Chapter XI: Transformation Products of Radium (2)

If observations of the active deposit are begun shortly after its formation, it is found that the activity, measured by the β rays, is small at first, but increases with the time, reaching a practical maximum about 40 days later. Experiments were made on a platinum plate, which was exposed for 3·75 days in a vessel containing the radium emanation. The observations of the β ray activity began 24 hours after removal. The results are shown in Fig. 94, where the time was measured from the middle of the time of exposure to the emanation. Similar results were obtained for a negatively charged wire exposed to the emanation. The curve, if produced back to the origin, is seen to be very similar to the recovery curves of Ur X, and other active products, and can be expressed by the equation

$$ \frac {I_t} {I₀} = 1 − e^{–λt} $$,

where _I₀_ is the maximum activity. The activity reaches half its final value in about six days, and the value of λ is equal to ·115 (day)⁻¹. We have shown in section 203 that a rising curve of this character indicates that the β ray activity arises from a product which is supplied at a constant rate from a primary source. Before discussing in detail the explanation of these curves, showing the rise with time of the α and β ray activity, further experimental results will be considered.

=232. Effect of temperature on the activity.= A platinum plate, made active in the manner described, was exposed to varying temperatures in an electric furnace, and the activity tested at atmospheric temperature after exposure. Four minutes’ exposure in the furnace, at first at 430° C., and afterwards at 800° C., had little, if any, effect on the activity. After four minutes at about 1000° C. the activity decreased about 20 per cent., and a further exposure of eight minutes at a temperature of about 1050° C. almost completely removed the α ray activity. On the other hand, the β ray activity, when measured immediately after removal, was not altered by the heating, but exposure to a still higher temperature caused it to decrease. These results show that the active matter consists of two kinds. The part which emits β rays is not volatile at 1000° C., but the other part, which emits α rays, is almost completely volatilized at that temperature.

It was found, however, that the β ray activity after heating to about 1000° was not permanent, but decayed according to an exponential law with the time, the activity decreasing to half value in about 4·5 days. From the recovery curve of the β ray activity already considered, it was to be expected that the activity would decay to half value in six days. This difference in the periods is possibly due to an effect of the high temperature in altering the rate of decay of radium E. The period of six days is more probably correct. The results obtained on the rise and decay of the β rays, taken together, show:—

(1) That the product giving β rays is supplied at a constant rate
from some parent matter of very slow rate of change.

(2) That this parent matter is volatilized at or below 1000° C., and
the β ray product is left behind. Since the parent matter is
removed, the product immediately begins to lose its activity at its
characteristic rate, viz. the activity falls to half value in about
six days.

=233. Separation of the constituents by means of a bismuth plate.= The active matter of slow decay was obtained in solution by introducing dilute sulphuric acid into a glass tube in which the emanation from 30 milligrams of radium bromide had been stored for a month. The solution showed strong activity and gave out both α and β rays, the latter, as in other cases, being present in an unusually large proportion.

When a polished bismuth disk was kept for some hours in the solution, it became strongly active. The active matter deposited on the bismuth gave out α rays, but no trace of β rays. After several bismuth disks had been successively left in the solution, the active matter, which emits α rays, was almost completely removed. This was shown by evaporating down the solution after treatment. The β ray activity remained unchanged, but that of the α rays had been reduced to about 10 per cent. of its original value. Three bismuth disks, made active in this way, were set aside and their activity measured at regular intervals. The activity fell off according to an exponential law with the time during the 200 days since their removal, while that of each fell to half value on an average in about 143 days.

At the same time it was observed that the solution, from which the α ray activity was removed, gradually regained its activity, showing that the active substance which gave out α rays was continuously produced from the matter left behind in the solution.

=234. Explanation of the results.= We have seen that a close examination of the active deposit of slow change has disclosed,

(1) the presence of a β ray product which loses half of its activity
in about six days;

(2) the presence of an α ray product, which is deposited on bismuth
and is volatilized at 1000° C. This product loses half of its
activity in 143 days;

(3) the presence of a parent substance, which produces the β ray
product at a constant rate.

This parent product must be transformed very slowly since the β ray product, which arises from it, soon reaches an equilibrium value, which does not change appreciably over a period of more than one year. The experimental evidence points to the conclusion that the parent product does not give rise to β rays, but that the β rays arise entirely from the next product. This parent product cannot give rise to α rays, for we have seen that the initial α ray activity is at first extremely small, but increases steadily with the time for a period of at least eighteen months. Thus the parent product does not give rise to either α or β rays, and must be a “rayless” product.

The first three transition products of the radium emanation, viz. radium A, B and C, have already been analysed, and shown to be consecutive. It thus seems probable that the active deposit of slow change must arise from the successive transformations of the last product radium C. The results already obtained can be completely explained if it is supposed that three transition products, viz. radium D, E and F, are present in the active deposit of slow rate of change. The properties of these products are summarized below.

_Radium D_ is a rayless product of very slow rate of change. It will
be shown later that it is half transformed in about 40 years. It is
volatile below 1000° C. and is soluble in strong acids.

_Radium E_ is produced from radium D. In breaking up, it emits β
(and probably γ) rays but no α rays. It is half transformed in about
6 days and is not so volatile as radium D and F.

_Radium F_ is produced from radium E. It emits only α rays and is
half transformed in 143 days. This substance in solution attaches
itself to bismuth. It is volatile at about 1000° C.

Apart from their value and interest in showing the stages of transformation of the radium atom, the results of this analysis have an important bearing upon the origin of some of the well-known radio-active substances separated from pitchblende; for it will be shown later that the product radium F is the radio-active substance present in radio-tellurium and probably also in polonium. In addition, there is very strong evidence that the radio-active lead obtained by Hofmann contains the three products radium D, E and F together.

The changes of radium as far as they are at present known, are shown diagrammatically in Fig. 95. It is possible that further investigation will show that the transformation does not end with radium F.

While we have shown that radium D is the parent of E, we have not given any conclusive evidence that E is the parent of F. This evidence is, however, supplied by the following experiment. A platinum plate, made active in the manner already described, was placed in an electric furnace and heated for four minutes at about 1000° C. Most of the products D and F were volatilized, but E was left behind. Since the parent matter D was removed, E at once commenced to lose its β ray activity. At the same time it was observed that the small α ray activity, left behind on the platinum plate, increased rapidly at first and then more slowly, as the activity of E became smaller and smaller. This experiment shows conclusively that E was the parent of F, the α ray product.

=235. Rate of transformation of radium D.= It has been observed experimentally that each of the products of radium, which emit α rays, supplies about an equal proportion of the activity of radium when in radio-active equilibrium. Since, when equilibrium is reached, the same number of particles of each of the successive products must break up per second, this is an expression of the fact that every atom of each product breaks up with the expulsion of an equal number (probably one) of α particles. Now radium D is directly derived from radium C, and, since the rate of change of D is very slow compared with that of C, the number of particles of D initially present must be very nearly equal to the number of particles of radium C which break up during the time that radium D is being formed. Now D does not itself give out rays, but the succeeding product E does. The products D and E are practically in radio-active equilibrium one month after D is set aside, and the variation of the β ray activity of E then serves as a measure of the variation of the parent product D. Suppose that a vessel is filled with a large quantity of radium emanation. After several hours, the product radium C, which emits β rays, reaches a maximum value, and then decreases at the same rate as the emanation loses its activity, _i.e._ it falls to half value in 3·8 days. If _N₁_ is the number of β particles expelled from radium C at its maximum value, the total number _Q₁_ of β particles expelled during the life of the emanation is given approximately by

$$ Q_1 = \int₀^{\infty} N_1 e^{–λ_1 t} dt = \frac {N_1} {λ_1} $$,

where λ₁ is the constant of change of the emanation.

After the emanation has disappeared, and the final products D + E are in radio-active equilibrium, suppose that the number of β particles _N₂_ expelled per second by radium E is determined. If _Q₂_ is the total number of particles expelled during the life of D + E, then _Q₂_ as before is approximately given by _Q₂_ = _N₂_/λ₂ where λ₂ is the constant of change of radium D. Now we have seen that if each particle of C and of E gives rise to one β particle, it is to be expected that

_Q₁_ = _Q₂_,

or

λ₂ _N₂_
---- = ---- .
λ₁ _N₁_

The ratio _N₂_/_N₁_ was determined by measuring the activity due to the β rays from C and E in the same testing-vessel. Then, since _N₂_/_N₁_ is known, and also the value of λ₁, the value of the constant of change, λ₂, of radium D is obtained. In this way it was calculated that D is half transformed in about 40 years.

In the above calculations it is assumed, as a first approximation, that the β rays from C and E have the same average velocity. This is probably not accurately the case, but the above number certainly serves to fix the order of magnitude of the period of the product D. This calculation is confirmed by observations to be given later on the amount of D and E in old radium.

It may be of interest to mention that the writer calculated the period of radium F by a similar method, before its value was experimentally determined, and found that F should be half transformed in about one year. This is not very different from the experimental value of 143 days found later. In addition, it was assumed in the calculation that the α particles from C and F were projected with the same velocity, and in consequence produced the same amount of ionization. In practice, however, it is found that the α particle of F is absorbed in about half the distance of the α particles of C, and in consequence produces only about half of the ionization of the latter. If this correction were made, the calculated period for half transformation would be six months instead of one year.

A table of the transformation products of radium, together with some of their physical and chemical properties, is given below.

Transformation Time to be Rays Chemical and
Products half Physical
transformed Properties

Radium 1200 years α rays —

Emanation 3·8 days α rays Chemically
inert gas;
condenses at
−150° C.

Radium A 3 mins. α rays Behaves as
(active deposit solid;
of rapid deposited on
change) the surface of
bodies;
concentrated on
cathode in
electric field.
Soluble in
strong acids;
volatile at a
white heat. B
is more
volatile than A
or C.

:: B (same) 21 mins. no rays Same

:: C (same) 28 mins. α, β, γ rays Same

:: D (active about 40 no rays Soluble in
deposit of slow years strong acids
change) and volatized
below 1000° C.

:: E (same) 6 days β (and γ) Non-volatile at
1000°C.

:: F (same) 143 days α rays Volatile at
1000° C;
deposited from
solution on to
bismuth plate.

? — — —

=236. Variation of the activity over long periods of time.= We are now in a position to calculate the variation of the α and β ray activity of the active deposit over long periods of time. If it is supposed that the matter initially deposited consists only of D, the amounts _P_, _Q_ and _R_ of radium D, E and F existing at any later time are given by the equations 3, 4, 5, section 197.

Since, however, the intermediate product E has a much more rapid rate of change than D or F, the equations can be simplified, without much loss of accuracy, by disregarding the change E, and by supposing that D gives out β rays and changes directly into the α ray product F.

Let λ₁, λ₂ be the constants of change D and F respectively. Let _n₀_ be the number of particles of D present initially. Then using the notation of section 197, the amount _P_ of radium D at any time _t_ is given by

$$ P = n₀ e^{–λ_1 t} $$ .

The amount _Q_ of radium F is given by

$$ Q = \frac {n₀ λ_1} {λ_1 − λ_2} (e^{–λ_2 t} - e^{–λ_1 t}) $$ .

The number of β particles emitted by D + E per second, some months afterwards, is

$$ λ_1 n₀ e^{–λ_1 t} $$,

and the number of α particles emitted by radium F is

$$ \frac {λ_1 λ_2 n₀} {λ_1 − λ_2} (e^{–λ_2 t} − e^{–λ_1 t} ) $$ .

The results are shown graphically in Fig. 96, by the curves _EE_ and _FF_, in which the ordinates represent the number of β and α particles expelled per second by the products D and F respectively. The complete calculation for three changes shows that the number of β particles soon reaches a practical maximum, and then decays nearly exponentially with the time, falling to half value in 40 years. The number of α particles expelled per second increases for several years, but reaches a maximum after 2·6 years and then diminishes, finally falling off exponentially with the time to half value in 40 years.

The experimental curve of the rise of α ray activity, shown in Fig. 93, as far as it has been determined, lies accurately on this curve, if the maximum is calculated from the above theory. The observed activity after a period of 250 days is marked by the point _X_ on the curve.

=237. Experiments with old radium.= Since the substance radium D is produced from radium at a constant rate, the amount present mixed with the radium will increase with its age. The writer had in his possession a small quantity of impure radium chloride, kindly presented by Professors Elster and Geitel four years before. The amount of radium D present in it was tested in the following way:—The substance was dissolved in water and kept continuously boiling for a period of about six hours. Under these conditions the emanation is removed as rapidly as it is formed, and the β rays from the radium, due to the product radium C, practically disappear. A newly prepared specimen of radium bromide under these conditions retains only a fraction of 1 per cent. of its original β radiation. The old radium, however, showed (immediately after this treatment) an activity measured by the β rays of about 8 per cent. of its original amount. The activity could not be reduced any lower by further boiling or aspiration of air through the solution. This residual β ray activity was due to the product radium E stored up in the radium. The β ray activity due to radium E was thus about 9 per cent. of that due to radium C. Disregarding the differences in the absorption of the β rays, when the activity of the product E in radium reaches a maximum value, the β ray activity due to it should be the same as that due to C. Since the parent product D is half transformed in forty years, the amount present in the radium after four years should be about 7 per cent. of the maximum amount; _i.e._ it should show a β ray activity of about 7 per cent. of that due to radium C. The observed and calculated values (7 and 9 per cent. respectively) are thus of the same order of magnitude. The amount of β rays from radium E present in pure radium bromide about one year old was about 2 per cent. of the total.

The amount of radium F present in old radium was measured by observations of the activity imparted to a bismuth disk left for several days in the solution, and was found to be of the same order as the theoretical value. Radium F is not deposited to an appreciable extent on the bismuth from a water solution of radium bromide. If, however, a trace of sulphuric acid is added to the solution, the radium F is readily deposited on the bismuth. The addition of sulphuric acid to the radium solution practically effected a separation of radium D, E and F from the radium proper; for the latter was precipitated as sulphate and the products D, E and F remained in solution. After filtering, the solution contained the greater proportion of the products D, E, and F and very little radium.

=238. Variation of the activity of radium with time.= It has been shown that the activity of freshly prepared radium increases at first with the time and practically reaches a maximum value after an interval of about one month. The results already considered show that there is a still further slow increase of activity with the time. This is the case whether the activity is measured by the α or β rays. It will be shown later that radium is probably half transformed in about 1000 years. From this it can readily be calculated that after a lapse of about 200 years the amount of the products radium D, E and F will have reached a maximum value. The same number of atoms of each of the products C and E will then break up per second. If each atom of these products in disintegrating throws off an equal number (probably one) of β particles, the number of β particles thrown off per second will be twice as great as from radium a few months old. The number will increase at first at the rate of about 2 per cent. a year.

Similar considerations apply to the α ray activity. Since, however, there are four other products of radium besides radium itself which expel α particles, the number of α particles emitted per second from old radium will not be more than 25 per cent. greater than the number from radium a few months old. The activity measured by the α rays will thus not increase more than 25 per cent., and probably still less, as the α particles from radium F produce less ionization than the α particles expelled from the other radium products. The activity of radium will consequently rise to a maximum after 200 years and then slowly die away with the time.

=239. Presence of these products in pitchblende.= The products radium D, E and F must be present in pitchblende in amounts proportional to the quantity of radium present, and should be capable of separation from the mineral by suitable chemical methods. The radio-active properties of these substances, if obtained in the pure state, are summarized below.

_Radium D_ when first separated, should give out very little α or β radiation. The β ray activity will rapidly increase, reaching half its maximum value in 6 days. The α ray activity will at first increase nearly proportionately with the time, and will reach a maximum value after an interval of about 3 years. The α and β ray activity, after reaching a maximum, will finally decay, the activity falling to half value in about 40 years. Since radium D is half transformed in 40 years, and radium in 1200 years, the maximum β ray activity of radium D, weight for weight, will be about 300 times that of radium.

The α ray activity, at any time, will be removed by placing a bismuth disk in the solution.

_Radium F_, after separation, will give out only α rays. Its activity, after separation, will decrease according to an exponential law, falling to half value in 143 days. Since radium in radio-active equilibrium contains four products which emit α rays, the number of α particles expelled per second from radium F will, weight for weight, be about 800 times as numerous as from new radium in radio-active equilibrium. Since the α particles from radium F produce only about half as much ionization as the α particles from the other radium products, the activity of radium F, measured by the electric method, will be about 400 times that of radium.

=240. Origin of radio-tellurium and polonium.= It is now necessary to consider whether these products of radium have been previously separated from pitchblende, and known by other names.

We shall first consider the α ray product, radium F. The radio-tellurium of Marckwald and the polonium of Mme Curie both resemble radium F in giving out only α rays, and in being deposited on a bismuth disk from a solution. If the active constituent present in radio-tellurium is the same as radium F, its activity should decay at the same rate as the latter. The writer[320] has carefully compared the rates of decay of the activity of radium F and of the radio-tellurium of Marckwald and found them to be the same within the limits of experimental error. Both lose half of their activity in about 143 days[321]. A similar value of the rate of decay of radio-tellurium has been obtained by Meyer and Schweidler[322].

The experiments on radio-tellurium were made upon the active bismuth plates supplied by Dr Sthamer of Hamburg, which were prepared under Marckwald’s directions.

An additional proof[323] of the identity of these two products was obtained by comparing the absorption of the α rays by aluminium foil. The α rays from different products are projected with different velocities, and, in consequence, are unequally absorbed by matter. The absorption of the rays from the two products by aluminium foil agreed very closely, indicating the probable identity of the substances from which they were emitted.

There can thus be no doubt that the active constituent present in the radio-tellurium of Marckwald is identical with the product radium F. This is a very interesting result, and shows how the close examination of the successive transformations of the radio-active bodies may throw light on the origin of the various substances found in pitchblende.

We have already seen (section 21) that Marckwald, by special chemical methods, was able to obtain a few milligrams of very active substance by working over 2 tons of pitchblende. We have already seen (section 239) that this substance, if obtained in the pure state, should be about 400 times as active as radium. Comparative measurements of the activity of this substance with radium will thus indicate the amount of impurity that is present with the former. This method should be of value in purifying radium F for the purpose of determining its spectrum, which has not yet been observed.

=241. Polonium.= Since the separation of the active substance by Marckwald, called by him radio-tellurium, there has been some discussion as to whether the active constituent is the same as that present in the polonium of Mme Curie. Both of these substances have similar radio-active and chemical properties, but the main objection to the view that the active constituents were identical has rested on an early statement of Marckwald that the activity of one of his very active preparations did not decay appreciably in the course of six months. This objection is now removed, for we have seen that the activity of radio-tellurium does decay fairly rapidly. It was early recognised that the activity of the polonium, separated from pitchblende by the methods of Mme Curie, was not permanent, but decayed with the time. Observations on the rate of decay have not been very precise, but Mme Curie states that some of her preparations lost half of their activity in about six months but in others the rate of decay was somewhat smaller. It is possible that the initial differences observed in the rates of decay of different specimens of polonium may be due to the presence of some radium D with the polonium. The polonium in my possession lost its activity fairly rapidly, and was reduced to a small portion of its value in the course of about four years. Rough observations of its activity, made from time to time, showed that its activity diminished to half value in about six months. If it is identical with radio-tellurium, the activity should decay to half value in 143 days, and I think there is little doubt that more accurate measurement will prove this to be the case.

While the proof of the identity of the active constituent in polonium is not so definite as for radio-tellurium, I think there can be no reasonable doubt that these substances both contain the same active substance, which is the seventh transformation product of radium. Marckwald has noticed some chemical differences in the behaviour of polonium and radio-tellurium, but little weight can be attached to such observations, for it must be remembered that the active constituent in both cases is present in minute quantity in the material under examination, and that the apparent chemical properties of the active substance are much influenced by the presence of impurities. The most important and trustworthy test rests upon the identity of the radiations and the period of decay.

=241 A. Origin of radio-active lead.= Some experiments will now be discussed which show that the radio-lead first separated from pitchblende by Hofmann (section 22) contains the products radium D, E and F. Hofmann has observed that the activity of this substance did not appreciably decay in the course of several years. In some recent experiments, Hofmann, Gonder and Wölfl[324] have made a close chemical examination of the radio-active lead, and have shown the presence of two radio-active constituents, which are probably identical with the products radium E and F. The radio-active measurements were unfortunately not very precise, and the periods of change of the separated products have not been examined very closely.

Experiments were made on the effect of adding substances to a solution of radio-lead, and then removing them by precipitation. Small quantities of iridium, rhodium, palladium, and platinum, in the form of chlorides, were left in the solution for three weeks, and then precipitated by formalin or hydroxylamine. All of these substances were found to give out both α and β rays, the activity being greatest for rhodium and least for platinum. A large proportion of the β ray activity disappeared in the course of six weeks, and of the α ray activity in one year. It is probable that the two products radium E and F were in part removed with the metals from the radio-lead. We have seen that radium E gives out β rays and loses half of its activity in about six days, while radium F gives out only α rays and its activity falls to half value in 143 days. This conclusion is further confirmed by experiments on the effect of heat on the activity of these substances. By heating to a full red heat, the α ray activity was lost in a few seconds. This is in agreement with the results (section 232) where we have seen that radium F is volatilized at about 1000° C. and radium E is left behind.

Salts of gold, silver and mercury added to the radio-lead were found to show only α ray activity on removal. This is in accordance with the view that radium F alone is removed with these substances. Bismuth salts on the other hand showed initially α and β ray activity, but the latter rapidly died away. The presence of β rays in freshly prepared polonium was early observed by Mme Curie. The α and β ray activity of the radio-lead is much reduced by the precipitation of bismuth added to the solution. The α and β ray activity of the radio-lead, however, recovers itself again. This result is exactly what is to be expected if radio-lead contains radium D, E and F. Radium E and F are removed with the bismuth, but the parent substance, radium D, is left behind, and, in consequence, a fresh supply of radium E and F is produced.

While further experiments are required to settle definitely whether the products separated from radio-lead are identical with radium E and F, there can be little doubt that such is the case. This conclusion is strengthened by some experiments which I have made on a specimen of radio-lead, which was kindly forwarded to me by Mr Boltwood of New Haven. This active lead gave out α and β rays, the latter being in unusually large proportion. The active lead was four months old when first tested. The β ray activity in the following six months has remained sensibly constant, but the α ray activity has steadily increased. These results are to be expected if the radio-lead contains radium D. Radium E will reach a practical maximum about 40 days after separation of the product radium D with the lead. The α ray activity due to radium F should increase to a maximum in about 2·6 years (see section 236).

Further experiments are required to settle whether the lead immediately after separation from pitchblende contains only radium D, or whether radium E also appears with it. It seems likely, however, that the bismuth, which is initially present in solution at the time of separation of the lead, will retain both radium E and F, and that the presence of these products in radio-lead is due to their production, after separation, by the parent substance, radium D.

It would be of scientific value to separate radium D from pitchblende and obtain it in the pure state, for, a month after removal, the β ray activity from it would be about 300 times as great as from an equal weight of radium. By placing a bismuth plate in a solution of this substance, radium F (polonium) should be separated, and, provided a sufficient interval is allowed to elapse, a fresh supply of radium F can at any time be obtained.

The rate of transformation of radium D (half transformed in 40 years) is sufficiently slow not to interfere seriously with its utility in most experiments.

The results of the comparison of the products of radium with those contained in polonium, radio-tellurium and radio-lead are summarized below.

Radium D = product in _new radio-lead_, no rays. Half transformed in
40 years.

Radium E gives out β rays, separated with bismuth, iridium and
platinum. Half transformed in 6 days.

Radium F = product in _polonium_ and _radio-tellurium_. Gives out
only α rays. Half transformed in 143 days.

=242. Temporary activity of inactive matter separated from radio-active substances.= We have seen in the last section that the platinum metals and bismuth acquire temporary activity by their admixture with a solution of radio-lead, and that these effects are very satisfactorily explained on the view that some of the products of change of radio-lead are removed with the inactive substances. Very similar effects have been observed by Pegram and von Lerch (section 186), when inactive substances were added to solutions of thorium and of the active deposit of thorium. These results, too, are almost certainly due to the removal of one or more of the products of thorium with the inactive matter. Examples of this character may readily be multiplied, and some of the more interesting and important of these will be briefly discussed later.

There have been two general points of view regarding the character of this activity which is temporarily acquired by inactive matter. Some people have supposed that the inactive molecules of the substance, mixed with the solution, acquire by “radio-active induction” temporary activity, the underlying idea being that the close admixture of an inactive and an active substance has communicated the property of radiating to some of the molecules of the former. According to the disintegration theory of radio-activity, on the other hand, the temporary activity of originally inactive matter is not due to any alteration of the inactive substance itself, but to an admixture with it of one or more of the numerous radio-active products. The idea of “radio-active induction” has no definite experimental evidence in support of it, while there is much indirect evidence against it.

We shall now consider how these facts are interpreted according to the disintegration theory. In a specimen of old radium, for example, there are present, besides radium itself, the seven successive products which arise from it. Each of these differs in chemical and physical properties from the others. If now, for example, a bismuth rod is introduced into the solution, one or more of these products are deposited on the bismuth. This action is most probably electrolytic in nature, and will depend upon the electro-chemical behaviour of the bismuth compared with that of the products in solution. An electro-negative substance will tend to remove the product or products which are strongly electro-positive. This point of view serves to explain why different metals are made active to different degrees, depending upon their position in the electro-chemical series.

It seems probable that the activity communicated to inactive matter by precipitation from an active solution occurs only during the precipitation. The correctness of this view could readily be tested by observing whether the time that the inactive substance is present in solution has any effect on the magnitude of the activity imparted to it.

When it is remembered that in pitchblende there are present the radio-elements uranium, thorium, radium and actinium and their numerous family of products, it is not surprising that many of the inactive substances separated from it may show very considerable activity due to the mixture of products which may be removed with them. In carrying out experiments on the separation of radium from pitchblende, M. and Mme Curie observed that the separation of the active substance is fairly complete if the stage of purification is not far advanced. Copper, antimony and arsenic can be separated only slightly active, but other substances like lead and iron always show activity. When the stage of precipitation is more advanced, every substance separated from the active solution shows activity.

One of the earliest observations in this direction was made by Debierne, who found that barium could be made active by solution with actinium. The active barium removed from the actinium still preserved its activity after chemical treatment, and, in this way, barium chloride was obtained whose activity was 6000 times that of uranium. Although the activity of the barium chloride could be concentrated in the same way as the activity of radiferous barium chloride, it did not show any of the spectroscopic lines of radium, and could not have been due to the admixture of that element with the barium. The activity of the barium was not permanent, and Debierne states that the activity fell to about one-third of its value in three months. It seems probable that the precipitated barium carried down with it the product actinium X, and also some of the actinium itself, and that the decay observed was due to the transformation of actinium X. It is interesting to note that barium is capable of removing a large number of products of the different radio-elements. This effect is probably connected with its position in the electro-chemical series, for barium is highly electro-positive.

Giesel showed in 1900 that bismuth could be made active by placing it in a radium solution, and considered that polonium was in reality bismuth made active by the process of induction. In later experiments, he found that the bismuth plate gave out only α rays, and that the activity of the bismuth could not be ascribed to radium, since no β rays were present. We have seen that this activity of the bismuth is due to the product radium F deposited on its surface.

Mme Curie also found that bismuth was made active by solution with a radium compound, and succeeded in fractionating the above bismuth in the same way as polonium. In this way bismuth was obtained 2000 times as active as uranium, but the activity, like that of polonium separated from pitchblende, decreased with the time. In the light of the experiments on the transformation products of radium, it is seen that these early experiments of Mme Curie add additional confirmation to the view that the product (radium F) separated from radium itself is identical with the polonium obtained directly from pitchblende.

Footnote 314:

Rutherford and Soddy, _Phil. Mag._ April, 1903.

Footnote 315:

Rutherford, _Phil. Trans._ A. p. 169, 1904. Curie and Danne, _C. R._
p. 748, 1904.

Footnote 316:

P. Curie and Danne, _Comptes Rendus_, 138, p. 748, 1904.

Footnote 317:

Miss Gates, _Phys. Rev._ p. 300, 1903.

Footnote 318:

Miss Brooks, _Nature_, July 21, 1904.

Footnote 319:

Rutherford, _Phil. Mag._ Nov. 1904. _Nature_, p. 341, Feb. 9, 1905.

Footnote 320:

Rutherford, _Nature_, p. 341, Feb. 9, 1905.

Footnote 321:

Marckwald (_Ber. d. D. Chem. Ges._ p. 591, 1905) has recently found
that the activity of his radio-tellurium falls to half value in 139
days.

Footnote 322:

Meyer and Schweidler, _Wien Ber._ Dec. 1, 1904.

Footnote 323:

Rutherford, _Phil. Trans._ A. p. 169, 1904.

Footnote 324:

Hofmann, Gonder and Wölfl, _Annal. d. Phys._ 15, p. 615, 1904.