Page:Radio-activity.djvu/326

 The amount of excited radio-activity reaches a maximum value when the rate of supply of fresh radio-active particles balances the rate of change of those already deposited.

180. Excited radio-activity produced by a short exposure. The initial portion of the recovery curve B, Fig. 64, is not accurately represented by the above equation. The activity for the first few hours increases more slowly than would be expected from the equation. This result, however, is completely explained in the light of later results. The writer found that, for a short exposure of a body to the thorium emanation, the excited activity upon it after removal, instead of at once decaying at the normal rate, increased for several hours. In some cases the activity of the body increased to three or four times its original value in the course of a few hours and then decayed with the time at the normal rate.

For an exposure of 41 minutes to the emanation the excited activity after removal rose to three times its initial value in about 3 hours and then fell again at about the normal rate to half value in 11 hours.

With a longer time of exposure to the emanation, the ratio of the increase after removal is much less marked. For a day's exposure, the activity after removal begins at once to diminish. In this case, the increase of activity of the matter deposited in the last few hours does not compensate for the decrease of activity of the active matter as a whole, and consequently the activity at once commences to decay. This increase of activity with time explains the initial irregularity in the recovery curve, for the active matter deposited during the first few hours takes some time to reach its maximum activity, and the initial activity is, in consequence, smaller than would be expected from the equation.

The increase of activity on a rod exposed for a short interval in the presence of the thorium emanation has been further investigated by Miss Brooks. The curve C in Fig. 65 shows the variation with time of the activity of a brass rod exposed for 10 minutes in the emanation vessel filled with dust-free air. The excited activity after removal increased in the course of 3·7 hours to five times its