=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.