ABSTRACT
The radioactivity of the element uranium has been examined in the light of the theory put forward by the authors to explain the radioactivity of thorium. Sir William Crookes shows that the activity of uranium to a photographic plate is caused by the presence of a minute amount of a foreign substance to which he gave the name Uranium X. The case of uranium presents some very interesting features for, unlike thorium, this substance produces neither an emanation nor excited activity, and the experimental analysis of the processes that occur is, in consequence, extremely simple. After several precipitations the activity of the uranium was found to be much enfeebled, whereas the first barium sulphate precipitates were more active weight for weight than the original uranium. The activity of the Uranium X, on the other hand, steadily diminished, until at the end of the period it possessed less than one percent of its original activity.
