XLI. The Diffusion of Uranium. By G. VON Hevesy, Ph.D., Hon. Research Fellow of the University of Manchester, and L. VON Putnoky [Communicated by Prof. E. Rutherford, F.R.S.]. BOLTWOOD found that uranium in disintegrating emits twice as many [alpha]-particles as the other bodies in the uranium-radium series, each of which emits one [alpha]-particle only. To explain this result we may assume, either that a uranium atom expels two [alpha]-particles on disintegrating, or that there is present with the uranium a second long-lived body which also expels an [alpha]-particle. Geiger and Nuttall [citation redacted] have shown recently that there are two different [alpha]-particles emitted by uranium, one having a range about 4 mm, greater than the other. The emission of an [alpha]-particle of definite range is a characteristic property of a radioelement. The obvious deduction, therefore, from Geiger and Nuttall's experiments is that ordinary uranium consists of two bodies, uranium 1 and uranium 2, each of which expels one [alpha]-particle. The same authors have shown further that there is a linear relation between the logarithm of the range of the [alpha]-particle in air and the logarithm of the transformation constant. From this relation they have deduced that the period of uranium 1 is 5 X 10 9 years, and that of uranium 2 2 x 10 6 years. Hence in 2500 parts of commercial uranium, there is one part of uranium 2, and the activity of this body, weight for weight, is 1250 times that of the uranium. The isolation of uranium 2, therefore, would be of considerable practical interest. In the preceding paper it is shown that after expulsion of an [alpha]-particle, the resultant product in many cases has a valency which differs by two from that of the disintegrating atom. It is of interest, therefore, to ascertain whether uranium 2 differs in valency from uranium 1. If it does it would diffuse at a different rate, and should therefore be capable of being partially separated from it. With this object in view the diffusion rates of the complex bivalent ion U0 2 ++ and the quadrivalent ion U ++++ have been compared. The bivalent ions are produced by dissolving uranyl salts in water, and the quadrivalent ions by dissolving the urano-salts. The diffusion constant either of the uranyl ion or of the urano ion may be obtained in two different ways : — (1) By determining the weight of uranium in equal volumes of the solution in the various layers. (2) By determining the activity of the uranium in equal volumes of the solution in the various layers. [footer] 416 [header] If uranium 1 diffuses at a different rate from that of uranium 2, some of the layers will be more active, weight for weight of uranium, than others, and the diffusion constant obtained by the second way will be very different from that obtained by the first. The diffusion apparatus employed in this experiment is described in the preceding paper. The amount of uranium in a layer of the liquid was deter- mined gravimetrically by evaporating the solution to dryness, igniting first in air and then in hydrogen, and weighing as U0 2 . The activity was determined by rubbing a small quantity of the oxide in a finely powdered condition on a ground-glass disk, and measuring it in an [alpha]-ray electroscope. The amount of uranium oxide per sq. cm. exceeded in no case more than 1/3. mg., as with thicker layers the absorption of the rays by the material becomes of consequence. The uranyl nitrate used was purified by crystallization. The urano salt was prepared by reducing a uranyl salt electrolytically at a lead cathode surrounded by a diaphragm. The current density employed was 0*15 amp. per sq. cm. Reduction took place in an atmosphere of C0 2 . During the diffusion of the urano salt also, the diffusion apparatus was filled with C0 2 . The following are the results of the experiments made : — (1) Diffusion velocity of [formula redacted] from a 1/5 molar solution of uranyl nitrate and an 8 molar nitric acid solution in 8 mol. HN0 3 . Diffusion constant at 18° calculated from the weight of the material used 0442 sq. cm. per day. Same calculated from the activity 0-438 + 0*005 sq. cm. per day. In this experiment the diffusion of the U0 2 ion takes place in an excess of the anion (N0 3 ) . In this case, as is shown in the foregoing paper, differences in the nature of the migrating cation are more pronounced than in the case when it diffuses in water. The diffusion velocity of the cation in water depends to the same extent on both ions. (2) Diffusion velocity of 1/5 molar [formula redacted] in water. Diffusion constant calculated from the weight of material, 0*576 sq. cm. per day. Same calculated from the activity, [formula redacted] sq. cm. per day. [header] 417 (3) Diffusion velocity o£ 1/6 molar U(SOJ 2 in water. Diffusion constant calculated from the weight of material, 0*480 sq. cm. per day. Same calculated from the activity, [formula redacted] sq. cm. per day. In these experiments diffusion was allowed to take place for periods of from two to eight days. The results show clearly that the differences in the diffusion constant calculated from the amount of material used and from the activity lie wholly within the error of experiment. If uranium 2 differs in valency from uranium 1 by two units, the diffusion constants calculated from the two sets of data should show a difference of about 30 per cent. The fact that the diffusion constant of the quadrivalent urano salts is only about 20 per cent, lower than that of the bivalent uranyl salts may seem to contradict the conclusions of the preceding paper. In these experiments, however, not only does diffusion take place in excess of the anion, but the solution used is so concentrated (1/6 molar) that the dissociation of the [formula redacted] into its quadrivalent ions is incomplete, and it is to be expected therefore that it has a correspondingly greater diffusion constant. It is difficult at present to say whether or not the results of these experiments contradict the hypothesis that loss of an [alpha]-particle from an atom causes the valency of the resultant atom to differ by two from the parent one. The existence of uranium 2, though rendered probable by the work of Geiger and Nuttall [citation redacted] and of Marsden and Barrett [citation redacted], is riot established with certainty. Again, its position in the disintegration series is at present uncertain. It may lie either between uranium 1 and uranium X, or between uranium X and ionium. The experiments of Soddy on the growth of radium from uranium X indicate that it comes after uranium X ; though naturally these results are capable of another interpretation. If this were so, uranium (hexavalent) produces uranium X (quadrivalent), and the latter, uranium 2 (hexavalent). The transformation of uranium 1 into uranium X, and that of uranium X into uranium 2, would then result in the valency changing by two units in each case. It is not improbable that this is really what takes place, for the expulsion of an [alpha]-ray should change the nature 418 [header] of an atom so much, that it is unlikely that the parent and the resultant products would differ in no way chemically. Experiments have been made to separate uranium 1 from uranium 2 by depositing; the oxide on electrodes at different potentials. The results have been negative. This agrees with the previous work of Boltwood, and of Soddy, on the same problem. Summary . The existence of uranium 2 cannot be shown chemically. Uranium 1 cannot be separated or concentrated from uranium 2 by diffusion. If uranium 2 really exists, a fact rendered very probable by the experiments of Geiger and Nuttall, it is not only very similar chemically to uranium 1, but has also the same valency. Physical Laboratories, The University, Manchester. November 1912.