THE 



AMERICAN JOURNAL OF SCIENCE 



[FOURTH SERIES.] 



Art. I. — Atmospheric Radio-activity • by EL A. Bumstead. 



It has recently been shown by a number of investigations, 

 made in widely separated localities, that the radio-active gas 

 obtained from the earth, from water, and from petroleum, has 

 the same properties as the emanation from radium.* Some of 

 the emanation must of course be present also in the air above- 

 ground, and it is an immediate inference that the radio-active 

 phenomena of the atmosphere may be due to its presence. 

 Thus one might expect that the activity deposited upon a nega- 

 tively-charged wire when it is exposed in the open air would 

 decay at the same rate as the excited activity due to radium, 

 and Elster and Geitel have recently made some measure- 

 mentsf which confirm this expectation, at least approximately. 

 On the other hand, Rutherford and Allan, J who were the first 

 to investigate carefully the rate of decay of the negatively- 

 charged wire, obtained a different result ; they found that the 

 activity falls off regularly according to an exponential law and 

 reaches half-value in about 45 minutes, whereas the radium- 

 induced activity does not fall off exponentially during the first 

 two hours, and, when it does begin to do so, its half -value time 

 is 28 minutes instead of 45. Quite recently, Allan has made 

 an extended study of atmospheric radio-activity§ in the course 

 of which he has, in the main, confirmed the previous results 

 obtained by Rutherford and himself with regard to the rate of 

 decay, although certain observations point to the conclusion 

 that the decay is not entirely regular. 



* Adams, Phil. Mag., Nov. 1903; Elster and Geitel, Phys. Zeitschr., v, 

 p. 11, 1904 ; Bumstead and Wheeler, this Journal, Feb. 1904 ; Himstedt, 

 Drud. Ann., xiii, p. 573, 1904. 



\ Elster and Geitel, loc. cit. 



% Eutherford and Allan, Phil. Mag., Dec. 1902. 



§ Allan, Phil. Mag., Feb. 1904. 



Am. Jour. Sci.— Fourth Series, Yol. XVIII. No. 103 —July, 1904. 



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