Joi^Y AND Smvth — Aviount of ttadium Emanation in the Soil. 14^ 



Tlie variations in tlie amount of active deposit from one locality to 

 another are very marked. lu hilly and mountainous regions — whether 

 observations are made in the valleys or on the hill-tops — high readings are 

 obtained. In sucli regions the surface-area of the ground is larger than 

 upon the plains; and the observations ai-e, therefore, in keeping with the 

 view that the active substances originate in the earth. Near the sea the 

 readings are generally lower. But the average amouut of radioactive con- 

 stituents in sea air cannot be said to be determinable from existing observa- 

 tions. The lately published observations of Simpson and Wright render it 

 certain that there is considerably less over the sea than over the land.' 



No steady interdependence between the amount of active constituents in 

 the atmosphere and the temperature, moisture, time of year, or daily period 

 of the potential gradient of the atmosphere, has been demonstrated. There 

 is some evidence that a steadily falling barometer is attended by an increase 

 in tiie amouut of active substances in the air ; but there are many discordant 

 observations. One fact may be regarded as supported by the majority of 

 observations : — that settled anticyclouic conditions, involving still weather, 

 are favourable to an accumulation of radioactive constituents in the lower 

 atmosphere. Eve,^ however, arrived at the opposite conclusion. It is 

 evident that if the soil is the source of the active constituents, the local con- 

 ditions will largely enter into the question as to whether a still atmosphere 

 is favourable or otherwise to the radioactivity of the atmosphere. 



The existence of radioactive matter in tlie atmosphere is connected with 

 the presence of radioactive substances in the rooks by observations upon the 

 active character of gases contained in the pores and capillaries of the soils. 

 There is a striking parallel between the results of tlie observations upon 

 " ground-air " and those we have been reviewing. 



The radioactivity of the ground-air has been directly observed. Elster 

 and Geitel found that air withdrawn from the soil possessed a conductivity 

 as much as thirty times that of the atmosphere, and that this conductivity 

 was not fixed in amount for a given sample of air, but at first increased for 

 some hours — a proof of its radioactive character. It is important to note 

 that the measure of the conductivity of the air does not express the emana- 

 tion content. The presence of moisture or dust in such gases will often, by 

 reducing the number of small ions, lower the conductivity, although much 

 emanation may be present.' Ebert and Ewers' deprived ground-air of its 

 small ions by passing it through a strong electric field, and found that such 

 air had, in three hours, regained its original conductivity — a result due 



1 Proc. Eoy. Soc, May, 1911. = Phil. Mag., Oct., 1908. 



3 A. Gookel, Die Luft Electiicitat. * Phys. Zeit. iv., 162, 1903. 



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