192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



it is not altogether easy to ionize a molecule in spite of this. The 

 trick is to concentrate the little bit of energy required on a single 

 molecule. You cannot do it by swinging a baseball bat. The energy 

 is there all right, but it gets spread around among billions of billions 

 of molecules, and no single one of them gets enough to break it in two. 

 The only two things that possess the required energy in sufficiently 

 compact packages to be concentrated on single molecules are the bul- 

 lets of ultraviolet light known as quanta, and fast-traveling atomic 

 particles of several types, which are known collectively as corpuscles. 

 The very existence of the ionosphere is sure evidence that the earth 

 is being showered continuously with ultraviolet quanta or corpuscles 

 or both. Until recently neither of these agents could be observed 

 directly. They are completely absorbed in the process of forming 

 the ionosphere and never reach the ground. To them the upper at- 

 mosphere is like a brick wall, and both ultraviolet quanta and cor- 

 puscles expend their energy on it in the process of forming ions. How- 

 ever, even though they were unseen, there were several good reasons 

 for supposing that both corpuscles and quanta were involved in the 

 formation of the ionosphere and that they all came from the sun. It 

 did not take a Sherlock Holmes to pin it on the sim. There simply 

 is no other likely source handy. Furthermore, it was observed that 

 certain events on the sun were often followed by sudden large changes 

 in the ionosphere with such perfect timing that there could be no 

 doubt of a connection. The timing also gave the clue to the nature 

 of the disturbing agents. When a flare flashed up on the solar sur- 

 face, the ionosphere on the daylit side of the earth responded instantly. 

 In other words, the disturbing agent had made the trip from the sun 

 to the earth along a straight-line path and with the same speed as 

 the light by which we saw the flare, and must therefore be ultraviolet 

 quanta. Then about a day later a second and more prolonged iono- 

 spheric disturbance would set in all over the world long after the flare 

 had died out and disappeared. The one-day time lag was interpreted 

 as the time required for a shower of material corpuscles to travel the 

 93,000,000 miles from the sun. They were electrically charged and 

 guided in to all parts of the earth by the earth's magnetic field. 

 Hence, both quanta and corpuscles were involved. All this is a 

 nice example of the power of careful step-by-step reasoning to ex- 

 plain phenomena that cannot be directly observed. First it was 

 observed that radio waves were transmitted around the world, and 

 Kennelly and Heaviside explained this by assuming the existence 

 of the ionosphere. Then radio experiments were devised to spe- 

 cifically test this assumption — and it proved correct. It was realized 

 that the ionosphere can be maintained only by incoming ultraviolet 

 quanta or corpuscles; and again it was assumed that the sun must 



