LXXVII. The Question of the Homogeneity of [gamma]-Rays. By F. and W. M. Soddy, and A. S. Russell, 1851 Exhibition Scholar and late Carnegie Scholar, University of Glasgow [Communicated by the Authors.]. Part I., by F. and W. M. Soddy and A. S. Russell. Part II., by F. and W. M. Soddy. Part III., by A. S. Russell. [Plate XII.] Introduction. IN the first systematic work on the absorption of the [gamma]-rays of radium [citation redacted], it was found that the absorption coefficient Xof the exponential equation [citation redacted] tended to diminish as thickness increased, especially for the denser substances, as though the rays were somewhat heterogeneous. The following values were given for lead : — From 0*8 to 1*05 cm. 0*64:1, from 1*05 to 1*3 cm. 0*563, from 1*3 to 1*8 cm. 0'480, from 1*8 to 2*3 cm. 0-440. Wigger [citation redacted] showed that absorption proceeded exponentially, the absorption coefficient being proportional to the density of the substance, if the ray- were first passed through 2"8 cm. of lead, but an 726 [header] Unfortunate arithmetical mistake in calculating the absorption coefficients caused him to assign values for these somewhat less than one-half the real value. In consequence of this a state of chaos exists in much of the literature of the subject. Quite recently Tuomikoski has published absorption curves of the radium [gamma]-rays in lead from 0*4 to 18 cm. in which the value of X decreased from 0'70 to 0*25. Still further support of the heterogeneity of the [gamma]-rays of radium is to be found in papers by Kleeman [citation redacted] and Madsen [citation redacted] who claim to have analysed the rays into three and two homogeneous radiations respectively of different penetrating power. On the other hand, Sir J. J. Thomson has worked out the theory of absorption of the [gamma]-rays, assuming that scattering takes place as in the case of the cathode and [beta]-rays, and has shown that departures of the absorption curves from the simple exponential law are to be expected if scattering takes place even for a homogeneous beam. [citation redacted]. With regard to the [gamma]-rays of uranium, Eve [citation redacted], working with uranyl nitrate drew attention to the relative poverty of uranium in [gamma]-rays as compared to [beta]-rays. He found the uranium [gamma]-rays were absorbed approximately exponentially in lead with a value for X 1'4, which is about three times the value for the radium [gamma]-rays, In our previous paper [citation redacted] the [gamma]-radiation of uranium X preparations from about 45 kilograms of uranyl nitrate was exhaustively examined. The [gamma]-rays w r ere naturally far more powerful than those at Eve's disposal, and in addition the rays had not first passed through a considerable thickness of material as in Eve's experiments with uranyl nitrate. We found the absorption did not proceed exponentially in any substance until a thickness equivalent to 1 cm. of lead had been penetrated and then the absorption coefficient remained constant (and, except for lead, mercury, and very light substances, proportional to the density) at least until 5 cm. of lead or the equivalent had been penetrated. The value of [lambda] for substances of density between copper and aluminium was only 1*18 times the corresponding value for radium [gamma]-rays, while lead was somewhat anomalous with the ratio 1*465. The value of [lambda] found for lead for the radium [gamma]-rays was 0'495 from 1 cm. to 9 cm. The results for the initial part of the absorption curves up to a thickness equivalent to 1 cm. of lead were reserved for the present communication. It was obvious that the whole question of the absorption of the [header] 727 [gamma]-rays needed careful re-examination before any conclusion as to their heterogeneity or homogeneity could be formed, and that the shape o£ the curves and values o£ the absorption coefficients depended greatly on then unknown disturbing influences. A quantitative comparison of the ratio of the intensity of the [beta]- to [gamma]-radiation for uranium and radium showed that the [gamma]-rays were relatively about 60 times less intense for uranium than for radium, and on account of the similarity of the penetrating power of the radiations of the two elements it was pointed out that the [beta]- and [gamma]-rays could hardly be regarded any longer as interdependent phenomena. In Part I. of the present communication the absorption curves of uranium [gamma]-rays for various substances up to thickness equivalent to 1 cm. of lead are given and discussed. It is concluded that although for certain metals, for example zinc, the curve can be represented as the sum of two positive exponential terms, one due to the "hard " [gamma]-rays previously studied and another due to a " soft " [gamma]-ray with absorption coefficient about 14 times greater, this explanation does not hold good for other substances. In any case the supposed soft [gamma]-radiation is relatively feeble and unimportant. In Part II. it is shown that with certain new experimental dispositions the [gamma]-rays of radium are absorbed by lead as a homogeneous non-scattered radiation with a constant value for the absorption coefficient [formula redacted] in the strict sense of the equation [formula redacted] over the whole range up to 7*7 cm. In these experiments the ionization vessel takes the form of two hemispheres with the radium at the centre and the absorbing plates are in the form of truncated hemispheres, this being a disposition for which the theoretical expressions can be evaluated. For zinc hemispheres again some evidence of a secondary penetrating radiation generated in the zinc, with a value for A, about 45 times greater than for the primary, has been obtained. The uranium [gamma]-ray curves show, however, small departures from the theoretical over the first part of the range. In Part III. further evidence of the homogeneity of the [gamma]-rays of radium is given. The departures of the curve from the simple exponential type both at the beginning and the end of the range in lead are shown to be due to disturbing factors. With regard to the departures at the end of the range (Tuomikoski) a simple exponential law has been found, with suitable means of measurement, to hold up to 22 cm. of lead with a value for [lambda], 0*498, practically identical with that given in the earlier paper. These results and others in this paper show that the [gamma]-rays are capable of measurement 728 [header] to a very high degree of accuracy under proper conditions. The whole laboratory in which the measurements have been carried out has been preserved scrupulously from contamination, and no radium, except in sealed tubes, has ever been brought in. With regard to the departures at the initial part of the range, it is shown that these are in opposite senses for different materials and vary greatly with the disposition employed. Although these departures have not yet been fully cleared up it is probable that they are due to equilibrium between the primary and secondary radiations, probably as regards distribution in space, not having yet been attained for the initial thickness of absorbing material. Confining attention to the [gamma]-rays of radium and lead for which most evidence is available it is difficult to escape from the conclusion that the rays are homogeneous and are exponentially absorbed, without scattering of the primary beam. In metals other than lead, softening and scattering of the primary [gamma]-rays may, it is true, occur. But this does not affect the important conclusion that initially the primary [gamma]-rays are probably homogeneous. Now there is considerable evidence that the [beta]-rays of radium are not homogeneous. Even if the very soft [beta]-rays due to radium itself and to radium B [citation redacted] are left out of consideration as possibly too feebly penetrating to produce a detectable [gamma]-radiation, there is evidence that the [beta]-radiation of radium itself is also complex [citation redacted]. The view that the [gamma]-rays of radium are homogeneous thus carries with it very strong support of our earlier view that the [beta]- and [gamma]-rays are not interdependent, It is not advocated that this point of view is as yet established but merely that a considerable body of evidence exists in its favour. We are now at work to see if Prof. Bragg's theory that some metals like zinc soften and scatter the [gamma]-rays before converting them into [beta]-rays while others like lead transform them into [beta]-rays at one step, will prove helpful in unravelling some of the intricate phenomena observed. It appears, however, if this is so, that zinc will be found to behave to the [gamma]-rays of uranium like lead to the [gamma]-rays of radium, Part I.-Initial part of Absorption Curves of the [gamma]-Rays of Uranium X. The disposition employed was identical with that used in Series I. of the earlier measurements [citation redacted], all the absorbing plates being clamped up to form the base of the [header] 729 electroscope, the uranium X preparations being 14*63 cm. below the upper surface of the base. The curves are shown in fig. 1 (PI. XII.). The ordinates represent logarithms of the ionization and the abscissae the thickness multiplied by density of material, i. e. what may be termed equivalent thickness. The ordinates have been corrected for the decay of the activity of the preparation during measurements and so the curves show the approximate relative magnitude of the ionization when equivalent thicknesses of various materials form the base of the electroscope. The actual results are plotted and no attempt has been made to smooth the curves. In the case of lead part of the irregularities are doubtless due to irregularities in the thickness of the only foil then available. It will be seen that the curves arrange themselves one above the other in order of the density of the material, the lighter substances giving for equivalent thicknesses far more ionization than the denser. The secondary radiation produced by the primary [gamma]-rays of: uranium X from the emergent surface of the absorbing plates obviously increases as the density of the material diminishes. According to the neutral-pair theory of [gamma]-rays the emergent radiations should all be equal if the density law of absorption holds true for both [beta]- and [gamma]-rays, and if the latter are homogeneous [citation redacted]. The dotted lines in the curve represent the prolongation back to the zero axis of the straight part of the absorption curves over the range at which absorption is exponential. The lead and iron dotted lines are from actual observations taken at the same time as the others with greater thicknesses of metal than can be shown on the curve. Unfortunately in other cases it was not possible to do this. The zinc dotted line is drawn with an arbitrary ordinate, from the value of [lambda], 0*37. before found. It will be seen that the curves differ from each other progressively as the density of the material decreases. For zinc the curve is not very different from what would be the case if it consisted of two superimposed curves, one due to the hard [gamma]-rays and a second due to a softer type with a coefficient of absorption about 14 times greater. But for the lighter materials, for example cardboard and aluminium, this is not the case. With diminishing thickness the slope of the curve at first is greater than that of the hard [gamma]-rays, then diminishes and runs for a little practically parallel to it, and then increases again. But just before the [beta]-rays begin to enter the electroscope a well-marked diminution of slope is again noticeable in the card curve. Attention may first he directed to the dotted lines. These 730 [header] represent the primary hard [gamma]-rays capable o£ penetrating 1 cm. o£ lead, together with the equilibrium amount of secondary radiation they produce in the absorbing material. I£ only hard [gamma]-rays were present, on the simple theory [citation redacted], assuming the ionization produced by secondary and primary to be directly proportional to their absorption coefficients, one would expect the following relation to hold: — [formula redacted] Here I T and I refer to the ionizations, [lambda]1 and [lambda] 2 to the coefficients of absorption of the primary and secondary radiations respectively and k is the coefficient of transformation, or fraction of the energy of absorbed primary transformed into secondary. For great thicknesses the second negative exponential term becomes negligible if [lambda] 2 is greater than [lambda]1 . The two radiations are in equilibrium and the relation becomes : — [formula redacted] This is the equation of the dotted-line curves. Since for thicknesses greater than 1 cm. nf lead the curves are exponential it may be assumed that [lambda] 2 is always much greater than [lambda]1 . The relation indicates that when the radiations are in equilibrium the ionization is due to the two parts, one, [formula redacted], due to the primary and the other, [formula redacted] due to the secondary. Now for different substances k may possess any value between and 1, and since [formula redacted]. is not very much greater than unity, it follows that for equivalent thicknesses of various materials the ionization cannot vary very much more than in the ratio of 1 to 2. So far as can be seen the results certainly bear this out. At equivalent thicknesses 56 — the greatest examined for card — the ratio of the ionizations of lead and card are 2'15, and, at 2'6, of pinewood and lead are 2'5. A similar 2:1 ratio in the emergence radiation from different elements for the [gamma]-rays of radium is to be seen in Bragg and Madsen's curves [citation redacted]. It was not possible with the lighter substances to extend the curve sufficiently far to give all the information required, for the source of radiation was too weak to be removed to a greater distance to allow of greater thicknesses of absorbing material being used. A [header] 731 prolonged attempt to interpret the curves obtained led to no very definite conclusion, partly perhaps on this account. If the ionizations shown by the dotted curves are subtracted from the observed values one would expect to obtain a curve having the relation [formula redacted] where C and [lambda] 3 refer to the soft primary [gamma]-radiation, if such exists. The zinc curve and to lesser extent the lead and iron curves so obtained are nearly straight lines. Either k, and hence B, must be very small (which cannot be true at least for iron as the dotted-line curve shows) or [lambda]2 must be relatively very large. In other words the secondary radiations are wholly [beta]-rays and secondary [gamma]-rays play no appreciable part. The zinc curve can be approximately represented by the equation [formula redacted] where [formula redacted] and C 0*7. The following table shows that the agreement between the observed and calculated values is fairly close. The deviations are not large but they are greater than the errors of measurement. [table redacted] The ratio [formula redacted] is 14, and if the energies of the two types of rays are proportional to the ionizations divided by the absorption coefficients it follows that the energy of the soft type initially is only one-sixth of that of the hard type. Even if such a soft type exists, therefore, in the [gamma]-radiation of 732 [header] uranium X, it must be unimportant relatively to the hard type. The curves on the whole, however, do not bear out the view of the existence of a soft primary radiation. It is possible by choosing the values of [lambda] 3 and [lambda] 3 near together to get a more or less close approximation of the curves for the lighter substances with the theoretical equation consisting of the sum of three exponential terms, two positive and one negative, but only by using values of B inconsistent with the dotted-line curves. Experiments were performed with the uncovered uranium X preparation between the poles of a powerful electromagnet to see if [beta]-rays had any influence on the results and to explore the curves over smaller values of the equivalent thickness in the hope of getting evidence of the negative exponential term. The electroscope rested on a thick cast-iron plate supported as a table above the magnet, over a circular hole cut in the plate, to the under side of which the absorbing plates were clamped up. Using just sufficient card or lead completely to absorb [beta]-rays it was found there was no difference in the value of the leak whether the magnet was excited or not. The same was true when card of equivalent thickness 4*4 was used. Hence the [beta]-rays do not affect the results, or produce any appreciable secondary [gamma]-radiation. The dotted part on the curve (fig. 1) indicates the reading obtained for card over the region usually masked by the [beta]-rays. The ordinate of the curve is naturally arbitrary. It must be remembered that [beta]-rays are never completely deviated by a magnet [citation redacted], although with the arrangement used they were reduced probably to less than one per cent, of their initial value. The results are therefore not of great value. For the metals sufficiently accurate results to be of service could not be obtained owing to the heat of the magnet disturbing the readings of the electroscope. In light of the results to be given in the succeeding sections it seems that the distribution of the emergent radiation as regards direction in space must be taken into account in the interpretation of such [gamma]-ray absorption curves. Bragg and Madsen [citation redacted] have suggested that the differences between light and heavy metals are due to the soft type of [gamma]-radiation, supposed to be present initially, being more readily absorbed, relatively to the hard type, by dense than by light substances, the absorption causing a proportional production of soft [beta]-radiation. The soft [gamma]-radiation is thus used up rapidly in lead, [header] 733 whereas in aluminium it is not used up so fast and it produces less secondary radiation. But such'a soft [beta]-radiation will always be in equilibrium with the soft [gamma]-radiation, except over the part of the curve not investigate owing to primary [beta]-rays. The logarithmic curves obtained by subtracting the effect due to the hard type from the observed effects should be straight for aluminium no less than for lead, whereas it is obvious that they are not. All that the explanation, if we understand it aright, would seem to involve is that the slopes of the curves, plotted against equivalent thickness, should be greater for dense than for light materials. Part II. — Absorption of [gamma] Rays in truncated Hemispheres. H. W. Schmidt [citation redacted] has given the solution of the problem of the absorption of a homogeneous radiation from a point source uniformly distributed in all directions, in a plate of uniform thickness placed directly over the source, assuming that the absorption proceeds exponentially as with light. His paper and the theoretical absorption curve he gives suggested at first sight that the departure of the experimental curves from the exponential form might be due, not to any want of homogeneity of the rays, but to the obliquity of part of the beam. This, however, as will be shown, is not the case. The error so introduced is trifling with the disposition employed, the found absorption coefficient being about 25 per cent. greater throughout the whole range than the true theoretical coefficient for a parallel beam of rays. Schmidt solved the problem in connexion with his study of [beta]-rays, to which it will be shown the solution does not apply owing to scattering. Experiments, however, on the absorption of the [gamma]-rays with some new dispositions for which the absorption according to the theoretical expressions can be calculated, have strongly confirmed the view that the [gamma] rays of radium are homogeneous and that a soft primary type does not exist, although still much remains to be cleared up. Unfortunately, in Schmidt's paper the accidental omission of a factor in the final expression makes the solution appear false, and we are indebted to Sir Joseph Larmor for solving the problem for us independently. A plate of infinite area and thickness t is in contact with a point source sending out radiation uniformly in all directions which are absorbed 734 [header] exponentially by the plate, the absorption coefficient being represented by [lambda] in the sense of the equation [figure redacted] It is required to find the quantity of radiation after it has passed through the plate. Let E be the intensity of the radiation at any point. Let I and I T , respectively, be the measure of the total radiation over the angle of 180°, with the absorbing plate absent and present. Within a solid angle [delta][omega] the intensity of the radiation is [formula redacted] with no plate present, and E . [formula redacted] with the plate. [formula redacted] The integral [formula redacted] is known as the Exponential Integral. It is expressed by the symbol [formula redacted]. Values both for [formula redacted] and [formula redacted] have been tabulated by J. W. L. Glaisher [citation redacted]. The result may [header] 735 thus be written : — [formula redacted] This is Schmidt's result, the factor [lambda]T in the second term having been omitted through an oversight. The graph of the function is shown by the lower curve in fig. 2 (PI- XII.), the lower curve representing [formula redacted]. The upper curve represents the value of [formula redacted], according to the simple exponential law. It will be seen that the curve somewhat resembles the experimental curve, the slope at first decreasing until a certain initial thickness has been penetrated, after which it remains nearly constant. As Schmidt has pointed out for values of [lambda]T greater than 5 the slope is about 1*1 6 times greater than for the simple exponential curve. As drawn, the slope over the range from [formula redacted] to , i. e., for a thickness of lead from 4 to 8 cm., is very nearly 1*25 times the slope of the exponential curve. The above solution, however, applies only to the case where the cone of rays included subtends an angle of 180°. The expression holding for a cone of any semi-angle 6 is, however, readily obtained. If I ' and 1 T ' represent respectively the quantities of radiation in a cone of semi-angle [theta], before and after passage through the absorbing plate, [formula redacted] In the experimental disposition described with cylindrical electroscope 1 2*8 cm. high and 10*5 cm. in diameter, and the preparation 8'25 cm. below the upper surface of the base, the base and top of the electroscope subtend respectively cones of semi-angle about 22° and 14°. The mean semi-angle may be taken as about 18°. A particular case was 736 [header] 21 worked out for [formula redacted]. In the following table the values of [formula redacted], and [formula redacted], the latter calculated from the simple exponential formula, are contrasted for various values of [lambda]T :— [table redacted] In the last column [formula redacted] represents the relation of the apparent to the true absorption coefficient, [lambda]' being the value that would be obtained for the absorption coefficient if calculated for the two values of [lambda]T indicated in the table according to the simple exponential law instead of from equation (5). It will be seen, therefore, that the general effect of using a cone of rays of semi-angle 18° is that the absorption coefficient experimentally obtained is from 2\ to 2 per cent, too high throughout the whole of the range. Thus the obliquity of part of the beam is completely insufficient to account for the initial irregularities of the absorption curves. Diagram B. [figure redacted] Equation (4) assumes the plate to be of infinite area, and it is scarcely possible working with the [gamma]-rays of radium to [header] 737 use plates so large that no radiation gets through the edges.. If instead of plates of infinite area we use truncated hemispheres, the part of the radiation escaping from the sides of the truncated hemisphere can be allowed for. The total initial radiation I over the whole 180° is divided into two parts, S and P, to represent the portions escaping: through the sides and top of the truncated hemisphere. Then [formula redacted] where T is the thickness of a truncated' hemisphere of radius R. Then [formula redacted] Equation (6) has been tested experimentally with hemispheres of zinc and lead built up of circular plates of diminishing radius from base to top, the radioactive preparation being placed at the centre of the base. Fig. 3 shows the apparatus employed. The ionization chamber consisted of two copper concentric hemispherical bowls, A and B, of diameter 40 cm. and 20 cm. respectively with the absorbing hemisphere C concentrically placed within the inner bowl. The wall thickness of B was about 0*5 mm. The radium was placed at the centre of C at the point marked D. Thus the paths of all rays over the whole 180° within the ionization chamber were equal. The ionization chamber was fixed to the under side of a table E, carrying the electroscope F and reading microscope. The base of the electroscope was protected by a circular block of lead, Gr, 5 cm. thick, of the same diameter as the electroscope, through central holes in which was placed an insulating plug H bearing the electrode K. The latter consisted of a construction of wires similar to an open umbrella frame at its lower end, which carried on its upper end a gold leaf. J was an additional lead plug. The absorbing hemisphere rested on a platform which could be raised or lowered at will. The dotted lines show the hemisphere in its lowered position. The platform consisted [footer] 738 [header] of a block of lead L, 10 cm. diameter and 5 cm. thick, in a small hole in which the radium tube was placed at D, so that the radium was in the centre and as near the surface as possible. For experiments with uranium X another similar [figure redacted] platform was used, with a rectangular depression 8 cm. x 4 cm. x 5 mm. deep, capable of taking the three platinum trays on which the uranium X was spread in the form of thin films. The apparatus was designed so that there should be as little material as possible round the base and sides of the ionization chamber, to produce secondary [gamma]-rays [citation redacted]. But owing to [header] 739 the weight o£ the absorbing hemispheres it was found necessary to support the platform by four T rods of iron MM running horizontally close beneath the base of the ionization chamber (which consisted of J inch sheet brass), and these probably produced a little secondary radiation capable of penetrating the base of the ionization chamber. Any such secondary radiation would, however, be constant, and would merely increase the value to be subtracted from the readings as " natural leak." The results with the lead hemisphere and the [gamma]-rays of radium bear out, in a remarkably close manner, equation (6), the value of A, being 0*5. This is practically the value before found, viz., 0495, and is identical with what, as the results of subsequent experiments given in Part III., has been found lead to hold accurately up to a thickness of 20 cm. of lead. The hemisphere was 7*7 cm. diameter. Fig. 4 (PL XII.) shows the results obtained up to the first 2 cm. of lead, and fig. 5 the results for the complete hemispheres with both lead and zinc. Dealing first with lead, the curves drawn are the theoretical curves calculated according to equation (6), with [formula redacted]. Fig. 4 may be first considered, as it is very remarkable. The observational points lie close on the curve, almost from the point at which all the [beta]-rays are absorbed. Instead of the large irregularity in the initial part of the curve, as with ordinary dispositions, there is only a very slight departure from the theoretical curve, the slope of the curve initially being a very little greater than the calculated. In fitting the observations to the curves the following procedure was adopted : — By repeated trial with different values of [lambda], that value was found which best fitted the results as a whole. In this way [lambda] was easily determined to about 2 per cent. Then in drawing fig. 4, one observation was put on the theoretical curve somewhere about the middle and the other points plotted from this point as the basis. Dealing now with fig. 5 for the whole lead hemisphere, only the logarithmic curve is shown. Measurements were done with 0*47 and and 6*7 mg. of radium, and showed virtually no difference. To fit the results to the curve and to find the correction for secondary radiation, which at the end of the curve is of some consequence, two observational points, one at the end and the other near the middle, were arbitrarily placed on the theoretical curve [formula redacted]. A simple calculation then showed what the amount was that had to be subtracted from the observations to allow for secondary radiation, assuming the theoretical law of absorption held true, and this amount was subtracted from all the measurements. It has also been [footer] 740 [header] subtracted in fig. 4, but there it is too small to be o£ any significance. With the 0*47 mg. radium this correction amounted to 1*78, and with the 6*7 mg. radium to 30*6. The corrected leak with the whole hemisphere in was 3*85 for the 0'47 mg., and 51*8 for the 6*7 mg. The natural leak was 2' 8 in most of the experiments. This shows that the correction is very small and is nearly proportional to the quantity of radium. The curve (fig. 5) shows that the observational points lie on the theoretical curve with very great accuracy. The gratifying result follows that according to these experiments the [gamma]-rays of radium are exponentially absorbed by lead as a single homogeneous radiation, with the value of the absorption coefficient 0*5, from the thickness at which the [beta]-radiation is completely absorbed up to 7*7 cm. This is so completely at variance with the result obtained with ordinary dispositions, and the conclusions of many investigators who claim to have evidence of the existence of two and even three types of homogeneous [gamma]-rays from radium, that it may be regarded as a very remarkable result. It also follows that the [gamma]-rays are not scattered at all in passage through matter. The sharpness and delicacy of [gamma]-ray photographs, when precautions are taken to remove [beta]-rays by a magnet, also bear this out [citation redacted] . To indicate the difference between the case of the [gamma]-rays and that of the [beta]-rays, which are well known to be scattered by thin films of absorbing materials, it is of interest to contrast the preceding result with some obtained with the [beta]-rays of uranium X, also in a hemispherical ionization chamber. A copper hemisphere, 16 cm. in diameter, was mounted above an electroscope as shown in fig. 6, with the absorbing plates clamped on to the top. The uranium X preparation, usually on a very small thin piece of micro-cover glass, was laid on the centre of the top plate, and could be covered at will with thick disks of zinc, lead, or aluminium, when it was desired to examine also the " reflected " radiation. Sometimes the uranium X was placed directly on the centre of the uppermost plate. To compare with these results others were taken in a brass electroscope of ordinary cylindrical pattern and size with the absorbing sheets clamped up to the base. The preparation was supported 9 cm. below the electroscope on a square of micro-cover glass 11 cm. sq. and 0*125 mm. thick by means of a light frame attached to the table on which the electroscope rested. The design was intended to minimize the reflected radiation as much as possible. The preparations [header] 741 were in the form of films upon micro-cover glass. Sometimes this was placed on the supporting square of micro-cover glass below the preparation. Sometimes the preparation was fastened beneath the support, film downward, so [figure redacted] that there was nothing below, but the rays had to penetrate 0'25 mm. of glass before entering the electroscope. Thick plates of lead, zinc, and aluminium could be clamped up immediately beneath the preparation at will. The curves for lead, for the hemispherical electroscope (fig. 6) only, are shown in fig. 7 (PI. XII.). The middle curve refers to the bare preparation with nothing below. The upper curve was obtained by placing close above the preparation a disk of lead to reflect back the rays. The lower curve is the difference curve, referring to the reflected radiation only. The middle curve is practically a straight line, if anything slightly concave to the origin. There is thus nothing abnormal about this result due to the use of the hemispherical ionization vessel and the cone of rays of angle 180°. Unfortunately no actual comparisons were done for lead, but for zinc and aluminium the results in the hemispherical apparatus were compared with those obtained with the ordinary disposition with cylindrical electroscope described. The curves are shown in figs. 8 & 9 (PI. XII.). In fig. 8, for zinc, the curves A, C, E refer to the hemisphere, the curves B, D, F to the cylinder. As before, the middle curves of each set C and D refer to the bare preparation, the upper A and B to the preparation with a thick plate of lead covering 742 [header] it, and the lower E and F to the reflected radiation only. There is practically very little difference between the curves C and D, though one refers to a comparatively narrow cone and the other to a cone of angle 180°. Fig. 9 shows similar results for aluminium, the lettering of the six curves referring to the same disposition as in the last figure. Here the hemisphere curves A and C are markedly more straight than the cylinder curves B and D, which, as is well known, are strongly concave to the axis [citation redacted], the absorption coefficient, indeed, increasing 2*5 times before all the rays are absorbed. So the general result of these experiments is to show that, on the whole, the simple exponential law is wore nearly followed with [beta]-rays when a hemispherical ionization chamber and cone of rays of angle 180° are used than in the common disposition. This sufficiently illustrates the great difference between the [beta]- and [gamma]-rays. In view of the controversy that is in progress as to the nature of the law of absorption of [beta]-rays, these results are of interest as showing how artificial the mathematical treatment of the question is which assumes a rectilinear propagation of the rays. The passage of [beta]-rays through matter probably resembles more a diffusion than a radiation, and before much real advance can be expected to be made it would seem necessary to obtain some information of the free-path of the [beta]-particle in various metals. With the apparatus shown in fig. 3 several other measurements of the [gamma]-rays both of uranium X and radium, for zinc hemispheres as well as lead, have been taken. Figs. 5 and 10, PL XII., show the result for radium with a zinc hemisphere 9 cm. in radius. For the latter two thirds of the hemisphere the curve agrees remarkably closely with the theoretical curve (X = 0*28). This again is identical with the value (0'278) given in Table II. of the last paper. When [lambda] was put equal to 0*30 the agreement was far less perfect. But for the first part of the curve the observed readings fall consistently below the theoretical. The lowest curve in fig. 10 is the difference curve between the observed and theoretical values of It, and it will be seen to be very approximately exponential up to the point at which [beta]-rays interfere. The value of the coefficient [lambda]' of these supposed secondary rays is 1*25, or 4*5 times that of the primary [citation redacted]. This result thus suggests that for zinc a secondary radiation is generated which does not come into equilibrium with the primary until about 2 cm. of zinc have been penetrated. Again there is no evidence of a soft primary [gamma]-radiation. The remaining two curves (fig. 11, [header] 743 PI. XII.) refer to the uranium X [gamma]-rays for lead and zinc. The fact that the uranium X was in the form of a large surface 5 mm. below the base of the hemisphere somewhat complicates the interpretation of these results. This would have been fatal for the radium [gamma]-rays, but for the less penetrating rays of uranium X it is probable that the disturbance so produced is not very great. In addition it was not found practicable to determine the curves over more than the first 2 cm. of the hemisphere, so that the position of the curve with reference to the theoretical is more or less arbitrary. This makes their interpretation rather uncertain. As drawn, both curves agree well over the latter half with the theoretical curve ([formula redacted] and [formula redacted]), while for the first half the observed values are consistently greater than the theoretical, the departure being greater for zinc than lead. The curves would not at present repay further discussion. All that can be said is that they bear out the view before expressed that if a soft [gamma]-radiation of uranium X exists it must be relatively feeble in comparison with the primary, while at least in the radiation of uranium X there is as yet no sufficient evidence either of its existence or of that of a secondary penetrating radiation. It has been thought advisable to append the observations, from which the [gamma]-ray curves in PI. XII. figs. 4, 5, 10, and 11 have been obtained, in the following two tables. The observations represent divisions of the scale per minute corrected only for the natural leak of the instrument. Table I. Pb and Zn Complete Hemispheres. Ra [gamma]-rays. See PI. XII. fig. 5. [table redacted] 744 [header] Table II. Pb and Zn Hemispheres (Initial Parts). Ra and UrX [gamma]-Rays. (See PI. XII. figs. 4, 10, and 11.) [table redacted] Part III. — The Variations in the Value of the Absorption Coefficient of Radium [gamma]-rays. In the former paper the absorption of the [gamma]-rays of radium by lead over a range of thickness of 1 cm. to 9 cm. was examined. The absorption was found to be strictly exponential, the value of X being 0*495 (cm.) -1 . This investigation has now been pushed further by employing much larger quantities of radium than before, to see if the exponential absorption held over as large a range as could be investigated accurately. In the first of the new experiments the same [gamma]-ray lead electroscope was used and the same method of measurement employed as had been used previously. 6*7 mg. of radium bromide was placed immediately below the electroscope at a distance of 21*5 cm. from the upper surface of the base. The lead used for absorbing the rays was in the form of circular plates 12'5 cm. in diameter and about 1*25 cm. thick. [header] 745 These plates were placed directly on the radium. The range of thickness of lead placed over the radium was from to 18*6 cm. Up to 11 cm. the exponential law of absorption held true. The value of X was 0'500 (cm.) -1 . Beyond 11 cm., however, the curve continued no longer straight but became convex to the origin (fig. 12, PI. XII.). The departure of the curve from the straight line occurred when the intensity leak (corrected for natural leak, which was 3'6 divisions per minute) was about one division per minute. This departure must be due, either to the presence of a very penetrating primary radiation from the radium, or to a constant secondary radiation entering the electroscope otherwise than through the base, and which, being very small, did not begin to affect the straightness of the curve until very great thicknesses of lead had been penetrated. The next experiment was carried out very similarly to the first except that an electroscope of brass was substituted for the lead one and no circular lead screens round the electroscope were used. The electroscope was a cylindrical one of the ordinary type, 13 cm. in height and 10*8 cm. in width. The thickness of the walls was 0*32 cm. and of the base 0*6 cm. The lead plates were placed on the radium as before (which was placed in a recess in a piece of wood) and a range of to 14 cm. of lead investigated. The curve obtained in the same way as the last is no longer straight over any part of the range but is convex to the origin, X varying from 0*62 (cm.) _1 to 0'08 (cm.) -1 over the range. The results are shown in fig. 13, curve A (PI. XII.). The difference between the character of this curve and of the last it would be natural to ascribe to the nature of the material of which the electroscope is made as the other factors of the disposition have been unchanged. Experiments as to the cause of this difference have shown, however, that it is due as much to the nature of the absorbing plates as to that of the material of which the electroscope is made. These will now be described shortly. A base 0'8 cm. was substituted for the 0'6 cm. base and the walls were increased in thickness to 1*8 cm., but the general convexity of the absorption curve was not affected. It was found impossible with this disposition to obtain the straight line relation by thickening the brass electroscope with brass. A series of measurements was next done under exactly similar conditions as last except that the two circular screens of lead before used were placed around the electroscope. The effect of the lead screens on the character of the curve was very marked (fig. 13, curve B). The curve which had been convex before became now straight over a large part of the 746 [header] range but finally became convex as before. The value of the leak at this point of departure from the straight line (T = 5*5 cm.) is about 10 divisions per minute. The departure in this case is not due to the same small effect as had caused the departure in the first experiments (fig. 12). Some measurements to illustrate the effect of the circular screens on the radiation are given in Table III. Table III. Thickness of Lead. Intensity, no lead screens Intensity, with lead screens [table redacted] Thus at the end of the range the radiation entering the brass electroscope is reduced to 0*37 of its value by merely placing lead screens round the side of the electroscope. Similar screens of brass of equivalent thicknesses were substituted for the lead but they had practically no effect on the leak in the electroscope. It may be noticed that the difference in the values in columns 2 and 3 of Table J. is practically constant (7*4 divisions per minute), as though caused by a constant reflected or secondary radiation entering the instrument, which is cut down by lead but which is capable of penetrating brass. Such a type of radiation has already been indicated by Kleeman [citation redacted]. The origin of this effect was sought next. With disposition otherwise identical, the radium was now mounted not in a block of wood but was placed in a shallow groove in a lead plate of the same diameter as the absorbing plates and with 3'5 cm. thickness of similar plates below. When the absorbing plates are now put on the radium it is surrounded in every direction by at least 1 cm. of lead. The absorption curve now obtained was exponential over the range 1 to 10*5 cm. (\ = 0'50). The screens of lead round the brass electroscope have now practically no effect, a very small difference between the leaks with or without lead round the electroscope being due to secondary [gamma]-radiation reflected from the wood of the stand, the existence of which has been established by Kleeman. This latter secondary [gamma]-radiation is quite distinct from the peculiar secondary radiation under discussion . [header] 747 Experiments were now conducted in the middle of the laboratory and the apparatus was set up so as to reduce secondary radiation effects to a minimum. Fig. 14 shows the disposition. The radium, mounted in a block of wood 2, [figure redacted] was placed on a wooden table 5. Over the radium were placed absorption plates of lead or of copper 1, and on the top of 1 was placed the electroscope. Two circular screens of metal could be placed either round the electroscope 4, or round the wooden stand and absorption screens 3. Throughout the experiments 4 was of lead and the electroscope of brass. 5 could be changed by lining the table with some other material than wood, and 3 could be varied in nature and in thickness by placing plates and circular screens of various bodies in position. The results are summarized in Table IV. (p. 748). For different thicknesses of 1 (lead) the value of the differences in leaks A and B, C and D, E and F varies, but in general it was about 5 to 10 per cent, when experiments were conducted in the centre of the laboratory and 10 per cent, and more when the electroscope was held in position by a stand of wood. Leaks H, G, B, and D are the smallest and correspond to the ionization due to the absorption of the primary [gamma]-rays only. A and C are the greatest, and correspond to the maximum amount of secondary radiation obtainable by the disposition employed, plus that due to the primary [gamma]-rays. The values of E and F depend on whether the thickness of 3 (lead) is great enough to ensure perfect E = F = G if this thickness be 2 cm., but 748 [header] E>F>G if thickness be only 0-3 cm. From VIII. of Table IV. it may be seen that if brass is used as absorbent instead of lead the variation of the other factors of the disposition have no effect on the value of the leak. Table IV. Disposition. Nature of the Effects. [table redacted] From these observations the following results have been deduced. The process of absorption of the [gamma]-rays by lead causes a secondary radiation, probably an incidence radiation, to be produced by the primary in the metal. This radiation is absorbed by the lead also and so cannot emerge from the lead in appreciable amount. In the experiments described in Table IV. the radium is inserted between thicknesses of lead and of wood. Through the wood this peculiar radiation escapes, and when it has escaped it can penetrate with ease [header] 749 every body tried of density less than that of lead, the absorption coefficient per cm. of equivalent thickness for brass, for instance, being at least 5 times less than that for lead. In the experiments in which the electroscope was mounted on a wooden stand this radiation is reflected from it through the sides of the electroscope. In those conducted without such a stand the radiation is reflected from the wood of the table, and after reflexion it still retains its capacity for penetrating brass. The placing of much lead, brass, or copper on the table cuts down the amount of this radiation, showing that reflexion from wood is an essential part of the phenomenon. It may be seen from Table IV. that this radiation may be prevented from masking absorption results by cutting it off, (1) at its point of production by surrounding the radium entirely by a sufficient thickness of lead, (2) at its point of reflexion by covering the table with lead, or (3) at the point it enters the electroscope by using either a thick electroscope of lead or by placing a 0*5 cm. circular screen of lead around one made of some other metal having a thick lead base. The thickness of lead required in (1) to prevent the escape of the radiation was found to be about 05 cm. A thickness sufficient to ensure the complete absorption of the [beta]-rays of radium made no appreciable difference. The effect cannot therefore be due to a secondary [gamma]-radiation generated by the action of [beta]-rays upon lead. It may be emphasized here that this peculiar radiation is not caused by the action of an untransformed primary radiation on a wood reflector, for if it were, the same or a similar effect would be given when brass absorbing plates were substituted for lead in I. (Table IV.). The important point about this peculiar radiation is that it appears to be produced only as an incidence radiation by lead, and that it is very penetrating to brass, retaining this power of penetration even after reflexion from wood. In addition to this radiation, secondary [gamma]-rays capable of penetrating the brass electroscope are generated by the action of primary emergent radiation from all bodies covering the radium when it falls upon wood, glass, and magnesia brick. Copper and brass produce such rays to a much less degree, while lead produces no appreciable amount. These points can be demonstrated by placing a thick plate of glass &c. vertically on the table and measuring the leak with it present and absent, taking care that none of the rays produced can get through the windows rather than the walls of the electroscope. Like the peculiar radiation described, these rays also are incapable of penetrating 0*5 cm. of lead. Tuomikoski, working with a very strong source of radium emanation, has shown [citation redacted] that the 750 [header] [gamma]-rays of radium are practically exponentially absorbed between a range of 2 2 to 12'0 cm. of lead (X=0*51), but that from 12*0 to 18*0 the value of [lambda] decreases continuously. The work described above suggests that the cause of this decreasing value of [lambda] is not due to the heterogeneous character of the [gamma]-rays, as has been supposed, but that the peculiar secondary radiation is giving an increasing effect relative to that due to the primary [gamma]-rays as the thickness of the absorbing material is increased. Tuomikoski worked with an electroscope of aluminium, which behaves towards this radiation very like brass. In the light of these results, experiments on the absorption of the [gamma]-rays, from 2 ±0 22 cm. in total thickness of lead, were undertaken. In order to prevent any secondary radiation from the stand, bench, or wails of the room from entering the one vital spot in the lead electroscope already described, namely the windows, the following experiments were conducted in the middle of the laboratory. Fig. 15 shows the disposition employed. An iron tripod of height 27 cm., its top being a narrow circular ring, 18' 3 cm. diameter, was placed on the work-table and all except the lower part of its legs covered with sheet lead (0*14 cm. thick). On the top of the tripod was placed a flat slab of lead 18 cm. square and 1*56 cm. thick. On the slab was placed the lead electroscope surrounded by the two circular screens of lead. The slab would absorb any secondary radiation due to the radium coming from the table, the reading microscope, or the iron stand, though such, if any, has been shown to be very small, while the circular screens protect the sides. 31 mg. of radium bromide were mounted in a lead disk and placed in a cylindrical stand of lead 12 - 5cm. in diameter and 4*1 cm. high. Over the radium were placed the lead absorbing screens, each of which was about 1*25 cm. thick and 12'5 cm. in diameter. The range of thickness of lead kid on between 10 and 20 cm. was investigated. The absorption was at first exponential (\ = 0*50), but it departed latterly just as it had done in fig. 12. It was found that if the natural leak were increased by a constant amount equal to half itself and the sum [figure redacted] [header] 751 subtracted from the gross leak obtained in the usual way the curve was exponential right up to 20 cm. The amount of this constant radiation bore about the same ratio to the amount obtained in the first experiment as the ratio of the quantities used now and before, and it seemed quite possible that it was due to a primary radiation more penetrating than [gamma]-rays, which could only be detected when the latter were very much reduced in intensity. This, however, was disproved by the next experiments. The two small windows of the electroscope were blocked up by 0'3 cm. of sheet lead as thoroughly as could be done without interfering with the microscope or 'completely shutting off the light from the lamp. This constant radiation was then greatly reduced. By removing everything from the table except the apparatus used, and by covering with sheet lead the microscope and also the rubber cork by which the leaf system and the charging rod were held in position, the constant radiation was entirely eliminated. Three sets of readings were obtained on different days for the absorption over the range of 10 to 20 cm., and in all the three cases the rays from the radium were absorbed exponentially (\= '050 as before), the leaks actually obtained being corrected only for the natural leak of the instrument, obtained by removing the radium out of the laboratory, and keeping everything else as it had been during the series of measurements. When it is considered that for the greatest thicknesses of lead employed the corrected leaks vary from about 0*1 to 1*0 division per minute, the natural leak being constant at 3*60 divisions per minute, the necessity of shutting off every secondary effect may be realised. An experiment was indeed made by exposing a corner of one of the windows, and the corrected leak was so doubled. As previous experiments indicate, brass or other metals would be useless for this work. Lead alone can be used with confidence in such measurements. Fig. 16 (PL XII.) represents the absorption of the [gamma]-rays by lead over the whole range explored. From 2 to 12 cm. total thickness of lead traversed, 6*7 mg. of radium bromide was used as source. From 10 to 22 cm., 31 mg. were used in two series of measurements and 45 mg. in a third. By making use of the values of the rates of leaks at two points common to two curves the composite absorption curve shown in fig. 10 is obtained. The second half of the curve is the better of the two series of readings obtained with the 31 mg. source. It is plotted as obtained. The first half is that obtained with the 6*7 mg. source, each value of which has been multiplied by a constant factor. In curve A (PI. XII. fig. 16), obtained with 31 mg., it may be noticed that there is a decided irregularity 752 [header] between 15*6 cm. and 17 cm., but the end point at 22*1 cm. is on the straight line. This effect is, no doubt, due to ineffectual shutting out of a small amount of secondary penetrating radiation which manifests itself at these points. As the air space between the top of the absorption plates and the electroscope base was filled in (thereby reducing the secondary penetrating radiation) the points came on to the straight line once more. In the main curve and curve B (fig. 16) greater precautions were taken to prevent any trace of such radiation from entering the electroscope. The value of [lambda] obtained from the principal curve of fig. 16 over a range from 2 to 22*1 cm. of lead is 0'498 (cm.) -1 , that is to say, 1*392 cm. of lead cuts down the [gamma]-rays of radium to half value. This value agrees (1) with that previously obtained [citation redacted] ; (2) with the value given by Tuomikoski, over a range of 22 to 12*0 cm. (0*51) ; (3) with the value in the present paper, using, however, a brass electroscope (0*50) ; and (4) in Part II. of the present paper with truncated hemispheres (•050). This value for [lambda] (cm.)" 1 0*50, making [formula redacted], may be used with confidence in calculations of the penetrating rays from the earth's crust. A new electroscope, made entirely of lead, was constructed in order to save trouble in blocking up the windows and the cork with lead. It is shown in section in fig. 17. It consists of a cylinder of lead of internal height 12*9 cm., internal diameter 9*0 cm. ; thickness of walls 1*30 cm. and of top 1*25 cm. Two cylinders of lead, 6*5 cm. long and 3*5 cm. in diameter, 0'4 cm. thick in the wall, were soldered into the sides of the electroscope to protect the windows, which were circular and of the same diameter as the lead cylinders. The latter were just large enough to allow the microscope to be inserted. The sulphur of the leaf system was surrounded by an earthed ring of brass. Over the cork and charging rod a third cylinder of lead, 0'4 cm. thick, was placed while measurements were being taken. To the instrument a permanent base could be soldered, or the thickness and nature of the base could be altered at will by clipping up different thickness to two pieces of brass attached to the sides of the electroscope at its base. This may be considered a standard form of electroscope for work on absorption of [gamma]-rays. [figure redacted] [header] 753 Initial Part of the Absorption Curves of the [gamma]-rays of Radium. Experiments involving four different dispositions were carried out with screens of lead, tin, zinc, and aluminium. The thick lead electroscope with base removed (fig. 17) was mounted on an iron tripod. Various thicknesses of metals were clamped up to form the base. In disposition 1, 6*7 mg.. of radium bromide were placed at the apex of a cone of height 11*5 cm. and of base 3 cm. in diameter, cut out of a cylindrical lead block 12*7 cm. long and 10*5 cm. diameter. The cylinder was placed immediately underneath the electroscope, the top of the former being about 3*5 cm, from the base of the latter. Disposition 2 was exactly the same as 1, except that 1*24 cm. of lead was placed on the lead cylinder over the base of the cone. In dispositions 3 and 4 the rays were not confined by a cone at all. In 3, 0*47 rag. of radium bromide, lying without cover in a recess in a lead block 1 cm. thick, was placed 14 cm. below the electroscope. 4 was the same as 3, except that 1*24 cm. of lead was placed directly over the radium. Table V. gives a summary of the results obtained with these dispositions over the ranges of thicknesses of metals described. Table V. Disposition. Metal. Range in cm. Shape of Absorption Curve. [table redacted] [footer] 754 [header] It is obvious from the results set forth in the Table that for the initial part of the range the absorption curves may be straight, or may depart in either direction from the exponential form according to the conditions of experiment and the absorbing metal used. Thus, if in the original investigations on [gamma]-rays, zinc or aluminium instead of lead had been used as the absorbing metal, the curve obtained over the first part of the range would have been found to be exponential. For disposition 1, the value of X rises in the case of aluminium to about the normal value ([formula redacted]), but for zinc, and still more for tin, it rises beyond the normal value, while for lead it diminishes, but does not reach the normal value. In disposition 3, which gives exponential curves for aluminium, zinc, and tin, the value of [lambda] is from 1/5 to twice the normal. A further result may be mentioned which has been obtained by a new disposition, in which a narrow cone of rays and a shorter cylindrical ionization chamber connected to a separate electroscope have been employed. The curves for aluminium and lead are concave and convex respectively, while those for zinc and tin are straight over the whole range. [labda] for zinc is 0'268, nearly the normal value, but for tin it is 0*355, about 26 per cent, too great. Contrasting this result for zinc with those given in Part II. with truncated hemispheres, we see that lead is normal in the hemispherical and abnormal in the cylindrical ionization chamber, while with zinc the converse is true. Zinc, in the disposition last described, is the only case so far found of a metal obeying the simple exponential law with the normal value for [lambda], from the thickness sufficient to absorb [beta]-rays up to the greatest thickness tried (6 cm.). Finally, a number of experiments may be referred to on the variation of the absorption coefficient [lambda] with variation of the different components of the disposition employed, over ranges of thickness greater than the equivalent of 1 cm. of lead. The chief disposition used (denoted by A) was to place the absorbing screens directly over the source of [gamma]-radiation at a distance of about 14 cm. below the electroscope, though some experiments have been carried out by clamping up the absorbing screens to act as base and leaving the ionizing source bare (B). It was found in general that [lambda] for any substance varies within certain limits with practically every important change in the disposition. The substances are divided into classes according to density. Class I. denotes lead and mercury, Class II. comprises substances in density from copper to magnesia brick, and Class III. from sulphur to pine-wood. For an electroscope made entirely of one material, if disposition A be employed with a radium source, the absorption coefficients vary slightly with the thickness of [header] 755 the base for all bodies used as absorbing screens, except the one of which the electroscope is made. In the case of brass a thin base (0'58 cm.) gives higher [lambda]/d than a thick base (1*2 cm.) for bodies of Class II., and lower for Class III. In the case of a lead base and Class III., X is independent of the thickness of the base, but for Class II. a thin base (1 cm.) gives higher A/s than a thicker one (2*85 cm.). For an electroscope made entirely of brass the absorption of all bodies except Class I. (which have already been discussed) for disposition A is absolutely exponential. Class III. has a higher [lambda]/d than Class II., for which the values of [lambda]/d are practically constant, as in the earlier experiments with the lead electroscope (ibid. p. 644). Using a thick lead electroscope base (1 cm.), but clamping up the absorbing metal, bare radium or uranium X gave exponential absorption in all cases, but with different values of X than had been obtained before (pp. 644 and 646) by method A (same electroscope). For copper and uranium X [lambda] is greater for B than A. For copper and radium [lambda] is smaller for B than A. For lead and uranium X [lambda] is smaller for B than A. For lead ana radium [lambda] is greater for B than A. These results also explain the abnormal value of the ratio [formula redacted] for lead (1*465) as compared with that for copper (1*186). With disposition B the value of the ratio is 1*28 for lead, while that for copper is 1*25. For radium (a thick brass electroscope and brass base were used) the value of [lambda] for a body laid on the radium depends to some extent on whether the rays have passed through other bodies first. In Table VL, given below, values of [lambda] for paraffin wax, magnesia brick, and zinc are given when the rays have first passed through lead, tin, graphite, and no other body, respectively, before entering the absorber. 1 cm. of lead was used and 2 cm. of tin and graphite. All the curves obtained were strictly exponential. Table VI. Rays first pass through Values of [lambda]. Paraffin Wax. Magnesia Brick. [table redacted] [footer] 756 [header] The very low results obtained when rays pass through 1 cm. of lead before entering the absorbing body is an example of the capacity of lead for " hardening the rays," which has been indicated by the work of numerous investigators already referred to. Similar results have been obtained with a brass electroscope and copper absorbing plates for disposition B and a radium source. The value of [lambda] for copper varied within narrow limits according to the amount of lead placed on the radium, the absorption in every case being strictly exponential. This short resume is not intended to be complete, but the results obtained are so definite and are so capable of accurate measurement that they may be accepted with some confidence. They serve to show how the value of [lambda], even for the range of thickness over which the absorption is in every case strictly in accordance with an exponential law, can be varied within fairly wide limits at the will of the experimenter. Summary of Results. (1) The general conclusion is reached that initially the primary [gamma]-rays (at least of radium) are homogeneous and,, since the [beta]-rays are not homogeneous, further support is obtained for the view that the two types of rays are probably not interdependent. (2) A detailed study of the initial part of the absorption curves of the uranium X [gamma]-rays failed to establish the existence of a soft [gamma]-radiation, and if such exists it must be relatively feeble and unimportant. (3) The absorption of the [gamma]-rays of radium in truncated hemispheres, using a cone of rays of angle 180°, has shown that absorption proceeds exponentially with constant value of [lambda] ( = 0*50) and no scattering takes place. In zinc hemispheres evidence was obtained of a secondary soft [gamma]-radiation generated by the primary, with absorption coefficient 4*5 times greater, the two radiations not coming into equilibrium till about 2 cm. of zinc have been penetrated. (4) The absorption of [gamma]-rays, using a cone of 180°, is not markedly different from that of an ordinary experiment, owing to scattering. For aluminium the simple exponential law, holding for a parallel beam, was rather more closely followed in the former case than in the latter. (5) With suitable methods the [gamma]-rays of radium are absorbed strictly exponentially ([formula redacted]) up to a thickness of 22 cm. of lead, and the variations at great thicknesses previously observed are due to the formation of a peculiar [header] 757 secondary radiation, apparently generated by lead and capable of penetrating readily all substances, except lead, even after reflexion from wood &c. (6) The initial variations in the absorption coefficient of the radium [gamma]-rays have been shown to depend much on the nature of the absorber and on the disposition employed. Zinc, tin, and aluminium, for a certain disposition, absorbed the rays quite exponentially but with somewhat high values of [lambda]. In another disposition zinc absorbed exponentially with normal value of [lambda] from the thickness sufficient to eliminate [beta]-rays up to 6 cm. (7) The value of the absorption coefficient, over the higher ranges, of thickness for which absorption is always strictly exponential, can be varied within fairly wide limits, being for example always diminished when the rays first traverse a denser substance ("hardening"). The abnormal ratio [formula redacted] for lead (1*465) previously obtained is so accounted for. In general, the ratio [formula redacted] may be taken to be from 1-2 to 1-3. Physical Chemistry Laboratory, Glasgow University.