Page:Radio-activity.djvu/41

 The active substance belongs to the group of cerium earths and is precipitated with them. By a succession of chemical operations, the active substance is separated mixed with lanthanum. While intensely active in comparison with thorium, the new active substance closely resembles it in radio-active properties, although, from the method of separation thorium cannot be present except in minute quantity. Giesel early observed that the substance gave off a radio-active emanation. On account of the intensity of the emanation it emits, he termed it the "emanating substance." Recently this name has been changed to "emanium," and under this title preparations of the active substance prepared by Giesel have been placed on the market.

Giesel found that the activity of this substance was permanent and seemed to increase during the six months' interval after separation. In this respect it is similar to radium compounds, for the activity of radium, measured by the electric method, increases in the course of a month's interval to four times its initial value at separation.

There can be no doubt that the "actinium" of Debierne and the "emanium" of Giesel contain the same radio-active constituent, for recent work has shown that they exhibit identical radio-active properties. Each gives out easily absorbed and penetrating rays, and emits a characteristic emanation of which the rate of decay is the same for both substances. The rate of decay of the emanation is the simplest method of distinguishing actinium from thorium, which it resembles so closely in radio-active as well as in chemical properties. The emanation of actinium loses its radiating power far more rapidly than that of thorium, the time taken for the activity to fall to half value being in the two cases 3·7 seconds and 52 seconds respectively.

The rapid and continuous emission of this short-lived emanation is the most striking radio-active property possessed by actinium. In still air, the radio-active effects of this emanation are confined to a distance of a few centimetres from the active material, as it is only able to diffuse a short distance through the air before losing its radiating power. With very active preparations of actinium,