f the
water that comes into play. Every liquid particle pushed against its
neighbour delivers up its motion with extreme rapidity, and the pulse
is propagated as a thrill. The incompressibility of water, as
illustrated by the famous Florentine experiment, is a measure of its
elasticity; and to the possession of this property, in so high a
degree, the rapid transmission of a sound-pulse through water is to be
ascribed.

But water, as you know, is not necessary to the conduction of sound;
air is its most common vehicle. And you know that when the air
possesses the particular density and elasticity corresponding to the
temperature of freezing water, the velocity of sound in it is 1,090
feet a second. It is almost exactly one-fourth of the velocity 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 know the length and frequency of the 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 discord.
In short, as regards sound in general, we have a very clear notion of
the external physical processes which correspond to our sensations.

In the phenomena of sound, we travel a very little way from downright
sensible experience. Still the imagination 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 is 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 extend our enquiries from the auditory 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 experience of this power. In the case