194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1926 
of the spectrum is described from time to time, has played a worthy part in 
the enormous development to which the following paper by Professor Millikan 
is so notable a continuation. 
It was as early as 1903 that the British physicists, McLennan and 
Burton? and Rutherford and Cooke ® noticed that the rate of leakage 
of an electric charge from an electroscope within an air-tight metal 
chamber could be reduced as much as 30 per cent by inclosing the 
chamber within a completely encircling metal shield or box with walls 
several centimeters thick. This meant that the loss of charge of the 
inclosed electroscope was not due to imperfectly insulating supports, 
but must rather be due to some highly penetrating rays, like the 
gamma rays of radium, which could pass through metal walls as 
much as a centimeter thick and ionize the gas inside. 
In view of this property of passing through relatively thick 
metal walls in measurable quantity, the radiation thus investigated 
was called the “ penetrating radiation ” of the atmosphere, and was 
at first quite naturally attributed to radioactive materials in the earth 
or air, and this is in fact the origin of the greater part of it. But 
in 1910 and 1911 it was found that it did not decrease as rapidly 
with altitude as it should upon this hypothesis. The first significant 
report upon this point was made by the Swiss physicist, Gockel,* 
who took an inclosed electroscope up in a balloon with him to a 
height of 18,000 feet and reported that he found the “ penetrating 
radiation ” about as large at this altitude as at the earth’s surface, 
and this despite the fact that according to Eve’s® calculation it 
ought to have fallen to half its surface value in going up 250 feet. 
In 1911, 1912, 1918, and 1914 two physicists, Hess,° a Swiss, and 
Kolhorster,’? a German, repeated these balloon measurements of 
Gockel’s, the latter going to a height of 9 kilometers, or 5.6 miles, 
and reported that they found this radiation decreasing a trifle for the 
first mile or so and then increasing until it reached a value at 9 
kilometers, according to Kolhérster’s measurements, eight times as 
great as at the surface. This seemed to indicate that the penetrating 
rays came from outside the earth, and were, therefore, of some sort 
of cosmic origin. If so it was computed ® that in order to fit the 
Hess and Kolhoérster data the rays had to have an absorption coef- 
ficient of 0.57 per meter of water and an ionizing power within a 
closed vessel sent to the top of our atmosphere of at least 500 ions 
per cubic centimeter per second in place of the 10 or 12 ions found 
2McLennan and Burton, Physic. Rey., 16, 184, 1903. 
8 Rutherford and Cooke, ibid., 16, 183, 1903. 
4Gockel, Physik Zeit., 11, 280, 1910. 
5 ive, Phil. Mag., 21, 26, 1911. 
6 Hess, Physik Zeit., 12, 998, 1911, and 138, 1084, 1912. 
7 Kolhorster, ibid., 14, 1158, 1918, and D. Physik Ges., July 30, 1914. 
81). v. Schweidler, Elster u. Geitel Fest schrift, p. 415, 1915. 
