CONTEMPORARY ADVANCES IN PHYSICS 183 



SO constant must come from vvitliout tlie terrestrial world, and on its 

 way be exempt from the influence of the earth's magnetic field.^* 



Of the origin of the cosmic rays I have not spoken in this article, 

 having so chosen its title as to exclude that mighty subject from its 

 scope. In the data and in the nature of these rays there is enough of 

 the mysterious and enough of the extraordinary, to suffice for an 

 introduction. Modest as the data seem, what they reveal is sen- 

 sational. It is found that the atmosphere of this earth is being 

 traversed by ionizing particles, of which the qualities are amazing. If 

 they are such familiar corpuscles as protons or electrons, their energy 

 must be of the order of tens of millions of equivalent volts, values 

 without precedent in our experience. If they are neutral particles 

 they are in themselves unprecedented. If they are electrons or 

 protons which derive their energy from photons or corpuscles of light, 

 these last must have energy greater, frequency higher and wave-length 

 smaller, than any form of light of w^hich we have previous knowledge. 

 Such quantities of energy, be it remembered, are enormously greater 

 than the largest which atoms can emit in the course of their normal 

 lives; they are several times larger even than those which are emitted by 

 collapsing atom-nuclei, in those processes of transmutation of which we 

 already have knowledge. If they come from individual atoms they 

 must arise from processes heretofore unknown, of transmutation or 

 synthesis or annihilation. The constancy of the effect of the ionizing 

 rays, its independence (be it absolute or only approximate) of weather 

 and time and direction and the earth's magnetic field, implies that 

 these processes are spread, not over the earth nor even over the solar 

 system by itself, but throughout the whole of the cosmos. 



Literature 



The literature of cosmic-ray research has already swollen to such proportions, that 

 a complete bibliography would cover several pages of this journal. There seems to be 

 no book devoted altogether to the subject, though in treatises on atmospheric 

 electricity — I name especially Die elektrische Leitfdhigkeit der Atmosphdre, by W F. 

 Hess, translated as The Electrical Conductivity of the Atmosphere and its Causes (Van 

 Nostrand, 1926) — a chapter or a section is usually assigned to it. 



The major publications of Alillikan's school have appeared in the Physical Revieiu, 

 as follows: 

 R. A. Millikan & I. S. Bowen, Phys. Rev. (2), 27, 353-361 (1926) (sounding-balloon 



observ'ations). 

 Millikan & R. AI. Otis, ibid., 27, 645-658 (1926) (mountain-peak and aeroplane ob- 

 servations). 

 Millikan & G. H. Cameron, ibid., 28, 851-868 (1926) (lakes in California). 

 Millikan & Cameron, ibid., 31, 163-173 (1928) and 921-930 (lakes in California and 

 Bolivia). 



-1 Nevertheless there have been opposing data; Clay, on a voyage to Java, from the 

 Mediterranean, found the ionization increasing as he approached the equator, and 

 Corlin is said to have observed an increase from south to north along the Scandinavian 

 peninsula; recall also Corlin's assertion mentioned on page 169. 



