tuning-fork which render its vibrations
audible. It is the waves of aether sent forth from those lamps
overhead which render them luminous to us; but so minute are these
waves, that it would take from 30,000 to 60,000 of them placed end to
end to cover a single inch. Their number, however, compensates for
their minuteness. Trillions of them have entered your eyes, and hit
the retina at the backs of your eyes, in the time consumed in the
utterance of the shortest sentence of this discourse. This is the
steadfast result of modern research; but we never could have reached
it without previous discipline. We never could have measured the
waves of light, nor even imagined them to exist, had we not previously
exercised ourselves among the waves of sound. Sound and light are now
mutually helpful, the conceptions of each being expanded,
strengthened, and defined by the conceptions of the other.

The aether which conveys the pulses of light and heat not only fills
celestial space, swathing suns, and planets, and moons, but it also
encircles the atoms of which these bodies are composed. It is the
motion of these atoms, and not that of any sensible parts of bodies,
that the aether conveys. This motion is the objective cause of what,
in our sensations, are light and heat. An atom, then, sending its
pulses through the aether, resembles a tuning-fork sending its pulses
through the air. Let us look for a moment at this thrilling medium,
and briefly consider its relation to the bodies whose vibrations it
conveys. Different bodies, when heated to the same temperature,
possess very different powers of agitating the aether: some are good
radiators, others are bad radiators; which means that some are so
constituted as to communicate their atomic motion freely to the
aether, producing therein powerful undulations; while the atoms of
others are unable thus to communicate their motions, but glide through
the medium without materially disturbing its repose. Recent
experiments have proved that elementary bodies, except under certain
anomalous conditions, belong to the class of bad radiators. An atom,
vibrating in the aether, resembles a naked tuning-fork vibrating in
the air. The amount of motion communicated to the air by the thin
prongs is too small to evoke at any distance the sensation of sound.
But if we permit the atoms to combine chemically and form molecules,
the result, in many cases, is an enormous change in the power of