Vili 
THE QUANTUM 
A 
hi. 
atomicity of electricity, in its dual aspect of positive and nega 
tive electrification, has also to be explained; and it may prove 
to be the case that these three types of atomicity represent 
only different aspects of some fundamental property of the 
electromagnetic field. 
In this book emphasis has been laid on the magnetic aspect 
of the interpretation of the quantum. It is, of course, recognized 
that a complete theory must take into account both electric 
and magnetic forces; indeed, in the theory of relativity these 
become most closely linked together. The genius of Faraday led to 
the development of a mode of representing electric and magnetic 
fields by physical lines of force, and these may still serve a 
useful purpose in forming mental pictures of certain atomic 
phenomena, even though we may be driven eventually to employ 
the four-dimensional tubes of electro-magnetic force introduced 
by Prof. E. T. Whittaker. There is still much for the mathe 
matician to accomplish in treating these “ calamoids ” as 
quanta. 
The greater part of this work is based upon lectures delivered 
to advanced students in the University of St. Andrews. This 
accounts for the mode of presentation, for the somewhat detailed 
description of fundamental principles, and for occasional repeti 
tion. I have endeavoured to take a middle course between a 
mathematical and a purely descriptive treatment. 
I wish to express my thanks to all those who have assisted 
me, and in particular to Mr. R. S. Maxwell, Mr. W. G. Robson 
and Dr. Ian Sandeman for help in the preparation of the manu 
script or in reading the proofs. For permission to reproduce 
diagrams, or for the loan of blocks, my thanks are due to the 
Royal Society of Edinburgh, the Editor of Nature, the Editors 
of the Philosophical Magazine, Mr. C. H. Douglas Clark, Sir 
Alfred Ewing, Mr. H. Moore, Prof. Arnold Sommerfeld, and 
Drs. Goudsmit and Uhlenbeck. I am indebted to Prof. A. 
Fowler for tables showing the arrangement of electrons in atoms. 
H. S. A. 
The University, 
St. Andrews, 
January, ig28 
ME