ATOMIC DISINTEGBATIOIT MILLIKAN" 283 



Churchill, Manitoba (lat. 59° N.), within 730 miles of the north 

 magnetic pole, showed to be not true, the mean intensity of the rays 

 there being not measurably different from that at Pasadena in 

 latitude 34° N. 



Nor is the conclusion that the cosmic rays enter the eartli's atmos- 

 phere as a practically pure photon beam dependent upon these meas- 

 urements of last summer alone. It follows also from the high alti- 

 tude sounding-balloon experiments of Millikan and Bowen in April, 

 1922, taken in connection with the lower balloon flights of Hess and 

 Kolhorster in 1911-1914. For in going to an altitude of 15.5 km we 

 got but one-fourth the total discharge of our electroscope which we 

 computed we should have obtained from the extrapolation of our 

 predecessors' curves. This shows that somewhere in the atmosphere 

 below a height of 15. 5 km the intensity of the ionization within a 

 closed vessel exposed to the rays goes through a maximum, and then 

 decreases, quite rapidly, too, in going to greater heights. We have 

 just taken very accurate observations up to the elevation of the top of 

 Pikes Peak (4.3 km), and found that within this range the rate of 

 increase with altitude is quite as large as that found in the Hess and 

 Kolhorster balloon flights, so that there can be no uncertainty at all 

 about the existence of this maximum. Such a maximum, however, 

 means that the rays, before entering the atmosphere, have not passed 

 through enough matter to begin to get into equilibrium with their 

 secondaries — electron-rays + or— , rays and photons of reduced fre- 

 quency — in other words, that they have not come through an appreci- 

 able amount of matter in getting from their place of origin to the 

 earth. 



This checks with the lack of effect of the earth's magnetic field on 

 the intensity of the rays; and the two phenomena, of quite unrelated 

 kinds and brought to light years apart, when taken together, prove 

 most conclusively, I think, that the cosmic rays can not originate even 

 in the outer atmospheres of the stars, though these are full of hydro- 

 gen and helium in a high-temperature state, but that they must origi- 

 nate rather in those portions of the universe from which the}' can 

 come to tlie earth without traversing matter in quantity that is ap- 

 preciable even as compared with the thickness of the earth's atmos- 

 phere — in other words, that they must originate in the intensely cold 

 regions in the depths of interstellar space. 



Further, the more penetrating the secondary rays produced by 

 photon encounters, the greater the thickness of matter that must be 

 traversed before the beam of pure photons which enters the atmos- 

 phere gets into equilibrium with its secondaries; and until such equi- 

 librium is reached, the apparent absorption coefficient must be less 

 than the coefficient computed for pure photons with the aid of any 



