Radio-Activity

138. Introduction. A most important and striking property possessed by radium, thorium, and actinium, but not by uranium or polonium, is the power of continuously emitting into the surrounding space a material emanation, which has all the properties of a radio-active gas. This emanation is able to diffuse rapidly through gases and through porous substances, and may be separated from the gas with which it is mixed by condensation by the action of extreme cold. This emanation forms a connecting link between the activity of the radio-elements themselves and their power of exciting activity on surrounding objects, and has been studied more closely than the other active products on account of its existence in the gaseous state. The emanations from the three active bodies all possess similar radio-active properties, but the effects are more marked in the case of the emanation from radium, on account of the very great activity of that element.

Thorium Emanation.

139. Discovery of the emanation. In the course of examination of the radiations of thorium, several observers had noted that some of the thorium compounds, and especially the oxide, were very inconstant sources of radiation, when examined in open vessels by the electrical method. Owens^[231] found that this inconstancy was due to the presence of air currents. When a closed vessel was used, the current, immediately after the introduction of the active matter, increased with the time, and finally reached a constant value. By drawing a steady stream of air through the vessel the value of the current was much reduced. It was also observed that the radiations could apparently pass through large thicknesses of paper, which completely absorbed the ordinary α radiation.

In an investigation of these peculiar properties of thorium compounds, the writer^[232] found that the effects were due to an emission of radio-active particles of some kind from the thorium compounds. This “emanation,” as it was termed for convenience, possesses the properties of ionizing the gas and acting on a photographic plate, and is able to diffuse rapidly through porous substances like paper and thin metal foil.

The emanation, like a gas, is completely prevented from escaping by covering the active matter with a thin plate of mica. The emanation can be carried away by a current of air; it passes through a plug of cotton-wool and can be bubbled through solutions without any loss of activity. In these respects, it behaves very differently from the ions produced in the gas by the rays from active substances, for these give up their charges completely under the same conditions.

Since the emanation passes readily through large thicknesses of cardboard, and through filters of tightly packed cotton-wool, it does not seem likely that the emanation consists of particles of dust given off by the active matter. This point was tested still further by the method used by Aitken and Wilson, for detecting the presence of dust particles in the air. The oxide, enclosed in a paper cylinder, was placed in a glass vessel, and the dust was removed by repeated small expansions of the air over a water surface. The dust particles act as nuclei for the formation of small drops and are then removed from the air by the action of gravity. After repeated expansions, no cloud was formed, and the dust was considered to be removed. After waiting for some time to allow the thorium emanation to collect, further expansions were made but no cloud resulted, showing that for the small expansions used, the particles were too small to become centres of condensation. The emanation then could not be regarded as dust emitted from thorium.

Since the power of diffusing rapidly through porous substances, and acting on a photographic plate, is also possessed by a chemical substance like hydrogen peroxide, some experiments were made to see if the emanation could be an agent of that character. It was found, however, that hydrogen peroxide is not radio-active, and that its action on the plate is a purely chemical one, while it is the radiation from the emanation and not the emanation itself that produces ionizing and photographic effects.

140. Experimental arrangements. The emanation from thorium is given off in minute quantity. No appreciable lowering of the vacuum is observed when an emanating compound is placed in a vacuum tube and no new spectrum lines are observed.

For an examination of the emanation, an apparatus similar in principle to that shown in Fig. 51 is convenient.

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.

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.

The rate of decay has been more accurately determined by Rossignol and Gimingham^[233] who found that the activity fell to half value in about 51 seconds. Bronson^[234], using the steady deflection method described in section 69, found the corresponding time 54 seconds.

The decrease of the current with the time is an actual measure of the decrease of the activity of the emanation, and is not in any way influenced by the time that the ions produced take to reach the electrodes. If the ions had been produced from a uranium compound the duration of the conductivity for a saturation voltage would only have been a fraction of a second.

The rate of decay of the activity of the emanation is independent of the electromotive force acting on the gas. This shows that the radio-active particles are not destroyed by the electric field. The current through the gas at any particular instant, after stoppage of the flow of air, was found to be the same whether the electromotive force had been acting the whole time or had been just applied for the time of the test.

The emanation itself is unaffected by a strong electric field and so cannot be charged. By testing its activity after passing it through long concentric cylinders, charged to a high potential, it was found that the emanation certainly did not move with a velocity greater than ·00001 cm. per second, for a gradient of 1 volt per cm., and there was no evidence to show that it moved at all. This conclusion has been confirmed by the experiments of McClelland^[235].

The rate at which the emanation is produced is independent of the gas surrounding the active matter. If in the apparatus of Fig. 51 air is replaced by hydrogen, oxygen, or carbonic acid, similar results are obtained, though the current observed in the testing vessel varies for the different gases on account of the unequal absorption by them of the radiation from the emanation.

If a thorium compound, enclosed in paper to absorb the α radiation, is placed in a closed vessel, the saturation current due to the emanation is found to vary directly as the pressure. Since the rate of ionization is proportional to the pressure for a constant source of radiation, this experiment shows that the rate of emission of the emanation is independent of the pressure of the gas. The effect of pressure on the rate of production of the emanation is discussed in more detail later in section 157.

142. Effect of thickness of layer. The amount of emanation emitted by a given area of thorium compound depends on the thickness of the layer. With a very thin layer, the current between two parallel plates, placed in a closed vessel as in Fig. 17, is due very largely to the α rays. Since the α radiation is very readily absorbed, the current due to it practically reaches a maximum when the surface of the plate is completely covered by a thin layer of the active material. On the other hand the current produced by the emanation increases until the layer is several millimetres in thickness, and then is not much altered by adding fresh active matter. This falling off of the current after a certain thickness has been reached is to be expected, since the emanation, which takes several minutes to diffuse through the layer above it, has already lost a large proportion of its activity.

With a thick layer of thorium oxide in a closed vessel, the current between the plates is largely due to the radiation from the emanation lying between the plates. The following tables illustrate the way in which the current varies with the thickness of paper for both a thin and a thick layer.

Table I. Thin Layer.

Thickness of sheets of paper ·0027.

Table II. Thick Layer.

Thickness of paper ·008 cm.

The initial current with the unscreened compound is taken as unity. In Table I, for a thin layer of thorium oxide, the current diminished rapidly with additional layers of thin paper. In this case the current is due almost entirely to the α rays. In Table II the current falls to ·74 for the first layer. In this case about 26% of the current is due to the α rays, which are practically absorbed by the layer ·008 cm. in thickness. The slow decrease with additional layers shows that the emanation diffuses so rapidly through a few layers of paper that there is little loss of activity during the passage. The time taken to diffuse through 20 layers is however appreciable, and the current consequently has decreased. After passing through a layer of cardboard 1·6 mms. in thickness the current is reduced to about one-fifth of its original value. In closed vessels the proportion of the total current, due to the emanation, varies with the distance between the plates as well as with the thickness of the layer of active material. It also varies greatly with the compound examined. In the nitrate, which gives off only a small amount of emanation, the proportion is very much smaller than in the hydroxide, which gives off a large amount of emanation.

143. Increase of current with time. The current due to the emanation does not reach its final value for some time after the active matter has been introduced into the closed vessel. The variation with time is shown in the following table. The saturation current due to thorium oxide, covered with paper, was observed between concentric cylinders of 5·5 cms. and ·8 cm. diameter.

Immediately before observations on the current were made, a rapid stream of air was blown through the apparatus. This removed most of the emanation. However, the current due to the ionization of the gas by the emanation, as it was carried along by the current of air, was still appreciable. The current consequently does not start from zero.

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

and the recovery curve by

where λ is the radio-active constant of the emanation.

This relation is to be expected, since the decay and recovery curves of the emanation are determined by exactly the same conditions as the decay and recovery curves of Ur X and Th X. In both cases there is:

(1) A supply of fresh radio-active particles produced at a constant rate.

(2) A loss of activity of the particles following an exponential law with the time.

In the case of Ur X and Th X, the active matter produced manifests its activity in the position in which it is formed; in this new phenomenon, a proportion of the active matter in the form of the emanation escapes into the surrounding gas. The activity of the emanation, due to a thorium compound kept in a closed vessel, thus reaches a maximum when the rate of supply of fresh emanation particles from the compound is balanced by the rate of change of those already present. The time for recovery of half the final activity is about 1 minute, the same as the time taken for the emanation, when left to itself, to lose half its activity.

If q₀ is the number of emanation particles escaping into the gas per second, and N₀ the final number when radio-active equilibrium is reached, then (section 133),

Since the activity of the emanation falls to half value in 1 minute

and N₀ = 87q₀, or the number of emanation particles present when a steady state is reached is 87 times the number produced per second.

Radium Emanation.

144. Discovery of the emanation. Shortly after the discovery of the thorium emanation, Dorn^[236] repeated the results, and, in addition, showed that radium compounds also gave off radio-active emanations, and that the amount given off was much increased by heating the compound. The radium emanation differs from the thorium emanation in the rate at which it loses its activity. It decays far more slowly, but in other respects the emanations of thorium and radium have much the same properties. Both emanations ionize the gas with which they are mixed, and affect a photographic plate. Both diffuse readily through porous substances but are unable to pass through a thin plate of mica; both behave like a temporarily radio-active gas, mixed in minute quantity with the air or other gas in which they are conveyed.

145. Decay of activity of the emanation. Very little emanation escapes from radium chloride in the solid state, but the amount is largely increased by heating, or by dissolving the compound in water. By bubbling air through a radium chloride solution, or passing air over a heated radium compound, a large amount of emanation may be obtained which can be collected, mixed with air, in a suitable vessel.

Experiments to determine accurately the rate of decay of activity of the emanation have been made by P. Curie^[237], and Rutherford and Soddy^[238]. In the experiments of the latter, the emanation mixed with air was stored over mercury in an ordinary gas-holder. From time to time, equal quantities of air mixed with the emanation were measured off by a gas pipette and delivered into a testing vessel. The latter consisted of an air-tight brass cylinder carrying a central insulated electrode. A saturation voltage was applied to the cylinder, and the inner electrode was connected to the electrometer with a suitable capacity in parallel. The saturation current was observed immediately after the introduction of the active gas into the testing vessel, and was taken as a measure of the activity of the emanation present. The current increased rapidly with the time owing to the production of excited activity on the walls of the containing vessel. This effect is described in detail in chapter VIII.

The measurements were made at suitable intervals over a period of 33 days. The following table expresses the results, the initial activity being taken as 100.

The activity falls off according to an exponential law with the time, and decays to half value in 3·71 days. With the usual notation

the mean value of λ deduced from the results is given by

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.

Now it will be shown later that the emanation itself gives off only α rays, and these rays are completely absorbed by the glass envelope, unless it is made extremely thin. The rays producing ionization in the testing vessel were thus not due to the α rays from the emanation at all, but to the β and γ rays due to the excited activity produced on the walls of the glass tube by the emanation inside it. What was actually measured was thus the decay of the excited activity derived from the emanation, and not the decay of activity of the emanation itself. Since, however, when a steady state is reached, the amount of excited activity is nearly proportional at any time to the activity of the emanation, the rate of decay of the excited activity on the walls of the vessel indirectly furnishes a measure of the rate of decay of the emanation itself. This is only true if the emanation is placed for four or five hours in the tube before observations begin, in order to allow the excited activity time to reach a maximum value.

Using this method P. Curie obtained results similar to those obtained by Rutherford and Soddy by the direct method. The activity decayed according to an exponential law with the time, falling to half value in 3·99 days.

The experiments were performed under the most varied conditions but the rate of decay was found to remain unaltered. The rate of decay did not depend on the material of the vessel containing the emanation or on the nature or pressure of the gas with which the emanation was mixed. It was unaffected by the amount of emanation present, or by the time of exposure to the radium, provided sufficient time had elapsed to allow the excited activity to reach a maximum value before the observations were begun. P. Curie^[239] found that the rate of decay of activity was not altered by exposing the vessel containing the emanation to different temperatures, ranging from +450° to -180° C.

In this respect the emanations of thorium and radium are quite analogous. The rate of decay seems to be unaffected by any physical or chemical agency, and the emanations behave in exactly the same way as the radio-active products Th X and Ur X, already referred to. The radio-active constant λ is thus a fixed and unalterable quantity for both emanations, although in one case its value is about 5000 times greater than in the other.

Emanations from Actinium.

146. Debierne^[240] found that actinium gives out an emanation similar to the emanation of thorium and radium. The loss of activity of the emanation is even more rapid than for the thorium emanation, for its activity falls to half value in 3·9 seconds. In consequence of the rapid decay of activity, the emanation is able to diffuse through the air only a short distance from the active matter before it loses the greater proportion of its activity. Giesel early observed that the radio-active substance separated by him, which we have seen (section 18) is identical in radio-active properties with actinium, gave off a large amount of emanation. It was in consequence of this property, that he gave it the name of the “emanating substance” and later “emanium.” The impure preparations of this substance emit the emanation very freely and in this respect differ from most of the thorium compounds. The emanation from actinium like those from thorium and radium possesses the property of exciting activity on inactive bodies, but it has not yet been studied so completely as the better known emanations of thorium and radium.

Experiments with large amounts of Radium Emanation.

147. With very active specimens of radium a large amount of emanation can be obtained, and the electrical, photographic, and fluorescent effects are correspondingly intense. On account of the small activity of thorium and the rapid decay of its emanation the effects due to it are weak, and can be studied only for a few minutes after its production. The emanation from radium, on the other hand, in consequence of the slow decay of its activity, may be stored mixed with air in an ordinary gas-holder, and its photographic and electrical actions may be examined several days or even weeks after, quite apart from those of the radium from which it was obtained.

It is, in general, difficult to study the radiation due to the emanation alone, on account of the fact that the emanation is continually producing a secondary type of activity on the surface of the vessel in which the emanation is enclosed. This excited activity reaches a maximum value several hours after the introduction of the emanation, and, as long as it is kept in the vessel, this excited activity on the walls decays at the same rate as the emanation itself, i.e. it falls to half its initial value in about 4 days. If, however, the emanation is blown out, the excited activity remains behind on the surface, but rapidly loses its activity in the course of a few hours. After several hours the intensity of the residual radiation is very small.

These effects and their connection with the emanation are discussed more fully in chapter VIII. Giesel^[241] has recorded some interesting observations of the effect of the radium emanation on a screen of phosphorescent zinc sulphide. When a few centigrams of moist radium bromide were placed on a screen any slight motion of the air caused the luminosity to move to and fro on the screen. The direction of phosphorescence could be altered at will by a slow current of air. The effect was still further increased by placing the active material in a tube and blowing the air through it towards the screen. A screen of barium platinocyanide or of Balmain’s paint failed to give any visible light under the same conditions. The luminosity was not altered by a magnetic field, but it was affected by an electric field. If the screen were charged the luminosity was more marked when it was negative than when it was positive.

Giesel states that the luminosity was not equally distributed, but was concentrated in a peculiar ring-shaped manner over the surface of the screen. The concentration of luminosity on the negative, rather than on the positive, electrode is probably due to the excited activity, caused by the emanation, and not to the emanation itself, for this excited activity is concentrated chiefly on the negative electrode in an electric field (see chapter VIII).

An experiment to illustrate the phosphorescence produced in some substances by the rays from a large amount of emanation is described in section 165.

148. Curie and Debierne^[242] have investigated the emanation from radium, and the excited activity produced by it. Some experiments were made on the amount of emanation given off from radium under very low pressures. The tube containing the emanation was exhausted to a good vacuum by a mercury pump. It was observed that a gas was given off from the radium which produced excited activity on the glass walls. This gas was extremely active, and rapidly affected a photographic plate through the glass. It caused fluorescence on the surface of the glass and rapidly blackened it, and was still active after standing ten days. When spectroscopically examined, this gas did not show any new lines, but generally those of the spectra of carbonic acid, hydrogen, and mercury. In the light of the results described in section 124 the gas, given off by the radium, was probably the non-active gases hydrogen and oxygen, in which the active emanation was mixed in minute quantity. It will be shown later (section 242) that the energy radiated from the emanation is enormous compared with the amount of matter involved, and that the effects observed, in most cases, are produced by an almost infinitesimal amount of the emanation.

In further experiments, Curie and Debierne^[243] found that many substances were phosphorescent under the action of the emanation and the excited activity produced by it. In their experiments, two glass bulbs A and B (Fig. 54) were connected with a glass tube. The active material was placed in the bulb A and the substance to be examined in the other.

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

where N_t is the number of emanation particles present at the time t, N₀ the number present when radio-active equilibrium is reached, and λ is the radio-active constant of the emanation. The phosphorescent action, which is due partly to the radiations from the emanation and partly to the excited activity on the walls, should thus reach half the maximum value in four days and should practically reach its limit after three weeks’ interval.

The variation of luminosity with different distances between the screens is to be expected. The amount of excited activity deposited on the boundaries is proportional to the amount of emanation present. Since the emanation is equally distributed, the amount of excited activity deposited on the screens, due to the emanation between them, varies directly as the distance, provided the distance between the screens is small compared with their dimensions. Such a result would also follow if the phosphorescence were due to the radiation from the emanation itself, provided that the pressure of the gas was low enough to prevent absorption of the radiation from the emanation in the gas itself between the screens.

149. Emanating power. The compounds of thorium in the solid state vary very widely in the amount of emanation they emit under ordinary conditions. It is convenient to use the term emanating power to express the amount of emanation given off per second by one gram of the compound. Since, however, we have no means of determining absolutely the amount of emanation present, all measurements of emanating power are of necessity comparative. In most cases, it is convenient to take a given weight of a thorium compound, kept under conditions as nearly as possible constant, and to compare the amount of emanation of the compound to be examined with this standard.

In this way comparisons of the emanating power of thorium compounds have been made by Rutherford and Soddy^[244], using an apparatus similar to that shown in Fig. 51 on page 240.

A known weight of the substance to be tested was spread on a shallow dish, placed in the glass tube C. A stream of dry dust-free air, kept constant during all the experiments, was passed over the compound and carried the emanation into the testing vessel. After ten minutes interval, the current due to the emanation in the testing vessel reached a constant value. The compound was then removed, and the standard comparison sample of equal weight substituted; the saturation current was observed when a steady state was again reached. The ratio of these two currents gives the ratio of the emanating power of the two samples.

It was found experimentally that, for the velocities of air current employed, the saturation current in the testing vessel was directly proportional to the weight of thorium, for weights up to 20 grams. This is explained by the supposition that the emanation is removed by the current of air from the mass of the compound, as fast as it is formed.

By means of this relation the emanating power of compounds which are not of equal weight can be compared.

It was found that thorium compounds varied enormously in emanating power, although the percentage proportion of thorium present in the compound was not very different. For example, the emanating power of thorium hydroxide was generally 3 to 4 times greater than that of ordinary thoria, obtained from the manufacturer. Thorium nitrate, in the solid state, had only ¹⁄₂₀₀ of the emanating power of ordinary thoria, while preparations of the carbonate were found to vary widely among themselves in emanating power, which depended upon slight variations in the method of preparation.

150. Effect of conditions on emanating power. The emanating power of different compounds of thorium and radium is much affected by the alteration of chemical and physical conditions. In this respect the emanating power, which is a measure of the rate of escape of the emanation into the surrounding gas, must not be confused with the rate of decay of the activity of the emanations themselves, which has already been shown to be unaffected by external conditions.

Dorn (loc. cit.) first observed that the emanating power of thorium and radium compounds was much affected by moisture. In a fuller investigation of this point by Rutherford and Soddy, it was found that the emanating power of thoria is from two to three times greater in a moist than in a dry gas. Continued desiccation of the thoria in a glass tube, containing phosphorus pentoxide, did not reduce the emanating power much below that observed in ordinary dry air. In the same way radium chloride in the solid state gives off very little emanation when in a dry gas, but the amount is much increased in a moist gas.

The rate of escape of emanation is much increased by solution of the compound. For example, thorium nitrate, which has an emanating power of only ¹⁄₂₀₀ that of thoria in the solid state, has in solution an emanating power of 3 to 4 times that of thoria. P. Curie and Debierne observed that the emanating power of radium was also much increased by solution.

Temperature has a very marked effect on the emanating power. The writer^[245] showed that the emanating power of ordinary thoria was increased three to four times by heating the substance to a dull red heat in a platinum tube. If the temperature was kept constant the emanation continued to escape at the increased rate, but returned to its original value on cooling. If, however, the compound was heated to a white heat, the emanating power was greatly reduced, and it returned on cooling to about 10% of the original value. Such a compound is said to be de-emanated. The emanating power of radium compounds varies in a still more striking manner with rise of temperature. The rate of escape of the emanation is momentarily increased even 10,000 times by heating to a dull red heat. This effect does not continue, for the large escape of the emanation by heating is in reality due to the release of the emanation stored up in the radium compound. Like thoria, when the compound has once been heated to a very high temperature, it loses its emanating power and does not regain it. It regains its power of emanating, however, after solution and re-separation.

A further examination of the effect of temperature was made by Rutherford and Soddy^[246]. The emanating power of thoria decreases very rapidly with lowering of temperature, and at the temperature of solid carbonic acid it is only about 10% of its ordinary value. It rapidly returns to its original value when the cooling agent is removed.

Increase of temperature from 80° C. to a dull red heat of platinum thus increases the emanating power about 40 times, and the effects can be repeated again and again, with the same compound, provided the temperature is not raised to the temperature at which de-emanation begins. De-emanation sets in above a red heat, and the emanating power is then permanently diminished, but even long-continued heating at a white heat never entirely destroys the emanating power.

151. Regeneration of emanating power. An interesting question arises whether the de-emanation of thorium and radium is due to a removal or alteration of the substance which produces the emanation, or whether intense ignition merely changes the rate of escape of the emanation from the solid into the surrounding atmosphere.

It is evident that the physical properties of the thoria are much altered by intense ignition. The compound changes in colour from white to pink; it becomes denser and also far less readily soluble in acids. In order to test if the emanating power could be regenerated by a cyclic chemical process, the de-emanated thoria was dissolved, precipitated as hydroxide and again converted into oxide. At the same time a specimen of the ordinary oxide was subjected to an exactly parallel process. The emanating power of both these compounds was the same, and was from two to three times greater than that of ordinary thoria.

Thus de-emanation does not permanently destroy the power of thorium of giving out an emanation, but merely produces an alteration of the amount of the emanation which escapes from the compound.

152. Rate of production of the emanation. The emanating power of thorium compounds, then, is a very variable quantity, much affected by moisture, heat, and solution. Speaking generally, increased temperatures and solution greatly increase the emanating power of both thorium and radium.

From the point of view developed in section 133, the exponential law of decay of the emanation expresses the result that N_t the number of particles remaining unchanged at the time t is given by