Page:Radio-activity.djvu/334

 decays approximately according to an exponential law, falling to half value in 41 minutes. Giesel examined the rate of decay of the excited activity of "emanium"—which, we have seen, probably contains the same radio-active constituents as actinium—and found that it decayed to half value in 34 minutes. Miss Brooks found that the curves of decay of the excited activity from Giesel's emanium varied with the time of exposure to the emanation. The results are shown graphically in Fig. 69, for time exposures of 1, 2, 2, 10 and 30 minutes, and also for a long exposure of 21 hours. After 10 minutes the curves have approximately the same rate of decay. For convenience, the ordinates of the curves are adjusted to pass through a common point. For a very short exposure, the activity is small at first, but reaches a maximum about 9 minutes later and finally decays exponentially to zero.

The curve of variation of activity for a very short exposure has been determined accurately by Bronson; it is shown later in Fig. 83. He found that the decay of activity is finally exponential, falling to half value in 36 minutes.

The explanation of these curves is discussed in detail in chapter, section 212.

185. Physical and chemical properties of the active deposit. On account of the slow decay of the activity of the active deposit from the thorium emanation, its physical and chemical properties have been more closely examined than the corresponding deposit from radium. It has already been mentioned that the active deposit of thorium is soluble in some acids. The writer found that the active matter was dissolved off the wire by strong or dilute solutions of sulphuric, hydrochloric and hydrofluoric acids, but was only slightly soluble in water or nitric acid. The active matter was left behind when the solvent was evaporated. The rate of decay of activity was unaltered by dissolving the active matter in sulphuric acid, and allowing it to decay in the solution. In the experiment, the active matter was dissolved off an active platinum wire; then equal portions of the solutions were taken at definite intervals, evaporated down in