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can be fully absorbed in air of the density and thickness involved in 

 these experiments. It seems necessary to suppose that as the pressure 

 is increased, an ever-rising fraction of the total number of generated 

 ions fails to make its way to the electrodes, positive ions and negative 



20 30 40 



60 70 80 90 100 110 120 130 140 150 160 170 

 PRESSURE IN ATMOSPHERES 



Fig. 11 — Curves of ionization versus quantity of contained air in ionization-chamber 



(Broxon). 



recombining with one another.^ If so, there is reason to fear that the 

 readings of a high-pressure ionization-chamber are affected by its 

 temperature — a possible cause of spurious fluctuations. 



I turn now to the other ways of experimentation: there are two of 

 them, both furnishing data more striking and spectacular than those of 

 electroscopes. They exhibit the immediate cause of the mysterious 

 ionization, and open the question of its ultimate cause. We must now 

 take up this question: is the ultimate cause to be compared with 

 electromagnetic waves or gamma-rays descending from above, as 

 in this article I have thus far let be assumed? or, is it a rain of fast- 

 flying material particles such as electrons or protons? or a mixture of 

 the two classes of radiation? or has it properties unlike those of any 

 radiation so far known? 



(This question is often put in the words: are the cosmic rays wavelike 

 or corpuscular? I think that this phrasing is very much to be con- 

 demned, gamma-rays as well as material particles being at one and the 

 same time both wavelike and corpuscular. However the reader should 

 know that in current usage "corpuscular" signifies "composed of 

 particles of electricity or matter, instead of photons.") 



'This theory, almost simullaneousiy published from Millikan's school and from 

 A. H. Compton's, is at present under test. 



