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Originally appearing in Volume V22, Page 799 of the 1911 Encyclopedia Britannica.
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TIME IX DAYS 4 roe 80 2. presence of radium in a body which contains only 10-11 gram of radium. With care, 10 12 gram can just be detected. This emanation method has been employed with great success in measuring the quantity of radium in minerals and in rocks. A very simple method has been devised of'determining the quantity of radium present when it is not less than 1/1 oo milligram. The tube containing the radium is placed some distance from an electroscope which is surrounded by a lead screen about 3 mms. thick. This cuts off the a and [3 rays and the effect in the electroscope is then due to the penetrating 7 rays. By comparison of the rate of discharge with that of a standard preparation of radium at the same distance, the quantity of radium can at once be deduced, provided the radium is in equilibrium with its emanation. This is usually the case if the radium preparation is one month old. This method is simple and direct, and has the great advantage that the radium tube under test need not he opened, nor its contents weighed. We shall see later that the amount of radium in an old mineral is always proportional to the amount of uranium present. Rutherford and Boltwood (8) found that 3.4 parts of radium by weight are present in ten million parts of uranium. Consequently an old mineral containing loco kilos of uranium should contain 340 milligrams of pure radium. In addition to radium and polonium, a number of other radioactive substances have been found in uranium minerals. With the exception of the radium emanation, none of these have yet been isolated in a pure state, although preparations of some of them have been obtained comparable in activity with radium itself. Debierne (9) found a radioactive substance which was separated from pitchblende with the rare earths and had chemical properties similar to those of thorium. This he called actinium. Giesel (lo) independently noted the presence of a new radio-active substance which was usually separated with lanthanum and cerium from the minerals. It possessed the property of giving out a radioactive emanation or gas, the activity of which died away in a few seconds. For this reason he called it the emanating substance and afterwards emanium. Later work has shown that emanium is identical in chemical and radioactive properities with actinium, so that the former name will be retained. We have already seen that Mme Curie gave the name polonium to a radioactive substance separated with bismuth. Later Marckwald found that a very radioactive substance was de-posited from a solution of a radioactive mineral on a polished bismuth plate. The active matter was found to be deposited in the bismuth with tellurium, and he gave the name radio-tellurium to this substance. In later work, he showed that the new substance could be chemically separated from tellurium. By treating the residues from 15 tons of Joachimsthal pitchblende, Marckwald (If) finally obtained 3 milligrams of intensely active material—far more active weight for weight than radium. It has been definitely settled that the active substance of Marckwald is identical with polonium. Both substances give out a type of easily absorbed a rays and both lose their activity at the same rate. The activity of polonium decays in a geometrical progression with the time and falls to half its initial value in 14o days. This law of decay, as we shall see, is characteristic of all radioactive products, although the period of decay is different in each case. Mme Curie and Debierne (12) have described further experiments with polonium. The latter substance was extracted from several tons of pitchblende and purified until 2 milligrams of material were obtained containing about 1/10 milligram of pure polonium. From a knowledge of the relative periods of transformation of radium and polonium, it can be calculated that the amount of polonium in a radium mineral is 1/5000 of the amount of radium, while the activity of pure polonium measured by the a rays should be 5000 times greater than that of radium. As we have seen, polonium is rapidly transformed, and it is of great interest to determine the nature of the substance into which polonium changes. We shall see later that there is considerable evidence that polonium changes into lead. Recently Boltwood (13) has separated another substance from uranium minerals which he has called " ionium." This substance is sometimes separated from the mineral with actinium and has chemical properties very similar to those of thorium. Preparations of ionium have been obtained several thousand times as active as uranium. Ionium emits a rays of short range and has a period of transformation probably much longer than that of radium. Ionium has a special interest inasmuch as it is the substance which changes directly into radium. A preparation of ionium initially free from radium grows radium at a rapid rate. Hofmann found that the lead separated from uranium minerals and named it radiolead. The active constituent in the lead is radium D, which changes into radium E' and then into radium F (polonium). Both radium D and radium F are products of the transformation of radium. In addition to these radioactive substances mentioned above, a large number of other radioactive substances have been discovered. Most of these lose their activity in the course of a few hours or days. The properties of these substances and their position in the radioactive series will be discussed later. Radiations from Radioactive Substances.—All the radioactive substances possess in common the property of emitting radiations which darken a photographic plate and cause a discharge of electrified bodies. Very active preparations of radium, actinium and polonium also 'possess the property of causing strong phosphorescence in some substances. Bodies which phosphoresce under X rays usually do so under the rays from radioactive matter. Barium platinocyanide, the mineral willemite (zinc silicate) and zinc sulphide are the best known examples. There are in general three types of radiation emitted by the radioactive bodies, called the a, /3 and y rays. Rutherford (2) in 1899 showed that the radiation from uranium was complex and consisted of (a) an easily absorbed radiation stopped by a sheet of paper or a few centimetres of air which he called the a rays and (b) a far more penetrating radiation capable of passing through several millimetres of aluminium, called the /3 rays. Later Villard found that radium emitted a very penetrating kind of radiation called the y rays capable of passing before absorption through twenty centimetres of iron and several centimetres of lead. Giesel and, later, Curie and Becquerel showed that the /3 rays of radium were deflected by a magnetic field. By the work of Becquerel and Kaufmann the 3 rays have been shown to' consist of negatively charged particles projected with a velocity approaching that of light, and having the same small mass as the electrons set free in a vacuum tube. In fact the Q rays are electrons spontaneously ejected from the radioactive matter at a speed on an average much greater than that observed in the electrons set free in a vacuum tube. The very penetrating 7 rays are not deflected in a magnetic or electric field and are believed to be a type of radiation similar to X rays. The y rays are only observed in radioactive substances which emit 3 rays, and the penetrating power of the y rays appears to be connected with the initial velocity of expulsion of the a rays. Two general theories have been advanced to account for the properties of these rays. On one view, they rays are to be regarded as electromagnetic pulses which have their origin in the expulsion of the 3 particle from the atom. On the other hand Bragg has collected evidence in support of the view that they rays are corpuscular and consist of uncharged particles or " neutral doublets." There is as yet no general consensus of opinion as to the true nature of the 7 rays. Rutherford (14) showed in 1903 that the a rays were deflected in a powerful magnetic or electric field. The amount of deflection is very small compared with the /3 rays under similar conditions. The direction of deflection in a magnetic field is opposite to that of the /3 rays, showing that the a rays consist of a stream of positively charged particles. A pencil of rays from a thick layer of radioactive matter is complex and consists of particles moving at varying velocities If, however, a thin film of radioactive matter of one kind is taken, . the particles which escape without absorption are found to be homogeneous and consist of particles projected at an identical speed. Observations of the velocity and mass of the particle have been made by Rutherford. The general method employed for this purpose is similar to that used for the determination of the velocity and mass of the electron in a vacuum tube. The deflection of a pencil of rays in a vacuum is determined for both a magnetic and electric field. From these observations the velocity and value e/m (the ratio of the charge carried by the particle to its mass) are determined. The value of elm has been found to be the same for the particles from all the types of radioactive matter that have been examined, indicating that the a particles from all radioactive substances are identical in mass. The value of e/m found for the a particle is 5.07 X ro3. Now the value of elm for the hydrogen atom set free in the electrolysis of water is 966o. On the assumption that the value of the charge e is the same for the a particle as for the hydrogen atom, the value would indicate that the a particle has about twice the mass of the hydrogen atom, i.e. has the same mass as the hydrogen molecule. If the charge on the a particle is twice that on the hydrogen atom, the value of elm indicates that the a particle is a helium atom, for the latter has an atomic weight of four times that of hydrogen. It was difficult at first to decide between these and other hypo-theses, but we shall show later that there is now no doubt that the a particle is in reality a helium atom carrying two elementary charges. We may consequently regard the a rays as a stream of helium atoms which are projected from a radio-active substance with a high velocity. The maximum velocity of the a particle from radium is 2 X Io9 ems. per second, or one-fifteenth of the velocity of light. Although the a rays are the least penetrating of the radiations, it will be seen that they play an extremely important part in radioactive phenomena. They are responsible for the greater part of the ionization and heating effects of radioactive matter and are closely connected with the transformations occurring in them. Under ordinary experimental conditions the greater part of the ionization observed in a gas is due to the a particles. This ionization due to' the a rays does not extend in air at atmospheric pressure for more than 7 cros. from radium, and 8.6 ems. from thorium. If a screen of aluminium about .or ems. thick is placed over the active material, the a rays are completely absorbed, and the ionization above the screen is then due to the (3 and •y rays alone. If a layer of lead about 2 mms. thick is placed over the active material, the (3 rays are stopped, and the ionization is then due almost entirely to the penetrating 7 rays. By the use of screens of suitable thickness we are thus able to sift out the various types of rays. These three types of radiations all set up secondary radiations in passing through matter. A pencil of /3 rays falling on matter is widely scattered in all directions. This scattered radiation is some-times called the secondary (3 rays. They rays give rise to secondary rays which consist in part of scattered 7 rays and in part electrons moving with a high velocity. These secondary rays in turn produce tertiary rays and so on. The impact of the a rays on matter sets free a number of slow moving electrons which are very easily deflected by a magnetic or electric field. This type of radiation was first observed by J. J. Thomson, and has been called by him the 5 rays. Emanations or Radioactive Gases. In addition to their power of emitting penetrating radiations, the substances thorium, actinium and radium possess another very striking and important property. Rutherford (i5) in 'goo showed that thorium compounds (especially the oxide) continuously emitted a radioactive emanation or gas. This emanation can he carried away by a current of air and its- properties tested apart from the substance which produces it. A little later Dorn showed that radium possesses a similar property, while Giesel and Debierne observed a similar effect with actinium. These emanations all possess the property of ionizing a gas and, if sufficiently intense, of producing marked photographic andphosphorescent action. The activity of , the radioactive gases is not permanent but disappears according to a definite law with the time, viz. the activity falls off in a geometric progression with the time. The emanations are distinguished by the different rates at which they lose their activity. The emanation of actinium is very shortlived, the time for the activity to fall to half value, i.e. the period of the emanation, being 3.7 seconds. The period of the thorium emanation is 54 seconds and of the radium emanation 3.9 days. This property of emitting an emanation is shown in a very striking manner by actinium. A compound of actinium is wrapped in a sheet of thin paper and laid on a screen of phosphorescent zinc sulphide. In a dark room the phosphorescence, marked by the characteristic scintillation, is seen to extend on all sides from the active body. A puff of air is seen to remove the emanation and with it the greater part of the phosphorescence. Fresh emanation immediately diffuses out and the experiment may be repeated indefinitely. The emanations have all the properties of radioactive gases. They can be transferred from point to point by currents of air. The emanations can be separated from the air or other gas with which they are mixed by the action of extreme cold. Rutherford and Soddy (16) showed that under ordinary conditions the temperature of condensation of the radium emanation mixed was – 15o° C. The emanations are produced from the parent matter and escape into the air under some conditions. Rutherford and Soddy (i7) made a systematic examination of the emanating power of thorium compounds under different conditions. The hydroxide emanates most freely, while in thorium nitrate, practically none of the emanation escapes into the air. Most of the compounds of actinium emanate very freely. Radium compounds, except in very thin films, retain most of the emanation in the compound. The occluded emanation can in all cases be released by solution or by heating. On account of its very slow period of decay, the emanation of radium can be collected like a gas and stored, when it retains its characteristic properties for a month or more. Induced Activity.—Curie (i8) showed that radium possessed another remarkable property. The surface of any body placed near radium, or still better, immersed in the emanation from it, acquires a new property. The surface after removal is found to be strongly active. Like the emanations, this induced activity in a body decays with the time, though at quite a different rate from the emanation itself. Rutherford (19) independently showed that thorium possessed a like property. He showed that the bodies made active behaved as if a thin film of intensely active matter were deposited on their surface. The active matter could be partly removed by rubbing, and could be dissolved off by strong acids. When the acid was evaporated the active matter remained behind. It was shown that induced activity was due to the emanations, and could not be produced if no emanation was present. We shall see. that induced activity on bodies is due to a deposit of non-gaseous matter derived from the transformation of the emanations. Each emanation gives a distinctive active deposit which decays at different rates. The active deposits of radium, thorium and actinium are very complex, and consist of several types of matter. Several hours after removal from the emanation the active deposit from radium decays to half-value—z6 minutes, for actinium half-value—34 minutes, for thorium half-value—10.5 hours. The active deposits obtained on a platinum wire or plate are volatilized before a white heat, and are again de-posited on the cooler bodies in the neighbourhood. Rutherford showed that the induced activity could be concentrated on the negative electrode in a strong electric field, indicating that the radioactive carriers had a positive charge. The distribution of the active deposit in a gas at low pressure has been investigated in detail by Makower and Russ. Theory of Radioactive Transformations.—We have seen that the radioactive bodies spontaneously and continuously emit a great number of a and ,B particles. In addition, new types of radioactive matter like the emanations and active deposits appear, and these are quite distinct in chemical and physical properties from the parent matter. The radiating power is an atomic property, for it is unaffected by combination of the active element with inactive bodies, and is uninfluenced by the most powerful chemical and physical agencies at our command. In order to explain these results, Rutherford and Soddy (20) in 1903 put forward a simple but comprehensive theory. The atoms of radioactive matter are unstable, and each second a definite fraction of the number of atoms present break up with explosive violence, in most cases expelling an a or R particle with great velocity. Taking as a simple illustration that an a particle is expelled during the explosion, the resulting atom has decreased in mass and possesses chemical and physical properties entirely distinct from the parent atom. A new type of matter has thus appeared as a result of the transformation. The atoms of this new matter are again unstable and break up in turn, the process of successive disintegration of the atom continuing through a number of distinct stages. On this view, a substance like the radium emanation is derived from the transformation of radium. The atoms of the emanation are far more unstable than the atoms of radium, and break up at a much quicker rate. We shall now consider the law of radio-active transformation according to this theory. It is experimentally observed that in all simple radioactive substances, the tensity of the radiation decreases in a geometrical progression with the time, i.e. I/Io = e-' where I is the intensity of the radiation at any time t, Io the initial intensity, and X a constant. Now according to this theory, the intensity of the radiation is proportional to the number of atoms breaking up per second. From this it follows that the atoms of active matter present decrease in a geometrical progression with the time, i.e. N/No=e-"where N is the number of atoms present at a time t, No the initial number, and X the same constant as .before. Differentiating, we have dNT/dt= -AN,i.e. A represents the fraction of the total number of atoms present which break up per second. The radioactive constant A has a definite and characteristic value for each type of matter. Since A is usually a very small fraction, it is convenient to distinguish the products by stating the time required for half the platter to be transformed. This will be called the period of the product, and is numerically equal to log s2/A. As far as our observation has gone, the law of radioactive change is applicable to all radioactive matter without exception. It appears to be an expression of the law of probability, for the average number breaking up per second is proportional to the number present. Viewed from this point of view, the number of atoms breaking up per second should have a certain average value, but the number from second to second should vary within certain limits according to the theory of probability. The theory of this effect was first put forward by Schweidler, and has since been verified by a number of experimenters, including Kohlrausch, Meyer, and Begener and H. Geiger. This variation in the number of atoms breaking up from moment to moment becomes marked with. weak radioactive matter, where only a few atoms break up per second. The variations observed are in good agreement with those to be expected from the theory of probability. This effect does not in any way invalidate the law of radioactive change. On an average the number of atoms of any simple kind of matter breaking up per second is proportional to the number present. We shall now consider how the amount of radioactive matter which is supplied at a constant rate from a source varies with the time. For clearness, we shall take the cffse of the production of emanation, by radium. The rate of transformation of radium is so slow compared with that of the emanation that we may assume without sensible error that the number of atoms of radium breaking up per second, i.e. the supply of fresh emanation, is on the average constant over the interval required. Suppose that initially radium is completely freed from emanation. In consequence of the steady supply, the amount of emanation present increases, but not at a constant rate, for the emanation is in turn breaking up. Let q be the number of atoms of emanation produced by the radium per second and N the number present after an interval t, then dN/dt = q-AN where A is the radio-active constant of the emanation. It is obvious that a steady state will ultimately be reached when the number of atoms of emanation supplied per second are on the average to the atoms which break up per second. If No be the maximum number, q=AN0. Integrating the above equation, it follows that N/No= i - e -Ai. If a curve be plotted with N as ordinates and time as abscissae, it is seen that the recovery curve is complementary to the decay curve. The two curves for the radium emanation period, 3.9 days, are shown in fig. r, the maximum ordinate being in each case roo. This process of production and disappearance of active matter holds for all the radioactive bodies. We shall now consider some special cases of the variation of the amount of active matter with time which have proved of great importance in the analysis of radioactive changes. (a) Suppose that initially the matter A is present, and this changes into B and B into C, it is required to find the number of atoms P, Q and R of A, B and C present at any subsequent time t. . Let A1, As, as be the constants of transformation of A, B and C respectively. Suppose n be the number of atoms of A initially present. From the law of radioactive change it follows: P =ne-'fit dQ/dt=X1P —X,Q (I) dR/dl=X2Q—X3R . (2) Substituting the value of Pin terms of n in (i ), dQ/dt =aine'alt-X2Q ; the solution of which is of the form Q=n(ae Ai +be A2t), where a and b are constants. By substitution it is seen that a=a1/(A2—Xi). Since Q=o when t=o, b=—X1/(N2—A1) Thus Q=~nT-s(e Alt-e Alt) (3) Similarly it can be shown that R=n(ae Ait-}-be —}—be—kg Fce A3t) (4) where a = alas b = T'~2 = ~1~2 (X1-X2)(Xi-as) (Xz-X1)(a2- c 7~s)' (A3-Ai) (As-a2). It will be seen from (3), that the value of Q, initially zero, increases to a maximum and then decays; finally, according to an exponential law, with the period of the more slowly transformed product, whether A or B. (b) A primary source supplies the matter A at a constant rate, and the process has continued so long that the amounts of the products A, B, C have reached a steady limiting value. The primary source is then suddenly removed. It is required to find the amounts of A, B and C remaining at any subsequent time t. In this case of equilibrium, the number no of particles of A supplied per second from the source is equal to the number of particles which change into B per second, and also of B into C. This requires the relation no =X1Po =Y2Qo =AsRo where Po, Q0, R. are the initial number of particles of A, B, C present, and a1, X2, As are their constants of transformation. Using the same quotations as in case (I), but remembering the new initial conditions, it can easily be shown that the number of particles P, Q and R of the matter A, B and C existing at the time t after removal are given by P=i e alt, Q ( — , a T2t-e xit , Al-T2 \^s / R n°(ae-Ait+be A2t-rce-'st), X2 -Al X1A2 where a= (x1-A2)(Xi-Xs)' b (Xi -Xs)(Xs-Xs)' c As(X1-X3)(A2-As)' The curves expressing the rate of variation of P, Q, R with time are in these cases very different from case (i). (c) The matter A is supplied at a constant rate from a primary source. Required to find the number of particles of A, B and C present at any time t later, when initially A, B, and C were absent. This is a converse case from case (2) and the solutions can be obtained from general considerations. Initially suppose A, B and C are in equilibrium with the primary source which supplied A at a constant rate. The source is then removed and the amounts of A, B and C vary according to the equation given in case (2). The source after removal continues to supply A at the same rate as before. Since initially the product A was in equilibrium with the source, and the radioactive processes are in no way changed by the removal of the source, it is clear that the amount of A present in the in which the matter is distributed is unchanged. If Pi be the amount of A produced by the source in the time t, and P the amount remaining in the part removed, then P1+P=P0 where P, is the equilibrium value. Thus PI/Po = i -P/P0. The ratio P/P0 can be written down from the solution given in case (2). Similarly the corresponding values of Ql/Qo, R1/Ro may be at once derived. It is obvious in these cases that the curve plotted with P/Po as ordinates and time as abscissae is complementary to the corresponding curve with PI/P0 as ordinates. This simple relation holds for all recovery and decay curves of radioactive products in general. We have so far considered the variation in the number of atoms of successive products with time when the periods of the products are known. In practice, the variation of the number of atoms is deduced from measurements of activity, usually made by the electric method. Using the same notation as before, the activity of any product is proportional to its rate of breaking up, i.e. to X1P where P is the number of atoms present. If two products are present, the activity is the sum of two corresponding terms X1P and X2Q. In practice, however, no two products emit a or 13 particles with the same velocity. The difference in ionizing power of a single a particle from the two products has thus to be taken into account. If, under the experimental conditions, the ionization produced by an a particle from the second product is K times that from the first product, the activity observed is proportional to X1P+KX2Q. In this way, it is possible to compare the theoretical activity curves of a mixture of products with those deduced experimentally. Analysis of Radioactive Changes.—The analysis of the successive changes occurring in uranium, thorium, radium and actinium has proved a very difficult matter. In order to establish the existence of a new product and to fix its position in the scheme of changes, it is necessary to show (a) that the new product has a distinctive period of decay and shows some distinctive physical or chemical properties; (b) that the product under consideration arises directly from the product preceding it in the scheme of changes, and is transformed into the product succeeding it. In general, it has been found that each product shows some distinctive chemical or physical behaviour which allows of its partial or complete separation from a mixture of other products. It must be remembered that in most cases the amount of radio-active matter under examination is too small to detect by weight, but its presence is inferred from its characteristic radiations and rate of change. In some cases, a separation may be effected by ordinary chemical methods; for example thorium X is separated from thorium by precipitation of thorium with ammonia. The Th X remains in the filtrate and is practically free from thorium. In other cases, a separation is effected by a separation of a metal in the solution of active matter. For example, polonium (radium F) always comes down with bismuth and may be separated by placing a bismuth plate in a solution. Radium C is separated from radium B by adding nickel filings to a solution of the two. Radium C is deposited on the nickel. In other cases, a partial separation may be effected by electrolysis or by differences in volatility when heated. For example, when radium A, B and C are deposited on a platinum plate, on heating the plate, radium B is volatilized and is deposited on any cold surface in the neighbourhood. A very striking method of separating certain products has been recently observed depending upon the recoil of an atom which breaks up with the expulsion of an a particle. The residual atom acquires sufficient velocity in consequence of the ejection of an a particle to escape and be deposited on bodies in the neighbourhood. This is especially marked in a low vacuum. This property was independently investigated by Russ and Makower (21) and by Hahn (22). The latter has shown that by means of the recoil, actinium C may be obtained pure from the active deposit containing actinium A, B and C, for B emits a rays, and actinium C is driven from the plate by the recoil. In a similar way a new product, thorium D, has been isolated. By the recoil method, radium B may be separated from radium A and C. The recoil method is one of the most definite and certain methods of settling whether an a ray product is simple or complex. While in the majority of cases the products break up either with the emission of a or g particles, some products have been observed which do not emit any characteristic radiation and have been called " rayless products." For example, radium D andthorium A are changing substances which break up without emitting either penetrating a or ,d rays. They appear to emit slow b rays which can only be detected by special methods. The presence and properties of a rayless product can be easily inferred if it is transformed into a product emitting a radiation, for the variation in activity of the latter affords a method of determining the amount of the parent product present. The distinction between a " ray " and a " rayless " product is not clear. It may be that the atom of a rayless product undergoes a re-arrangement of its constituent parts giving rise to an atom of the same mass but of different properties. In the case of an a ray or ,(3 ray product, the expulsion of an a or particle affords an obvious explanation of the appearance of a new product with distinctive physical properties. In the table a list of the known products of transformation is given. In each case, the half period of transformation is given and the type of radiation emitted. If the product emits a rays, the range of ionization of the a particle in air is given. Half Period Range Product. of Rays. of Rays Transforma- in Air in tion. Cms. URANIUM— 5 X lo' years a 3'5 Uranium X 22 days 0+7 Ionium . ? a 2.8 RADIUM— 176o years a 3'5 Ra Emanation. 3.86 days a 4'33 Radium A 3 mins. a 4'83 Radium B 26 mins. slow ,B 7.06 Radium C 19 mins. a+ +y Radium D 17 years slow # Radium E 5 days 13 Radium F 140 days a 3.86 Radium G =lead? about IOl° yrs. 3.5 THORIUM— (Th. I) 5'5 years rayless Mesothorium (Th. 2) 6.2 hours /3+7 Radiothorium . 737 days a 3'9 Thorium X 3.6 days a 5.7 Th Emanation 54 secs a 5'5 Thorium A Io•6 hours slow 13 5.0 Thorium B 55 mins. a Thorium C very short? a 8.6 Thorium D 3 mins. 13+7 ACTINIUM— ? rayless Radioactinium . 19.5 days a-FP 4.8 Actinium X 11.8 days a 6.55 Act Emanation 3.7 secs. a 5.8 Actinium A 36 mins. slow 13 5.50 Actinium B 2.15 mins. a Actinium C 5.1 mins. 13+7 In each of the groups under the heading uranium, thorium and actinium, each product is derived from the direct transformation of the product above it. Products of Radium.—Radium is transformed directly into the emanation which in turn goes through a rapid series of trans-formations called radium A, B and C. The complete analysis of these changes has involved a large amount of work. The emanation changes first into radium A, a substance of period 3 minutes emitting only a rays. Radium A changes into radium B, a product of period 26 minutes emitting 13 rays of penetrating power small compared with those emitted, from the next product radium C. The product radium C has proved of considerable importance, for it not only emits very penetrating a rays and rays, but is the origin of the 'y rays arising from radium in equilibrium. When a wire charged negatively has been exposed for some time in the presence of the radium emanation, it becomes coated with an invisible film of radium A, B and C. After removal from the emanation for 20 minutes, radium A has present in the active deposit, but no chemical separation of B and C has yet been found possible. Hahn has shown that thorium D—a 13 ray product of period 3 minutes—can easily be separated by the recoil method. A special interest attaches to the product thorium X (30), which was first separated by Rutherford and Soddy, since experiments with this substance laid the foundation of the general theory of radioactive trans-formations. A close analysis of thorium has led to the separation of a number of new products. Hahn (31) found that a very active substance emitting a rays, which gave rise to thorium X, could be separated from thorium minerals. This active substance, called radiothorium, has been closely examined by Hahn and Blanc. Its period of decay was found by Hahn to be about 2 years, and by Blanc to be 737 days. From an examination of the activity of commercial thorium nitrate of different ages, Hahn showed that another product must be present, which he called mesothorium. This is separated from thorium with Th X by precipitation with ammonia. Thorium is first transformed into the rayless product mesothorium, of period about 5 years. This gives rise to a 13 ray product of quick transformation, which in turn changes into radiothorium. This changes into thorium X, and so on through a long series of changes. When isolated in the pure state, radiothorium would have an activity about a thousand times greater than radium, but would lose its activity with time with a period of about 2 years. Mesothorium, when first separated, would be inactive, but in consequence of the production of radiothorium, its activity would rapidly increase for several years. After reaching a maximum, it would finally decay with a period of five years. Since a large amount, of thorium is separated annually from thorium minerals, it would be of great importance at the same time to separate the radiothorium and mesothorium present. For many purposes active preparations of these substances would be as valuable as radium itself, and the amount of active matter from this source would be greater than that at present available from the separation of radium from uranium minerals. Actinium.—The transformations observed in actinium are very analogous to those in thorium. Actinium itself is a rayless product which changes into radioactinium, an a ray product of period 19.5 days, first separated by Hahn (32). This changes into actinium X, of period 10.2 days, first separated by Godlewski (33). Actinium X is transformed into the emanation which in turn gives rise to three further products, called actinium A, B and C. Although very active preparations of actinium have been prepared, it has so far not been found possible to separate the actinium from the rare earths with which it is mixed. We do not in consequence know its atomic weight or spectrum. Origin of Radium.—According to the transformation theory, radium, like all other radioactive products, must be regarded as a changing element. Preliminary calculations showed that radium must have a period of transformation of several thousand years. Consequently in order that any radium could exist in old minerals, the supply must be kept up by the transformation of some other substance. Since radium is always found associated with uranium minerals, it seemed probable from the, beginning that uranium must be the primary element from which radium is derived. If this were the case, in old minerals which have not been altered by the action of percolating waters, the ratio of the amount of radium to uranium in a mineral must be a constant. This must evidently be the case, for in a state of equilibrium the rate of breaking up of radium must equal the rate of supply of radium from uranium. If P, Q be the number of atoms of uranium and radium respectively in equilibrium, and X1, X their constants of change, then
End of Article: TIME IX

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