150 On the Radioactivity of the Atmosphere. 
appreciable excited activity produced on a rod suspended in it. 
It takes a far greater volume of air than the tank held to 
produce any measurable amount of excited activity from the 
air, unless some radioactive substance, such as thorium or 
radium, is present. There is certainly an increase of con- 
ductivity produced, which dies away quickly, and which is 
undoubtedly caused by the mixture of the water-spray with 
the air. The water from the tap, when evaporated down to 
dryness and tested, gave no signs of any radioactivity. 
Conclusion. 
From these results we may conclude that the excited 
activity from the atmosphere behaves in many respects like 
the radioactivity from thorium and radium. It contains, as. 
they do, an easily-absorbed « radiation, and a more penetrating 
B radiation. The « radiation is probably responsible for the 
greater part of the total energy radiated, and it is completely 
absorbed in about ‘004 cm. of aluminium and 10 ems. of air. 
The 8 rays are cut down to half value in ‘007 cm. of 
aluminium, and completely absorbed by 06 cm. The # rays. 
probably consist of negatively-charged particles, similar to 
cathode rays, and projected with great velocity. The ioniza- 
tion produced by them is too small to test whether they are 
deviable in a magnetic field. 
The difference in the rates of decay of the excited activity 
obtained under different conditions seems to point to the fact 
that the radioactivity of the atmosphere is of a very complex 
nature. 
The radioactivity of snow and rain must be derived from 
some radioactive matter in the air which adheres to the surface 
of the snow-flake or rain-drop, and is brought down with it 
in its descent. A possible explanation of the difference 
observed in the rate of decay of the radioactivity from snow 
and rain, and that of the excited activity on a wire, may be 
based on the view that the radioactive matter in the air is of 
different kinds, having different rates of decay. Snow and 
rain may owe their activity to one kind while the negatively- 
charged wire removes all the active carriers to its surface. 
The rate of decay of the charged wire might thus be the 
resultant of several different rates. 
In conclusion, I wish to thank Prof. Rutherford for his 
kindly interest in the work. 
McDonald Physics Building, 
McGill University, 
Aug. 1905. 
