Those of them which are of an abstergent nature, and
purge the whole surface of the tongue, if they do it in excess,
and so encroach as to consume some part of the flesh itself, like
potash and soda, are all termed bitter. But the particles which
are deficient in the alkaline quality, and which cleanse only
moderately, are called salt, and having no bitterness or
roughness, are regarded as rather agreeable than otherwise.
Bodies which share in and are made smooth by the heat of the
mouth, and which are inflamed, and again in turn inflame that
which heats them, and which are so light that they are carried
upwards to the sensations of the head, and cut all that comes in
their way, by reason of these qualities in them, are all termed
pungent. But when these same particles, refined by putrefaction,
enter into the narrow veins, and are duly proportioned to the
particles of earth and air which are there, they set them
whirling about one another, and while they are in a whirl cause
them to dash against and enter into one another, and so form
hollows surrounding the particles that enter-which watery vessels
of air (for a film of moisture, sometimes earthy, sometimes pure,
is spread around the air) are hollow spheres of water; and those
of them which are pure, are transparent, and are called bubbles,
while those composed of the earthy liquid, which is in a state of
general agitation and effervescence, are said to boil 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 vapor 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 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 reverse. Respecting the harmonies of sound I must
hereafter speak.
There is a fourth class of sensible things, having many
intricate varieties, which must now be distinguished. They are
called by the general name of colours, and are a flame which
emanates from every sort of body, and has particles corresponding
to the sense of sight. I have spoken already, in what has
preceded, of the causes which generate sight, and in this place
it will be natural and suitable to give a rational theory of
colours.
