XXXIV. Heating Effect of Radium and its Emanation. By Prof. E. Rutherford, F.R.S., and H. Robinson, M.Sc., Demonstrator and Assistant Lecturer in the University of Manchester. [Communicated by the Authors. This paper was read before the KK. Akad. d. Wissenschaft. in Wien, July 4, 1912, and published in the Wien. Ber. October 1912.] Since the initial discovery of the rapid and continuous emission of heat from radium by P. Curie and Laborde in 1903, a number of investigations have been made by various methods to determine with accuracy the rate of emission of heat. Among the more important of these may be mentioned the determination of Curie and Dewar [citation redacted] by means of a liquid air and liquid hydrogen calorimeter ; of Angstrom [citation redacted]; and of Schweidler and Hess [citation redacted] by balancing the (heating effect of radium against that due to an electric current ; and of Callendar [citation redacted] by a special balance method. It is difficult to compare the actual values found on account of the uncertainty as to the relative purity of the radium preparations employed by the different experimenters. The most definite value is that recently obtained by Meyer and Hess [citation redacted] using part of the material purified by Hönigschmid in his determination of the atomic weight of radium. As a result of a series of: measurements, they found that 1 gram of radium in equilibrium with its short-lived products produces heat at the rate of 132*3 gram calories per hour. Rutherford and Barnes [citation redacted] in 1904 made an analysis of the distribution of the heat emission between radium and its products. They showed that less than one quarter of the heat emission of radium in radioactive equilibrium was due to radium itself. The emanation and its products, radium A, B, and C, supplied more than three quarters of the total. The heating effect of the emanation was shown to decay exponentially with the same period as its activity, while the heating effect of the active deposit after removal of the emanation was found to decrease very approximately at the same rate as its activity measured by the [alpha] rays. The results showed clearly that the heat emission of radium was a [header] 313 necessary consequence of the emission of [alpha] rays, and was approximately a measure of the kinetic energy of the expelled [alpha] particles. If this were the case, all radioactive substances should emit heat in amount proportional to the energy of their own radiations absorbed by the active matter or the envelope surrounding them. This general conclusion has been indirectly confirmed by measurements of the heating effect of a number of radioactive substances. Duane [citation redacted] showed that the heating effect of a preparation of polonium was of about the value to be expected from the energy of the [alpha] particles emitted, while the experiments of Pegram and Webb [citation redacted] on thorium and of Poole [citation redacted] on pitchblende showed that the heat emission in these cases was of about the magnitude to be expected theoretically from their activity. It is of great interest to settle definitely whether the heat of radium and other radioactive substances is a direct measure of the energy of the absorbed radiations. Since the emission of the radiations accompanies the transformation of the atoms, it is not a priori impossible that, quite apart from the energy emitted in the form of [alpha], [beta], or [gamma] rays, heat may be emitted or absorbed in consequence of the rearrangements of the constituents to form new atoms. The recent proof by Greiger and Nuttall [citation redacted] that there appears to be a definite relation between the period of transformation of a substance and the velocity of expulsion of its [alpha] particles, suggests the possibility that the heating effect of any [alpha]-ray product might not after all be a measure of the energy of the expelled [alpha] particles. For example, it might be supposed that the slower velocity of expulsion of the [alpha] particle from a long-period product might be due to a slow and long-continued loss of energy by radiation from the [alpha] particle before it escaped from the atom. If this were the case, it might be expected that the total heating effect of an [alpha]-ray product might prove considerably greater than the energy of the expelled [alpha] particles. In order to throw light on these points, experiments have been made to determine as accurately as possible: — (1) The distribution of the heating effect amongst its three quick-period products, radium A, radium B, and radium C (2) The heating effect of the radium emanation. [footer] 314 [header] (3) The agreement between the observed heat emission of the emanation and its products and the value calculated on the assumption that the heat emission is a measure of the absorbed radiations. (4) The heating effect due to the [beta] and [gamma] rays. It was also of interest to test whether the product radium B, which emits no [alpha] rays but only [beta] and [gamma] rays, contributed a detectable amount to the heat emission of the active deposit. Method of Experiment. In order to test these points, it was essential to employ a method whereby rapidly changing heating effects could be followed with ease and accuracy. A sufficient quantity of radium emanation was available to produce comparatively large heating effects. It was consequently not necessary to employ one of the more sensitive methods for measuring small heating effects, such as have been devised by Callendar and Duane. The general arrangement was similar to that employed in 1904 by Rutherford and Barnes for a like purpose. Two equal coils, P, P, fig. 1, about 2*5 cm. long, were made of covered platinum wire of diameter *004 cm. and length 100-300 cm., and wound on thin glass tubes of 5*5 mm. diameter. These platinum coils of nearly equal resistance formed two arms of a Wheatstone bridge, while the ratio arms consisted of two equal coils, M, M, of manganin wire, each of about the same resistance as the platinum coils, and wound together on the same spool and immersed in oil. The platinum coils had a resistance varying between 15 and 45 ohms in the various experiments. The glass tubes on which the platinum coils were wound were placed in brass tubes passing through a water-bath. When a specially steady balance was required, the water-bath was completely enclosed in a box and surrounded with lagging to reduce the changes of temperature to a minimum. In most of the experiments the correction for change of balance during the time of a complete experiment was small and easily allowed for. By means of an adjustable resistance in parallel with one of the coils, a nearly exact balance was readily obtained. A Siemens and Halske moving-coil galvanometer was employed of resistance 100 ohms. This had the sensibility required, and was found to be very steady and proved in every way suitable. [header] 315 The current through the platinum coils never exceeded 1/100 ampere, and was generally about 1/200 ampere. A calibration of the scale of the galvanometer was made by placing a heating-coil of small dimensions of covered manganin wire within one of the platinum coils, and noting the steady deflexion when known currents were sent through it. [figure redacted] It was found that the deflexion of the galvanometer from the balance zero was very nearly proportional to the heating effect of the manganin coil, for the range of deflexion employed, viz. 400 scale-divisions. The deflexion thus served as a direct measure of the heating effect. [footer] 316 [header] §1. Distribution of the Heat Emission between the Emanation and its Products. A quantity of emanation of about 50 millicuries was introduced into a thin-walled glass tube T (fig. 2) connected by a capillary tube C to a small stopcock S. The tube was attached to a mercury-pump by the aid of which the emanation could be purified and compressed into the emanation-tube. The position of the latter was adjusted to lie in the centre of one of the platinum coils. [figure redacted] As it was necessary to leave the emanation for about 5 hours in the tube for equilibrium to be reached with its successive products, it was desirable to arrange that the emanation-tube could be removed outside the water-bath at intervals to test for any change in the balance. The emanation-tube was kept fixed to the pump, but the water-bath was moved backwards along the direction of the axis of the emanation-tube. For this purpose, the water-bath was mounted on metal guides, and was moved backwards or forwards by means of a screw. The general procedure of an experiment was as follows. The balance was adjusted as nearly as possible and the tube, which had been filled with emanation for more than five hours, was introduced within the platinum coil. In about ten minutes a steady deflexion of the galvanometer was obtained, proportional to the heating effect of the emanation. The emanation was then suddenly expanded into the exhausted pump by opening the stopcock S, and condensed in a U- tube by liquid air. The removal of the emanation caused a rapid decrease of the deflexion, followed by a slower decrease due to the decay of activity of the deposit. Observations of the deflexion were continued in some experiments for over two hours. In this time the heating effect had decayed to about 6 per cent, of the initial value. At the conclusion of the experiment, the emanation-tube was removed and the balance point [header] 317 again obtained. Special experiments showed that any change of balance was very slow and regular, so that a correction of the readings for the small change of balance during the observations could be made with accuracy. The readings or the galvanometer were remarkably steady, and observations of the deflexion could be made to about 1/5 of a scale-division. A typical example showing the variation of the deflexion with time for the first 18 minutes after removal of the emanation is shown in fig. 3, curve E + A + B + C. [figure redacted] On account of the lag of the apparatus, the deflexion of the galvanometer at any moment is always greater than corresponds to the heat emission of the emanation-tube. A number of experiments were made to determine the amount of this lag. For this purpose a manganin coil was wound on a glass tube of the same size and thickness as the emanation-tube T and introduced into the platinum coil. A current from an accumulator was sent through this coil so as to give an effect of about the same magnitude as that of the emanation used in the experiments. The circuit was then broken and the decrease of deflexion with time was noted. For this coil the deflexion fell to half value in about 45 seconds, and after that decreased approximately according to an exponential law with a half-value period of about 30 seconds. The lag of the manganin coil was found by experiment to be 318 [header] slightly greater than the lag of the bare emanation-tube. It was found that for a slow decrease of heating effect the deflexion lagged about one minute behind the actual heat, emission. Analysis of the Curve. The curve given in fig. 3 is typical of a number of curves obtained which agreed closely with one another. The relative heating effects of the emanation and its products can be deduced from the observed curve by comparison with the theoretical curve of decay of the components of the active deposit. The heating effect of the emanation itself has practically disappeared three minutes after its removal. The variation of the heating effect of the tube C resulting from the removal of the emanation alone is shown in the dotted side curve CC, where the maximum heating effect is taken as 29 per cent, of the total. This curve was deduced from a knowledge of the cooling curve of the tube under the experimental conditions when heated above its surroundings. After subtracting the heating effect due to the emanation alone, the resulting curve A + B + C gives the heating effect due to radium A-fB + C. After about 20 minutes the heating effect due to radium A has practically vanished, and the effect observed is then due to radium B + C. It was found that the curve after 20 minutes followed closely the theoretical curve to be expected if the heating effect was provided mainly by radium C. Assuming this to be the case, the heating effect due to radium alone is shown by the curve C C, which cuts the axis of ordinates at 40. A lag of 1 minute is assumed between the observed and true heating effects after 20 minutes. The difference of the ordinates of the curves A + B + C, and C C, must be due to the heating effect supplied by radium A. After an initial lag, the heating effect of radium A should ultimately decay exponentially with its known period of transformation, viz. 3 minutes. This is shown clearly in the curve of fig. 4. The difference-curve is plotted, allowing an initial interval of 3 minutes for the emanation effect to decay. Plotting the logarithms of the deflexions as ordinates and the time as abscissae., the curve is a straight line, showing that the heating- effect due to radium A decays exponentially with a half-value period of 3 minutes. The maximum heating effect of radium A was deduced to be 31 per cent, of the total. The variation of heating effect of the emanation-tube with time brings out clearly that about 29 per cent, of the initial [header] 319 heating effect of the tube is due to the emanation alone, 31 per cent, to radium A, and 40 per cent, to radium B+C together. It should be mentioned that it is difficult to deduce [figure redacted] with certainty the exact ratio of the heating effects due to the emanation and radium A on account of small errors in the determination of the lag. It is clear, however, from the experiments that the heating effects are nearly equal. These deductions were verified by another method which avoided the necessity of any correction for lag or for galvanometer or scale errors. The emanation-tube was replaced by a glass tube of the same dimensions, over which was wound a layer of fine insulated manganin wire whose resistance was determined. The variation of the heating effect of the emanation-tube after removal of the emanation was then calculated from the known periods of transformation of radium A, B, and C, assuming that the emanation provided 29 per cent, of the total, radium A, 31 per cent., radium C, 40 per cent The current through the resistance-coil to give a corresponding heating effect was then calculated, and the external resistance to be added in the battery-circuit at any moment deduced. The initial amount through the coil was adjusted to give nearly the same deflexion of the galvanometer as that due to the emanation. By means of a dial resistance-box, the resistance could be rapidly varied to give at any moment the required heating effect. For example, to imitate the removal of the emanation the resistance was 320 [header] suddenly increased so that the heating effect of the current changed to 71 per cent, of the initial amount. In this way the variation of the deflexion of the galvanometer to be expected for the assumed distribution of the heating effect was directly determined. The values obtained are marked by crosses in the main curve fig. 3. It is seen that the observations lie close to the curve throughout the whole range. The lag of the resistance-coil was slightly greater than that due to the emanation-tube, and in consequence the initial points lie slightly above the curve. The agreement between the two curves shows clearly that the assumed distribution of heating effect between the emanation and its products is in close accord with direct experiment. It is seen from the curve fig. 3 that the heating effects due to the emanation and radium A have practically vanished after 20 minutes, and the remaining heating effect is due to radium B and radium C together. A number of experiments were made to test whether radium B provided an appreciable part of the heating effect observed. For this purpose the decay of the heating effect was carefully followed for about three hours after removal of the emanation and the results compared with those to be expected theoretically for any assumed distribution between the heating effects of radium B and radium C. The decay curve observed was found to agree closely with that to be expected if all the heating effect arose from radium C alone. The experiments were rendered difficult by the fact that a small fraction of the emanation adhered to the walls of the emanation-tube, and was gradually released during the time of the experiment. The effect of this became appreciable after two hours, when the heating effect due to radium C had decayed to about 14 per cent. of its initial value. In addition, the method is not very sensitive, for the decay curve over the region examined is not much affected even if radium B provides 5 per cent, of the heating effect of radium C. It was concluded from the observations that radium B could not provide more than 5 per cent, of the heating effect due to radium C; but for the reasons mentioned the actual percentage could not be deduced with any confidence. Agreement of Experiment with (Calculation. The relative heating effects of the emanation, radium A and radium C in equilibrium can be readily calculated if it be assumed that the heating effect is a measure of the kinetic energy of the expelled [alpha] particles. Since the expulsion of [header] 321 an [alpha] particle causes a recoil of the residual atom, the kinetic energy of the latter should he included in the calculation. If m, M be the masses of the [alpha] particle and recoil atom respectively, and u, U the corresponding velocities, [mu] = MU, and the kinetic energy of the [alpha] particle and recoil atom is given by [formula redacted]. The ratio m/M is slightly less than *02, so that the heating effect due to recoil is about 2 per cent, of that due to the [alpha] particle itself. When the emanation is in transient equilibrium with its products, it has been shown [citation redacted] that radium C is in excess of the true equilibrium amount by 0'89 per cent., so that a slight correction is necessary for this factor. The velocity u of the [alpha] particle is deduced from the relation found by Geiger, u3 =.KR, where R is the range in air. Taking the ranges of the [alpha] particles from the emanation and its products given by Bragg and correcting for the factors mentioned -above, it can be simply shown that the heating effect of the emanation in transient equilibrium with its products is distributed as follows: — Emanation ... 28*8 per cent. Radium A ... 30*9 „ Radium C ... 40*3 „ The experimental values observed are 29, 31, and 40 per cent, respectively, and thus appear in good accord with theory. In this comparison no account is taken of the heating effect contributed by the [beta] rays from radium B and radium C. From experiments described later, it appears probable that the [beta] rays from these two products contributed under the experimental conditions about 4 per cent, of the total heating effect. It follows that the percentage of the heating effect included under radium C should be 42'6 per cent, instead of 40-3. As the result of a number of observations, the heating effect due to radium B + C was found to be certainly not greater than 40*5 per cent, and probably nearer 40*0 percent. It thus appears that radium C provides slightly less heating effect than that to be expected theoretically. While the difference between observation and calculation is not large, it may prove to be significant: for in making the calculations no account is taken of the heating effect of the p rays or of a possible small heating effect due to radium B. 322 [header] If the heating effect of radium B were 5 per cent, of that contributed by C, the discrepancy between theory and experiment would be quite marked, and would indicate that the heating effect of a product was not entirely a measure of the energy of the expelled [alpha] particles and the recoil atoms. In order to settle this point with certainty, it would be necessary to isolate radium C from radium B, and to measure accurately its heating effect. It is hoped to continue experiments in this direction, for the question to be settled is of great importance in connexion with the general theory. § 2. Heating Effect of the Radium Emanation. A series of experiments were made to determine accurately the heating; effect of the radium emanation in absolute measure in order to test how far the calculated heating; effect is in agreement with experiment. The general method employed was similar to that described in the earlier part of the paper. A quantity of emanation of 100 to 150 millicuries was concentrated in a small glass tube about 2*2 cm. long, 2 mm. bore, and of thickness 0*2 mm. This was attached to a long thin glass cylinder of small diameter for convenience of handling. In order to calibrate the heating effect observed, a coil of silk-covered manganin wire about 127 cm. long and 41*45 ohms resistance was wound uniformly for a length of 2*2 cm. on a long thin glass tube of 2*5 nun. bore. The heating-coil was of exactly the same length as the emanation-tube, but in order to make sure that the heat distribution was the same for the emanation-tube and for the heating-coil, a copper cylinder 2*7 cm. long and 0'2 mm. thick was placed over the heating-coil. The whole arrangement was placed symmetrically in the glass tube of 5 mm. bore, over which was wound one of the platinum balance coils P (fig. 1). The procedure of an experiment was as follows. The balance of the platinum coils was accurately adjusted and the current through the coils kept constant. The emanation tube in equilibrium with the active deposit was introduced in the platinum coil in a definite position, and the maximum deflexion of the galvanometer observed. A steady deflexion was reached in less than ten minutes. The emanation-tube was then withdrawn by the glass handle, and a known constant current from a storage-cell passed through the manganin coil to give nearly the same maximum deflexion as that due to the emanation. The current was then cut off and the emanation-tube again introduced. Alternate measurements [header] 323 of the heating effect of the emanation and of the current were made for a period of two hours. The emanation-tube was then removed and the change of the balance in the interval determined. The change of balance due to slight alterations of the temperature of the room was usually found to be quite regular and small, and could be easily corrected for if necessary. In order to determine the heating effect, it was necessary to measure accurately the amount of emanation at any moment in the tube and the current! through the heating-coil. The [gamma]-ray effect of the emanation-tube at a definite time was compared in terms of the Rutherford-Boltwood standard by the electroscopic method, and also by the balance method developed by Rutherford and Chadwick. The authors are indebted to Mr. Chadwick for his kind assistance in these measurements. The results obtained by the two methods were in good agreement. A correction of 0*3 per cent, was made for the absorption of the [gamma] rays in the walls of the emanation-tube. The heating effect of the emanation was assumed to decrease exponentially with a half-value period of 3*85 days. This period of decay was verified on several occasions by direct measurement of the heating effect. The current through the heating-coil was determined by measurement of the E.M.F. of the accumulator by a carefully standardized voltmeter, and the total resistance of the circuit. The measurements of the heating effect made with different quantities of emanation were in close accord, and the mean of each series of measurements agreed within 1 part in 500. In this way it was found that the heating effect of a quantity of emanation which gave the same [gamma]-ray effect as one gram of radium (Rutherford-Boltwood standard) was 95*95 + '05 gram calories per hour under the experimental conditions. It is necessary, however, to correct this value to obtain the heating effect of one curie of emanation, i.e. of the quantity of emanation in radioactive equilibrium with one gram of radium. The amount of the products radium A, B,. and C in transient equilibrium with the emanation are somewhat greater than the amounts in equilibrium with the same quantity of emanation which is maintained constant. This point has been discussed by Moseley and Makower [citation redacted], and by Rutherford and Chadwick [citation redacted]. The amount of radium B is 0*54 per cent, and of radium G 0'89 per cent, in excess of the true equilibrium amount. Moseley and Makower showed 324 [header] that under ordinary experimental conditions, radium B provides about 11 per cent, of the [gamma]-ray effect due to the emanation, and radium C 89 per cent. We have seen earlier that radium C contributes about 40 per cent, of the heating effect of the emanation. Taking these factors into account, it can be deduced that the heating effect of the emanation which gives a [gamma]-ray effect equal to that of one gram of radium is about 0*54 per cent, less than corresponds to one curie of emanation in equilibrium with radium. The heat emission of one curie of emanation thus reduces to 98*5 gram calories per hour in terms of the laboratory standard. By the kindness of Professor Stefan Meyer of Vienna, the laboratory standard has been compared in terms of the pure radium salt prepared by Hönigschmid. Expressed in terms of the Vienna standard, the heat emission of one curie of emanation is equal to 103*5 gram calories per hour. In the experimental arrangement the ft rays traversed a thickness of glass, copper, &c., equivalent to a weight of *354 gram per square cm. More than 90 per cent, of the energy of the [beta] rays was absorbed and added its heating effect to that of the [alpha] rays. Heating Effect of the [beta] and [gamma] Rays. Before comparing the observed heating effect with the calculated value, it is necessary to determine how much of the heating effect observed is to be ascribed to [beta] and [gamma] rays. A number of experiments were made to form an estimate of the magnitude of these effects. In the experimental arrangement described the greater part of the [beta] rays was absorbed in the glass tubes, heating-coil, and copper tube surrounding it. The heating effect under these conditions will be taken as 1. Experiments were first made to determine the alteration of the heating effect when a lead cylinder 12 mm. thick, which completely absorbed the [beta] rays and some of the soft [gamma] rays, was substituted for the copper cylinder. As a result of a series of measurements, the heating effect was found to be 1*02. A series of measurements were then made to determine the heating effect of the [gamma] rays. For this purpose about 4 metres of platinum wire were wound on the outside of two similar thin-walled test-tubes of 1*5 cm. diameter. Each of these was inside a metal cylinder of 6*5 cm. diameter, closed at one end and immersed in a water-bath (fig. 5). Each of the test-tubes was filled with mercury so that the mercury extended about 5 mm. above and below the platinum coil. After the balance had been obtained the emanation-tube, surrounded by its heating-coil, as in previous experiments, was fixed in [header] 325 the centre of one of the mercury columns and the steady deflexion of the galvanometer determined. The thickness of mercury traversed by the rays was 4*4 millimetres. On account of the large quantity of mercury and the distance between the platinum coils and the outside cylinder, there was a marked lag between the deflexion and the heating effect. For example, the deflexion reached half its maximum value in ten minutes. [figure redacted] It is difficult to determine the heating effect with the same accuracy as for smaller coils ; but a number of fairly concordant experiments gave a heating effect of 1*034. Similar experiments were made with a thickness of mercury of 1*46 cm. The deflexion in this case reached half its maximum value in 23 minutes. On account of the large lag, the small variations in the balance point during the time of observation became more important. Suitable corrections were made for the lag of the apparatus, and also for the effect of the decay of the emanation. As a mean of several observations the heating effect was found to be about 1*05. It is, however, difficult to fix the value of the heating effect in this case closer than half of one per cent. The heating effect due to the [gamma] rays from radium could be determined with greater accuracy with a preparation of radium in equilibrium, for under such conditions there is no 326 [header] necessity, as in the case of the emanation, to take into account the decrease of the heat emission of the source. It is now necessary to consider what fraction of the energy of the [beta] rays was absorbed in the arrangement of the heating apparatus shown in fig. 1, by which the heating effect of the emanation was accurately determined. Eve [citation redacted] has made an estimate of the relative energy emitted by the [alpha], [beta], and [gamma] rays from one gram of radium on the assumption that the energy of the radiation emitted is proportional to the total ionization produced. By this method he deduced that the [beta] rays from radium contributed about 2 per cent., and the [gamma] rays about 4*5 per cent, of the total heat emission. In his arrangement, however, the radium was enclosed in a glass tube which must have absorbed a considerable fraction of the soft [beta] rays, and the correction for the ionization of these soft rays was uncertain. It was thought desirable to repeat the experiments made by Eve, using the radium emanation in place of radium, and compressing the former in a very thin glass tube, which allowed the [alpha] rays to escape freely. The stopping power of the glass tube corresponded to only 2 cm. of air. Mr. Moseley kindly assisted in these experiments, a more detailed account of which will be published later by Mr. Moseley and Mr. Robinson. The method originally used by Eve is very suitable for the purpose for which it was designed, and was employed in these experiments. The ionization produced by the [beta] and [gamma] rays from the emanation-tube in a thin-walled ionization-chamber supported in the middle of a room was measured for different distances of the tube extending up to 12 metres. The ionization current in the chamber was directly measured by an electrometer using a balance method. Taking the ionization in air due to the [alpha] rays from radium in equilibrium as 100, the total ionization in air due to complete absorption of the [beta] rays was about 3'8, and for the [gamma] rays about 5*2. The ionization due to the [gamma] rays is somewhat greater than that found by Eve, but the ionization due to the [beta] rays is nearly twice as large. In Eve's experiment a large part of the soft [beta] rays was absorbed in the radium tube. By placing over the emanation-tube the copper and glass tubes &c. used in measuring the heating effect of the emanation, it was found that about 85 per cent, of the total ionization due to the [beta] rays was cut out. This was an unexpectedly large fraction, but was confirmed in several experiments. Remembering that the [alpha] rays from the emanation and its products provide about 80 per cent, of the heating effect due [header] 327 to the [alpha] rays from radium in equilibrium, it follows that the heating effect of the [beta] and [gamma] rays absorbed in the experimental arrangement described in § 2 was 4*2 per cent, of that due to the [alpha] rays. This is, of course, based on the assumption that the total ionization produced by the [alpha] and [beta] rays is a measure of their relative energy. From the ionization measurements, it follows that the total heating effect of the [gamma] rays from the emanation should be 6*5 per cent, of that due to the [alpha] rays. It is difficult to estimate with certainty the fraction of the [gamma] rays absorbed by the thickness of mercury of 1*46 cm., but it was probably about 70 per cent. The heating effect of the [gamma] rays should thus be about 4*6 per cent, of that of the emanation. This is in fair accord with the observed increase of heating: effect of: 5 per cent., of which probably about *5 per cent, was due to [beta] rays. It should be pointed out that the increase of the heating effect of 2 per cent, observed when the copper cylinder was replaced by a lead cylinder 1*2 mm. thick, is greater than would be expected from the ionization results. The value was undoubtedly nearly correct, for it was verified in a number of experiments. It is well known that lead shows an abnormal absorption for soft [gamma] rays, and the heating effect observed is no doubt due partly to the absorption of the more penetrating j3 rays and partly to the soft [gamma] rays. The heating effect observed for [gamma] rays is in reasonable accord with the value calculated from the ionization, and indicates that the underlying assumption is not much in error. Since the ionization observed for [gamma] rays is mainly, if not entirely, due to the liberation of [beta] rays from the matter which the [gamma] rays traverse, it seems probable that the ionization method can be used with confidence to estimate also the energy of the [beta] rays. In 1910 Pettersson [citation redacted] made a number of careful observations by balance methods of the heating effect of [beta] and [gamma] rays from a radium preparation. The heating effect of the radium preparation was 116'4 when the rays were absorbed in 4 mm. of lead, and 114*5 when the lead was replaced by aluminium 2 mm. thick. The rays in both cases passed through absorbing material equivalent to 4 mm. of aluminium before entering the lead or aluminium cylinder. From the measurement of ionization, it is clear that the difference is due not to the heating effect of the [beta] rays as assumed by Pettersson, but mainly to the absorption of the [gamma] rays by the lead. It seems certain that nearly all the energy of the [beta] rays is absorbed in traversing aluminium 4 mm. thick. 328 [header] The results obtained for the heating effect of one curie of emanation under various conditions are tabulated below. Heat emission of one curie of emanation in gram calories per hour. Screen Equivalent to 1.3 mm. Of aluminium 0.7 mm. Al + 1.2 mm. Of lead +4.4 mm. Of mercury +14.6 mm. Of mercury [alpha] rays [beta] rays [gamma] rays Total [table redacted] These results are expressed in terms of the Vienna Radium Standard. § 3. Calculation of Heating Effect of Radium and its Emanation. The energy of the [alpha] particles and recoil atoms liberated from one gram of radium or of any of its products in equilibrium with it can readily be calculated. This energy is equal to [formula redacted] for each of the [alpha]-ray products concerned. The value [formula redacted] was directly determined by Rutherford for the [alpha] particle from radium C by measurement of the electrostatic deflexion of the rays, and found to be 4'21 X 10 u . Taking the velocity of an [alpha] particle to be proportional to the cube root of its range the corresponding values of [formula redacted] for radium, emanation, and radium A, are 2*56, 2-95, 3'13 x 10 14 respectively. The masses of the recoil atoms from radium, emanation, radium A, and radium 0, are 222, 218, 214, and 210 respectively. The value of [formula redacted] is consequently slightly less than 1*02 for each product. The value of ne, the total charge carried by the [alpha] particles from one gram of radium itself, has been found by Rutherford and Geiger to be 1*054 x 10~ 9 e.m. unit. Substituting these values, the emission of energy is for one gram of radium in equilibrium 1*38 x 10° ergs per second, and for the emanation in equilibrium with it 1'10 x 10 6 ergs. The corresponding heating effect of one gram of radium [header] 329 for complete absorption of [alpha] particles is 118 gram calories per hour, and for the emanation 94*5 gram calories per hour. These results are expressed in terms of the Rutherford standard, for the value ne depends on this standard. Correcting in terms of the Vienna standard, the corresponding heating effects are 124 and 100 gram calories per hour. We have seen (§ 2) that the heating effect of one curie of emanation on the Vienna standard is 103*5 gram calories per hour Since under the experimental conditions probably 5 per cent, of this is due to the [beta] rays, it is seen that the calculated and measured values for one curie of emanation are in good agreement. St. Meyer and Hess found that the heating effect of one gram of radium in terms of the Vienna standard was 132*3 gram calories per hour. This includes the heating effect of the [beta] rays and 15 per cent, of the [gamma] rays. This is in excellent accord with the value deduced from the observed heating effect of the emanation. The heating effect of one curie of emanation surrounded by 1*2 mm. of lead was 105*5 gram calories per hour. The heating effect due to [alpha] particles and recoil atoms is probably about 98*5 gram calories. The theoretical rates of the heating effects for radium in equilibrium compared with its emanation is 1*255. The heating effect of one gram of radium surrounded by 1*2 mm. of lead thus comes out to be 123*6 + 7 = 130*6 gram calories per hour. Allowing a small correction for the extra absorption of [gamma] rays in the Vienna experiments, this value is in close accord with that found by Meyer and Hess. Too much stress should not be laid on the agreement of the calculated value of the heating effect of radium with that deduced experimentally, for the data used in the calculations are not fixed with the accuracy required. For example, the calculation depends on the accuracy of the values [formula redacted] and n.e. The former was determined by measuring the electrostatic deflexion of the [alpha] rays. A combination of the value so found with the value mu/e found by deflexion of the same rays in a magnetic field gave a value e/m = 5'01 x 10 3 . There is now no doubt that the [alpha] particle is a helium atom carrying two charges, and the value of e/m should be 4*84 x 10 3 . Taking the value[formula redacted] found by Rutherford for the [alpha] particle from radium C as correct, and assuming [formula redacted] , the value [formula redacted] instead of the value 4*21 xlO 14 used in the calculation. Taking the new [footer] 330 [header] value, the calculated heating effect is reduced 5 per cent., and the agreement between calculation and experiment is not so good. In order to settle this point, experiments are now in progress to redetermine the values of u and e\m of the [alpha] particle from radium C. From the data already given, the distribution of the heating effect between radium and its products and radiations is given below. Heating effect in gram calories per hour corresponding to one gram of radium. [alpha] rays. [beta] rays. [gamma] rays. Total. Radium [citation redacted] Emanation Radium A Radium B Radium C [table redacted] It follows that the total heating effect of one gram of radium for complete absorption of the [alpha], [beta], and [gamma] rays should be about 135 gram calories per hour per gram on the Vienna standard. University of Manchester, 1912.