ODORS FROM THE VEGETABLE KINGDOM.
The odors occurring in plants have their seat mostly in peculiar
receptacles called oil glands in which the aromatic substances are
stored and seem to take no further part in the vital processes of the
plant. As has been intimated, the parts of the plant in which the
aromatic substances are stored differ greatly; but in general it may be
said that in most cases the flowers and fruits contain the odors; more
rarely they may be found in the roots, in the bark, or in the wood,
and in very few instances equally distributed throughout the whole
plant. In some cases, however, we can obtain totally different odors
from various parts of the same plant; this applies, for instance, to
the orange-tree, whose blossoms furnish a different odor from the ripe
fruits, and the latter must be distinguished from that obtainable from
the leaves. The odorous substances occurring in the vegetable kingdom
are either mobile liquids (essential oils), or they have a thicker
consistence ranging from that of cream to that of soft cheese (balsams
or gum-resins), or they are solid (resins). Aside from the fact that
the term “essential oils” is quite incorrect, since the substances
called by that name have nothing in common with oils except perhaps
the liquid state, we are forced from a chemical standpoint to include
among them even solid substances; the well-known camphor, a firm and
waxy-looking body, belongs according to all its chemical properties
into the same group as the so-called essential oils. The name
“essential (or volatile) oils” is due to the fact that the volatile
vegetable aromatic substances cause a stain on paper similar to that
produced by oils and fats; but the stain made by the former disappears
spontaneously after some time, while that due to true oils and fats
persists. The disappearance of the stain depends on the evaporation
of the vegetable aromatic substances—a quality not possessed by fats.
Hence the volatile vegetable aromatic substances, in contradistinction
from non-volatile fixed or fatty oils, have been designated as
essential or volatile or ethereal oils. Inasmuch as the latter terms
are the ordinary trade names for these substances, we are compelled to
retain them despite their incorrectness. The French name for essential
oils is _essences_; “essence de lavande,” for instance, is the French
name for essential oil of lavender, and not for an alcoholic solution
of the oil, as might be inferred from the usually accepted meaning of
the English terms “essence of lavender,” “essence of peppermint,” etc.,
which mean solutions of these essential oils in alcohol.
As the localities where the raw materials—that is, the aromatic
plants—are cultivated on a large scale naturally constitute the places
of manufacture of essential oils, we find in southern France and
in England the most extensive factories devoted exclusively to the
preparation of perfumes. In the countries named, a favorable influence
is exerted, too, by their situation near the sea, as well as by their
trade with tropical lands from which additional aromatic plants are
imported.
We have stated above that the manufacture of essential oils forms
almost a monopoly in France and England; but there is no doubt that
this country (the United States) likewise possesses many localities
favorable to the cultivation of certain aromatic plants and the
preparation of essential oils from them, so that this branch of
industry could be carried on at a profit. For this reason we have in
our descriptions devoted some attention to the conditions of growth
required by such plants as might be raised here. We even find that some
advantages are derived from the hot-house cultivation of some tropical
plants.
An exact knowledge of the chemical properties of a substance is in all
cases the first and fundamental condition for its preparation; it would
appear necessary, therefore, that we should endeavor to gain complete
information about the nature of vegetable aromatic substances before we
enter upon the description of the various methods of their preparation.
THE CHEMICAL CONSTITUTION OF VEGETABLE AROMATIC SUBSTANCES.
The sources of the odors derived from the vegetable kingdom can be
divided, as stated above, into so-called essential oils, balsams,
gum-resins or soft resins, and hard resins. Since the latter bear a
certain relation to the essential oils from which they are formed
through chemical combinations, we must consider them first.
The flowers, the fruits and their rinds, or even the wood of some
plants form the receptacles of essential oils; if they are liquid they
are called essential oils _par excellence_; if they are firm they
are called camphors. Besides, there are intermediate states between
them: oil of rose is always viscid and solidifies even at temperatures
considerably above the freezing-point of water (see under Oil of Rose).
The bodies which are generally called essential oils are usually
mixtures of a hydrocarbon with an oxygenated body, or an unchanged oil
with another which has become altered by the influence of the oxygen of
the air—a condition to which we shall recur later on. With reference
to their elementary composition, essential oils may be divided into two
groups:
1. Non-oxygenated essential oils.
2. Oxygenated essential oils.
The non-oxgenated essential oils consist only of two elements—carbon
and hydrogen; the other group, as the name indicates, contains a third
element in chemical combination, and consist of carbon, hydrogen, and
oxygen. Most of the essential oils of the first group have the same
chemical composition: C_{10}H_{16} (10 atoms of carbon combined with
16 atoms of hydrogen). Despite the like chemical composition, all the
essential oils display different physical qualities; they vary in
density, in refractive power, in boiling-point (often by many degrees),
and, a matter of the greatest importance for our purposes, in their
odor. We may state at once that but few essential oils can be said
to have a pleasant odor; that of most of them is even disagreeable
and narcotic to the olfactory nerves; it is only after the oil has
been extremely diluted that the odor begins to become pleasant and to
resemble that of the plant from which the oil was derived.
According to their physical qualities, essential oils may be described
as fluids of a specific narcotic odor, colorless but very refractive,
and easily inflammable. Only a few essential oils can be produced
in such a state of purity as to appear perfectly colorless; usually
they are more or less dark yellow in color, and some even possess a
characteristic tint; thus oil of acacia is reddish-brown, oils of
rose and absinth are green, oil of chamomile is blue. But a simple
experiment will show that the color is not inseparably connected with
the oil, for certain tinted oils can be obtained perfectly colorless
by being distilled with another, less volatile oil which retains the
coloring matter.
The boiling-point of essential oils is in general very high —between
160° and 288° of the centigrade thermometer (C.), or 320° to 550°
F. The fact that we smell the essential oils in aromatic plants so
distinctly despite their high boiling-point is an evidence of their
exceedingly strong influence on the olfactory nerves.
A peculiar property of essential oils, which is of great importance
in their preparation, is that of distilling over in large quantities
with steam—both ordinary and superheated—that is, at temperatures
at most only slightly exceeding 100° C. or 212° F. For this reason
essential oils are usually obtained in this way, since they are but
slightly soluble in water. Still, most of the oils dissolve in water in
sufficient amount to impart to it their characteristic odor and thus
to render it often very fragrant. Aqua Naphæ triplex (orange-flower
water), rose water, etc., are such as have been distilled over with the
essential oils, contain a small quantity of the latter in solution, and
hence have a very agreeable odor.
All essential oils dissolve readily in strong alcohol, petroleum ether,
benzol, bisulphide of carbon, in liquid and solid fats, in glycerin,
etc.; we shall again recur to this important subject under the head of
the preparation of the essential oils.
If a freshly prepared essential oil is at once excluded from the air
by being placed in hermetically sealed vessels which it completely
fills, and is kept from the light, the oil will remain unchanged for
any length of time. But if an essential oil is exposed to the air, a
peculiar, chemical alteration begins, which proceeds more rapidly and
obviously if direct light acts upon the oil at the same time. The odor
becomes less intense, the oil grows darker in color and more viscous,
and also acquires a peculiar quality: it has a strong bleaching
effect which is easily seen on the cork closing the bottle, which is
beautifully bleached. After a certain time the oil changes to a viscid,
less odorous mass, into balsam, and the latter, after the prolonged
influence of the air, finally changes into a brownish, odorless
substance, into resin.
These remarkable physical and chemical alterations depend on the fact
that the essential oil absorbs oxygen from the air, which it puts into
a peculiar condition in which it exerts increased chemical activity
and is termed ozonized oxygen. One of the most marked of these effects
is the uncommonly strong bleaching power of ozonized or active oxygen.
When an essential oil that has altered so far as to contain ozonized
oxygen—which is shown by its bleaching vegetable coloring matters such
as the juice of cherries, red beets, tincture of litmus, etc., agitated
with it—is cooled, we notice the separation from it of a usually
crystalline, colorless, and odorless body called stearopten, while the
remaining liquid part is called elæopten. Stearopten always contains
oxygen, while elæopten still consists only of carbon and hydrogen.
In the formation of the stearopten we distinctly see the beginning
process of resinification, which, therefore, is nothing but an
oxidation (combination of the essential oil with oxygen). It should,
however, be stated that as to many essential oils this is not proven
by actual observation. Many of them are not known to us as naturally
existing without any stearopten. Balsams are essential oils which
have to a great extent changed into resin, which they contain in
solution, and thereby have become more or less viscid. If the process
of oxidation goes still farther, eventually the greater portion of the
essential oil becomes oxidized, the entire mass grows firm, and then
possesses only a very faint odor which is due to the last remnants of
the unchanged essential oil.
Since aromatic substances during evaporation become mixed with air, it
appears probable that they act upon the olfactory nerves only at the
moment when they become oxidized.
The entire process of resinification of oil of turpentine can be
followed very clearly on the pitch pine (Pinus austriaca, or other
species of Pinus), just as oil of turpentine in general can be taken
as an example of an essential oil on which the peculiarities of the
non-oxygenated essential oils may be easily studied. In many localities
the pitch pine is partly deprived of its bark when it has reached a
certain age. From the trunk exudes oil of turpentine which in the air
becomes more and more viscid by the absorption of oxygen and changes
into balsam, called turpentine. The latter is collected and distilled
with water, when the unchanged oil of turpentine passes over with the
steam, while the odorless resin (rosin or colophony) remains behind in
the stills.
The above-mentioned qualities of the essential oils indicate naturally
how those used in perfumery, which are often very costly, are to be
preserved. For this purpose small strong bottles should be chosen which
are closed with well-fitting glass stoppers, over which is applied a
glass capsule ground to fit tightly over the neck of the bottle. _These
bottles should always be completely filled_ (hence small bottles should
be selected), _and kept tightly closed, in the dark_. As the action of
oxygen is retarded by low temperatures, it is advisable to keep bottles
containing essential oils in a cool cellar. But care must be had never
to pour out an essential oil in the cellar near an open candle light.
The vapors are very apt to take fire, as they are quite inflammable.
As there are a great many aromatic vegetable substances, so there
are numerous odors, or, to retain the customary though incorrect
appellation, numerous essential oils. All of these, however, cannot be
used in the art of perfumery, as some of them do not possess a pleasant
odor, as is the case, for instance, with oil of turpentine. (We may
state here, however, that very pure oil of turpentine, distilled from
certain Coniferæ, has an agreeable, refreshing odor which at present
has found application in perfumery under the title of forest perfume
or pine-needle essence.) Besides, there are numerous essential oils
which, while possessing a very pleasant odor, still cannot be used in
perfumery except for very cheap preparations, though they are employed
in much larger quantities in the manufacture of liqueurs. Such oils
are: oil of cumin, fennel, juniper, absinth, etc.
As we shall return to this subject in connection with the essential
oils which are used in perfumery in general, we will now consider at
greater length the aromatic vegetable substances which are employed for
the manufacture of fragrant odors.