216 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1928 



rays are a phenomenon the origin of which is to be souglit in the 

 cosmos." ^* 



Millikan and Otis, on the other hand, conchided from their new 

 Pikes Peak absorption data that if any of the penetrating rays which 

 they found on the Peak were of cosmic origin they had to be more 

 penetrating, or less strong, than corresponded even with the reduced 

 values now found by Kolhorster, namely, 2 ions at sea level, absorp- 

 tion coefficient 0.25 per meter of water. The mean coefficient of the 

 radiation which they found on Pikes Peak was but slightly less 

 than that of thorium D, and a large part of it was certainly of local 

 origin. They brought to liglit in these experiments no new evidence 

 for the existence of rays of cosmic origin. Indeed, up to 1925, 

 there appears from the literature to have been no feeling of certainty 

 in any quarter that the existence of rays of cosmic origin had been 

 proved. The increase in ionization in closed vessels up to nearly 

 10 miles was an undoubted fact, and Kolhorster's glacier experiments 

 were favorable to the cosmic ray interpretation ; but the possibilities 

 of the radioactive contamination of a glacier are not small, nor do 

 its irregular shape and proximity to land masses adapt it well to 

 trustworthy absorption-coefficient measurements. Further, Hoff- 

 mann,^^ in Germany, with an extraordinarily fine technique, had in 

 1925 pronounced against the existence of rays of cosmic origin. 

 Also in America, Swann ^"^ was convinced that the work of himself 

 and his collaborators with the ionization in vessels at pressures up to 

 75 atmospheres was incompatible with the cosmic ray interpretation 

 of the penetrating radiation. 



OBSERVATIONS IN MOUNTAIN LAKES 



In 1925, however, Millikan and Cameron got unambiguous evi- 

 dence from their own point of view of a penetrating radiation which 

 had to be of cosmic origin. It was indeed weaker and more pene- 

 trating than had corresjoonded to preceding estimates, having an 

 ionizing power at sea level of but 1.4 ions per cubic centimeter per 

 second, an absorption coefficient which became as small as 0.18 per 

 meter of water, and a definite spectral distribution, the longest wave 

 lengths found having a value, computed from A. H. Compton's 

 formula, A= 0.00063 A, the shortest 0.00038 A. This last is but one- 

 thirtieth the wave length of the very hardest gamma rays. 



M " Neuerdings neigt man immer mehr der Ansicht zu die Hohenstrahlung als eine 

 Erscheinung aufzufassen, deren tlrspruug im Kosmos zu suchen ist." Again : " Da fiii 

 die erstere Auflfassung der Hiihenstrahlung als einer aus den hijheren Atmosphiirensetiich- 

 ten stammenden bisher keinerlei direkta Audeutung gefunden wurde, so spreehen die 

 augonblicklichen Verliiillnisse nielir zugunsteu einer kosmischen Erkliirung." 



"Hoffmann, Phys. Zeit., 26, 40, 669; 1925. 



'6 Downey, Phys. Rev., 20, 186; 1922. Fruth, Pliys. Rev., 22, 109; 1923. 



