made simply and readily. The curve of decay of activity, measured by the α rays, for an exposure of 1 minute in the presence of the radium emanation is shown in Fig. 86, curve _BB_. The curve exhibits three stages:— (1) A rapid decay in the course of 15 minutes to less than 10 per cent. of the value immediately after removal; (2) A period of 30 minutes in which the activity varies very little; (3) A gradual decrease almost to zero. The initial drop decays very approximately according to an 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. [Illustration: 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. The curves of variation with time of the excited activity when measured by the β _rays_ are shown graphically in Figs. 87 and 88. Fig. 87 is for a short exposure of 1 minute. Fig. 88 shows the decay for a long exposure of about 24 hours. [Illustration: Fig. 87.] The curves obtained for the β rays are quite different from those obtained for the α rays. For a short exposure, the activity measured by the β rays is at first small, then passes through a maximum about 36 minutes after removal. There is then a gradual decrease, and after several hours the activity decays according to an exponential law, falling, as in the other cases, to half value in 28 minutes. The curve shown in Fig. 88 for the β rays is very similar in shape to the corresponding curve, Fig. 86, curve _AA_, for the α rays, with the exception that the rapid initial drop observed for the α-ray curve is quite absent. The later portions of the curve are similar in shape, and, disregarding the first 15 minutes after removal, the activity decays at exactly the same rate in both cases. The curves obtained by means of the γ rays are identical with those obtained for the β rays. This shows that the β and γ rays always occur together and in the same proportion.