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.
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