
366 RADIO-ACTIVITY OF THE ATMOSPHERE [CH. 
from the capillaries of the earth into the atmosphere. This, how- 
ever, need not necessarily be the case if the conditions of the escape 
of the emanation into the atmosphere are altered by the variation 
of the position of underground water or by a heavy fall of rain. 
The amount of excited activity to be derived from the air on 
the Baltic Coast was only one-third of that observed inland at 
Wolfenbiittel. Experiments on the radio-activity of the air in 
mid-ocean would be of great importance in order to settle whether 
the radio-activity observed in the air is due to the emanations 
from the soil alone. It is to be expected that the radio-activity 
of the air at different points of the earth would vary widely, and 
would largely depend on the nature of the soil. 
Some interesting experiments have been made by McLennan! 
on the amount of excited radio-activity to be derived from the air 
when filled with fine spray. The experiments were made at the 
foot of the American Fall at Niagara. An insulated wire was 
suspended near the foot of the Fall, and the amount of excited 
activity on the wire compared with the amount to be obtained on 
the same wire for the same exposure in Toronto. The amount of 
activity obtained from the air at Toronto was generally five or six 
times that obtained from the air at the Falls. In these experi- 
ments it was not necessary to use an electric machine to charge 
the wire negatively, for the falling spray kept the insulated wire 
permanently charged to a potential of about — 7500 volts. These 
results indicate that the falling spray had a negative charge and 
electrified the wire. The small amount of the excited radio- | 
activity at the Falls was probably due to the fact that the 
negatively charged drops abstracted the positively charged radio- 
active carriers from the atmosphere, and in falling carried them 
to the river below. On collecting the spray and evaporating it, 
no active residue was obtained. Such a result is, however, to be 
expected on account of the minute proportion of the spray tested 
compared with that present in the air. 
215. A very penetrating radiation from the earth’s 
surface. McLennan’, and Rutherford and Cooke*® independently 
1 Phys. Rev. 16, p. 184, 1903, and Phil. Mag. 5, p. 419, 1903. 
2 Phys. Rev. No. 4, 1903. 3 Americ. Phys. Soc. Dec. 1902. 


