Radio-activity/Chapter 13

by Ernest Rutherford

 

CHAPTER XIII.

RADIO-ACTIVE PROCESSES.

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^[1], 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^[2] 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^[3] 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^[4] 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^[5] 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^[6] 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^[7] 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^[8] 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^[9]. The discovery of a condensation of the radio-active emanations^[10] gave additional support to the view that the emanations were gaseous in character. In the meantime, the writer^[11] 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^[12] 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^[13] 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^[14], 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^[15], 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^[16] 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^[17] 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^[18] 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 N = N_{0}e^{-λt}, where N_{0] 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 N_{0} metabolons have been set aside, the number which change in the time dt is equal to λNdt or λN_{0}e^{-λt}dt. Each metabolon has a life t, so that the average life of the whole number is given by

[integral]_{0}^[infinity] λte^{-λt}dt = 1/λ.

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 450.

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.

+—————————+—————————+—————+————————————+—————————————————-+
| | | | Nature of | Chemical and Physical properties |
| Products |λ(sec)^{-1}| T | the rays | of the product |
+—————————+—————————+—————+————————————+—————————————————-+
|Uranium | — | — | α |Soluble in excess of ammonium |
| [v] | | | | carbonate, soluble in ether. |
|Uranium X | 3·6 × 10^{-7} | 22 days | β and γ |Insoluble in excess of ammonium |
| | | | | | carbonate, soluble in |
| [v] | | | | ether and water. |
|  ? | — | — | — | |
+—————————+—————————+—————+————————————+—————————————————-+
|Thorium | — | — | α |Insoluble in ammonia. |
| [v] | | | | |
|Thorium X | 2·0 × 10^{-6} | 4 days | α |Soluble in ammonia and water. |
| [v] | | | | |
|Emanation | 1·3 × 10^{-2} | 53 secs. | α |Chemically inert gas of heavy |
| | | | | | molecular weight. Condenses |
| | | | | | at -120° C. |
| [v] | | | |} Deposited on bodies; |
|Thorium A | 1·74 × 10^{-5} | 11 hours | no rays |} concentrated on the cathode in |
| | | | | |} an electric field. Soluble in |
| [v] | | | |} some acids; Th A more volatile |
|Thorium B | 2·2 × 10^{-4} | 55 mins. |α, β, γ|} than Th B; shows definite |
| | | | | |} electro-chemical behaviour. |
| [v] | | | | |
|  ? | — | — | — | |
+—————————+—————————+—————+————————————+—————————————————-+
|Actinium | — | — | no rays |Insoluble in ammonia. |
| [v] | | | | |
|Actinium X | 7·8 × 10^{-7} |10·2 days | α (and β?) |Soluble in ammonia. |
| [v] | | | | |
|Emanation | ·17 |3·9 secs. | α |Behaves like a gas. |
| | | | | | |
| [v] | | | |} Deposited on bodies; concentrated|
|Actinium A | 3·2 × 10^{-4} | 36 mins. | no rays |} on the cathode in an electric |
| | | | | |} field, soluble in ammonia |
| [v] | | | |} and strong acids; volatilized |
|Actinium B | 5·4 × 10^{-3} |2·15 mins.|α, β, γ|} at a temperature of 100° C., |
| | | | | |} A and B can be separated by |
| [v] | | | |} electrolysis. |
|  ? | — | — | — | |
+—————————+—————————+—————+————————————+—————————————————-+
|Radium | — |1300 years| α |Allied chemically to barium. |
| [v] | | | | |
|Emanation | 2·1 × 10^{-6} | 3·8 days | α |Chemically inert gas of heavy |
| | | | | | molecular weight; condenses |
| [v] } Active | | | | at -150° C. |
|Radium A } deposit| 3·85 × 10^{-3} | 3 mins. | α |} Deposited on surface of bodies; |
| [v] } of | | | |} concentrated on cathode in |
|Radium B } rapid | 5·38 × 10^{-4} | 21 mins. | no rays |} electric field; soluble in |
| [v] } change | | | |} strong acids; B volatilized at |
|Radium C } | 4·13 × 10^{-4} | 28 mins. |α, β, γ|} about 700° C., A and C at about |
| [v] | | | |} 1000° C. |
|Radium D } Active | — | about 40 | no rays |Soluble in acids; volatile below |
| [v] } deposit| | years | | 1000° C. |
|Radium E } of | 1·3 × 10^{-6} | 6 days | β and γ |Non-volatile at 1000° C. |
| [v] } slow | | | | |
|Radium F } change | 5·6 × 10^{-8} | 143 days | α |Deposited on bismuth from |
| [v] | | | | solution; volatile at about |
|  ? | — | — | — | 1000° C., same properties as |
| | | | | radio-tellurium and polonium. |
+—————————+—————————+—————+————————————+—————————————————-+

Fig. 102. 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 × 10^8. 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,

q = λ_{A}N_{A} = λ_{B}N_{B} = λ_{C}N_{C}.

Consider the case of the radium products, where the value of q is 6·2 × 10^{10} (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 × 10^{21} atoms (section 39). The results are shown in the following table:—

+————————————————-+——————————-+—————————-+
| | Value of | Number of atoms, | Weight of product |
| Product | λ | N, present per | in milligrams per |
| | (sec)^{-1} | gram | gram of radium |
+————————-+———————-+——————————-+—————————-+
|Radium emanation | 2·0 × 10^{-6} | 3·2 × 10^{16} | 8 × 10^{-3} |
| [v] | | | |
| Radium A | 3·8 × 10^{-3} | 1·7 × 10^{13} | 4 × 10^{-6} |
| [v] | | | |
| Radium B | 5·4 × 10^{-4} | 1·3 × 10^{14} | 3 × 10^{-5} |
| [v] | | | |
| Radium C | 4·1 × 10^{-4} | 1·6 × 10^{14} | 4 × 10^{-5} |
+————————————————-+——————————-+—————————-+

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 × 10^6 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 × 10^8 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.

Since the rayless changes are not accompanied by any appreciable ionization of the gas, their presence cannot be detected by direct means. The rate of change of the substance can, however, be determined indirectly, as we have seen, by measurement of the variation with time of the activity of the succeeding product. The law of change has been found to be the same as for the changes which give rise to α rays. The rayless changes are thus analogous, in some respects, to the monomolecular changes observed in chemistry, with the difference that the changes are in the atom itself, and are not due to the decomposition of a molecule into simpler molecules or into its constituent atoms.

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 re-arrangement 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 10^9 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.

There seems to be no escape from this conclusion. Let us consider, for example, the case of radium. The radium is continuously producing from itself the radium emanation, the rate of production being always proportional to the amount of radium present. All the radium must ultimately be changed into emanation, which in turn is transformed through a succession of stages into other kinds of matter. There is no doubt that the emanation is chemically quite different from radium itself. The quantity of radium must diminish, to compensate for the emanation which is formed; otherwise it is necessary to assume that matter in the form of emanation is created from some unknown source.

An approximate estimate of the rate of change of radium can easily be made by two different methods depending upon (1) the number of atoms of radium breaking up per second, and (2) the amount of emanation produced per second.

It has been shown experimentally (section 93) that 1 gram of radium at its minimum activity expels 6·2 × 10^{10} 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 × 10^{10} 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 × 10^{19} molecules. Taking the atomic weight of radium as 225, the number of atoms in 1 gram of radium is equal to 3·6 × 10^{21}. 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 λN_{0}, where N_{0} 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^{-6}λ = 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 × 10^8 years.

If thorium is a true radio-active element, the time for half transformation is about 2·4 × 10^9 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^[19] 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^[20], McCoy^[21] and Strutt^[22]. 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.

+—————————+———————+———+——+———+—-+
| Substance | Locality | I | II | III |IV |
+—————————+———————+———+——+———+—-+
|Uraninite |North Carolina|170·0 |11·3|0·7465|228|
|Uraninite |Colorado |155·1 | 5·2|0·6961|223|
|Gummite |North Carolina|147·0 |13·7|0·6538|225|
|Uraninite |Joachimsthal |139·6 | 5·6|0·6174|226|
|Uranophane |North Carolina|117·7 | 8·2|0·5168|228|
|Uraninite |Saxony |115·6 | 2·7|0·5064|228|
|Uranophane |North Carolina|113·5 |22·8|0·4984|228|
|Thorogummite |North Carolina| 72·9 |16·2|0·3317|220|
|Carnotite |Colorado | 49·7 |16·3|0·2261|220|
|Uranothorite |Norway | 25·2 | 1·3|0·1138|221|
|Samarskite |North Carolina| 23·4 | 0·7|0·1044|224|
|Orangite |Norway | 23·1 | 1·1|0·1034|223|
|Euxinite |Norway | 19·9 | 0·5|0·0871|228|
|Thorite |Norway | 16·6 | 6·2|0·0754|220|
|Fergusonite |Norway | 12·0 | 0·5|0·0557|215|
|Aeschynite |Norway | 10·0 | 0·2|0·0452|221|
|Xenotine |Norway | 1·54|26·0|0·0070|220|
|Monazite (sand) |North Carolina| 0·88| |0·0043|205|
|Monazite (crys.) |Norway | 0·84| 1·2|0·0041|207|
|Monazite (sand) |Brazil | 0·76| |0·0031|245|
|Monazite (massive)|Conn. | 0·63| |0·0030|210|
+—————————+———————+———+——+———+—-+

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^[23].

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 RaBr_{2}, 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^[24] 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^[25] 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 inter-

• mediary 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.

While much work still remains to be done, a promising beginning has already been made in determining the origin and relation of the radio-elements. We have seen that the connection between polonium, radio-tellurium, and radio-lead with radium has already been established. Radium itself is now added to the list, and it is probable that actinium will soon follow.

While the experiments undoubtedly show that there is a definite relation between the amount of uranium and radium present in the ordinary radio-active minerals, Danne^[26] has recently called attention to a very interesting apparent exception. Considerable quantities of radium were found in certain deposits in the neighbourhood of Issy-l'Evêque in the Saône-Loire district, although no trace of uranium was present. The active matter is found in pyromorphite (phosphate of lead), in clays containing lead, and in pegmatite, but the radium is usually present in greater quantities in the former. The pyromorphite is found in veins of the quartz and felspar rocks. The veins are always wet owing to the presence of a number of springs in the neighbourhood. The content of uranium in the pyromorphite varies considerably, but Danne considers that about a centigram of radium is present per ton. It seems probable that the radium found in this locality has been deposited from water flowing through it, possibly in past times. The presence of radium is not surprising, since crystals of autunite have been found about 40 miles distant, and probably there are deposits containing uranium in that region. This result is of interest, as suggesting that radium may be removed with water and deposited by physical or chemical action some distance away.

It will be shown in the next chapter that radium has been found very widely distributed over the surface of the earth, but generally in very small quantities.

263. Does the radio-activity of radium depend upon its concentration? We have seen that the radio-active constant λ of any product is independent of the concentration of the product. This result has been established over a very wide range for some substances, and especially for the radium emanation. No certain difference in the rate of decay of the emanation has been observed, although the amount present in unit volume of the air has been varied a millionfold.

It has been suggested by J. J. Thomson^[27] that the rate of disintegration of radium may be influenced by its own radiations. This, at first sight, appears very probable, for a small mass of a pure radium compound is subjected to an intense bombardment by the radiations arising from it, and the radiations are of such a character that they might be expected to produce a breaking up of the atoms of matter which they traverse. If this be the case, the radio-activity of a given quantity of radium should be a function of its concentration, and should be greater in the solid state than when disseminated through a large mass of matter.

The writer has made an experiment to examine this question. Two glass tubes were taken, in one of which was placed a few milligrams of pure radium bromide in a state of radio-active equilibrium, and in the other a solution of barium chloride. The two tubes were connected near the top by a short cross tube, and the open ends sealed off. The activity of the radium in the solid state was tested immediately after its introduction by placing it in a definite position near an electroscope made of thin metal of the type shown in Fig. 12. The increased rate of discharge of the electroscope due to the β and γ rays from the radium was observed. When a lead plate 6 mms. in thickness was placed between the radium and the electroscope, the rate of discharge observed was due to the γ rays alone. By slightly tilting the apparatus, the barium solution flowed into the radium tube and dissolved the radium. The tube was well shaken, so as to distribute the radium uniformly throughout the solution. No appreciable change of the activity measured by the γ rays was observed over the period of one month. The activity measured by the β and γ rays was somewhat reduced, but this was not due to a decrease of the radio-activity, but to an increased absorption of the β rays in their passage through the solution. The volume of the solution was at least 1000 times greater than that of the solid radium bromide, and, in consequence, the radium was subjected to the action of a much weaker radiation. I think we may conclude from this experiment that the radiations emitted by radium have little if any influence in causing the disintegration of the radium atoms.

Voller^[28] recently published some experiments which appeared to show that the life of radium varied enormously with its concentration. In his experiments, solutions of radium bromide of known strengths were evaporated down in a platinum vessel 1·2 sq. cms. in area, and their activity tested from time to time. The activity of the radium, so deposited, at first showed the normal rise to be expected on account of the production of the emanation, but after reaching a maximum, it rapidly decayed. For a weight of 10^{-6} mgrs. of radium bromide, the activity for example, practically disappeared in 26 days after reaching its maximum. The time taken for the activity to disappear increased rapidly with the amount of radium present. In another set of experiments, he states that the activity observed on the vessel was not proportional to the amount of radium present. For example, the activity only increased 24 times for a millionfold increase of the radium present, from 10^{-9} mgrs. to 10^{-3} mgrs.

These results, however, have not been confirmed by later experiments made by Eve. He found that, over the range examined, the activity was directly proportional to the amount of radium present, within the limits of experimental error. The following table illustrates the results obtained. The radium was evaporated down in platinum vessels 4·9 sq. cms. in area.

Weight of radium Activity in
 in milligrams arbitrary units
    10^{-4} 1000
    10^{-5} 106
    10^{-6} 11·8
    10^{-7} 1·25

For an increase of one-thousandfold of the quantity of radium, the activity increased 800 times, while Voller states that the activity, in his experiments, only increased 3 to 4 times.

In the experiments of Eve, the activity was measured by observing the increased rate of discharge of a gold-leaf electroscope when the platinum vessel containing the active deposit was placed inside the electroscope. The activity of 10^{-8} mgrs. was too small to be measured with accuracy in the electroscope employed, while 10^{-3} mgrs. gave too rapid a rate of discharge. On the other hand, the method of measurement employed by Voller was unsuitable for the measurement of very weak radio-activity.

Eve also found that a small quantity of radium kept in a closed vessel did not lose its activity with time. A silvered glass vessel contained a gold-leaf system, such as is shown in Fig. 12. A solution containing 10^{-6} mgrs. of radium bromide was evaporated over the bottom of the vessel of area 76 sq. cms. The activity, after reaching a maximum, has remained constant over the 100 days during which observations have so far been made.

These experiments of Eve, as far as they go, show that the activity of radium is proportional to the amount of radium present, and that radium, kept in a closed vessel, shows no signs of decreasing in activity. On the other hand, I think there is no doubt that a very small quantity of radium deposited on a plate and left in the open air does lose its activity fairly rapidly. This loss of activity has nothing whatever to do with the shortness of life of the radium itself, but is due to the escape of the radium from the plate into the surrounding gas. Suppose, for example, that a solution containing 10^{-9} mgrs. of radium bromide is evaporated in a vessel of one sq. cm. in area. This amount of radium is far too small to form even a layer of molecular thickness. It seems likely that, during the process of evaporation, the radium would tend to collect in small masses and be deposited on the surface of the vessel. These would very readily be removed by slow currents of air and so escape from the plate. The disappearance of such minute amounts of radium is to be expected, and would probably occur with all kinds of matter present in such minute amount. Such an effect has nothing to do with an alteration of the life of radium and must not be confused with it.

The result that the total radiation from a given quantity of radium depends only on the quantity of radium and not on the degree of its concentration is of great importance, for it allows us to determine with accuracy the content of radium in minerals and in soils in which the radium exists in a very diffused state.

264. Constancy of the radiations. The early observations on uranium and thorium had shown that their radio-activity remained constant over the period of several years during which they were examined. The possibility of separating from uranium and thorium the active products Ur X and Th X respectively, the activity of which decayed with the time, seemed at first sight to contradict this. Further observation, however, showed that the total radio-activity of these bodies was not altered by the chemical processes, for it was found that the uranium and thorium from which the active products were removed, spontaneously regained their radio-activity. At any time after removal of the active product, the sum total of the radio-activity of the separated product together with that of the substance from which it has been separated is always equal to that of the original compound before separation. In cases where active products, like Ur X and the radium emanation, decay with time according to an exponential law, this follows at once from the experimental results. If I_{1} is the activity of the product at any time t after separation, and I_{0} the initial value, we know that I_{1}/I_{0} = e^{-λt}. At the same time the activity I_{2} recovered during the same interval t is given by I_{2}/I_{0} = 1 - e^{-λt}, where λ is the same constant as before. It thus follows that I_{1} + I_{2} = I_{0}, which is an expression of the above result. The same is also true whatever the law of decay of activity of the separated product (see section 200). For example, the activity of Th X after separation from thorium at first increases with the time. At the same time, the activity of the residual thorium compound at first decreases, and at such a rate that the sum of the activities of the thorium and its separated product is always equal to that of the original thorium.

This apparent constancy of the total radiation follows from the general result that the radio-active processes cannot in any way be changed by the action of known forces. It may be recalled that the constant of decay of the activity of a radio-active product has a definite fixed value under all conditions. It is independent of the concentration of the active matter, of the pressure, and of the nature of the gas in which the substance is placed, and it is not affected by wide ranges of temperature. The only observed exception is the product radium C. Its value of λ increases with temperature to some extent at about 1000° C., but at 1200° C. returns nearly to the normal value. In the same way, it has not been found possible to alter the rate of production of active matter from the radio-elements. In addition, there is not a single well-authenticated case where radio-activity has been altered or destroyed in any active body or created in an inactive element.

Certain cases have been observed, which at first sight seem to indicate a destruction of radio-activity. For example, the excited radio-activity is removed from a platinum wire when heated above a red heat. It has been shown, however, by Miss Gates (section 187) that the radio-activity is not destroyed, but is deposited in unaltered amount on the colder bodies surrounding it. Thorium oxide has been shown to lose to a large extent its power of emanating by ignition to a white heat. But a close examination shows that the emanation is still being produced at the same rate, but is occluded in the compound.

The total radio-activity of a given mass of a radio-element, measured by the peculiar radiations emitted, is a quantity which can neither be increased nor diminished, although it may be manifested in a series of products which are capable of separation from the radio-element. The term "conservation of radio-activity" is thus a convenient expression of the facts known at the present time. It is quite possible, however, that further experiments at very high or very low temperatures may show that the radio-activity does vary.

Although no difference has been observed in the radio-activity of uranium over an interval of five years, it has been shown (section 261) that on theoretical grounds the radio-activity of a given quantity of a radio-element should decrease with the time. The change will, however, be so slow in uranium, that probably millions of years must elapse before a measurable change can take place, while the total radio-activity of a given quantity of matter left to itself should thus decrease, but it ought to be constant for a constant mass of the radio-element. It has already been pointed out (section 238) that the activity of radium, measured by the α and β rays, will probably increase for several hundred years after its separation. This is due to the appearance of fresh products in the radium. Ultimately, however, the activity must decrease according to an exponential law with the time, falling to half value in about 1300 years.

The conservation of radio-activity applies not only to the radiations taken as a whole, but also to each specific type of radiation. If the emanation is removed from a radium compound, the amount of β radiation of the radium at once commences to decrease, but this is compensated by the appearance of β rays in the radiations from the vessel in which the separated emanation is stored. At any time the sum total of the β radiations from the radium and the emanation vessel is always the same as that from the radium compound before the emanation was removed.

Similar results have also been found to hold for the γ rays. This was tested by the writer in the following way. The emanation from some solid radium bromide was released by heat, and condensed in a small glass tube which was then sealed off. The radium so treated, and the emanation tube, were placed together under an electroscope, with a screen of lead 1 cm. thick interposed in order to let through only the γ rays. The experiments were continued over three weeks, but the sum total of the γ rays from the radium and the emanation tube was, over the whole interval, equal to that of the original radium. During this period the amount of γ rays from the radium at first decreased to only a few per cent. of the original value, and then slowly increased again, until at the end of the three weeks it had nearly regained its original value, before the emanation was removed. At the same time the amount of γ rays from the emanation tube rose from zero to a maximum and then slowly decreased again at the same rate as the decay of the activity of the emanation in the tube. This result shows that the amount of γ rays from radium was a constant quantity over the interval of observation, although the amount of γ rays from the radium and emanation tube had passed through a cycle of changes.

There is one interesting possibility in this connection that should be borne in mind. The rays from the active substances carry off energy in a very concentrated form, and this energy is dissipated by the absorption of the rays in matter. The rays might be expected to cause a disintegration of the atoms of inactive matter on which they fall and thus give rise to a kind of radio-activity. This effect has been looked for by several observers. Ramsay and W. T. Cooke^[29] state that they have noticed such an action, using about a decigram of radium as a source of radiation. The radium, sealed in a glass vessel, was surrounded by an external glass tube and exposed to the action of the β and γ rays of radium for several weeks. The inside and outside of the glass tube were found to be active, and the active matter was removed by solution in water. The radio-activity observed was very minute, corresponding to only about 1 milligram of uranium. The writer has, at various times, tried experiments of this character but with negative results. The greatest care is necessary in such experiments to ensure that the radio-activity is not due to other causes besides the rays from the radium. This care is especially necessary in laboratories where considerable quantities of the radium emanation have been allowed to escape into the air. The surface of every substance becomes coated with the slow transformation products of radium, viz. radium D, E, and F. The activity communicated in this way to originally inactive matter is often considerable. This infection by the radium emana-*

• tion extends throughout the whole laboratory, on account of the

distribution of the emanation by convection and diffusion. For example, Eve^[30] found that every substance which he examined in the laboratory of the writer showed much greater activity than the normal. In this case the radium had been in use in the building for about two years.

265. Loss of weight of the radio-elements. Since the radio-elements are continually throwing off α particles atomic in size, an active substance, enclosed in a vessel sufficiently thin to allow the α particles to escape, must gradually lose in weight. This loss of weight will be small under ordinary conditions, since the greater proportion of the α rays produced are absorbed in the mass of the substance. If a very thin layer of a radium compound were spread on a very thin sheet of substance, which did not appreciably absorb the α particles, a loss of weight due to the expulsion of α particles might be detectable. Since e/m = 6 × 10^3 for the α particle and e = 1·1 × 10^{-20} electro-magnetic units and 2·5 × 10^{11} α particles are expelled per second per gram of radium, the proportion of the mass expelled is 4·8 × 10^{-13} per second and 10^{-5} per year. There is one condition, however, under which the radium should lose in weight fairly rapidly. If a current of air is slowly passed over a radium solution, the emanation produced would be removed as fast as it was formed. Since the atom of the emanation has a mass probably not much smaller than the radium atom, the fraction of the mass removed per year should be nearly equal to the fraction of the radium which changes per year, i.e. one gram of radium should diminish in weight about half a milligram (section 261) per year.

If it is supposed that the β particles have weight, the loss of weight due to their expulsion is very small compared with that due to the emission of α particles. The writer has shown (section 253) that about 7 × 10^{10} β particles are projected per second from 1 gram of radium. The consequent loss of weight would only be about 10^{-9} grams per year.

Except under very special experimental conditions, it would thus be difficult to detect the loss of weight of radium due to the expulsion of β particles from its mass. There is, however, a possibility that radium might change in weight even though none of the radio-active products were allowed to escape. For example, if the view is taken that gravitation is the result of forces having their origin in the atom, it is possible that, if the atom were disintegrated, the weight of the parts might not be equal to that of the original atom.

A large number of experiments have been made to see if radium preparations, kept in a sealed tube, alter in weight. With the small quantities of radium available to the experimenter, no difference of weight of radium preparations with time has yet been established with certainty. Heydweiller stated that he had observed a loss of weight of radium and Dorn also obtained a slight indication of change in weight. These results have not, however, been confirmed. Forch, later, was unable to observe any appreciable change.

J. J. Thomson^[31] has made experiments to see if the ratio of weight to mass for radium is the same as for inactive matter. We have seen in section 48 that a charge in motion possesses an apparent mass which is constant for slow speeds but increases as the speed of light is approached. Now radium emits some electrons at a velocity comparable with the velocity of light, and presumably these electrons were in rapid motion in the atom before their expulsion. It might thus be possible that the ratio for radium would differ from that for ordinary matter. The pendulum method was used, and the radium was enclosed in a small light tube suspended by a silk fibre. Within the limit of experimental error the ratio of weight to mass was found to be the same as for ordinary matter, so that we may conclude that the number of electrons moving with a velocity approaching that of light is small compared with the total number present.

266. Total emission of energy from the radio-element. It has been shown that 1 gram of radium emits energy at the rate of 100 gram-calories per hour or 876,000 gram-calories per year. If 1 gram of radium in radio-active equilibrium be set apart, its radio-activity and consequent heat emission is given at a time t by qe^{-λt}, where λ is the constant of decay of activity of radium and of the initial heating effect; the total heat emission from 1 gram of radium is given by [integral]_{0}^[infinity] qe^{-λt} dt = q/λ.

Now on the estimate of the life of radium given in section 261 the value of λ is 1/1850 when 1 year is taken as the unit of time. The total heat emission from 1 gram of radium during its life is thus 1·6 × 10^9 gram-calories. The heat emitted in the union of hydrogen and oxygen to form 1 gram of water is about 4 × 10^3 gram-calories, and in this reaction more heat is given out for equal weights than in any other chemical reaction known. It is thus seen that the total energy emitted from 1 gram of radium during its changes is about one million times greater than in any known molecular change. That matter is able, under special conditions, to emit an enormous amount of energy, is well exemplified by the case of the radium emanation. Calculations of the amount of this energy have already been given in section 249.

Since the other radio-elements only differ from radium in the slowness of their change, the total heat emission from uranium and thorium must be of a similar high order of magnitude. There is thus reason to believe that there is an enormous store of latent energy resident in the atoms of the radio-elements. This store of energy could not have been recognized if the atoms had not been undergoing a slow process of disintegration. The energy emitted in radio-active changes is derived from the internal energy of the atoms. The emission of this energy does not disobey the law of the conservation of energy, for it is only necessary to suppose that, when the radio-active changes have ceased, the energy stored up in the atoms of the final products is less than that of the original atoms of the radio-elements. The difference between the energy originally possessed by the matter which has undergone the change, and the final inactive products which arise, is a measure of the total amount of energy released.

There seems to be every reason to suppose that the atomic energy of all the elements is of a similar high order of magnitude. With the exception of their high atomic weights, the radio-elements do not possess any special chemical characteristics which differentiate them from the inactive elements. The existence of a latent store of energy in the atoms is a necessary consequence of the modern view developed by J. J. Thomson, Larmor, and Lorentz, of regarding the atom as a complicated structure consisting of charged parts in rapid oscillatory or orbital motion in regard to one another. The energy may be partly kinetic and partly potential, but the mere concentration of the charged particles, which probably constitute the atom, in itself implies a large store of energy in the atom, in comparison with which the energy emitted during the changes of radium is insignificant.

The existence of this store of latent energy does not ordinarily manifest itself, since the atoms cannot be broken up into simpler forms by the physical or chemical agencies at our disposal. Its existence at once explains the failure of chemistry to transform the atoms, and also accounts for the rate of change of the radio-active processes being independent of all external agencies. It has not so far been found possible to alter in any way the rate of emission of energy from the radio-elements. If it should ever be found possible to control at will the rate of disintegration of the radio-elements, an enormous amount of energy could be obtained from a small quantity of matter.

267. Production of helium from radium and the radium emanation. Since the final products, resulting from a disintegration of the radio-elements, are not radio-active, they should in the course of geologic ages collect in some quantity, and should always be found associated with the radio-elements. Now the inactive products resulting from the radio-active changes are the α particles expelled at each stage, and the final inactive product or products which remain, when the process of disintegration can no longer be traced by the property of radio-activity.

Pitchblende, in which the radio-elements are mostly found, contains in small quantity a large proportion of all the known elements. In searching for a possible disintegration product common to all the radio-elements, the presence of helium in the radio-active minerals is noteworthy; for helium is only found in the radio-active minerals, and is an invariable companion of the radio-elements. Moreover, the presence in minerals of a light, inert gas like helium had always been a matter of surprise. The production by radium and thorium of the radio-active emanations, which behave like chemically inert gases of the helium-argon family, suggested the possibility that one of the final inactive products of the disintegration of the radio-elements might prove to be a chemically inert gas. The later discovery of the material nature of the α rays added weight to the suggestion; for the measurement of the ratio e/m of the α particle indicated that if the α particle consisted of any known kind of matter, it must either be hydrogen or helium. For these reasons, it was suggested in 1902 by Rutherford and Soddy^[32] that helium might be a product of the disintegration of the radio-elements.

Sir William Ramsay and Mr Soddy in 1903 undertook an investigation of the radium emanation, with the purpose of seeing if it were possible to obtain any spectroscopic evidence of the presence of a new substance. First of all, they exposed the emanation to very drastic treatment (section 158), and confirmed and extended the results previously noted by Rutherford and Soddy that the emanation behaved like a chemically inert gas, and in this respect possessed properties analogous to the gases of the helium-argon group.

On obtaining 30 milligrams of pure radium bromide (prepared about three months previously) Ramsay and Soddy^[33] examined the gases, liberated by solution of the radium bromide in water, for the presence of helium. A considerable quantity of hydrogen and oxygen was released by the solution (see section 124). The hydrogen and oxygen were removed by passing the liberated gases over a red-hot spiral of partially oxidized copper-wire and the resulting water vapour was absorbed in a phosphorus pentoxide tube.

The gas was then passed into a small vacuum tube which was in connection with a small U tube. By placing the U tube in liquid air, most of the emanation present was condensed, and also most of the CO_{2} present in the gas. On examining the spectrum of the gas in the vacuum tube, the characteristic line D_{3} of helium was observed. This experiment was repeated with 30 milligrams of radium bromide about four months old, lent for the purpose by the writer. The emanation and CO_{2} were removed by passing them through a U tube immersed in liquid air. A practically complete spectrum of helium was observed, including the lines of wave-lengths 6677, 5876, 5016, 4972, 4713 and 4472. There were also present three other lines of wave-lengths about 6180, 5695, 5455 which have not yet been identified.

In later experiments, the emanation from 50 milligrams of the radium bromide was conveyed with oxygen into a small U tube, cooled in liquid air, in which the emanation was condensed. Fresh oxygen was added, and the U tube again pumped out. The small vacuum tube, connected with the U tube, showed at first no helium lines when the liquid air was removed. The spectrum obtained was a new one, and Ramsay and Soddy considered it to be probably that of the emanation itself. After allowing the emanation tube to stand for four days, the helium spectrum appeared with all the characteristic lines, and in addition, three new lines present in the helium obtained by solution of the radium. These results have since been confirmed. The experiments, which have led to such striking and important results, were by no means easy of performance, for the quantity of helium and of emanation released from 50 mgrs. of radium bromide is extremely small. It was necessary, in all cases, to remove almost completely the other gases, which were present in sufficient quantity to mask the spectrum of the substance under examination. The success of the experiments has been largely due to the application, to this investigation, of the refined methods of gas analysis, previously employed by Sir William Ramsay with so much skill in the separation of the rare gases xenon and krypton, which exist in minute proportions in the atmosphere. The fact that the helium spectrum was not present at first, but appeared after the emanation had remained in the tube for some days, shows that the helium must have been produced from the emanation. The emanation cannot be helium itself, for, in the first place, helium is not radio-active, and in the second place, the helium spectrum was not present at first, when the quantity of emanation in the tube was at its maximum. Moreover, the diffusion experiments, already dis-

• cussed, point to the conclusion that the emanation is of high

molecular weight. There can thus be no doubt that the helium is derived from the emanation of radium in consequence of changes of some kind occurring in it.

These results were confirmed later by other observers. Curie and Dewar^[34] performed the following experiment: A weight of about ·42 gr. of radium bromide was placed in a quartz tube, and the tube exhausted until no further gas came off. The radium was then heated to fusion, about 2·6 c.c. of gas being liberated in the process. The tube was then sealed, and some weeks afterwards the spectrum of the gas liberated in the tube by the radium was examined by Deslandres and found to give the entire spectrum of helium. The gas, liberated during the initial heating of the radium, was collected and found to contain a large amount of emanation, although the gas had been passed through two tubes immersed in liquid air. The tube containing these gases was very luminous and rapidly turned violet, while more than half of the gases was absorbed. The spectrum of the phosphorescent light was found to be discontinuous, consisting of three nitrogen bands. No sign of the helium spectrum was observed, although helium must have been present.

Himstedt and Meyer^[35] placed 50 mgrs. of radium bromide in a U tube connected with a small vacuum tube. The tube was carefully exhausted and then sealed off. The spectrum of hydrogen and carbon dioxide alone was observed for three months, but after four months the red, yellow, green and blue lines of the helium spectrum were visible. The slow appearance of the helium spectrum was probably due to the presence in the tube of a considerable quantity of hydrogen. In another experiment, some radium sulphate which had been heated to a bright red heat in a quartz tube was connected with a small vacuum tube. After three weeks, some of the lines of helium were clearly seen, and increased in brightness with time.

268. Connection between helium and the α particles. The appearance of helium in a tube containing the radium emanation may indicate either that the helium is one of the final products, which appear at the end of the series of radio-active changes, or that the helium is in reality the expelled α particle. The evidence at present points to the latter as being the more probable explanation. In the first place, the emanation diffuses like a gas of heavy molecular weight, and it appears probable that after the expulsion of a few α particles, the atomic weight of the final product is comparable with that of the emanation. On the other hand, the value of e/m determined for the projected α particle points to the conclusion that, if it consists of any known kind of matter, it is either hydrogen or helium.

There has been a tendency to assume that the helium produced from the radium emanation is the last transformation product of that substance. The evidence, however, does not support this view. We have seen that the emanation, after the initial rapid changes, is transformed very slowly. If the helium were the final product, the amount present in the emanation tube after a few days or weeks would be insignificant, since the product radium D intervenes, which takes 40 years to be half transformed. Since the helium cannot be the final product of the series of changes, and since all the other products are radio-active, and almost certainly of high atomic weight, it is difficult to see what position the helium atom occupies in the scheme of transformation, unless it be the α particle expelled during the successive changes.

It is a matter of great difficulty to settle definitely whether the α particle is a projected helium atom or not. On account of the very small deflection of the α rays in an electric field, and the complex nature of the α radiation from radium, an accurate determination of the value e/m for the α particle is beset with difficulties.

It may be possible to settle the question by accurate measurements of the volume of gas in a tube, filled originally with the radium emanation. Since the emanation itself, and two of the rapidly changing products which result from it, emit α particles, the final volume of the α particles, if they can exist in the gaseous state, would be three times the volume of the emanation. Ramsay and Soddy (section 172) have made experiments of this kind, but the results obtained were very contradictory, depending upon the kind of glass employed. In one case, the volume of the residual gases shrank almost to zero, in another the initial volume increased to about ten times its initial value. In the latter experiment a brilliant spectrum of helium was observed in the residual gas. This difference of behaviour is probably due to different degrees of absorption of helium by the glass tubes.

If the α particles are helium atoms, we may expect that a large proportion of the helium, which is produced in a tube containing the radium emanation, will be buried in the wall of the glass tube; for the α particles are projected with sufficient velocity to penetrate some distance into the glass. This helium may either remain in the glass, or in some cases rapidly diffuse out again. In any case, a fraction of the helium would be liberated when an intense electric discharge is passed through the tube. Ramsay and Soddy have in some instances observed that a slight amount of helium is liberated on heating the walls of the tube in which the emanation had been stored for some time.

The volume of helium produced per year by 1 gram of radium can easily be calculated on the assumption that the α particle is in reality a helium atom.

It has been shown that 2·5 × 10^{11} α particles are projected per second from 1 gram of radium. Since there are 3·6 × 10^{19} molecules in one cubic centimetre of any gas at standard pressure and temperature, the volume of the α particles released per second is 7 × 10^{-9} c.c. and per year 0·24 c.c. It has already been pointed out that, on this hypothesis, the volume of helium released by the emanation is three times the volume of the latter. The amount of helium to be obtained from the emanation released from 1 gram of radium in radio-active equilibrium is thus about 3 cubic mms.

Ramsay and Soddy have tried to estimate experimentally the probable volume of helium produced per second by one gram of radium. The helium, obtained from 50 mgrs. of radium bromide, which had been kept in solution in a closed vessel for 60 days, was introduced into a vacuum tube. Another similar tube was placed in series with it, and the amount of the helium in the latter adjusted until on passing a discharge through the two tubes in series the helium lines in each tube were of about the same brightness. In this way they calculated that the amount of helium present was 0·1 cubic mm. On this estimate, the amount of helium produced per year per gram of radium is about 20 cubic mms. We have seen that the calculated amount is about 240 cubic mms., on the assumption that the α particle is a helium atom. Ramsay and Soddy consider that the presence of argon in one of the tubes may have seriously interfered with the correctness of the estimation. On account of the great uncertainty attaching to estimates of the above character, the value deduced by Ramsay and Soddy does not exclude the probability that the calculated volume may be of the right order of magnitude.

In order to explain the presence of helium in radium on ordinary chemical lines, it has been suggested that radium is not a true element, but a molecular compound of helium with some substance known or unknown. The helium compound gradually breaks down, giving rise to the helium observed. It is at once obvious that this postulated helium compound is of a character entirely different from that of any other compound previously observed in chemistry. Weight for weight, it emits during its change an amount of energy at least one million times greater than any molecular compound known (see section 249). In addition, it must be supposed that the rate of breaking up of the helium compound is independent of great ranges of temperature—a result never before observed in any molecular change. The helium compound in its breaking up must give rise to the peculiar radiations and also pass through the successive radio-active changes observed in radium.

Thus in order to explain the production of helium and radio-activity on this view, a unique kind of molecule must be postulated—a molecule, in fact, which is endowed with every single property which on the disintegration theory is ascribed to the atom of the radio-elements. On the other hand, radium as far as it has been examined, has fulfilled every test required for an element. It has a well-marked and characteristic spectrum, and there is no reason to suppose that it is not an element in the ordinarily accepted sense of the term.

On the theory that the radio-elements are undergoing atomic disintegration, the helium must be considered to be a constituent of the radium atom, or, in other words, the radium atom is built up of parts, one of which, at least, is the atom of helium. The theory that the heavy atoms are all built up of some simple fundamental unit of matter or protyle has been advanced at various times by many prominent chemists and physicists. Prout's hypothesis that all elements are built up out of hydrogen is an example of this point of view of regarding the subject.

On the disintegration theory, the changes occurring in the radio-atoms involve an actual transformation of the atoms through successive changes. This change is so slow in uranium and thorium that at least a million years would be required before the amount of change could be measured by the balance. In radium it is a million times faster, but even in this case it is doubtful whether any appreciable change would have been observed by ordinary chemical methods for many years had not the possibility of such a change been suggested from other lines of evidence.

The similarity of the α particles from the different radio-elements indicates that they consist of expelled particles of the same kind. On this view, helium should be produced by each of the radio-elements. Its presence in minerals containing thorium, for example in monazite sand and the Ceylon mineral described by Ramsay, indicates that helium may be a product of thorium as well as of radium. Strutt^[36] has recently suggested that most of the helium observed in radio-active minerals may be a decomposition product of thorium rather than of uranium and radium; for he finds that minerals rich in helium always contain thorium, while many uranium minerals nearly free from thorium contain little helium. The evidence in support of this view is, however, not altogether satisfactory, for some of the uranium minerals in question are secondary uranium minerals (see Appendix B), deposited by the action of water or other agencies at a comparatively late date, and are also, in many cases, highly emanating, and consequently could not be expected to retain more than a fraction of the helium produced in them.

Taking the view that the α particles are projected helium atoms, we must regard the atoms of the radio-elements as compounds of some known or unknown substance with helium. These compounds break up spontaneously, and at a very slow rate even in the case of radium. The disintegration takes place in successive stages, and at most of the stages a helium atom is projected with great velocity. This disintegration is accompanied by an enormous emission of energy. The liberation of such a large amount of energy in the radio-active changes at once explains the constancy of the rate of change under the action of any of the physical and chemical agencies at our command. On this view, uranium, thorium and radium are in reality compounds of helium. The helium, however, is held in such strong combination that the compound cannot be broken up by chemical or physical forces, and, in consequence, these bodies behave as chemical elements in the ordinary accepted chemical sense.

It appears not unlikely that many of the so-called chemical elements may prove to be compounds of helium, or, in other words, that the helium atom is one of the secondary units with which the heavier atoms are built up. In this connection it is of interest to note that many of the elements differ in their atomic weight by four—the atomic weight of helium.

If the α particle is a helium atom, at least three α particles must be expelled from uranium (238·5) to reduce its atomic weight to that of radium (225). It is known that five α particles are expelled from radium during its successive transformations. This would make the atomic weight of the final residue 225 - 20 = 205. This is very nearly the atomic weight of lead, 206·5. I have, for some time, considered it probable that lead is the end or final product of radium. The same suggestion has recently been made by Boltwood^[37]. This point of view is supported by the fact that lead is always found in small quantity in all uranium minerals, and that the relative proportions of lead and helium in the radio-active minerals are about the same as would be expected if lead and helium were both decomposition products of radium. Dr Boltwood has drawn my attention to the fact that the proportion of lead in many radio-active minerals varies with the content of helium. A mineral rich in helium in nearly all cases contains more lead than a mineral poor in helium. This cannot be considered, at present, more than a speculation, but the facts as they stand are very suggestive. 269. Age of radio-active minerals. Helium is only found in the radio-active minerals, and this fact, taken in conjunction with the liberation of helium by radium, indicates that the helium must have been produced as a result of the transformation of radium and the other radio-active substances contained in the minerals. Now in a mineral about half the helium is, in many cases, released by heat and the residue by solution. It seems probable that the helium produced throughout the mass of the mineral is mechanically imprisoned in it. Moss^[38] found that, by grinding pitchblende in vacuo, helium is evolved, apparently showing that the helium exists in cavities of the mineral. Travers^[39] has suggested that, since helium is liberated on heating, the effect may be due to the heat generated by grinding. The escape of the helium from the heated mineral is probably connected with the fact observed by Jaquerod^[40] that helium passes through the walls of a quartz tube, heated above 500° C. The substance of the mineral probably possesses a similar property. Travers considers that helium is present in the mineral in a state of supersaturated solid solution, but the facts are equally well explained by assuming that the helium is mechanically imprisoned in the mass of the mineral.

The sudden rise of temperature observed in the mineral fergusonite, at the time the helium is released, has been found to have nothing to do with the presence of helium, for it also takes place in minerals not containing helium. The old view that helium was in a state of chemical combination with the mineral must be abandoned in the light of these more recent experiments.

Since the helium is only released from some minerals by the action of high temperatures and solution, it appears probable that a large proportion of the helium found in the minerals is unable to escape under normal conditions. Thus if the rate of production of helium by the radio-active substance were definitely known, it should be possible to calculate the age of the mineral by observing the volume of helium liberated from it by solution.

In the absence of such definite information, an approximate calculation will be made to indicate the order of magnitude of the time that has elapsed since the mineral was formed or was at a temperature low enough to prevent the escape of the helium.

Let us take, for example, the mineral fergusonite, which was found by Ramsay and Travers^[41] to evolve 1·81 c.c. of helium. The fergusonite contained about 7 per cent. of uranium. Now uranium in old minerals probably contains about 8 × 10^{-7} of its weight of radium (see section 262). One gram of the mineral thus contained about 5·6 × 10^{-8} grams of radium. Now if the α particle is helium, it has been shown that 1 gram of radium produces 0·24 c.c. of helium per year. The volume of helium produced per year in 1 gram of fergusonite is thus 1·3 × 10^{-8} c.c. Assuming that the rate of production of helium has been uniform, the time required to produce 1·81 c.c. per gram is about 140 million years. If the calculated rate of production of helium by radium is an overestimate, the time is correspondingly lengthened.

I think that, when the constants required for these calculations are more definitely fixed, this method will probably give fairly trustworthy information as to the probable age of some of the radio-active minerals of the earth's crust, and indirectly as to the age of the strata in which they are found.

In this connection it is of interest to note that Ramsay^[42] found that a Ceylon mineral, thorianite, contained as much as 9·5 c.c. of helium per gram. According to the analysis by Dunstan, this mineral contains about 76 per cent. of thorium and 12 per cent. of uranium. The unusually large amount of helium evolved from this mineral would indicate that it was formed at an earlier date than the fergusonite previously considered.

270. Possible causes of disintegration. In order to explain the phenomena of radio-activity, it has been supposed that a certain small fraction of the radio-atoms undergoes disintegration per second, but no assumptions have been made as to the cause which produces the instability and consequent disintegration. The instability of the atoms may be supposed to be brought about either by the action of external forces or by that of forces inherent in the atoms themselves. It is conceivable, for example, that the application of some slight external force might cause instability and consequent disintegration, accompanied by the liberation of a large amount of energy, on the same principle that a detonator is necessary to start some explosives. It has been shown that the number of atoms of any radio-active product which break up per second is always proportional to the number present. This law of change does not throw any light on the question, for it would be expected equally on either hypothesis. It has not been found possible to alter the rate of change of any product by the application of any known physical or chemical forces, unless possibly it is assumed that the force of gravitation which is not under our control may influence in some way the stability of the radio-atoms.

It seems likely therefore that the cause of the disruption of the atoms of the radio-elements and their products resides in the atoms themselves. According to the modern views of the constitution of the atom, it is not so much a matter of surprise that some atoms disintegrate as that the atoms of the elements are so permanent as they appear to be. In accordance with the hypothesis of J. J. Thomson, it may be supposed that the atoms consist of a number of small positively and negatively charged particles in rapid internal movement, and held in equilibrium by their mutual forces. In a complex atom, where the possible variations in the relative motion of the parts are very great, the atom may arrive at such a phase that one part acquires sufficient kinetic energy to escape from the system, or that the constraining forces are momentarily neutralised, so that the part escapes from the system with the velocity possessed by it at the instant of its release.

Sir Oliver Lodge^[43] has advanced the view that the instability of the atom may be a result of radiation of energy by the atom. Larmor has shown that an electron, subject to acceleration, radiates energy at a rate proportional to the square of its acceleration. An electron moving uniformly in a straight line does not radiate energy, but an electron, constrained to move in a circular orbit with constant velocity, is a powerful radiator, for in such a case the electron is continuously accelerated towards the centre. Lodge considered the simple case of a negatively charged electron revolving round an atom of mass relatively large but having an equal positive charge and held in equilibrium by electrical forces. This system will radiate energy, and, since the radiation of energy is equivalent to motion in a resisting medium, the particle tends to move towards the centre, and its speed consequently increases. The rate of radiation of energy will increase rapidly with the speed of the electron. When the speed of the electron becomes very nearly equal to the velocity of light, according to Lodge, another effect supervenes. It has been shown (section 82) that the apparent mass of an electron increases very rapidly as the speed of light is approached, and is theoretically infinite at the speed of light. There will be at this stage a sudden increase of the mass of the revolving atom, and, on the supposition that this stage can be reached, a consequent disturbance of the balance of forces holding the system together. Lodge considers it probable that, under these conditions, the parts of the system will break asunder and escape from the sphere of one another's influence.

It seems probable that the primary cause of the disintegration of the atom must be looked for in the loss of energy of the atomic system due to electro-magnetic radiation (section 52). Larmor^[44] has shown that the condition to be fulfilled in order that a system of rapidly moving electrons may persist without loss of energy is that the vector sum of the accelerations towards the centre should be permanently zero. While a single electron moving in a circular orbit is a powerful radiator of energy, it is remarkable how rapidly the radiation of energy diminishes if several electrons are revolving in a ring. This has recently been shown by J. J. Thomson^[45], who examined mathematically the case of a system of negatively electrified corpuscles, situated at equal intervals round the circumference of a circle, and rotating in one plane with uniform velocity round its centre. For example, he found that the radiation from a group of six particles moving with a velocity of 1/10 of the velocity of light is less than one-millionth part of the radiation from a single particle describing the same orbit with the same velocity. When the velocity is 1/100 of that of light the amount of radiation is only 10^{-16} that of a single particle moving with the same velocity in the same orbit.

Results of this kind indicate that an atom consisting of a large number of revolving electrons may radiate energy extremely slowly, and yet, finally, this minute but continuous drain of energy from the atom must result either in a rearrangement of its component parts into a new system, or of an expulsion of electrons or groups of electrons from the atom.

Simple models of atoms to imitate the behaviour of polonium in shooting out α particles, and of radium in shooting out β particles have been discussed by Lord Kelvin^[46]. It is possible to devise certain stable arrangements of the positively and negatively electrified particles, supposed to constitute an atom, which, on the application of some disturbing force, break up with the expulsion of a part of the system with great velocity.

J. J. Thomson^[47] has mathematically investigated the possible stable arrangements of a number of electrons moving about in a sphere of uniform positive electrification. The properties of such a model atom are very striking, and indirectly suggest a possible explanation of the periodic law in chemistry. He has shown that the electrons, if in one plane, arrange themselves in a number of concentric rings; and generally, if they are not constrained to move in one plane, in a number of concentric shells like the coats of an onion.

The mathematical problem is much simplified if the electrons are supposed to rotate in rings in one plane, the electrons in each ring being arranged at equal angular intervals. The ways in which the number of electrons group themselves, for numbers ranging from 60 to 5 at intervals of 5, are shown in the following table:—

+——————————————+——+——+——+——+——+——+
| Number of electrons | 60 | 55 | 50 | 45 | 40 | 35 |
+——————————————+——+——+——+——+——+——+
| Number in successive rings | 20 | 19 | 18 | 17 | 16 | 16 |
| | 16 | 16 | 15 | 14 | 13 | 12 |
| | 13 | 12 | 11 | 10 | 8 | 6 |
| | 8 | 7 | 5 | 4 | 3 | 1 |
| | 3 | 1 | 1 | | | |
+——————————————+——+——+——+——+——+——+

+——————————————+——+——+——+——+——+——+
| Number of electrons | 30 | 25 | 20 | 15 | 10 | 5 |
+——————————————+——+——+——+——+——+——+
| Number in successive rings | 15 | 13 | 12 | 10 | 8 | 5 |
| | 10 | 9 | 7 | 5 | 2 | |
| | 5 | 3 | 1 | | | |
+——————————————+——+——+——+——+——+——+

In the next table is given the possible series of arrangements of electrons which can have an outer ring of 20:—

+——————————————+——+——+——+——+——+——+——+——+——+
| Number of electrons | 59 | 60 | 61 | 62 | 63 | 64 | 65 | 66 | 67 |
+——————————————+——+——+——+——+——+——+——+——+——+
| Number in successive rings | 20 | 20 | 20 | 20 | 20 | 20 | 20 | 20 | 20 |
| | 16 | 16 | 16 | 17 | 17 | 17 | 17 | 17 | 17 |
| | 13 | 13 | 13 | 13 | 13 | 13 | 14 | 14 | 15 |
| | 8 | 8 | 9 | 9 | 10 | 10 | 10 | 10 | 10 |
| | 2 | 3 | 3 | 3 | 3 | 4 | 4 | 5 | 5 |
+——————————————+——+——+——+——+——+——+——+——+——+

The smallest number of electrons which can have an outer ring of 20 is 59, while 67 is the greatest.

The various arrangements of electrons can be classified into families, in which the groupings of the electrons have certain features in common. Thus the group of 60 electrons consists of the same arrangement of electrons as the group of 40 with the addition of an outer ring of 20 electrons; the group of 40 is the same as the group of 24 with an additional ring outside; and the group of 24 in turn is the same as the group of 11 with an extra ring. A series of model atoms may be formed in this way, in which each atom is derived from the preceding member by an additional ring of electrons. Such atoms would be expected to possess many properties in common, and would correspond to the elements in the same vertical column of the periodic table of Mendeléef.

Different arrangements of electrons vary widely in stability. Some may acquire an extra electron or two and yet remain stable, others readily lose an electron without disturbing their stability. The former would correspond to an electro-negative atom, the latter to an electro-positive.

Certain arrangements of electrons are stable if the electrons move with an angular velocity greater than a certain value, but become unstable when the velocity falls below this value. Four electrons in motion, for example, are stable in one plane, but when the velocity falls below a certain critical value, the system is unstable, and the electrons tend to arrange themselves at the corners of a regular tetrahedron. J. J. Thomson (loc. cit.) applies this property to explain why an atom of radio-active matter breaks up, as follows:—

"Consider now the properties of an atom containing a system of corpuscles (electrons) of this kind. Suppose the corpuscles were originally moving with velocities far exceeding the critical velocity; in consequence of the radiation from the moving corpuscles, their velocity will slowly—very slowly—diminish; when, after a long interval, the velocity reaches the critical velocity, there will be what is equivalent to an explosion of the corpuscles, the corpuscles will move far away from their original position, their potential energy will decrease, while their kinetic energy will increase. The kinetic energy gained in this way might be sufficient to carry the system out of the atom, and we should have, as in the case of radium, a part of the atom shot off. In consequence of the very slow dissipation of energy by radiation the life of the atom would be very long. We have taken the case of the four corpuscles as the type of a system which, like a top, requires for its stability a certain amount of rotation. Any system possessing this property would, in consequence of the gradual dissipation of energy by radiation, give to the atom containing it radio-active properties similar to those conferred by the four corpuscles."

271. Heat of the sun and earth. It was pointed out by Rutherford and Soddy^[48] that the maintenance of the sun's heat for long intervals of time did not present any fundamental difficulty if a process of disintegration, such as occurs in the radio-elements, were supposed to be taking place in the sun. In a letter to Nature (July 9, 1903) W. E. Wilson showed that the presence of 3·6 grams of radium in each cubic metre of the sun's mass was sufficient to account for the present rate of emission of energy by the sun. This calculation was based on the estimate of Curie and Laborde that 1 gram of radium emits 100 gram-calories per hour, and on the observation of Langley that each square centimetre of the sun's surface emits 8·28 × 10^6 gram-calories per hour. Since the average density of the sun is 1·44, the presence of radium in the sun, to the extent of 2·5 parts by weight in a million, would account for its present rate of emission of energy.

An examination of the spectrum of the sun has not so far revealed any of the radium lines. It is known, however, from spectroscopic evidence that helium is present, and this indirectly suggests the existence of radio-active matter also. It can readily be shown^[49] that the absence of penetrating rays from the sun at the surface of the earth does not imply that the radio-elements are not present in the sun. Even if the sun were composed of pure radium, it would hardly be expected that the γ rays emitted would be appreciable at the surface of the earth, since the rays would be almost completely absorbed in passing through the atmosphere, which corresponds to a thickness of 76 centimetres of mercury.

In the Appendix E of Thomson and Tait's Natural Philosophy, Lord Kelvin has calculated the energy lost in the concentration of the sun from a condition of infinite dispersion, and concludes that it seems "on the whole probable that the sun has not illuminated the earth for 100,000,000 years and almost certain that he has not done so for 500,000,000 years. As for the future we may say, with equal certainty, that inhabitants of the earth cannot continue to enjoy the light and heat essential to their life for many million years longer, unless sources now unknown to us are prepared in the great storehouses of creation."

The discovery that a small mass of a substance like radium can emit spontaneously an enormous quantity of heat renders it possible that this estimate of the age of the sun's heat may be much increased. In a letter to Nature (Sept. 24, 1903) G. H. Darwin drew attention to this probability, and at the same time pointed out that, on Kelvin's hypotheses, his estimate of the duration of the sun's heat was probably much too high, and stated that, "The lost energy of the sun, supposed to be a homogeneous sphere of mass M and radius a, is (3/5)μM^2/a where μ is the constant of gravitation. On introducing numerical values for the symbols in this formula, I find the lost energy to be 2·7 × 10^7 M calories where M is expressed in grams. If we adopt Langley's value of the solar constant, this heat suffices to give a supply for 12 million years. Lord Kelvin used Pouillet's value for that constant, but if he had been able to use Langley's, his 100 million would have been reduced to 60 million. The discrepancy between my results of 12 million and his of 60 million is explained by a conjectural augmentation of the lost energy to allow for the concentration of the solar mass towards its central parts." Now it has been shown (section 266) that one gram of radium emits during its life an amount of heat corresponding to 1·6 × 10^9 gram-calories. It has also been pointed out that there is every reason to suppose that a similar amount of energy is resident in the chemical atoms of the inactive elements. It is not improbable that, at the enormous temperature of the sun, the breaking up of the elements into simpler forms may be taking place at a more rapid rate than on the earth. If the energy resident in the atoms of the elements is thus available, the time during which the sun may continue to emit heat at the present rate may be at least 50 times longer than the value computed from dynamical data.

Similar considerations apply to the question of the age of the earth. A full discussion of the probable age of the earth, computed from its secular cooling from a molten mass, is given by Lord Kelvin in Appendix D of Thomson and Tait's Natural Philosophy. He has there shown that about 100 million years after the earth was a molten mass, the gradual cooling due to radiation from its surface would account for the average temperature gradient of 1/50° F. per foot, observed to-day near the earth's surface.

Some considerations will now be discussed which point to the probability that the present temperature gradient observed in the earth cannot be used as a guide to estimate the length of time that has elapsed since the earth has been at a temperature capable of supporting animal and vegetable life; for it will be shown that probably there is sufficient radio-active matter on the earth to supply as much heat to the earth as is lost by radiation from its surface. Taking the average conductivity K of the materials of the earth as ·004 (C.G.S. units) and the temperature gradient T near the surface as ·00037° C. per cm., the heat Q in gram-calories conducted to the surface of the earth per second is given by

Q = 4πR^2KT,

where R is the radius of the earth.

Let X be the average amount of heat liberated per second per cubic centimetre of the earth's volume owing to the presence of radio-active matter. If the heat Q radiated from the earth is equal to the heat supplied by the radio-active matter in the earth,

       X . (4/3)πR^3 = 4πR^2KT,
or X = 3KT/R.

Substituting the values of these constants,

X = 7 × 10^{-15} gram-calories per second
    = 2·2 × 10^{-7} gram-calories per year.

Since 1 gram of radium emits 876,000 gram-calories per year, the presence of 2·6 × 10^{-13} grams of radium per unit volume, or 4·6 × 10^{-14} grams per unit mass, would compensate for the heat lost from the earth by conduction.

Now it will be shown in the following chapter that radio-active matter seems to be distributed fairly uniformly through the earth and atmosphere. In addition, it has been found that all substances are radio-active to a feeble degree, although it is not yet settled whether this radio-activity may not be due mainly to the presence of a radio-element as an impurity. For example, Strutt^[50] observed that a platinum plate was about 1/3000 as active as a crystal of uranium nitrate, or about 2 × 10^{-10} as active as radium. This corresponds to a far greater activity than is necessary to compensate for the loss of heat of the earth. A more accurate deduction, however, can be made from data of the radio-activity exhibited by matter dug out of the earth. Elster and Geitel^[51] filled a dish of volume 3·3 × 10^3 c.c. with clay dug up from the garden, and placed it in a vessel of 30 litres capacity in which was placed an electroscope to determine the conductivity of the enclosed gas. After standing for several days, they found that the conductivity of the air reached a constant maximum value, corresponding to three times the normal. It will be shown later (section 284) that the normal conductivity observed in sealed vessels corresponds to the production of about 30 ions per c.c. per second. The number of ions produced per second in the vessel by the radio-active earth was thus about 2 × 10^6. This would give a saturation current through the gas of 2·2 × 10^{-14} electro-magnetic units. Now the emanation from 1 gram of radium stored in a metal cylinder gives a saturation current of about 3·2 × 10^{-5} electro-magnetic units. Elster and Geitel considered that most of the conductivity observed in the gas was due to a radio-active emanation, which gradually diffused from the clay into the air in the vessel. The increased conductivity in the gas observed by Elster and Geitel would thus be produced by the emanation from 7 × 10^{-10} gram of radium. Taking the density of clay as 2, this corresponds to about 10^{-13} gram of radium per gram of clay. But it has been shown that if 4·6 × 10^{-14} gram of radium were present in each gram of earth, the heat emitted would compensate for the loss of heat of the earth by conduction and radiation. The amount of activity observed in the earth is thus about the right order of magnitude to account for the heat emission required. In the above estimate, the presence of uranium and thorium minerals in the earth has not been considered. Moreover, it is probable that the total amount of radio-activity in the clay was considerably greater than that calculated, for it is likely that other radio-active matter was present which did not give off an emanation.

If the earth is supposed to be in a state of thermal equilibrium in which the heat lost by radiation is supplied from radio-active matter, there must be an amount of radio-active matter in the earth corresponding to about 270 million tons of radium. If there were more radium than this in the earth, the temperature gradient would be greater than that observed to-day. This may appear to be a very large quantity of radium, but recent determinations (section 281) of the amount of radium emanation in the atmosphere strongly support the view that a large quantity of radium must exist in the surface soil of the earth. Eve found, on a minimum estimate, that the amount of emanation always present in the atmosphere is equivalent to the equilibrium amount derived from 100 tons of radium. There is every reason to believe that the emanation found in the atmosphere is supplied both by the diffusion of the emanation from the soil and by the action of springs. Since the emanation loses half its activity in four days, it cannot diffuse from any great depth. Assuming that the radium is uniformly distributed throughout the earth, the quantity of the radium emanation produced in a thin shell of earth about thirteen metres in depth, is sufficient to account for the amount ordinarily observed in the atmosphere.

I think we may conclude that the present rate of loss of heat of the earth might have continued unchanged for long periods of time in consequence of the supply of heat from radio-active matter in the earth. It thus seems probable that the earth may have remained for very long intervals of time at a temperature not very different from that observed to-day, and that, in consequence, the time during which the earth has been at a temperature capable of supporting the presence of animal and vegetable life may be very much longer than the estimate made by Lord Kelvin from other data.

272. Evolution of matter. Although the hypothesis that all matter is composed of some elementary unit of matter or protyle has been advanced as a speculation at various times by many prominent physicists and chemists, the first definite experimental evidence showing that the chemical atom was not the smallest unit of matter was obtained in 1897 by J. J. Thomson in his classic research on the nature of the cathode rays produced by an electric discharge in a vacuum tube. We have seen that Sir William Crookes, who was the first to demonstrate the remarkable properties of these rays, had suggested that they consisted of streams of projected charged matter and represented—as he termed it—a new or "fourth state of matter."

J. J. Thomson showed by two distinct methods (section 50), that the cathode rays consisted of a stream of negatively charged particles projected with great velocity. The particles behaved as if their mass was only about 1/1000 of the mass of the atom of hydrogen, which is the lightest atom known. These corpuscles, as they were termed by Thomson, were found at a later date to be produced from a glowing carbon filament and from a zinc plate exposed to the action of ultra-violet light. They acted as isolated units of negative electricity, and, as we have seen, may be identified with the electrons studied mathematically by Larmor and Lorentz. Not only were these electrons produced by the action of light, heat, and the electric discharge, but similar bodies were also found to be emitted spontaneously from the radio-elements with a velocity far greater than that observed for the electrons in a vacuum tube.

The electrons produced in these various ways were all found to carry a negative charge, and to be apparently identical; for the ratio e/m of the charge of the electron to its mass was in all cases the same within the limits of experimental error. Since electrons, produced from different kinds of matter and under different conditions, were in all cases identical, it seemed probable that they were a constituent part of all matter. J. J. Thomson suggested that the atom is built up of a number of these negatively charged electrons combined in some way with corresponding positively charged bodies.

On this view the atoms of the chemical elements differ from one another only in the number and arrangement of the component electrons.

The removal of an electron from the atom in the case of ionization does not appear to affect permanently the stability of the system, for no evidence has so far been obtained to show that the passage of an intense electric discharge through a gas results in a permanent alteration of the structure of the atom. On the other hand, in the case of the radio-active bodies, a positively charged particle of mass about twice that of the hydrogen atom escapes from the heavy radio-atom. This loss appears to result at once in a permanent alteration of the atom, and causes a marked change in its physical and chemical properties. In addition there is no evidence that the process is reversible. The expulsion of a β particle with great velocity from an atom of radio-active matter also results in a transformation of the atom. For example radium E emits a β particle, and, in consequence, gives rise to a distinct substance radium F (polonium). A case of this kind, where the expulsion of a β particle with great velocity causes a complete rearrangement of the parts of an atom, is probably quite distinct from the process which occurs during ionization, where a slow speed electron escapes from the atom without apparently affecting the stability of the atom left behind.

The only direct experimental evidence of the transformation of matter has been derived from a study of the radio-active bodies. If the disintegration theory, advanced to account for the phenomena of radio-activity, is correct in the main essentials, then the radio-elements are undergoing a spontaneous and continuous process of transformation into other and different kinds of matter. The rate of transformation is slow in uranium and thorium, but is fairly rapid in radium. It has been shown that the fraction of a mass of radium which is transformed per year is about 1/2000 of the total amount present. In the case of uranium and thorium probably a million years would be required to produce a similar amount of change. Thus the process of transformation in uranium and thorium is far too slow to be detected within a reasonable time by the use of the balance or spectroscope, but the radiations which accompany the transformation can easily be detected. Although the process of change is slow it is continuous, and in the course of ages the uranium and thorium present in the earth must be transformed into other types of matter.

Those who have considered the possibility of atoms undergoing a process of transformation have generally thought that the matter as a whole would undergo a progressive change, with a gradual alteration of physical and chemical properties of the whole mass of substance. On the theory of disintegration this is not the case. Only a minute fraction of the matter present breaks up in unit time, and in each of the successive stages through which the disintegrated atoms pass, there is in most cases a marked alteration in the chemical and physical properties of the matter. The transformation of the radio-elements is thus a transformation of a part per saltum, and not a progressive change of the whole. At any time after the process of transformation has been in progress there will thus remain a part of the matter which is unchanged, and, mixed with it, the products which have resulted from the transformation of the remainder.

The question naturally arises whether the process of degradation of matter is confined to the radio-elements or is a universal property of matter. It will be shown in chapter XIV that all matter, so far examined, exhibits the property of radio-activity to a slight degree. It is very difficult, however, to make certain that the observed radio-activity is not due to the presence in the matter of a slight trace of a radio-element. If ordinary matter is radio-active, it is certain that its activity is much less than that of uranium, and consequently that its rate of transformation must be excessively slow. There is, however, another possibility to be considered. The changes occurring in the radio-elements would probably never have been detected if the change had not been accompanied by the expulsion of charged particles with great velocity. It does not seem unlikely that an atom may undergo disintegration without projecting a part of its system with sufficient velocity to ionize the gas. In fact, we have seen that, even in the radio-elements, several of the series of changes in both thorium, radium, and actinium are unaccompanied by ionizing rays. The experimental results given in Appendix A strongly support this point of view. It may thus be possible that all matter is undergoing a slow process of transformation, which has so far only been detected in the radio-elements on account of the expulsion of charged particles with great velocity during the change. This process of degradation of matter continuing for ages must reduce the constituents of the earth to the simpler and more stable forms of matter.

The idea that helium is a transformation product of radium suggests the probability that helium is one of the more elementary substances of which the heavier atoms are composed. Sir Norman Lockyer, in his interesting book on "Inorganic Evolution," has pointed out that the spectra of helium and of hydrogen predominate in the hottest stars. In the cooler stars the more complex types of matter appear. Sir Norman Lockyer has based his theory of evolution of matter on evidence of a spectroscopic examination of the stars, and considers that temperature is the main factor in breaking up matter into its simpler forms. The transformation of matter occurring in the radio-elements is on the other hand spontaneous, and independent of temperature over the range examined.

1. Perrin, Revue Scientifique, April 13, 1901.
2. Becquerel, C. R. 133, p. 979, 1901.
3. Rutherford and McClung, Phil. Trans. A, p. 25, 1901.
4. Rutherford, Phil. Mag. Jan. and Feb. 1900.
5. P. Curie, C. R. 136, p. 223, 1903.
6. Rutherford, Phil. Mag. April, 1903.
7. M. and Mme Curie, C. R. 134, p. 85, 1902.
8. Rutherford and Soddy, Trans. Chem. Soc. 81, pp. 321, 837, 1902. Phil. Mag. Sept. and Nov. 1902.
9. Rutherford and Soddy, Phil. Mag. April, 1903.
10. Rutherford and Soddy, Phil. Mag. May, 1903.
11. Rutherford, Phys. Zeit. 4, p. 235, 1902. Phil. Mag. Feb. 1903.
12. Rutherford, Phil. Mag. May, 1903.
13. Curie and Laborde, C. R. 136, p. 673, 1903.
14. J. J. Thomson, Nature, p. 601, 1903.
15. Crookes, C. R. 128, p. 176, 1899.
16. F. Re, C. R. p. 136, p. 1393, 1903.
17. Richarz and Schenck, Berl. Ber. p. 1102, 1903. Schenck, Berl. Ber. p. 37, 1904.
18. Armstrong and Lowry, Proc. Roy. Soc. 1903. Chem. News, 88, p. 89, 1903.
19. Rutherford and Soddy, Phil. Mag. May, 1903.
20. Boltwood, Nature, May 25, p. 80, 1904. Phil. Mag. April, 1905.
21. McCoy, Ber. d. D. Chem. Ges. No. 11, p. 2641, 1904.
22. Strutt, Nature, March 17 and July 7, 1904. Proc. Roy. Soc. March 2, 1905.
23. Strutt, Proc. Roy. Soc. March 2, 1905.
24. Soddy, Nature, May 12, 1904; Jan. 19, 1905.
25. Whetham, Nature, May 5, 1904; Jan. 26, 1905.
26. Danne, C. R. Jan. 23, 1905.
27. J. J. Thomson, Nature, April 30, p. 601, 1903.
28. Voller, Phys. Zeit. 5, No. 24, p. 781, 1904.
29. Ramsay and Cooke, Nature, Aug. 11, 1904.
30. Eve, Nature, March 16, 1905.
31. J. J. Thomson, International Electrical Congress, St Louis, Sept. 1904.
32. Rutherford and Soddy, Phil. Mag. p. 582, 1902; pp. 453 and 579, 1903.
33. Ramsay and Soddy, Nature, July 16, p. 246, 1903. Proc. Roy. Soc. 72, p. 204, 1903; 73, p. 346, 1904.
34. Curie and Dewar, C. R. 138, p. 190, 1904. Chem. News, 89, p. 85, 1904.
35. Himstedt and Meyer, Ann. d. Phys. 15, p. 184, 1904.
36. Strutt, Proc. Roy. Soc. March 2, 1905.
37. Boltwood, Phil. Mag. April, 1905.
38. Moss, Trans. Roy. Soc. Dublin, 1904.
39. Travers, Nature, p. 248, Jan. 12, 1905.
40. Jaquerod, C. R. p. 789, 1904.
41. Ramsay and Travers, Zeitsch. Physik. Chem. 25, p. 568, 1898.
42. Ramsay, Nature, April 7, 1904.
43. Lodge, Nature, June 11, p. 129, 1903.
44. Larmor, Aether and Matter, p. 233.
45. J. J. Thomson, Phil. Mag. p. 681, Dec. 1903.
46. Lord Kelvin, Phil. Mag. Oct. 1904.
47. Thomson, Phil. Mag. March, 1904.
48. Rutherford and Soddy, Phil. Mag. May, 1903.
49. See Strutt and Joly, Nature, Oct. 15, 1903.
50. Strutt, Phil. Mag. June, 1903.
51. Elster and Geitel, Phys. Zeit. 4, No. 19, p. 522, 1903. Chem. News, July 17, p. 30, 1903.