“If this be the case, we must conclude that the nitrogen atom is disintegrated under the intense forces developed in a close collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a constituent part of the nitrogen nucleus.... The results as a whole suggest that, if alpha particles or similar projectiles of still greater energy were available for experiment, we might expect to break down the nuclear structure of many of the lighter atoms.” This prediction has certainly been verified through the use of the atomic artillery provided by extremely powerful particle accelerators, or “atom smashers.”[1] [Illustration: _The Bevatron accelerator at the University of California’s Lawrence Radiation Laboratory, Berkeley, California, shown after recent remodeling in which it was enclosed in concrete shielding._ Courtesy Lawrence Radiation Laboratory] Patrick Blackett in England and W. D. Harkins in the United States soon proved independently that, during the nuclear event reported by Rutherford in his 1919 paper, an alpha particle combines with a nitrogen nucleus and that the resulting unstable combination immediately emits a proton and ends up as one of the isotopes of oxygen. This was the first instance of deliberate transmutation of one stable chemical element into another. Since that time practically every known element has been transmuted by bombardment. The dream of the alchemists has been partially fulfilled in that mercury has been changed into gold. We say “partially fulfilled” because the process is much too expensive to be economically profitable. Some Particles Have No Electric Charge During the early 1920s a number of investigators, including Harkins in the United States, Orme Masson in Australia, and Rutherford and his assistant James Chadwick in England, seriously considered the possibility that a neutral particle might exist in nature, possibly formed by the very close association of a proton and an electron. However, strenuous efforts to produce such particles by combining protons and electrons were unsuccessful. During these years the new technique of bombarding all kinds of matter with alpha particles to see what would happen was widely exploited, and it gradually became clear that in a few instances a peculiar and highly penetrating kind of radiation was produced. In 1932, Chadwick succeeded in showing that the peculiar radiation must consist of a stream of particles, each weighing about the same as a proton but having no electrical charge. The name “neutron” for a possible neutral particle of this type was suggested by Harkins in the United States in 1921. Much evidence now exists that the neutron is a fundamental particle in its own right and that it should not be thought of merely as a particle formed by a very close association between a proton and an electron.