Prof. Owens on Thorium Radiation. 385 



The absorption of thorium radiation as a function of the 

 depth of the absorbing layer of gas for air at different 

 pressures is given below. 



The method used was the same as that employed for 

 studying the absorption by aluminium foil. The depth of 

 the absorbing layer of air was varied by altering the distance 

 between the active material and two insulated parallel brass 

 plates attached to the rod R, fig. 8, and shown below the 

 figure. A circular hole, 8 cm. in diameter, was cut in the 

 lower plate, and covered with a single layer of aluminium 

 foil to admit the radiations. The lower plate was connected 

 to a Lattei'v of i)5 volts. The upper plate was connected to 

 the electrometer as usual. 



Using a thick layer of thorium oxide, the l'elation between 

 the conduction-current between the two plates and the 

 distance of the lower one from the active material for pres- 

 sures of ^, I, 2, and 3 atmospheres is given in Tables XI a, 

 XI b, XI c, XI d, and on fig. 13. 



It will be noticed that for pressures of ^ and 1 atmosphere 

 the conduction-current varies inversely in geometrical pro- 

 gression very nearly as the distance of the lower plate from 

 the active material increases in arithmetical progression. At 

 higher pressures the current does not decrease with distance 

 so rapidly, due to the fact that at high pressures the current 

 is produced in larger part by the more penetrating kind of 

 radiations, which are absorbed much less rapidly. 



Table XI a. 



Thorium Oxide. Thick layer. 



Absorption of radiations in air. 



Distance apart of fixed plates 3 cm. Lower plate 95 volts + 



Distance from oxide to lowest position of movable plate 



= d = 2*25 mm. 



Pressure \ atmosphere. 



Distance in mm. 



Time of 100 div. in sec. 



Current. 



rf= 2-25 

 <2 + 10-0 

 r/ + 20-0 



22-0 

 31-8 

 46-8 



1000 

 700 

 47-0 



1 



J'hil. Mag. S. 5. Vol. 48. No. 293. Oct. 1899. 2 E 



