Sxcrion IIL., 1902 [97] Trans. RB. S. C. 
X.— On Exciled Radioactivity. 
By Mr. R. M. STEWART. 
Advanced Student, University of Toronto. 
(Communicated by President Loudon and read by Professor J. C. McLennan 
May 27, 1902.) 
(1.) INTRODUCTION. 
Professor Rutherford! has shown that air which has been passed 
over thorium oxide, or other compounds of thorium, possesses the 
power, in the absence of an electric field, of imparting radioactivity 
to any body with which it comes in contact. 
He also found that when this air is drawn into a receiver con- 
taining a negatively charged conductor, the excited radioactivity at 
ordinary pressures is confined entirely to the negatively charged body 
and does not appear to be induced in any degree on the walls of the 
receiver. 
On the other hand, when a positively charged wire is inserted 
in the receiver containing air which has been passed over thorium, 
the walls of the containing vessel become radioactive but the posi- 
tively charged wire does not. 
Quite recently Elster and Geitel? have found an analogous effect 
when a negatively charged body is exposed in atmospheric air. They 
find that negatively charged conductors, on being exposed for some 
hours in the open air, gradually become radioactive, while conductors 
which are positively charged remain inactive. 
This radioactivity which is excited or induced is of a temporary 
character and does not persist after the exposure, but decays in the 
course of a few hours. In this respect it is very similar to the 
activity excited by the action of thorium compounds. 
From these results it would appear that excited radioactivity 
can be communicated in an electric field only to conductors which 
are negatively charged. 
In his paper on radioactivity induced by the action of thorium 
compounds, however, Rutherford® describes an experiment which 
suggested the possibility of concentrating radioactivity under certain 
conditions upon positively charged conductors, as well as on those 

1 E. Rutherford, Phil. Mag., 49, page 1 and page 161, 1900. 
? Elster & Geitel, Physikalische Zeitschrift, No. 40, page 590. 
* —. Rutherford, Phil. Mag., page 186, Feb., 1900. 
