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＇sapid＇ moisture has been burnt out of them； their slag is； in all
cases； less odorous the metals ＇than the metals themselves＇。 Silver
and tin are more odorous than the one class of metals； less so than
the other； inasmuch as they are water ＇to a greater degree than the
former； to a less degree than the latter＇。
Some writers look upon Fumid exhalation； which is a compound of
Earth and Air； as the essence of Odour。 ＇Indeed all are inclined to
rush to this theory of Odour。＇ Heraclitus implied his adherence to
it when he declared that if all existing things were turned into
Smoke； the nose would be the organ to discern them with。 All writers
incline to refer odour to this cause ＇sc。 exhalation of some sort＇；
but some regard it as aqueous； others as fumid； exhalation； while
others； again； hold it to be either。 Aqueous exhalation is merely a
form of moisture； but fumid exhalation is； as already remarked；
composed of Air and Earth。 The former when condensed turns into water；
the latter； in a particular species of earth。 Now； it is unlikely that
odour is either of these。 For vaporous exhalation consists of mere
water ＇which； being tasteless； is inodorous＇； and fumid exhalation
cannot occur in water at all； though； as has been before stated；
aquatic creatures also have the sense of smell。
Again； the exhalation theory of odour is analogous to the theory
of emanations。 If； therefore； the latter is untenable； so； too； is the
former。
It is clearly conceivable that the Moist； whether in air （for air；
too； is essentially moist） or in water； should imbibe the influence
of； and have effects wrought in it by； the Sapid Dryness。 Moreover； if
the Dry produces in moist media； i。e。 water and air； an effect as of
something washed out in them； it is manifest that odours must be
something analogous to savours。 Nay； indeed； this analogy is； in
some instances； a fact ＇registered in language＇； for odours as well as
savours are spoken of as pungent； sweet； harsh； astringent rich
＇='savoury'＇； and one might regard fetid smells as analogous to bitter
tastes； which explains why the former are offensive to inhalation as
the latter are to deglutition。 It is clear； therefore； that Odour is
in both water and air what Savour is in water alone。 This explains why
coldness and freezing render Savours dull； and abolish odours
altogether； for cooling and freezing tend to annul the kinetic heat
which helps to fabricate sapidity。
There are two species of the Odorous。 For the statement of certain
writers that the odorous is not divisible into species is false； it is
so divisible。 We must here define the sense in which these species are
to be admitted or denied。
One class of odours； then； is that which runs parallel； as has
been observed； to savours： to odours of this class their
pleasantness or unpleasantness belongs incidentally。 For owing to
the fact that Savours are qualities of nutrient matter； the odours
connected with these ＇e。g。 those of a certain food＇ are agreeable as
long as animals have an appetite for the food； but they are not
agreeable to them when sated and no longer in want of it； nor are they
agreeable； either； to those animals that do not like the food itself
which yields the odours。 Hence； as we observed； these odours are
pleasant or unpleasant incidentally； and the same reasoning explains
why it is that they are perceptible to all animals in common。
The other class of odours consists of those agreeable in their
essential nature； e。g。 those of flowers。 For these do not in any
degree stimulate animals to food； nor do they contribute in any way to
appetite； their effect upon it； if any； is rather the opposite。 For
the verse of Strattis ridiculing Euripides…
Use not perfumery to flavour soup；
contains a truth。
Those who nowadays introduce such flavours into beverages deforce
our sense of pleasure by habituating us to them； until； from two
distinct kinds of sensations combined； pleasure arises as it might
from one simple kind。
Of this species of odour man alone is sensible； the other； viz。 that
correlated with Tastes； is； as has been said before； perceptible
also to the lower animals。 And odours of the latter sort； since
their pleasureableness depends upon taste； are divided into as many
species as there are different tastes； but we cannot go on to say this
of the former kind of odour； since its nature is agreeable or
disagreeable per se。 The reason why the perception of such odours is
peculiar to man is found in the characteristic state of man's brain。
For his brain is naturally cold； and the blood which it contains in
its vessels is thin and pure but easily cooled （whence it happens that
the exhalation arising from food； being cooled by the coldness of this
region； produces unhealthy rheums）； therefore it is that odours of
such a species have been generated for human beings； as a safeguard to
health。 This is their sole function； and that they perform it is
evident。 For food； whether dry or moist； though sweet to taste； is
often unwholesome； whereas the odour arising from what is fragrant；
that odour which is pleasant in its own right； is； so to say； always
beneficial to persons in any state of bodily health whatever。
For this reason； too； the perception of odour ＇in general＇
effected through respiration； not in all animals； but in man and
certain other sanguineous animals； e。g。 quadrupeds； and all that
participate freely in the natural substance air； because when
odours； on account of the lightness of the heat in them； mount to
the brain； the health of this region is thereby promoted。 For odour；
as a power； is naturally heat…giving。 Thus Nature has employed
respiration for two purposes： primarily for the relief thereby brought
to the thorax； secondarily for the inhalation of odour。 For while an
animal is inhaling；… odour moves in through its nostrils； as it were
'from a side…entrance。'
But the perception of the second class of odours above described
＇does not belong to all animal； but＇ is confined to human beings；
because man's brain is； in proportion to his whole bulk； larger and
moister than the brain of any other animal。 This is the reason of
the further fact that man alone； so to speak； among animals
perceives and takes pleasure in the odours of flowers and such things。
For the heat and stimulation set up by these odours are commensurate
with the excess of moisture and coldness in his cerebral region。 On
all the other animals which have lungs； Nature has bestowed their
due perception of one of the two kinds of odour ＇i。e。 that connected
with nutrition＇ through the act of respiration； guarding against the
needless creation of two organs of sense； for in the fact that they
respire the other animals have already sufficient provision for
their perception of the one species of odour only； as human beings
have for their perception of both。
But that creatures which do not respire have the olfactory sense
is evident。 For fishes； and all insects as a class； have； thanks to
the species of odour correlated with nutrition； a keen olfactory sense
of their proper food from a distance； even when they are very far away
from it； such is the case with bees； and also with the class of
small ants； which some denominate knipes。 Among marine animals； too；
the murex and many other similar animals have an acute perception of
their food by its odour。
It is not equally certain what the organ is whereby they so
perceive。 This question； of the organ whereby they perceive odour； may
well cause a difficulty； if we assume that smelling takes place in
animals only while respiring （for that this is the fact is manifest in
all the animals which do respire）