_Of Oil._
Oil is a liquid inflammable substance, of great tenacity, disposed to
pour in a stream rather than in drops. It is little, if at all, soluble
in water. It burns with smoke and soot, and leaves a residuum of a coaly
substance. It consists of acid and water combined with phlogiston.
All oil is the produce of the vegetable or animal kingdom, no proper
mineral substance containing any of it.
By distillation oil is in part decomposed, and by this means the thicker
kinds of oil are rendered thinner and more volatile, the acid, to which
their consistence is chiefly owing, being lost in the process. By
repeated distillation it is supposed that all oils may be brought almost
to the state of aether, and even of ardent spirit.
Acids act powerfully upon oils, but very differently, according to the
nature of each. Alkalies also combine with oils, and the less thin and
volatile they are, the more easily are they soluble in alkalies. The
union of alkali and oil makes _soap_. All oil dissolves sulphur, and
with it makes what is called a _balsam_. Oils also dissolve metallic
substances, but most sensibly copper and lead. United with the calx of
lead, it is used in painting.
Oil not readily mixing with water, it will diffuse itself over its
surface, and, notwithstanding its tenacity, it will do this very
rapidly, and to a great extent; and then it has the extraordinary
effect of preventing the action of the wind upon the water, so as to
prevent the forming of waves. If a quantity of oil and water be put into
a glass vessel and swung, the surface of the water below the oil will be
seen to change with respect to the vessel, but not that of the oil. If
spirit of wine be put upon them, that will be at rest, and both the
lower fluids in motion.
Vegetable oil is of two kinds, the _soft_, or _mild_, which has little
or no taste or smell, and the _essential_ oil, which is thin, and
retains the smell and taste of the plant from which it was extracted.
Mild or sweet oil is expressed from the grains or kernels of vegetables,
and requires a considerable degree of heat to convert it into vapour, in
which state alone it is capable of being inflamed.
_Essential oil_ is volatile in the heat of boiling water, and is
generally obtained by means of distillation from the most odoriferous
sorts of plants; but is sometimes found in their vesicles, as in the
rind of an orange. The strong taste of this kind of oil arises from the
disengaged acid which abounds in it; and by this means it is soluble in
spirit of wine, which sweet oil is not; but it loses much of this
property by repeated distillations. By long exposure to the air it loses
its more volatile parts, and thereby approaches to the nature of a
resin. This volatile odoriferous principle has been called the _spiritus
rector_ of the plant.
The essential oils of different plants differ much in their specific
gravity, and also in the manner by which they are affected by cold, some
being heavier and others lighter than water, and some being more
difficultly, and others more easily, congealed. Though the differences
with respect to _weight_ and _consistency_ in these oils is probably
owing to the state of the acid that is combined with them, these two
properties are wholly independent of each other; some essential oils
being very thin and yet heavy, and others thick and yet light. Essential
oils are used in perfumes, and also in medicine, acting powerfully the
nervous system.
Essential oils are very apt to be adulterated. If it be with sweet oil,
it may be discovered by evaporation on white paper, or by a solution in
spirit of wine, which will not act upon the sweet oil. If spirit of wine
be mixed with it, it will be discovered by a milky appearance upon
putting water to it, which uniting with the spirit, will leave the oil
much divided. If oil of turpentine, which is the cheapest of essential
oils, be mixed with any of the more valuable kinds, it will be
discovered by evaporation; a strong smell of turpentine being left on
the paper, or cloth, upon which the evaporation was made.
Animal oil, like the vegetable, is of two kinds; the first _butter_, or
_fat_, which is easily congealed, owing to the quantity of acid that is
intimately combined with it. It resembles the sweet oil of vegetables in
having no smell or taste. The other kind of animal oil is extracted by
distillation from the flesh, the tendons, the bones, and horns, &c. of
animals. It differs essentially from the other kind of animal oil, by
containing an alkali instead of an acid. By repeated distillation it
becomes highly attenuated and volatile; and in this state it is called
the _oil of Dippel_, the discoverer of it.
All oil exposed to much heat is in part decomposed, and acquires a
disagreeable smell; and in this state it is said to be _empyreumatic_:
but this property is lost by repeated distillations.
Besides the vegetable and animal oils above described, there is a fossil
oil called _bitumen_, the several kinds of which differ much in colour
and consistence; the most liquid is called _petroleum_, from being found
in the cavities of rocks, and the more solid kinds are _amber_, _jet_,
_asphaltum_, and _pit-coal_. When distilled, the principal component
parts of all these substances are an oil and an acid. But all fossil oil
is probably of vegetable or animal origin, from masses of vegetables or
animals long buried in the earth. Their differences from resins and
other oily matters are probably owing to _time_; the combinations of
mineral acids and oils so nearly resembling bitumens, the principal
difference being their insolubility in spirit of wine.
That the most solid of these, as amber, has been formerly in a liquid
state, is evident, from insects and other substances being frequently
found in them; and pit-coal has been often found with both the internal
texture and external appearance of wood; so that strata of pit-coal have
probably been beds of peat in some former state of the earth.