124-126] 
Highly Penetrating Radiation 
137 
radiation. The irregular nebulae and the outer shells of the planetary nebulae 
form obvious instances, but the most important for our present purpose are 
the spiral and other extra-galactic nebulae. All the radiation generated in 
the transparent parts of these nebulae ought to pass without appreciable 
absorption or softening into outer space, so that we should expect to find 
space filled with high-frequency radiation of wave-length of the order of 
1-3 x 10~ 13 eras. Here and there this radiation may devastate isolated atoms, 
ejecting 940-million volt electrons as it passes, but the greater part of it 
will pass through space unhindered until it meets a medium of substantial 
absorbing powers. Thus we should expect the atmospheres of the stars, sun 
and earth, and even the solid body of the earth, to be under continual bom 
bardment by highly penetrating radiation of nebular origin. 
125. Such radiation has been detected in the earth’s atmosphere by 
McLennan, Burton, Cook, Rutherford, Kolhorster, Millikan, and many others. 
It seems to be satisfactorily established that the radiation is of extra-terrestrial 
origin, since it does not decrease in intensity with increasing height as it would 
if, as at first was thought, it originated in the earth’s radioactivity. Indeed, 
by sending up balloons, Kolhorster, and later Millikan and Bowen, have shewn 
that the intensity of the radiation is substantially greater at high altitudes 
than at low, proving clearly that the radiation comes into the earth’s atmo 
sphere from outside. It does not come from the stars, for if it did the main 
part would come from the sun, and the amount received by day would be far 
greater than that received at night; this is not found to be the case. Thus 
the radiation must originate in nebulae or cosmic masses other than stars. 
Kolhorster and von Salis have recently found* that the intensity of the 
radiation received at any point on the earth’s surface varies with the orienta 
tion of the earth in space in a way which indicates that the radiation is 
received largely from regions near the Milky Way, especially the regions of 
Andromeda and Hercules. 
126. This is entirely consistent with the radiation being the direct product 
of the annihilation of matter in the transparent parts of the great nebulae. 
If g ergs per second are generated in any small region of space, the flux of 
radiation per square cm. at a distance r will be g/4nrr 2 . If a shell of matter 
surrounding the earth and having radii r lf r 2 generates G ergs per gramme per 
second, the amount falling on a square cm. of the earth’s atmosphere will be 
We can form a rough estimate of the actual flux of this highly penetrating 
radiation through the earth’s atmosphere. At sea-level it is found to produce 
T4 ions per cubic centimetre per second, so that if it underwent no absorption 
Highly Penetrating Radiation. 
(126T).