i50 Scientific Proceedings^ Uoyal t)ublin Society, 



to the break-up of tlie emanation wliicli, owing to its purely gaseous 

 properties, Lad escaped the electric field. 



The radioactivity of ground-air is very variable from time to time in the 

 one locality. H. Brandes' found that tlie emanation content of ground-air 

 increases witli a slow fall of the barometer, and vice versa ; but sudden changes 

 of pressure maj' produce effects of the opposite cliaracter. No connexion 

 between the content of emanation and the actual height of the barometer 

 could be traced. GrockeP confirms the observations as to the variability 

 of tlie ground-air from time to time, and finds tliat the most effective 

 influences are tliose wliich affect tlie permeability of the soil, sucli as rain or 

 frost. These tend to increase the emanation content, by choking the 

 capillaries. 



Eve' estimated the amount of radium emanation in the atmosphere by 

 abstracting the active deposit from the air enclosed in a large tank. By a 

 comparison of the ionization produced by a known amount of the active 

 deposit of radium with that due to the active deposit derived from the air in 

 the tank, he concluded that in a cubic metre of air the amount of emanation 

 which would be in equilibrium with 80 x 10"''- gram of radium must be 

 present. 



Since this observation of Eve's, measurements of the radioactive sub- 

 stances in the atmosplicre have been made by direct abstraction of the emana- 

 tion. Thus Eve,^ Ashman,^ and Satterly" have used either tlie absorptive 

 properties of charcoal, discovered by Rutherford, or the low temperature of 

 liquid air, to condense it directly from tlie atmospliere. In this manner the 

 emanation from 100 to 200 litres of air has been brought into tlie electro- 

 scope. These observatious confirm Eve's original measurement. Satterly 

 finds 100 X 10"'" per cub. metre. The variations from day to day are con- 

 siderable. Ashman finds the equivalent radium to vary from 45 x 10"'^ to 

 200 X 10"'' gram per cubic metre. Eve' finds tlie variation as mucli as 1 to 7. 



Of the reliability of these results there can be no doubt ; and, following Eve, 

 we may accept 80x10"'- gram as the amount of radium in equilibrium 

 with the emanation contained in one cubic metre of average air. How high 

 in the atmosphere tliese conditions extend is not known. Tlie emanation has 

 a half-life period of 3'7 days. The circulatory movements of the atmosphere 

 are comparatively rapid, so that a marked falliug-off in tlie contents of radium 

 emanation upwards is not to be expected. Nevertheless, if we assume tiiat 



' H. Brandes, Inaugural Dissertation, Kiel, 1900. ■ A Gockel, Phys. Zeit. ix., 30i, 1908. 



= Eve, Phil. Mag., July, 1905. * Eve, Phil. Mag., Oct., 1908. 



6 Ashman, Am. J. Sci., xxvi, 119, 1908. « Satterly, Phil. Mag., Oct., 1908. 

 ' Eve, loc. cii. 



