H. M. Dadonrian— Atmospheric Radio-activity. 337 



testing vessel. On account of the rapid decay of thorium ema- 

 nation it is very difficult to detect its presence in the ground 

 air by direct observation on the rate of decay of the activity 

 of the radio-active gas. This difficulty was avoided by meas- 

 uring the rate of decay of the active-deposit obtained from 

 underground air. 



A cavity 50 cm in diameter and 200 cm deep was dug in the 

 earth near the Physical Laboratory. A negatively charged 

 wire wound around a cylindrical frame was suspended in the 

 cavity. The top of the cavity was sealed and the air was 

 removed by means of a filter pump in order to keep the wire 

 in contact with fresh ground air. After exposures of three 

 hours duration the wire was taken out and the rate of decay of 

 its activity observed. About 5 per cent of the total initial 

 activity was found to be due to the disintegration products of 

 thorium emanation, the balance being due to those of radium 

 emanation. 



It should be observed that unless account is taken of the 

 time of exposures Bumstead's results do not give a measure 

 of the relative amounts of radium and thorium emanations 

 present in the open air in New Haven, nor do the results ob- 

 tained by the writer measure the relative amounts of these 

 emanations present in the underground air. The activity of a 

 body which is negatively charged in the open or underground 

 air is due to at least six disintegration products of radium 

 and thorium, e. g., Ea A, Ea B, Ea C, Th A, Th B, and Th C. 

 Each of these accumulates and decays at a perfectly definite 

 rate, which is characteristic of that particular substance. So 

 that the amounts of these disintegration products present on 

 the negatively charged body at any instant of the time of ex- 

 posure are not, in general, proportional to the amounts of the 

 corresponding products present in a unit volume of the air. 



Suppose P to represent the number of particles of any one 

 of these radio-active products present on the charged wire 

 at any instant of the time of exposure. Then the observed 

 rate of decay is represented by the exponential law 



P = P'e~ Xt 



where P' is the number of particles at the time t=0, and \ is 

 the constant of disintegration. Now if p represents the num- 

 ber of particles which are transformed per sec, then 



t+i 

 p = fdP 



XP 



