28 RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 



to compare the variations in the penetrating radiation from earth, 

 studied by Wright ^^^ in 1908. Measurements of this radiation made 

 in different localities on the north and south shores of lake Ontario, 

 and at different points on the surface of the lake during the passage 

 of a steamer, show that there is greater activity inland and at points 

 along the shore than over the lake between Toronto harbor and the 

 Niagara river bell-buoy. The activity was measured by the number 

 of ions generated per c.c. per second within a lead cylinder.* 



Studies on the effect of altitude enabled Saake ^""^ to state that there 

 was a larger amount of emanation in the high altitudes of the valley 

 of Arosa, Switzerland, than is normal at lower levels. Eberf*" has 

 found that a radioactive emanation can be removed from the atmos- 

 phere by condensation in liquid air, while Ramsay and Soddy ^^ have 

 demonstrated that the inert gases in the atmosphere (helium, neon, 

 argon, krypton, and xenon) have no radioactivity of their own. 

 That the atmosphere at New Haven, Connecticut, contains the emana- 

 tion of both thorium and radium was announced by Bumstead ^**' *^ in 



1904- 



Antedating these discoveries, was the paper of Elster and Geitel^^ 



on the existence of electrical ions in the atmosphere. The authors 



found that the free atmosphere contains positively electrical masses, 



and considered the existence of these free ions as the most suitable 



basis for a rational theory of atmospheric electricity. This theory 



was further elaborated by Geitel®* in 1901. 



Elster and Geitel did not explain the origin of these ions in the 

 free atmosphere, but, in the year following their discovery, Lenard^^ 

 found that the rays of ultra-violet light generate cathode rays, and 

 two years subsequently he"*^ published the results of further investi- 

 gations along the same line, stating that the cathode rays thus gen- 

 erated show diffusiveness, and must be largely absorbed by gases. 



In the light of Lenard's experiments C. T. A. Wilson "^concluded 

 that the sunlight ionizes the atmosphere through which it passes, espe- 

 cially in the upper layers, where the sunlight is still strong in ultra- 

 violet rays. 



Certain facts, says Wilson, render it not improbable that pene- 



*It has not been thought necessary, in reviewing the above literature, to explain 

 that electrical conductivity', through wires or air, or whatever else, is accomplished by 

 the passage of electrons, or other ions. Increase in the conductivity of air and water 

 means an increase in the number of the ions they contain. The fundamental ideas 

 here involved are clearly set forth bvFournier (Bibliography, p. 17, No. 53), and others. 



