=254. Theories of radio-activity.= In previous chapters, a detailed account has been given of the nature and properties of the radiations, and of the complex processes taking place in the radio-active substances. The numerous products arising from the radio-elements have been closely examined, and have been shown to result from a transformation of the parent element through a number of well-marked stages. In this chapter, the application of the disintegration theory to the explanation of radio-active phenomena will be considered still further, and the logical deductions to be drawn from the theory will be discussed briefly.
A review will first be given of the working hypotheses which have served as a guide to the investigators in the field of radio-activity. These working theories have in many cases been modified or extended with the growth of experimental knowledge.
The early experiments of Mme Curie had indicated that radio-activity was an atomic and not a molecular phenomenon. This was still further substantiated by later work, and the detection and isolation of radium from pitchblende was a brilliant verification of the truth of this hypothesis.
The discovery that the β rays of the radio-elements were similar to the cathode rays produced in a vacuum tube was an important advance, and has formed the basis of several subsequent theories. J. Perrin[333], in 1901, following the views of J. J. Thomson and others, suggested that the atoms of bodies consisted of parts and might be likened to a miniature planetary system. In the atoms of the radio-elements, the parts composing the atoms more distant from the centre might be able to escape from the central attraction and thus give rise to the radiation of energy observed. In December 1901, Becquerel[334] put forward the following hypothesis, which, he stated, had served him as a guide in his investigations. According to the view of J. J. Thomson, radio-active matter consists of negatively and positively charged particles. The former have a mass about ¹⁄₁₀₀₀ of the mass of the hydrogen atom, while the latter have a mass about one thousand times greater than that of the negative particle. The negatively charged particles (the β rays) would be projected with great velocity, but the larger positive particles with a much lower velocity forming a sort of gas (the emanation) which deposits itself on the surface of bodies. This in turn would subdivide, giving rise to rays (excited activity).
In a paper communicated to the Royal Society in June 1900, Rutherford and McClung[335] estimated that the energy, radiated in the form of ionizing rays into the gas, was 3000 gram-calories per year for radium of activity 100,000 times that of uranium. Taking the latest estimate of the activity of a pure radium compound as 2,000,000, this would correspond to an emission of energy into the gas in the form of α rays of about 66,000 gram-calories per gram per year. The suggestion was made that this energy might be derived from a re-grouping of the constituents of the atom of the radio-elements, and it was pointed out that the possible energy to be derived from a greater concentration of the components of the atom was large compared with that given out in molecular reactions.
In the original papers[336] giving an account of the discovery of the emanation of thorium and the excited radio-activity produced by it, the view was taken that both of these manifestations were due to radio-active material. The emanation behaved like a gas, while the matter which caused excited activity attached itself to solids and could be dissolved in some acids but not in others. Rutherford and Miss Brooks showed that the radium emanation diffused through air like a gas of heavy molecular weight. At a later date Rutherford and Soddy showed that the radium and thorium emanations behaved like chemically inert gases, since they were unaffected by the most drastic physical and chemical treatment.
On the other hand, P. Curie, who, in conjunction with Debierne, had made a series of researches on the radium emanation, expressed dissent from this view. P. Curie[337] did not consider that there was sufficient evidence that the emanation was material in nature, and pointed out that no spectroscopic evidence of its presence had yet been obtained, and also that the emanation disappeared when contained in a sealed vessel. It was pointed out by the writer[338] that the failure to detect spectroscopic lines was probably a consequence of the minute quantity of the emanation present, under ordinary conditions, although the electrical and phosphorescent actions produced by this small quantity are very marked. This contention is borne out by later work. P. Curie at first took the view that the emanation was not material, but consisted of centres of condensation of energy attached to the gas molecules and moving with them.
M. and Mme Curie have throughout taken a very general view of the phenomena of radio-activity, and have not put forward any definite theory. In Jan. 1902, they gave an account of the general working theory[339] which had guided them in their researches. Radio-activity is an atomic property, and the recognition of this fact had created their methods of research. Each atom acts as a constant source of emission of energy. This energy may either be derived from the potential energy of the atom itself, or each atom may act as a mechanism which instantly regains the energy which is lost. They suggested that this energy may be borrowed from the surrounding air in some way not accounted for by the principle of Carnot.
In the course of a detailed study of the radio-activity of thorium, Rutherford and Soddy[340] found that it was necessary to suppose that thorium was continuously producing from itself new kinds of active matter, which possess temporary activity and differ in chemical properties from the thorium itself. The constant radio-activity of thorium was shown to be the result of equilibrium between the processes of production of active matter and the change of that already produced. At the same time, the theory was advanced that the production of active matter was a consequence of the disintegration of the atom. The work of the following year was devoted to an examination of the radio-activity of uranium and radium on similar lines, and it was found that the conclusions already advanced for thorium held equally for uranium and radium[341]. The discovery of a condensation of the radio-active emanations[342] gave additional support to the view that the emanations were gaseous in character. In the meantime, the writer[343] had found that the rays consisted of positively charged bodies atomic in size, projected with great velocity. The discovery of the material nature of these rays served to strengthen the theory of atomic disintegration, and at the same time to offer an explanation of the connection between the α rays and the changes occurring in the radio-elements. In a paper entitled “Radio-active Change,” Rutherford and Soddy[344] put forward in some detail the theory of atomic disintegration as an explanation of the phenomena of radio-activity, and at the same time some of the more important consequences which follow from the theory were discussed.
In a paper announcing the discovery of the heat emission of radium, P. Curie and Laborde[345] state that the heat energy may be equally well supposed to be derived from a breaking up of the radium atom or from energy absorbed by the radium from some external source.
J. J. Thomson in an article on “Radium,” communicated to _Nature_[346], put forward the view that the emission of energy from radium is probably due to some change within the atom, and pointed out that a large store of energy would be released by a contraction of the atom.
Sir William Crookes[347], in 1899, proposed the theory that the radio-active elements possess the property of abstracting energy from the gas. If the moving molecules, impinging more swiftly on the substance, were released from the active substance at a much lower velocity, the energy released from the radio-elements might be derived from the atmosphere. This theory was advanced again later on to account for the large heat emission of radium, discovered by P. Curie and Laborde.
F. Re[348] recently advanced a very general theory of matter with a special application to radio-active bodies. He supposes that the parts of the atom were originally free, constituting a nebula of extreme tenuity. These parts have gradually become united round centres of condensation, and have thus formed the atoms of the elements. On this view an atom may be likened to an extinct sun. The radio-active atoms occupy a transitional stage between the original nebula and the more stable chemical atoms, and in the course of their contraction give rise to the heat emission observed.
Lord Kelvin in a paper to the British Association meeting, 1903, has suggested that radium may obtain its energy from external sources. If a piece of white paper is put into one vessel and a piece of black paper into an exactly similar vessel, on exposure of both vessels to the light the vessel containing the black paper is found to be at a higher temperature. He suggests that radium in a similar manner may keep its temperature above the surrounding air by its power of absorption of unknown radiations.
Richarz and Schenck[349] have suggested that radio-activity may be due to the production and breaking up of ozone which is known to be produced by radium salts.
=255. Discussion of Theories.= From the survey of the general hypotheses advanced as possible explanations of radio-activity, it is seen that they may be divided broadly into two classes, one of which assumes that the energy emitted from the radio-elements is obtained at the expense of the internal energy of the atom, and the other that the energy is derived from external sources, but that the radio-elements act as mechanisms capable of transforming this borrowed energy into the special forms manifested in the phenomena of radio-activity. Of these two sets of hypotheses the first appears to be the more probable, and to be best supported by the experimental evidence. Up to the present not the slightest experimental evidence has been adduced to show that the energy of radium is derived from external sources.
J. J. Thomson (_loc. cit._) has discussed the question in the following way:—
“It has been suggested that the radium derives its energy from the air surrounding it, that the atoms of radium possess the faculty of abstracting the kinetic energy from the more rapidly moving air molecules while they are able to retain their own energy when in collision with the slowly moving molecules of air. I cannot see, however, that even the possession of this property would explain the behaviour of radium; for imagine a portion of radium placed in a cavity in a block of ice; the ice around the radium gets melted; where does the energy for this come from? By the hypothesis there is no change in the air-radium system in the cavity, for the energy gained by the radium is lost by the air, while heat cannot flow into the cavity from the outside, for the melted ice round the cavity is hotter than the ice surrounding it.”
The writer has recently found that the activity of radium is not altered by surrounding it with a large mass of lead. A cylinder of lead was cast 10 cms. in diameter and 10 cms. high. A hole was bored in one end of the cylinder to the centre, and the radium, enclosed in a small glass tube, was placed in the cavity. The opening was then hermetically closed. The activity was measured by the rate of discharge of an electroscope by the γ rays transmitted through the lead, but no appreciable change was observed during a period of one month.
M. and Mme Curie early made the suggestion that the radiation of energy from the radio-active bodies might be accounted for by supposing that space is traversed by a type of Röntgen rays, and that the radio-elements possess the property of absorbing them. Recent experiments (section 279) have shown that there is present at the surface of the earth a very penetrating type of rays, similar to the γ rays of radium. Even if it were supposed that the radio-elements possessed the power of absorbing this radiation, the energy of the rays is far too minute to account even for the energy radiated from an element of small activity like uranium. In addition, all the evidence so far obtained points to the conclusion that the radio-active bodies do not absorb the type of rays they emit to any greater extent than would be expected from their density. It has been shown (section 86) that this is true in the case of uranium. Even if it were supposed that the radio-elements possess the property of absorbing the energy of some unknown type of radiation, which is able to pass through ordinary matter with little absorption, there still remains the fundamental difficulty of accounting for the peculiar radiations from the radio-elements, and the series of changes that occur in them. It is not sufficient for us to account for the heat emission only, for it has been shown (chapter XII) that the emission of heat is directly connected with the radio-activity.
In addition, the distribution of the heat emission of radium amongst the radio-active products which arise from it is extremely difficult to explain on the hypothesis that the energy emitted is borrowed from external sources. It has been shown that more than two-thirds of the heat emitted by radium is due to the emanation together with the active deposit which is produced by the emanation. When the emanation is separated from the radium, its power of emitting heat, after reaching a maximum, decreases with the time according to an exponential law. It would thus be necessary on the absorption hypothesis to postulate that most of the heat emission of radium, observed under ordinary conditions, is not due to the radium itself but to something produced by the radium, whose power of absorbing energy from external sources diminishes with time.
A similar argument also applies to the variation with time of the heating effect of the active deposit produced from the emanation. It has been shown in the last chapter that most of the heating effect observed in radium and its products must be ascribed to the bombardment of the α particles expelled from these substances. It has already been pointed out (section 136) that it is difficult to imagine any mechanism, either internal or external, whereby such enormous velocity can suddenly be impressed upon the α particles. We are forced to the conclusion that the α particle did not suddenly acquire this energy of motion, but was initially in rapid motion in the atom, and for some reason, was suddenly released with the velocity which it previously possessed in its orbit.
The strongest evidence against the hypothesis of absorption of external energy is that such a theory ignores the fact, that, whenever radio-activity is observed, it is always accompanied by some change which can be detected by the appearance of new products having chemical properties distinct from those of the original substances. This leads to some form of “chemical” theory, and other results show that the change is atomic and not molecular.
=256. Theory of radio-active change.= The processes occurring in the radio-elements are of a character quite distinct from any previously observed in chemistry. Although it has been shown that the radio-activity is due to the spontaneous and continuous production of new types of active matter, the laws which control this production are different from the laws of ordinary chemical reactions. It has not been found possible in any way to alter either the rate at which the matter is produced or its rate of change when produced. Temperature, which is such an important factor in altering the rate of chemical reactions, is, in these cases, almost entirely without influence. In addition, no ordinary chemical change is known which is accompanied by the expulsion of charged atoms with great velocity. It has been suggested by Armstrong and Lowry[350] that radio-activity may be an exaggerated form of fluorescence or phosphorescence with a very slow rate of decay. But no form of phosphorescence has yet been shown to be accompanied by radiations of the character of those emitted by the radio-elements. Whatever hypothesis is put forward to explain radio-activity must account not only for the production of a series of active products, which differ in chemical and physical properties from each other and from the parent element, but also for the emission of rays of a special character. Besides this, it is necessary to account for the large amount of energy continuously radiated from the radio-elements.
The radio-elements, besides their high atomic weights, do not possess in common any special chemical characteristics which differentiate them from the other elements, which do not possess the property of radio-activity to an appreciable degree. Of all the known elements, uranium, thorium, and radium possess the greatest atomic weights, viz.: radium 225, thorium 232·5, and uranium 240.
If a high atomic weight is taken as evidence of a complicated structure of the atom, it might be expected that disintegration would occur more readily in heavy than in light atoms. At the same time, there is no reason to suppose that the elements of the highest atomic weight must be the most radio-active; in fact, radium is far more active than uranium, although its atomic weight is less. This is seen to be the case also in the radio-active products; for example, the radium emanation is enormously more active weight for weight than the radium itself, and there is every reason to believe that the emanation has an atom lighter than that of radium.
In order to explain the phenomena of radio-activity, Rutherford and Soddy have advanced the theory that the atoms of the radio-elements suffer spontaneous disintegration, and that each disintegrated atom passes through a succession of well-marked changes, accompanied in most cases by the emission of α rays.
A preliminary account of this hypothesis has already been given in section 136, while the mathematical theory of successive changes, which is based upon it, has been discussed in chapter IX. The general theory has been utilized in chapters X and XI to account for the numerous active substances found in uranium, thorium, actinium and radium.
The theory supposes that, on an average, a definite small proportion of the atoms of each radio-active substance becomes unstable at a given time. As a result of this instability, the atoms break up. In most cases, the disintegration is explosive in violence and is accompanied by the ejection of an α particle with great velocity; in a few cases, α and β particles are expelled together, while in others a β particle alone escapes. In a few cases, the change in the atom appears to be less violent in character, and is not accompanied by the expulsion of either an α or β particle. The explanation of these rayless changes is considered in section 259. The expulsion of an α particle, of mass about twice that of the hydrogen atom, leaves behind it a new system lighter than the original one, and possessing chemical and physical properties quite different from those of the original element. This new system again becomes unstable, and expels another α particle. The process of disintegration, once started, proceeds from stage to stage at a definite measurable rate in each case.
At any time after the disintegration has commenced, there exists a proportion of the original matter, which is unchanged, mixed with the part which has undergone change. This is in accordance with the observed fact that the spectrum of radium, for example, does not change progressively with time. The radium breaks up so slowly that only a small fraction has been transformed in the course of a few years. The unchanged part still shows its characteristic spectrum, and will continue to do so as long as any radium exists. At the same time it is to be expected that, in old radium, the spectrum of those products which exist in any quantity should also appear.
The term metabolon has been suggested as a convenient expression for each of these changing atoms, derived from the successive disintegration of the atoms of the radio-elements. Each metabolon, on an average, exists only for a limited time. In a collection of metabolons of the same kind the number _N_, which are unchanged at a time _t_ after production, is given by
$$ N = N₀ e^{–λt} $$,
where _N₀_ is the original number. Now _dN_/_dt_ = -λ_N_, or the fraction of the metabolons present, which change in unit time, is equal to λ. The value 1/λ may be taken as the _average life_ of each metabolon.
This may be simply shown as follows:—At any time _t_ after _N₀_ metabolons have been set aside, the number which change in the time _dt_ is equal to λ_Ndt_ or
$$ λ N₀ e^{–λt} dt $$ .
Each metabolon has a life _t_, so that the average life of the whole number is given by
$$ \int₀^{\infty} λ te^{–λt} dt = \frac {1} {λ} $$
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.
Products λ(sec)⁻¹ T Nature of Chemical and the rays Physical properties of the product
──────────────────────────────────────────────────────────────── Uranium — — α Soluble in excess of ammonium carbonate, soluble in ether. Uranium X 3·6 × 10⁻⁷ 22 days β and γ Insoluble in excess of ammonium carbonate, soluble in ether and water. ──────────────────────────────────────────────────────────────── Thorium — — α Insoluble in ammonia. Thorium X 2·0 × 10⁻⁶ 4 days α Soluble in ammonia and water. Emanation 1·3 × 10⁻² 53 secs. α Chemically inert gas of heavy molecular weight. Condenses at −120° C. Thorium A 1·74 × 10⁻⁵ 11 hours no rays Deposited on bodies; concentrated on the cathode in an electric field. Soluble in some acids; Th A more volatile than Th B; shows definite electro-chemical behaviour. Thorium B 2·2 × 10⁻⁴ 55 mins. α, β, γ Same ? — — — ──────────────────────────────────────────────────────────────── Actinium — — no rays Insoluble in ammonia. Actinium X 7·8 × 10⁻⁷ 10·2 days α (and β?) Soluble in ammonia. Emanation ·17 3·9 secs. α Behaves like a gas. Actinium A 3·2 × 10⁻⁴ 36 mins. no rays Deposited on bodies; concentrated on the cathode in an electric field, soluble in ammonia and strong acids; volatilized at a temperature of 100° C., A and B can be separated by electrolysis. Actinium B 5·4 × 10⁻³ 2·15 mins. α, β, γ Same ? — — — ──────────────────────────────────────────────────────────────── Radium — 1300 years α Allied chemically to barium. Emanation 2·1 × 10⁻⁶ 3·8 days α Chemically inert gas of heavy molecular weight; condenses at −150° C. Radium A 3·85 × 10⁻³ 3 mins. α Deposited on (active surface of deposit of bodies; rapid concentrated on change) cathode in electric field; soluble in strong acids; B volatized at about 700° C., A and C at about 1000° C. Radium B 5·38 × 10⁻⁴ 21 mins. no rays Same (same) Radium C 4·13 × 10⁻⁴ 28 mins. α, β, γ Same (same) Radium D — about 40 no rays Soluble in (active acids; volatile deposit of below 1000° C. slow change) Radium E 1·3 × 10⁻⁶ 6 days β and γ Non-volatile at (same) 1000° C. Radium F 5·6 × 10⁻⁸ 143 days α Deposited on (same) bismuth from solution; volatile at about 1000° C., same properties as radio-tellurium and polonium.
The products and their radiations are indicated graphically in Fig. 102 on page 450.
[Illustration: Fig. 102.]
One product has been observed in uranium, four in thorium, four in actinium and seven in radium. It is not improbable that a closer examination of the radio-elements may reveal still further changes. If any very rapid transformations exist, they would be very difficult to detect. The change of thorium X into the emanation, for example, would probably not have been discovered if the product of the change had not been gaseous in character. The electrolysis of solutions is, in many cases, a very powerful method of separating active products from one another, and its possibilities have not yet been exhausted. The main family of changes of the radio-elements, as far as they are known, have been investigated closely, and it is not likely that any product of comparatively slow rate of change has been overlooked. There is a possibility, however, that two radio-active products may in some cases arise from the disintegration of a single substance. This point is discussed further in section 260.
The remarkable way in which the disintegration theory can be applied to unravel the intricacies of the succession of radio-active changes is very well illustrated in the case of radium. Without its aid, it would not have been possible to disentangle the complicated processes which occur. We have already seen that this analysis has been instrumental in showing that the substances polonium, radio-tellurium and radio-lead are in reality products of radium.
After the radio-active substances have undergone the succession of changes traced above, a final stage is reached where the atoms are either permanently stable, or change so slowly that it is difficult to detect their presence by means of their radio-activity. It is probable, however, that the process of transformation still continues through further slow stages.
There is now considerable evidence that the elements uranium, radium and actinium are intimately connected together. The two latter probably result from the breaking up of uranium. The evidence in support of this idea is given in section 262, but there still remains much work to be done to bridge over the gaps which at present appear to separate these elements from one another.
After the series of transformations have come to an end, there will probably remain a product or products which will be inactive, or active only to a minute extent. In addition, since the α particles, expelled during the transformation, are material in nature, and are non-radio-active, they must collect in some quantity in radio-active matter. The probability that the α particles consist of helium is considered later in section 268.
The value of _T_, the time for a product to be half-transformed, may be taken as a comparative measure of the stability of the different metabolons. The stability of the products varies over a very wide range. For example, the value of _T_ for radium D is 40 years, and for the actinium emanation 3·9 secs. This corresponds to a range of stability measured by 3·8 × 10⁸. The range of stability is still further extended, when it is remembered that the atoms of the radio-elements themselves are very slowly changing.
The only two metabolons of about the same stability are thorium X and the radium emanation. In each case, the transformation is half completed in about four days. I consider that the approximate agreement of the numbers is a mere coincidence, and that the two types of matter are quite distinct from one another; for, if the metabolons were identical, it would be expected that the changes which follow would take place in the same way and at the same rate, but such is not the case. Moreover, Th X and the radium emanation have chemical and physical properties quite distinct from one another.
It is very remarkable that the three radio-active substances, radium, thorium and actinium, should exhibit such a close similarity in the succession of changes which occur in them. Each of them at one stage of its disintegration emits a radio-active gas, and in each case this gas is transformed into a solid which is deposited upon the surface of bodies. It would appear that, after disintegration of an atom of any of these has once begun, there is a similar succession of changes, in which the resulting systems have allied chemical and physical properties. Such a connection is of interest as indicating a possible origin of the recurrence of properties in the atoms of the elements, as exemplified by the periodic law. The connection between thorium and actinium is especially close both as regards the number and nature of the products. The period of transformation of the successive products, though differing in magnitude, rises and falls in a very analogous manner. This indicates that the atoms of these two elements are very similarly constituted.
=258. Amount of the products.= By application of the theory of successive changes, the probable amount of each of the products present in radium and the other radio-elements can readily be estimated.
Since each radio-atom expels one α particle of atomic weight about that of hydrogen or helium, the atoms of the intermediate products will not differ much in weight from the parent atom.
The approximate weight of each product present in a gram of radium can be readily deduced. Let _N_{A}_, _N_{B}_, _N_{C}_ be the number of atoms of the products A, B, C present per gram in radio-active equilibrium. Let λ_{_A_}, λ_{_B_}, λ_{_C_} be the corresponding constants of change. Then if _q_ is the number of the parent atoms breaking up per second, per gram,
_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 × 10¹⁰ (section 93). Knowing the value of λ and _q_, the value of _N_ can at once be calculated. The corresponding weight can be deduced, since in one gram of matter of atomic weight about 200, there are about 4 × 10²¹ atoms (section 39). The results are shown in the following table:—
Product Value of λ Number of Weight of (sec)⁻¹ atoms, _N_, product gram present per of radium gram
Radium 2·0 × 10⁻⁶ 3·2 × 10¹⁶ 8 × 10⁻³ emanation
Radium A 3·8 × 10⁻³ 1·7 × 10¹³ 4 × 10⁻⁶
Radium B 5·4 × 10⁻⁴ 1·3 × 10¹⁴ 3 × 10⁻⁵
Radium C 4·1 × 10⁻⁴ 1·6 × 10¹⁴ 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 × 10⁶ times the value for thorium. Even with a kilogram of thorium, the amount of Th X is far too small to be detected by its weight.
This method can be used generally to calculate the relative amounts of any successive products in radio-active equilibrium, provided the value of λ for each product is known. For example, it will be shown later that uranium is the parent of radium and is half transformed in about 6 × 10⁸ years, while radium and radium D are half transformed in 1300 and 40 years respectively. The weight of radium present in one gram of uranium, when equilibrium is established, is thus 2 × 10⁻⁶ grams, and the weight of radium D is 7 × 10⁻⁸ grams. In a mineral containing a ton of uranium there should be about 1·8 grams of radium and ·063 grams of radium D. Some recent experiments described in section 262 show that these theoretical estimates are about twice too great.
=259. Rayless Changes.= The existence of well-marked changes in radium, thorium, and actinium, which are not accompanied by the expulsion of α or β particles, is of great interest and importance.
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 rearrangement of the components of the atom, or that the atom breaks up without the expulsion of its parts with sufficient velocity to produce ionization by collision with the gas. The latter point of view, if correct, at once indicates the possibility that undetected changes of a similar character may be taking place slowly in the non-radio-active elements; or, in other words, that all matter may be undergoing a slow process of change. The changes taking place in the radio-elements have been observed only in consequence of the expulsion with great velocity of the parts of the disintegrated atom. Some recent experiments described in Appendix A show that the α particle from radium ceases to ionize the gas when its velocity falls below about 10⁹ cms. per second. It is thus seen that α particles may be projected with a great velocity, and yet fail to produce ionization in the gas. In such a case, it would be difficult to follow the changes by the electrical method, as the electrical effects would be very small in comparison with those produced by the known radio-active bodies.
=260. Radiations from the products.= We have seen that the great majority of the radio-active products break up with the expulsion of α particles, and that the β particle with its accompaniment of the γ ray appears in most cases only in the last rapid change. In the case of radium, for example, which has been most closely investigated on account of its great activity, radium itself, the emanation and radium A emit only α particles; radium B emits no rays at all; while radium C emits all three kinds of rays. It is difficult to settle with certainty whether the products thorium X and actinium X emit β particles or not, but the β and γ rays certainly appear in each case in the last rapid change in the active deposit, and, in this respect, behave in a similar manner to radium.
The very slow moving electrons which accompany the particles emitted from radium (section 93) are not taken into account, for they appear to be liberated as a result of the impact of α particles on matter, and are expelled with a speed insignificant compared with that of the β particles emitted from radium C.
The appearance of β and γ rays only in the last rapid changes of the radio-elements is very remarkable, and cannot be regarded as a mere coincidence. The final expulsion of a β particle results in the appearance of a product of great stability, or, in the case of radium, of a product (radium D) which has far more stability than the preceding one. It would appear that the initial changes are accompanied by the expulsion of an α particle, and that once the β particle is expelled, the components of the residual atom fall into an arrangement of fairly stable equilibrium, where the rate of transformation is very slow. It thus appears probable that the β particle, which is finally expelled, may be regarded as the active agent in promoting the disintegration of the radio-atom through the successive stages. A discussion of this question will be given with more advantage later (section 270), when the general question of the stability of the atom is under consideration.
It is significant that the change in which the three types of rays appear is far more violent in character than the preceding changes. Not only are the α particles expelled with greater velocity than in any other change, but the β particles are projected with a velocity very closely approaching that of light.
There is always a possibility that, in such a violent explosion in the atom, not only may the α and β particles be expelled, but the atom itself may be disrupted into several fragments. If the greater proportion of the matter resulting from the disintegration is of one kind, it would be difficult to detect the presence of a small quantity of rapidly changing matter from observations of the rate of decay; but, if the products have distinctive electro-chemical behaviour, a partial separation should, in some cases, be effected by electrolysis. It has already been pointed out that the results of Pegram and von Lerch (section 207) on the electrolysis of thorium solutions may be explained on the supposition that thorium A and B have distinctive electro-chemical behaviour. Pegram, however, in addition observed the presence of a product which decayed to half value in six minutes. This active product was obtained by electrolysing a solution of pure thorium salt, to which a small quantity of copper nitrate had been added. The copper deposit was slightly active and lost half of its activity in about six minutes.
The presence of such radio-active products, which do not come under the main scheme of changes, indicates that, at some stage of the disintegration, more than one substance results. In the violent disintegration which occurs in radium C and thorium B, such a result is to be expected, for it is not improbable that there are several arrangements whereby the constituents of the atom form a system of some slight stability. The two products resulting from the disintegration would probably be present in unequal proportion, and, unless they gave out different kinds of rays, would be difficult to separate from each other.
=261. Life of radium.= Since the atoms of the radio-elements are continuously breaking up, they must also be considered to be metabolons, the only difference between them and metabolons such as the emanations Th X and others being their comparatively great stability and consequent very slow rate of change. There is no evidence that the process of change, traced above, is reversible under present conditions, and in the course of time a quantity of radium, uranium, or thorium left to itself must gradually be transformed into other types of matter.
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 × 10¹⁰ α particles per second. The heating effect of radium and also its volume agree closely with calculation, if it is supposed that each atom of each product in breaking up emits one α particle. On this supposition it is seen that 6·2 × 10¹⁰ atoms of radium break up per second.
Now it has been shown experimentally (section 39) that one cubic centimetre of hydrogen at standard pressure and temperature contains 3·6 × 10¹⁹ molecules. Taking the atomic weight of radium as 225, the number of atoms in 1 gram of radium is equal to 3·6 × 10²¹. The fraction λ of radium which breaks up is thus 1·95 × 10⁻¹¹ per second, or 5·4 × 10⁻⁴ per year. It follows that in each gram of radium about half a milligram breaks up per year. The average life of radium is about 1800 years, and half of the radium is transformed in about 1300 years.
We shall now consider the calculation, based on the observed result of Ramsay and Soddy, that the volume of emanation to be obtained from one gram of radium is about 1 cubic millimetre. The experimental evidence based on diffusion results indicates that the molecular weight of the emanation is about 100. If the disintegration theory is correct, the emanation is an atom of radium minus one particle, and therefore must have a molecular weight of at least 200. This high value is more likely to be correct than the experimental number, which is based on evidence that must necessarily be somewhat uncertain. Now the rate of production of emanation per second is equal to λ_N₀_, where _N₀_ is the equilibrium amount. Taking the molecular weight as 200, the weight of emanation produced per second from 1 gram of radium = 8·96 × 10⁻⁶ 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 × 10⁸ years.
If thorium is a true radio-active element, the time for half transformation is about 2·4 × 10⁹ years, since thorium has about the same activity as uranium but contains four products which emit α rays. If the activity of thorium is due to some radio-active impurity, no estimate of the length of its life can be made until the primary active substance has been isolated and its activity measured.
=262. Origin of radium.= The changes in radium are thus fairly rapid, and a mass of radium if left to itself should in the course of a few thousand years have lost a large proportion of its radio-activity. Taking the above estimate of the life of radium, the value of λ is 5·4 × 10⁻⁴, with a year as the unit of time. A mass of radium left to itself should be half transformed in 1300 years and only one-millionth part would remain after 26,000 years. Thus supposing, for illustration, that the earth was originally composed of pure radium, its activity per gram 26,000 years later would not be greater than the activity observed to-day in a good specimen of pitchblende. Even supposing this estimate of the life of radium is too small, the time required for the radium practically to disappear is short compared with the probable age of the earth. We are thus forced to the conclusion that radium is being continuously produced in the earth, unless the very improbable assumption is made, that radium was in some way suddenly formed at a date recent in comparison with the age of the earth. It was early suggested by Rutherford and Soddy[351] that radium might be a disintegration product of one of the radio-elements found in pitchblende. Both uranium and thorium fulfil the conditions required in a possible source of production of radium. Both are present in pitchblende, have atomic weights greater than that of radium, and have rates of change which are slow compared with that of radium. In some respects, uranium fulfils the conditions required better than thorium; for it has not been observed that minerals rich in thorium contain much radium, while on the other hand, the pitchblendes containing the most radium contain a large proportion of uranium.
If radium is not produced from uranium, it is certainly a remarkable coincidence that the greatest activity of pitchblende yet observed is about five or six times that of uranium. Since radium has a life short compared with that of uranium, the amount of radium produced should reach a maximum value after a few thousand years, when the rate of production of fresh radium—which is also a measure of the rate of change of uranium—balances the rate of change of that product. In this respect the process would be exactly analogous to the production of the emanation by radium, with the difference that the radium changes much more slowly than the emanation. But since radium itself in its disintegration gives rise to at least five changes with the corresponding production of α rays, the activity due to the radium (measured by the α rays), when in a state of radio-active equilibrium with uranium, should be about five times that of the uranium that produces it; for it has been shown that only one change has so far been observed in uranium in which α rays are expelled. Taking into account the presence of actinium in pitchblende, the activity observed in the best pitchblende is about the same as would be expected if the radium were a disintegration product of uranium. If this hypothesis is correct, the amount of radium in any pitchblende should be proportional to the amount of uranium present, provided the radium is not removed from the mineral by percolating water.
This question has been experimentally attacked by Boltwood[352], McCoy[353] and Strutt[354]. McCoy measured the relative activities of different minerals in the form of powder by means of an electroscope, and determined the amount of uranium present by chemical analysis. His results indicated that the activity observed in the minerals was very approximately proportional to their content of uranium. Since actinium is present as well as uranium and its products, this would indicate that the amount of radium and actinium taken together is proportional to the amount of uranium. This problem has been attacked more directly by Boltwood and Strutt by measuring the relative amount of the radium emanation evolved by different minerals. By dissolving the mineral and then setting it aside in a closed vessel, the amount of emanation present reaches a maximum value after about a month’s interval. The emanation is then introduced into a closed vessel containing a gold-leaf electroscope similar to that shown in Fig. 12. The rate of movement of the gold-leaf is proportional to the amount of emanation from the solution, and this in turn is proportional to the amount of radium. Boltwood has made in this way a very complete and accurate comparison of the radium content of different varieties of pitchblende and other ores containing radium. It was found that many of the minerals in the solid state allowed a considerable fraction of the emanation to escape into the air. The percentage fraction of the total amount of emanation lost in this way is shown in Column II of the following table. Column I gives the maximum amount of emanation present in 1 gram of the mineral in arbitrary units when none of the emanation escapes; Column III the weight in grams of uranium contained in 1 gram; and Column IV the ratio obtained by dividing the quantity of emanation by the quantity of uranium. The numbers in Column IV should be constant, if the amount of radium is proportional to the amount of uranium.
Locality I II III IV Substance
Uraninite North 170·0 11·3 0·7465 228 Carolina
Uraninite Colorado 155·1 5·2 0·6961 223
Gummite North 147·0 13·7 0·6538 225 Carolina
Uraninite Joachimsthal 139·6 5·6 0·6174 226
Uranophane North 117·7 8·2 0·5168 228 Carolina
Uraninite Saxony 115·6 2·7 0·5064 228
Uranophane North 113·5 22·8 0·4984 228 Carolina
Thorogummite North 72·9 16·2 0·3317 220 Carolina
Carnotite Colorado 49·7 16·3 0·2261 220
Uranothorite Norway 25·2 1·3 0·1138 221
Samarskite North 23·4 0·7 0·1044 224 Carolina
Orangite Norway 23·1 1·1 0·1034 223
Euxenite 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 North 0·88 0·0043 205 (sand) Carolina
Monazite Norway 0·84 1·2 0·0041 207 (crys.)
Monazite Brazil 0·76 0·0031 245 (sand)
Monazite Conn. 0·63 0·0030 210 (massive)
With the exception of some of the monazites, the numbers show a surprisingly good agreement, and, taking into consideration the great variation of the content of uranium in the different minerals, and the wide range of locality from which they were obtained, the results afford a direct and satisfactory proof that the amount of radium in the minerals is directly proportional to the amount of uranium.
In this connection, it is of interest to note that Boltwood found that a considerable quantity of radium existed in various varieties of monazite, although most of the previous analyses agreed in stating that no uranium was present. A careful examination was in consequence made to test this point, and it was found by special methods that uranium was present, and in about the amount to be expected from the theory. The ordinary methods of analysis failed to give correct results on account of the presence of phosphates. Results of a similar character have recently been given by Strutt[355].
The weight of radium in a mineral per gram of uranium is thus a definite constant of considerable practical importance. Its value was recently determined by Boltwood by comparison of the emanation, liberated from a known weight of uraninite, with that liberated from a known quantity of pure radium bromide, supplied for the purpose by the writer. A measured weight of radium bromide was taken from a stock which gave out heat at a rate of slightly over 100 gram calories per hour per gram, and was thus probably pure. This was dissolved in water, and, by successive dilutions, a standard solution was made up containing 10⁻⁷ gram of radium bromide per c.c. Taking the constitution of radium bromide as RaBr₂, it was deduced that the weight of radium per gram of uranium in any mineral was 8·0 × 10⁻⁷ gram. The amount of radium in a mineral per ton of uranium is thus 0·72 gram.
Strutt (_loc. cit._) obtained a value nearly twice as great, but he had no means of ascertaining the purity of his radium bromide.
This amount of radium per gram of uranium is of the right order of magnitude to be expected on the disintegration theory, if uranium is the parent of radium. The activity of pure radium, compared with uranium, is not known with sufficient accuracy to determine with accuracy the theoretical proportion of radium to uranium.
The production of radium from uranium, while very strongly supported by these experiments, cannot be considered definitely established until direct experimental evidence is obtained of the growth of radium in uranium. The rate of production of radium to be expected on the disintegration theory can readily be estimated. The fraction of uranium breaking up per year has been calculated (section 261) and shown to be about 10⁻⁹ per year. This number represents the weight of radium produced per year from 1 gram of uranium. The emanation, released from the amount of radium produced in one year from 1 gram of uranium, would cause an ordinary gold-leaf electroscope to be discharged in about half-an-hour. If a kilogram of uranium is used, the amount of radium produced in a single day should be easily detectable.
Experiments to detect the growth of radium in uranium have been made by several observers. Soddy[356] examined the amount of emanation given off at different times from one kilogram of uranium nitrate in solution, which was originally freed from the small trace of radium present by a suitable chemical process. The solution was kept stored in a closed vessel, and the amount of emanation which collected in the solution was measured at regular intervals.
Preliminary experiments showed that the actual rate of production of radium was far less than the amount to be expected theoretically, and at first very little indication was obtained that radium was produced at all. After allowing the uranium to stand for eighteen months, Soddy states that the amount of emanation was distinctly greater than at first. The solution after this interval contained about 1·5 × 10⁻⁹ gram of radium. This gives the value of about 2 × 10⁻¹² for the fraction of uranium changing per year, while the theoretical value is about 10⁻⁹.
Whetham[357] also found that a quantity of uranium nitrate which had been set aside for a year showed an appreciable increase in the content of radium, and considers that the rate of production is faster than that found by Soddy. In his case, the uranium was not originally completely freed from radium.
Observations extending over years will be required before the question can be considered settled, for the accurate estimation of small quantities of radium by the amount of emanation is beset with difficulties. This is especially the case where observations are made over wide intervals of time.
The writer has made an examination to see if radium is produced from actinium or thorium. It was thought possible that actinium might prove to be an intermediate product between uranium and radium. The solutions, freed from radium, have been set aside for a year, but no certain increase in the content of radium has been observed.
There is little doubt that the production of radium by uranium first proceeds at only a small fraction of the rate to be expected from theory. This is not surprising when we consider that probably several changes intervene between the product Ur X and the radium. In the case of radium, for example, it has been shown that a number of slow changes follow the rapid changes ordinarily observed. On account of the feeble activity of uranium, it would not be easy to detect directly the occurrence of such changes. If, for example, one or more rayless products occurred between Ur X and radium, which were removed from the uranium by the same chemical process used to free it from radium, the rate of production of radium would be very small at first, but would be expected to increase with time as more of the intermediary products were stored up in the uranium. The fact that the contents of uranium and radium in radio-active minerals are always proportional to each other, coupled with definite experimental evidence that radium is produced from uranium, affords an almost conclusive proof that uranium is in some way the parent of radium.
The general evidence which has been advanced to show that radium must be continuously produced from some other substance applies also to actinium, which has an activity of the same order of magnitude as that of radium. The presence of actinium with radium in pitchblende would indicate that this substance also is in some way derived from uranium. It is possible that actinium may prove to be produced either from radium or to be the intermediary substance between uranium and radium. If it could be shown that the amount of actinium in radio-active minerals is, like radium, proportional to the amount of uranium, this would afford indirect proof of such a connection. It is not so simple to settle this point for actinium as for radium, since actinium gives out a very short-lived emanation, and the methods adopted to determine the content of radium in minerals cannot be applied without considerable modifications to determine the amount of actinium present.
The experimental data, so far obtained, do not throw much light upon the origin of the primary active matter in thorium. Hofmann and others (section 23) have shown that thorium separated from minerals containing uranium is always more active the greater the quantity of uranium present. This would indicate that the active substance in thorium also may be derived from uranium.
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[358] 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 feldspar 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[359] 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[360] 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 Activity in radium in arbitrary units milligrams
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 _I₁_ is the activity of the product at any time t after separation, and _I₀_ the initial value, we know that
$$ \frac {I_1} {I₀} = e^{–λt} $$ .
At the same time the activity _I₂_ recovered during the same interval _t_ is given by
$$ \frac {I_2} {I₀} = 1 − e^{–λt} $$,
where λ is the same constant as before. It thus follows that _I₁_ + _I₂_ = _I₀_, 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[361] 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 emanation extends throughout the whole laboratory, on account of the distribution of the emanation by convection and diffusion. For example, Eve[362] 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 × 10³ for the α particle and _e_ = 1·1 × 10⁻²⁰ electromagnetic units and 2·5 × 10¹¹ 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 × 10¹⁰ 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[363] 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
$$ \int₀^{\infty} qe^{–λt} dt = \frac {q} {λ} $$ .
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 × 10⁹ gram-calories. The heat emitted in the union of hydrogen and oxygen to form 1 gram of water is about 4 × 10³ 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[364] 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[365] 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 CO₂ present in the gas. On examining the spectrum of the gas in the vacuum tube, the characteristic line _D₃_ 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 CO₂ 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 discussed, 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[366] 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[367] 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 × 10¹¹ α particles are projected per second from 1 gram of radium. Since there are 3·6 × 10¹⁹ 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[368] 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[369]. 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[370] found that, by grinding pitchblende in vacuo, helium is evolved, apparently showing that the helium exists in cavities of the mineral. Travers[371] 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[372] 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[373] 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 over-estimate, 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[374] 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[375] 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 electromagnetic radiation (section 52). Larmor[376] 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[377], 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 ⅒ 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[378]. 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[379] 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 60 55 50 45 40 35 of electrons
Number in 20 19 18 17 16 16 successive rings
16 16 15 14 13 12
13 12 11 10 8 6
8 7 5 4 3 1
3 1 1
Number 30 25 20 15 10 5 of electrons
Number in 15 13 12 10 8 5 successive rings
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 59 60 61 62 63 64 65 66 67 of electrons
Number in 20 20 20 20 20 20 20 20 20 successive rings
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[380] 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 × 10⁶ 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[381] 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 (⅗)μ_M²_/_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 × 10⁷ _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 × 10⁹ 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_ = 4π_R²KT_,
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)π_R³_ = 4π_R²KT_,
or 3_KT_ _X_ = ------ . _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[382] 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[383] filled a dish of volume 3·3 × 10³ 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 × 10⁶. This would give a saturation current through the gas of 2·2 × 10⁻¹⁴ electromagnetic units. Now the emanation from 1 gram of radium stored in a metal cylinder gives a saturation current of about 3·2 × 10⁻⁵ electromagnetic 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.
Footnote 333:
Perrin, _Revue Scientifique_, April 13, 1901.
Footnote 334:
Becquerel, _C. R._ 133, p. 979, 1901.
Footnote 335:
Rutherford and McClung, _Phil. Trans._ A, p. 25, 1901.
Footnote 336:
Rutherford, _Phil. Mag._ Jan. and Feb. 1900.
Footnote 337:
P. Curie, _C. R._ 136, p. 223, 1903.
Footnote 338:
Rutherford, _Phil. Mag._ April, 1903.
Footnote 339:
M. and Mme Curie, _C. R._ 134, p. 85, 1902.
Footnote 340:
Rutherford and Soddy, _Trans. Chem. Soc._ 81, pp. 321, 837, 1902. _Phil. Mag._ Sept. and Nov. 1902.
Footnote 341:
Rutherford and Soddy, _Phil. Mag._ April, 1903.
Footnote 342:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 343:
Rutherford, _Phys. Zeit._ 4, p. 235, 1902. _Phil. Mag._ Feb. 1903.
Footnote 344:
Rutherford, _Phil. Mag._ May, 1903.
Footnote 345:
Curie and Laborde, _C. R._ 136, p. 673, 1903.
Footnote 346:
J. J. Thomson, _Nature_, p. 601, 1903.
Footnote 347:
Crookes, _C. R._ 128, p. 176, 1899.
Footnote 348:
F. Re, _C. R._ p. 136, p. 1393, 1903.
Footnote 349:
Richarz and Schenck, _Berl. Ber._ p. 1102, 1903. Schenck, _Berl. Ber._ p. 37, 1904.
Footnote 350:
Armstrong and Lowry, _Proc. Roy. Soc._ 1903. _Chem. News_, 88, p. 89, 1903.
Footnote 351:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 352:
Boltwood, _Nature_, May 25, p. 80, 1904. _Phil. Mag._ April, 1905.
Footnote 353:
McCoy, _Ber. d. D. Chem. Ges._ No. 11, p. 2641, 1904.
Footnote 354:
Strutt, _Nature_, March 17 and July 7, 1904. _Proc. Roy. Soc._ March 2, 1905.
Footnote 355:
Strutt, _Proc. Roy. Soc._ March 2, 1905.
Footnote 356:
Soddy, _Nature_, May 12, 1904; Jan. 19, 1905.
Footnote 357:
Whetham, _Nature_, May 5, 1904; Jan. 26, 1905.
Footnote 358:
Danne, _C. R._ Jan. 23, 1905.
Footnote 359:
J. J. Thomson, _Nature_, April 30, p. 601, 1903.
Footnote 360:
Voller, _Phys. Zeit._ 5, No. 24, p. 781, 1904.
Footnote 361:
Ramsay and Cooke, _Nature_, Aug. 11, 1904.
Footnote 362:
Eve, _Nature_, March 16, 1905.
Footnote 363:
J. J. Thomson, International Electrical Congress, St Louis, Sept. 1904.
Footnote 364:
Rutherford and Soddy, _Phil. Mag._ p. 582, 1902; pp. 453 and 579, 1903.
Footnote 365:
Ramsay and Soddy, _Nature_, July 16, p. 246, 1903. _Proc. Roy. Soc._ 72, p. 204, 1903; 73, p. 346, 1904.
Footnote 366:
Curie and Dewar, _C. R._ 138, p. 190, 1904. _Chem. News_, 89, p. 85, 1904.
Footnote 367:
Himstedt and Meyer, _Ann. d. Phys._ 15, p. 184, 1904.
Footnote 368:
Strutt, _Proc. Roy. Soc._ March 2, 1905.
Footnote 369:
Boltwood, _Phil. Mag._ April, 1905.
Footnote 370:
Moss, _Trans. Roy. Soc. Dublin_, 1904.
Footnote 371:
Travers, _Nature_, p. 248, Jan. 12, 1905.
Footnote 372:
Jaquerod, _C. R._ p. 789, 1904.
Footnote 373:
Ramsay and Travers, _Zeitsch. Physik. Chem._ 25, p. 568, 1898.
Footnote 374:
Ramsay, _Nature_, April 7, 1904.
Footnote 375:
Lodge, _Nature_, June 11, p. 129, 1903.
Footnote 376:
Larmor, _Aether and Matter_, p. 233.
Footnote 377:
J. J. Thomson, _Phil. Mag._ p. 681, Dec. 1903.
Footnote 378:
Lord Kelvin, _Phil. Mag._ Oct. 1904.
Footnote 379:
Thomson, _Phil. Mag._ March, 1904.
Footnote 380:
Rutherford and Soddy, _Phil. Mag._ May, 1903.
Footnote 381:
See Strutt and Joly, _Nature_, Oct. 15, 1903.
Footnote 382:
Strutt, _Phil. Mag._ June, 1903.
Footnote 383:
Elster and Geitel, _Phys. Zeit._ 4, No. 19, p. 522, 1903. _Chem. News_, July 17, p. 30, 1903.
CHAPTER XIV. RADIO-ACTIVITY OF THE ATMOSPHERE AND OF ORDINARY MATERIALS.
=273. Radio-activity of the atmosphere.= The experiments of Geitel[384] and C. T. R. Wilson[385] in 1900 showed that a positively or negatively charged conductor placed inside a closed vessel gradually lost its charge. This loss of charge was shown to be due to a small ionization of the air inside the vessel. Elster and Geitel also found that a charged body exposed in the open air lost its charge rapidly, and that the rate of discharge was dependent on the locality and on atmospheric conditions. A more detailed description and discussion of these results will be given later in section 284.
In the course of these experiments, Geitel observed that the rate of discharge increased slightly for some time after the vessel had been closed. He considered that this might possibly be due to the existence of some radio-active substances in the air, which produced excited activity on the walls of the vessel and so increased the rate of dissipation of the charge. In 1901 Elster and Geitel[386] tried the bold experiment of seeing whether it were possible to extract a radio-active substance from the air. The experiments of the writer had shown that the excited radio-activity from the thorium emanation could be concentrated on the negative electrode in a strong electric field. This result indicated that the carriers of the radio-activity had a positive charge of electricity. Elster and Geitel therefore tried an experiment to see whether positively charged carriers, possessing a similar property, were present in the atmosphere. For this purpose a cylinder of wire-netting, charged negatively to 600 volts, was exposed for several hours in the open air. The cylinder was then removed, and quickly placed in a large bell-jar, inside which was placed an electroscope to detect the rate of discharge. It was found that the rate of discharge was increased to a slight extent. In order to multiply the effect a wire about 20 metres in length was exposed at some height from the ground, and was kept charged to a high potential by connecting it to the negative terminal of an influence machine. After exposure for some hours, this wire was removed and placed inside the dissipation vessel. The rate of discharge was found to be increased many times by the presence of the wire. No increase was observed when the wire was charged positively instead of negatively. The results also showed that the radio-active matter could be removed from the wire in the same way as from a wire made active by exposure in the presence of the thorium emanation. A piece of leather moistened with ammonia was rubbed over the active wire. On testing the leather, it was found to be strongly radio-active. When a long wire was used, the amount of activity obtained on the leather was comparable with that possessed by a gram of uranium oxide.
The activity produced on the wire was not permanent, but disappeared to a large extent in the course of a few hours. The amount of activity produced on a wire of given size, exposed under similar conditions, was independent of the material of the wire. Lead, iron and copper wires gave about equal effects.
The amount of activity obtained was greatly increased by exposing a negatively charged wire in a mass of air which had been undisturbed for a long time. Experiments were made in the great cave of Wolfenbüttel, and a very large amount of activity was observed. By transferring the activity to a piece of leather it was found that the rays could appreciably light up a screen of barium platinocyanide in the dark[387]. The rays also darkened a photographic plate through a piece of aluminium 0·1 mm. in thickness.
These remarkable experiments show that the excited radio-activity obtained from the atmosphere is very similar in character to the excited activity produced by the emanations of radium and thorium. No investigators have contributed more to our knowledge of the radio-activity and ionization of the atmosphere than Elster and Geitel. The experiments here described have been the starting-point of a series of researches by them and others on the radio-active properties of the atmosphere, which have led to a great extension of our knowledge of that important subject.
Rutherford and Allan[388] determined the rate of decay of the excited activity produced on a negatively charged wire exposed in the open air. A wire about 15 metres long was exposed in the open air, and kept charged by an influence machine to a potential of about −10,000 volts. An hour’s exposure was sufficient to obtain a large amount of excited activity on the wire. The wire was then rapidly removed and wound on a framework which formed the central electrode in a large cylindrical metal vessel. The ionization current for a saturation voltage was measured by means of a sensitive Dolezalek electrometer. The current, which is a measure of the activity of the wire, was found to diminish according to an exponential law with the time, falling to half value in about 45 minutes. The rate of decay was independent of the material of the wire, of the time of exposure, and of the potential of the wire.
An examination was also made of the nature of the rays emitted by the radio-active wire. For this purpose a lead wire was made radio-active in the manner described, and then rapidly wound into the form of a flat spiral. The penetrating power of the rays was tested in a vessel similar to that shown in Fig. 17. Most of the ionization was found to be due to some very easily absorbed rays, which were of a slightly more penetrating character than the α rays emitted from a wire made active by the radium or thorium emanations. The intensity of the rays was cut down to half value by about 0·001 cm. of aluminium. The photographic action observed by Elster and Geitel through 0·1 mm. of aluminium showed that some penetrating rays were also present. This was afterwards confirmed by Allan, who used the electric method. These penetrating rays are probably similar in character to the β rays from the radio-elements.
=274.= The excited activity produced on the negatively charged wire cannot be due to an action of the strong electric field on the surface of the wire; for very little excited activity is produced if the wire is charged to the same potential inside a closed cylinder.
We have seen that the excited activity produced on the wire can be partially removed by rubbing and by solution in acids, and, in this respect, it is similar to the excited activity produced in bodies by the emanations of radium and thorium. The very close similarity of the excited activity obtained from the atmosphere to that obtained from the radium and thorium emanations suggests the probability that a radio-active emanation exists in the atmosphere. This view is confirmed by a large amount of indirect evidence discussed in sections 276, 277 and 280.
Assuming the presence of a radio-active emanation in the atmosphere, the radio-active effects observed receive a simple explanation. The emanation in the air gradually breaks up, giving rise in some way to positively charged radio-active carriers. These are driven to the negative electrode in the electric field, and there undergo a further change, giving rise to the radiations observed at the surface of the wire. The matter which causes excited activity will thus be analogous to the active deposit of radium and thorium.
Since the earth is negatively electrified with regard to the upper atmosphere, these positive radio-active carriers produced in the air are continuously deposited on the surface of the earth. Everything on the surface of the earth, including the external surface of buildings, the grass, and leaves of trees, must be covered with an invisible deposit of radio-active material. A hill, or mountain peak, or any high mass of rock or land, concentrates the earth’s electric field at that point and consequently will receive more excited radio-activity per unit area than the plain. Elster and Geitel have pointed out that the greater ionization of the air observed in the neighbourhood of projecting peaks receives a satisfactory explanation on this view.
If the radio-active carriers are produced at a uniform rate in the atmosphere, the amount of excited activity _I_{t}_, produced on a wire exposed under given conditions, will, after exposure for a time _t_, be given by
$$ I_t = I₀ (1 − e^{–λt} ) $$,
where _I₀_ is the maximum activity on the wire and λ is the constant of decay of the excited activity. Since the activity of a wire after removal falls to half value in about 45 minutes, the value of λ is 0·92 (hour)⁻¹. Some experiments made by Allan[389] are in rough agreement with the above equation. Accurate comparative results are difficult to obtain on account of the inconstancy of the radio-activity of the open air. After an exposure of a wire for several hours, the activity reached a practical maximum, and was not much increased by continued exposure.
We have seen (section 191) that the carriers of the active deposit of radium and thorium move in an electric field with about the same velocity as the ions. We should expect therefore that a long wire charged to a high negative potential would abstract the active carriers from the atmosphere for a considerable distance. This does not appear to be the case, for Eve (see section 281) has found that the carriers are only abstracted from the air for a radius of less than one metre, for a potential of the wire of −10,000 volts. It seems probable that the carriers of the active matter are deposited on the numerous fine dust particles present in the air and thus move very slowly even in a strong electric field.
The amount of excited activity produced on a wire, supported some distance from the surface of the earth, should increase steadily with the voltage, for the greater the potential, the greater the volume of air from which the radio-active carriers are abstracted.
The presence of radio-active matter in the atmosphere will account for a considerable portion of the ionization of the air observed near the earth. This important question is discussed in more detail in section 281.
=275. Radio-activity of freshly fallen rain and snow.= C. T. R. Wilson[390] tried experiments to see if any of the radio-active material from the air was carried down by rain. For this purpose a quantity of freshly fallen rain was collected, rapidly evaporated to dryness in a platinum vessel, and the activity of the residue tested by placing the vessel in an electroscope. In all cases, the rate of discharge of the electroscope was considerably increased. From about 50 c.c. of rain water, an amount of activity was obtained sufficient to increase the rate of discharge of the electroscope four or five times, after the rays had traversed a thin layer of aluminium or gold-leaf. The activity disappeared in the course of a few hours, falling to half value in about 30 minutes. Rain water, which had stood for some hours, showed no trace of activity. Tap water, when evaporated, left no active residue.
The amounts of activity obtained from a given quantity of rain water were all of the same order of magnitude, whether the rain was precipitated in fine or in large drops, by night or by day, or whether the rain was tested at the beginning or at the end of a heavy rainfall lasting several hours.
The activity obtained from rain is not destroyed by heating the platinum vessel to a red heat. In this and other respects it resembles the excited activity obtained on negatively charged wires exposed in the open air.
C. T. R. Wilson[391] obtained a radio-active precipitate from rain water by adding a little barium chloride and precipitating the barium with sulphuric acid. An active precipitate was also obtained when alum was added to the water, and the aluminium precipitated by ammonia. The precipitates obtained in this way showed a large activity. The filtrate when boiled down was quite inactive, showing that the active matter had been completely removed by precipitation. This effect is quite analogous to the production of active precipitates from a solution containing the active deposit of thorium (see section 185).
The radio-activity of freshly fallen snow was independently observed by C. T. R. Wilson[392] in England, and Allan[393] and McLennan[394] in Canada. In order to obtain a large amount of activity, the surface layer of snow was removed, and evaporated to dryness in a metal vessel. An active residue was obtained with radio-active properties similar to those observed for freshly fallen rain. Both Wilson and Allan found that the activity of rain and snow decayed at about the same rate, the activity falling to half value in about 30 minutes. McLennan states that he found a smaller amount of radio-activity in the air after a prolonged fall of snow.
Schmauss[395] has observed that drops of water falling through air ionized by Röntgen rays acquire a negative charge. This effect is ascribed to the fact that the negative ions in air diffuse faster than the positive. On this view the drops of rain and flakes of snow would acquire a negative charge in falling through the air. They would in consequence act as collectors of the positive radio-active carriers from the air. On evaporation of the water the radio-active matter would be left behind.
=276. Radio-active emanations from the earth.= Elster and Geitel observed that the air in caves and cellars was, in most cases, abnormally radio-active, and showed very strong ionization. This action might possibly be due to an effect of stagnant air, by which it produced a radio-active emanation from itself, or to a diffusion of a radio-active emanation from the soil. To test whether this emanation was produced by the air itself, Elster and Geitel shut up the air for several weeks in a large boiler, but no appreciable increase of the activity or ionization was observed. To see whether the air imprisoned in the capillaries of the soil was radio-active, Elster and Geitel[396] put a pipe into the earth and sucked up the air into a testing vessel by means of a water pump.
The apparatus employed to test the ionization of the air is shown in Fig. 103. _C_ is an electroscope connected with a wire net, _Z_. The active air was introduced into a large bell-jar of 27 litres capacity, the inside of which was covered with wire netting, _MM´_. The bell-jar rested on an iron plate _AB_. The electroscope could be charged by the rod _S_. The rate of discharge of the electroscope, before the active air was introduced, was noted. On allowing the active air to enter, the rate of discharge increased rapidly, rising in the course of a few hours in one experiment to 30 times the original value. They found that the emanation produced excited activity on the walls of the containing vessel. The air sucked up from the earth was even more active than that observed in caves and cellars. There can thus be little doubt that the abnormal activity observed in caves and cellars is due to a radio-active emanation, present in the earth, which gradually diffuses to the surface and collects in places where the air is not disturbed.
Results similar to those obtained by Elster and Geitel for the air removed from the earth at Wolfenbüttel were also obtained later by Ebert and Ewers[397] at Munich. They found a strongly active emanation in the soil, and, in addition, examined the variation with time of the activity due to the emanation in a sealed vessel. After the introduction of the active air into the testing vessel, the activity was observed to increase for several hours, and then to decay, according to an exponential law, with the time, falling to half value in about 3·2 days. This rate of decay is more rapid than that observed for the radium emanation, which decays to half value in a little less than four days. The increase of activity with time is probably due to the production of excited activity on the walls of the vessel by the emanation. In this respect it is analogous to the increase of activity observed when the radium emanation is introduced into a closed vessel. No definite experiments were made by Ebert and Ewers on the rate of decay of this excited activity. In one experiment the active emanation, after standing in the vessel for 140 hours, was removed by sucking ordinary air of small activity through the apparatus. The activity rapidly fell to about half value, and this was followed by a very slow decrease of the activity with time. This result indicates that about half the rate of discharge observed was due to the radiation from the emanation and the other half to the excited activity produced by it.
The apparatus employed by Ebert and Ewers in these experiments was very similar to that employed by Elster and Geitel, shown in Fig. 103. Ebert and Ewers observed that, when the wire net attached to the electroscope was charged negatively, the rate of discharge observed was always greater than when it was charged positively. The differences observed between the two rates of discharge varied between 10 and 20 per cent. A similar effect has been observed by Sarasin, Tommasina and Micheli[398] for a wire made active by exposure to the open air. This difference in the rates of discharge for positive and negative electricity is probably connected with the presence of particles of dust or small water globules suspended in the gas. The experiments of Miss Brooks (section 181) have shown that the particles of dust present in the air containing the thorium emanation become radio-active. A large proportion of these dust particles acquire a positive charge and are carried to the negative electrode in an electric field. This effect would increase the rate of discharge of the electroscope when charged negatively. In later experiments, Ebert and Ewers noticed that, in some cases, when the air had been kept in the vessel for several days, the effect was reversed, and the electroscope showed a great rate of discharge when charged positively.
[Illustration: Fig. 103.]
J. J. Thomson[399] has observed that the magnitude of the ionization current depends on the direction of the electric field, if fine water globules are suspended in the ionized gas.
In later experiments, Ebert[400] found that the radio-active emanation could be removed from the air by condensation in liquid air. This property of the emanation was independently discovered by Ebert before he was aware of the results of Rutherford and Soddy on the condensation of the emanations of radium and thorium. To increase the amount of radio-active emanation in a given volume of air, a quantity of the active air, obtained by sucking the air from the soil, was condensed by a liquid air machine. The air was then allowed partially to evaporate, but the process was stopped before the point of volatilization of the emanation was reached. This process was repeated with another quantity of air and the residues added together. Proceeding in this way, he was able to concentrate the emanation in a small volume of air. On allowing the air to evaporate, the ionization of the air in the testing vessel increased rapidly for a time and then slowly diminished. Ebert states that the maximum for the emanation which had been liquefied for some time was reached earlier than for fresh air. The rate of decay of activity of the emanation was not altered by keeping it at the temperature of liquid air for some time. In this respect it behaves like the emanations of radium and thorium.
J. J. Thomson[401] found that air bubbled through Cambridge tap water showed much greater conductivity than ordinary air. The air was drawn through the water by means of a water pump into a large gasometer, when the ionization current was tested with a sensitive electrometer. When a rod charged negatively was introduced into this conducting air it became active. After an exposure for a period of 15 to 30 minutes in the conducting gas, the rod, when introduced into a second testing vessel, increased the saturation current in the vessel to about five times the normal amount. Very little effect was produced when the rod was uncharged or charged positively for the same time. The activity of the rod decayed with the time, falling to half value in about 40 minutes. The amount of activity produced on a wire under constant conditions was independent of the material of the wire. The rays from the rod were readily absorbed in a few centimetres of air.
These effects were, at first, thought to be due to the action of the small water drops suspended in the gas, for it was well known that air rapidly drawn through water causes a temporary increase in its conductivity. Later results, however, showed that there was a radio-active emanation present in Cambridge tap water. This led to an examination of the waters from deep wells in various parts of England, and J. J. Thomson found that, in some cases, a large amount of emanation could be obtained from the well water. The emanation was released either by bubbling air through the water or by boiling the water. The gases obtained by boiling the water were found to be strongly active. A sample of air mixed with the radio-active emanation was condensed. The liquefied gas was allowed to evaporate, and the earlier and later portions of the gas were collected in separate vessels. The final portion was found to be about 30 times as active as the first portion.
An examination of the radio-active properties of the active gases so obtained has been made by Adams[402]. He found that the activity of the emanation decayed, according to an exponential law, with the time, falling to half value in about 3·4 days. This is not very different from the rate of decay of the activity of the radium emanation, which falls to half value in a little less than four days. The excited activity produced by the emanation decayed to half value in about 35 minutes. The decay of the excited activity from radium is at first irregular, but after some time falls off, according to an exponential law, diminishing to half value in 28 minutes. Taking into account the uncertainty attaching to measurements of the very small ionization observed in these experiments, the results indicate that the emanation obtained from well water in England is similar to, if not identical with, the radium emanation. Adams observed that the emanation was slightly soluble in water. After well water had been boiled for a while and then put aside, it was found to recover its power of giving off an emanation. The amount obtained after standing for some time was never more than 10 per cent. of the amount first obtained. Thus it is probable that the well water, in addition to the emanations mixed with it, has also a slight amount of a permanent radio-active substance dissolved in it. Ordinary rain water or distilled water does not give off an emanation.
Bumstead and Wheeler[403] have made a very careful examination of the radio-activity of the emanation obtained from the surface water and soil at New Haven, Connecticut. The emanation, obtained from the water by boiling, was passed into a large testing cylinder, and measurements of the current were made by means of a sensitive electrometer. The current gradually rose to a maximum, after the introduction of the emanation, in exactly the same way as the current increases in a vessel after the introduction of the radium emanation. The decay of activity of the emanations obtained from the water and soil was carefully measured, and, within the limits of experimental error, agreed with the rate of decay of activity observed for the radium emanation. The identity of the emanations from the water and soil with the radium emanation was still further established by experiments on the rate of diffusion of the emanation through a porous plate. By comparative tests it was found that the coefficient of diffusion of the emanations from the water and soil was the same as for the radium emanation. Also, by comparison of the rate of diffusion of carbonic acid, it was found that the density of the emanation was about four times that of carbonic acid, a result in good agreement with that found for the radium emanation (sections 161 and 162).
Bumstead[404] has found that a considerable amount of thorium as well as radium emanation exists in the air of New Haven. For a three hour exposure in the open air, 3 to 5 per cent. of the excited activity on the wire is due to thorium. For a twelve hour exposure, the thorium activity was sometimes 15 per cent. of the whole. On account of the comparatively slow decay of the excited activity of thorium, the activity on the wire after removal for three or four hours was due almost entirely to thorium. The rate of decay could then be measured accurately, and was found to be the same as for a wire exposed in the presence of the thorium emanation.
Dadourian[405] has made an examination of the underground air in New Haven, and has found that this too contains a large quantity of the thorium emanation. A circular hole about 50 cms. in diameter and 2 metres deep was dug in the ground. A number of wires were wound on an insulated frame and suspended in the hole, the top of the hole then being covered over. The wire was charged negatively by a Wimshurst machine. After a long exposure the excited activity on the wire diminished at a rate that showed it to be a mixture of the excited activities of thorium and radium.
A very large amount of work has been done in examining various hot and mineral springs for the presence of the radium emanation, and it is not possible here to refer more than briefly to a few of the very numerous papers that have been published on this subject both in Europe and America. H. S. Allen and Lord Blythswood[406] have observed that the hot springs at Bath and Buxton gave off a radio-active emanation. This was confirmed by Strutt[407], who found that the escaping gases contained the radium emanation, and also that the mud deposited from the springs contained a trace of radium salts. These results are of considerable interest, for Lord Rayleigh has observed that helium is contained among the gases evolved by the springs. It appears probable that the helium observed is produced from the radium or radio-active deposits through which the water flows. Many mineral and hot springs which are famous for their curative properties have been found to contain traces of radium and also considerable amounts of radium emanation. It has been suggested that the curative properties may be due to some extent to the presence of these minute quantities of radium.
Himstedt[408] found that the thermal springs at Baden Baden contained the radium emanation, while Elster and Geitel[409] examined the deposits formed by these springs and found them to contain small quantities of radium salts. Results of a similar character were obtained for a number of waters in Germany by Dorn[410], Schenck[411], and H. Mache[412].
Curie and Laborde[413] have tested the waters of a large number of mineral springs and found that the great majority contain the radium emanation. In this connection, it is of interest to note that Curie and Laborde found very little emanation in the waters of Salins-Moutiers, while Blanc[414] observed, on the other hand, that the sediment from the spring was very active. A closer examination of this deposit by Blanc revealed the fact that it contained a considerable quantity of thorium. This was proved by finding that it gave out an emanation, which lost half of its activity in one minute, and produced excited activity, which fell to half value in about 11 hours. Boltwood[415] has tested a number of samples of spring water from different sources in America and has found that many of them contain the radium emanation.
Most of the results upon the amount of radium emanation from different sources have been expressed in arbitrary units without, in many cases, any comparative standard being given. Boltwood (_loc. cit._) has described a satisfactory method for collecting and testing the emanation from different waters, and has suggested that the rate of discharge observed by the electroscope or the electrometer should be expressed in terms of the effect due to the emanation liberated on solution of a definite weight of the mineral uraninite. Since in every mineral so far examined, the amount of radium present is proportional to the amount of uranium, such a standard would be sufficiently definite for practical purposes. The emanation liberated from a few centigrams of the mineral is sufficient to give a convenient rate of discharge of an electroscope. Such a method is preferable to using a known quantity of a radium compound as a standard, since it is difficult to know with certainty the activity of the preparations of radium which may be in the possession of the different experimenters.
=277. Radio-activity of constituents of the earth.= Elster and Geitel[416] observed that, although in many cases the conductivity of the air was abnormally high in underground enclosures, the conductivity varied greatly in different places. In the Baumann Cave, for example, the conductivity of the air was nine times the normal, but in the Iberg Cave only three times the normal. In a cellar at Clausthal the conductivity was only slightly greater than the normal, but the excited radio-activity obtained on a negatively charged wire exposed in it was only ¹⁄₁₁ of the excited radio-activity obtained when the wire was exposed in the free air. They concluded from these experiments that the amount of radio-activity in the different places probably varied with the nature of the soil. Observations were then made on the conductivity of the air sucked up from the earth at different parts of the country. The clayey and limestone soils at Wolfenbüttel were found to be strongly active, the conductivity varying from four to sixteen times the normal amount. A sample of air from the shell limestone of Würzburg and from the basalt of Wilhelmshöhe showed very little activity.
Experiments were made to see whether any radio-active substance could be detected in the soil itself. For this purpose some earth was placed on a dish and introduced under a bell-jar, similar to that shown in Fig. 103. The conductivity of the air in the bell-jar increased with the time, rising to three times the normal value after several days. Little difference was observed whether the earth was dry or moist. The activity of the soil seemed to be permanent, for no change in the activity was observed after the earth had been laid aside for eight months.
Attempts were then made to separate the radio-active constituent from the soil by chemical treatment. For this purpose a sample of clay was tested. By extraction with hydrochloric acid all the calcium carbonate was removed. On drying the clay the activity was found to be reduced, but it spontaneously regained its original activity in the course of a few days. It seems probable, therefore, that an active product had been separated from the soil by the acid. Elster and Geitel consider that an active substance was present in the clay, which formed a product more readily soluble in hydrochloric acid than the active material itself. There seemed to be a process of separation analogous to that of Th X from thorium by precipitation with ammonia.
Experiments were also made to see whether substances placed in the earth acquired any radio-activity. For this purpose samples of potter’s clay, whitening, and heavy spar, wrapped in linen, were placed in the earth 50 cms. below the surface. After an interval of a month, these were dug up and their activity examined. The clay was the only substance which showed any activity. The activity of the clay diminished with the time, showing that activity had been excited in it by the emanations present in the soil.
Elster and Geitel[417] have found that a large quantity of the radio-active emanation can be obtained by sucking air through clay. In some cases, the conductivity of the air in the testing vessel was increased over 100 times. They have also found that the so-called “fango”—a fine mud obtained from hot springs in Battaglia, Northern Italy—gives off three or four times as much emanation as clay. By treating the fango with acid, the active substance present was dissolved. On adding some barium chloride to the solution, and precipitating the barium as sulphate, the active substance was removed, and in this way a precipitate was obtained over 100 times as active, weight for weight, as the original fango. Comparisons were made of the rate of decay of the excited activity, due to the emanation from fango, with that due to the radium emanation, and within the limits of error, the decay curves obtained were found to be identical. There can thus be no doubt that the activity observed in fango is due to the presence of a small quantity of radium. Elster and Geitel calculate that the amount of radium, contained in it, is only about one-thousandth of the amount to be obtained from an equal weight of pitchblende from Joachimsthal.
Vincenti and Levi Da Zara[418] have found that the waters and sediments of a number of hot springs in Northern Italy contain the radium emanation. Elster and Geitel observed that natural carbonic acid obtained from great depths of old volcanic soil was radio-active, while Burton[419] found that the petroleum from a deep well in Ontario, Canada, contained a large quantity of emanation, probably of radium, since its activity fell to half value in 3·1 days, while the excited activity produced by the emanation fell to half value in about 35 minutes. A permanently active deposit was left behind after volatilization of the oil, indicating that probably one or more of the radio-elements were present in minute quantity.
Elster and Geitel[420] have found that the active sediments obtained from springs at Nauheim and Baden Baden showed abnormal rates of decay of the excited activity. This was finally traced to the presence in the deposit of both thorium and radium. By suitable chemical methods, the two active substances were separated from each other and were then tested separately.
=278. Effect of meteorological conditions upon the radio-activity of the atmosphere.= The original experiments of Elster and Geitel on the excited radio-activity derived from the atmosphere were repeated by Rutherford and Allan[421] in Canada. It was found that a large amount of excited radio-activity could be derived from the air, and that the effects were similar to those observed by Elster and Geitel in Germany. This was the case even on the coldest day in winter, when the ground was covered deeply with snow and wind was blowing from the north over snow-covered lands. The results showed that the radio-activity present in the air was not much affected by the presence of moisture, for the air during a Canadian winter is extremely dry. The greatest amount of excited activity on a negatively charged wire was obtained in a strong wind. In some cases the amount produced for a given time of exposure was ten to twenty times the normal amount. A cold bright day of winter usually gave more effect than a warm dull day in summer.
Elster and Geitel[422] have made a detailed examination of the effect of meteorological conditions on the amount of excited radio-activity to be derived from the atmosphere. For this purpose a simple portable apparatus was devised by them and used for the whole series of experiments. A large number of observations were taken, extending over a period of twelve months. They found that the amount of excited activity obtained was subject to great variations. The extreme values obtained varied in the ratio of 16 to 1. No direct connection could be traced between the amount of ionization in the atmosphere and the amount of excited activity produced. They found that the greatest amount of excited activity was obtained during a fog, when the amount of ionization in the air was small. This result, however, is not necessarily contradictory to the view that the ionization and activity of the air are to a certain extent connected. From the experiments of Miss Brooks on the effect of dust in acting as carriers of excited activity, more excited activity should be obtained during a fog than in clear air. The particles of water become centres for the deposit of radio-active matter. The positive carriers are thus anchored and are not removed from the air by the earth’s field. In a strong electric field, these small drops will be carried to the negative electrode and manifest their activity on the surface of the wire. On the other hand, the distribution of water globules throughout the air causes the ions in the air to disappear rapidly in consequence of their diffusion to the surface of the drops (see section 31). For this reason the denser the fog, the smaller will be the conductivity observed in the air.
Lowering the temperature of the air had a decided influence. The average activity observed below 0° C. was 1·44 times the activity observed above 0° C. The height of the barometer was found to exert a marked influence on the amount of excited activity to be derived from the air. The lower the barometer the greater was the amount of excited activity in the air. The effect of variation of the height of the barometer is intelligible, when it is considered that probably a large proportion of the radio-activity observed in the air is due to the radio-active emanations which are continuously diffusing from the earth into the atmosphere. Elster and Geitel have suggested that a lowering of the pressure of the air would cause the air from the ground to be drawn up from the capillaries of the earth into the atmosphere. This, however, need not necessarily be the case if the conditions of the escape of the emanation into the atmosphere are altered by the variation of the position of underground water or by a heavy fall of rain.
The amount of excited activity to be derived from the air on the Baltic Coast was only one-third of that observed inland at Wolfenbüttel. Experiments on the radio-activity of the air in mid-ocean would be of great importance in order to settle whether the radio-activity observed in the air is due to the emanations from the soil alone. It is probable that the radio-activity of the air at different points of the earth may vary widely, and may largely depend on the nature of the soil.
Saake[423] has found that the amount of emanation present in the air at high altitudes in the valley of Arosa in Switzerland is much greater than the normal amount at lower levels. Elster and Geitel have observed that there is also a larger number of ions in the air at high altitudes, and suggest that the curative effect of thermal springs and the physiological actions of the air at high levels may be connected with the presence of an unusual amount of radio-active matter in the atmosphere. Simpson[424] made experiments on the amount of excited activity at Karasjoh, Norway, at a height of about 150 feet above sea level. The sun did not rise above the level of the horizon during the time the observations were taken. The average amount of excited activity obtained from the air was considerably greater than the normal amount observed by Elster and Geitel in Germany. This was the more surprising as the ground was frozen hard and covered with deep snow. Allan, working in Montreal, Canada, early observed that the amount of activity to be obtained from the air was about the same in summer as in winter, although, in the latter case, the whole earth was deeply frozen and covered with snow, and the winds blew from the north over snow-covered lands. Under such conditions, a diminution of the amount of activity is to be expected since the diffusion of the emanation must be retarded, if not altogether stopped, by the freezing of the soil. On the other hand, it appears difficult to escape from the conclusion of Elster and Geitel that the emanation present in the atmosphere is evolved from the earth itself.
Some interesting experiments have been made by McLennan[425] on the amount of excited radio-activity to be derived from the air when filled with fine spray. The experiments were made at the foot of the American Fall at Niagara. An insulated wire was suspended near the foot of the Fall, and the amount of excited activity on the wire compared with the amount to be obtained on the same wire for the same exposure in Toronto. The amount of activity obtained from the air at Toronto was generally five or six times that obtained from the air at the Falls. In these experiments it was not necessary to use an electric machine to charge the wire negatively, for the falling spray kept the insulated wire permanently charged to a potential of about −7500 volts. These results indicate that the falling spray had a negative charge and electrified the wire. The small amount of the excited radio-activity at the Falls was probably due to the fact that the negatively charged drops abstracted the positively charged radio-active carriers from the atmosphere, and in falling carried them to the river below. On collecting the spray and evaporating it, no active residue was obtained. Such a result is, however, to be expected on account of the minute proportion of the spray tested compared with that present in the air.
=279. A very penetrating radiation from the earth’s surface.= McLennan[426], and Rutherford and Cooke[427] independently, observed the presence of a very penetrating radiation inside buildings. McLennan measured the natural conductivity of the air in a large closed metal cylinder by means of a sensitive electrometer. The cylinder was then placed inside another and the space between filled with water. For a thickness of water between the cylinders of 25 cms. the conductivity of the air in the inner cylinder fell to about 63 per cent. of its initial value. This result shows that part of the ionization in the inner cylinder was due to a penetrating radiation from an external source, which radiation was partially or wholly absorbed in water.
Rutherford and Cooke observed that the rate of discharge of a sealed brass electroscope was diminished by placing a lead screen around the electroscope. A detailed investigation of the decrease of the rate of discharge in the electroscope, when surrounded by metal screens, was made later by Cooke[428]. A thickness of 5 cms. of lead round the electroscope decreased the rate of discharge about 30 per cent. Further increase of the thickness of the screen had no effect. When the apparatus was surrounded by 5 tons of pig-lead the rate of discharge was about the same as when it was surrounded by a plate about 3 cms. thick. An iron screen also diminished the rate of discharge to about the same extent as the lead. By suitably arranging lead screens it was found that the radiation came equally from all directions. It was of the same intensity by night as by day. In order to be sure that this penetrating radiation did not arise from the presence of radio-active substances in the laboratory, the experiments were repeated in buildings in which radio-active substances had never been introduced, and also on the open ground far removed from any building. In all cases a diminution of the rate of discharge of the electroscope, when surrounded by lead screens, was observed. These results show that a penetrating radiation is present at the surface of the earth, arising partly from the earth itself and partly from the atmosphere.
The result is not surprising when the radio-activity of the earth and atmosphere is taken into account. The writer has found that bodies made active by exposure to the emanations from thorium and radium give out γ rays. We may expect then that the very similar excited radio-activity which is present in the earth and atmosphere should also give rise to γ rays of a similar character. More recent work, however (section 286), indicates that this explanation is not sufficient to explain all the facts observed.
=280. Comparison of the radio-activity of the atmosphere with that produced by the radio-elements.= The radio-active phenomena observed in the earth and atmosphere are very similar in character to those produced by thorium and radium. Radio-active emanations are present in the air of caves and cellars, in natural carbonic acid, and in deep well water, and these emanations produce excited radio-activity on all bodies in contact with them. The question now arises whether these effects are due entirely to known radio-elements present in the earth or to unknown kinds of radio-active matter. The simplest method of testing this point is to compare the rate of decay of the radio-active product in the atmosphere with those of the known radio-active products of thorium and radium. A cursory examination of the facts at once shows that the radio-activity of the atmosphere is much more closely allied to effects produced by radium than to those due to thorium. The activity of the emanation released from well water, and also that sucked up from the earth, decays to half value in about 3·3 days, while the activity of the radium emanation decays to half value in an interval of 3·7 to 4 days. Considering the difficulty of making accurate determinations of these quantities, the rates of decay of the activity of the emanations from the earth and from radium agree within the limits of experimental error. A large number of observers have found that the radium emanation is present in the water of thermal springs and in the sediment deposited by them. Bumstead and Wheeler have shown that the emanation from the soil and surface water of New Haven is identical with that from radium. If the emanations from the earth and from radium are the same, the excited activities produced should have the same rate of decay. The emanation from well water in England approximately fulfils this condition (section 276), but an observation recorded by Ebert and Ewers (section 276) seems to show that the excited activity due to the emanation sucked up from the earth decays at a very slow rate compared with that due to radium.
Bumstead has given undoubted evidence that the thorium as well as the radium emanation is also present in the atmosphere at New Haven, while Dadourian has shown that it is emitted by New Haven soil. Blanc, and Elster and Geitel, have also found that thorium is present in the sediment from some thermal springs.
If the active matter in the atmosphere consists mainly of the radium emanation, the active deposit on a negatively charged wire, exposed in the open air, should initially consist of radium A, B and C. The curve of decay should be identical with the decay curve of the excited activity of radium, measured by the α rays, that is, there should be a rapid initial drop corresponding to the initial 3 minute change, then a slow rate of variation, the activity after several hours decaying to half value in about 28 minutes (see section 222). The rapid initial drop has been observed by Bumstead for the air at New Haven. Allan[429] did not observe this initial drop in Montreal, but found the activity fell to half value in about 45 minutes, reckoning from a time about 10 minutes after the removal of the active wire. This is about the rate of decay to be expected for the active deposit of radium over the same interval. Allan obtained evidence that there were several kinds of active matter deposited on the wire. For example, the activity transferred from the active wire to a piece of leather, moistened with ammonia, fell to half value in 38 minutes; for a piece of absorbent felt treated similarly, the activity fell to half value in 60 minutes, the normal time for the untreated wire being 45 minutes.
It is probable that this variation of the rate of decay is due to the fact that unequal proportions of radium B and C were transferred from the wire to the rubber. If a greater proportion of B than of C were removed, the decay would be slower and _vice versa_.
The fact that the activity of rain and snow falls to half value in about 30 minutes is a strong indication that the radium emanation is present in the atmosphere. The active matter with the rain and snow after standing some time would consist mainly of radium C and this should decay exponentially with the time, falling to half value in 28 minutes.
On account of the rapid decay of the thorium emanation—half value in one minute—it is not likely that much of the activity of the atmosphere can be ascribed to it. Its effect would be most marked near the surface of the soil.
There can be little doubt, that a large part of the radio-activity of the atmosphere is due to the radium emanation, which is continually diffusing into the atmosphere from the pores of the earth. Since radio-activity has been observed in the atmosphere at all points at which observations have, so far, been made, radio-active matter must be distributed in minute quantities throughout the soil of the earth. The volatile emanations escape into the atmosphere by diffusion, or are carried to the surface in spring water or by the escape of underground gases, and cause the radio-active phenomena observed in the atmosphere. The observation of Elster and Geitel that the radio-activity of the air is much less near the sea than inland is explained at once, if the radio-activity of the atmosphere is due mainly to the diffusion of emanations from the soil into the air above it.
The rare gases helium and xenon which exist in the atmosphere have been tested and found to be non-radio-active. The radio-activity of the air cannot be ascribed to a slight radio-activity possessed by either of these gases.
=281. Amount of the radium emanation in the atmosphere.= It is a matter of great interest to form an estimate of the amount of radium emanation present in the atmosphere, for since it comes from the earth, it indirectly serves as a means of estimating the amount of radium which is distributed over a thin crust of the earth.
Some experiments in this direction have been made by Eve in the laboratory of the writer. The experiments are not yet completed but the results so far obtained allow us to calculate the probable amount of emanation per cubic kilometre of the atmosphere near the earth.
Experiments were first made with a large iron tank 154 cms. square and 730 cms. deep, in a building in which no radium or other radio-active material had ever been introduced. The saturation ionization current for the air in the tank was first measured by means of an electroscope, connected with an insulated electrode passing up the centre of the closed tank. Assuming that the ionization in the tank was uniform, the number of ions produced per c.c. of the air in the tank was found to be 10. This is a considerably lower value than has usually been observed in a small closed vessel (see section 284). Cooke obtained the value 10 for a well cleaned brass electroscope, surrounded by lead, while Schuster obtained a value about 12 for the air in the laboratory of Owens College, Manchester.
In order to measure the amount of the excited activity from the tank, a central insulated wire was charged negatively to about 10,000 volts by a Wimshurst machine. After two hours, the wire was removed and wound on an insulated frame connected with a gold-leaf electroscope. The rate of decay of the activity on the wire was found to be about the same as for the excited activity produced by the radium emanation. In order to estimate the amount of radium emanation present in the large tank, special experiments were made with a smaller tank in which a known quantity of the radium emanation was introduced by employing a solution of pure radium bromide of known concentration. A central wire was made the negative electrode as before, and, after removal, it was wound on the frame and its activity tested. In this way it was found that the amount of radium emanation present in the large tank, in order to produce the excited activity observed, must have been equal to the equilibrium or maximum amount to be obtained from 9·5 × 10⁻⁹ grams of pure radium bromide. The volume of the large tank was 17 cubic metres, so that the amount of emanation present per cubic metre was equivalent to that liberated from 5·6 × 10⁻¹⁰ grams of radium bromide in radio-active equilibrium.
If the amount of the emanation in the tank is taken as the average amount existing in the outside air, _the amount of radium emanation present per cubic kilometre of the air is equivalent to that supplied by 0·56 grams of radium bromide_.
For the purpose of calculation, suppose the emanation is uniformly distributed over the land portion of the earth (¼ of the total surface), and to extend to an average height of 5 kilometres. The air over the sea is not taken into account as its radio-activity has not been examined. The total amount of emanation present in the atmosphere under these conditions corresponds to that supplied by about 400 tons of radium bromide. In order to maintain this amount of emanation in the atmosphere, it must be supplied at a constant rate from the earth’s surface. Since the greater amount of the emanation probably escapes into the air by transpiration and diffusion through the soil, the emanation cannot reach the surface except from a very thin layer of the earth. The probable thickness of this layer can be estimated if it is assumed that the present loss of heat from the earth is supplied from the radio-active matter contained in it. We have seen (section 271) that, on this hypothesis, there must be an amount of active matter in the earth corresponding to about 300 million tons of radium. If this is supposed to be uniformly distributed, a thickness of layer of about 13 metres will suffice to maintain the calculated amount of emanation in the atmosphere. This thickness of layer is about the order of magnitude to be expected from general considerations.
These results lead indirectly to the conclusion that a large amount of emanation does undoubtedly exist in the surface crust of the earth.
Experiments were also made by Eve with a large zinc cylinder exposed in the open air. Volume for volume, the average amount of excited activity derived from it was only about one-third of that obtained from the large iron tank. This would reduce the amount of emanation, previously deduced, to about one-third.
Before such calculations can be considered at all definite, it will be necessary to make comparative measurements of the amount of emanation in the atmosphere at various parts of the earth. The air at Montreal is not abnormally active, so that the calculations probably give the right order of magnitude of the quantities.
Eve also observed that the amount of activity to be obtained per unit length of the wire in the zinc cylinder of about 70 cms. in diameter was about the same as for a wire ·5 mms. in diameter charged to 10,000 volts in the open air, supported 20 feet from the ground. This shows that such a potential does not draw in the carriers of excited activity which are more than half a metre away, and probably the range is even less.
It is of great importance to find how large a proportion of the number of ions produced in the atmosphere is due to the radio-active matter distributed throughout it. The results of Eve with the large iron tank, already referred to, indicate that a large proportion of the ionization in the tank was due to the radio-active matter contained in it, for the ratio of the excited activity on the central electrode to the total ionization current in the tank was about ⁷⁄₁₀ of the corresponding ratio for a smaller tank into which a supply of the radium emanation had been introduced.
This result requires confirmation by experiments at other parts of the earth, but the results point to the conclusion that a large part, if not all, of the ionization at the earth’s surface is due to radio-active matter distributed in the atmosphere. A constant rate of production of 30 ions per second per c.c. of air, which has been observed in the open air at the surface of the earth in various localities, would be produced by the presence in each c.c. of the air of the amount of emanation liberated from 2·4 × 10⁻¹⁵ grams of radium bromide in radio-active equilibrium. It is not likely, however, that the ionization of the upper part of the atmosphere is due to this cause alone. In order to explain the maintenance of the large positive charge, which generally exists in the upper atmosphere, there must be a strong ionization of the upper air, which may possibly be due to ionizing radiations emitted by the sun.
=282. Ionization of atmospheric air.= A large number of measurements have been made during the last few years to determine the relative amount of ionization in the atmosphere in different localities and at different altitudes. Measurements of this character were first undertaken by Elster and Geitel with a special type of electroscope. A charged body exposed to the air was attached to a portable electroscope, and the rate of loss of charge was observed by the movement of the gold or aluminium leaf. The rates of discharge of the electroscope for positive and negative electricity were generally different, the ratio depending on the locality and the altitude, and on the meteorological conditions. This apparatus is not suitable for quantitative measurements and the deductions to be drawn from the observations are of necessity somewhat indefinite.
Ebert[430] has designed a portable apparatus in which the number of ions per c.c. of the air can be determined easily. A constant current of air is drawn between two concentric cylinders by means of a fan actuated by a falling weight. The inner cylinder is insulated and connected with an electroscope. Knowing the capacity of the apparatus, and the velocity of the current of air, the rate of movement of the gold-leaf affords a measure of the number of ions present in unit volume of the air drawn between the cylinders.
In this way Ebert found that the number of ions in the air was somewhat variable, but on an average corresponded to about 2600 per c.c. in the particular locality where the measurements were made.
This is the equilibrium number of ions present per c.c. when the rate of production balances the rate of recombination. If _q_ is the number of ions produced per second per unit volume of the air and _n_ is the equilibrium number, then _q_ = α_n²_ where α is the constant of recombination (section 30).
By a slight addition to the apparatus of Ebert, Schuster[431] has shown that the constant of recombination for the particular sample of air under investigation can be determined. The value so obtained for air in the neighbourhood of Manchester was variable, and two or three times as great as for dust-free air. The results of some preliminary measurements showed that the number of ions present per c.c. of the air in different localities varied from 2370 to 3660, while the value of _q_, the number of ions produced per c.c. per second, varied between 12 and 38·5.
Rutherford and Allan and Eberts showed that the ions in the air had about the same mobility as the ions produced in air by Röntgen rays and radio-active substances. In some recent determinations by Mache and Von Schweidler[432], the velocity of the positive ion was found to be about 1·02 cms. per second, and that of the negative 1·25 cms., for a potential gradient of one volt per cm.
Langevin[433] has recently shown that in addition to these swift moving ions, there are also present in the atmosphere some ions which travel extremely slowly in an electric field. The number of these slowly moving ions in the air in Paris is about 40 times as great as the number of the swifter ions. This result is of great importance, for in the apparatus of Ebert these ions escape detection, since the electric field is not strong enough to carry them to the electrodes during the time of their passage between the cylinders.
=283. Radio-activity of ordinary materials.= It has been shown that radio-active matter seems to be distributed fairly uniformly over the surface of the earth and in the atmosphere. The very important question arises whether the small radio-activity observed is due to known or unknown radio-elements present in the earth and atmosphere, or to a feeble radio-activity of matter in general, which is only readily detectable when large quantities of matter are present. The experimental evidence is not yet sufficient to answer this question, but undoubted proof has been obtained that many of the metals show a very feeble radio-activity. Whether this radio-activity is due to the presence of a slight trace of the radio-elements or is an actual property of the metals themselves will be discussed in more detail in section 286.
Schuster[434] has pointed out that every physical property hitherto discovered for one element has been found to be shared by all the others in varying degrees. For example, the property of magnetism is most strongly marked in iron, nickel, and cobalt, but all other substances are found to be either feebly magnetic or diamagnetic. It might thus be expected on general principles that all matter should exhibit the property of radio-activity in varying degrees. On the view developed in chapter X., the presence of this property is an indication that the matter is undergoing change accompanied by the expulsion of charged particles. It does not, however, by any means follow that because the atom of one element in the course of time becomes unstable and breaks up, that, therefore, the atoms of all the other elements pass through similar phases of instability.
It has already been mentioned (section 8), that Mme Curie made a very extensive examination of most of the elements and their compounds for radio-activity. The electric method was used, and any substance possessing an activity of ¹⁄₁₀₀ of that of uranium would certainly have been detected. With the exception of the known radio-elements and the minerals containing uranium and thorium, no other substances were found to be radio-active even to that degree.
Certain substances like phosphorus[435] possess the property of ionizing a gas under special conditions. The air which is drawn over the phosphorus is conducting, but it has not yet been settled whether this conductivity is due merely to ions formed at the surface of the phosphorus or to ions produced by the phosphorus nuclei or emanations, as they have been termed, which are carried along with the current of air. It does not however appear that the ionization of the gas is in any way due to the presence of a penetrating type of radiation such as is emitted by the radio-active bodies. Le Bon (section 8) observed that quinine sulphate, after being heated to a temperature below the melting point and then allowed to cool, showed for a time strong phosphorescence and was able rapidly to discharge an electroscope. The discharging action of quinine sulphate under varying conditions has been very carefully examined by Miss Gates[436]. The ionization could not be observed through thin aluminium foil or gold-leaf, but appeared to be confined to the surface of the sulphate. The current observed by an electrometer was found to vary with the direction of the electric field, indicating that the positive and negative ions had very different mobilities. The discharging action appears to be due either to an ionization of the gas very close to the surface by some short ultra-violet light waves, accompanying the phosphorescence, or to a chemical action taking place at the surface.
Thus, neither phosphorus nor quinine sulphate can be considered to be radio-active, even under the special conditions when they are able to discharge an electrified body. No evidence in either case has been found that the ionization is due to the emission of a penetrating radiation.
No certain evidence has yet been obtained that any body can be made radio-active by exposure to Röntgen rays or cathode rays. A metal exposed to the action of Röntgen rays gives rise to a secondary radiation which is very readily absorbed in a few centimetres of air. It is possible that this secondary radiation may prove to be analogous in some respects to the α rays from the radio-elements. The secondary radiation, however, ceases immediately the Röntgen rays are cut off. Villard[437] stated that a piece of bismuth produced a feeble photographic action after it had been exposed for some time to the action of the cathode rays in a vacuum. It has not however been shown that the bismuth gives out rays of a character similar to those of the radio-active bodies. The experiments of Ramsay and Cooke on the production of apparent activity in inactive matter by the radiations from radium have already been discussed in section 264.
The existence of a very feeble radio-activity of ordinary matter has been deduced from the study of the conductivity of gases in closed vessels. The conductivity is extremely minute, and special methods are required to determine it with accuracy. A brief account will now be given of the gradual growth of our knowledge on this important question.
=284. Conductivity of air in closed vessels.= Since the time of Coulomb onwards several investigators have believed that a charged conductor placed inside a closed vessel lost its charge more rapidly than could be explained by the conduction leak across the insulating support. Matteucci, as early as 1850, observed that the rate of loss of charge was independent of the potential. Boys, by using quartz insulators of different lengths and diameters, arrived at the conclusion that the leakage must in part take place through the air. This loss of charge in a closed vessel was believed to be due in some way to the presence of dust particles in the air.
On the discovery that gases become temporary conductors of electricity under the influence of Röntgen rays and the rays from radio-active substances, attention was again drawn to this question. Geitel[438] and C. T. R. Wilson[439] independently attacked the problem, and both came to the conclusion that the loss of charge was due to a constant ionization of the air in the closed vessel. Geitel employed in his experiments an apparatus similar to that shown in Fig. 103. The loss of charge of an Exner electroscope, with the cylinder of wire netting _Z_ attached, was observed in a closed vessel containing about 30 litres of air. The electroscope system was found to diminish in potential at the rate of about 40 volts per hour, and this leakage was shown not to be due to a want of insulation of the supports.
Wilson, on the other hand, used a vessel of very small volume, in order to work with air which could be completely freed from dust. In the first experiments a silvered glass vessel with a volume of only 163 c.c. was employed. The experimental arrangement is shown in Fig. 104.
[Illustration: Fig. 104.]
The conductor, of which the loss of charge was to be measured, was placed near the centre of the vessel _A_. It consisted of a narrow strip of metal with a gold-leaf attached. The strip of metal was fixed to the upper rod by means of a small sulphur bead. The upper rod was connected with a sulphur condenser with an Exner electroscope _B_ attached to indicate its potential. The gold-leaf system was initially charged to the same potential as the upper rod and condenser by means of a fine steel wire which was caused to touch the gold-leaf system by the attraction of a magnet brought near it. The rate of movement of the gold-leaf was measured by means of a microscope provided with a micrometer eye-piece. By keeping the upper rod at a slightly higher potential than the gold-leaf system, it was ensured that the loss of charge of the gold-leaf system should not be due in any way to a conduction leakage across the sulphur bead.
The method employed by Wilson in these experiments is very certain and convenient when an extremely small rate of discharge is to be observed. In this respect the electroscope measures with certainty a rate of loss of charge much smaller than can be measured by a sensitive electrometer.
Both Geitel and Wilson found that the leakage of the insulated system in dust-free air was the same for a positive as for a negative charge, and was independent of the potential over a considerable range. The leakage was the same in the dark as in diffuse daylight. The independence of leakage of the potential is strong evidence that the loss of charge is due to a constant ionization of the air. When the electric field acting on the gas exceeds a certain value, all the ions are carried to the electrodes before recombination occurs. A saturation current is reached, and it will be independent of further increase of the electric field, provided, of course, a potential sufficiently high to cause a spark to pass is not applied.
C. T. R. Wilson has recently devised a striking experiment to show the presence of ions in dust-free air which is not exposed to any external ionizing agency. Two large metal plates are placed in a glass vessel connected with an expansion apparatus similar to that described in section 34. On expanding the air, the presence of the ions is shown by the appearance of a slight cloud between the plates. These condensation nuclei carry an electric charge, and are apparently similar in all respects to the ions produced in gases by X rays, or by the rays from active substances.
Wilson found that the loss of charge of the insulated system was independent of the locality. The rate of discharge was unaltered when the apparatus was placed in a deep tunnel, so that it did not appear that the loss of charge was due to an external radiation. From experiments already described, however (section 279), it is probable that about 30 per cent. of the rate of discharge observed was due to a very penetrating radiation. This experiment of Wilson’s indicates that the intensity of the penetrating radiation was the same in the tunnel as at the earth’s surface. Wilson found that the ionization of the air was about the same in a brass vessel as in one of glass, and came to the conclusion that the air was spontaneously ionized.
Using a brass vessel of volume about 471 c.c., Wilson determined the number of ions that must be produced in air per unit volume per second, in order to account for the loss of charge of the insulated system. The leakage system was found to have a capacity of about 1·1 electrostatic units, and lost its charge at the rate of 4·1 volts per hour for a potential of 210 volts, and 4·0 volts per hour for a potential of 120 volts. Taking the charge on an ion as 3·4 × 10⁻¹⁰ electrostatic units, this corresponds to a production of 26 ions per second.
Rutherford and Allan[440] repeated the results of Geitel and Wilson, using an electrometer method. The saturation current was observed between two concentric zinc cylinders of diameter 25·5 and 7·5 cms. respectively and length 154 cms. It was found that the saturation current could practically be obtained with a potential of a few volts. Saturation was however obtained with a lower voltage after the air had remained undisturbed in the cylinders for several days. This was probably due to the gradual settling of the dust originally present in the air.
Later observations of the number of ions produced in air in sealed vessels have been made by Patterson[441], Harms[442], and Cooke[443]. The results obtained by different observers are shown in the following table. The value of the charge on an ion is taken as 3·4 × 10⁻¹⁰ electrostatic units:
Material of Number of ions Observer vessel produced per c.c. per second
Silvered 36 C. T. R. Wilson glass
Brass 26 „ „
Zinc 27 Rutherford and Allan
Glass 53 to 63 Harms
Iron 61 Patterson
Cleaned 10 Cooke brass
It will be shown later that the differences in these results are probably due to differences in the radio-activity of the containing vessel.
=285. Effect of pressure and nature of gas.= C. T. R. Wilson (_loc. cit._) found that the rate of leakage of a charged conductor varied approximately as the pressure of the air between the pressures examined, viz. 43 mms. and 743 mms. of mercury. These results point to the conclusion that, in a good vacuum, a charged body would lose its charge extremely slowly. This is in agreement with an observation of Crookes, who found that a pair of gold-leaves retained their charge for several months in a high vacuum.
Wilson[444] at a later date investigated the leakage for different gases. The results are included in the following table, where the ionization produced in air is taken as unity:
Gas Relative (Relative ionization ionization) / (density)
Air 1·00 1·00
Hydrogen 0·184 2·7
Carbon dioxide 1·69 1·10
Sulphur 2·64 1·21 dioxide
Chloroform 4·7 1·09
With the exception of hydrogen, the ionization produced in different gases is approximately proportional to their density. The relative ionization is very similar to that observed by Strutt (section 45) for gases exposed to the influence of the α and β rays from radio-active substances, and points to the conclusion that the ionization observed may be due either to a radiation from the walls of the vessel or from external sources.
Jaffé[445] has made a careful examination of the natural ionization in the very heavy gas nickel-carbonyl, Ni(CO)₄, in a small silvered glass vessel. The ionization of this gas was 5·1 times that of air at normal pressure while its density is 5·9 times that of air. The leak of the electroscope was nearly proportional to the pressures except at low pressure, when the leak was somewhat greater than would be expected if the pressure law held. The fact that a gas of such high density and complicated structure behaves like the simpler and lighter gases is a strong indication that the ionization itself is due to a radiation from the walls of the vessel and not to a spontaneous ionization of the gas.
Patterson[446] examined the variation of the ionization of air with pressure in a large iron vessel of diameter 30 cms. and length 20 cms. The current between a central electrode and the cylinder was measured by means of a sensitive Dolezalek electrometer. He found that the saturation current was practically independent of the pressure for pressures greater than 300 mms. of mercury. Below a pressure of 80 mms. the current varied directly as the pressure. For air at atmospheric pressure, the current was independent of the temperature up to 450° C. With further increase of temperature, the current began to increase, and the increase was more rapid when the central electrode was charged negatively than when it was charged positively. This difference was ascribed to the production of positive ions at the surface of the iron vessel. The results obtained by Patterson render it very improbable that the ionization observed in air is due to a spontaneous ionization of the enclosed air: for we should expect the amount of this ionization to depend on the temperature of the gas. On the other hand, these results are to be expected if the ionization of the enclosed air is mainly due to an easily absorbed radiation from the walls of the vessel. If this radiation had a penetrating power about equal to that observed for the α rays of the radio-elements, the radiation would be absorbed in a few centimetres of air. With diminution of pressure, the radiations would traverse a greater distance of air before complete absorption, but the total ionization produced by the rays would still remain about the same, until the pressure was reduced sufficiently to allow the radiation to traverse the air space in the vessel without complete absorption. With still further diminution of pressure, the total ionization produced by the radiation, and in consequence the current observed, would vary directly as the pressure.
=286. Examination of ordinary matter for radio-activity.= Strutt[447], McLennan and Burton[448], and Cooke[449], independently observed about the same time that ordinary matter is radio-active to a slight degree. Strutt, by means of an electroscope, observed that the ionization produced in a closed vessel varied with the material of the vessel. A glass vessel with a removable base was employed and the vessel was lined with the material to be examined. The following table shows the relative results obtained. The amount of leakage observed is expressed in terms of the number of scale divisions of the eye-piece passed over per hour by the gold-leaf:
Material of Leakage in lining of scale divisions vessel per hour
Tinfoil 3·3
„ another 2·3 sample
Glass coated 1·3 with phosphoric acid
Silver 1·6 chemically deposited on glass
Zinc 1·2
Lead 2·2
Copper (clean) 2·3
„ (oxidized) 1·7
Platinum 2·0, 2·9, 3·9 (various samples)
Aluminium 1·4
There are thus marked differences in the leakage observed for different materials and also considerable differences in different samples of the same metal. For example, one specimen of platinum caused nearly twice the leakage of another sample from a different stock.
McLennan and Burton, on the other hand, measured by means of a sensitive electrometer the ionization current produced in the air in a closed iron cylinder 25 cms. in diameter and 130 cms. in length, in which an insulated central electrode was placed. The open cylinder was first exposed for some time at the open window of the laboratory. It was then removed, the top and bottom closed, and the saturation current through the gas determined as soon as possible. In all cases it was observed that the current diminished for two or three hours to a minimum and then very slowly increased again. In one experiment, for example, the initial current observed corresponded to 30 on an arbitrary scale. In the course of four hours the current fell to a minimum of 6·6, and 44 hours later had risen to a practical maximum of 24. The initial decrease observed is probably due to a radio-activity of the enclosed air or walls of the vessel, which decayed rapidly with the time. The decay of the excited activity produced on the interior surface of the cylinder when exposed to the air was probably responsible for a part of the decrease observed. McLennan ascribes the increase of current with time to a radio-active _emanation_ which is given off from the cylinder, and ionizes the enclosed air. On placing linings of lead, tin, and zinc in the iron cylinder, considerable differences were observed both for the minimum current and also for the final maximum. Lead gave about twice the current due to zinc, while tin gave an intermediate value. These results are similar in character to those obtained by Strutt.
McLennan and Burton also investigated the effect of diminution of pressure on the current. The cylinder was filled with air to a pressure of 7 atmospheres, and allowed to stand until the current reached a constant value. The air was then allowed to escape and the pressure reduced to 44 mms. of mercury. The current was found to vary approximately as the pressure over the whole range. These results are not in agreement with the results of Patterson already described, nor with some later experiments of Strutt. McLennan’s results however point to the conclusion that the ionization was mainly due to an emanation emitted from the metal. Since the air was rapidly removed, a proportionate amount of the emanation would be removed also, and it might thus be expected that the current would vary directly as the pressure. If this is the case the current through the gas at low pressures should increase again to a maximum if time is allowed for a fresh emanation to form.
H. L. Cooke, using an electroscopic method, obtained results very similar to those given by Strutt. Cooke observed that a penetrating radiation was given out from brick. When a brass vessel containing the gold-leaf system was surrounded by brick, the discharge of the electroscope was increased by 40 to 50 per cent. This radiation was of about the same penetrating power as the rays from radio-active substances. The rays were completely absorbed by surrounding the electroscope with a sheet of lead 2 mms. in thickness. This result is in agreement with the observation of Elster and Geitel, already mentioned, that radio-active matter was present in clay freshly dug up from the earth.
Cooke also observed that the ionization of the air in a brass electroscope could be reduced to about one-third of its usual value if the interior surface of the brass was carefully cleaned. By removing the surface of the brass he was able to reduce the ionization of the enclosed air from 30 to 10 ions per c.c. per second. This is an important observation, and indicates that a large proportion of the radio-activity observed in ordinary matter is due to a deposit of radio-active matter on its surface. It has already been shown that bodies which have been exposed in the presence of the radium emanation retain a residual activity which decays extremely slowly. There can be no doubt that the radium emanation is present in the atmosphere, and the exposed surface of matter, in consequence, will become coated with an invisible film of radio-active matter, deposited from the atmosphere. On account of the slow decay of this activity it is probable that the activity of matter exposed in the open air would steadily increase for a long interval. Metals, even if they are originally inactive, would thus acquire a fairly permanent activity, but it should be possible to get rid of this by removing the surface of the metal or by chemical treatment. The rapid increase of activity of all matter left in a laboratory in which a large quantity of emanation has been released has been drawn attention to by Eve[450]. This superficial activity, due to the products radium D, E, and F, was mainly removed by placing the metal in strong acid.
A number of experiments have been made by J. J. Thomson, N. R. Campbell, and A. Wood in the Cavendish laboratory to examine whether the radio-activity observed in ordinary matter is a specific property of such matter or is due to the presence of some radio-active impurity. An account of these experiments was given by Professor J. J. Thomson in a discussion on the Radio-activity of Ordinary Matter at the British Association meeting at Cambridge, 1904. The results[451], as a whole, support the view that each substance gives out a characteristic type or types of radiation and that the radiation is a specific property of the substance. J. J. Thomson[452] has made experiments to observe the action of different substances in cutting off the external very penetrating radiation (section 279) observed by Cooke and McLennan. He found that some substances cut off this external radiation, while others had little if any effect. For example, the ionization in a closed vessel was reduced 17 per cent. by surrounding it with a thick lead envelope; but, on surrounding it with an equivalent absorbing thickness of water, or water mixed with sand, no sensible diminution was observed. In other experiments Wood[453] found that the diminution of the ionization by a given screen depended upon the material of the vessel. For example, the ionization in a lead vessel, surrounded by a lead screen, was reduced 10 per cent., while in an iron vessel it was reduced 24 per cent. He concludes from his experiments that the ionization observed in a closed vessel has a threefold origin. Part of it is due to an external penetrating radiation, part to a secondary radiation set up by it, while the remainder is due to an intrinsic radiation from the walls, altogether independent of the external radiation.
In some experiments of Campbell[454], the variation of the ionization current between two parallel plates was observed for a progressive increase of the distance between them. The effects observed are shown in Fig. 105. The curves at first rise rapidly, then bend over and finally become a straight line. The knee of the curve is at a different distance for the different substances. The shape of these curves indicates that two types of radiation are present, one of which is readily absorbed in the gas while the other, a more penetrating type of radiation, extends over the whole distance between the plates. In another series of experiments, one side of the testing vessel was of thin aluminium, and the ionization current was observed when an exterior screen was brought up to it. Lead gave a considerable increase, but the radiation from it was readily absorbed by an interposed screen. The radiation emitted by carbon and zinc was more than twice as penetrating as from lead.
[Illustration: Fig. 105.]
Attempts were made to see whether a radio-active emanation was given off by dissolving solid substances and then keeping the solutions in a closed vessel and afterwards testing the activity of the air drawn from them. In some cases an emanation was observed, but the amount varied with different specimens of the same material; in others no effect was detected.
When linings of different substances were placed in a closed testing vessel, the ionization current in most cases fell at first, passed through a minimum, and then slowly increased to a maximum. For lead the maximum was reached in 9 hours, for tin in 14 and for zinc in 18 hours. These results indicate that an emanation is given off from the metal, and that the amount reaches a maximum value at different intervals in the various cases. This was confirmed by an examination of a piece of lead which was left in radium-free nitric acid. Twenty times the normal effect was observed after this treatment. This is probably due to the increase of porosity of the lead which allows a greater fraction of the emanation produced in the metal to diffuse out with the gas.
The activity observed in ordinary matter is extremely small. The lowest rate of production of ions yet observed is 10 per cubic centimetre per second in a brass vessel. Suppose a spherical brass vessel is taken of capacity 1 litre. The area of the interior surface would be about 480 sq. cms. and the total number of ions produced per second would be about 10⁴. Now it has been shown, in section 252, that an α particle projected from radium itself gives rise to 8·6 × 10⁴ ions before it is absorbed in the gas. An expulsion of one α particle every 8 seconds from the whole vessel, or of one α particle from each square centimetre of surface _per hour_ would thus account for the minute conductivity observed. Even if it were supposed that this activity is the result of a breaking up of the matter composing the vessel, the disintegration of one atom per second per gram, provided it was accompanied by the expulsion of an α particle, would fully account for the conductivity observed.
While the experiments, already referred to, afford strong evidence that ordinary matter does possess the property of radio-activity to a feeble degree, it must not be forgotten that the activity observed is excessively minute, compared even with a weak radio-active substance like uranium or thorium. The interpretation of the results is complicated, too, by the presence of the radium emanation in the atmosphere, for we have seen that the surface of every body exposed to the open air must become coated with the slowly changing transformation products of the radium emanation. The distribution of radio-active matter throughout the constituents of the earth renders it difficult to be certain that any substance, however carefully prepared, is freed from radio-active impurities. If matter in general is radio-active, it must be undergoing transformation at an excessively slow rate, unless it be supposed (see Appendix A) that changes of a similar character to those observed in the radio-elements may occur without the appearance of their characteristic radiations.
Footnote 384:
Geitel, _Phys. Zeit._ 2, p. 116, 1900.
Footnote 385:
C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 32, 1900. _Proc. Roy. Soc._ 68, p. 151, 1901.
Footnote 386:
Elster and Geitel, _Phys. Zeit._ 2, p. 590, 1901.
Footnote 387:
Elster and Geitel, _Phys. Zeit._ 3, p. 76, 1901.
Footnote 388:
Rutherford and Allan, _Phil. Mag._ Dec. 1902.
Footnote 389:
Allan, _Phil. Mag._ Feb. 1904.
Footnote 390:
C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 428, 1902.
Footnote 391:
C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 428, 1902; 12, p. 17, 1903.
Footnote 392:
C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 12, p. 85, 1903.
Footnote 393:
Allan, _Phys. Rev._ 16, p. 106, 1903.
Footnote 394:
McLennan, _Phys. Rev._ 16, p. 184, 1903.
Footnote 395:
Schmauss, _Annal. d. Phys._ 9, p. 224, 1902.
Footnote 396:
Elster and Geitel, _Phys. Zeit_. 3, p. 574, 1902.
Footnote 397:
Ebert and Ewers, _Phys. Zeit._ 4, p. 162, 1902.
Footnote 398:
Sarasin, Tommasina and Micheli, _C. R._ 139, p. 917, 1905.
Footnote 399:
J. J. Thomson, _Phil. Mag._ Sept. 1902.
Footnote 400:
Ebert, _Sitz. Akad. d. Wiss. Munich_, 33, p. 133, 1903.
Footnote 401:
J. J. Thomson, _Phil. Mag._ Sept. 1902.
Footnote 402:
Adams, _Phil. Mag._ Nov. 1903.
Footnote 403:
Bumstead and Wheeler, _Amer. Journ. Science_, 17, p. 97, Feb. 1904.
Footnote 404:
Bumstead, _Amer. Journ. Science_, 18, July, 1904.
Footnote 405:
Dadourian, _Amer. Journ. Science_, 19, Jan. 1905.
Footnote 406:
H. S. Allen and Lord Blythswood, _Nature_, 68, p. 343, 1903; 69, p. 247, 1904.
Footnote 407:
Strutt, _Proc. Roy. Soc._ 73, p. 191, 1904.
Footnote 408:
Himstedt, _Ann. d. Phys._ 13, p. 573, 1904.
Footnote 409:
Elster and Geitel, _Phys. Zeit._ 5, No. 12, p. 321, 1904.
Footnote 410:
Dorn, _Abhandl. d. Natur. Ges. Halle_, 25, p. 107, 1904.
Footnote 411:
Schenck, Thesis Univ. Halle, 1904.
Footnote 412:
Mache, _Wien. Ber._ 113, p. 1329, 1904.
Footnote 413:
Curie and Laborde, _C. R._ 138, p. 1150, 1904.
Footnote 414:
Blanc, _Phil. Mag._ Jan. 1905.
Footnote 415:
Boltwood, _Amer. Journ. Science_, 18, Nov. 1904.
Footnote 416:
Elster and Geitel, _Phys. Zeit._ 4, p. 522, 1903.
Footnote 417:
Elster and Geitel, _Phys. Zeit._ 5, No. 1, p. 11, 1903.
Footnote 418:
Vincenti and Levi Da Zara, _Atti d. R. Instit. Veneto d. Scienze_, 54, p. 95, 1905.
Footnote 419:
Burton, _Phil. Mag._ Oct. 1904.
Footnote 420:
Elster and Geitel, _Phys. Zeit._ 6, No. 3, p. 67, 1905.
Footnote 421:
Rutherford and Allan, _Phil. Mag._ Dec. 1902.
Footnote 422:
Elster and Geitel, _Phys. Zeit._ 4, p. 138, 1902; 4, p. 522, 1903.
Footnote 423:
Saake, _Phys. Zeit._ 4, p. 626, 1903.
Footnote 424:
Simpson, _Proc. Roy. Soc._ 73, p. 209, 1904.
Footnote 425:
McLennan, _Phys. Rev._ 16, p. 184, 1903, and _Phil. Mag._ 5, p. 419, 1903.
Footnote 426:
McLennan, _Phys. Rev._ No. 4, 1903.
Footnote 427:
Rutherford and Cooke, _Americ. Phys. Soc._ Dec. 1902.
Footnote 428:
Cooke, _Phil. Mag._ Oct. 1903.
Footnote 429:
Allan, _Phil. Mag._ Feb. 1904.
Footnote 430:
Ebert, _Phys. Zeit._ 2, p. 622, 1901. _Zeitschr. f. Luftschiffahrt_, 4, Oct. 1902.
Footnote 431:
Schuster, _Proc. Manchester Phil. Soc._ p. 488, No. 12, 1904.
Footnote 432:
Mache and Von Schweidler, _Phys. Zeit._ 6, No. 3, p. 71, 1905.
Footnote 433:
Langevin, _C. R._ 140, p. 232, 1905.
Footnote 434:
Schuster, British Assoc. 1903.
Footnote 435:
J. J. Thomson, _Conduction of Electricity through Gases_, p. 324, 1903.
Footnote 436:
Miss Gates, _Phys. Rev._ 17, p. 499, 1903.
Footnote 437:
Villard, _Société de Physique_, July, 1900.
Footnote 438:
Geitel, _Phys. Zeit._ 2, p. 116, 1900.
Footnote 439:
C. T. R. Wilson, _Proc. Camb. Phil. Soc._ 11, p. 52, 1900. _Proc. Roy. Soc._ 68, p. 152, 1901.
Footnote 440:
Rutherford and Allan, _Phil. Mag._ Dec. 1902.
Footnote 441:
Patterson, _Phil. Mag._ August, 1903.
Footnote 442:
Harms, _Phys. Zeit._ 4, No. 1, p. 11, 1902.
Footnote 443:
Cooke, _Phil. Mag._ Oct. 1903.
Footnote 444:
Wilson, _Proc. Roy. Soc._ 69, p. 277, 1901.
Footnote 445:
Jaffé, _Phil. Mag._ Oct. 1904.
Footnote 446:
Patterson, _Phil. Mag._ Aug. 1903.
Footnote 447:
Strutt, _Phil. Mag._ June, 1903. _Nature_, Feb. 19, 1903.
Footnote 448:
McLennan and Burton, _Phys. Rev._ No. 4, 1903. J. J. Thomson, _Nature_, Feb. 26, 1903.
Footnote 449:
Cooke, _Phil. Mag._ Aug. 6, 1903. Rutherford, _Nature_, April 2, 1903.
Footnote 450:
Eve, _Nature_, March 16, 1905.
Footnote 451:
See article in _Le Radium_, No. 3, p. 81, Sept. 15, 1904.
Footnote 452:
J. J. Thomson, _Proc. Camb. Phil. Soc._ 12, p. 391, 1904.
Footnote 453:
Wood, _Phil. Mag._ April, 1905.
Footnote 454:
Campbell, _Nature_, p. 511, March 31, 1904. _Phil. Mag._ April, 1905.
APPENDIX A. PROPERTIES OF THE α RAYS.
A brief account is given here of some investigations made by the writer on the properties of the α rays from radium—investigations which were not completed in time for the results to be incorporated in the text.
The experiments were undertaken primarily with a view of determining accurately the value of _e_/_m_ of the α particle from radium, in order to settle definitely whether or not it is an atom of helium. In the previous experiments of the writer, Becquerel, and Des Coudres, on this subject (sections 89, 90, and 91), a thick layer of radium in radio-active equilibrium has been used as a source of α rays. Bragg (section 103) has shown that the rays emitted from radium under such conditions are complex, and consist of particles projected over a considerable range of velocity. In order to obtain a homogeneous pencil of rays it is necessary to use a very thin layer of a simple radio-active substance as a source of rays. In the experiments that follow, this condition was fulfilled by using a fine wire which was made active by exposure for several hours in the presence of a large quantity of radium emanation. By charging the wire negatively the active deposit was concentrated upon the wire, which was made intensely active. The active deposit initially contains radium A, B, and C. The activity of radium A practically disappears in about fifteen minutes, and the α radiation is then due entirely to the single product radium C, since radium B is a rayless product. The activity of radium C decreases to about 15 per cent. of its initial value after two hours.
=Magnetic deflection of the α rays.= The photographic method was employed to determine the deviation of the pencil of rays in a magnetic field. The experimental arrangement is shown in Fig. 106. The rays from the active wire, which was placed in a slot, passed through a narrow slit and fell normally on a photographic plate, placed at a known distance above the slit. The apparatus was enclosed in a brass tube which could be exhausted rapidly to a low pressure by means of a Fleuss pump. The apparatus was placed in a strong uniform magnetic field parallel to the plane of the slit. The magnetic field was reversed every ten minutes, so that on developing the plate two narrow bands were observed, the distance between which represented twice the deviation from the normal of the pencil of rays by the magnetic field. The width of the band was found to be the same whether the magnetic field was applied or not, showing that the pencil of rays was homogeneous and consisted of α particles projected with the same velocity.
[Illustration: Fig. 106.]
By placing the photographic plate at different distances from the slit it was found that the rays, after entering the magnetic field, described the arc of a circle of radius ρ equal to 42·0 cms. The strength of field _H_ was 9470 C.G.S. units, so that the value of _H_ρ for the α particles expelled from radium C is 398,000. This is in good agreement with the maximum values of _H_ρ, previously found for radium rays (see section 92).
The electric deviation of the rays from radium C has not yet been accurately measured, but an approximate determination of _e_/_m_ for the α particles can be obtained by assuming that the heating effect of radium C is a measure of the kinetic energy of the α particles expelled from it. We have seen in section 246 that the heating effect of the radium C present in one gram of radium in radio-active equilibrium is 31 gram calories per hour, which corresponds to an emission of energy of 3·6 × 10⁵ ergs per second. Now when radio-active equilibrium is reached, the number of α particles expelled from radium C per second is equal to the number of α particles expelled per second from radium at its minimum activity. This number, _n_, is 6·2 × 10¹⁰ (section 93).
Then ½ _mnv²_ = 3·6 × 10⁵,
or (_m_/_e_)_v²_ = 1·03 × 10¹⁶,
substituting the value of _n_, and the value of the ionic charge _e_. The value of _e_ in this case has not been assumed, since _n_ = _i_/_e_, where _i_ was the measured current due to the charge carried by the α rays.
From the magnetic deflection, it is known that
(_m_/_e_)_v_ = 3·98 × 10⁵.
From these two equations we obtain
_v_ = 2·6 × 10⁹ cms. per second. _e_/_m_ = 6·5 × 10³ electromagnetic units.
These values are in surprisingly good agreement with the previous values of the writer and Des Coudres (section 91). On account of the uncertainty attaching to the value of _n_, not much weight can be attached to the determination by this method of the constants of the α particles.
=Decrease of velocity of the α particles in passing through matter.= Some experiments were made to determine the velocity of the α particles from radium C after passing through known thicknesses of aluminium. The previous apparatus was employed, and the distance between the photographic bands was observed for successive layers of aluminium foil, each ·00031 cms. thick, placed over the active wire. The photographic plate was placed 2 cms. above the slit, and the magnetic field extended 1 cm. below the slit. The amount of deviation of the rays is inversely proportional to their velocity after traversing the aluminium screens. The impressions on the plate were clear and distinct, and about the same in all cases, showing that the rays were still homogeneous after passing through the aluminium.
A clear photographic impression was obtained for 12 layers of foil, but it was not found possible to obtain any effect through 13 layers. This result shows that the photographic action of the rays, like the ionizing action, ceases very abruptly.
The results obtained are shown in the following table. Assuming that the value of _e_/_m_ is constant, the third column gives the velocity of the α particles after traversing the aluminium. This is expressed in terms of _V₀_, the velocity of the α particle when the screens are removed.
Number of Distance Velocity layers of between of α aluminum bands on the particles foil plate
0 1·46 mms. 1·00 _V₀_
5 1·71 „ ·85 „
8 1·91 „ ·76 „
10 2·01 „ ·73 „
12 2·29 „ ·64 „
13 No photographic effect
The velocity of the α particle is thus reduced only about 36 per cent. of its initial value when it fails to produce any action on the photographic plate.
Now Bragg has shown (section 104) that the α particle produces approximately the same number of ions per cm. of path in air over its whole range. Consequently, the simplest assumption to make is that the energy of the α particle is diminished by a constant amount in traversing each layer of foil. After passing through 12 layers the kinetic energy is reduced to 41 per cent. of the maximum. Each layer of foil thus absorbs 4·9 per cent. of the maximum energy. The observed kinetic energy of the α particle after passing through successive layers of foil, and the value calculated on the above assumptions, are shown in the following table.
Number of Observed Calculated layers of energy energy aluminum foil
0 100 100
5 73 75
8 58 61
10 53 51
12 41 41
The experimental and theoretical values agree within the limits of experimental error. We may thus conclude, as a first approximation, that the same proportion of the total energy is abstracted from the α particles in passing through equal distances of the absorbing screen.
=Range of ionization and photographic action in air.= The abrupt falling off of the photographic impression after the rays had passed through 12 layers of foil suggested that it might be directly connected with the corresponding abrupt falling off of the ionization in air, so clearly brought out by Bragg. This was found to be the case. It was found experimentally that the absorption in each layer of aluminium foil was equivalent to that produced by a distance of ·54 cms. of air. Twelve layers of foil thus corresponded to 6·5 cms. of air. Now Bragg found that the α rays from radium C ionize the air for a distance 6·7 cms., and that the ionization then falls off very rapidly. We may thus conclude that the α rays cease to affect the photographic plate at the same velocity as that at which they cease to ionize the gas. This is a very important result, and, as we shall see later, suggests that the action on the photographic plate is due to an ionization of the photographic salts.
The velocity of the α particles from the different radio-active products can at once be calculated, knowing the maximum range in air of the α rays from each product. The latter have been experimentally determined by Bragg. The velocity is expressed in terms of _V₀_, the initial velocity of the α particles from radium C. The rays from radium C are projected with a greater velocity than the rays from the other products of radium.
Product Maximum Velocity range of α of α particles particles in air
Radium 3 cms. ·82 _V₀_
Emanation 3·8 or 4·4 ·87 or ·90 cms. _V₀_
Rad. A 4·4 or 3·8 ·90 or ·87 „ _V₀_
Rad. C 6·7 1·00 _V₀_ „
It is difficult to determine from the experiments whether the range 3·8 cms. belongs to the rays from the emanation or from radium A. The mean velocity of the α particles is thus ·90 _V₀_, and the maximum variation for the individual products does not vary more than 10 per cent. from the mean value.
The results of Becquerel, discussed in section 92, at once receive an explanation on the above results. The α particles, expelled from radium in radio-active equilibrium, have all ranges lying between 0 and 6·7 cms. of air. The velocity of the α particles which are able to produce a photographic impression varies between ·64 _V₀_ and _V₀_. The particles which have only a short range in air are projected with a smaller velocity than those which have a greater range. The former are in consequence more bent by a magnetic field. It is thus to be expected that the apparent curvature of the path of rays in a uniform magnetic field will be greater close to the radium than at some distance away.
=Range of phosphorescent action in air.= Some experiments were also made to see whether the action of the α rays in producing luminosity in substances like zinc sulphide, barium platinocyanide, and willemite, ceased at the same distance as the ionizing action.
A very active wire was placed on a movable plate, the distance of which from a fixed screen of phosphorescent substance could be varied. The distance at which the phosphorescent action ceased could be determined fairly accurately. Different thicknesses of aluminium foil were then placed over the active wire, and the corresponding distance at which the luminosity disappeared was measured. The results are shown graphically in Fig. 107, where the ordinates represent the distance of the phosphorescent screen from the active wire, and the abscissae the number of layers of aluminium foil, each ·00031 cms. thick.
[Illustration: Fig. 107.]
It is seen that the curve joining the points is a straight line. 12·5 thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of air, so that each thickness of aluminium corresponded in absorbing power to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent action ceased at a distance of air of 6·8 cms., showing that the photographic and phosphorescent ranges of the α rays in air were practically identical.
The experiments with barium platinocyanide and willemite were more difficult, as the β and γ rays from the active wire produced a luminosity comparable with that produced by the α rays. Fairly concordant results, however, were obtained by introducing a thin sheet of black paper between the active wire and the screen. If the luminosity was sensibly changed, it was concluded that the α rays still produced an effect, and in this way the point of cessation of phosphorescent action could be approximately determined. For example, with eight thicknesses of foil over the active wire the additional thickness of air required to cut off the phosphorescent effect of the a rays was 2·5 cms. for willemite, and 2·1 cms. for barium platinocyanide.
The corresponding distance for zinc sulphide was 2·40 cms., a value intermediate between the other two.
Since eight layers of foil are equivalent to 4·3 cms. of air, the ranges in air of phosphorescent action for zinc sulphide, barium platinocyanide, and willemite correspond to 6·7, 6·8, and 6·4 cms. respectively. The differences observed are quite likely to be due to experimental error.
=Discussion of results.= We have seen that the ionizing, phosphorescent, and photographic actions of the α rays emitted from radium C cease after traversing very nearly the same distance of air. This is a surprising result when it is remembered that the α particle, after passing through this depth of air, still possesses a velocity of at least 60 per cent. of its initial value. Taking the probable value of the initial velocity of the α particle from radium C as 2·5 × 10⁹ cms. per sec., the ionizing, phosphorescent, and photographic actions cease when the velocity of the α particle falls below 1·5 × 10⁹ cms. per second, that is, a velocity of about ¹⁄₂₀ of that of light. The particle still possesses nearly 40 per cent. of its initial energy of projection at this stage.
These results show that the property of the α rays of producing ionization in gases, of producing luminosity in some substances, and of affecting a photographic plate, ceases when the velocity of the α particle falls below a certain fixed value which is the same in each case. It seems reasonable, therefore, to suppose that these three properties of the α rays must be ascribed to a common cause. Now the absorption of the α rays in gases is mainly a consequence of the energy absorbed in the production of ions in the gas. When the α particles are completely absorbed in the gas, the same total amount of ionization is produced, showing that the energy required to produce an ion is the same for all gases. On the other hand, for a constant source of radiation, the ionization per unit volume of the gas is approximately proportional to its density. Since the absorption of the α rays in solid matter is approximately proportional to the density of the absorbing medium compared with air, it is probable that this absorption is also a result of the energy used up in producing ions in the solid matter traversed, and that about the same amount of energy is required to produce an ion in matter whether solid, liquid, or gaseous.
It is probable, therefore, that the production of ions in the phosphorescent material and in the photographic film would cease at about the same velocity for which the α particle is unable to ionize the gas. On this view, then, the experimental results receive a simple explanation. The action of the α rays in producing photographic and phosphorescent actions is primarily a result of ionization. This ionization may possibly give rise to secondary actions which influence the effects observed.
This point of view is of interest in connection with the origin of the “scintillations” observed in zinc sulphide and other substances when exposed to the action of the α rays. This effect is ascribed by Becquerel to the cleavage of the crystals under the bombardment of the α particles. These results, however, show that we must look deeper for the explanation of this phenomenon. The effect is primarily due to the production of ions in the phosphorescent material and not to direct bombardment, for we have seen that the α particle produces no scintillations when it still possesses a large amount of kinetic energy. It seems not unlikely that the scintillations produced by the α rays must be ascribed to the recombination of the ions which are produced by the α particle in the crystalline mass. It is difficult to see how this ionization could result in a cleavage of the crystals.
This close connection of the photographic and phosphorescent actions of the α rays with their property of producing ions, raises the question whether photographic and phosphorescent actions in general may not, in the first place, be due to a production of ions in the substance.
=Ionization curve for the α rays from radium C.= Mr McClung, working in the laboratory of the writer, has recently determined the relative ionization per unit path of the α particles projected from radium C, using the method first employed by Bragg and discussed in section 104. An active wire, exposed for several hours to the emanation from radium, was used as a source of rays. The α particles were homogeneous, since the film of radio-active matter was extremely thin.
The relation between the ionization observed over the cross section of the narrow cone of rays and the distance from the source of rays is shown in Fig. 108.
[Illustration: Fig. 108.]
The curve exhibits the same peculiarities as those given by Bragg for a thin film of matter of one kind. The ionization of the α particle per unit path increases slowly for about 4 cms. There is then a more rapid increase just before the α particle ceases to ionize the gas, and then a rapid falling off. The ionization does not appear to end so abruptly as is really the case, since there is a correction to be applied for the angle subtended by the cone of rays. The maximum range of the α rays in air was 6·7 cms., a number in agreement with that obtained by Bragg by measurements on the range of the rays from radium.
These results show that the ionization per unit path of the α particle increases at first slowly and then rapidly with decrease of velocity until the rays cease to ionize the gas.
=Energy required to produce an ion.= From the above results the energy required to produce an ion by collision of the α particle with the gas molecules can readily be deduced. The α particles, emitted from radium itself, are initially projected with a velocity ·88_V₀_ where _V₀_ is the initial velocity of projection of the α particles from radium C. The α particles cease to ionize the gas at a velocity ·64_V₀_. From this it can at once be deduced that ·48 of the total energy of the α particle, shot out by radium itself, is absorbed when it ceases to ionize the gas. Assuming that the heating effect of radium at its minimum activity—25 gram calories per hour per gram—is a measure of the kinetic energy of the expelled α particles, it can be calculated that the kinetic energy of each α particle is 4·7 × 10⁻⁶ ergs. The amount of energy absorbed when the α particle just ceases to ionize the gas is 2·3 × 10⁻⁶ ergs. Assuming that this energy is used up in ionization, and remembering that the α particle from radium itself produces 86000 ions in its path (section 252), the average energy required to produce an ion is 2·7 × 10⁻¹¹ ergs. This is equivalent to the energy acquired by an ion moving freely between two points differing in potential by 24 volts.
Townsend found that fresh ions were produced by an electron for a corresponding difference of potential of 10 volts. Stark, from other data, obtained a value 45 volts, while Langevin considers that 60 volts is an average value. The value obtained by Rutherford and McClung for ionization by X-rays was 175 volts, and is probably too high.
=Rayless changes.= We have seen that the α particles from the radio-active substances are projected with an average velocity not more than 30 per cent. greater than the minimum velocity, below which the α particles are unable to produce any ionizing, photographic, or phosphorescent action. Such a conclusion suggests that the property of the radio-active substances of emitting α particles has been detected because the α particles were projected slightly above this minimum velocity. A similar disintegration of matter may be taking place in other substances at a rate much greater than in uranium without producing much electrical effect, provided the α particles are projected below the critical velocity.
The α particle, on an average, produces about 100,000 ions in the gas before it is absorbed, so that the electrical effect observed is about 100,000 times as great as that due to the charge carried by the α particles alone.
It is not unlikely that the numerous rayless products which have been observed may undergo disintegration of a similar character to the products which obviously emit α rays. In the rayless product the α particle may be expelled with a velocity less than 1·5 × 10⁹ cms. per second and so fail to produce much electrical effect.
These considerations have an important bearing on the question whether matter in general is radio-active. The property of emitting α particles above the critical velocity may well be a property only of a special class of substances, and need not be exhibited by matter in general. At the same time the results suggest that ordinary matter may be undergoing transformation accompanied by the expulsion of α particles at a rate much greater than that shown by uranium, without producing appreciable electrical or photographic action.
