One establishes a potential difference of 100 volts between the plates. The absolute value of the current which traversed the condenser was measured by means of an electrometer and a piezoelectric quartz. I examined a large number of metals, salts, oxides, and minerals. The substances which I studied but omitted from the Table are at least 100 times less active than uranium. The compounds of thorium are very active.
Thorium oxide surpasses even metallic uranium in activity.
It is remarkable that the two most active elements, uranium and thorium, are the ones which possess the greatest atomic weight. Cerium, niobium, and tantalum appear to be slightly active. White phosphorus is very active, but its action is probably of a different nature than that of uranium and thorium. In fact, phorphorus is not active in the form of red phosphorus or in the state of phosphates. All the minerals which demonstrate activity contain active elements.