
354 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
210. The excited activity produced on the negatively charged 
wire cannot be due to an action of the strong electric field on the 
surface of the wire; for very little excited activity is produced if 
the wire is charged to the same potential inside a closed cylinder. 
We have seen that the excited activity produced on the wire 
can be partially removed by rubbing, and by solution in acids, and, 
in this respect, 1t is similar to the excited activity produced. in 
bodies by the emanations of radium and thorium. ‘The very close 
similarity of the excited activity obtained from the atmosphere 
to that obtained from the radium and thorium emanations sug- 
gests the probability that a radio-active emanation exists in the 
atmosphere. This view is confirmed by a large amount of indirect 
evidence discussed in sections 212 and 213. 
Assuming the presence of a radio-active emanation in the 
atmosphere, the radio-active effects observed receive a simple 
explanation. The emanation in the air gradually breaks up, 
giving rise in some way to positively charged radio-active carriers. 
These are driven to the negative electrode in the electric field, 
and there undergo a further change, giving rise to the radiations 
observed at the surface of the wire. The matter which causes 
excited activity will thus be analogous to the emanation X of 
radium and thorium. 
Since the earth is negatively electrified with regard to the 
upper atmosphere, these positive radio-active carriers produced in 
the air are continuously deposited on the surface of the earth. 
Everything on the surface of the earth, including the external | 
surface of buildings, the grass, and leaves of trees, must be covered 
with an invisible deposit of radio-active material. A hill or 
mountain peak, or any high mass of rock or land, concentrates the 
earth’s electric field at that point and consequently will receive 
more excited radio-activity per unit area than the plain. Elster 
and Geitel have pointed out that the greater ionization of the air 
observed in the neighbourhood of projecting peaks receives a 
satisfactory explanation on this view. 
If the radio-active carriers are produced at a uniform rate in 
the atmosphere, the amount of excited activity J;, produced on 
a wire exposed under given conditions, will, after exposure for a 
time t, be given by J,=J,(1—e-™) where J, is the maximum 

