64. McCiLettaAnp—Jonization in Atmospheric Air. 
p 
ionizing a gas. This would suggest that the ionization in the atmosphere may 
be due entirely to a radiation from the surface of the Earth. However, the 
radiation observed by Strutt is weak, and would not seem sufficiently to account 
for the atmospheric ionization. Wilson’s results, showing the radio-active 
properties of the residue obtained by boiling freshly-fallen rain, point to the 
presence in the atmosphere of some strongly radio-active substance, and we have 
seen above that the greatest ionization occurs when this radio-active substance 
has been deposited on the surface by a heavy fall of rain. The facts, on the whole, 
point to some radio-active substance disseminated through the atmosphere, 
