McCoy and Ross — Thorium Compounds. • 43 7 



in the electroscope case for 5 or 6 minutes in order to allow 

 the maximum amount of emanation to accumulate. The elec- 

 troscope was then charged and as soon as the leaf had reached 

 the beginning of the scale, in the subsequent discharge, the 

 film was rapidly withdrawn from the case, by means of a 

 thread. The small door of the case opened and closed auto- 

 matically. At the instant of removing the film the position of 

 the leaf on the scale was noted and the stop-watch was started. 

 The leaf continued to move with gradually decreasing velocity 

 for about 5 minutes ; at the end of which period the emana- 

 tion had almost completely decayed and the motion of the leaf 

 had become very slow and uniform, corresponding to the nat- 

 ural leak of the instrument. Calling D the number of 

 divisions of the scale traversed by the leaf in 5 min., corrected 

 for the natural leak, and E the initial activity of the emana- 

 tion, then E = *173 D ; as shown by the following calculation. 

 Unit activity discharged the electroscope *075 division per sec. 



Ad 

 Therefore ==- = '075E t 

 dt f 



E t - Ee~ u 



dD ■ -At 



—r- = -0l5Ee 

 dt 



-075E -u, 



D t = — — (l-e ). 



For the thorium emanation \ = -013. If t = 300 sec. e~ 11 = 

 practically, and therefore 



E= -173D 



E is very small except for the thicker films. Table II gives 

 the results for some thick films of sample A of thorium dioxide. 





Table II. 







w 



E 



w 



~E 



F 



•8893 



2-17 



•87 



•68 



•9182 



1-20 



•77 



•80 



•2798 



•43 



•65 



•90 



(|) repre* 



w 

 esents the value of the ratio -= for an infinitely thin 



film, curve II. For such an ideal film of unit weight, the 



activity of the evolved emanation would be — = 1 - 71. This 



"58 



is also the activity of the emanated emanation of unit weight 



