the experiment of Fizeau, from which one was obliged to infer
that the luminiferous ether does not take part in the movements of
bodies. The phenomenon of aberration also favoured the theory of
the quasi-rigid ether.

The development of the theory of electricity along the path opened
up by Maxwell and Lorentz gave the development of our ideas concerning
the ether quite a peculiar and unexpected turn. For Maxwell himself
the ether indeed still had properties which were purely mechanical,
although of a much more complicated kind than the mechanical
properties of tangible solid bodies. But neither Maxwell nor his
followers succeeded in elaborating a mechanical model for the ether
which might furnish a satisfactory mechanical interpretation of
Maxwell's laws of the electro-magnetic field. The laws were clear
and simple, the mechanical interpretations clumsy and contradictory.
Almost imperceptibly the theoretical physicists adapted themselves
to a situation which, from the standpoint of their mechanical
programme, was very depressing. They were particularly influenced
by the electro-dynamical investigations of Heinrich Hertz. For
whereas they previously had required of a conclusive theory that
it should content itself with the fundamental concepts which belong
exclusively to mechanics (e.g. densities, velocities, deformations,
stresses) they gradually accustomed themselves to admitting electric and
magnetic force as fundamental concepts side by side with those of
mechanics, without requiring a mechanical interpretation for them.
Thus the purely mechanical view of nature was gradually abandoned.
But this change led to a fundamental dualism which in the long-run
was insupportable. A way of escape was now sought in the reverse
direction, by reducing the principles of mechanics to those
of electricity, and this especially as confidence in the strict