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the luminous body. But here a certain reserve is necessary.
Many chemists of the present day refuse to speak of atoms
and molecules as real things. Their caution leads them to
stop short of the clear, sharp, mechanically intelligible
atomic theory enunciated by Dalton, or any form of that
theory, and to make the doctrine of multiple proportions
their intellectual bourne. I respect the caution, though I
think it is here misPlaced. The chemists who recoil from
these notions of atoms and molecules accept without hesita-
tion the Undulatory Theory of Light. Like you and me,
they one and all believe in an ether and its light-producing
waves. Let us consider what this belief involves. Bring
your imaginations once more into play and figure a series
of sound-waves passing through air. Follow them up to
their origin, and what do you there find ? A definite, tan-
gible, vibrating body. It may be the vocal chords of a
human being, it may be an organ-pipe, or it may be a
stretched string. Follow in the same manner a train of
ether-waves to their source; remembering at the same time
that your ether is matter, dense, elastic, and capable of
motions subject to and determined by mechanical laws.
What then do you expect to find as the source of a series
of ether-waves ? Ask your imagination if it will accept a
vibrating multiple proportion—a numerical ratio in a state
of oscillation ? I do not think it will. You cannot crown
the edifice by this abstraction. The scientific imagination,
which is here authoritative, demands as the origin and cause
of a series of ether-waves a particle of vibrating matter
quite as definite, though it may be excessively minute, as
that which gives origin to a musical sound. Such a particle
we name an atom or a molecule. I think the seeking intel-
lect when focussed so as to give definition without penum-
bral haze, is sure to realize this image at the last.

With a View of preserving thought continuous through-
out this discourse, and of preventing either failure of knowl--

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