Page:Radio-activity.djvu/324

 proportionality which always exists between the activity of the emanation and the excited activity to which it gives rise, is at once explained, if one substance be the parent of the other.

179. Decay of the excited activity produced by thorium. The excited activity produced in a body after a long exposure to the emanations of thorium, decays in an exponential law with the time, falling to half value in about 11 hours. The following table shows the rate of decay of the excited activity produced on a brass rod.

Time in hours   Current 0           100     7·9           64    11·8           47·4    23·4           19·6    29·2           13·8    32·6           10·3    49·2            3·7    62·1            1·86    71·4            0·86

The results are shown graphically in Fig. 64, Curve A.

Fig. 64.