﻿Thorium and Radium Emanations. 351 



been standing for at least four hours in liquid air, so that it 

 could be assumed that any activity more than the irreducible 

 quantity mentioned above was decaying exponentially with 

 the period of thorium B. The tube was withdrawn from 

 liquid air, and during the ensuing 40 minutes the activity, L> 

 in excess of the irreducible activity at the beginning of that 

 interval was determined. At the end of that period of time 

 the experimental end was placed 5 cm. deep in a pentane 

 bath previously cooled to some low temperature. After a 

 40 minutes' exposure at this temperature, the experimental 

 end was withdrawn and the P 2 5 end replaced in liquid air 

 for another definite time, 3J hours, when the tube was again 

 withdrawn from liquid air and the activity, N, of the experi- 

 mental end determined as quickly as possible. Then the 

 P 2 5 end was again placed in liquid air for at least four , 

 hours before another determination at some other temperature 

 was made. 



When the measurement N is being made, the quantity L will 

 have decayed to L X e~* f , where X is the radioactive constant 

 of thorium B and t is the time that has elapsed between 

 making the measurements L and JS". In determining the 

 initial leak prior to the exposure, the emanation is uniformly 

 distributed throughout the tube and a certain quantity of 

 active deposit will be obtained on the experimental end. The 

 activity from this active deposit is exceedingly small while 

 that measurement is being made but will not be, by any 

 means, negligible when the final measurement is taken. If r 

 however, the time during which the initial measurement is 

 made is always constant, the final measurement will be the 

 sum of three quantities, the quantity he~ xt , a variable 

 quantity due to the 40 minutes' exposure at some low tempe- 

 rature, and a constant quantity due to the uniform distribu- 

 tion of the emanation during the initial measurement. The 

 activity, D, due to the 40 minutes' exposure is therefore 

 given by 



D = N-Le-*'-Q, (2) 



where Q is a constant activity due to active deposit collected 

 during the uniform distribution while the initial measure- 

 ment is being made. The fraction of emanation condensed 

 at any temperature T is obtained by finding the values of D 

 for that temperature, room and liquid air temperatures, and 

 substituting the values D T , D L , and D E for A T , A L , and A R in 

 expression (1). The numerator and the denominator of this 

 expression therefore consist of the difference of two values 

 for the expression (2), and therefore the quantity Q does not 



