Page:Radio-activity.djvu/400

 exponential law with the time, falling to half value in about 3 minutes. Three or four hours after removal the activity again decays according to an exponential law with the time, falling to half value in about 28 minutes. The family of curves obtained for different times of exposure have already been shown in Fig. 67. These results thus indicate:—

(1) An initial change in which half the matter is transformed in 3 minutes;

(2) A final change in which half the matter is transformed in 28 minutes.

Decay of Excited Activity of Radium measured by α rays.

Fig. 86.

Before considering the explanation of the intermediate portion of the curve further experimental results will be considered.

The curve of decay of the excited activity for a long exposure (24 hours) is shown graphically in Fig. 86, curve AA. There is at first a rapid decrease for the first 15 minutes to about 50 per cent. of the initial value, then a slower decay, and, after an interval of about 4 hours, a gradual decay nearly to zero, according to an exponential law with the time, falling to half value in 28 minutes.