I wish to express my best thanks to Dr A. D. Udden, University of Pennsylvania, who has undertaken the translation of the original addresses into English, and to Mr C. D. Ellis, Trinity College, Cambridge, who has looked through the manuscript and suggested many valuable improvements in the exposition of the subject.

N. BOHR.

COPENHAGEN,
May 1922.

CONTENTS

ESSAY I
ON THE SPECTRUM OF HYDROGEN
PAGE
Empirical Spectral Laws[1]
Laws of Temperature Radiation[4]
The Nuclear Theory of the Atom[7]
Quantum Theory of Spectra[10]
Hydrogen Spectrum[12]
The Pickering Lines[15]
Other Spectra[18]
ESSAY II
ON THE SERIES SPECTRA OF THE ELEMENTS
I.INTRODUCTION[20]
II.GENERAL PRINCIPLES OF THE QUANTUM THEORY OF SPECTRA[23]
Hydrogen Spectrum[24]
The Correspondence Principle[27]
General Spectral Laws[29]
Absorption and Excitation of Radiation[32]
III.DEVELOPMENT OF THE QUANTUM THEORY OF SPECTRA[36]
Effect of External Forces on the Hydrogen Spectrum[37]
The Stark Effect[39]
The Zeeman Effect[42]
Central Perturbations[44]
Relativity Effect on Hydrogen Lines[46]
Theory of Series Spectra[48]
Correspondence Principle and Conservation of Angular Momentum[50]
The Spectra of Helium and Lithium[54]
Complex Structure of Series Lines[58]
IV.CONCLUSION[59]
ESSAY III
THE STRUCTURE OF THE ATOM AND THE PHYSICAL AND CHEMICAL PROPERTIES OF THE ELEMENTS
I.PRELIMINARY[61]
The Nuclear Atom[61]
The Postulates of the Quantum Theory[62]
Hydrogen Atom[63]
Hydrogen Spectrum and X-ray Spectra[65]
The Fine Structure of the Hydrogen Lines[67]
Periodic Table[69]
Recent Atomic Models[74]
II.SERIES SPECTRA AND THE CAPTURE OF ELECTRONS BY ATOMS[75]
Arc and Spark Spectra[76]
Series Diagram[78]
Correspondence Principle[81]
III.FORMATION OF ATOMS AND THE PERIODIC TABLE[85]
First Period. Hydrogen—Helium[85]
Second Period. Lithium—Neon[89]
Third Period. Sodium—Argon[95]
Fourth Period. Potassium—Krypton[100]
Fifth Period. Rubidium—Xenon[108]
Sixth Period. Caesium—Niton[109]
Seventh Period[111]
Survey of the Periodic Table[113]
IV.REORGANIZATION OF ATOMS AND X-RAY SPECTRA[116]
Absorption and Emission of X-rays and Correspondence Principle[117]
X-ray Spectra and Atomic Structure[119]
Classification of X-ray Spectra[121]
Conclusion[125]

ESSAY I
ON THE SPECTRUM OF HYDROGEN[1]

Empirical spectral laws. Hydrogen possesses not only the smallest atomic weight of all the elements, but it also occupies a peculiar position both with regard to its physical and its chemical properties. One of the points where this becomes particularly apparent is the hydrogen line spectrum.

The spectrum of hydrogen observed in an ordinary Geissler tube consists of a series of lines, the strongest of which lies at the red end of the spectrum, while the others extend out into the ultra-violet, the distance between the various lines, as well as their intensities, constantly decreasing. In the ultra-violet the series converges to a limit.

Balmer, as we know, discovered (1885) that it was possible to represent the wave lengths of these lines very accurately by the simple law