United States Atomic Energy Commission Division of Technical Information Library of Congress Catalog Card Number: 63-64918 1963; 1964 (Rev.) [Illustration: The cover is a time-exposed photograph of an animated model of a uranium-235 atom. The center represents the nucleus, greatly exaggerated in size. The fine lines represent the electrons whirling about the nucleus. Courtesy Union Carbide Corporation] C. JACKSON CRAVEN is a teacher’s teacher as well as a student’s teacher, and has had an active career aiding understanding of atomic energy as a member of the University of Tennessee faculty and on the staff of the Oak Ridge Institute of Nuclear Studies. He has conducted short courses to instruct groups of high school science teachers in nuclear energy, and has served in a key capacity in training Institute demonstration-lecturers who visit high schools throughout the nation. Dr. Craven worked during World War II for the Manhattan Project, which built the first atomic bomb. He earned bachelor’s and graduate degrees at the University of North Carolina, and later taught physics and mathematics at Delta State Teachers College and at Furman and Emory Universities. His research interests include infrared spectroscopy, gaseous diffusion through porous media, and the physical properties of fibers. OUR ATOMIC WORLD By C. Jackson Craven _The story of atomic energy evolves from the curiosity of people concerning the nature and structure of matter, the stuff of which all material things are made._ The Greeks Were Curious About Matter Certain philosophers of ancient Greece—Democritus for one—were fascinated by the question: _what is matter?_ You can imagine one of the philosophers saying to his pupils: “Gentlemen, let us consider a piece of cheese. With a knife we can cut it in two, thus obtaining smaller pieces. We can then cut one of these smaller pieces in two, obtaining still smaller pieces. We can _think_ about repeating this process over and over to get smaller and smaller pieces of cheese. Now can this process be continued without limit, or will a time come when we arrive at the smallest possible piece of cheese? In other words, is there a piece so small that we must have at least that much or none, with no choice in between?” It is probable that most people who thought about this question at all during the next two thousand years answered the last question in the negative. The prevailing notion was that matter was continuous, with no theoretical limit as to how small a piece of cheese, or anything else, might be. This concept was humorously expressed by the British mathematician Augustus De Morgan (1806-1871) in these lines: _Great fleas have little fleas upon their backs to bite ’em, And little fleas have lesser fleas, and so, ad infinitum._