Soil on Local Atmospheric Radioactivity. 175 



is so drawn in the figure. The decay curve for radium A and 

 C is superposed on this straight line to show the combined 

 activities. 



The values in the last part of the curve correspond to a half- 

 value period for thorium active deposit of about 12 hours, 

 which is in sufficiently good agreement with the accepted 

 value, namely 106 hours. 



Points surrounded by circles are observed points ; those with 

 double circles are mean values of two or more readings ; a 

 point in a square is one which was extrapolated from the 

 curve. 



Standardization of the Thorium Emanation. 



In order to compare the thorium emanation in the under- 

 ground air with that produced by a known amount of thorium, 

 it was necessary to obtain a relation between the amount of 

 emanation produced per cubic centimeter of the earth or sand 

 and the corresponding rate of leak of the electroscope. The 

 calculation was simplified by substituting for the mercury 

 flask and connecting tube a brass tube, 140 centimeters long 

 and with an internal diameter of about Ol centimeter. This 

 tube was provided with a sharp, conical head of iron, 3 cen- 

 timeters long, with a shank which fitted snugly in the tube and 

 a shoulder which rested firmly against its end. The tube was 

 driven into the earth with a heavy mallet until only a cen- 

 timeter or so remained above ground. It was then withdrawn 

 a short distance and the conical head driven out with a blow 

 from a long iron rod passed into the tube. The ionization 

 chamber of the electroscope was then connected to the brass 

 tube by a length of rubber tubing and observations were made 

 as follows : 



The filter pump was started (running about 3 liters per 

 minute) and the current of air continued for several minutes 

 until the ionization vessel was filled with underground air. 

 The pump was then shut off, and the vessel with the under- 

 ground air allowed to stand for half an hour, when the supply 

 of air was renewed by running the pump for another short 

 interval. This operation was repeated every half hour. At 

 the end of about three hours the radium A, B and C had come 

 into equilibrium with the emanation and the radium part of 

 the observed activity had attained a constant value. 



With the pump running, a series of readings was then made 

 which gave the combined activities of radium emanation, 

 radium A, B, C, and thorium emanation. The pump was 

 stopped and five or six minutes allowed for the thorium 

 emanation to decay. A series of readings then gave the 

 constant radium part of the activity alone. The difference 



