even in the focus of the largest
burning-glasses, which in a moment vitrifies a particle of opaque matter.

IV. _Of the Organ of Hearing._

It is generally believed, that the tympanum of the ear vibrates
mechanically, when exposed to audible sounds, like the strings of one
musical instrument, when the same notes are struck upon another. Nor is
this opinion improbable, as the muscles and cartilages of the larynx are
employed in producing variety of tones by mechanical vibration: so the
muscles and bones of the ear seem adapted to increase or diminish the
tension of the tympanum for the purposes of similar mechanical vibrations.

But it appears from dissection, that the tympanum is not the immediate
organ of hearing, but that like the humours and cornea of the eye, it is
only of use to prepare the object for the immediate organ. For the portio
mollis of the auditory nerve is not spread upon the tympanum, but upon the
vestibulum, and cochlea, and semicircular canals of the ear; while between
the tympanum and the expansion of the auditory nerve the cavity is said by
Dr. Cotunnus and Dr. Meckel to be filled with water; as they had frequently
observed by freezing the heads of dead animals before they dissected them;
and water being a more dense fluid than air is much better adapted to the
propagation of vibrations. We may add, that even the external opening of
the ear is not absolutely necessary for the perception of sound: for some
people, who from these defects would have been completely deaf, have
distinguished acute or grave sounds by the tremours of a stick held between
their teeth propagated along the bones of the head, (Haller. Phys. T. V. p.
295).

Hence it appears, that the immediate organ of hearing is not affected by
the particles of the air themselves, but is stimulated into animal motion
by the vibrations of them. And it is probable from the loose bones, which
are found in the heads of some fishes, that the vibrations of water are
sensible to the inhabitants of that element by a similar organ.

The motions of the atmosphere, which we become acquainted with by the sense
of touch, are combined with its solidity, weight, or vis intertiae; whereas
those, that are perceived by this organ, depend alone on its elasticity.
But though the vibration of the air is the immediate object of the sense of
hearing, yet the ideas, we receive by this sense, like those received from
light, are only as a language, which by acquir