﻿350 Mr. A. Fleck on the Condensation of 



there for three days, and then the activity of the experi- 

 mental end was determined. This quantity was found to be 

 be 2*90 d.p.m., and it must be treated as an additional 

 " natural leak " and always deducted from any subsequent 

 measurements. 



Test for instantaneous diffusion. — An experiment to de- 

 termine whether the assumption as to instantaneous diffusion 

 was justified was carried out with this tube. 



The P 2 5 end was allowed to stand in liquid air for three 

 days, and then the activity (I F ) of 10 cm. of the P 2 5 end 

 was measured. This gave the activity due to the equilibrium 

 amount of active deposit that could be collected at this end. 

 The experimental end was then immersed for the same time 

 in liquid air and the activity (I N ) of the equilibrium amount 

 of active deposit that could be collected at this end found. 

 The values of these quantities were respectively 332'6 d.p.m. 

 and 319'1 d.p.m. 



Let k be the fraction of the emanation present in one half 

 of the tube crossing the glass-wool plug per second, 

 to the other half ; 

 Q, the total amount of emanation set free from the 



drum ; 

 n the number of molecules that cross the glass-wool 

 plug per second from the P 2 5 end to the experi- 

 mental end ; 

 and in the number that cross in the reverse direction. 



For the P 2 5 end in liquid air, I p = Q, and for the experi- 

 mental end in liquid air XI^ = n — m ; but in this case m = 0, 

 and therefore ~\I^ = n : 



but n^k(Q-I^=\I^ 



Since I p is nearly equal to I N , it follows that k must be very 

 large compared with X ; i.e. that the fraction of emanation 

 diffusing through the glass-wool plug is very large compared 

 with the fraction decaying per unit time. The assumption 

 as to instantaneous diffusion in the case of this thorium 

 emanation tube is therefore justified, and it follows that in 

 the case of radium emanation, where there is no glass-wool 

 plug, instantaneous diffusion also takes place. 



Method of Experiment. — The method of making a determi- 

 nation of the fraction of emanation condensed at any 

 particular temperature was as follows. The P 2 5 end had 



