Radio-activity/Chapter 8

by Ernest Rutherford

 

CHAPTER VIII.

EXCITED RADIO-ACTIVITY.

175. Excited radio-activity. One of the most interesting and remarkable properties of thorium, radium, and actinium, is their power of "exciting" or "inducing" temporary activity on all bodies in their neighbourhood. A substance which has been exposed for some time in the presence of radium or thorium behaves as if its surface were covered with an invisible deposit of intensely radio-active material. The "excited" body emits radiations capable of affecting a photographic plate and of ionizing a gas. Unlike the radio-elements themselves, however, the activity of the body does not remain constant after it has been removed from the influence of the exciting active material, but decays with the time. The activity lasts for several hours when due to radium and several days when due to thorium.

This property was first observed by M. and Mme. Curie^[1] for radium, and independently by the writer^[2] for thorium^[3]. If any solid body is placed inside a closed vessel containing an emanating compound of thorium or radium, its surface becomes radio-active. For thorium compounds the amount of excited activity on a body is in general greater the nearer it is to the active material. In the case of radium, however, provided the body has been exposed for several hours, the amount of excited activity is to a large extent independent of the position of the body in the vessel containing the active material. Bodies are made active whether exposed directly to the action of the radio-active substance or screened from the action of the direct rays. This has been clearly shown in some experiments of P. Curie. A small open vessel a (Fig. 62) containing a solution of radium is placed inside a larger closed vessel V.

Fig. 62.

Plates A, B, C, D, E are placed in various positions in the enclosure. After exposure for a day, the plates after removal are found to be radio-active even in positions completely shielded from the action of the direct rays. For example, the plate D shielded from the direct radiation by the lead plate P is as active as the plate E, exposed to the direct radiation. The amount of activity produced in a given time on a plate of given area in a definite position is independent of the material of the plate. Plates of mica, copper, cardboard, ebonite, all show equal amounts of activity. The amount of activity depends on the area of the plate and on the amount of free space in its neighbourhood. Excited radio-activity is also produced in water if exposed to the action of an emanating compound.

176. Concentration of excited radio-activity on the negative electrode. When thorium or radium is placed in a closed vessel, the whole interior surface becomes strongly active. In a strong electric field, on the other hand, the writer found that the activity was confined entirely to the negative electrode. By suitable arrangements, the whole of the excited activity, which was previously distributed over the surface of the vessel, can be concentrated on a small negative electrode placed inside the vessel. An experimental arrangement for this purpose is shown in Fig. 63.

Fig. 63.

The metal vessel V containing a large amount of thoria is connected with the positive pole of a battery of about 300 volts. The wire AB to be made active is fastened to a stouter rod BC, passing through an ebonite cork inside a short cylinder D, fixed in the side of the vessel. This rod is connected with the negative pole of the battery. In this way the wire AB is the only conductor exposed in the field with a negative charge, and it is found that the whole of the excited activity is concentrated upon it.

In this way it is possible to make a short thin metal wire over 10,000 times as active per unit surface as the thoria from which the excited activity is derived. In the same way, the excited activity due to radium can be concentrated mainly on the negative electrode. In the case of thorium, if the central wire be charged positively, it shows no appreciable activity. With radium, however, a positively charged body becomes slightly active. In most cases, the amount of activity produced on the positive electrode is not more than 5% of the corresponding amount when the body is negatively charged. For both thorium and radium, the amount of excited activity on electrodes of the same size is independent of their material.

All metals are made active to equal extents for equal times of exposure. When no electric field is acting, the same amount of activity is produced on insulators like mica and glass as on conductors of equal dimensions.

177. Connection between the emanations and excited activity. An examination of the conditions under which excited activity is produced shows that there is a very close connection between the emanation and the excited activity. If a thorium compound is covered with several sheets of paper, which cut off the α rays but allow the emanation to pass through, excited activity is still produced in the space above it. If a thin sheet of mica is waxed down over the active material, thus preventing the escape of the emanation, no excited activity is produced outside it. Uranium and polonium which do not give off an emanation are not able to produce excited activity on bodies. Not only is the presence of the emanation necessary to cause excited activity, but the amount of excited activity is always proportional to the amount of emanation present. For example, de-emanated thoria produces very little excited activity compared with ordinary thoria. In all cases the amount of excited activity produced is proportional to the emanating power. When passing through an electric field the emanation loses its property of exciting activity at the same rate as the radiating power diminishes. This was shown by the following experiment.

A slow constant current of air from a gasometer, freed from dust by its passage through cotton-wool, passed through a rectangular wooden tube 70 cms. long. Four equal insulated metal plates A, B, C, D, were placed at regular intervals along the tube. The positive pole of a battery of 300 volts was connected with a metal plate placed in the bottom of the tube, while the negative pole was connected with the four plates. A mass of thoria was placed in the bottom of the tube under the plate A, and the current due to the emanation determined at each of the four plates. After passing a current of air of 0·2 cm. per second for 7 hours along the tube, the plates were removed and the amount of excited activity produced on them was tested by the electric method. The following results were obtained.

+—————-+————————+————————+
| |Relative current|Relative excited|
| |due to emanation| activity |
+—————-+————————+————————+
| Plate A | 1 | 1 |
| " B | ·55 | ·43 |
| " C | ·18 | ·16 |
| " D | ·072 | ·061 |
+—————-+————————+————————+

Within the errors of measurement, the amount of excited activity is thus proportional to the radiation from the emanation, i.e. to the amount of emanation present. The same considerations hold for the radium emanation. The emanation in this case, on account of the slow loss of its activity, can be stored mixed with air for long periods in a gasometer, and its effects tested quite independently of the active matter from which it is produced. The ionization current due to the excited activity produced by the emanation is always proportional to the current due to the emanation for the period of one month or more that its activity is large enough to be measured conveniently by an electrometer.

If, at any time during the interval, some of the emanation is removed and introduced into a new testing vessel, the ionization current will immediately commence to increase, rising in the course of four or five hours to about twice its original value. This increase of the current is due to the excited activity produced on the walls of the containing vessel. On blowing out the emanation, the excited activity is left behind, and at once begins to decay. Whatever its age, the emanation still possesses the property of causing excited activity, and in amount always proportional to its activity, i.e. to the amount of emanation present.

These results show that the power of exciting activity on inactive substances is a property of the radio-active emanations, and is proportional to the amount of emanation present.

The phenomenon of excited activity cannot be ascribed to a type of phosphorescence produced by the rays from the emanation on bodies; for it has been shown that the activity can be concentrated on the negative electrode in a strong electric field, even if the electrode is shielded from the direct radiation from the active substance which gives off the emanation. The amount of excited activity does not seem in any way connected with the ionization produced by the emanation in the gas with which it is mixed. For example, if a closed vessel is constructed with two large parallel insulated metal plates on the lower of which a layer of thoria is spread, the amount of the excited activity on the upper plate when charged negatively, is independent of the distance between the plates when that distance is varied from 1 millimetre to 2 centimetres. This experiment shows that the amount of excited activity depends only on the amount of emanation emitted from the thoria; for the ionization produced with a distance of 2 centimetres between the plates is about ten times as great as with a distance of 1 millimetre.

178. If a platinum wire be made active by exposure to the emanation of thoria, its activity can be removed by treating the wire with certain acids^[4]. For example, the activity is not much altered by immersing the wire in hot or cold water or nitric acid, but more than 80% of it is removed by dilute or concentrated solutions of sulphuric or hydrochloric acid. The activity has not been destroyed by this treatment but is manifested in the solution. If the solution be evaporated, the activity remains behind on the dish.

These results show that the excited activity is due to a deposit on the surface of bodies of radio-active matter which has definite properties as regards solution in acids. This active matter is dissolved in some acids, but, when the solvent is evaporated, the active matter is left behind. This active matter is deposited on the surface of bodies, for it can be partly removed by rubbing the body with a cloth, and almost completely by scouring the plate with sand or emery paper. If a negatively charged wire is placed in the presence of a large quantity of radium emanation, it becomes intensely active. If the wire, after removal, is drawn across a screen of zinc sulphide, or willemite, a portion of the active matter is rubbed off, and a luminous trail is left behind on the screen. The amount of active matter deposited is extremely small, for no difference of weight has been detected in a platinum wire when made extremely active. On examining the wire under a microscope, no trace of foreign matter is observed. It follows from these results that the matter which causes excited activity is many thousand times more active, weight for weight, than radium itself.

It is convenient to have a definite name for this radio-active matter, for the term "excited activity" only refers to the radiation from the active matter and not to the matter itself. The term "active deposit" will be generally applied to this matter. The active deposit from the three substances thorium, radium, and actinium is, in each case, derived from its respective emanation, and possesses the same general property of concentration on the negative electrode in an electric field and of acting as a non-volatile type of matter which is deposited from the gas on to the surface of bodies. These active deposits, while all soluble in strong acids, are chemically distinct from each other.

The term "active deposit" can, however, only be used when the matter is spoken of as a whole; for it will be shown later that the matter, under ordinary conditions, is complex and contains several constituents which have distinctive physical and chemical properties and also a distinctive rate of change. According to the theory advanced in section 136, we may suppose that the emanation of thorium, radium, and actinium is unstable and breaks up with the expulsion of an α particle. The residue of the atom of the emanation diffuses to the sides of the vessel or is removed to the negative electrode in an electric field. This active deposit is in turn unstable and breaks up in several successive stages.

The "excited activity" proper is the radiation set up by the active deposit in consequence of the changes occurring in it. On this view, the emanation is the parent of the active deposit in the same way that Th X is the parent of the emanation. The proportionality which always exists between the activity of the emanation and the excited activity to which it gives rise, is at once explained, if one substance be the parent of the other.

179. Decay of the excited activity produced by thorium. The excited activity produced in a body after a long exposure to the emanations of thorium, decays in an exponential law with the time, falling to half value in about 11 hours. The following table shows the rate of decay of the excited activity produced on a brass rod.

Time in hours Current
     0 100
     7·9 64
    11·8 47·4
    23·4 19·6
    29·2 13·8
    32·6 10·3
    49·2 3·7
    62·1 1·86
    71·4 0·86

The results are shown graphically in Fig. 64, Curve A.

Fig. 64. The intensity of the radiation I after any time t is given by I/I_{0} = e^{-λt}, where λ is the radio-active constant.

The rate of decay of excited activity, like that of the activity of other radio-active products, is not appreciably affected by change of conditions. The rate of decay is independent of the concentration of the excited activity, and of the material of the body on which it is produced. It is independent also of the nature and pressure of the gas in which it decays. The rate of decay is unchanged whether the excited activity is produced on the body with or without an electric field.

The amount of excited activity produced on a body increases at first with the time, but reaches a maximum after an exposure of several days. An example of the results is given in the following table. In this experiment a rod was made the cathode in a closed vessel containing thoria. It was removed at intervals for the short time necessary to test its activity and then replaced.

Time in hours Current
    1·58 6·3
    3·25 10·5
    5·83 29
    9·83 40
   14·00 59
   23·41 77
   29·83 83
   47·00 90
   72·50 95
   96·00 100

These results are shown graphically in Curve B, Fig. 64. It is seen that the decay and recovery curves may be represented approximately by the following equations.

For the decay curve A, I/I_{0} = e^{-λt}.
For the recovery curve B, I/I_{0} = 1 - e^{-λt}.

The two curves are thus complementary to one another; they are connected in the same way as the decay and recovery curves of Ur X, and are susceptible of a similar explanation. The amount of excited radio-activity reaches a maximum value when the rate of supply of fresh radio-active particles balances the rate of change of those already deposited.

180. Excited radio-activity produced by a short exposure. The initial portion of the recovery curve B, Fig. 64, is not accurately represented by the above equation. The activity for the first few hours increases more slowly than would be expected from the equation. This result, however, is completely explained in the light of later results. The writer^[5] found that, for a short exposure of a body to the thorium emanation, the excited activity upon it after removal, instead of at once decaying at the normal rate, increased for several hours. In some cases the activity of the body increased to three or four times its original value in the course of a few hours and then decayed with the time at the normal rate.

For an exposure of 41 minutes to the emanation the excited activity after removal rose to three times its initial value in about 3 hours and then fell again at about the normal rate to half value in 11 hours.

With a longer time of exposure to the emanation, the ratio of the increase after removal is much less marked. For a day's exposure, the activity after removal begins at once to diminish. In this case, the increase of activity of the matter deposited in the last few hours does not compensate for the decrease of activity of the active matter as a whole, and consequently the activity at once commences to decay. This increase of activity with time explains the initial irregularity in the recovery curve, for the active matter deposited during the first few hours takes some time to reach its maximum activity, and the initial activity is, in consequence, smaller than would be expected from the equation.

The increase of activity on a rod exposed for a short interval in the presence of the thorium emanation has been further investigated by Miss Brooks. The curve C in Fig. 65 shows the variation with time of the activity of a brass rod exposed for 10 minutes in the emanation vessel filled with dust-free air. The excited activity after removal increased in the course of 3·7 hours to five times its initial value, and afterwards decayed at the normal rate. The dotted line curve D represents the variation of activity to be expected if the activity decayed exponentially with the time. The explanation of this remarkable action is considered in detail in section 207.

Fig. 65.

181. Effect of dust on the distribution of excited activity. Miss Brooks^[6], working in the Cavendish Laboratory, observed that the excited activity due to the thorium emanation appeared in some cases on the anode in an electric field, and that the distribution of excited activity varied in an apparently capricious manner. This effect was finally traced to the presence of dust in the air of the emanation vessel. For example, with an exposure of 5 minutes the amount of excited activity to be observed on a rod depended on the time that the air had been allowed to remain undisturbed in the emanation vessel beforehand. The effect increased with the time of standing, and was a maximum after about 18 hours. The amount of excited activity obtained on the rod was then about 20 times as great as the amount observed for air freshly introduced. The activity of this rod did not increase after removal, but with fresh air, the excited activity, for an exposure of 5 minutes, increased to five or six times its initial value.

This anomalous behaviour was found to be due to the presence of dust particles in the air of the vessel, in which the bodies were made radio-active. These particles of dust, when shut up in the presence of the emanation, become radio-active. When a negatively charged rod is introduced into the vessel, a part of the radio-active dust is concentrated on the rod and its activity is added to the normal activity produced on the wire. After the air in the vessel has been left undisturbed for an interval sufficiently long to allow each of the particles of dust to reach a state of radio-active equilibrium, on the application of an electric field, all the positively charged dust particles will at once be carried to the negative electrode. The activity of the electrode at once commences to decay, since the decay of the activity of the dust particles on the wire quite masks the initial rise of the normal activity produced on the wire.

Part of the radio-active dust is also carried to the anode, and the proportion increases with the length of time during which the air has been undisturbed. The greatest amount obtained on the anode was about 60% of that on the cathode.

These anomalous effects were found to disappear if the air was made dust-free by passing through a plug of glass wool, or by application for some time of a strong electric field.

182. Decay of excited activity from radium. The excited activity produced on bodies by exposure to the radium emanation decays much more rapidly than the thorium excited activity. For short times of exposure^[7] to the emanation the decay curve is very irregular. This is shown in Fig. 66.

It was found that the intensity of the radiation measured by the α rays decreased rapidly for the first 10 minutes after removal, but about 15 minutes after removal reached a value which remained nearly constant for an interval of about 20 minutes. It then decayed to zero, finally following an exponential law, the intensity falling to half value in about 28 minutes. With longer times of exposure, the irregularities in the curve are not so marked.

Decay of Excited Activity of Radium—short exposure

Fig. 66.

Miss Brooks has recently determined the decay curves of the excited activity of radium for different times of exposure, measured by the α rays. The results are shown in Fig. 67, where the initial ordinates represent the activity communicated to the body from different times of exposure to a constant supply of emanation. It will be observed that in all cases there is a sudden initial drop of activity, which becomes less marked with increasing time of exposure. The activity, several hours after removal, decreases exponentially in all cases, falling to half value in about 28 minutes.

Not only do the curves of variation of the excited activity after removal depend upon the time of exposure to the emanation, but they also depend upon whether the α or β and γ rays are used as a means of measurement. The curves obtained for the γ rays are identical with those from the β rays, showing that these two types of rays always occur together and in the same proportion. The curves measured by the β rays are very different, especially for the case of a short exposure to the emanation. This is clearly shown in Fig. 68, which gives the β and γ ray curves for exposures of 10 minutes, 40 minutes, and 1 hour, and also the limiting case of an exposure of 24 hours.

Fig. 67.

About 25 minutes after removal, the activity decays approximately

at the same rate in each case. For convenience of repre-

Fig. 68.

• sentation, the ordinates of the curves were adjusted so that they

all passed through a common point. We shall see later (chapter XI) that the rates of decay are not identically the same until several hours after removal; but, in the above figure, it is difficult to represent the slight variations. It will be observed that for the short exposure of 10 minutes the activity measured by the β rays is small at first but rises to a maximum in about 22 minutes, and then dies away with the time. The curve of decay of activity, measured by the β rays for a long exposure, does not show the rapid initial drop which occurs in all the α ray curves. Curie and Danne^[8] made an investigation of the curves of decay of excited activity for different times of exposure to the radium emanation, but apparently did not take into account the fact that measurements made by the α and β rays give quite different curves of decay. Some of the family of curves, given in their paper, refer to the α rays and others to the β rays. They showed, however, the important fact that the curve of decay obtained by them for a long exposure (which is identical with the β ray curve) could be empirically expressed by an equation of the form

I_{t}/I_{0} = ae^{-λ_{1}t} - (a - 1)e^{-λ_{2}t},

where I_{0} is the initial intensity and I_{t} the intensity after any time t; λ_{1} = 1/2420, λ_{2} = 1/1860. The numerical constant a = 4·20. After an interval of 2·5 hours, the logarithmic decay curve is nearly a straight line, that is, the activity falls off according to an exponential law with the time, decreasing to half value in about 28 minutes.

The full explanation of this equation, and of the peculiarities of the various decay curves of the excited activity of radium, will be discussed in detail in chapter XI.

As in the case of the excited activity from thorium, the rate of decay of the excited activity from radium is for the most part independent of the nature of the body made active. Curie and Danne (loc. cit.) observed that the active bodies gave off an emanation itself capable of exciting activity in neighbouring bodies. This property rapidly disappeared, and was inappreciable 2 hours after removal. In certain substances like celluloid and caoutchouc, the decay of activity is very much slower than for the metals. This effect becomes more marked with increase of time of exposure to the emanation. A similar effect is exhibited by lead, but to a less marked degree. During the time the activity lasts, these substances continue to give off an emanation.

It is probable that these divergencies from the general law are not due to an actual change in the rate of decay of the true excited activity but to an occlusion of the emanation by these substances during the interval of exposure. After exposure the emanation gradually diffuses out, and thus the activity due to this occluded emanation and the excited activity produced by it decays very slowly with the time. 183. Active deposit of very slow decay. M. and Mme Curie^[9] have observed that bodies which have been exposed for a long interval in the presence of the radium emanation do not lose all their activity. The excited activity at first decays rapidly at the normal rate, falling to half value in about 28 minutes, but a residual activity, which they state is of the order of 1/20,000 of the initial activity, always remains. A similar effect was observed by Giesel. The writer has examined the variation of this residual activity, and has found that it increases for several years. The results are discussed in detail in chapter XI. It will there be shown that this active deposit of slow transformation contains the radio-active constituents present in polonium, radio-tellurium and radio-lead.

Fig. 69.

184. The excited activity from actinium. The emanation of actinium, like that of thorium and radium, produces excited activity on bodies, which is concentrated on the negative electrode in an electric field. Debierne^[10] found that the excited activity decays approximately according to an exponential law, falling to half value in 41 minutes. Giesel^[11] examined the rate of decay of the excited activity of "emanium"—which, we have seen, probably contains the same radio-active constituents as actinium—and found that it decayed to half value in 34 minutes. Miss Brooks^[12] found that the curves of decay of the excited activity from Giesel's emanium varied with the time of exposure to the emanation. The results are shown graphically in Fig. 69, for time exposures of 1, 2, 2, 10 and 30 minutes, and also for a long exposure of 21 hours. After 10 minutes the curves have approximately the same rate of decay. For convenience, the ordinates of the curves are adjusted to pass through a common point. For a very short exposure, the activity is small at first, but reaches a maximum about 9 minutes later and finally decays exponentially to zero.

The curve of variation of activity for a very short exposure has been determined accurately by Bronson; it is shown later in Fig. 83. He found that the decay of activity is finally exponential, falling to half value in 36 minutes.

The explanation of these curves is discussed in detail in chapter X, section 212.

185. Physical and chemical properties of the active deposit. On account of the slow decay of the activity of the active deposit from the thorium emanation, its physical and chemical properties have been more closely examined than the corresponding deposit from radium. It has already been mentioned that the active deposit of thorium is soluble in some acids. The writer^[13] found that the active matter was dissolved off the wire by strong or dilute solutions of sulphuric, hydrochloric and hydrofluoric acids, but was only slightly soluble in water or nitric acid. The active matter was left behind when the solvent was evaporated. The rate of decay of activity was unaltered by dissolving the active matter in sulphuric acid, and allowing it to decay in the solution. In the experiment, the active matter was dissolved off an active platinum wire; then equal portions of the solutions were taken at definite intervals, evaporated down in a platinum dish, and the activity of the residue tested by the electric method. The rate of decay was found to be exactly the same as if the active matter had been left on the wire. In another experiment, an active platinum wire was made the cathode in a copper sulphate solution, and a thin film of copper deposited on it. The rate of decay of the activity was unchanged by the process.

A detailed examination of the physical and chemical properties of the active deposit of thorium has been made by F. von Lerch^[14] and some important and interesting results have been obtained. A solution of the active deposit was prepared by dissolving the metal which had been exposed for some time in the presence of the thorium emanation. In most cases the active matter was precipitated with the metal. For example, an active copper wire was dissolved in nitric acid and then precipitated by caustic potash. The precipitate was strongly active. An active magnesium wire, dissolved in hydrochloric acid and then precipitated as phosphate, also gave an active precipitate. The activity of the precipitates decayed at the normal rate, i.e. the activity fell to half value in about 11 hours.

Experiments were also made on the solubility of the active deposit in different substances. A platinum plate was made active and then placed in different solutions, and the decrease of the activity observed. In addition to the acids already mentioned, a large number of substances were found to dissolve the active deposit to some extent. The active matter was however not dissolved to an appreciable extent in ether or alcohol. Many substances became active if added to the active solution and then precipitated. For example, an active solution of hydrochloric acid was obtained by dissolving the deposit on an active platinum wire. Barium chloride was then added and precipitated as sulphate. The precipitate was strongly active, thus suggesting that the active matter was carried down by the barium.

186. Electrolysis of solutions. Dorn showed that, if solutions of radiferous barium chloride were electrolysed, both electrodes became temporarily active, but the anode to a greater degree than the cathode. F. von Lerch has made a detailed examination of the action of electrolysis on a solution of the active deposit of thorium. The matter was dissolved off an active platinum plate by hydrochloric acid, and then electrolysed between platinum electrodes. The cathode was very active, but there was no trace of activity on the anode. The cathode lost its activity at a rate much faster than the normal. With an amalgamated zinc cathode on the other hand, the rate of decay was normal. When an active solution of hydrochloric acid was electrolysed with an electromotive force smaller than that required to decompose water, the platinum became active. The activity decayed to half value in 4·75 hours while the normal fall is to half value in 11 hours. These results point to the conclusion that the active matter is complex and consists of two parts which have different rates of decay of activity, and can be separated by electrolysis.

Under special conditions it was found possible to make the anode active. This was the case if the anion attached itself to the anode. For example, if an active hydrochloric solution was electrolysed with a silver anode, the chloride of silver formed was strongly active and its activity decayed at a normal rate. The amount of activity obtained by placing different metals in active solutions for equal times varied greatly with the metal. For example, it was found that if a zinc plate and an amalgamated zinc plate, which show equal potential differences with regard to hydrochloric acid, were dipped for equal times in two solutions of equal activity, the zinc plate was seven times as active as the other. The activity was almost removed from the solution in a few minutes by dipping a zinc plate into it. Some metals became active when dipped into an active solution while others did not. Platinum, palladium, and silver remained inactive, while copper, tin, lead, nickel, iron, zinc, cadmium, magnesium, and aluminium became active. These results strongly confirm the view that excited activity is due to a deposit of active matter which has distinctive chemical behaviour.

G. B. Pegram^[15] has made a detailed study of the active deposits obtained by electrolysis of pure and commercial thorium salts. The commercial thorium nitrate obtained from P. de Haen gave, when electrolysed, a deposit of lead peroxide on the anode. This deposit was radio-active, and its activity decayed at the normal rate of the excited activity due to thorium. From solutions of pure thorium nitrate, no visible deposit was obtained on the anode, but it was, however, found to be radio-active. The activity decayed rapidly, falling to half value in about one hour. Some experiments were also made on the effect of adding metallic salts to thorium solutions and then electrolysing them. Anode and cathode deposits of the oxides or metals obtained in this way were found to be radio-active, but the activity fell to half value in a few minutes. The gases produced by electrolysis were radio-active, but this was due to the presence of the thorium emanation. The explanation of the results obtained by Pegram and von Lerch will be considered later in section 207. It will be shown that the active deposit of thorium contains two distinct substances which have different rates of transformation.

187. Effect of temperature. The activity of a platinum wire which has been exposed in the presence of the thorium emanation is almost completely lost by heating the wire to a white heat. Miss F. Gates^[16] found that the activity was not destroyed by the intense heat, but manifested itself on neighbouring bodies. When the active wire was heated electrically in a closed cylinder, the activity was transferred from the wire to the interior surface of the cylinder in unaltered amount. The rate of decay of the activity was not altered by the process. By blowing a current of air through the cylinder during the heating, a part of the active matter was removed from the cylinder. Similar results were found for the excited activity due to radium.

F. von Lerch (loc. cit.) determined the amount of activity removed at different temperatures. The results are shown in the following table for a platinum wire excited by the thorium emanation^[17].

+—————————————-+—————-+————————+
| |Temperature| Percentage of |
| | |activity removed|
+—————————————-+—————-+————————+
| Heated 2 minutes | 800° C. | 0 |
|then " 1/2 minute more| 1020° C. | 16 |
| " " 1/2 " " | 1260° C. | 52 |
| " " 1/2 " " | 1460° C. | 99 |
+—————————————-+—————-+————————+

The effect of heat on the volatilization of the active deposit of

radium has been examined in detail by Curie and Danne. The interesting and important results obtained by them will be discussed in chapter XI, section 226.

188. Effect of variation of E.M.F. on amount of excited activity from thorium. It has been shown that the excited activity is confined to the cathode in a strong electric field. In weaker fields the activity is divided between the cathode and the walls of the vessel. This was tested in an apparatus^[18] shown in Fig. 70.

Fig. 70.

A is a cylindrical vessel of 5·5 cms. diameter, B the negative electrode passing through insulating ends C, D. For a potential difference of 50 volts, most of the excited activity was deposited on the electrode B. For about 3 volts, half of the total excited activity was produced on the rod B, and half on the walls of the vessel. Whatever the voltage applied, the sum of the activities on the central rod and the walls of the cylinder was found to be a constant when a steady state was reached.

When no voltage was applied, diffusion alone was operative, and in that case about 13 per cent. of the total activity was on the rod B. The application of an electric field has thus no influence on the sum total of excited activity, but merely controls the proportion concentrated on the negative electrode.

A more detailed examination of the variation with strength of field of the amount on the negative electrode was made in a similar manner by F. Henning^[19]. He found that in a strong electric field the amount of excited activity was practically independent of the diameter of the rod B, although the diameter varied between ·59 mm. and 6·0 mms. With a small voltage, the amount on the negative electrode varied with its diameter. The curves showing the relation between the amount of excited activity and voltage are very similar in character to those obtained for the variation of the current through an ionized gas with the voltage applied.

The amount of excited activity reaches a maximum when all the active matter is removed from the gas as rapidly as it is formed. With weaker fields, a portion diffuses to the sides of the vessel, and produces excited activity on the positive electrode.

189. Effect of pressure on distribution of excited activity. In a strong electric field, the amount of excited activity produced on the cathode is independent of the pressure down to a pressure of about 10 mms. of mercury. In some experiments made by the writer^[20], the emanating thorium compound was placed inside a closed cylinder about 4 cms. in diameter, through which passed an insulated central rod. The central rod was connected to the negative pole of a battery of 50 volts. When the pressure was reduced below 10 mms. of mercury, the amount of excited activity produced on the negative electrode diminished, and was a very small fraction of its original value at a pressure of 1/10 mm. Some excited activity was in this case found to be distributed over the interior surface of the cylinder. It may thus be concluded that at low pressures the excited activity appears on both anode and cathode, even in a strong electric field. The probable explanation of this effect is given in the next section.

Curie and Debierne^[21] observed that when a vessel containing an emanating radium compound was kept pumped down to a low pressure, the amount of excited activity produced on the vessel was much reduced. In this case the emanation given off by the radium was removed by the pump with the other gases continuously evolved from the radium compound. On account of the very slow decay of activity of the emanation, the amount of excited activity produced on the walls of the vessel, in the passage of the emanation through it, was only a minute fraction of the amount produced when none of the emanation given off was allowed to escape. 190. Transmission of excited activity. The characteristic property of excited radio-activity is that it can be confined to the cathode in a strong electric field. Since the activity is due to a deposit of radio-active matter on the electrified surface, the matter must be transported by positively charged carriers. The experiments of Fehrle^[22] showed that the carriers of excited activity travel along the lines of force in an electric field. For example, when a small negatively charged metal plate was placed in the centre of a metal vessel containing an emanating thorium compound, more excited activity was produced on the sides and corners of the plate than at the central part.

A difficulty however arises in connection with the positive charge of the carrier. According to the view developed in section 136 and later in chapters X and XI, the active matter which is deposited on bodies and gives rise to excited activity, is itself derived from the emanation. The emanations of thorium and radium emit only α rays, i.e. positively charged particles. After the expulsion of an α particle, the residue, which is supposed to constitute the primary matter of the active deposit, should retain a negative charge, and be carried to the anode in an electric field. The exact opposite however is observed to be the case. The experimental evidence does not support the view that the positively charged α particles, expelled from the emanation, are directly responsible for the phenomena of excited activity; for no excited activity is produced in a body exposed to the α rays of the emanation, provided the emanation itself does not come in contact with it.

There has been a tendency to attach undue importance to this apparent discrepancy between theory and experiment. The difficulty is not so much to offer a probable explanation of the results as to select from a number of possible causes. While there can be little doubt that the main factor in the disintegration of the atom consists in the expulsion of an α particle carrying a positive charge, a complicated series of processes probably occurs before the residue of the atom is carried to the negative electrode. The experimental evidence suggests that one or more negative electrons of slow velocity escape from the atom at the same time as the particle. This is borne out by the recent discovery that the particle expelled from radium, freed from the ordinary β rays, and also from polonium, is accompanied by a number of slowly moving and consequently easily absorbed electrons. If two negative electrons escaped at the same time as the α particle, the residue would be left with a positive charge and would be carried to the negative electrode. There is also another experimental point which is of importance in this connection. In the absence of an electric field, the carriers remain in the gas for a considerable time and undergo their transformation in situ. There is also some evidence (section 227) that, even in an electric field, the carriers of the active deposit are not swept to the electrode immediately after the break up of the emanation, but remain some time in the gas before they gain a positive charge. It must be remembered that the atoms of the active deposit do not exist as a gas and by the process of diffusion would tend to collect together to form aggregates. These aggregates would act as small metallic particles, and, if they were electro-positive in regard to the gas, would gain a positive charge from the gas.

There can be little doubt that the processes occurring between the break up of the emanation and the deposit of the residue in the cathode in an electric field are complicated, and further careful experiment is required to elucidate the sequence of the phenomena.

Whatever view is taken of the process by which these carriers obtain a positive charge, there can be little doubt that the expulsion of an α particle with great velocity from the atom of the emanation must set the residue in motion. On account of the comparatively large mass of this residue, the velocity acquired will be small compared with that of the expelled α particle, and the moving mass will rapidly be brought to rest at atmospheric pressure by collision with the gas molecules in its path. At low pressures, however, the collisions will be so few that it will not be brought to rest until it strikes the boundaries of the vessel. A strong electric field would have very little effect in controlling the motion of such a heavy mass, unless it has been initially brought to rest by collision with the gas molecules. This would explain why the active matter is not deposited on the cathode at low pressures in an electric field. Some direct evidence of a process of this character, obtained by Debierne on examination of the excited activity produced by actinium, is discussed in section 192.

191. The following method has been employed by the writer^[23] to determine the velocity of the positive carriers of excited activity of radium and thorium in an electric field. Suppose A and B (Fig. 71) are two parallel plates exposed to the influence of the emanation, which is uniformly distributed between them. If an alternating E.M.F. E_{0} is applied between the plates, the same amount of excited activity is produced on each electrode. If, in series with the source of the alternating E.M.F., a battery of E.M.F. E_{1} less than E_{0} is placed, the positive carrier moves in a stronger electric field in one half alternation than in the other. A carrier consequently moves over unequal distances during the two half alternations, since the velocity of the carrier is proportional to the strength of the electric field in which it moves. The excited activity will in consequence be unequally distributed over the two electrodes. If the frequency of alternation is sufficiently great, only the positive carriers within a certain small distance of one plate can be conveyed to it, and the rest, in the course of several succeeding alternations, are carried to the other plate.

Fig. 71.

When the plate B is negatively charged, the E.M.F. between

the plates is E_{0} - E_{1}, when B is positive the E.M.F. is E_{0} + E_{1}.

Let d = distance between the plates,
    T = time of a half alternation,
    ρ = ratio of the excited radio-activity on the plate B to the
                 sum of the radio-activities on the plates A and B,
    K = velocity of the positive carriers for a potential-gradient
                 of 1 volt per centimetre.

On the assumption that the electric field between the plates is uniform, and that the velocity of the carrier is proportional to the electric field, the velocity of the positive carrier towards B is

((E_{0} - E_{1})/d)K,

and, in the course of the next half alternation,

((E_{0} + E_{1})/d)K

towards the plate A.

If x_{1} is less than d, the greatest distances x_{1}, x_{2} passed over by the positive carrier during two succeeding half alternations is thus given by

x_{1} = ((E_{0} - E_{1})/d)KT, and x_{2} = ((E_{0} + E_{1})/d)KT.

Suppose that the positive carriers are produced at a uniform rate of q per second for unit distance between the plates. The number of positive carriers which reach B during a half alternation consists of two parts:

(1) One half of those carriers which are produced within the distance x_{1} of the plate B. This number is equal to

(1/2)x_{1}qT.

(2) All the carriers which are left within the distance x_{1} from B at the end of the previous half alternation. The number of these can readily be shown to be

(1/2)x_{1}(x_{1}/x_{2})qT.

The remainder of the carriers, produced between A and B during a complete alternation, will reach the other plate A in the course of succeeding alternations, provided no appreciable recombi-

• nation takes place. This must obviously be the case, since the

positive carriers travel further in a half alternation towards A than they return towards B during the next half alternation. The carriers thus move backwards and forwards in the changing electric field, but on the whole move towards the plate A. The total number of positive carriers produced between the plates during a complete alternation is 2dqT. The ratio ρ of the number which reach B to the total number produced is thus given by

 ρ = ((1/2)x_{1}qT + (1/2)x_{1}(x_{1}/x_{2})qT)/(2dqT) = (1/4)(x_{1}/d)(x_{1} + x_{2})/x_{2}.

Substituting the values of x_{1} and x_{2}, we find that

 K = (2(E_{0} + E_{1}))/(E_{0}(E_{0} - E_{1}))(d^2/T)ρ.

In the experiments, the values of E_{0}, E_{1}, d, and T were varied, and the results obtained were in general agreement with the above equation. The following were the results for thorium: Plates 1·30 cms. apart.

+————————-+————————-+——————+——-+——+
|E_{0} + E_{1}|E_{0} - E_{1}|Alternations|ρ|K | | | | per second | | | +————————-+————————-+——————+——-+——+ | 152 | 101 | 57 | ·27 |1·25| | 225 | 150 | 57 | ·38 |1·17| | 300 | 200 | 57 | ·44 |1·24| +————————-+————————-+——————+——-+——+

Plates 2 cms. apart.

+————————-+————————-+——————+——-+——+
|E_{0} + E_{1}|E_{0} - E_{1}|Alternations|ρ|K |
| | | per second | | |
+————————-+————————-+——————+——-+——+
| 273 | 207 | 44 | ·37 |1·47|
| 300 | 200 | 53 | ·286|1·45|
+————————-+————————-+——————+——-+——+

The average mobility K deduced from a large number of experiments was 1·3 cms. per sec. per volt per cm. for atmospheric pressure and temperature. This velocity is about the same as the velocity of the positive ion produced by Röntgen rays in air, viz. 1·37 cms. per sec. The results obtained with the radium emanation were more uncertain than those for thorium on account of the distribution of some excited activity on the positive electrode. The values of the velocities of the carriers were however found to be roughly the same for radium as for thorium.

These results show that the carriers of the active deposit travel in the gas with about the same velocity as the positive or negative ions produced by the radiations in the gas. This indicates either that the active matter becomes attached to positive ions, or that the active matter itself, acquiring in some way a positive charge, collects a cluster of neutral molecules which travel with it.

192. Carriers of the excited activity from actinium and "emanium." Giesel^[24] observed that "emanium" gave off a large quantity of emanation, and that this emanation gave rise to a type of radiation which he termed the E rays. A narrow metal cylinder containing the active substance was placed with the open end downwards, about 5 cms. above the surface of a zinc sulphide screen. The screen was charged negatively to a high potential by an electric machine, and the cylinder connected with earth. A luminous spot of light was observed on the screen, which was brighter at the edge than at the centre. A conductor, connected with earth, brought near the luminous spot apparently repelled it. An insulator did not show such a marked effect. On removal of the active substance, the luminosity of the screen persisted for some time. This was probably due to the excited activity produced on the screen.

The results obtained by Giesel support the view that the carriers of excited activity of "emanium" have a positive charge. In a strong electric field the carriers travel along the lines of force to the cathode, and there cause excited activity on the screen. The movement of the luminous zone on the approach of a conductor is due to the disturbance of the electric field. Debierne^[25] found that actinium also gave off a large amount of emanation, the activity of which decayed very rapidly with the time, falling to half value in 3·9 seconds.

This emanation produces excited activity on surrounding objects, and at diminished pressure the emanation produces a uniform distribution of excited activity in the enclosure containing the emanation. The excited activity falls to half value in 41 minutes.

Debierne observed that the distribution of excited activity was altered by a strong magnetic field. The experimental arrangement is shown in Fig. 71A. The active matter was placed at M, and two plates A and B were placed symmetrically with regard to the source. On the application of a strong magnetic field normal to the plane of the paper, the excited activity was unequally distributed between the plates A and B. The results showed that the carriers of excited activity were deviated by a magnetic field in the opposite sense to the cathode rays, i.e. the carriers were positively charged. In some cases, however, the opposite effect was obtained. Debierne considers that the excited activity of actinium is due to "ions activants," the motion of which is altered by a magnetic field. Other experiments showed that the magnetic field acted on the "ions activants" and not on the emanation.

Fig. 71A.

The results of Debierne thus lead to the conclusion that the carriers of excited activity are derived from the emanation and are projected with considerable velocity. This result supports the view, advanced in section 190, that the expulsion of α particles from the emanation must set the part of the system left behind in rapid motion. A close examination of the mode of transference of the excited activity by actinium and the emanation substance is likely to throw further light on the processes which give rise to the deposit of active matter on the electrodes.

1. M. and Mme. Curie, C. R. 129, p. 714, 1899.
2. Rutherford, Phil. Mag. Jan. and Feb. 1900.
3. As regards date of publication, the priority of the discovery of "excited activity" belongs to M. and Mme. Curie. A short paper on this subject, entitled "Sur la radioactivité provoquée par les rayons de Becquerel," was communicated by them to the Comptes Rendus, Nov. 6, 1899. A short note was added to the paper by Becquerel in which the phenomena of excited activity were ascribed to a type of phosphorescence. On my part, I had simultaneously discovered the emission of an emanation from thorium compounds and the excited activity produced by it, in July, 1899. I, however, delayed publication in order to work out in some detail the properties of the emanation and of the excited activity and the connection between them. The results were published in two papers in the Philosophical Magazine (Jan. and Feb. 1900) entitled "A radio-active substance emitted from thorium compounds," and "Radio-activity produced in substances by the action of thorium compounds."
4. Rutherford, Phil. Mag. Feb. 1900.
5. Rutherford, Phys. Zeit. 3, No. 12, p. 254, 1902. Phil. Mag. Jan. 1903.
6. Miss Brooks, Phil. Mag. Sept. 1904.
7. Rutherford and Miss Brooks, Phil. Mag. July, 1902.
8. Curie and Danne, C. R. 136, p. 364, 1903.
9. Mme Curie, Thèse, Paris, 1903, p. 116.
10. Debierne, C. R. 138, p. 411, 1904.
11. Giesel, Ber. d. D. Chem. Ges. No. 3, p. 775, 1905.
12. Miss Brooks, Phil. Mag. Sept. 1904.
13. Rutherford, Phys. Zeit. 3, No. 12, p. 254, 1902.
14. F. von Lerch, Annal. d. Phys. 12, p. 745, 1903.
15. Pegram, Phys. Review, p. 424, Dec. 1903.
16. Miss Gates, Phys. Review, p. 300, 1903.
17. A more complete examination of the effect of temperature on the excited activity of thorium has been made by Miss Slater (section 207).
18. Rutherford, Phil. Mag. Feb. 1900.
19. Henning, Annal d. Phys. 7, p. 562, 1902.
20. Rutherford, Phil. Mag. Feb. 1900.
21. Curie and Debierne, C. R. 132, p. 768, 1901.
22. Fehrle, Phys. Zeit. 3, No. 7, p. 130, 1902.
23. Rutherford, Phil. Mag. Jan. 1903.
24. Giesel, Ber. d. D. Chem. Ges. 36, p. 342, 1903.
25. Debierne, C. R. 136, pp. 446 and 671, 1903; 138, p. 411, 1904.