THE A B C OF ATOMS BY BERTRAND RUSSELL, F.R.S. AUTHOR OF “MYSTICISM AND LOGIC,” “THE ANALYSIS OF MIND,” ETC. NEW YORK E. P. DUTTON & COMPANY 681 FIFTH AVENUE COPYRIGHT, 1923 By E. P. DUTTON & COMPANY All rights reserved First Printing Oct., 1923 Second Printing Jan., 1924 Third Printing March, 1924 Fourth Printing Oct., 1924 Printed in the United States of America CONTENTS CHAPTER PAGE I. INTRODUCTORY 1 II. THE PERIODIC LAW 11 III. ELECTRONS AND NUCLEI 22 IV. THE HYDROGEN SPECTRUM 36 V. POSSIBLE STATES OF THE HYDROGEN ATOM 49 VI. THE THEORY OF QUANTA 60 VII. REFINEMENTS OF THE HYDROGEN SPECTRUM 72 VIII. RINGS OF ELECTRONS 86 IX. X-RAYS 97 X. RADIO-ACTIVITY 108 XI. THE STRUCTURE OF NUCLEI 123 XII. THE NEW PHYSICS AND THE WAVE THEORY OF LIGHT 134 XIII. THE NEW PHYSICS AND RELATIVITY 144 APPENDIX BOHR’S THEORY OF THE HYDROGEN SPECTRUM 159 [Pg 1] I. INTRODUCTORY To the eye or to the touch, ordinary matter appears to be continuous; our dinner-table, or the chairs on which we sit, seem to present an unbroken surface. We think that if there were too many holes the chairs would not be safe to sit on. Science, however, compels us to accept a quite different conception of what we are pleased to call “solid” matter; it is, in fact, something much more like the Irishman’s definition of a net, “a number of holes tied together with pieces of string.” Only it would be necessary to imagine the strings cut away until only the knots were left. When science seeks to find the units of which matter is composed, it is led to continually smaller particles. The largest unit is the molecule, but a molecule is as a rule composed of “atoms” of several [Pg 2] different “elements.” For example, a molecule of water consists of two atoms of hydrogen and one of oxygen, which can be separated from each other by chemical methods. An atom, in its turn, is found to be a sort of solar system, with a sun and planets; the empty regions between the Sun and the planets fill up vastly more space than they do, so that much the greater part of the volume that seems to us to be filled by a solid body is really unoccupied. In the solar system that constitutes an atom, the planets are called “electrons” and the Sun is called the “nucleus.” The nucleus itself is not simple except in the case of hydrogen; in all other cases, it is a complicated system consisting, in all likelihood, of electrons and hydrogen nuclei (or protons, as they are also called).