produced by the Action of Thorium Compounds. 185 



exhausted to the required pressure as rapidly as possible, 

 and rod A exposed for several hours. The rod was then 

 removed, and the current due to its radioactivity tested inside 

 another cylinder D (fig. lib). The battery and electrometer 



Fig. 1 1 b. — Testing- Apparatus. 



e**r// 



connexions are seen in figure. On account of the press of 

 other work, it was not found possible to take observations at 

 regular intervals, but the table given below suffices to show 

 the general nature of the results. 



Pressure. 



Time of 

 Exposure. 



Divisions 

 per second. 



Divisions per sec. 



Interpolated 



values. 



milliin. 

 760 



175 



16 

 45 

 1-7 

 •45 

 •04 



hours. 

 525 



20 



4-1 



5-4 



4-8 

 14-3 

 25 



2-37 

 9-83 

 2-15 

 196 



•72 

 •38 

 •34 



131 



139 



15-1 



10-3 



4-5 



•65 



•44 



The third column gives the current in divisions per second 

 due to radio-active rod in the testing vessel. In the fourth 

 column are given the divisions per second corresponding to 

 an exposure of the rod for the same time (three days) in each 

 case, at the particular pressure. The results are interpolated 

 from the first two columns with the help of the curve given 

 in fig. 8. The results must only be considered approximate, and 

 merely serve to give a comparative estimate of the radioactivity 

 at each pressure. The general results are clear. The radio- 

 activity is about the same at 16 millim. as at 760 millim. 

 Between the pressures o£ 16 and 4*5 millim. the amount 

 begins to diminish, until at '45 millim. it is only ^V of the 



