HIGH FREQUENCY RAYS—MILLIKAN 197 
ments on Pike’s Peak showed to be more than twice as copious there 
as at Pasadena. 
The only possible absorbing material obtainable in the immense 
quantities needed, and of homogeneous and nonradioactive constitu- 
tion, were the waters of very deep snow-fed lakes—snow-fed be- 
cause the results of underwater experiments which we had previously 
carried on near Pasadena had been vitiated by our discovery that the 
waters were appreciably radioactive. We felt that there was much 
uncertainty as to how much this cause might have affected the 
Huropean observations in and about glaciers. Further, our Pike’s 
Peak experiments had demonstrated that if any of the penetrating 
rays were of cosmic origin the ionization due to them in our electro- 
scope at sea level had to be much less than the 2 ions, assumed above, 
out of the 11.6 observed, the experimental error being, say, half an 
ion. No crucial tests could, therefore, possibly be made unless we 
could find very deep, nonradioactive lakes at very high altitudes 
where cosmic rays, if they existed, had two or three times the ionizing 
effect to be expected from them at sea level. We needed at least three 
ions due to cosmic rays, to vary with absorbing materials, if we were 
to obtain unambiguous evidence. 
We chose for the first experiments Muir Lake (11,800 feet high), 
just under the brow of Mount Whitney, the highest peak in the 
United States, a beautiful snow-fed lake hundreds of feet deep and 
some 2,000 feet in diameter. Here we worked for the last 10 days in 
August, sinking our electroscopes to various depths down to 67 feet. 
Our experiments brought to light altogether unambiguously a radia- 
tion of such extraordinary penetrating power that the electroscope 
readings kept decreasing down to a depth of 50 feet below the sur- 
face. The atmosphere above the lake was equivalent in absorbing 
power to 23 feet of water, so that here were rays so penetrating that, 
if they came from outside the atmosphere, they had the power of 
passing through 50+-23—73 feet of water, or the equivalent of 6 feet 
of lead, before being completely absorbed. The most penetrating 
X rays that we produce in our hospitals can not go through half an 
inch of lead. Here were rays at least a hundred times more pene- 
trating than these, and having an absorption coefficient but one 
twenty-fifth, instead of “about one-tenth of that of the hardest 
known gamma rays.” ® 
How unambiguous was the experimental evidence may be seen 
from the fact that with the aid of a new electroscope of high sensi- 
tivity the change in ions per cubic centimeter per second in going 
from the surface of Muir Lake to the depth of 15 meters (50 feet) 
® Kolhorster, Sitz.-Ber. Preuss. Akad. Wiss., 34, 366, 23. 
