SUCCESSION OF CHANGES IN RADIOACTIVE i:oi>ll^ 177 



to alworb all the a rays. For measurements %vitli (lit- y rays, the electroscope was 

 placed on a lead plate OT, rvntiin. tliick, and the active rod placed under the lead 

 plate. The a and ft rays were completely stopped by the lead, and the discharge in 

 the electroscope was then due to the y rays alone. The electroscope is very 

 advantageous for measurements of this character, and accurate observations can be 

 simply and readily made. It, however, cannot be used with advantage to follow very 

 rapid changes in activity. 



7. The curve of decay of activity, measured by the a rays, for an exposure of 

 1 minute in the presence of the radium emanation is shown in fig. 4, 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 4 hours after removal 

 the activity again decays according to an exponential law with the time, falling to 

 half value in about 28 minutes. 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. 



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. 4, curve A A. 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 

 ft rays are shown graphically in figs. 5 and 6. 



Fig. 5 is for a short exposure of 1 minute. Fig. 6 shows the decay for a long 

 exposure of about 24 hours. 



The curves obtained for the ft rays are quite different from those obtained for the 

 rays. For a short exposure, the activity measured by the ft 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. 6 for the ft rays is very similar in shape to the corre- 

 sponding curve, fig. 4, curve A A, for the a rays, with the exception that the rapid 

 initial drop observed for the -ray curve is quite absent. The later portions of the 



VOL. cciv. A. 2 A 



