or ferment--of all these affections the
cause is termed acid. And there is the opposite affection arising from
an opposite cause, when the mass of entering particles, immersed in the
moisture of the mouth, is congenial to the tongue, and smooths and
oils over the roughness, and relaxes the parts which are unnaturally
contracted, and contracts the parts which are relaxed, and disposes
them all according to their nature;--that sort of remedy of violent
affections is pleasant and agreeable to every man, and has the name
sweet. But enough of this.

The faculty of smell does not admit of differences of kind; for all
smells are of a half-formed nature, and no element is so proportioned
as to have any smell. The veins about the nose are too narrow to admit
earth and water, and too wide to detain fire and air; and for this
reason no one ever perceives the smell of any of them; but smells always
proceed from bodies that are damp, or putrefying, or liquefying, or
evaporating, and are perceptible only in the intermediate state, when
water is changing into air and air into water; and all of them are
either vapour or mist. That which is passing out of air into water is
mist, and that which is passing from water into air is vapour; and hence
all smells are thinner than water and thicker than air. The proof of
this is, that when there is any obstruction to the respiration, and a
man draws in his breath by force, then no smell filters through, but the
air without the smell alone penetrates. Wherefore the varieties of smell
have no name, and they have not many, or definite and simple kinds;
but they are distinguished only as painful and pleasant, the one sort
irritating and disturbing the whole cavity which is situated between the
head and the navel, the other having a soothing influence, and restoring
this same region to an agreeable and natural condition.

In considering the third kind of sense, hearing, we must speak of the
causes in which it originates. We may in general assume sound to be a
blow which passes through the ears, and is transmitted by means of the
air, the brain, and the blood, to the soul, and that hearing is the
vibration of this blow, which begins in the head and ends in the region
of the liver. The sound which moves swiftly is acute, and the sound
which moves slowly is grave, and that which is regular is equable and
smooth, and the reverse is harsh. A great body of sound is loud, and
a small body of sound the reve