RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 29 



trating cathode rays traverse our atmosphere without being absorbed 

 until they encounter the solid mass of the earth which thus becomes 

 negatively charged. An excess of negative electricity is also carried 

 from the atmosphere to the earth by rain. The positive charge thus 

 left behind in the air is carried by convection currents to other 

 regions. Thus the negative charge of the earth is maintained. As 

 Wilson states, the fact that the earth's surface is negatively charged, 

 and that free positive ions exist in the atmosphere, must result in a 

 continuous flow of positive electricity from the atmosphere into the 

 ground. 



The hypothesis has been proposed by Villard ^^"^ that cathode rays 

 giving rise to the aurora borealis have their origin, not in the sun, but 

 in the earth itself. Even the spraying of liquids may produce ioni- 

 zation, the negative ions, as Eve'^' has shown, being greatly in excess 

 of the positive. Strong "-• "'^ thinks that the penetrating radiation that 

 causes ionization in closed vessels is probably due to gamma rays 

 from radioactive products in the air, rather than in the ground, but it 

 is considered probable that these products originate in the ground, as 

 the theory of Elster and Geitel indicates. The products vary much 

 in quantity according to atmospheric conditions. Eve ^* states that 

 the ionization of the air is due to radioactive changes in both the air 

 and the soil. Of course the amount of emanation coming into the air 

 from the soil would vary with the rise and fall of the water-table in 

 the soil, the soil temperature, the entrance into the soil of rain water, 

 and the decrease of barometric pressure, all of which would be accom- 

 panied by a flow of emanation out of the soil into the lower layers of 

 the atmosphere.* According to McLennan's *^ calculation, " approx- 

 imately 9 ions per c.c. per second are generated in free air by the 

 penetrating radiation from the earth." 



Eve ^^ has called attention to the fact that the radium C in the air 

 is carried to the earth not only by falling rain, snow, dust, or smoke, 

 but also by the potential difference in the atmosphere. Thus the 

 radioactive matter in the air is decreased, that in the soil increased. 

 Thorium C has been found in the atmosphere of both hemispheres, 

 with an activity about one half that of the radium C present. Since 



* Another paper by Strong "3" has appeared since the above sentence was written. 

 He finds a relatively enormous amount ot " external radiation " during the forenoon 

 from 8 A. M. until 2 or 3 P. M. " This may be due," he says, " to the expansion of 

 the air in the soil and an increase of emanating power due to the heating by sunshine^ 

 or it may be due to a change of barometric pressure." 



