RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 27 



C. T. R. Wilson '^'^ calculated that the number of ions being pro- 

 duced per second in each cubic centimeter of air is about twenty. 

 The actual number of ions present per c.c. of air varies with the 

 meteorological conditions, being greater in clear, sunny weather. 

 The number usually fluctuates between 500 and several thousands, 

 and the positive ions are more numerous than the negative.^'' 

 Schuster ^"^ found the following numbers of ions per c.c. of air in 

 Manchester (Eng.), under the conditions indicated: 2,370 (snowing 

 at intervals, with spells of sunshine. East wind. In a field). 3,600 

 (cold, bright, gusty. East wind. On a roof). 3,660 (cold, dull. 

 East wind, on a roof). In addition to swiftly moving ions, Lange- 

 vin '^^ found slowly moving ones also present. The latter are about 

 forty times as numerous as the former. Langevin's experiments 

 were made on top of the Eiffel tower. 



A negatively charged conductor placed in the open air becomes 

 temporarily radioactive,^' and this radioactivity can be removed by 

 solution in an acid. This experiment was repeated in Canada by 

 Rutherford and Allan, ^'^ who found that the greatest amount of ex- 

 cited radioactivity on a negatively charged wire was obtained during a 

 strong wind. Allan^ thinks the activity of the air is " probably due 

 to an emanation of positive electrons in the air, ever present though 

 in varying quantities."* Elster and Geitel ^^ found that the radioac- 

 tivity of the air of any given place varies with the nature of the soil 

 of the locality, and later in the same year '^'* found that sea air is only 

 about one third as active as air over the land at Wolfenbiittel. Soon 

 after this Himstedt*^^ showed that air through which water is blown, 

 or air that has passed through or over surfaces moistened with water 

 has its electrical conductivity increased (by ionization) more than 100 

 times. This, he thinks, is the origin of the conductivity of the air 

 from cellars and soil, observed by Elster and Geitel, as well as of the 

 conductivity constantly present in the free atmosphere. In the fol- 

 lowing year Himstedt^^ reported that air sent through the water from 

 many springs, and fresh ground water has its conductivity increased, 

 and he contends that there is widely (perhaps universally) distributed 

 in the earth a radioactive material from which a gaseous emanation 

 proceeds which is absorbed by water and oil, is carried by these to 

 the surface, and thence distributed to the atmosphere. 



In connection with Elster and Geitel's observation it is interesting 



* Cf . also Stewart, R. M.i" 



