30 RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 



the emanation of thorium decays about 6,000 times as fast as that of 

 radium, and "has a poor chance of escaping from the soil," the 

 amount of thorium C in the ground exceeds the amount of radium C, 

 and the thorium C in the ground will be more than 15 times that in 

 the air. Eve says that, ♦' in most localities the penetrating radiation 

 due to active matter in the air is less than one fifteenth that due to 

 active matter in the earth." Recent determinations by Dadourian^* 

 indicate that the amount of radium emanation present in the air at 

 New Haven, Conn. (U. S. A.), is from 20,000 to 50,000 times as 

 great as the amount of thorium emanation. 



Radioactivity in Snow and Rain: In 1902 Professor J. J. 

 Thomson observed that water drops falling through air that contains 

 ions remove the ions, and in the same year C. T. R. Wilson ^'"' ^" tested 

 freshly fallen rain and found it radioactive. By adding barium 

 chloride to rain water and precipitating the barium with HgSO^ he 

 found the precipitate to be radioactive. Subsequently Allan * ob- 

 tained a radioactive residue by evaporating freshly fallen snow to 

 dryness. From about one liter of snow that fell during a heavy storm, 

 there could be obtained about the same effect as from o.i gr. of 

 uranium. The amount of activity varied with the amount of snow 

 falling per second, and was constant so long as the fall of snow was 

 constant. Later in that year the same writer "^ found that the radioac- 

 tive residue could be rubbed off onto a piece of cotton, and when this 

 was burned the ashes were still radioactive. Allan supported the 

 theory that some process is continually going on in the air pro- 

 ducing radioactive carriers which are removed by the snow-flakes. 

 C. T. R. Wilson ^" also detected the activity of freshly fallen snow. 

 Kaufmann has determined that such snow is, under similar cir- 

 cumstances, as active as rain, but snow falling on roofs loses its 

 activity sooner than snow that falls on the ground.* The loss of 

 activity appears to vary with atmospheric pressure. Righi ^* found 

 that, during a snowfall, the electrical conductivity of the air is more 

 than doubled. 



Soil air from Wolfenbiittel was found to contain a radioactive 

 material by Elster and Geitel,^^ and by Ebert and Ewers *^ at Munich. 

 GeiteP" found that the activity of the air from the soil in his garden 

 did not apparently diminish in eight months, but the ashes of plants 

 which had grown on active earth gave off no appreciable rays. 



* This result was confirmed by Constanzo and Negro, ^^ in 1906. 



