tter elucidation of the terms employed, or for giving account
of discoveries made while these pages were passing through the
press, may be distinguished from the author's own by the
signature "ED."

THE EDITOR.

ROYAL INSTITUTION OF GREAT BRITAIN,
April 1907.

Author's Preface

During the last ten years so many works have accumulated in the
domain of Physics, and so many new theories have been propounded,
that those who follow with interest the progress of science, and
even some professed scholars, absorbed as they are in their own
special studies, find themselves at sea in a confusion more
apparent than real.

It has therefore occurred to me that it might be useful to write
a book which, while avoiding too great insistence on purely
technical details, should try to make known the general results
at which physicists have lately arrived, and to indicate the
direction and import which should be ascribed to those
speculations on the constitution of matter, and the discussions
on the nature of first principles, to which it has become, so to
speak, the fashion of the present day to devote oneself.

I have endeavoured throughout to rely only on the experiments in
which we can place the most confidence, and, above all, to show
how the ideas prevailing at the present day have been formed, by
tracing their evolution, and rapidly examining the successive
transformations which have brought them to their present
condition.

In order to understand the text, the reader will have no need to
consult any treatise on physics, for I have throughout given the
necessary definitions and set forth the fundamental facts.
Moreover, while strictly employing exact expressions, I have
avoided the use of mathematical language. Algebra is an admirable
tongue, but there are many occasions where it can only be used
with much discretion.

Nothing would be easier than to point out many great omissions
from this little volume; but some, at all events, are not
involuntary.

Certain questions which are still too confused have been put on
one side, as have a few others which form an important collection
for a special study to be possibly made later. Thus, as regards
electrical phenomena, the relations between electricity and
optics, as also the theories of ionization, the electronic
hypothesis, etc., have been treated at some length; but it has
not been thought necessary to dilate upon the modes of production
and utilization of the cu