JoLY AND Smyth — Amount of Radium Emanation in tlie Soil. lo5 



The observations are, so far, confined to the amount of radium emanation 

 contained in the gases of the soil. It is necessarily part of the inquiry to 

 determine whether these are of sufficient richness to allow of the atmospheric 

 emanation being ascribed to this source. The experiments show that in every 

 case the emanation content of the soil-gas is very much greater than that of 

 the atmosphere — in most cases some thousands of times. Obviously this is 

 but a step in the investigation. Tlie emanation in the capillaries of the soil 

 might pursue its changes within the soil, and never escape into the atmo- 

 sphere. It will presently be seen that the variations in the emanation content 

 of the ground-gases, taken in connexion with the weather changes progressing 

 during the experiments, afford strong evidence that the emanation escapes 

 freely into the atmosphere. More direct proof of this is, however, very desirable. 

 Accordingly, observations were made both in the College Park and at Somerset 

 House to determine, if possible, the rate at which the emanation quitted the 

 surface of the soil. 



The observation of this phenomenon presents some difficulties. Un- 

 doubtedly under normal conditions the motion of the air over the surface of 

 the ground must greatly facilitate, if it does not actually occasion, the escape 

 of ground-gases. A collector in the form of a bell-shaped vessel placed mouth 

 downwards upon the surface of the ground does not, therefore, realize the 

 natural conditions when there is any motion of the air. It does not even 

 realize the conditions of the stillest weather, for the diffusion outwards, 

 attending the differing composition of the ground-gases from those of the 

 atmosphere, must ultimately be brought to an end by the attainment of 

 uniformity of chemical composition immediately beneath and above the 

 surface. A collecting-vessel of this sort left in situ for an hour will 

 accumulate but little emanation, as may be determined by filling an electro- 

 scope from its contents. Doubtless if left in situ long enough the result would 

 be different. The point is that this arrangement does not realize the con- 

 ditions which go towards accelerating and facilitating the escape of emanation 

 at the surface. It permits, in fact, only of diffusive movements, and these 

 under very unfavourable circumstances. 



A modification of this arrangement which secures a gentle motion of the 

 air over the surface of the soil, without at the same time creating a local 

 reduction of atmospheric pressure, serves to show that during the space of one 

 hour a very considerable amount of emanation actually leaves a soil freely 

 exposed to the atmosphere. 



The collector designed on these principles will be easily understood from 

 the figure. It consists of a cylindrical vessel of tin plate, about 25 cms. in 

 diameter and 8 cms. deep. One end is closed save tor a central opening 



SCIENT. PKOC. E.D.S., VOL. XHI., NO. XI. 2 A 



