Uranium has not yet been obtained inactive when tested by the electric method. Becquerel[298] has stated that he was able to obtain inactive uranium, but in his experiments the uranium was covered with a layer of black paper, which would entirely absorb the α rays. There is no evidence that the α radiation of uranium has been altered either in character or amount by any chemical treatment. The α rays appear to be inseparable from the uranium, and it will be shown later that thorium and radium as well as uranium also possess a non-separable activity consisting entirely of α rays. The changes occurring in uranium must then be considered to be of two kinds, (1) the change which gives rise to the α rays and the product Ur X, (2) the change which gives rise to the β rays from Ur X. The possibility of separating the Ur X, which gives rise to the β rays of uranium, shows that the α and β rays are produced quite independently of one another, and by matter of different chemical properties. Following the general considerations discussed in section 136 we may suppose that every second some of the atoms of uranium—a very minute fraction of the total number present will suffice—become unstable and break up, expelling an α particle with great velocity. The uranium atom, minus one α particle, becomes the atom of the new substance, Ur X. This in turn is unstable and breaks up with the expulsion of the β particle and the appearance of a γ ray. The changes occurring in uranium are graphically shown in Fig. 77. [Illustration: 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.