by Fresnel, the hypothesis of the luminous ether, which
had so great a struggle at the outset to overcome the stubborn
resistance of the partisans of the then classic theory of emission,
seemed, on the contrary, to possess in the sequel an unshakable
strength. Lame, though a prudent mathematician, wrote: "_The
existence_ of the ethereal fluid is _incontestably demonstrated_ by
the propagation of light through the planetary spaces, and by the
explanation, so simple and so complete, of the phenomena of
diffraction in the wave theory of light"; and he adds: "The laws of
double refraction prove with no less certainty that the _ether exists_
in all diaphanous media." Thus the ether was no longer an hypothesis,
but in some sort a tangible reality. But the ethereal fluid of which
the existence was thus proclaimed has some singular properties.

Were it only a question of explaining rectilinear propagation,
reflexion, refraction, diffraction, and interferences notwithstanding
grave difficulties at the outset and the objections formulated by
Laplace and Poisson (some of which, though treated somewhat lightly at
the present day, have not lost all value), we should be under no
obligation to make any hypothesis other than that of the undulations
of an elastic medium, without deciding in advance anything as to the
nature and direction of the vibrations.

This medium would, naturally--since it exists in what we call the
void--be considered as imponderable. It may be compared to a fluid of
negligible mass--since it offers no appreciable resistance to the
motion of the planets--but is endowed with an enormous elasticity,
because the velocity of the propagation of light is considerable. It
must be capable of penetrating into all transparent bodies, and of
retaining there, so to speak, a constant elasticity, but must there
become condensed, since the speed of propagation in these bodies is
less than in a vacuum. Such properties belong to no material gas, even
the most rarefied, but they admit of no essential contradiction, and
that is the important point.[20]

[Footnote 20: Since this was written, however, men of science have
become less unanimous than they formerly were on this point. The
veteran chemist Professor Mendeleeff has given reasons for thinking
that the ether is an inert gas with an atomic weight a million times
less than that of hydrogen, and a velocity of 2250 kilometres per
second (_Principles of Chemistry_, Eng. ed., 19