174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192 8 



Modern physical theory shows that the annihilation of an electron 

 must pro(hice a single flash of radiation of wave length far shorter 

 than any we can produce on earth. As this radiation threads its way 

 through a star its wave length is continually increased, or to use 

 the technical term the radiation is continually softened. In time it 

 becomes 7 radiation, then hard X radiation, then soft X radiation, 

 and finally it emerges from the surface of the star as ordinary light 

 and heat. Consider, however, an electron which is annihilated, not 

 inside a star but outside in free space, or in one of the almost trans- 

 parent nebulse. The short wave-length radiation now undergoes no 

 softening, but travels on until it meets something capable of checking 

 it. Thus all astronomical bodies, including the surface of the earth, 

 ought to be under continual bombardment by radiation of shorter 

 wave length, and consequently of greater penetrating powers, than 

 any we can produce on earth. 



Many years ago such radiation was detected in the earth's atmos- 

 phere by McLennan, Rutherford, and other observers ; it has recently 

 been studied in detail by Millikan and others. There is no reason 

 to doubt that it originates just Avhere it ought to, namely, in the 

 great nebulae, and its amount is about what it ought to be if it is 

 evidence of the Avhole universe melting away into radiation. The 

 wave length of the radiation might be expected to reveal the physical 

 process by which it is generated, but the evidence is a bit puz/Jing. 

 The hardest terrestrial radiation penetrates inches of lead and 

 corresponds to a voltage of hundreds of thousands of volts. The 

 cosmic radiation penetrates about 5 yards of lead, and the hai'dest 

 rays are now found to correspond to about 60,000,000 volts.^ Millikan 

 was at one time inclined to attribute the rays to the combinat'on 

 of 4 atoms of hydrogen to form an atom of helium, but rays so 

 produced would only be of the hardness corresponding to 30,000,000 

 volts. There are many Avays known to physics of softening radia- 

 tion, but none of hardening it. Thus we must look for some source 

 more energetic than the sjm thesis of hydrogen into helium, and I 

 can see no possible stopping place short of the annihilation of 

 matter. Again, we are not dealing with a minute phenomenon of 

 mere academic interest. In a sense this radiation is the most funda- 

 mental physical phenomenon of the whole universe, most regions of 

 space containing more of it than of visible light or heat. Our 

 bodies are traversed b}' it night and day. Short of going down 

 into a mine or in a submarine we can not escape it, and it is so 

 intense that it breaks up several million atoms in each of our bodies 

 every second. It may be essential to life or it may be killing us. 



1 Added in proof : The recent complete theory of Klein and Nishina gives about 

 940,000,000 volts, which is precisely the volt.nge to be expected if the rays originate in 

 the annihilation of matter. — J. H. J. 



