Radio-Activity

254. Theories of radio-activity. In previous chapters, a detailed account has been given of the nature and properties of the radiations, and of the complex processes taking place in the radio-active substances. The numerous products arising from the radio-elements have been closely examined, and have been shown to result from a transformation of the parent element through a number of well-marked stages. In this chapter, the application of the disintegration theory to the explanation of radio-active phenomena will be considered still further, and the logical deductions to be drawn from the theory will be discussed briefly.

A review will first be given of the working hypotheses which have served as a guide to the investigators in the field of radio-activity. These working theories have in many cases been modified or extended with the growth of experimental knowledge.

The early experiments of Mme Curie had indicated that radio-activity was an atomic and not a molecular phenomenon. This was still further substantiated by later work, and the detection and isolation of radium from pitchblende was a brilliant verification of the truth of this hypothesis.

The discovery that the β rays of the radio-elements were similar to the cathode rays produced in a vacuum tube was an important advance, and has formed the basis of several subsequent theories. J. Perrin^[333], in 1901, following the views of J. J. Thomson and others, suggested that the atoms of bodies consisted of parts and might be likened to a miniature planetary system. In the atoms of the radio-elements, the parts composing the atoms more distant from the centre might be able to escape from the central attraction and thus give rise to the radiation of energy observed. In December 1901, Becquerel^[334] put forward the following hypothesis, which, he stated, had served him as a guide in his investigations. According to the view of J. J. Thomson, radio-active matter consists of negatively and positively charged particles. The former have a mass about ¹⁄₁₀₀₀ of the mass of the hydrogen atom, while the latter have a mass about one thousand times greater than that of the negative particle. The negatively charged particles (the β rays) would be projected with great velocity, but the larger positive particles with a much lower velocity forming a sort of gas (the emanation) which deposits itself on the surface of bodies. This in turn would subdivide, giving rise to rays (excited activity).

In a paper communicated to the Royal Society in June 1900, Rutherford and McClung^[335] estimated that the energy, radiated in the form of ionizing rays into the gas, was 3000 gram-calories per year for radium of activity 100,000 times that of uranium. Taking the latest estimate of the activity of a pure radium compound as 2,000,000, this would correspond to an emission of energy into the gas in the form of α rays of about 66,000 gram-calories per gram per year. The suggestion was made that this energy might be derived from a re-grouping of the constituents of the atom of the radio-elements, and it was pointed out that the possible energy to be derived from a greater concentration of the components of the atom was large compared with that given out in molecular reactions.

In the original papers^[336] giving an account of the discovery of the emanation of thorium and the excited radio-activity produced by it, the view was taken that both of these manifestations were due to radio-active material. The emanation behaved like a gas, while the matter which caused excited activity attached itself to solids and could be dissolved in some acids but not in others. Rutherford and Miss Brooks showed that the radium emanation diffused through air like a gas of heavy molecular weight. At a later date Rutherford and Soddy showed that the radium and thorium emanations behaved like chemically inert gases, since they were unaffected by the most drastic physical and chemical treatment.

On the other hand, P. Curie, who, in conjunction with Debierne, had made a series of researches on the radium emanation, expressed dissent from this view. P. Curie^[337] did not consider that there was sufficient evidence that the emanation was material in nature, and pointed out that no spectroscopic evidence of its presence had yet been obtained, and also that the emanation disappeared when contained in a sealed vessel. It was pointed out by the writer^[338] that the failure to detect spectroscopic lines was probably a consequence of the minute quantity of the emanation present, under ordinary conditions, although the electrical and phosphorescent actions produced by this small quantity are very marked. This contention is borne out by later work. P. Curie at first took the view that the emanation was not material, but consisted of centres of condensation of energy attached to the gas molecules and moving with them.

M. and Mme Curie have throughout taken a very general view of the phenomena of radio-activity, and have not put forward any definite theory. In Jan. 1902, they gave an account of the general working theory^[339] which had guided them in their researches. Radio-activity is an atomic property, and the recognition of this fact had created their methods of research. Each atom acts as a constant source of emission of energy. This energy may either be derived from the potential energy of the atom itself, or each atom may act as a mechanism which instantly regains the energy which is lost. They suggested that this energy may be borrowed from the surrounding air in some way not accounted for by the principle of Carnot.

In the course of a detailed study of the radio-activity of thorium, Rutherford and Soddy^[340] found that it was necessary to suppose that thorium was continuously producing from itself new kinds of active matter, which possess temporary activity and differ in chemical properties from the thorium itself. The constant radio-activity of thorium was shown to be the result of equilibrium between the processes of production of active matter and the change of that already produced. At the same time, the theory was advanced that the production of active matter was a consequence of the disintegration of the atom. The work of the following year was devoted to an examination of the radio-activity of uranium and radium on similar lines, and it was found that the conclusions already advanced for thorium held equally for uranium and radium^[341]. The discovery of a condensation of the radio-active emanations^[342] gave additional support to the view that the emanations were gaseous in character. In the meantime, the writer^[343] had found that the rays consisted of positively charged bodies atomic in size, projected with great velocity. The discovery of the material nature of these rays served to strengthen the theory of atomic disintegration, and at the same time to offer an explanation of the connection between the α rays and the changes occurring in the radio-elements. In a paper entitled “Radio-active Change,” Rutherford and Soddy^[344] put forward in some detail the theory of atomic disintegration as an explanation of the phenomena of radio-activity, and at the same time some of the more important consequences which follow from the theory were discussed.

In a paper announcing the discovery of the heat emission of radium, P. Curie and Laborde^[345] state that the heat energy may be equally well supposed to be derived from a breaking up of the radium atom or from energy absorbed by the radium from some external source.

J. J. Thomson in an article on “Radium,” communicated to Nature^[346], put forward the view that the emission of energy from radium is probably due to some change within the atom, and pointed out that a large store of energy would be released by a contraction of the atom.

Sir William Crookes^[347], in 1899, proposed the theory that the radio-active elements possess the property of abstracting energy from the gas. If the moving molecules, impinging more swiftly on the substance, were released from the active substance at a much lower velocity, the energy released from the radio-elements might be derived from the atmosphere. This theory was advanced again later on to account for the large heat emission of radium, discovered by P. Curie and Laborde.

F. Re^[348] recently advanced a very general theory of matter with a special application to radio-active bodies. He supposes that the parts of the atom were originally free, constituting a nebula of extreme tenuity. These parts have gradually become united round centres of condensation, and have thus formed the atoms of the elements. On this view an atom may be likened to an extinct sun. The radio-active atoms occupy a transitional stage between the original nebula and the more stable chemical atoms, and in the course of their contraction give rise to the heat emission observed.

Lord Kelvin in a paper to the British Association meeting, 1903, has suggested that radium may obtain its energy from external sources. If a piece of white paper is put into one vessel and a piece of black paper into an exactly similar vessel, on exposure of both vessels to the light the vessel containing the black paper is found to be at a higher temperature. He suggests that radium in a similar manner may keep its temperature above the surrounding air by its power of absorption of unknown radiations.

Richarz and Schenck^[349] have suggested that radio-activity may be due to the production and breaking up of ozone which is known to be produced by radium salts.

255. Discussion of Theories. From the survey of the general hypotheses advanced as possible explanations of radio-activity, it is seen that they may be divided broadly into two classes, one of which assumes that the energy emitted from the radio-elements is obtained at the expense of the internal energy of the atom, and the other that the energy is derived from external sources, but that the radio-elements act as mechanisms capable of transforming this borrowed energy into the special forms manifested in the phenomena of radio-activity. Of these two sets of hypotheses the first appears to be the more probable, and to be best supported by the experimental evidence. Up to the present not the slightest experimental evidence has been adduced to show that the energy of radium is derived from external sources.

J. J. Thomson (loc. cit.) has discussed the question in the following way:—

“It has been suggested that the radium derives its energy from the air surrounding it, that the atoms of radium possess the faculty of abstracting the kinetic energy from the more rapidly moving air molecules while they are able to retain their own energy when in collision with the slowly moving molecules of air. I cannot see, however, that even the possession of this property would explain the behaviour of radium; for imagine a portion of radium placed in a cavity in a block of ice; the ice around the radium gets melted; where does the energy for this come from? By the hypothesis there is no change in the air-radium system in the cavity, for the energy gained by the radium is lost by the air, while heat cannot flow into the cavity from the outside, for the melted ice round the cavity is hotter than the ice surrounding it.”

The writer has recently found that the activity of radium is not altered by surrounding it with a large mass of lead. A cylinder of lead was cast 10 cms. in diameter and 10 cms. high. A hole was bored in one end of the cylinder to the centre, and the radium, enclosed in a small glass tube, was placed in the cavity. The opening was then hermetically closed. The activity was measured by the rate of discharge of an electroscope by the γ rays transmitted through the lead, but no appreciable change was observed during a period of one month.

M. and Mme Curie early made the suggestion that the radiation of energy from the radio-active bodies might be accounted for by supposing that space is traversed by a type of Röntgen rays, and that the radio-elements possess the property of absorbing them. Recent experiments (section 279) have shown that there is present at the surface of the earth a very penetrating type of rays, similar to the γ rays of radium. Even if it were supposed that the radio-elements possessed the power of absorbing this radiation, the energy of the rays is far too minute to account even for the energy radiated from an element of small activity like uranium. In addition, all the evidence so far obtained points to the conclusion that the radio-active bodies do not absorb the type of rays they emit to any greater extent than would be expected from their density. It has been shown (section 86) that this is true in the case of uranium. Even if it were supposed that the radio-elements possess the property of absorbing the energy of some unknown type of radiation, which is able to pass through ordinary matter with little absorption, there still remains the fundamental difficulty of accounting for the peculiar radiations from the radio-elements, and the series of changes that occur in them. It is not sufficient for us to account for the heat emission only, for it has been shown (chapter XII) that the emission of heat is directly connected with the radio-activity.

In addition, the distribution of the heat emission of radium amongst the radio-active products which arise from it is extremely difficult to explain on the hypothesis that the energy emitted is borrowed from external sources. It has been shown that more than two-thirds of the heat emitted by radium is due to the emanation together with the active deposit which is produced by the emanation. When the emanation is separated from the radium, its power of emitting heat, after reaching a maximum, decreases with the time according to an exponential law. It would thus be necessary on the absorption hypothesis to postulate that most of the heat emission of radium, observed under ordinary conditions, is not due to the radium itself but to something produced by the radium, whose power of absorbing energy from external sources diminishes with time.

A similar argument also applies to the variation with time of the heating effect of the active deposit produced from the emanation. It has been shown in the last chapter that most of the heating effect observed in radium and its products must be ascribed to the bombardment of the α particles expelled from these substances. It has already been pointed out (section 136) that it is difficult to imagine any mechanism, either internal or external, whereby such enormous velocity can suddenly be impressed upon the α particles. We are forced to the conclusion that the α particle did not suddenly acquire this energy of motion, but was initially in rapid motion in the atom, and for some reason, was suddenly released with the velocity which it previously possessed in its orbit.

The strongest evidence against the hypothesis of absorption of external energy is that such a theory ignores the fact, that, whenever radio-activity is observed, it is always accompanied by some change which can be detected by the appearance of new products having chemical properties distinct from those of the original substances. This leads to some form of “chemical” theory, and other results show that the change is atomic and not molecular.

256. Theory of radio-active change. The processes occurring in the radio-elements are of a character quite distinct from any previously observed in chemistry. Although it has been shown that the radio-activity is due to the spontaneous and continuous production of new types of active matter, the laws which control this production are different from the laws of ordinary chemical reactions. It has not been found possible in any way to alter either the rate at which the matter is produced or its rate of change when produced. Temperature, which is such an important factor in altering the rate of chemical reactions, is, in these cases, almost entirely without influence. In addition, no ordinary chemical change is known which is accompanied by the expulsion of charged atoms with great velocity. It has been suggested by Armstrong and Lowry^[350] that radio-activity may be an exaggerated form of fluorescence or phosphorescence with a very slow rate of decay. But no form of phosphorescence has yet been shown to be accompanied by radiations of the character of those emitted by the radio-elements. Whatever hypothesis is put forward to explain radio-activity must account not only for the production of a series of active products, which differ in chemical and physical properties from each other and from the parent element, but also for the emission of rays of a special character. Besides this, it is necessary to account for the large amount of energy continuously radiated from the radio-elements.

The radio-elements, besides their high atomic weights, do not possess in common any special chemical characteristics which differentiate them from the other elements, which do not possess the property of radio-activity to an appreciable degree. Of all the known elements, uranium, thorium, and radium possess the greatest atomic weights, viz.: radium 225, thorium 232·5, and uranium 240.

If a high atomic weight is taken as evidence of a complicated structure of the atom, it might be expected that disintegration would occur more readily in heavy than in light atoms. At the same time, there is no reason to suppose that the elements of the highest atomic weight must be the most radio-active; in fact, radium is far more active than uranium, although its atomic weight is less. This is seen to be the case also in the radio-active products; for example, the radium emanation is enormously more active weight for weight than the radium itself, and there is every reason to believe that the emanation has an atom lighter than that of radium.

In order to explain the phenomena of radio-activity, Rutherford and Soddy have advanced the theory that the atoms of the radio-elements suffer spontaneous disintegration, and that each disintegrated atom passes through a succession of well-marked changes, accompanied in most cases by the emission of α rays.

A preliminary account of this hypothesis has already been given in section 136, while the mathematical theory of successive changes, which is based upon it, has been discussed in chapter IX. The general theory has been utilized in chapters X and XI to account for the numerous active substances found in uranium, thorium, actinium and radium.

The theory supposes that, on an average, a definite small proportion of the atoms of each radio-active substance becomes unstable at a given time. As a result of this instability, the atoms break up. In most cases, the disintegration is explosive in violence and is accompanied by the ejection of an α particle with great velocity; in a few cases, α and β particles are expelled together, while in others a β particle alone escapes. In a few cases, the change in the atom appears to be less violent in character, and is not accompanied by the expulsion of either an α or β particle. The explanation of these rayless changes is considered in section 259. The expulsion of an α particle, of mass about twice that of the hydrogen atom, leaves behind it a new system lighter than the original one, and possessing chemical and physical properties quite different from those of the original element. This new system again becomes unstable, and expels another α particle. The process of disintegration, once started, proceeds from stage to stage at a definite measurable rate in each case.

At any time after the disintegration has commenced, there exists a proportion of the original matter, which is unchanged, mixed with the part which has undergone change. This is in accordance with the observed fact that the spectrum of radium, for example, does not change progressively with time. The radium breaks up so slowly that only a small fraction has been transformed in the course of a few years. The unchanged part still shows its characteristic spectrum, and will continue to do so as long as any radium exists. At the same time it is to be expected that, in old radium, the spectrum of those products which exist in any quantity should also appear.

The term metabolon has been suggested as a convenient expression for each of these changing atoms, derived from the successive disintegration of the atoms of the radio-elements. Each metabolon, on an average, exists only for a limited time. In a collection of metabolons of the same kind the number N, which are unchanged at a time t after production, is given by

where N₀ is the original number. Now dN/dt = -λN, or the fraction of the metabolons present, which change in unit time, is equal to λ. The value 1/λ may be taken as the average life of each metabolon.

This may be simply shown as follows:—At any time t after N₀ metabolons have been set aside, the number which change in the time dt is equal to λNdt or

Each metabolon has a life t, so that the average life of the whole number is given by

The various metabolons from the radio-elements are distinguished from ordinary matter by their great instability and consequent rapid rate of change. Since a body which is radio-active must ipso facto be undergoing change, it follows that none of the active products, for example, the emanations and Th X, can consist of any known kind of matter; for there is no evidence to show that inactive matter can be made radio-active, or that two forms of the same element can exist, one radio-active and the other not. For example, half of the matter constituting the radium emanation has undergone change after an interval of four days. After the lapse of about one month the emanation as such has nearly disappeared, having been transformed through several stages into other and more stable types of matter, which are in consequence difficult to detect by their radio-activity.

The striking difference in chemical and physical properties which exists in many cases between the various products themselves, and also between the primary active substance and its products, has already been drawn attention to in chapter IX. Some of the products show distinctive electro-chemical behaviour and can be removed from a solution by electrolysis. Others show differences in volatility which have been utilized to effect a partial separation. There can be no doubt that each of these products is a definite new chemical substance, and if it could be collected in sufficient quantity to be examined by ordinary chemical means, would be found to behave like a distinct chemical element. It would differ, however, from the ordinary chemical element in the shortness of its life, and the fact that it is continuously changing into another substance. We shall see later (section 261) that there is every reason to believe that radium itself is a metabolon in the true sense of the term, since it is continuously changing, and is itself produced from another substance. The main point of difference between it and the other products lies in the comparative slowness of its rate of change.

It is for this reason that radium exists in pitchblende in greater quantity than the other more rapidly changing products. By working up a large amount of the mineral, we have seen that a sufficient quantity of the pure product has been obtained for chemical examination.

On account of the short life of the emanation, it exists in pitchblende in much less quantity than radium, but it, too, has been isolated chemically and its volume measured. The extraordinary properties of this emanation, or gas, have already been discussed, and there can be no doubt that, while it exists, it must be considered a new element allied in chemical properties to the argon-helium group of gases.

There can be no doubt that in the radio-elements we are witnessing the spontaneous transformation of matter, and that the different products which arise mark the stages or halting-places in the process of transformation, where the atoms are able to exist for a short time before again breaking up into new systems.

257. Radio-active products. The following table gives the list of the active products or metabolons known to result from the disintegration of the three radio-elements. In the second column is given the value of the radio-active constant λ for each active product, i.e. the proportion of the active matter undergoing change per second; in the third column the time T required for the activity to fall to one-half, i.e. the time taken for half the active product to undergo change; in the fourth column, the nature of the rays from each active product, not including the rays from the products which result from it; in the fifth column, a few of the more marked physical and chemical properties of each metabolon.

The products and their radiations are indicated graphically in Fig. 102 on page 448.

One product has been observed in uranium, four in thorium, four in actinium and seven in radium. It is not improbable that a closer examination of the radio-elements may reveal still further changes. If any very rapid transformations exist, they would be very difficult to detect. The change of thorium X into the emanation, for example, would probably not have been discovered if the product of the change had not been gaseous in character. The electrolysis of solutions is, in many cases, a very powerful method of separating active products from one another, and its possibilities have not yet been exhausted. The main family of changes of the radio-elements, as far as they are known, have been investigated closely, and it is not likely that any product of comparatively slow rate of change has been overlooked. There is a possibility, however, that two radio-active products may in some cases arise from the disintegration of a single substance. This point is discussed further in section 260.

The remarkable way in which the disintegration theory can be applied to unravel the intricacies of the succession of radio-active changes is very well illustrated in the case of radium. Without its aid, it would not have been possible to disentangle the complicated processes which occur. We have already seen that this analysis has been instrumental in showing that the substances polonium, radio-tellurium and radio-lead are in reality products of radium.

After the radio-active substances have undergone the succession of changes traced above, a final stage is reached where the atoms are either permanently stable, or change so slowly that it is difficult to detect their presence by means of their radio-activity. It is probable, however, that the process of transformation still continues through further slow stages.

There is now considerable evidence that the elements uranium, radium and actinium are intimately connected together. The two latter probably result from the breaking up of uranium. The evidence in support of this idea is given in section 262, but there still remains much work to be done to bridge over the gaps which at present appear to separate these elements from one another.

After the series of transformations have come to an end, there will probably remain a product or products which will be inactive, or active only to a minute extent. In addition, since the α particles, expelled during the transformation, are material in nature, and are non-radio-active, they must collect in some quantity in radio-active matter. The probability that the α particles consist of helium is considered later in section 268.

The value of T, the time for a product to be half-transformed, may be taken as a comparative measure of the stability of the different metabolons. The stability of the products varies over a very wide range. For example, the value of T for radium D is 40 years, and for the actinium emanation 3·9 secs. This corresponds to a range of stability measured by 3·8 × 10⁸. The range of stability is still further extended, when it is remembered that the atoms of the radio-elements themselves are very slowly changing.

The only two metabolons of about the same stability are thorium X and the radium emanation. In each case, the transformation is half completed in about four days. I consider that the approximate agreement of the numbers is a mere coincidence, and that the two types of matter are quite distinct from one another; for, if the metabolons were identical, it would be expected that the changes which follow would take place in the same way and at the same rate, but such is not the case. Moreover, Th X and the radium emanation have chemical and physical properties quite distinct from one another.

It is very remarkable that the three radio-active substances, radium, thorium and actinium, should exhibit such a close similarity in the succession of changes which occur in them. Each of them at one stage of its disintegration emits a radio-active gas, and in each case this gas is transformed into a solid which is deposited upon the surface of bodies. It would appear that, after disintegration of an atom of any of these has once begun, there is a similar succession of changes, in which the resulting systems have allied chemical and physical properties. Such a connection is of interest as indicating a possible origin of the recurrence of properties in the atoms of the elements, as exemplified by the periodic law. The connection between thorium and actinium is especially close both as regards the number and nature of the products. The period of transformation of the successive products, though differing in magnitude, rises and falls in a very analogous manner. This indicates that the atoms of these two elements are very similarly constituted.

258. Amount of the products. By application of the theory of successive changes, the probable amount of each of the products present in radium and the other radio-elements can readily be estimated.

Since each radio-atom expels one α particle of atomic weight about that of hydrogen or helium, the atoms of the intermediate products will not differ much in weight from the parent atom.

The approximate weight of each product present in a gram of radium can be readily deduced. Let N_A, N_B, N_C be the number of atoms of the products A, B, C present per gram in radio-active equilibrium. Let λ_A, λ_B, λ_C be the corresponding constants of change. Then if q is the number of the parent atoms breaking up per second, per gram,

Consider the case of the radium products, where the value of q is 6·2 × 10¹⁰ (section 93). Knowing the value of λ and q, the value of N can at once be calculated. The corresponding weight can be deduced, since in one gram of matter of atomic weight about 200, there are about 4 × 10²¹ atoms (section 39). The results are shown in the following table:—

With the small quantities of radium available, the amounts of the products radium A, B and C are too small to weigh. It may be possible, however, to detect their presence by means of the spectroscope.

In the case of thorium, the weight of the product Th X, which is present in greatest quantity, is far too small to be detected. Since the value of λ for Th X is about the same as for the radium emanation, the maximum weight present per gram is about 4 × 10^-12 of a gram, remembering that q for radium is about 2 × 10⁶ times the value for thorium. Even with a kilogram of thorium, the amount of Th X is far too small to be detected by its weight.

This method can be used generally to calculate the relative amounts of any successive products in radio-active equilibrium, provided the value of λ for each product is known. For example, it will be shown later that uranium is the parent of radium and is half transformed in about 6 × 10⁸ years, while radium and radium D are half transformed in 1300 and 40 years respectively. The weight of radium present in one gram of uranium, when equilibrium is established, is thus 2 × 10^-6 grams, and the weight of radium D is 7 × 10^-8 grams. In a mineral containing a ton of uranium there should be about 1·8 grams of radium and ·063 grams of radium D. Some recent experiments described in section 262 show that these theoretical estimates are about twice too great.

259. Rayless Changes. The existence of well-marked changes in radium, thorium, and actinium, which are not accompanied by the expulsion of α or β particles, is of great interest and importance.

It must be supposed that a rayless change is not of so violent a character as one which gives rise to the expulsion of α or β particles. The change may be accounted for either by supposing that there is a rearrangement of the components of the atom, or that the atom breaks up without the expulsion of its parts with sufficient velocity to produce ionization by collision with the gas. The latter point of view, if correct, at once indicates the possibility that undetected changes of a similar character may be taking place slowly in the non-radio-active elements; or, in other words, that all matter may be undergoing a slow process of change. The changes taking place in the radio-elements have been observed only in consequence of the expulsion with great velocity of the parts of the disintegrated atom. Some recent experiments described in Appendix A show that the α particle from radium ceases to ionize the gas when its velocity falls below about 10⁹ cms. per second. It is thus seen that α particles may be projected with a great velocity, and yet fail to produce ionization in the gas. In such a case, it would be difficult to follow the changes by the electrical method, as the electrical effects would be very small in comparison with those produced by the known radio-active bodies.

260. Radiations from the products. We have seen that the great majority of the radio-active products break up with the expulsion of α particles, and that the β particle with its accompaniment of the γ ray appears in most cases only in the last rapid change. In the case of radium, for example, which has been most closely investigated on account of its great activity, radium itself, the emanation and radium A emit only α particles; radium B emits no rays at all; while radium C emits all three kinds of rays. It is difficult to settle with certainty whether the products thorium X and actinium X emit β particles or not, but the β and γ rays certainly appear in each case in the last rapid change in the active deposit, and, in this respect, behave in a similar manner to radium.

The very slow moving electrons which accompany the particles emitted from radium (section 93) are not taken into account, for they appear to be liberated as a result of the impact of α particles on matter, and are expelled with a speed insignificant compared with that of the β particles emitted from radium C.

The appearance of β and γ rays only in the last rapid changes of the radio-elements is very remarkable, and cannot be regarded as a mere coincidence. The final expulsion of a β particle results in the appearance of a product of great stability, or, in the case of radium, of a product (radium D) which has far more stability than the preceding one. It would appear that the initial changes are accompanied by the expulsion of an α particle, and that once the β particle is expelled, the components of the residual atom fall into an arrangement of fairly stable equilibrium, where the rate of transformation is very slow. It thus appears probable that the β particle, which is finally expelled, may be regarded as the active agent in promoting the disintegration of the radio-atom through the successive stages. A discussion of this question will be given with more advantage later (section 270), when the general question of the stability of the atom is under consideration.

It is significant that the change in which the three types of rays appear is far more violent in character than the preceding changes. Not only are the α particles expelled with greater velocity than in any other change, but the β particles are projected with a velocity very closely approaching that of light.

There is always a possibility that, in such a violent explosion in the atom, not only may the α and β particles be expelled, but the atom itself may be disrupted into several fragments. If the greater proportion of the matter resulting from the disintegration is of one kind, it would be difficult to detect the presence of a small quantity of rapidly changing matter from observations of the rate of decay; but, if the products have distinctive electro-chemical behaviour, a partial separation should, in some cases, be effected by electrolysis. It has already been pointed out that the results of Pegram and von Lerch (section 207) on the electrolysis of thorium solutions may be explained on the supposition that thorium A and B have distinctive electro-chemical behaviour. Pegram, however, in addition observed the presence of a product which decayed to half value in six minutes. This active product was obtained by electrolysing a solution of pure thorium salt, to which a small quantity of copper nitrate had been added. The copper deposit was slightly active and lost half of its activity in about six minutes.

The presence of such radio-active products, which do not come under the main scheme of changes, indicates that, at some stage of the disintegration, more than one substance results. In the violent disintegration which occurs in radium C and thorium B, such a result is to be expected, for it is not improbable that there are several arrangements whereby the constituents of the atom form a system of some slight stability. The two products resulting from the disintegration would probably be present in unequal proportion, and, unless they gave out different kinds of rays, would be difficult to separate from each other.

261. Life of radium. Since the atoms of the radio-elements are continuously breaking up, they must also be considered to be metabolons, the only difference between them and metabolons such as the emanations Th X and others being their comparatively great stability and consequent very slow rate of change. There is no evidence that the process of change, traced above, is reversible under present conditions, and in the course of time a quantity of radium, uranium, or thorium left to itself must gradually be transformed into other types of matter.

It has been shown experimentally (section 93) that 1 gram of radium at its minimum activity expels 6·2 × 10¹⁰ α particles per second. The heating effect of radium and also its volume agree closely with calculation, if it is supposed that each atom of each product in breaking up emits one α particle. On this supposition it is seen that 6·2 × 10¹⁰ atoms of radium break up per second.

Now it has been shown experimentally (section 39) that one cubic centimetre of hydrogen at standard pressure and temperature contains 3·6 × 10¹⁹ molecules. Taking the atomic weight of radium as 225, the number of atoms in 1 gram of radium is equal to 3·6 × 10²¹. The fraction λ of radium which breaks up is thus 1·95 × 10^-11 per second, or 5·4 × 10^-4 per year. It follows that in each gram of radium about half a milligram breaks up per year. The average life of radium is about 1800 years, and half of the radium is transformed in about 1300 years.

We shall now consider the calculation, based on the observed result of Ramsay and Soddy, that the volume of emanation to be obtained from one gram of radium is about 1 cubic millimetre. The experimental evidence based on diffusion results indicates that the molecular weight of the emanation is about 100. If the disintegration theory is correct, the emanation is an atom of radium minus one particle, and therefore must have a molecular weight of at least 200. This high value is more likely to be correct than the experimental number, which is based on evidence that must necessarily be somewhat uncertain. Now the rate of production of emanation per second is equal to λN₀, where N₀ is the equilibrium amount. Taking the molecular weight as 200, the weight of emanation produced per second from 1 gram of radium = 8·96 × 10^-6λ = 1·9 × 10^-11 gram.

Now the weight of emanation produced per second is very nearly equal to the weight of radium breaking up per second. Thus the fraction of radium breaking up per second is about 1·9 × 10^-11, which is in agreement with the number previously calculated by the first method.

We may thus conclude that radium is half transformed in about 1300 years.

Taking the activity of pure radium as about two million times that of uranium, and remembering that only one change, which gives rise to α rays, occurs in uranium and four in radium, it can readily be calculated that the fraction of uranium changing per year is about 10^-9. From this it follows that uranium should be half transformed in about 6 × 10⁸ years.

If thorium is a true radio-active element, the time for half transformation is about 2·4 × 10⁹ years, since thorium has about the same activity as uranium but contains four products which emit α rays. If the activity of thorium is due to some radio-active impurity, no estimate of the length of its life can be made until the primary active substance has been isolated and its activity measured.

262. Origin of radium. The changes in radium are thus fairly rapid, and a mass of radium if left to itself should in the course of a few thousand years have lost a large proportion of its radio-activity. Taking the above estimate of the life of radium, the value of λ is 5·4 × 10^-4, with a year as the unit of time. A mass of radium left to itself should be half transformed in 1300 years and only one-millionth part would remain after 26,000 years. Thus supposing, for illustration, that the earth was originally composed of pure radium, its activity per gram 26,000 years later would not be greater than the activity observed to-day in a good specimen of pitchblende. Even supposing this estimate of the life of radium is too small, the time required for the radium practically to disappear is short compared with the probable age of the earth. We are thus forced to the conclusion that radium is being continuously produced in the earth, unless the very improbable assumption is made, that radium was in some way suddenly formed at a date recent in comparison with the age of the earth. It was early suggested by Rutherford and Soddy^[351] that radium might be a disintegration product of one of the radio-elements found in pitchblende. Both uranium and thorium fulfil the conditions required in a possible source of production of radium. Both are present in pitchblende, have atomic weights greater than that of radium, and have rates of change which are slow compared with that of radium. In some respects, uranium fulfils the conditions required better than thorium; for it has not been observed that minerals rich in thorium contain much radium, while on the other hand, the pitchblendes containing the most radium contain a large proportion of uranium.

If radium is not produced from uranium, it is certainly a remarkable coincidence that the greatest activity of pitchblende yet observed is about five or six times that of uranium. Since radium has a life short compared with that of uranium, the amount of radium produced should reach a maximum value after a few thousand years, when the rate of production of fresh radium—which is also a measure of the rate of change of uranium—balances the rate of change of that product. In this respect the process would be exactly analogous to the production of the emanation by radium, with the difference that the radium changes much more slowly than the emanation. But since radium itself in its disintegration gives rise to at least five changes with the corresponding production of α rays, the activity due to the radium (measured by the α rays), when in a state of radio-active equilibrium with uranium, should be about five times that of the uranium that produces it; for it has been shown that only one change has so far been observed in uranium in which α rays are expelled. Taking into account the presence of actinium in pitchblende, the activity observed in the best pitchblende is about the same as would be expected if the radium were a disintegration product of uranium. If this hypothesis is correct, the amount of radium in any pitchblende should be proportional to the amount of uranium present, provided the radium is not removed from the mineral by percolating water.

This question has been experimentally attacked by Boltwood^[352], McCoy^[353] and Strutt^[354]. McCoy measured the relative activities of different minerals in the form of powder by means of an electroscope, and determined the amount of uranium present by chemical analysis. His results indicated that the activity observed in the minerals was very approximately proportional to their content of uranium. Since actinium is present as well as uranium and its products, this would indicate that the amount of radium and actinium taken together is proportional to the amount of uranium. This problem has been attacked more directly by Boltwood and Strutt by measuring the relative amount of the radium emanation evolved by different minerals. By dissolving the mineral and then setting it aside in a closed vessel, the amount of emanation present reaches a maximum value after about a month’s interval. The emanation is then introduced into a closed vessel containing a gold-leaf electroscope similar to that shown in Fig. 12. The rate of movement of the gold-leaf is proportional to the amount of emanation from the solution, and this in turn is proportional to the amount of radium. Boltwood has made in this way a very complete and accurate comparison of the radium content of different varieties of pitchblende and other ores containing radium. It was found that many of the minerals in the solid state allowed a considerable fraction of the emanation to escape into the air. The percentage fraction of the total amount of emanation lost in this way is shown in Column II of the following table. Column I gives the maximum amount of emanation present in 1 gram of the mineral in arbitrary units when none of the emanation escapes; Column III the weight in grams of uranium contained in 1 gram; and Column IV the ratio obtained by dividing the quantity of emanation by the quantity of uranium. The numbers in Column IV should be constant, if the amount of radium is proportional to the amount of uranium.

With the exception of some of the monazites, the numbers show a surprisingly good agreement, and, taking into consideration the great variation of the content of uranium in the different minerals, and the wide range of locality from which they were obtained, the results afford a direct and satisfactory proof that the amount of radium in the minerals is directly proportional to the amount of uranium.

In this connection, it is of interest to note that Boltwood found that a considerable quantity of radium existed in various varieties of monazite, although most of the previous analyses agreed in stating that no uranium was present. A careful examination was in consequence made to test this point, and it was found by special methods that uranium was present, and in about the amount to be expected from the theory. The ordinary methods of analysis failed to give correct results on account of the presence of phosphates. Results of a similar character have recently been given by Strutt^[355].

The weight of radium in a mineral per gram of uranium is thus a definite constant of considerable practical importance. Its value was recently determined by Boltwood by comparison of the emanation, liberated from a known weight of uraninite, with that liberated from a known quantity of pure radium bromide, supplied for the purpose by the writer. A measured weight of radium bromide was taken from a stock which gave out heat at a rate of slightly over 100 gram calories per hour per gram, and was thus probably pure. This was dissolved in water, and, by successive dilutions, a standard solution was made up containing 10⁻⁷ gram of radium bromide per c.c. Taking the constitution of radium bromide as RaBr₂, it was deduced that the weight of radium per gram of uranium in any mineral was 8·0 × 10⁻⁷ gram. The amount of radium in a mineral per ton of uranium is thus 0·72 gram.

Strutt (loc. cit.) obtained a value nearly twice as great, but he had no means of ascertaining the purity of his radium bromide.

This amount of radium per gram of uranium is of the right order of magnitude to be expected on the disintegration theory, if uranium is the parent of radium. The activity of pure radium, compared with uranium, is not known with sufficient accuracy to determine with accuracy the theoretical proportion of radium to uranium.

The production of radium from uranium, while very strongly supported by these experiments, cannot be considered definitely established until direct experimental evidence is obtained of the growth of radium in uranium. The rate of production of radium to be expected on the disintegration theory can readily be estimated. The fraction of uranium breaking up per year has been calculated (section 261) and shown to be about 10^-9 per year. This number represents the weight of radium produced per year from 1 gram of uranium. The emanation, released from the amount of radium produced in one year from 1 gram of uranium, would cause an ordinary gold-leaf electroscope to be discharged in about half-an-hour. If a kilogram of uranium is used, the amount of radium produced in a single day should be easily detectable.

Experiments to detect the growth of radium in uranium have been made by several observers. Soddy^[356] examined the amount of emanation given off at different times from one kilogram of uranium nitrate in solution, which was originally freed from the small trace of radium present by a suitable chemical process. The solution was kept stored in a closed vessel, and the amount of emanation which collected in the solution was measured at regular intervals.

Preliminary experiments showed that the actual rate of production of radium was far less than the amount to be expected theoretically, and at first very little indication was obtained that radium was produced at all. After allowing the uranium to stand for eighteen months, Soddy states that the amount of emanation was distinctly greater than at first. The solution after this interval contained about 1·5 × 10^-9 gram of radium. This gives the value of about 2 × 10^-12 for the fraction of uranium changing per year, while the theoretical value is about 10^-9.

Whetham^[357] also found that a quantity of uranium nitrate which had been set aside for a year showed an appreciable increase in the content of radium, and considers that the rate of production is faster than that found by Soddy. In his case, the uranium was not originally completely freed from radium.

Observations extending over years will be required before the question can be considered settled, for the accurate estimation of small quantities of radium by the amount of emanation is beset with difficulties. This is especially the case where observations are made over wide intervals of time.

The writer has made an examination to see if radium is produced from actinium or thorium. It was thought possible that actinium might prove to be an intermediate product between uranium and radium. The solutions, freed from radium, have been set aside for a year, but no certain increase in the content of radium has been observed.

There is little doubt that the production of radium by uranium first proceeds at only a small fraction of the rate to be expected from theory. This is not surprising when we consider that probably several changes intervene between the product Ur X and the radium. In the case of radium, for example, it has been shown that a number of slow changes follow the rapid changes ordinarily observed. On account of the feeble activity of uranium, it would not be easy to detect directly the occurrence of such changes. If, for example, one or more rayless products occurred between Ur X and radium, which were removed from the uranium by the same chemical process used to free it from radium, the rate of production of radium would be very small at first, but would be expected to increase with time as more of the intermediary products were stored up in the uranium. The fact that the contents of uranium and radium in radio-active minerals are always proportional to each other, coupled with definite experimental evidence that radium is produced from uranium, affords an almost conclusive proof that uranium is in some way the parent of radium.

The general evidence which has been advanced to show that radium must be continuously produced from some other substance applies also to actinium, which has an activity of the same order of magnitude as that of radium. The presence of actinium with radium in pitchblende would indicate that this substance also is in some way derived from uranium. It is possible that actinium may prove to be produced either from radium or to be the intermediary substance between uranium and radium. If it could be shown that the amount of actinium in radio-active minerals is, like radium, proportional to the amount of uranium, this would afford indirect proof of such a connection. It is not so simple to settle this point for actinium as for radium, since actinium gives out a very short-lived emanation, and the methods adopted to determine the content of radium in minerals cannot be applied without considerable modifications to determine the amount of actinium present.

The experimental data, so far obtained, do not throw much light upon the origin of the primary active matter in thorium. Hofmann and others (section 23) have shown that thorium separated from minerals containing uranium is always more active the greater the quantity of uranium present. This would indicate that the active substance in thorium also may be derived from uranium.