Radioactivity and Ionization of the Atmosphere, 711 



on a bright warm day in the spring. The most powerful 

 factor in determining the amount of activity given to the 

 wire is the presence or absence of wind. A windy day 

 always gave much greater effects than a quiet day, when 

 other conditions were the same. This is true whether the 

 air was cold or warm, or the day bright or dull. 



Most of the experiments were made during the Canadian 

 winter, when there was about two feet of snow over the ground. 

 The prevailing wind was from the north, and had been carried 

 over snow-covered lands. The fact that the amount of 

 activity was uninfluenced by the presence of snow shows that 

 the excited activity is not likely to be due to any effect arising 

 from vegetation. The amount of water-vapour in the air 

 appears to have little influence on the result, for at — 20° C, 

 the air is extremely dry. 



Penetrating Power of Excited Radiation, 



It has been shown in a previous paper* that the penetrating- 

 power of the excited radiations of thorium and radium was 

 the same. As the penetrating power is one of the method.^- 

 of distinguishing between the various radiations a special 

 experiment was made to compare the penetrating power of 

 the excited radiation from the air with that of other known 

 radiations from radioactive substances. 



Lead wire was employed in these experiments as it could 

 readily be retained in the form of a flat helix. The wire was 

 excited by exposure of two to three hours at —30.000 volts. 

 It was then wound to form a flat helix and placed between a 

 parallel plate apparatus. The ionization current between the 

 plates was observed for different numbers of sheets of thin 

 aluminium foil placed over the helix. The average thickness 

 of the aluminium foil was •00034 cm. 



The results are shown in Curve I. rig. -1 (p. 712) where 

 the penetrating power of other known types of radiation 

 are added for comparison. 



The excited radiation due to air has greater penetrating- 

 power than any of the types of radiations, not deviated by a 

 magnetic field, from the radioactive substances uranium, 

 thorium, and radium, and i- also more penetrating than the 

 excited radiation produced by radium and thorium. 



No special experiment has been made to determine the 

 absorption of the excited radiation in its passage through the 

 air, but its approximate amount can be readily deduced from 

 known data. In all the different types of radiations examined 



* E. Rutherford and Miss II. T. Brooks, Phil. Mag. July 1902. 



