xchanges between the cell and
the outer medium, cause new chapters in physics to appear, and suggest
researches adapted to the necessities of actual life.

The evolution of the different parts of physics does not, however,
take place with equal speed, because the circumstances in which they
are placed are not equally favourable. Sometimes a whole series of
questions will appear forgotten, and will live only with a languishing
existence; and then some accidental circumstance suddenly brings them
new life, and they become the object of manifold labours, engross
public attention, and invade nearly the whole domain of science.

We have in our own day witnessed such a spectacle. The discovery of
the X rays--a discovery which physicists no doubt consider as the
logical outcome of researches long pursued by a few scholars working
in silence and obscurity on an otherwise much neglected subject--
seemed to the public eye to have inaugurated a new era in the history
of physics. If, as is the case, however, the extraordinary scientific
movement provoked by Roentgen's sensational experiments has a very
remote origin, it has, at least, been singularly quickened by the
favourable conditions created by the interest aroused in its
astonishing applications to radiography.

A lucky chance has thus hastened an evolution already taking place,
and theories previously outlined have received a singular development.
Without wishing to yield too much to what may be considered a whim of
fashion, we cannot, if we are to note in this book the stage actually
reached in the continuous march of physics, refrain from giving a
clearly preponderant place to the questions suggested by the study of
the new radiations. At the present time it is these questions which
move us the most; they have shown us unknown horizons, and towards the
fields recently opened to scientific activity the daily increasing
crowd of searchers rushes in rather disorderly fashion.

One of the most interesting consequences of the recent discoveries has
been to rehabilitate in the eyes of scholars, speculations relating to
the constitution of matter, and, in a more general way, metaphysical
problems. Philosophy has, of course, never been completely separated
from science; but in times past many physicists dissociated themselves
from studies which they looked upon as unreal word-squabbles, and
sometimes not unreasonably abstained from joining in discussions which
seemed to them i