Page:Radio-activity.djvu/363

 *portional to R, and is initially equal to that of A. The activity curve due to C is thus represented by the curve CC, which is the same curve as the upper curve CC of Fig. 73. The activity of A and C together is represented by the upper curve A + C (Fig. 75), where the ordinates are equal to the sum of the ordinates of the curves A and C. This theoretical curve is seen to be very similar in shape to the experimental curve (Fig. 67) showing the decay of activity of the active deposit from a long exposure measured by the α rays.

Fig. 75.

203. Effect of a rayless change on the activity curves. Certain important cases occur in the analysis of radio-active changes, when one of the products does not give rise to rays and so cannot be detected directly. The presence of this rayless change can, however, be readily observed by the variations which occur in the activity of the succeeding product.

Let us consider, for example, the case where the inactive matter A, initially all of one kind, changes into the matter B which gives out rays. The inactive matter A is supposed to be transformed according to the same law as the radio-active products. Let λ_{1}, λ_{2} be the constants of the change of A and B respectively. If n is the number of particles of A, initially present, we see from