ELECTRONS (+ and-), PROTONS, PHOTONS, NEUTRONS, MESOTRONS, AND COSMIC RAYS. originally published in 1934. PREFACE: In 1917 the University pf Chicago Press published a small volume entitled The Electron which was intended to be a rather simple presentation of some of the newer de velopments in physics with which my own work had been closely associated. In 1924 a revised edition of this work appeared. A few years later I had the honor of giving The Messenger Lectures at Cornell University and in them still further expanded and brought up to date these newer developments. The present volume grows immediately out of these Messenger Lectures, but is, of course, an attempt to make the presentation as true a picture as I am able to give of the situation as it exists at the date of publication, Janu ary i, 1935. In a sense this work may be looked upon as a third re vision of The Electron, but it differs from most revisions in two particulars, First, an effort has been made at the request of the publishers to introduce into The Electron portion only such changes as are demanded by correct ness of presentation today, and in fact I have been pleased and somewhat surprised to find that the historical mode of presentation originally adopted has rendered radical changes even today both unnecessary and undesirable. Second, the growth of discovery and the rapidity of the advance in physics from the base occupied in 1924 has made it altogether necessary to add six entirely new chap ters ( ad to rvi) on ' Waves and Particles, on The Dis ' ccfvby anVl OrigJ* i ; 6f the'Cosmic Rays/' on The Spin ning Elecr& h$ on The Positron, on The Neutron an< l the Translation of the Elements/' and on The Xa ture of the Cosmic Rays, so that in this particular tin book is not at all a revision. The illust rativc material, ton has been much more than doubled, and now hrrnmr* quite a notable feature. It is hoped that this volume, like its preclcresmni. may be of some interest both to the physicist and to the reader of somewhat less technical training. It has been thought desirable for the sake of both classes of readers not to break the thread of the discussion in the body of the book with the detailed analyses that the careful student de mands. It is for this reason that all mathematical proofs have been thrown into appendixes. If, in spite of tht*, the general student finds that certain chapters, such * vii, viii, and xii, require more familiarity with the general background of physics than he possesses, it is still hoped that without them he may yet gain some idea of certain phases, at least of the fascinating progress of modern science, ROBERT A. MILUKAN CALIFORNIA INSTITUTE OP TECHNOLOGY PASADENA, CALIFORNIA December, 1934. PREFACE TO SECOND EDITION: Again the University of Chicago Press has called for a reprinting of this volume, but neither the Press nor I could tolerate a reprinting without an attempt to bring it up to dale through full discussion of the extraordinary advances of the last twelve years. Believing profoundly in the historical approach both in science and in its teach ing, I have made no changes in the first 400 pages save those necessitated by new knowledge, mostly in the values of units, but have replaced the last 50 pages by 200 pages of entirely new material under the five new chapter headings ( xvi-xx) : The Release and Utilization of Nuclear Energy, Geomagnetic Studies on Cosmic Rays at Low Altitudes, The Discovery and the Signifi cance of the Mesotron, The Nature and Number of the Incoming Primary Rays, and The Atom-Annihilation Hypothesis as to the Origin of Cosmic Rays.