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.

=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[278] found that the excited activity decays approximately according to an exponential law, falling to half value in 41 minutes. Giesel[279] 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[280] 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, 5, 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[281] 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[282] 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[283] 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[284] 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[285].

Temperature Percentage of activity removed

Heated 2 minutes 800° C. 0

then „ ½ minute 1020° C. 16 more

„ „ ½ „ 1260° C. 52 „

„ „ ½ „ 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[286] shown in 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[287]. 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[288], 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 ⅒ 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[289] 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[290] 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[291] 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. _E₀_ 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. _E₁_ less than _E₀_ 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.

When the plate _B_ is negatively charged, the E.M.F. between the plates is _E₀_ − _E₁_, when _B_ is positive the E.M.F. is _E₀_ + _E₁_.

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

_E₀_ − _E₁_ ---------- _K_ _d_

and, in the course of the next half alternation,

_E₀_ + _E₁_ ---------- _K_ _d_

towards the plate _A_.

If _x₁_ is less than _d_, the greatest distances _x₁_, _x₂_ passed over by the positive carrier during two succeeding half alternations is thus given by

_E₀_ − _E₁_ _x₁_ = ---------- _KT_ _d_

and

_E₀_ + _E₁_ _x₂_ = ---------- _KT_ _d_

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 _x₁_ of the plate _B_. This number is equal to

1 --- _x₁_ _qT_ 2

(2) All the carriers which are left within the distance _x₁_ from _B_ at the end of the previous half alternation. The number of these can readily be shown to be

1 _x₁_ --- _x₁_ ---- _qT_ 2 _x₂_

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 recombination 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 2_dqT_. The ratio ρ of the number which reach _B_ to the total number produced is thus given by

$$ \rho = \frac {\frac {1}{2} x_1qT + \frac {1}{2} x_1 \frac {x_1}{x_2} qT} {2dqT} = \frac {1}{4} \frac {x_1}{d} \frac {x_1 + x_2} {x_2} $$ .

Substituting the values of _x₁_ and _x₂_, we find that

$$ K = \frac {2 (E₀ + E_1) d^2} {E₀ (E₀ − E_1) T} \rho $$ .

In the experiments, the values of _E₀_, _E₁_, _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._

_E₀_ + _E₀_ − Alternations ρ _K_ _E₁_ _E₁_ per second

152 101 57 ·27 1·25

225 150 57 ·38 1·17

300 200 57 ·44 1·24

_Plates 2 cms. apart._

_E₀_ + _E₀_ − Alternations ρ _K_ _E₁_ _E₁_ 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[292] 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[293] 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.

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.

Footnote 269:

M. and Mme. Curie, _C. R._ 129, p. 714, 1899.

Footnote 270:

Rutherford, _Phil. Mag._ Jan. and Feb. 1900.

Footnote 271:

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.”

Footnote 272:

Rutherford, _Phil. Mag._ Feb. 1900.

Footnote 273:

Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902. _Phil. Mag._ Jan. 1903.

Footnote 274:

Miss Brooks, _Phil. Mag._ Sept. 1904.

Footnote 275:

Rutherford and Miss Brooks, _Phil. Mag._ July, 1902.

Footnote 276:

Curie and Danne, _C. R._ 136, p. 364, 1903.

Footnote 277:

Mme Curie, _Thèse_, Paris, 1903, p. 116.

Footnote 278:

Debierne, _C. R._ 138, p. 411, 1904.

Footnote 279:

Giesel, _Ber. d. D. Chem. Ges._ No. 3, p. 775, 1905.

Footnote 280:

Miss Brooks, _Phil. Mag._ Sept. 1904.

Footnote 281:

Rutherford, _Phys. Zeit._ 3, No. 12, p. 254, 1902.

Footnote 282:

F. von Lerch, _Annal. d. Phys._ 12, p. 745, 1903.

Footnote 283:

Pegram, _Phys. Review_, p. 424, Dec. 1903.

Footnote 284:

Miss Gates, _Phys. Review_, p. 300, 1903.

Footnote 285:

A more complete examination of the effect of temperature on the excited activity of thorium has been made by Miss Slater (section 207).

Footnote 286:

Rutherford, _Phil. Mag._ Feb. 1900.

Footnote 287:

Henning, _Annal. d. Phys._ 7, p. 562, 1902.

Footnote 288:

Rutherford, _Phil. Mag._ Feb. 1900.

Footnote 289:

Curie and Debierne, _C. R._ 132, p. 768, 1901.

Footnote 290:

Fehrle, _Phys. Zeit._ 3, No. 7, p. 130, 1902.

Footnote 291:

Rutherford, _Phil. Mag._ Jan. 1903.

Footnote 292:

Giesel, _Ber. d. D. Chem. Ges._ 36, p. 342, 1903.

Footnote 293:

Debierne, _C. R._ 136, pp. 446 and 671, 1903; 138, p. 411, 1904.