Chapter VII: Radio-Active Emanations (2)

A large amount of emanation was introduced into a cylinder made of sheet copper ·005 cm. thick, which absorbed all the α rays but allowed the β and γ rays, if present, to pass through with but little loss. The external radiation from the cylinder was determined at intervals, commencing about two minutes after the introduction of the emanation. The amount observed at first was extremely small, but increased rapidly and practically reached a maximum in three or four hours. Thus the radium emanation only gives out α rays, the β rays appearing as the excited activity is produced on the walls of the vessel. On sweeping out the emanation by a current of air, there was no immediately appreciable decrease of the radiation. This is another proof that the emanation does not emit any β rays. In a similar way it can be shown that the emanation does not give out γ rays; these rays always make their appearance at the same time as the β rays.

The method of examination of the radiations from the emanations has been given in some detail, as the results are of considerable importance in the discussion, which will be given later in chapters X and XI, of the connection between the changes occurring in radio-active products and the radiations they emit. There is no doubt that the emanations, apart from the excited activity to which they give rise, only give out α rays, consisting most probably of positively charged bodies projected with great velocity.

Effect of pressure on the rate of production of the Emanation.

=157.= It has already been mentioned that the conductivity due to the thorium emanation is proportional to the pressure of the gas, pointing to the conclusion that the rate of production of the emanation is independent of the pressure, as well as of the nature of the surrounding gas. This result was directly confirmed with the apparatus of Fig. 55. When the pressure of the gas under the vessel was slowly reduced, the radiation, tested outside the window, increased to a limit, and then remained constant over a wide range of pressure. This increase, which was far more marked in air than in hydrogen, is due to the fact that the α rays from the emanation were partially absorbed in the gas inside the vessel when at atmospheric pressure. At pressures of the order of 1 millimetre of mercury the external radiation decreased, but experiment showed that this must be ascribed to a removal of the emanation by the pump, and not to a change in the rate of production. The thorium compounds very readily absorb water-vapour, which is slowly given off at low pressures, and in consequence some of the emanation is carried out of the vessel with the water-vapour.

Curie and Debierne[250] found that both the amount of excited activity produced in a closed vessel containing active samples of radium, and also the time taken to reach a maximum value, were independent of the pressure and nature of the gas. This was true in the case of a solution down to the pressure of the saturated vapour, and in the case of solid salts to very low pressures. When the pump was kept going at pressures of the order of ·001 mm. of mercury, the amount of excited activity was much diminished. This was probably not due to any alteration of the rate of escape of the emanation, but to the removal of the emanation by the action of the pump as fast as it was formed.

Since the amount of excited activity, when in a state of radio-active equilibrium, is a measure of the amount of emanation producing it, these results show that the amount of emanation present when the rate of production balances the rate of decay is independent of the pressure and nature of the gas. It was also found that the time taken to reach the point of radio-active equilibrium was independent of the size of the vessel or the amount of active matter present. This proves that the state of equilibrium cannot in any way be ascribed to the possession by the emanation of any appreciable vapour pressure; for if such were the case, the time taken to reach the equilibrium value should depend on the size of the vessel and the amount of active matter present. The results are, however, in agreement with the view that the emanation is present in minute quantity in the tube, and that the equilibrium is governed purely by the radio-active constant λ, the constant of decay of activity of the emanation. This has been seen to be the same under all conditions of concentration, pressure and temperature, and, provided the rate of supply of the emanation from the active compound is not changed, the time-rate of increase of activity to the equilibrium value will always be the same, whatever the size of the vessel or the nature and pressure of the surrounding gas.

Chemical Nature of the Emanations.

=158.= We shall now consider some experiments on the physical and chemical properties of the emanations themselves, without reference to the material producing them, in order to see if they possess any properties which connect them with any known kind of matter.

It was soon observed that the thorium emanation passed unchanged through acid solutions, and later the same result was shown to hold true in the case of both emanations for every reagent that was tried. Preliminary observations[251] showed that the thorium emanation, obtained in the usual way by passing air over thoria, passed unchanged in amount through a platinum tube heated electrically to the highest temperature obtainable. The tube was then filled with platinum-black, and the emanation passed through it in the cold, and with gradually increasing temperatures, until the limit was reached. In another experiment, the emanation was passed through a layer of red-hot lead-chromate in a glass tube. The current of air was replaced by a current of hydrogen, and the emanation was sent through red-hot magnesium-powder and red-hot palladium-black, and, by using a current of carbon dioxide, through red-hot zinc-dust. In every case the emanation passed through without sensible change in the amount. If anything, a slight increase occurred, owing to the time taken for the gas-current to pass through the tubes when hot being slightly less than when cold, the decay _en route_ being consequently less. The only known gases capable of passing in unchanged amount through all the reagents employed are the recently discovered members of the argon family.

But another possible interpretation might be put upon the results. If the emanation were the manifestation of a type of excited radio-activity on the surrounding atmosphere, then, since from the nature of the experiments it was necessary to employ in each case as the atmosphere, a gas not acted on by the reagent employed, the result obtained might be expected. Red-hot magnesium would not retain an emanation consisting of radio-active hydrogen, nor red-hot zinc-dust an emanation consisting of radio-active carbon dioxide. The incorrectness of this explanation was shown in the following way. Carbon dioxide was passed over thoria, then through a =T=-tube, where a current of air met and mixed with it, both passing on to the testing-cylinder. But between this and the =T=-tube a large soda-lime tube was introduced, and the current of gas was thus freed from its admixed carbon dioxide, before being tested in the cylinder for the emanation. The amount of emanation found was quite unchanged, whether carbon dioxide was sent over thoria in the manner described, or whether, keeping the other arrangements as before, an equally rapid current of air was substituted for it. The theory that the emanation is an effect of the excited activity on the surrounding medium is thus excluded.

Experiments of a similar kind on the radium emanation were made later. A steady stream of gas was passed through a radium chloride solution and then through the reagent to be employed, into a testing-vessel of small volume, so that any change in the amount of emanation passing through could readily be detected. The radium emanation, like that of thorium, passed unchanged in amount through every reagent used.

In later experiments by Sir William Ramsay and Mr Soddy[252], the emanation from radium was exposed to still more drastic treatment. The emanation in a glass tube was sparked for several hours with oxygen over alkali. The oxygen was then removed by ignited phosphorus and no visible residue was left. When, however, another gas was introduced, mixed with the minute amount of emanation in the tube and withdrawn, the activity of emanation was found to be unaltered. In another experiment, the emanation was introduced into a magnesium lime tube, which was heated for three hours at a red heat. The emanation was then removed and tested, but no diminution in its discharging power was observed.

The emanations of thorium and radium thus withstand chemical treatment in a manner hitherto unobserved except in gases of the argon family.

=159.= Ramsay and Soddy (_loc. cit._) record an interesting experiment to illustrate the gaseous nature of the emanation. A large amount of the radium emanation was collected in a small glass tube. This tube phosphoresced brightly under the influence of the rays from the emanation. The passage of the emanation from point to point was observed in a darkened room by the luminosity excited in the glass. On opening the stop-cock connecting with the Töpler pump, the slow flow through the capillary tube was noticed, the rapid passage along the wider tubes, the delay in passing through a plug of phosphorous pentoxide, and the rapid expansion into the reservoir of the pump. When compressed, the luminosity of the emanation increased, and became very bright as the small bubble containing the emanation was expelled through the fine capillary tube.

Diffusion of the Emanations.

=160.= It has been shown that the emanations of thorium and radium behave like radio-active gases, distributed in minute amount in the air or other gas in which they are tested. With the small quantities of active material so far investigated, the emanations have not yet been collected in sufficient amount to determine their density. Although the molecular weight of the emanations cannot yet be obtained by direct chemical methods, an indirect estimate of it can be made by determining the rate of their inter-diffusion into air or other gases. The coefficients of inter-diffusion of various gases have long been known, and the results show that the coefficient of diffusion of one gas into another is, for the simpler gases, approximately inversely proportional to the square root of the product of their molecular weights. If, therefore, the coefficient of diffusion of the emanation into air is found to have a value, lying between that of two known gases _A_ and _B_, it is probable that the molecular weight of the emanation lies between that of _A_ and _B_.

Although the volume of the emanation given off from radium is very small, the electrical conductivity produced by the emanation in the gas, with which it is mixed, is often very large, and offers a ready means of measuring the emanation present.

Some experiments have been made by Miss Brooks and the writer[253] to determine the rate of the diffusion of the radium emanation into air, by a method similar to that employed by Loschmidt[254] in 1871, in his investigations of the coefficient of inter-diffusion of gases.

Fig. 56 shows the general arrangement. A long brass cylinder _AB_, of length 73 cms., and diameter 6 cms., was divided into two equal parts by a movable metal slide _S_. The ends of the cylinder were closed with ebonite stoppers. Two insulated brass rods, _a_ and _b_, each half the length of the tube, passed through the ebonite stoppers and were supported centrally in the tube. The cylinder was insulated and connected with one pole of a battery of 300 volts, the other pole of which was earthed. The central rods could be connected with a sensitive quadrant electrometer. The cylinder was covered with a thick layer of felt, and placed inside a metal box filled with cotton wool in order to keep temperature conditions as steady as possible.

In order to convey a sufficient quantity of emanation into the half-cylinder _A_, it was necessary to heat the radium slightly. The slide _S_ was closed and the side tubes opened. A slow current of dry air from a gasometer was passed through a platinum tube, in which a small quantity of radium compound was placed. The emanation was carried with the air into the cylinder _A_. When a sufficient quantity had been introduced, the stream of air was stopped. The side tubes were closed by fine capillary tubes. These prevented any appreciable loss of gas due to the diffusion, but served to keep the pressure of the gas inside _A_ at the pressure of the outside air. The three entrance tubes into the cylinder, shown in the figure, were for the purpose of initially mixing the emanation and gas as uniformly as possible.

After standing several hours to make temperature conditions steady, the slide was opened, and the emanation began to diffuse into the tube _B_. The current through the tubes _A_ and _B_ was measured at regular intervals by an electrometer, with a suitable capacity in parallel. Initially there is no current in _B_, but after the opening of the slide, the amount in _A_ decreased and the amount in _B_ steadily increased. After several hours the amount in each half is nearly the same, showing that the emanation is nearly uniformly diffused throughout the cylinder.

It can readily be shown[255] that if

_K_ = coefficient of diffusion of the emanation into air,
_t_ = duration of diffusion experiments in secs.,
_a_ = total length of cylinder,
_S₁_ = partial pressure of emanation in tube _A_ at end of
diffusion,
_S₂_ = partial pressure of emanation in tube _B_ at end of
diffusion,

then

$$ \frac {S_1 − S_2} {S_1 + S_2} = \frac {8} {\pi^2} (e^{\frac {\pi^2 Kt} {a^2}} + \frac {1}{9}e^{-\frac {9 \pi^2 Kt} {a^2}} + ...) $$ .

Now the values of _S₁_ and _S₂_ are proportional to the saturation ionization currents due to the emanations in the two halves of the cylinder. From this equation _K_ can be determined, if the relative values of _S₁_ and _S₂_ are observed after diffusion has been in progress for a definite interval _t_.

The determination of _S₁_ and _S₂_ is complicated by the excited activity produced on the walls of the vessel. The ionization due to this must be subtracted from the total ionization observed in each half of the cylinder, for the excited activity is produced from the material composing the emanation, and is removed to the electrodes in an electric field. The ratio of the current due to excited activity to the current due to the emanation depends on the time of exposure to the emanation, and is only proportional to it for exposures of several hours.

The method generally adopted in the experiments was to open the slide for a definite interval, ranging in the experiments from 15 to 120 minutes. The slide was then closed and the currents in each half determined at once. The central rods, which had been kept negatively charged during the experiments, had most of the excited activity concentrated on their surfaces. These were removed, new rods substituted and the current immediately determined. The ratio of the currents in the half cylinders under these conditions was proportional to _S₁_ and _S₂_, the amounts of emanation present in the two halves of the cylinder.

The values of _K_, deduced from different values of _t_, were found to be in good agreement. In the earlier experiments the values of _K_ were found to vary between ·08 and ·12. In some later experiments, where great care was taken to ensure that temperature conditions were very constant, the values of _K_ were found to vary between ·07 and ·09. The lower value ·07 is most likely nearer the true value, as temperature disturbances tend to give too large a value of _K_. No certain differences were observed in the value of _K_ whether the air was dry or damp, or whether an electric field was acting or not.

=161.= Some experiments on the rate of diffusion of the radium emanation into air were made at a later date by P. Curie and Danne[256]. If the emanation is contained in a closed reservoir, it has been shown that its activity, which is a measure of the amount of emanation present, decreases according to an exponential law with the time. If the reservoir is put in communication with the outside air through a capillary tube, the emanation slowly diffuses out, and the amount of emanation in the reservoir is found to decrease according to the same law as before, but at a faster rate. Using tubes of different lengths and diameters, the rate of diffusion was found to obey the same laws as a gas. The value of _K_ was found to be 0·100. This is a slightly greater value of _K_ than the lowest value 0·07 found by Rutherford and Miss Brooks. No mention is made by Curie and Danne of having taken any special precautions against temperature disturbances, and this may account for the higher value of _K_ obtained by them.

They also found that the emanation, like a gas, always divided itself between two reservoirs, put in connection with one another, in the proportion of their volumes. In one experiment one reservoir was kept at a temperature of 10° C. and the other at 350° C. The emanation divided itself between the two reservoirs in the same proportion as would a gas under the same conditions.

=162.= For the purpose of comparison, a few of the coefficients of inter-diffusion of gases, compiled from Landolt and Bernstein’s tables, are given below.

Gas or vapour Coefficient of Molecular
diffusion into weight
air

Water vapour 0·198 18

Carbonic acid 0·142 44
gas

Alcohol vapour 0·101 46

Ether vapour 0·077 74

Radium 0·07 ?
emanation

The tables, although not very satisfactory for the purpose of comparison, show that the coefficient of inter-diffusion follows the inverse order of the molecular weights. The value of _K_ for the radium emanation is slightly less than for ether vapour, of which the molecular weight is 74. We may thus conclude that the emanation is of greater molecular weight than 74. It seems likely that the emanation has a molecular weight somewhere in the neighbourhood of 100, and is probably greater than this, for the vapours of ether and alcohol have higher diffusion coefficients compared with carbonic acid than the theory would lead us to anticipate. Comparing the diffusion coefficients of the emanation and carbonic acid into air, the value of the molecular weight of the emanation should be about 176 if the result observed for the simple gases, viz. that the coefficient of diffusion is inversely proportional to the square root of the molecular weights, holds true in the present case. Bumstead and Wheeler[257] compared the rates of diffusion of the radium emanation and of carbon dioxide through a porous plate, and concluded that the molecular weight of the emanation was about 180. On the disintegration theory, the atom of the emanation is derived from the radium atom by the expulsion of one α particle. Thus, it is to be expected that its molecular weight would be over 200.

It is of interest to compare the value of _K_ = ·07 with the value of _K_ determined by Townsend (section 37) for the gaseous ions produced in air at ordinary pressure and temperature, by Röntgen rays or by the radiations from active substances. Townsend found that the value of _K_ in dry air was ·028 for the positive ions and ·043 for the negative ions. The radium emanation thus diffuses more rapidly than the ions produced by its radiation in the gas, and behaves as if its mass were smaller than that of the ions produced in air, but considerably greater than that of the air molecules with which it is mixed.

It is not possible to regard the emanation as a temporarily modified condition of the gas originally in contact with the active body. Under such conditions a much larger value of _K_ would be expected. The evidence derived from the experiments on diffusion strongly supports the view that the emanation is a gas of heavy molecular weight.

Makower[258] has recently attacked the question of the molecular weight of the radium emanation by another method. The rate of diffusion of the emanation through a porous plug of plaster-of-Paris was compared with that of the gases oxygen, carbon dioxide, and sulphur dioxide. It was found that Graham’s law, viz. that the coefficient of diffusion _K_ is inversely proportional to the square root of its molecular weight _M_, was not strictly applicable. The value of _K_ √_M_ was not found to be constant for these gases, but decreased with increase of molecular weight of the gas. If, however, a curve was plotted with _K_ √_M_ as ordinate and _K_ as abscissa, the points corresponding to the values of O, CO₂ and SO₂ were found to lie on a straight line. By linear extrapolation, the molecular weight of the emanation was estimated. The value obtained from experiments on three different porous plugs was 85·5, 97, and 99 respectively. This method indicates that the molecular weight of the radium emanation is about 100; but in all the experiments on diffusion, it must be remembered that the emanation, whose rate of inter-diffusion is being examined, exists in minute quantity mixed with the gas, and is compared with the rate of inter-diffusion of gases which are present in large quantity. For this reason, deductions of the molecular weight of the emanation may be subject to comparatively large errors, for which it is difficult to make correction.

Diffusion of the Thorium Emanation.

=163.= On account of the rapid decay of the activity of the thorium emanation, it is not possible to determine the value of _K_ its coefficient of diffusion into air by the methods employed for the radium emanation. The value of _K_ has been determined by the writer in the following way. A plate _C_, Fig. 57, covered with thorium hydroxide, was placed horizontally near the base of a long vertical brass cylinder _P_. The emanation released from the thorium compound diffuses upwards in the cylinder.

Let _p_ be the partial pressure of the emanation at a distance _x_ from the source _C_. This will be approximately uniform over the cross section of the cylinder. From the general principles of diffusion we get the equation

_d²p_ _dp_
_K_ ----- = − ---- .
_dx²_ _dt_

The emanation is continuously breaking up and expelling α particles. The emanation-residue gains a positive charge, and, in an electric field, is removed at once from the gas to the negative electrode.

Since the activity of the emanation at any time is always proportional to the number of particles which have not broken up, and since the activity decays with the time according to an exponential law,

$$ p = p_1 e^{–λt} $$,

where _p₁_ is the value of _p_ when _t_ = 0 and λ is the _radio-active constant_ of the emanation.

Then

_dp_
---- = -λ_p_,
_dt_

and

_d²p_
K ----- = λ_p_.
_dx²_

Thus

$$ p = Ae^{-\sqrt {\frac {λ} {K} x}} + Be^{\sqrt {\frac {λ} {K} x}} $$ .

Since _p_ = 0 when _x_ = infinity, _B_ = 0. If _p_ = _p₀_ when _x_ = 0, _A_ = _p₀_.

Thus

$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .

It was not found convenient in the experiments to determine the activity of the emanation along the cylinder, but an equivalent method was used which depends upon measuring the distribution of “excited activity,” produced along a central rod _AB_, which is charged negatively.

It will be shown later (section 177) that the amount of excited activity at any point is always proportional to the amount of emanation at that point. The distribution of “excited activity” along the central rod from the plate _C_ upwards thus gives the variation of _p_ for the emanation along the tube.

In the experiments, the cylinder was filled with dry air at atmospheric pressure and was kept at a constant temperature. The central rod was charged negatively and exposed from one to two days in the presence of the emanation. The rod was then removed, and the distribution of the excited activity along it determined by the electric method. It was found that the amount of excited activity fell off with the distance _x_ according to an exponential law, falling to half value in about 1·9 cms. This is in agreement with the above theory.

Since the activity of the emanation falls to half value in 1 minute, λ = ·0115. The value _K_ = ·09 was deduced from the average of a number of experiments. This is a slightly greater value than _K_ = ·07, obtained for the radium emanation, but the results show that the two emanations do not differ much from one another in molecular weight.

Makower (_loc. cit._) compared the rates of diffusion of the thorium and radium emanation through a porous plate, and concluded that the two emanations were of about the same molecular weight, thus confirming the results obtained by the above method.

Diffusion of the Emanation into Liquids.

=164.= Experiments have been made by Wallstabe[259] on the coefficient of diffusion of the radium emanation into various liquids. The radium emanation was allowed to diffuse into a closed reservoir, containing a cylinder of the liquid under observation. The cylinder was provided with a tube and a stop-cock extending beyond the closed vessel, so that different layers of the liquid could be removed. The liquid was then placed in a closed testing vessel, where the ionization current due to the escape of the emanation from the liquid was observed to rise to a maximum after several hours, and then to decay. This maximum value of the current was taken as a measure of the amount of emanation absorbed in the liquid.

The coefficient of diffusion _K_ of the emanation into the liquid can be obtained from the same equation used to determine the diffusion of the thorium emanation into air,

$$ p = p₀ e^{-\sqrt {\frac {λ} {K} x}} $$ .

where λ is the constant of decay of activity of the radium emanation and _x_ the depth of the layer of water from the surface.

Putting

$$ a = \sqrt {\frac {λ} {K}} $$,

it was found that

for water α = 1·6,
for toluol α = ·75.

The value of λ expressed in terms of a day as the unit of time is about ·17.

Thus the value of _K_ for the diffusion of the radium emanation into water = ·066 cm.² / day.

The value of _K_ found by Stefan[260] for the diffusion of carbon dioxide into water was 1·36 cm.²/day. These results are thus in harmony with the conclusion drawn from the diffusion of the radium emanation into air, and show that the radium emanation behaves as a gas of high molecular weight.

Condensation of the Emanations.

=165. Condensation of the emanations.= During an investigation of the effect of physical and chemical agencies on the thorium emanation, Rutherford and Soddy[261] found that the emanation passed unchanged in amount through a white-hot platinum tube and through a tube cooled to the temperature of solid carbon dioxide. In later experiments the effects of still lower temperatures were examined, and it was then found that at the temperature of liquid air both emanations were condensed[262].

If either emanation is conveyed by a slow stream of hydrogen, oxygen, or air through a metal spiral immersed in liquid air, and placed in connection with a testing vessel as in Fig. 51, no trace of emanation escapes in the issuing gas. When the liquid air is removed and the spiral plunged into cotton-wool, several minutes elapse before any deflection of the electrometer needle is observed, and then the condensed emanation volatilizes rapidly, and the movement of the electrometer needle is very sudden, especially in the case of radium. With a fairly large amount of radium emanation, under the conditions mentioned, a very few seconds elapse after the first sign of movement before the electrometer needle indicates a deflection of several hundred divisions per second. It is not necessary in either case that the emanating compound should be retained in the gas stream. After the emanation is condensed in the spiral, the thorium or radium compound may be removed and the gas stream sent directly into the spiral. But in the case of thorium, under these conditions, the effects observed are naturally small owing to the rapid loss of the activity of the emanation with time, which proceeds at the same rate at the temperature of liquid air as at ordinary temperatures.

If a large amount of radium emanation is condensed in a glass =U= tube, the progress of the condensation can be followed by the eye, by means of the phosphorescence which the radiations excite in the glass. If the ends of the tube are sealed and the temperature allowed to rise, the glow diffuses uniformly throughout the tube, and can be concentrated at any point to some extent by local cooling of the tube with liquid air.

=166. Experimental arrangements.= A simple experimental arrangement to illustrate the condensation and volatilization of the emanation and some of its characteristic properties is shown in Fig. 58. The emanation obtained from a few milligrams of radium bromide by solution or heating is condensed in the glass =U= tube _T_ immersed in liquid air. This =U= tube is then put into connection with a larger glass tube _V_, in the upper part of which is placed a piece of zinc sulphide screen _Z_, and in the lower part of the tube a piece of the mineral willemite. The stop-cock _A_ is closed and the =U= tube and the vessel _V_ are partially exhausted by a pump through the stop-cock _B_. This lowering of the pressure causes a more rapid diffusion of the emanation when released. The emanation does not escape if the tube _T_ is kept immersed in liquid air. The stop-cock _B_ is then closed, and the liquid air removed. No luminosity of the screen or the willemite in the tube _V_ is observed for several minutes, until the temperature of _T_ rises above the point of volatilization of the emanation. The emanation is then rapidly carried into the vessel _V_, partly by expansion of the gas in the tube _T_ with rising temperature, and partly by the process of diffusion. The screen _Z_ and the willemite _W_ are caused to phosphoresce brilliantly under the influence of the rays from the emanation surrounding them.

If the end of the vessel _V_ is then plunged into liquid air, the emanation is again condensed in the lower end of the tube, and the willemite phosphoresces much more brightly than before. This is not due to an increase of the phosphorescence of willemite at the temperature of the liquid air, but to the effect of the rays from the emanation condensed around it. At the same time the luminosity of the zinc sulphide gradually diminishes, and practically disappears after several hours if the end of the tube is kept in the liquid air. If the tube is removed from the liquid air, the emanation again volatilizes and lights up the screen _Z_. The luminosity of the willemite returns to its original value after the lapse of several hours. This slow change of the luminosity of the zinc sulphide screen and of the willemite is due to the gradual decay of the “excited activity” produced by the emanation on the surface of all bodies exposed to its action (chapter VIII). The luminosity of the screen is thus due partly to the radiation from the emanation and partly to the excited radiation caused by it. As soon as the emanation is removed from the upper to the lower part of the tube, the “excited” radiation gradually diminishes in the upper and increases in the lower part of the tube.

The luminosity of the screen gradually diminishes with the time as the enclosed emanation loses its activity, but is still appreciable after an interval of several weeks.

An apparatus of a similar character to illustrate the condensation of the radium emanation has been described by P. Curie[263].

=167. Determination of the temperature of condensation.= A detailed investigation was made by Rutherford and Soddy (_loc. cit._) of the temperatures at which condensation and volatilization commenced for the two emanations. The experimental arrangement of the first method is shown clearly in Fig. 59. A slow constant stream of gas, entering at _A_, was passed through a copper spiral _S_, over 3 metres in length, immersed in a bath of liquid ethylene. The copper spiral was made to act as its own thermometer by determining its electrical resistance. The resistance temperature curve was obtained by observation of the resistances at 0°, the boiling point of liquid ethylene −103·5°, the solidification point of ethylene −169° and in liquid air. The temperature of the liquid air was deduced from the tables given by Baly for the boiling point of liquid air for different percentages of oxygen. The resistance-temperature curve, for the particular spiral employed, was found to be nearly a straight line between 0° and −192°C., cutting the temperature axis if produced nearly at the absolute zero. The resistance of the spiral, deduced from readings on an accurately calibrated Weston millivoltmeter, with a constant current through the spiral, was thus very approximately proportional to the absolute temperature. The liquid ethylene was kept vigorously stirred by an electric motor, and was cooled to any desired temperature by surrounding the vessel with liquid air.

The general method employed for the radium emanation was to pass a suitable amount of emanation, mixed with the gas to be used, from the gas holder _B_ into the spiral, cooled below the temperature of condensation. After the emanation was condensed in the spiral, a current of electrolytic hydrogen or oxygen was passed through the spiral. The temperature was allowed to rise gradually, and was noted at the instant when a deflection of the electrometer, due to the presence of emanation in the testing vessel _T_, was observed. The resistance, subject to a slight correction due to the time taken for the emanation to be carried into the testing vessel, gave the temperature at which some of the emanation commenced to volatilize. The ionization current in the testing vessel rose rapidly to a maximum value, showing that, for a small increase of temperature, the whole of the radium emanation was volatilized. The following table gives an illustration of the results obtained for a current of hydrogen of 1·38 cubic centimetres per second.

Temperature Divisions per
second of the
electrometer

−160° 0

−156° 0

−154°·3 1

−153°·8 21

−152°·5 24

The following table shows the results obtained for different currents of hydrogen and oxygen.

Current of Gas _T₁_ _T₂_

Hydrogen ·25 c.c. per sec. −151·3 −150
„ ·32 „ „ −153·7 −151
„ ·92 „ „ −152 −151
„ 1·38 „ „ −154 −153
„ 2·3 „ „ −162·5 −162
Oxygen ·34 „ „ −152·5 −151·5
„ ·58 „ „ −155 −153

The temperature _T₁_ in the above table gives the temperature of initial volatilization, _T₂_ the temperature for which half of the condensed emanation had been released. For slow currents of hydrogen and oxygen, the values of _T₁_ and _T₂_ are in good agreement. For a stream of gas as rapid as 2·3 cubic centimetres per second the value of _T₁_ is much lower. Such a result is to be expected; for, in too rapid a stream, the gas is not cooled to the temperature of the spiral, and, in consequence, the inside surface of the spiral is above the mean temperature, and some of the emanation escapes at a temperature apparently much lower. In the case of oxygen, this effect appears for a gas stream of 0·58 cubic centimetres per second.

In the experiments on the thorium emanation, on account of the rapid loss of activity, a slightly different method was necessary. The steady stream of gas was passed over the thorium compound, and the temperature was observed at the instant when an appreciable movement of the electrometer appeared. This gave the temperature at which a small fraction of the thorium emanation escaped condensation, and not the value _T₁_ observed for the radium emanation, which gave the temperature for which a small fraction of the previously condensed emanation was volatilized.

The following table illustrates the results obtained.

Current of Gas Temperature

Hydrogen ·71 c.c. per −155° C.
sec.

„ 1·38 „ „ −159° C.

Oxygen ·58 „ „ −155° C.

On comparing these results with the values obtained for the radium emanation, it will be observed that with equal gas streams the temperatures are nearly the same.

A closer examination of the thorium emanation showed, however, that this apparent agreement was only accidental, and that there was, in reality, a very marked difference in the effect of temperature on the two emanations. It was found experimentally that the radium emanation was condensed very near the temperature at which volatilization commenced, and that the points of condensation and volatilization were defined fairly sharply.

On the other hand, the thorium emanation required a range of over 30° C. after condensation had started in order to ensure complete condensation. Fig. 60 is an example of the results obtained with a steady gas stream of 1·38 c.c. per sec. of oxygen. The ordinates represent the percentage proportion of the emanation uncondensed at different temperatures. It will be observed that condensation commences about −120°, and that very little of the emanation escapes condensation at −155° C.

To investigate this difference of behaviour in the two emanations, a static method was employed, which allowed an examination of the two emanations to be made under comparable conditions. The emanation, mixed with a small amount of the gas to be used, was introduced into the cool spiral, which had been exhausted previously by means of a mercury pump. The amount of emanation remaining uncondensed after definite intervals was rapidly removed by means of the pump, and was carried with a constant auxiliary stream of gas into the testing vessel.

Tested in this way, it was found that the volatilization point of the radium emanation was very nearly the same as that obtained by the blowing method, viz. −150° C. With thorium, on the other hand, the condensation started at about −120° C., and, as in the blowing method, continued over a range of about 30° C. The proportion of the emanation condensed at any temperature was found to depend on a variety of conditions, although the point at which condensation commenced, viz. −120° C., was about the same in each case. It depended on the pressure and nature of the gas, on the concentration of the emanation, and on the time for which it was left in the spiral. For a given temperature a greater proportion of the emanation was condensed, the lower the pressure and the longer the time it was left in the spiral. Under the same conditions, the emanation was condensed more rapidly in hydrogen than in oxygen.

=168.= Thus there is no doubt that the thorium emanation begins to condense at a temperature higher than that at which the radium emanation condenses. The explanation of the peculiar behaviour of the thorium emanation is clear when the small number of emanation particles present in the gas are taken into consideration. It has been shown that both emanations give out only α rays. It is probable that the α particles from the two emanations are similar in character and produce about the same number of ions in their passage through the gas. The number of ions produced by each α particle before its energy is dissipated is probably about 70,000. (See section 252.)

Now, in the experiment, the electrometer readily measured a current of 10⁻³ electrostatic units. Taking the charge on an ion as 3·4 × 10⁻¹⁰ electrostatic units, this corresponds to a production in the testing vessel of about 3 × 10⁶ ions per sec., which would be produced by about 40 expelled α particles per second. Each radiating particle cannot expel less than one α particle and may expel more, but it is likely that the number expelled by an atom of the thorium emanation is not greatly different from that expelled by an atom of the radium emanation.

In section 133 it has been shown that, according to the law of decay, λ_N_ particles change per second when _N_ are present. Thus, to produce 40 α particles, λ_N_ cannot be greater than 40. Since for the thorium emanation λ is ¹⁄₈₇, it follows that _N_ cannot be greater than 3500. The electrometer thus detected the presence of 3500 particles of the thorium emanation, and since in the static method the volume of the condensing spiral was about 15 c.c., this corresponded to a concentration of about 230 particles per c.c. An ordinary gas at atmospheric pressure and temperature probably contains about 3·6 × 10¹⁹ molecules per c.c. Thus the emanation would have been detected on the spiral if it had possessed a partial pressure of less than 10⁻¹⁷ of an atmosphere.

It is not surprising then that the condensation point of the thorium emanation is not sharply defined. It is rather a matter of remark that condensation should occur so readily with so sparse a distribution of emanation particles in the gas; for, in order that condensation may take place, it is probable that the particles must approach within one another’s sphere of influence.

Now in the case of the radium emanation, the rate of decay is about 5000 times slower than that of the thorium emanation, and consequently the actual number of particles that must be present to produce the same ionization per second in the two cases must be about 5000 times greater in the case of radium than in the case of thorium. This conclusion involves only the assumption that the same number of rays is produced by a particle of emanation in each case, and that the expelled particles produce in their passage through the gas the same number of ions. The number of particles present, in order to be detected by the electrometer, in this experiment, must therefore have been about 5000 × 3500, _i.e._ about 2 × 10⁷. The difference of behaviour in the two cases is well explained by the view that, _for equal electrical effects_, the number of radium emanation particles must be far larger than the number of thorium emanation particles. The probability of the particles coming into each other’s sphere of influence will increase very rapidly as the concentration of the particles increases, and, in the case of the radium emanation, once the temperature of condensation is attained, all but a small proportion of the total number of particles present will condense in a very short time. In the case of the thorium emanation, however, the temperature might be far below that of condensation, and yet a considerable portion remain uncondensed for comparatively long intervals. On this view the experimental results obtained might reasonably be expected. A greater proportion of emanation condenses the longer the time allowed for condensation under the same conditions. The condensation occurs more rapidly in hydrogen than in oxygen, as the diffusion is greater in the former gas. For the same reason the condensation occurs faster the lower the pressure of the gas present. Finally, when the emanation is carried by a steady stream of gas, a smaller proportion condenses than in the other cases, because the concentration of emanation particles per unit volume of gas is less under these conditions.

It is possible that the condensation of the emanations may not occur in the gas itself but at the surface of the containing vessel. Accurate observations of the temperature of condensation have so far only been made in a copper spiral, but condensation certainly occurs in tubes of lead or glass at about the same temperature as in tubes of copper.

=169.= In experiments that were made by the static method with a very large quantity of radium emanation, a slight amount of escape of the condensed emanation was observed several degrees below the temperature at which most of the emanation was released. This is to be expected, since, under such conditions, the electrometer is able to detect a very minute proportion of the whole quantity of the emanation condensed.

Special experiments, with a large quantity of emanation, that were made with the spiral immersed in a bath of rapidly boiling nitric oxide, showed this effect very clearly. For example, the condensed emanation began to volatilize at −155° C. In 4 minutes the temperature had risen to −153·5°, and the amount volatilized was four times as great as at −155°. In the next 5½ minutes the temperature had increased to −152·3° and practically the whole quantity, which was at least fifty times the amount at the temperature of −153·5°, had volatilized.

It thus seems probable that, if the temperature were kept steady at the point at which volatilization was first observed, and the released emanation removed at intervals, the whole of the emanation would in course of time be liberated at that temperature. Curie and Dewar and Ramsay have observed that the emanation condensed in a =U= tube, immersed in liquid air, slowly escapes if the pump is kept steadily working. These results point to the probability that the condensed emanation possesses a true vapour pressure, but great refinements in experimental methods would be necessary before such a conclusion could be definitely established.

The true temperature of condensation of the thorium emanation is probably about −120° C., and that of radium about −150° C. Thus there is no doubt that the two emanations are quite distinct from each other in this respect, and also with regard to their radio-activity, although they both possess the property of chemical inertness. These results on the temperatures of condensation do not allow us to make any comparison of the condensation points of the emanations with those of known gases, since the lowering of the condensation points of gases with diminution of pressure has not been studied at such extremely minute pressures.

=170.= It has been found[264] that the activity of the thorium emanation, when condensed in the spiral at the temperature of liquid air, decayed at the same rate as at ordinary temperatures. This is in accord with results of a similar kind obtained by P. Curie for the radium emanation (section 145), and shows that the value of the radio-active constant is unaffected by wide variations of temperature.

Amount of Emanation from Radium and Thorium.

=171.= It has been shown in section 93 from experimental data that 1 gram of radium bromide at its minimum activity emits about 3·6 × 10¹⁰ α particles per second. Since the activity due to the emanation stored up in radium, when in a state of radio-active equilibrium, is about one quarter of the whole and about equal to the minimum activity, the number of α particles projected per second by the emanation from 1 gram of radium bromide is about 3·6 × 10¹⁰. It has been shown in section 152 that 463,000 times the amount of emanation produced per second is stored up in the radium. But, in a state of radio-active equilibrium, the number of emanation particles breaking up per second is equal to the number produced per second. Assuming that each emanation particle in breaking up expels one α particle, it follows that the number of emanation particles present in 1 gram of radium bromide in radio-active equilibrium is 463,000 × 3·6 × 10¹⁰, _i.e._ 1·7 × 10¹⁶. Taking the number of hydrogen molecules in 1 c.c. of gas at atmospheric pressure and temperature as 3·6 × 10¹⁹ (section 39), the volume of the emanation from 1 gram of radium bromide is 4·6 × 10⁻⁴ cubic centimetres at atmospheric pressure and temperature. Assuming the composition of radium bromide as RaBr₂, the amount from 1 gram of radium in radio-active equilibrium is 0·82 cubic millimetres. Quite independently of any method of calculation it was early evident that the volume of the emanation was very small, for all the earlier attempts made to detect its presence by its volume were unsuccessful. It will be seen, however, that, when larger quantities of radium were available for experiment, the emanation has been collected in volume sufficiently large to measure.

In the case of thorium, the maximum quantity of emanation to be obtained from 1 gram of the solid is very minute, both on account of the small activity of thorium and of the rapid break up of the emanation after its production. Since the amount of emanation, stored in a non-emanating thorium compound, is only 87 times the rate of production, while in radium it is 463,000 times, and the rate of production of the emanation by radium is about 1 million times faster than by thorium, it follows that the amount of emanation to be obtained from 1 gram of thorium is not greater than 10⁻¹⁰ of the amount from an equal weight of radium, _i.e._ its volume is not greater than 10⁻¹³ c.c. at the ordinary pressure and temperature. Even with large quantities of thorium, the amount of emanation is too small ever to be detected by its volume.

=172. Volume of the emanation from radium.= The evidence already considered points very strongly to the conclusion that the emanation possesses all the properties of a chemically inert gas of high molecular weight.

Since the emanation continuously breaks up, and is transformed into a solid type of matter, which is deposited on the surface of bodies, the volume of the emanation, when separated from radium, should contract at the same rate as it loses its activity, _i.e._ it should decrease to half value in about four days. The amount of emanation to be obtained from a given quantity of radium is a maximum when the rate of production of new emanation balances its rate of change. This condition is practically attained when the emanation has been allowed to collect for an interval of one month. The probable volume of the emanation to be obtained from 1 gram of radium was early calculated on certain assumptions, and from data then available the writer[265] deduced that the volume of the emanation from 1 gram of radium lay between ·06 and ·6 cubic millimetre at atmospheric pressure and temperature, and was probably nearer the latter value. The volume to be expected on the latest data has been discussed in the preceding section and shown to be about ·82 cubic mm. The volume of the emanation is thus very small, but not too small to be detected if several centigrams of radium are available. This has been proved to be the case by Ramsay and Soddy[266] who, by very careful experiment, finally succeeded in isolating a small quantity of the emanation and in determining its volume. The experimental method employed by them will now be briefly described.

The emanation from 60 milligrams of radium bromide in solution was allowed to collect for 8 days and then drawn off through the inverted siphon _E_ (Fig. 61) into the explosion burette _F_. This gas consisted for the most part of hydrogen and oxygen, produced by the action of the radiations on the water of the solution. After explosion, the excess of hydrogen mixed with emanation was left some time in contact with caustic soda, placed in the upper part of the burette, in order to remove all trace of carbon dioxide. In the meantime the upper part of the apparatus had been completely evacuated. The connection _C_ to the pump was closed, and the hydrogen and emanation were allowed to enter the apparatus, passing over a phosphorous pentoxide tube _D_. The emanation was condensed in the lower part of the capillary tube _A_, by surrounding it with the tube _B_ filled with liquid air. The process of condensation was rendered manifest by the brilliant luminosity of the lower part of the tube. The mercury from the burette was then allowed to run to _G_, and the apparatus again completely evacuated. The connection of the pump was again closed, the liquid air was removed and the volatilized emanation forced into the fine capillary tube _A_. Observations were then made, from day to day, of the volume of the emanation. The results are given in the table below.

Time Volume Time Volume

Start 0·124 cub. 7 days 0·0050 cub. mm.
mm.

1 day 0·027 „ 9 „ 0·0041 „

3 „ 0·011 „ 11 „ 0·0020 „

4 „ 0·0095 „ 12 „ 0·0011 „

6 „ 0·0063 „ 28 „ 0·0004 „

The volume contracted with the time, and was very small after a month’s interval, but the minute bubble of the emanation still retained its luminosity to the last. The tube became deep purple in colour, which rendered readings difficult except with a strong light. There was a sudden decrease in the first day, which may have been due to the mercury sticking in the capillary tube.