112 Radioactive Matter present in the Atmosphere. 



(5) Assuming that the radium in the earth is equally 

 distributed in amounts sufficient to maintain the temperature 

 gradients actually observed in the earth, then the radium 

 emanation in the air is derived from an average depth 

 estimated to be between 5 and 17 metres. 



All these estimates and results are based on the assumption 

 that the conditions at Montreal are normal, and represent the 

 average. It most be remembered that the country was 

 covered with snow from one to three feet deep, and that the 

 temperature was sometimes as low as 10 degrees, or 15 degrees 

 below zero Fahrenheit. But after the snow had melted, and 

 the frost had left the ground, the active deposit on a wire in 

 the zinc cylinder on the College Campus was almost equal in 

 value to that obtained in the depth of winter. Allan also 

 found that the summer and winter values at Montreal showed 

 but slight variation. 



It is most desirable that experiments should be repeated 

 elsewhere similar to those described in this paper, particularly 

 that suggested by Professor Rutherford, and described in 

 Section IV. If any investigators have at their disposal a 

 very large tank which can be rendered absolutely air-tight, 

 it would be interesting to observe whether the natural leak 

 of a well-insulated wire suspended in it and connected to an 

 electroscope would fall in a few days at the same rate as the 

 decay of radium emanation. Fresh air could then be intro- 

 duced from without, and the experiment repeated. Such an 

 investigation would determine absolutely whether the natural 

 ionization is entirely due to the emanation present in the 

 air. There might be a small residual ionization due to the 

 sides of the vessel. A large steam-boiler might serve as a 

 tank. Observations at sea are also needed, both on the rate 

 of production of ions, and on the excited activity which can 

 be collected. 



In conclusion, I am indebted and grateful to Professor 

 Rutherford, both for suggesting the experiment, and for his 

 advice, encouragement, and resourcefulness when difficulties 

 occurred. 



McGill University, Montreal. 

 3rd April ,'1905. 



