H. M. Dadourian — Atmospheric Radio-activity. 335 



Art. XXXYIII. — On the Constituents of Atmospheric Radio- 

 activity ; by H. M. Dadourian. 



[Contributions from the Physical Laboratory of 

 The Sheffield Scientific School of Yale University.] 



1. The study of the atmospheric radio-activity may be said 

 to have had its origin in an interesting discovery made by Elster 

 and Geitel* in the spring of 1901. They found that if a nega- 

 tively charged conductor be exposed to the open air for a short 

 time, it temporarily exhibits the properties of radio-active 

 bodies. They showed further in a series of investigationsf that 

 this activity is due to a substance in the solid state, which is 

 attracted towards and deposited upon the charged body under 

 the action of the electric field. The active deposit was isolated 

 by dissolving it in acid solutions and evaporating the solution 

 to dryness ; the residue was found to possess the general prop- 

 erties of the rapidly disintegrating radio-active products. 



A number of investigators,^: working under different experi- 

 mental conditions and in widely separated localities, conducted 

 experiments similar to those of Elster and Geitel, and obtained 

 similar results. Rutherford and Allan, £ who made a study 

 of the rate of decay of the active-deposit, found that its 

 activity fell off exponentially to half value in about 45 minutes. 

 This is not close enough to the half-value periods of radium 

 and thorium emanations to justify one to ascribe the atmos- 

 pheric radio-activity to the presence in the atmosphere of either 

 of these emanations alone. Therefore they arrived at the fol- 

 lowing conclusion : " From the differences observed for the 

 penetrating power and the rate of decay we can conclude that 

 the excited radiation from air cannot be ascribed to the presence 

 of any known radio-active substance in the atmosphere." § 



Later it was shown by Elster and Geitel | that the decay 

 curves of active-deposits obtained from the air and from radium 

 emanations agreed fairly well. But the observations on the 

 decay were not carried beyond two hours after the removal of the 

 potential difference from the negatively charged body. Thus 

 the agreement was confined to within a rather small region of 

 the activity and iime-diagram. Furthermore the agreement 

 was not as close as would be expected from the precision of the 

 experimental data, if the active-deposits obtained from the air 

 were due to radium emanation. Soon after the appearance of 



*Elster and Geitel, Phys. Zeitschr., ii, 590, 1901. 



f Elster and Geitel, ibid., iii, 305, 1902, iv, 522. 



% Rutherford and Allan, Phil. Mag., iv, 352, 1902 ; Gockel, Phys. Zeitschr., 

 v. 591, 1904; J. J. Thomson, Phil. Mag., iv, 352, 1902; Himstedt, Phys. 

 Zeitschr., iv, 482, 1903. 



§Loc. cit., 712. 



|| Elster and Geitel, Phys. Zeitschr., v, 11, 1904. 



