Page:Radio-activity.djvu/271

 activity on the walls of the vessel indirectly furnishes a measure of the rate of decay of the emanation itself. This is only true if the emanation is placed for four or five hours in the tube before observations begin, in order to allow the excited activity time to reach a maximum value.

Using this method P. Curie obtained results similar to those obtained by Rutherford and Soddy by the direct method. The activity decayed according to an exponential law with the time, falling to half value in 3·99 days.

The experiments were performed under the most varied conditions but the rate of decay was found to remain unaltered. The rate of decay did not depend on the material of the vessel containing the emanation or on the nature or pressure of the gas with which the emanation was mixed. It was unaffected by the amount of emanation present, or by the time of exposure to the radium, provided sufficient time had elapsed to allow the excited activity to reach a maximum value before the observations were begun. P. Curie found that the rate of decay of activity was not altered by exposing the vessel containing the emanation to different temperatures, ranging from +450° to -180° C.

In this respect the emanations of thorium and radium are quite analogous. The rate of decay seems to be unaffected by any physical or chemical agency, and the emanations behave in exactly the same way as the radio-active products Th X and Ur X, already referred to. The radio-active constant λ is thus a fixed and unalterable quantity for both emanations, although in one case its value is about 5000 times greater than in the other.

Emanations from Actinium.

146. Debierne found that actinium gives out an emanation similar to the emanation of thorium and radium. The loss of activity of the emanation is even more rapid than for the thorium emanation, for its activity falls to half value in 3·9 seconds. In consequence of the rapid decay of activity, the emanation is able to diffuse through the air only a short distance from the active matter before it loses the greater proportion of its