For a long time we
have used for transmissions to a distance the elastic properties of
the air, and more recently the electric conductivity of the soil and
of water, particularly that of the sea.
Modern physics leads us on the other hand, as we have seen, to
consider that there exists throughout the whole of the universe
another and more subtle medium which penetrates everywhere, is endowed
with elasticity _in vacuo_, and retains its elasticity when it
penetrates into a great number of bodies, such as the air. This medium
is the luminous ether which possesses, as we cannot doubt, the
property of being able to transmit energy, since it itself brings to
us by far the larger part of the energy which we possess on earth and
which we find in the movements of the atmosphere, or of waterfalls,
and in the coal mines proceeding from the decomposition of carbon
compounds under the influence of the solar energy. For a long time
also before the existence of the ether was known, the duty of
transmitting signals was entrusted to it. Thus through the ages a
double evolution is unfolded which has to be followed by the historian
who is ambitious of completeness.
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