PAGE PREFACE vii FOREWORD xi CHAP. I. ATOMS AND MOLECULES 1 II. LIGHT WAVES AND THE SPECTRUM 34 III. IONS AND ELECTRONS 61 IV. THE NUCLEAR ATOM 83 V. THE BOHR THEORY OF THE HYDROGEN SPECTRUM 105 VI. VARIOUS APPLICATIONS OF THE BOHR THEORY 153 VII. ATOMIC STRUCTURE AND THE CHEMICAL PROPERTIES OF THE ELEMENTS 180 INTERPRETATION OF SYMBOLS AND PHYSICAL CONSTANTS 209 COLOURED PLATES PLATE I. SPECTRUM PLATES ACCORDING TO THE ORIGINAL DRAWINGS OF BUNSEN AND KIRCHHOFF _At end_ II. PRINCIPAL FEATURES OF ATOMIC STRUCTURE IN SOME OF THE ELEMENTS—ATOMIC STRUCTURE OF RADIUM _At end_ THE ATOM AND THE BOHR THEORY OF ITS STRUCTURE CHAPTER I ATOMS AND MOLECULES Introduction. As early as 400 B.C. the Greek philosopher, Democritus, taught that the world consisted of empty space and an infinite number of small invisible particles. These particles, differing in form and magnitude, by their arrangements and movements, by their unions and disunions, caused the existence of physical bodies with different characteristics, and also produced the observed variations in these bodies. This theory, which no doubt antedated Democritus, later became known as the Atomic Theory, since the particles were called atoms, _i.e._ the “indivisible.”