IX, A, 1 Wright and Smith: Radium Emanation 71 



great oceans the air contains a considerable amount of radium 

 emanation. Since tests of sea water have shown that its radium 

 content is very low, the greater part of the emanation over the 

 sea must be transported by air currents from the land. The 

 question also arises, to what extent is the emanation carried 

 upward before much of its activity is lost? The rate of diffusion 

 of the emanation is too slow for it to reach any considerable 

 height before losing most of its activity, but upward air currents 

 may carry it to a comparatively high altitude. Nevertheless, 

 it is to be expected that the amount of radium emanation in the 

 atmosphere diminishes with altitude. 



Flemming,^" from balloon observations, found that a negatively 

 charged wire collected about the same amount of radioactive 

 deposit at an elevation of 3,000 meters as at the earth's surface. 

 Saake ^^ and Gockel," from observations taken on mountain 

 peaks, by the same method, found that the active deposit was 

 greater at high altitudes than at sea level. W. Knoche,^^ likewise, 

 obtained a high value for the active deposit at an elevation of 

 5,200 meters in the Bollivian Cordilleras. 



The active-deposit method, however, is not adapted to an 

 accurate determination of the radium-emanation content of the 

 atmosphere, and it is very doubtful if the results at different 

 altitudes have even a comparative value. Rutherford^* appends 

 his discussion of the results of Flemming, Saake, and of Gockel 

 with the following remarks: 



It does not necessarily follow that the air at great altitudes contains 

 more radium or thorium emanation, for the amount of active matter 

 collected for a given volume distribution of emanation will depend on the 

 pressure of the air and the amount of dust or other nucleP'' in the at- 

 mosphere. 



From the above discussion the importance of a direct deter- 

 mination of the radium-emanation content of the air at high 

 altitudes is perfectly evident. The charcoal-absorption method 

 is well adapted for obtaining observations at different altitudes 

 which will be directly comparable. Since the method is a com- 

 parative one, each observation on the emanation in the air being 

 taken simultaneously with a determination of the emanation 



"Phys. Zeitschr. (1908), 9, 801. 

 "Ibid. (1903), 4, 626. 

 "Ibid. (1907), 8, 701. 

 "Ibid. (1912), 13, 440. 



" Radioactive Substances and their Radiations. University Press, Cam- 

 bridge (1913), 631. 



" The italics are ours. 



