700 Messrs. McLennan and Burton on the 



bodies exposed under negative electrification. This phenome- 

 non of induced or excited radioactivity had been previously 

 observed by Rutherford in bodies exposed to air drawn from 

 the neighbourhood of thorium compounds, and had been 

 connected by him very directly with an emanation which 

 these salts emit. This emanation he found possessed the 

 property not only of exciting radioactivity in all solid sub- 

 stances in its neighbourhood, but also of ionizing any gas 

 with which it was in contact. 



Since atmospheric air has been shown by Elster and 

 Greitel*, C. T. R. AYilsont. and others to be continually 

 ionized by some agent, and since it has also been shown to 

 possess the property of exciting radioactivity, one is forced to 

 conclude there is present in the air an emanation possessing' 

 properties similar to that emitted by thorium compounds. 



Hitherto the source of such an emanation has not been de- 

 termined, but, as the phenomena of induced radioactivity and 

 spontaneous ionization universally characterize atmospheric 

 air, it seems evident, since thorium compounds are but sparsely 

 distributed in nature, that sources other than these must 

 exist. 



Recalling the experiments of Elster and Greitel, it sterns 

 probable that the earth's surface, and possibly too the materials 

 used in the construction of their apparatus, are sources of 

 this emanation. As but little evidence existed in favour of 

 this conclusion, the writers recently made a series of observa- 

 tions upon atmospheric air confined in air-tight vessels of 

 different metals. The result of the investigation showed that 

 the conductivity of the inclosed air depended very largely 

 upon the material of which the receiver was made, and the 

 effects observed would seem to indicate that all metals, in 

 varying degree, are the sources of a marked though feeble 

 radioactive emanation. 



2. Apijaratus. 



In these observations the air whose conductivity was to be 

 measured was confined in a cylinder, 125 cms. in length and 

 2b cms. in diameter, similar to that shown in fig. 1. In the 

 first experiment it was made of sheet-iron coated with zinc, 

 and in the later experiments linings of various metals were 

 inserted in order to examine their effect upon the conductivity 

 of the inclosed air. The bottom and cover were removable, 

 and, when in position, were made air-tight by means of 

 cement. Into a flanged opening in the cover was fitted an 



* Loc. cit. 



f Proc. Roy. Soc. March 1901. 



