§ 1. It is proposed in the following paper to describe experiments made to test the electric state of the air in the neighbourhood of metallic uranium, or of other metals on which a salt of uranium had been deposited from a solution, when these were charged to a positive or a negative potential.

§ 2. Method employed. To test the electric state of the air the electric filter method* due to Kelvin, Maclean, and Gait was employed. The special filter used in the experiments to be described was a block-tin pipe, 10 cms. long and 1 cm. diameter, filled with brass filings. This was insulated on two tunnelled pieces of paraffin, and put in metallic connection with the insulated pair of quadrants of a quadrant electrometer. From one of the tunnelled pieces of paraffin a metal tube led to an air pump; from the other a piece of india-rubber tubing led to the place where the air to be tested was. This air was then drawn through the electric filter, and the deviation from the metallic zero of the electrometer, when the two pairs of quadrants were insulated, was noted. To give some idea of the efficiency of the filter, the results obtained when air was pumped away from the neighbourhood of the electrodes of a Ruhmkorff inductorium (Apps. pattern, 10-inch spark) will be given.