Soil on Local Atmospheric Radioactivity. 171 



radioactive properties of the underground air have been studied 

 by drawing a current of air directly through an electroscope. 



The object of the present investigation was to determine, by 

 an elaboration of the early experiment of Elster and Geitel 

 just cited, the character and amount of the gaseous radioactive 

 constituents of the soil. 



The general scheme of the investigation was as follows : 



A current of underground air was drawn from a small iron 

 vessel, buried about 120 centimeters beneath the surface of the 

 earth under the basement floor of the Physical Laboratory, and 

 was conducted through the ionization chamber of a gold leaf 

 electroscope. Immediately following the introduction of the 

 air there was a rise in the activity to about eighteen times the 

 natural leak of the electroscope. This was due to radioactive 

 emanations in the underground air. The activity continued to 

 rise, at first rapidly, and then more slowly, for about three 

 hours, as the result of the production of the radium active 

 deposit. The air current was continued and a very gradual 

 rise in the activity was noted for about three days, when it 

 reached a maximum. If, at this stage, the flow of air was 

 suddenly stopped, a sharp falling off of the activity was noticed 

 which continued for five or six minutes and then reached a 

 minimum. This part of the activity fell to half value in about 

 one minute and was easily identified as thorium emanation. 



A relation was found between the amount of thorium ema- 

 nation set free per cubic centimeter of the soil and the corre- 

 sponding activity as measured in the electroscope. It was thus 

 possible to compare the amount of thorium emanation set free 

 per cubic centimeter of the soil with that liberated by a known 

 amount of thorium. 



The case of the radium emanation is more simple ; since the 

 half value period of radium emanation is comparatively long, 

 it suffers no appreciable loss of activity in the time taken to 

 reach the electroscope. It is therefore only necessary to stand- 

 ardize the electroscope by introducing the radium emanation 

 in equilibrium with a known amount of radium and determin- 

 ing the rate of leak after the emanation has stood for three 

 hours. 



Some additional experiments were made on a number of 

 soils, sands and minerals, with a view to comparing their ema- 

 nating power. 



Apparatus. 



The electroscope consisted of two cylindrical metal vessels, 

 30 centimeters high and 13 centimeters in diameter, placed one 

 above the other. The lower vessel, or ionization chamber, 

 was provided with a central electrode which extended, through 



