88 MR. GEORGE C. SIMPSON ON THE 



I found it impossible to make observations of the ionization and dissipation at the 

 same time as those of the radio-activity. This is much to be regretted, as it is very 

 important to decide if the emanation in the atmosphere is the cause of the permanent 

 ionization. That the ionization does not depend on the amount of emanation alone is 

 quite clear from the yearly variations of the two, for the ionization is at a minimum 

 during the winter, exactly the season when the activity is at its maximum. But that 

 does not prove that the ionization is not due to the emanation ; we can only say that 

 if it is, then the increase in the production of ions owing to the excess of emanation is 

 overbalanced by the increased rate of recombination due to the winter conditions. 



That all the relations shown by the above analysis should be as they are gives an 

 exceedingly strong support to ELSTER and GEITEL'S theory of the origin of the 

 atmospheric radio-active emanation. According to their theory, the air which is 

 mixed up with the soil of the ground becomes highly charged with radium 

 emanation.* When the barometer falls, this air passes out of the ground into the 

 atmosphere, bringing with it its charge of emanation. 



All the facts of the above analysis receive a very simple explanation by this 

 theory if one extends it to include the fact that, as the emanation is a gas contained 

 in the soil, it must constantly diffuse into the atmosphere above quite independently 

 of the state of the barometer. Assuming this constant diffusion, we at once see that 

 everything which tenth to reduce the atmospheric circulation, i.e., to keep the air 

 stagnant, tends a/so to increase the quantity of emanation in the lower layers of the 

 atmosphere. 



Looking now at each of the tables in order, we see that the temperature does not 

 have a direct, but an indirect influence on the radio-activity. This is explained by 

 the fact that the low temperature of the winter produces a nearly permanent 

 temperature inversion, as mentioned above, which entirely prevents ascending 

 currents of air. Thus the emanation on leaving the ground in cold weather cannot 

 rise, but collects in the lower atmosphere, causing the high winter values of the radio- 

 activity. 



The reason why the radio-activity is high with high relative humidity is easily 

 found when one considers that each evening, as the temperature rapidly falls, two 

 things happen : first there is a rapid rise in the relative humidity and secondly 

 ascending currents of air are cut off. The latter fact gives rise to the high radio- 

 activity. Also a mist or fog is always a sign of stagnant air. 



A high wind is naturally accompanied by low activity, for it acts as a stirrer, and 

 rapidly mixes the escaped emanation with a large volume of air. 



ELSTER and GEITEL'S theory explains the relation found between radio-activity and 

 a rising and falling barometer. If air stream out of the ground when the barometer 

 falls, it must charge the atmosphere with its emanation. 



* 'Phys. Zeit., 1 5, p. 11, 1904; 'Terr. Mag.,' vol. 9, p. 49, 1904. 



