In the present edition of this book I have endeavored to present a simple treatment of all the developments in physics to date which have caused a modification or extension of any of the viewpoints expressed just seven years ago. In its preparation I have been very much impressed to find how uniformly the changes represent additions rather than subtractions—a striking illustration of the great truth that science, like a plant, grows in the main by the process of accretion. If I have succeeded in interesting some old friends and making a few new ones for one of the most fascinating of subjects, I shall be content. ROBERT ANDREWS MILLIKAN NORMAN BRIDGE LABORATORY OF PHYSICS CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA MAY 18, 1924 CONTENTS PAGE INTRODUCTION 1 CHAPTER I. EARLY VIEWS OF ELECTRICITY 6 II. THE EXTENSION OF THE ELECTROLYTIC LAWS TO CONDUCTION IN GASES 25 III. EARLY ATTEMPTS AT THE DIRECT DETERMINATION OF 45 IV. GENERAL PROOF OF THE ATOMIC NATURE OF ELECTRICITY 66 V. THE EXACT EVALUATION OF 90 VI. THE MECHANISM OF IONIZATION OF GASES BY X-RAYS AND RADIUM RAY 125 VII. BROWNIAN MOVEMENTS IN GASES 145 VIII. IS THE ELECTRON ITSELF DIVISIBLE? 158 IX. THE STRUCTURE OF THE ATOM 182 X. THE NATURE OF RADIANT ENERGY 232 APPENDIX A. FROM MOBILITIES AND DIFFUSION COEFFICIENTS 262 APPENDIX B. TOWNSEND’S FIRST ATTEMPT AT A DETERMINATION OF 265 APPENDIX C. THE BROWNIAN-MOVEMENT EQUATION 268 APPENDIX D. THE INERTIA OR MASS OF AN ELECTRICAL CHARGE ON A SPHERE OF RADIUS 272[Pg xiii] APPENDIX E. MOLECULAR CROSS-SECTION AND MEAN FREE PATH 275 APPENDIX F. NUMBER OF FREE POSITIVE ELECTRONS IN THE NUCLEUS OR AN ATOM BY RUTHERFORD’S METHOD 277 APPENDIX G. BOHR’S THEORETICAL DERIVATION OR THE VALUE OR THE RYDBERG CONSTANT 282 APPENDIX H. A. H. COMPTON’S THEORETICAL DERIVATION OF THE CHANGE IN THE WAVE-LENGTH OR ETHER-WAVES BECAUSE OR SCATTERING BY FREE ELECTRON 284 APPENDIX I. THE ELEMENTS, THEIR ATOMIC NUMBERS, ATOMIC WEIGHTS, AND CHEMICAL POSITION 286 INDEXES. 287[Pg xiv] [Pg 1] INTRODUCTION