Soil on Local Atmospheric Radioactivity. 179 



sand. Each layer will therefore acquire a concentration 



1), %ks . . 



— (1 — e ~z~) before leaving the sand, where n is the emana- 

 tion per cubic centimeter of sand and P the volume of air in 

 one cubic centimeter of sand. The terms relating to the decay 

 in transit and the mean value in the ionization chamber will 

 keep the same form. Hence 



(IL) N = ai4 0- e "T) fl "T 0-«~T) 



where k t = 347, \ = 2750, k 5 = 201, P = -366. 



With the pump drawing 3*03 liters per minute the rate of 

 the electroscope was 6*865 divisions per minute ; for the same 

 value of q, equation II, gives 0*0886 n. This ratio was used 

 to determine the ordinates, and equation II was plotted 

 (v. curve II, figure 2). Two other observed values of the 

 activity, for different values of q, fell near enough this curve 

 to show the correctness of the assumptions. 



Since 6*865 scale divisions per minute correspond to a con- 

 centration of 0*0886 n, we see at once that one scale division 

 corresponds to a concentration of 0*0129 n. But n is the ema- 



0*52 X 0' 1 

 nation in equilibrium with — gram of Th0 2 = 9*4:5 xlO -0 



gram Th0 2 , and hence one scale division per minute corre- 

 sponds to a concentration equal to the emanation in equilibrium 

 with 1*22 Xl0~ 6 gram of Th0 2 or 1*07 XlO - gram of pure tho- 

 rium under the conditions specified. 



Other Materials. 



A larger cylinder, made from a section of galvanized iron 

 pipe, with a diameter of 17*6 centimeters, a height of 71 centi- 

 meters, and a capacity of about 15 liters, was provided with a 

 gauze screen and cotton pad, as before, and used to test the 

 thorium content of other materials. 



The earth under the cellar floor consists of about 85 per cent 

 disintegrated red sandstone, none of which disintegrated in 

 situ, but which was carried by streams from a melting glacier. 

 The grains were worn and rounded in transit. The remaining 

 15 per cent consists of fragments of trap rock and various crys- 

 talline gneisses and schists. The sample designated " Sand- 

 stone II " was taken from some excavations about a mile from 

 the physical laboratory. This earth has the same constituents 

 as the above, but disintegrated where it was found and was not 

 carried by glacial streams. 



