Excited Act/city of Thorium, 637 



green all over. When these thick disks were fitted into the 

 end of a metal cylinder (the rays falling again on the inactive 

 side ) they did not lose any appreciable amount of activity, 

 while the thin disks in the same circumstances lost 15 per 

 rent. Here the disturbances would tend to spread over the 

 surface of the cylinder rather than through the metal, and 

 the difference caused by the extra thickness of aluminium 

 becomes more noticeable. When thin copper disks were 

 used, there was less loss of activity from the back than was 

 the case with aluminium of about the same thickness. 



Separation of two Stages in the Radioactive Change. 



The majority of the experiments, and especially those in 

 which the active disk was enclosed in a metal box, were more 

 useful in showing the nature than the cause of the change 

 occurring, which was found to be less simple than was at 

 6rst supposed. When the activity of a disk was tested for 

 some time after its exposure in the tube, it was nearly always 

 found that the rate of decay was diminished, so that after an 

 hour or two a part of the activity lost had been recovered. 

 In many cases there was an actual increase of activity for the 

 first hour or so, but this was only temporary, and the rate of 

 decay always returned eventually to its normal value. Curve I. 

 (diagram 'I, PI. VIII.) is typical of the larger rises; there is 

 a maximum about two hours after exposure, the activity then 

 diminishes, and after about four hours its rate of decay is the 

 same as that given by the unaltered disk (Curve IV.). In 

 other cases (Curve II.) there is a less marked rise, and the 

 maximum is sooner reached; Carve III. shows hardly any 

 perceptible rise, but the rate of decay is at first abnormally 

 slow. In the experiments made with the disk enclosed in a 

 metal box. a large proportion of the activity lost by the disk 

 was generally found on the box, and its rate of decay could 

 parately tested. This was found in nearly all cases to 

 1m- greater than the normal value, but gradually diminished 

 BO that any activity remaining after three or four hours had 

 the normal rate of decay. Diagram 3 shows a typical curve ; 

 ili- activity of the box is plotted on a larger scale, to show 

 the Bhape of the curve clearly. 



Similar results were, obtained by placing a second (inactive) 



disk in the discharge-tube close to the active surface ; part 



of the activity was transferred from one disk to the other, 



ibsequent rat.- of decay of the firsi being smaller, and 



the second greater, than the normal. 



Tli'- general similarity of the curve obtained after exposure 

 PldL Man. S. 6. Vol. 9. No. 53. May 1905. 2 U 



