There was one way out. In 1911 the Austrian physicist Victor Francis Hess (1883-1964) had discovered that earth was bombarded from every side by “cosmic rays”. These consisted of speeding atomic nuclei (“cosmic particles”) of enormous energies—in some cases, billions of times as intense as any energies available through particles produced by mankind. If a cosmic particle of sufficient energy struck an atomic nucleus in the atmosphere, it might knock mesons out of it. In 1936 the American physicists Carl David Anderson (1905- ) and Seth Henry Neddermeyer (1907- ), studying the results of cosmic-particle bombardment of matter, detected the existence of particles of intermediate mass. This particle turned out to be lighter than Yukawa had predicted; it was only about 207 times as massive as an electron. Much worse, it lacked other properties that Yukawa had predicted. It did not interact with the nucleus in the manner expected. [Illustration: _Hideki Yukawa_] [Illustration: _Victor F. Hess_] [Illustration: _C. D. Anderson_] In 1947, however, the English physicist Cecil Frank Powell (1903-1969) and his co-workers, also studying cosmic-particle bombardment, located another intermediate-sized body, which had the right mass and all the other appropriate properties to fit Yukawa’s theories. Anderson’s particle was called a “mu-meson”, soon abbreviated to “muon”. Powell’s particle was called a “pi-meson”, soon abbreviated to “pion”. With the discovery of the pion, Yukawa’s theory was nailed down and any lingering doubt as to the validity of the proton-neutron theory vanished. [Illustration: _C. F. Powell_] (Actually, it turns out that there are two forces. The one with the pion as exchange particle is the “strong nuclear interaction”. Another, involved in beta particle emission, for instance, is a “weak interaction”, much weaker than the electromagnetic but stronger than the gravitational.) The working out of the details of the strong nuclear interaction explains further the vast energies to be found resulting from nuclear reactions. Ordinary chemical reactions, with the electron shifts that accompany them, involve the electromagnetic interaction only. Nuclear energy, with the shifts of the particles inside the nucleus, involves the much stronger nuclear interaction. Neutron Bombardment As soon as neutrons were discovered, it seemed to physicists that they had another possible bombarding particle of extraordinary properties. Since the neutron lacked any electric charge, it could not be repelled by either electrons on the outside of the atoms or by the nuclei at the center. The neutron was completely indifferent to the electromagnetic attraction and it just moved along in a straight line. If it happened to be headed toward a nucleus it would strike it no matter how heavy a charge that nucleus might have and very often it would, as a result, induce a nuclear reaction where a proton would not have been able to.