﻿Thorium and Radium Emanations. 353 



in the bulb of the tube by sealing-wax. ft rays only were 

 used and the electroscope was in the position relative to the 

 lead cylinder shown in fig. 3. 



The short limb of the tube is the P 2 5 end, and 10 cm. 

 of the long limb is the experimental end. 



Fig. 9. 



.tu-77i 



The advantage of this tube is that the activity can be 

 determined prior to the exposure without removing the 

 P 2 5 end from liquid air. 



The P 2 5 end was, as before, immersed for three days in 

 liquid air, and the irreducible activity of the experimental 

 end determined, and this quantity is always treated as a 

 natural leak. As in the thorium straight tube, the P 2 5 

 end must always have been four hours at least in liquid air 

 before a determination at some temperature is carried out. 



The following is the method of carrying out such a deter- 

 mination. The activity, L, of the experimental end was 

 measured. The tube was then removed from the lead 

 cylinder and the experimental end placed as quickly as 

 possible 5 cm. deep in a pentane bath previously cooled to 

 some low temperature T. After 40 minutes of exposure 

 at this constant temperature, the tube was replaced in the 

 lead cylinder and the P 2 5 end immediately immersed in 

 liquid air. The activity of the experimental end is deter- 

 mined during the 15 minutes following this replacement, and 

 then the final measurement is made after the P 2 5 has been 

 immersed in liquid air for 3J hours. 



The curve obtained from the measurements made during 

 the time immediately following the exposure is extrapolated 

 back to the instant of withdrawal of the tube from the pen- 

 tane bath. Let the quantity so obtained be M, and let the 

 final activity be N. M is made up of three factors, (a) the 

 activity contributed by L, i.e. L x e-°'% since t for the time 

 of exposure = 40 minutes ; (b) the activity contributed by 

 thorium active deposit obtained during the 40 minutes' expo- 

 sure ; and (c) the active deposit from the radium emanation 

 condensed during that exposure. (c) when found can be 

 taken as a measure of the radium emanation condensed. It 

 is better, however, for the sake of accuracy to allow (/>) to 

 increase to its maximum and to measure it after 3^ hours. 

 If E is the maximum activity obtained from the thorium 



Phil. Mao. S. 6. Vol. 29. No. 171. March 1915. 2 A 



