were the next two instruments devised in the search for efficient bombarding projectiles. However, the impasse continued: neither instrument allowed scientists to crack the nuclei of the heavier elements.

Ernest O. Lawrence's cyclotron, built in 1931, was the first device capable of accelerating positive ions to the very high energies needed. Its basic principle of operation is not difficult to understand. A charged particle accelerated in a cyclotron is analogous to a ball being whirled on a string fastened to the top of a pole. A negative electric field attracts the positively charged particle (ball) towards it and then switches off until the particle swings halfway around; the field then becomes negative in front of the particle again, and again attracts it. As the particle moves faster and faster it spirals outward in an ever increasing circle, something like a tether ball unwinding from a pole. The energies achieved would have seemed fantastic to earlier scientists. The Bevatron, a modern offspring of the first cyclotron, accelerates protons to 99.13% the speed of light, thereby giving them 6.2 billion electron volts (BeV).

Another instrument, the heavy-ion linear accelerator (Hilac), accelerates ions as heavy as neon to about 15% the speed of light. It is called a linear accelerator because it accelerates particles in a straight line. Stanford University is currently (1963) in the process of building a linear accelerator approximately two miles long which will accelerate charged particles to 99.9% the speed of light.

But highly accelerated charged particles did not solve all of science's questions about the inner workings of the nucleus.

In 1932, during the early search for more efficient ways to bombard nuclei, James Chadwick discovered the neutron. This particle, which is neutral in charge and is approximately the same mass as a proton, has the remarkable quality of efficiently producing nuclear reactions even at very low energies. No one exactly knowns why. At low energies, protons, alpha particles, or other charged particles do not interact with nuclei because they cannot penetrate the electrostatic energy barriers. For example, slow positive particles pick up electrons, become neutral, and lose their ability to cause nuclear transformations. Slow neutrons, on the other hand, can enter nearly all atomic nuclei and induce fission of certain of the heavier ones. It is, in fact, these properties of the neutron which have made possible the utilization of atomic energy.

With these tools, researchers were not long in accurately identifying the missing elements 43, 61, 85, and 87 and more—indeed, the list of new elements, isotopes, and particles now seems endless.

Element 43 was "made" for the first time as a result of bombarding molybdenum with deuterons in the Berkeley cyclotron. The chemical work of identifying the element was done by Emilio Segrè and others then working at Palermo, Sicily, and they chose to call it technetium, because it was the element first made by artificial technical methods.

Element 61 was made for the first time from the fission disintegration products of uranium in the Clinton (Oak Ridge) reactor. Marinsky and Glendenin, who did the chemical work of identification, chose to call it promethium because they wished to point out that just as Prometheus stole fire (a great force for good or evil) from the hidden storehouse of the gods and presented it to man, so their newly assembled reactor delivered to mankind an even greater force, nuclear energy.

Element 85 is called astatine, from the Greek astatos, meaning "unstable," because astatine is unstable (of course all other elements having a nuclear charge number greater than 84 are unstable, too). Astatine was first made at Berkeley by bombarding bismuth with alpha particles, which produced astatine and released two neutrons. The element has since been