
- ↑ Bronson, Amer. Journ. Science, Feb. 1905.
- ↑ Phil. Mag. April, 1904.
- ↑ Dorn, Abh. der. Naturforsch. Ges. für Halle-a-S., 1900.
- ↑ P. Curie, C. R. 135, p. 857, 1902.
- ↑ Rutherford and Soddy, Phil. Mag. April, 1903.
- ↑ P. Curie, C. R. 136, p. 223, 1903.
- ↑ Debierne, C. R. 136, p. 146, 1903.
- ↑ Giesel, Ber. D. deutsch. Chem. Ges. p. 3608, 1902.
- ↑ Curie and Debierne, C. R. 132, pp. 548 and 768, 1901.
- ↑ Curie and Debierne, C. R. 133, p. 931, 1901.
- ↑ Rutherford and Soddy, Trans. Chem. Soc. p. 321, 1902. Phil. Mag. Sept. 1902.
- ↑ Rutherford, Phys. Zeit. 2, p. 429, 1901.
- ↑ Rutherford and Soddy, Phil. Mag. Nov. 1902.
- ↑ Rutherford and Soddy, Phil. Mag. April, 1903.
- ↑ Rutherford and Soddy, Phil. Mag. Nov. 1902.
- ↑ Rutherford and Soddy, Phil. Mag. April, 1903.
- ↑ Curie and Debierne, C. R. 133, p. 931, 1901.
- ↑ Rutherford and Soddy, Phil. Mag. Nov. 1902.
- ↑ Ramsay and Soddy, Proc. Roy. Soc. 72, p. 204, 1903.
- ↑ Rutherford and Miss Brooks, Trans. Roy. Soc. Canada 1901, Chem. News 1902.
- ↑ Loschmidt, Sitzungsber. d. Wien. Akad. 61, II. p. 367, 1871.
- ↑ See Stefan, Sitzungsber. d. Wien. Akad. 63, II. p. 82, 1871.
- ↑ P. Curie and Danne, C. R. 136, p. 1314, 1903.
- ↑ Bumstead and Wheeler, Amer. Jour. Science, Feb. 1904.
- ↑ Makower, Phil. Mag. Jan. 1905.
- ↑ Wallstabe, Phys. Zeit. 4, p. 721, 1903.
- ↑ Stefan, Wien. Ber. 2, p. 371, 1878.
- ↑ Rutherford and Soddy, Phil. Mag. Nov. 1902.
- ↑ Phil. Mag. May, 1903.
- ↑ P. Curie, Société de Physique, 1903.
- ↑ Rutherford and Soddy, Phil. Mag. May, 1903.
- ↑ Nature, Aug. 20, 1903.
- ↑ Proc. Roy. Soc. 73, No. 494, p. 346, 1904.
- ↑ Proc. Roy. Soc. 73, No. 495, p. 470, 1904.
- ↑ Pickering, Astrophys. Journ. Vol. 14, p. 368, 1901.