Fig. 77.
On this view the α ray activity of uranium should be an inherent property of the uranium, and should be non-separable from it by physical or chemical means. The β and γ ray activity of uranium is a property of Ur X, which differs in chemical properties from the parent substance and can at any time be completely removed from it. The final product, after the decay of Ur X, is so slightly active that its activity has not yet been observed. We shall see later ([chapter XIII.]) that there is some reason to believe that the changes in uranium do not end at this point but continue through one or more stages, finally giving rise to radium, or in other words that radium is a product of the disintegration of the uranium atom. Meyer and Schweidler[[299]], in a recent paper, state that the activity due to uranium preparations increases somewhat in a closed vessel. On removing the uranium no residual activity, however, was observed. They consider that this effect may be due to a very short-lived emanation emitted by uranium.
206. Effect of crystallization on the activity of uranium. Meyer and Schweidler[[300]] recently observed that uranium nitrate, after certain methods of treatment, showed remarkable variations of its activity, measured by the β rays. The α ray activity, on the other hand, was unaltered. Some uranium nitrate was dissolved in water and then shaken up with ether, and the ether fraction drawn off. The early experiments of Crookes showed that, by this method, the uranium in the ether portion was photographically inactive. This is simply explained by supposing that the uranium X is insoluble in ether, and consequently remained behind in the water fraction. The ether fraction gradually regained its β ray activity at the normal rate to be expected if Ur X was produced by the uranium at a constant rate, for it recovered half its final activity in about 22 days. Some of the uranium in the water fraction was crystallized and placed under an electroscope. The β ray activity fell rapidly at first to half its value in the course of four days. The activity then remained constant, and no further change was observed over an interval of one month. Other experiments were made with crystals of uranium nitrate, which had not been treated with ether. The nitrate was dissolved in water and a layer of crystals separated. The β ray activity of these crystals fell rapidly at first, the rate varying somewhat in different experiments, but reached a minimum value after about five days. The β ray activity then rose again at a slow rate for several months.
The rapid drop of activity of the crystals seemed, at first sight, to indicate that crystallization was able in some way to alter the activity of uranium.
Dr Godlewski, working in the laboratory of the writer, repeated the work of Meyer and Schweidler, and obtained results of a similar character, but the initial drop of activity was found to vary both in rate and amount in different experiments. These results were at first very puzzling and difficult to explain, for the mother liquor, left behind after removal of the crystals, did not show the corresponding initial rise, which would be expected if the variation of activity were due to the partial separation of some new product of uranium.
The cause of this effect was, however, rendered very evident by a few well-considered experiments made by Godlewski. The uranium nitrate was dissolved in hot water in a flat dish, and allowed to crystallize under the electroscope. Up to the moment of crystallization the β ray activity remained constant, but as soon as the crystals commenced to form at the bottom of the solution the β ray activity rapidly rose in the course of a few minutes to five times the initial value. After reaching a maximum, the activity very gradually decreased again to the normal value. If, however, the plate of crystals was reversed, the β ray activity was found at first to be much smaller than the normal, but increased as fast as that of the other side diminished.
The explanation of this effect is simple. Ur X is very soluble in water and, at first, does not crystallize with the uranium, but remains in the solution, and, consequently, when the crystallization commences at the bottom of the vessel the upper layer of liquid becomes richer in uranium X. Since the β rays arise only from the product Ur X and not from the uranium itself, and the Ur X is mostly confined to the upper layer, a much greater proportion of the β rays escape than if the Ur X were uniformly distributed throughout the thick layer of uranium. When the amount of water added is just sufficient to supply the water of crystallization, the Ur X in the upper layer of crystals gradually diffuses back through the mass and, in consequence, the activity of the upper surface diminishes and of the lower surface rises. A similar explanation applies to the effects observed by Meyer and Schweidler. The water fraction, left behind after treatment with ether, contained all the Ur X. The first layer of crystals formed in it contained some Ur X, and this was for the most part confined to the top layer of crystals. The amount of β rays at first diminished owing to the gradual diffusion of the Ur X from the surface. In the first experiment, the amount of Ur X present was in radio-active equilibrium with the uranium, and, after the initial drop, the β ray activity remained constant. In the second experiment, the gradual rise is due to the fact that the crystals of uranium first formed contained less than the equilibrium amount of Ur X. After falling to a minimum, the β ray activity, in consequence, slowly rose again to the equilibrium value.
These effects exhibited by uranium are of great interest, and illustrate in a striking manner the difference in properties of Ur X and the uranium. The gradual diffusion of the Ur X throughout the mass of crystals is noteworthy. By measurements of the variation with time of the β ray activity, it should be possible to deduce its rate of diffusion into the crystallized mass.
Transformation products of Thorium.
207. Analysis of the active deposit. The radio-active processes occurring in thorium are far more complicated than those in uranium. It has already been shown in chapter vi that a radio-active product Th X is continuously produced from the thorium. This Th X breaks up, giving rise to the radio-active emanation. The emanation produces from itself a type of active matter which is deposited on the surface of bodies, where it gives rise to the phenomena of excited or induced activity. This active deposit possesses some distinctive chemical and physical properties which distinguish it from the emanation and the Th X. We have seen ([section 180]) that the rate at which the active deposit loses its activity depends upon the time of exposure of the body made active to the emanation. The explanation of the activity curves for different time of exposure will now be considered.