26 RADIOACTIVITY A FACTOR OF PLANT ENVIRONMENT 



in the emanation of thorium or radium can collect a deposit formed 

 by the disintegration of the atoms of the emanation, hence the con- 

 clusion from the above experiments, that the air contains a radio- 

 active emanation. 



Evidence of the presence of negative ions in the air was known, 

 however, long before radioactivit}'^ was discovered. Thus Giese, ^^ in 

 1882, observed the electrical conductivity of gases from flames, and in 

 1897 Kelvin and MacLean,''^ investigating the flames of a bunsen 

 burner, a candle, an alcohol lamp, and a paraffin lamp, found a small 

 negative charge in the gases drawn from them. Charcoal and coal 

 " both gave negative electrification when there was a flame ; and both 

 gave positive electrification when they were glowing without flame." 

 These investigations were extended by MacLean and Goto,^" who 

 showed that air is electrified by the burning of matches, wood, paper, 

 and many other substances. So also McClelland ^^ in 1898. Waves 

 of ultra-violet light, and point-discharges of electricity produce nega- 

 tive ions in the air.^^* 



The formation of ions by a candle flame was demonstrated by 

 Ayrton,* who observed that such a flame can discharge an electro- 

 scope in 40 seconds at a distance of 40 cm. " The flame of a match 

 had no less power, and an electric arc no more power than an unin- 

 sulated candle flame placed at the same distance." 



Traubenberg ^^^ found the atmosphere in the vicinity of the crater 

 of Vesuvius strongly ionized, and in 1904 Allan ^ showed that the 

 excited radioactivity from the atmosphere behaves like that from 

 thorium and radium, and contains both a and /9 rays. In the same 

 year Elster and Geitel ^^ pointed out that the electrical conductivity of 

 the atmosphere is due largely, if not wholly, to a radioactive emana- 

 tion which issues from the earth's crust. These authors are of the 

 opinion that the outer layers of the atmosphere doubtless become 

 ionized by the sun's rays of short wave-length, and that perhaps ^ 

 and '( rays also proceed from the sun and produce a like effect. In 

 this connection, it is known, from the investigation of Bacon, ^ that 

 an electroscope discharges nearly 18 times as fast in sunlight as in 

 ordinary diffuse light in the middle hours of the day. McClelland ^^ 

 believed that the ionization of the atmosphere points to the presence 

 in it of some radioactive substance, and Blanc ^^ showed experi- 

 mentally that transformation products of radio-thorium are, at least 

 in the vicinity of Rome (Italy), a most important agent in atmos- 

 pheric radioactivity. 



