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and elasticity corresponding to the temperature of freezing
water the velocity of sound in it is ten hundred and ninety
feet a second. It is almost exactly one-fourth of the ve-
locity in water; the reason being that though the greater
weight of the water tends to diminish the velocity, the
enormous molecular elasticity of the liquid far more than
atones for the disadvantage due to weight. By various
contrivances we can compel the vibrations of the air to
declare themselves; we knowthe length and frequency of
sonorous waves, and we have also obtained great mastery
over the various methods by which the air is thrown into
vibration. We know the phenomena and laws of vibrating
rods, of organ-pipes, strings, membranes, plates, and bells.
We can abolish one sound by another. We know the
physical meaning of music and noise, of harmony and dis-
cord. In short, as regards sounds we have a very clear
notion of the external physical processes which corre-
spond to our sensations.

In these phenomena of sound we travel a very little
way from downright sensible experience. Still the imagi-
nation is to some extent exercised. The bodily eye, for
example, cannot see the condensations and rarefactions of
the waves of sound. We Construct them in thought, and
we believe as firmly in their existence as in that of the air
itself. But now our experience has to be carried into a
new region, where a new use is to be made of it. Having
mastered the cause and mechanism of sound, we desire to
know the cause and mechanism of light. We Wish to ex-
tend our inquiries from the auditory nerve to the optic nerve.
There is in the human intellect a power of expansion—I
might almost call it a power of creation—which is brought
into play by the simple brooding upon facts. The legend
of the Spirit brooding over chaos may have originated in a
knowledge of this power. In the case now before us it has
manifested itself by transplanting into space, for the pur-

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