RADIANT ENERGY 397 



The apparent dual nature of light is best viewed not as such 

 but rather in terms of the philosophy of as if (a concept fre- 

 quently employed in physics and applicable to such expressions 

 as ''curved space")- Light behaves as if it were wavelike in 

 character. Diffraction phenomena prove this. Yet it is cer- 

 tainly corpuscular in the sense that it can strike a single electron 

 and give up all of its energy and momentum to that electron 

 (as in the photoelectric effect). In other words, the particle 

 moves as if it were a wave; but when it collides, it behaves like 

 a particle, i.e., it has mass as well as energy. We may now 

 transfer the discussion of the particle vs. the wave theory of 

 matter from the photon (light) to the electron. 



The first constructive hypothesis which led directly to modern 

 theories of atomic structure was that of the English physicist 

 Ernest Rutherford. He, in 1911, conceived of the atom as a 

 miniature replica of our solar system. The "sun," or nucleus 

 of the atomic "solar" system, was presumed to be a compact 

 cluster of positively charged particles, and the surrounding 

 "planets" were negative electrons. (It is said that the Japanese 

 Nagaoka, in 1904, independently advanced the same idea, but 

 credit is given to Rutherford, for he supported the theory with 

 experimental facts.) Such a postulate was not possible without 

 the experimental work that preceded it — the discovery of X rays 

 by Roentgen (1895), of the radioactivity of uranium by Becquerel 

 (1896), the recognition by J. J. Thomson of the electron as the 

 unit of electricity (1897), and the conception of energy quanta 

 by Planck (1900). Then followed much speculation on the 

 arrangement of the positive and negative electrons and on the 

 energy relationships within the atom. G. N. Lewis and Irving 

 Langmuir advocated a static atom ; that is to say, they assumed 

 that the electrons, or planets, are stationary or move only within 

 very restricted regions. The Danish physicist Niels Bohr took 

 the opposite point of view and regarded the atom as a dynamic 

 system in which the electrons travel in definite orbits around 

 the nucleus, just as do the planets of our universe around their 

 sun. Chemists at first rather preferred the static atom; it fitted 

 in better with their ideas of valence. If atoms unite by sharing 

 an electron, it is easier to visualize their doing so if the electron 

 is relatively quiet rather than traveling at terrific speed in an 

 orbit. Objection was also raised on the ground that the electrons 



