IT' 1 J. C. Sanderson Probable Inilu< nee of the 



barometer, varying direction and velocity of winds, etc. The 

 radium emanation is evidently the one which is affected by 

 such changes in atmospheric conditions, since it has a compar- 

 atively long halt' value period, can he carried great distances 

 and diffuses to ureal heights before completely disintegrating. 

 The thorium emanation, on the other hand, with its compar- 

 atively short life, will only he found near the surface of the 

 earth. Dadonrian* and W. Wilsonf used the method of the 

 negatively charged wire to determine the ratio of the amounts 

 of radium and thorium emanations in the atmosphere. Knrz,J 

 using much the same method, obtained accurate curves, as well 

 as some quantitative data, for the active deposits. 



Since the emanations present in the atmosphere must have 

 their origin in the soils and rocks of the earth's surface, a 

 study of the radioactive properties of the underground air 

 might be expected to furnish valuable data on the nature and 

 origin of atmospheric radioactivity. 



Ebert,§ Dadourian|| and Blanc^f have investigated the radio- 

 activity of the underground air. In each instance the active 

 deposit was collected upon a negatively charged electrode over 

 which underground air was drawn, or which was allowed to 

 stand in a vessel filled with underground air. Blanc placed a 

 large vessel, opened at the bottom, upon the ground and col- 

 lected, upon a negatively charged electrode, the active deposit, 

 produced by the emanation diffusing into it. He plotted the 

 decay curve of this active deposit and, from the curve, deduced 

 the amount of thorium active deposit initially present. He 

 then allowed thorium emanation to diffuse into the same vessel 

 from a tube containing a known amount of thorium hydroxide 

 mixed with sand. On comparing the amounts thus obtained, 

 he concludes that there must be at least l - 45 X 10" 5 gram of 

 pure thorium per gram of vegetal earth in Route. 



In one of the earlier experiments of Elster and Geitel,** the 

 underground air was drawn continuously into the electroscope 

 for an hour. The rise of the activity, due to the production 

 of active deposit, was measured and, when this had reached a 

 maximum, the current of air was stopped and the decay of the 

 activity followed for many hours. They afterward devised 

 and employed the method of collecting the active deposit on a 

 negatively charged electrode which is subsequently placed in 

 the ionization vessel. An examination of the literature of the 

 subject has failed to disclose any other instance where the 



*This Journal, xxv, 335, 1908. f Phil. Mag., xvii, 321, 1909. 



JPhys. Zeitschr., ix, 177, 1908. §Phys. Zeitsckr., iv, 162, 1902-3. 



|| This Journal, xix, 1G, 1905. 



^"Phys. Zeitschr., ix, 294, 1908; Phil. Mag., xviii, 146. 1909. 

 **Phys. Zeitschr., iii, 574, 1901-2. 



