Excited Activity of Thorium. 631 



and cutting the glass each time. The discharge was generally 

 obtained from an induction-coil, and the times of exposure 

 were usually from half an hour to an hour. 



Effect of the Discharge. 



In the earlier qualitative experiments, a fall in activity of 

 from 10 per cent, to 20 per cent, was obtained in almost all 

 cases after exposure to the rays, the curves of decay being 

 similar to that of diagram 1, where the surface A was ex- 

 posed to the discharge for J hour, the control-piece B being 

 tested while A was in the discharge-tube. 



The effect was then examined in more detail, so as to 

 determine as far as possible the exact nature of the action. 

 The first point which it was important to settle was whether 

 the activity lost by the metal disks was destroyed — i. e., the 

 rate of decay increased during the discharge — or whether it 

 was merely driven off, in which case it should be found on 

 the walls of the tube. To decide this point, a very small 

 gold-leaf electroscope was made, consisting of a little brass 

 plate and gold-leaf supported on an insulating stem of sealing- 

 wax. This could be placed inside the tube, while the rate of 

 collapse of the leaf was observed through a reading-micro- 

 scope outside, both before and after the exposure of the active 

 disk to the discharge. 



The results obtained could hardly be regarded as quanti- 

 tative, as no special precautions were taken to prevent leakage 

 from the leaf along the insulating stand, and the capacity also 

 varied somewhat according to the exact position of the elec- 

 troscope relative to the metal in the tube ; the nature of the 

 change occurring was, however, clearly shown. The electro- 

 scope readings could be roughly standardized by observing 

 the increase in the rate of fall of the leaf when a disk of 

 known activity was introduced into the tube. In these ex- 

 periments, the observed deflexions of the electrometer were 

 corrected for change of sensitiveness by means of the uranium 

 cell. The observations of January 28th will serve as an 

 example. 



Hate of collapse of gold-leaf in 1 .,,. , ,. 



.i i ,. i- , ft > = 'oo scaie-div. per mm. 



tube before discharge J l 



Elate of collapse with disk Oin tube =3*24 div. per min. 



Therefore increase due to r=2*88 per min. 



( gave an electrometer leak of 263 scale-divisions per 

 minute, the leak with the uranium cell being 27*75 div. 

 per min. 



