I4 THE QUANTUM [i. I0 
at the present time. As pictured by Bohr the atom is a solar 
system in miniature in which electrons are circling in rapid orbital 
motion about a massive nucleus. Does this picture represent the 
actual facts, or is it possible to substitute stationary electrons, 
or perhaps magnetons, so as to give an approximately statical 
model as imagined by A. L. Parson, or by Lewis, and subsequently 
by Langmuir ? And supposing we have satisfactorily applied 
the quantum theory to the arrangement of the electrons in the 
atom, what are we to say as to its bearing upon the still more 
difficult question of the structure of the nucleus itself ? 
All these problems will have to be reviewed in the light of 
more recent work on the spinning electron and on the mathe 
matical expression of the quantum theory by Heisenberg, 
Schrodinger and others. The new quantum mechanics is a 
severe mathematical discipline but it has already met with 
considerable success in explaining experimental facts, and it 
may be hoped that in course of time the mathematicians may 
be able to express the theory in a form more acceptable to the 
physicist. 
According to Kramers: * " This work clearly demonstrates 
the limited applicability of a picture of atomic structure, in 
which the behaviour of the electrons inside the atom is visualized 
by orbits possessing definite kinematical properties.” The atomic 
model of Bohr has, however, played such a prominent part 
in the development of the quantum theory, that it will be 
necessary to devote a large portion of our space to its con 
sideration. For the guidance of the reader it may be said here 
that the present volume falls naturally into three divisions. Part I 
contains a general account of the physical phenomena with 
which we are mainly concerned, and shows how they are related 
to atomic theories in general, and to the quantum theory in 
particular. It is designed for the student who has little previous 
knowledge of the latter theory. Part II is of a more specialized 
character and deals with certain investigations and speculations 
of a less orthodox character. It may therefore be omitted by 
the reader who is interested only in following the well-trodden 
path of what may be termed the classical quantum theory. 
Part III is an attempt to show how the theory has developed 
in more recent times, and includes an account of the new quantum 
mechanics. The last chapter is a discussion of the views advanced 
as to the interpretation of the quantum, emphasis being laid on 
some of the philosophical aspects of the questions involved. 
The reader of this volume will find in it much that is 
speculative, and probably much that will require revision as 
our knowledge increases, but it is the fascination of the unknown 
* Kramers, International Critical Tables, vol. i, p. 47, 1926.