speak, _in potentia_. Science is in some sort a living organism, which
gives birth to an indefinite series of new beings taking the places of
the old, and which evolves according to the nature of its environment,
adapting itself to external conditions, and healing at every step the
wounds which contact with reality may have occasioned.

Sometimes this evolution is rapid, sometimes it is slow enough; but it
obeys the ordinary laws. The wants imposed by its surroundings create
certain organs in science. The problems set to physicists by the
engineer who wishes to facilitate transport or to produce better
illumination, or by the doctor who seeks to know how such and such a
remedy acts, or, again, by the physiologist desirous of understanding
the mechanism of the gaseous and liquid exchanges 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