, is brought to a potential of 500 volts. The metallic tube,
, connected with the earth, serves as a guard-tube. When the tube,
, is sufficiently active it is removed from the radium, and the intensity of the current passing through the condenser is measured. Then the active air which fills the condenser is rapidly driven out, inactive air is admitted, and a new measurement of the intensity of the current is made immediately. It is found that the current has become six times more feeble. Thus, during the second measurement the radiation of the excited walls acts solely to ionize the air in the condenser, while during the first measurement the emanation acts as well. We may, then, suppose that it also emits an emanation. This radiation is necessarily very slightly penetrating, for its action cannot be detected on the exterior.
When a solid plate which has been excited by the emanation becomes inactive in free air, the law of loss of activity depends on the time during which the plate has been left in contact with the emanation. If the action of the emanation is prolonged (more than twenty-four hours, for example), the law of loss of activity is given by the difference between two exponentials. The intensity of the radiation,
, may in this case be represented as a function of the time,
