of them owe their virtues to the efflorescences they have washed off
from pyrites, that have grown hot and begun to be decomposed, they are
impregnated with the saline matters produced by pyrites in that state:
that is, with aluminous, vitriolic, and sulphureous substances, which
have the same effect on Soap as the Selenites have.
Mineral waters containing Neutral Salts only, such as Sea-salt, Epsom
Salt, Glauber's Salt, are nevertheless hard with regard to Soap,
though the Acids of those Salts, being united with Fixed Alkalis, are
incapable of decompounding it. The reason is, that those Neutral Salts
are more soluble in water than Soap is; so much indeed as even to
exclude it: because each of the two principles that composed them hath
a very great affinity with water; whereas only one of the principles
of Soap, namely, its Alkali, hath that affinity; the other, to wit,
the oily principle, having none at all. Thus water impregnated with
an Acid, or with any Neutral Salt, is hard with regard to Soap, and
incapable of dissolving it; and hence it follows, that Soap is a sort
of touchstone for trying the purity of water.
Wine dissolves Soap; but imperfectly, because it contains an acid or
tartarous part. Spirit of Wine also dissolves it: but neither is this
dissolution perfect; because it contains too little water: for its
spirituous part can dissolve nothing but the Oil of the Soap; and the
Alkali is not at all, or at least in a very small quantity, soluble in
this menstruum. The true solvent of Soap is therefore a liquor that is
partly spirituous, partly aqueous, and not acid.
Brandy has these qualities: and accordingly it is the solvent that
unites best with Soap, dissolves the greatest quantity, and makes the
most limpid solution thereof. Yet even this solution hath something
of a milky cast, occasioned by its not being entirely free from an
Acid, or the tartarous principle. This fault may be easily corrected,
by mixing with it a little Alkali to absorb the Acid. A dram of
crystallized salt of kelp mixed with three ounces and a half of good
brandy, renders it capable of dissolving an ounce and two drams of good
hard Soap, into a perfectly limpid liquor. This experiment also we owe
to Mr. Geoffroy.
Some years ago it was discovered that Soap might be used with great
success in Medicine, and that it possesses the property of dissolving
the stony concretions that form in several parts of the body,
particularly in the kidneys and bladder. Soap is the basis of the
composition known by the name of _Mrs. Stephen's Remedy_, and in this
one ingredient its whole virtue resides.
From what hath been said on the nature of this compound, as well as on
the cause and phenomena of its dissolution, it plainly appears to be
of the last consequence, in administering it to a patient, that his
constitution be considered, and a proper regimen ordered. All Acids
should be absolutely forbid him; as we know they hinder the Soap from
dissolving, and decompound it; and if the patient have any acidities
in the first passages, matters capable of neutralizing them should be
prescribed him: as prepared crabs eyes, and other absorbents known in
Medicine: in such cases those with which the Soap is compounded in Mrs.
Stephen's remedy may be of use.
PROCESS IV.
_To combine Fat Oils with Sulphur._
Put any Fat Oil whatever into an earthen vessel; add to it about the
fourth part of its weight of Flower of Sulphur, and set the vessel in
a furnace, with lighted coals under it. When the Oil hath acquired
a certain degree of heat, the Sulphur will melt, and you will see
it fall immediately to the bottom of the Oil, in the form of a very
red fluid. The two substances will remain thus separated, without
mixing together, while the heat is no greater than is necessary to
keep the Sulphur in fusion. Increase it therefore; but slowly and with
circumspection, lest the matter take fire. When the Oil begins to
smoke, the two liquors will begin to mix and look turbid: at last they
will unite so as to appear one homogeneous whole. If you keep up the
heat so that the mixture shall always continue smoking and ready to
boil, you may add more Sulphur, which will perfectly incorporate with
it: and thus may a pretty considerable quantity thereof be introduced
into this composition.
_OBSERVATIONS._
The Phlogiston and the Vitriolic Acid have each an affinity with Oils.
It is not therefore surprising that Sulphur, which is a compound of
these two substances, should be soluble in oily matters. Yet it is
remarkable, that Essential Oils, which are much thinner than the Fat
Oils, dissolve Sulphur with much more difficulty; as will be shewn when
we come to treat of those Oils; and that Spirit of Wine, which contains
an exceeding subtile Oil, doth not act upon Sulphur at all.
Oil, by contracting an union, with Sulphur, produces a considerable
alteration in that mineral: a phenomenon so much the more surprising,
that we know it to be in some sort unalterable by any other solvent,
of what kind soever, add, that its nature admits of no change but by
burning. We shall say more on this subject under the head of Essential
Oils.
PROCESS V.
_To combine Fat Oils with Lead, and the Calces of Lead. The Basis of
Plasters. The Decomposition of this Combination._
Into an earthen vessel put granulated Lead, Litharge, Ceruse, or
Minium; and pour thereon twice its weight of any Fat Oil whatever. If
you set the vessel over a brisk fire, the Lead at bottom will melt
before the Oil begin to boil. When it boils, stir the matter with a
stick: the Lead, or the Calx of Lead, will gradually disappear, and
at last be totally dissolved by the Oil, to which it will give a very
thick consistence.
_OBSERVATIONS._
Fat Oils dissolve not only Lead, but its calces also: nay, they
dissolve the latter more readily than Lead in substance; probably
because they are more divided. The result of a combination of these
matters is a thick, tenacious mass, that grows in some degree hard in
the cold, and soft by heat. This composition is known in Pharmacy by
the name of _Plaster_. It is made up with several drugs into plasters,
which partake of the virtues of those drugs; so that it is the basis of
almost all plasters.
Lead itself is seldom used to make plasters: Ceruse, Litharge, or
Minium, are preferred to it; because these matters unite, as hath been
said, more readily and more easily with Oils.
It sometimes happens, that the Oil is burnt in the operation, and that
the calx of Lead is partly resuscitated: and this gives the plaster
a black colour, which however it ought not to have. This accident is
occasioned by an excess of heat: and as it is very difficult to keep
the Oil and the Lead in the proper degree of heat, seeing both these
matters are apt to grow very hot, it hath been contrived to put into
the vessel, in which the coction is to be performed, a pretty large
quantity of water; which being susceptible only of a much smaller and
a certain degree of heat, that is constantly the same when it boils,
procures the advantage of having the composition very uniform and very
white.
It is necessary to stir the mixture incessantly, in order to prevent
the burning of the combined Oil and Lead; which, as they unite, sink
in the water by their greater weight. If the water happen to be wasted
before the Oil hath dissolved all the Lead, or before the plaster hath
acquired a proper degree of consistence, you must remove the vessel
from the fire, and let the mixture cool, before you add more: for, if
this precaution be neglected, the heat of the matter, which is now
much greater than that of boiling water, will occasion a considerable
explosion and extravasation thereof, though the water poured into it be
as hot as possible.
The combination of Fat Oil with a Calx of Lead may be considered as
a sort of metallic Soap, having a metalline Calx, instead of a Fixed
Alkali, for its basis. Mr. Geoffroy hath observed, that if a pound of
Litharge, rubbed very fine and well washed, be incorporated with two
pounds of Olive Oil, in the same manner as plaster is made, keeping
water enough in the vessel to hinder the mixture from burning, there
rises a smoke, while the Oil is uniting with the Calx of Lead, smelling
much like that which rises from Soap.
The Oil may be separated from the Calx of Lead, by the methods used to
separate it from a Fixed Alkali: and when it is so separated, it hath
the same properties as that separated from common Soap.
This species of metallic Soap, formed by the union of a Fat Oil with
the Calx of Lead, is not soluble in water, and communicates nothing to
it but a greasy taste. Therefore, if you would decompound it by the
means of an Acid, you must pour that Acid immediately on the compound.
The Acid will attack and dissolve the Calx of Lead; and the Oil, being
thus set at liberty, will rise clear and limpid to the surface of the
acid liquor. Distilled vinegar effects this separation better than any
other Acid, because it is the true solvent of Lead.
CHAP. IV.
_Of the_ SUBSTANCES _obtained from_ VEGETABLES _with a Degree of Heat
not exceeding that of boiling Water_.
PROCESS I.
_To obtain from Plants, by distilling them with the mean Degree of
Heat between freezing and boiling Water, a Liquor impregnated with
their Principle of Odour._
In the morning, before sun-rise, gather the plant from which you design
to extract its odoriferous water. Chuse the plant in its full vigour,
perfectly sound, and free from all adventitious matters, except dew.
Put this plant, without squeezing it, into the body of a tinned copper
alembic, and set it in a water-bath. Fit on its head, and to the nose
thereof lute a glass receiver with wet bladder.
Warm the bath to the mean degree between freezing and boiling water.
You will see a liquor distil and fall drop by drop into the receiver.
Continue the distillation with this degree of heat, till no more drops
fall from the nose of the alembic. Then unlute the vessels; and if
you have not as much liquor as you want, take out of the cucurbit the
plant already distilled, and put a fresh one in its place. Distil
as before, and go on thus till you have a sufficient quantity of
odoriferous liquor. Put it into a bottle; stop it close; and set it in
a cool place.
_OBSERVATIONS._
The liquor obtained from plants, with the degree of heat here
prescribed, consists of the dew that was on the plant, and some of the
phlegm of the plant itself, together with its odorous principle. Mr.
Boerhaave, who examined this odoriferous part of plants with great
care, calls it the _Spiritus Rector_. The nature of this Spirit is
not yet thoroughly ascertained; because it is so very volatile, that
it cannot easily be subjected to the experiments that are necessary
to analyze it, and to discover all its properties. If the bottle
containing the liquor, which may be considered as the vehicle of this
Spirit, be not exceeding carefully stopped, it flies quite off: so that
in a few days nothing will be found but an insipid inodorous water.
Great part of the virtue of plants resides in this their principle
of odour; and to it must be ascribed the most singular and the most
wonderful effects we every day see produced by them. Every body
knows, that a great number of odorous plants affect, in a particular
manner, by their scent only, the brain and the _genus nervosum_, of
such especially whose nerves are very sensible, and susceptible of
the slightest impression; such as hypochondriacal or melancholy men,
and hysterical women. The smell of the Tuberose, for instance, is
capable of throwing such persons into fits, so as to make them drop
down and swoon away. The smell of Rue, again, which is equally strong
and penetrating, but of a different kind, is a specific remedy against
the ill effects of the Tuberose; and brings those persons to life
again, with as quick and as surprising an efficacy, as that by which
they were reduced to a state not unlike death. This is Mr. Boerhaave's
observation.
The odorous exhalations of plants must be considered as a continual
emanation of their _Spiritus Rector_: but as growing plants are in a
condition to repair, every instant, the losses they sustain by this
means, as well as by transpiration, it is not surprising that they
are not soon exhausted, while they continue in vigour. Those, on the
contrary, which we distil, having no such resource, are very soon
entirely deprived of this principle.
The separation of the _Spiritus Rector_ from plants requires but a
very gentle heat, equally distant from the freezing point and from
the heat of boiling water. Accordingly the heat of the sun in summer
is sufficient to dissipate it almost entirely. This shews why it is
dangerous to stay long in fields, or woods, where many noxious plants
grow. The virtues of plants residing chiefly in their exhalations,
which the heat of the sun increases considerably, a sort of atmosphere
is formed round them, and carried by the air and the wind to very great
distances.
For the same reason the air of a country may be rendered salutary and
medicinal, by the exhalations of wholesome plants growing therein. From
the facility with which the odorous principle of plants evaporates, we
learn what care ought to be taken in drying those intended for medical
uses, so as to preserve their virtues. They must by no means be exposed
to the sun, or laid in a warm place: a cool, dry place, into which the
rays of the sun never penetrate, is the properest for drying plants,
with as little loss of their virtue as possible.
Though there is reason to believe that every vegetable matter hath
a _Spiritus Rector_, seeing each hath its particular scent, yet
this principle is not very perceptible in any but those which have
a very manifest odour: and accordingly it is extracted chiefly from
aromatic plants, or the most odoriferous parts of plants. I say the
most odoriferous parts; because, in most plants and trees, there are
generally certain parts that have a much more sensible, and much
stronger scent than the rest. The odour of a plant, or of a tree, hath
its principal residence sometimes in the root, sometimes in the leaves,
at other times in the bark or wood, and very frequently in the flowers
and seeds. Therefore, when you design to extract the principle of odour
from a vegetable that is not equally odoriferous in every part, you
must chuse those parts that have the most perceptible and strongest
scent.
PROCESS II.
_To extract the Fat Oils of Plants by Decoction in boiling Water. Cacao
Butter._
Pound or bruise in a marble mortar your vegetable substances, abounding
with the Fat Oil which you intend to extract by decoction: tie them
up in a linen cloth; put this packet into a pan, with seven or eight
times as much water, and make the water boil. The Oil will be separated
by the ebullition, and float on the surface of the water. Skim it off
carefully with a ladle, and continue boiling till no more Oil appear.
_OBSERVATIONS._
The heat of boiling water is capable of separating the Fat Oils from
vegetable matters that contain any: but this is to be effected by
actual decoction only, and not by distillation; because these Oils will
not rise in an alembic with the heat of boiling water. We are therefore
necessitated to collect them from the surface of the water, as above
directed. By this means a much greater quantity of Fat Oil may be
obtained than by expression alone; because the degree of heat applied
greatly facilitates the separation of the Oil. For a convincing proof
of this truth, take the remains of any vegetable matters, from which
the Oil hath been so thoroughly expressed that they would yield no
more; boil them in this manner, and you will obtain a great deal more
Oil.
The water used in this coction generally becomes milky, like an
emulsion; because it contains many oily particles, that are dispersed
in it just as in an emulsion. Nevertheless, this way of obtaining the
Fat Oils is not generally practised; because the heat, to which they
are exposed in the operation, occasions their being less mild than they
naturally are: but it is an excellent method, and indeed the only one
that can be employed, for extracting from particular vegetables certain
concrete oily matters, in the form of Butter or Wax; which matters are
no other than Fat Oils in a fixed state. The Cacao yields, by this
means, a very mild butter; and in the same manner is a Wax obtained
from a certain shrub in America.
The heat of boiling water melts these oily matters, which then ascend
to the surface of the liquor, and float on it like other Oils. They
afterwards fix as they cool, and resume their natural consistence. We
shall see in the sequel, that they cannot be extracted in a concrete
form by distillation, which requires a greater degree of heat than
that of boiling water; because distillation changes their nature,
partly decomposes them, and prevents their returning to their proper
consistence as they cool.
PROCESS III.
_To extract Essential Oils of Plants by Distillation with the Heat of
boiling Water. Distilled Waters._
Put into a cucurbit the plant from which you design to extract the
Essential Oil. Add as much water as will fill two thirds of your
vessel, and dissolve therein half an ounce of Sea-salt for every quart
of water you use. To this body fit on an alembic-head, and to the nose
thereof lute a receiver, with sized paper, or wet bladder. Set it in a
furnace, and let the whole digest together, in a very gentle warmth,
for twenty-four hours.
This being done, light a wood-fire under your vessel, brisk enough to
make the water in it boil immediately. Then slacken your fire, and
leave it just strong enough to keep the water simmering. There will
come over into the receiver a liquor of a whitish colour, somewhat
milky; on the surface of which, or at the bottom, will be found an Oil;
which is the Essential Oil of the vegetable you put into the cucurbit.
Continue your distillation with the same degree of heat, till you
perceive the liquor come off clear, and unaccompanied with any Oil.
When the distillation is finished, unlute the receiver; and, if the
Essential Oil be of that sort that it is lighter than water, fill the
vessel up to the top with water. On this occasion a long-necked matrass
should be used for a receiver; that the Oil which floats on the water
may collect together in its neck, and rise up to its mouth. Then in the
neck of this vessel put the end of a thread of cotton-twine, so that
the depending part without the vessel may be longer than that in the
Oil, and the extremity thereof hang within the mouth of a little phial,
just big enough to contain your quantity of Oil. The Oil will rise
along the yarn as in a siphon, filter through it, and fall drop by drop
into the little phial. When all the Oil is thus come over, stop your
little bottle very close, with a cork coated over with a mixture of wax
and a little pitch.
If your Oil be ponderous, and of the sort that sinks in water, pour the
whole contents of the receiver into a glass funnel, the pipe of which
must terminate in a very small aperture that may be stopped with your
fore-finger. All the Oil will be collected in the lower part of the
funnel: then remove your finger, and let the Oil run out into a little
bottle through another small funnel. When you see the water ready to
come, stop the pipe of the funnel, and cork the bottle containing your
Oil.
_OBSERVATIONS._
Essential Oils, though they all resemble each other in their principal
properties, are nevertheless very different in some respects: for which
reason almost every one of them requires a particular management, for
obtaining it with the greatest advantage possible, both as to quality
and quantity.
One of the first things requisite is, to chuse the proper time for
distilling the plant, from which you desire to extract the Essential
Oil; because the quantity of Oil varies considerably, according to
the season of the year, as well as the age of the plant. For example,
the most favourable time for obtaining these Oils from the leaves of
ever-green plants or trees, such as Thyme, Sage, Rosemary, the Orange,
the Bay, the Fir, &c. is the end of Autumn; because these vegetables
contain a great deal more Oil at that season than at any other. With
regard to annual plants, they must be chosen when in their prime, and
just before they begin to decline. The time therefore of gathering them
is when they begin to flower: and if you want to extract the Oil from
the flowers themselves, you must pull them just when they are newly
blown.
Secondly, it must be observed, that the Essential Oils of plants
are, as it were, the chief residence and reservoir of their odorous
principle; that they are to be found wherever that principle exists,
and never where it is not: so that what we said concerning the
_Spiritus Rector_ of plants is applicable here. It must be remembered,
that all the parts of some vegetables are odoriferous. Such plants may
be put into the alembic all together, and the Essential Oil distilled
from all their parts at once. But others, and indeed the greatest
number, have no odour, or at least none that is very perceptible,
except in some particular parts; as in their leaves, flowers, roots,
or seeds: therefore, when you want to have the Essential Oil of such a
plant, you must chuse that part in which the Odour resides. The sense
of smelling must be the artist's principal guide on this occasion.
Thirdly, all vegetables, and all the parts of vegetables, have not the
same texture: some are hard and compact, as woods, barks, and some
roots; others are tender and succulent, as most annual plants, and
some fruits. For this reason, they must be differently prepared for
distillation. It may be laid down as a general rule, that the closer
and more compact their texture is, the more they require to be opened
and divided, either by comminuting them into small particles, or by
digesting them a considerable time in water acuated with Salt.
Fourthly, though all Essential Oils be capable of rising in
distillation with the heat of boiling water, yet they have not all
an equal degree of levity and weight: on the contrary, they vary
exceedingly in this respect: some, as, for instance, those of all our
European aromatics, being lighter than water, so that they always float
on its surface; whereas others, such as those of Cloves, Sassafras,
&c. which are Indian aromatics, are heavier than water, and always
sink in it by their specific gravity. These differences therefore
require different methods of distillation. It is proper, for example,
to make use of a low alembic in distilling such Essential Oils as are
heavier than water; and, moreover, to facilitate their separation, by
applying a degree of heat somewhat stronger than that of boiling water.
This is easily done by impregnating the water with a proper quantity
of Sea-salt, or the Vitriolic Acid; for, the more saline matters are
contained in water, the more will the degree of heat it acquires, by
being brought to boil, exceed that of pure boiling water.
Fifthly, Essential Oils differ from one another in point of fluidity.
Some are as thin and as fluid as Spirit of Wine: of this number is the
Essential Oil of Turpentine. Others, again, are thick, and even congeal
as they cool: such, for instance, is the Oil of Roses. In distilling
Oils of this latter sort, care must be taken that the spout of the
alembic head do not grow too cold, but be kept always in such a degree
of warmth as may prevent the Oil from fixing in it, and stopping it up;
which would interrupt the distillation, and might also occasion some
other more considerable inconveniencies, of which we shall take notice
presently.
From what hath been said it appears, that the distillation of Essential
Oils cannot be regulated by any one general rule; but that the manner
of operating must be a little varied, according to the nature of the
Oil to be distilled, and to that of the vegetable from which it is to
be drawn.
The time of day fittest to gather plants for this distillation is the
morning before sun-rise; because the coolness of the night hath shut
all their pores, and concentrated their odour: whereas in the evening,
after the plants have been exposed all day to the heat of the sun,
their odorous principle is in a great measure dissipated, and they
are left almost quite exhausted of it. Now, the more of the odorous
principle the plants contain, the more Essential Oil will they yield,
and the more virtue will that Oil have.
Plants fresh gathered, and as yet full of moisture, do not yield so
much Oil in distillation as they do when dried; because the oily
particles in a very moist plant are more diffused, and even separated
from each other, by the interposition of the aqueous parts: whence
it comes to pass that, in distillation, they ascend in a state of
separation from each other; so that being dispersed through the water
they give it a milky colour, like that of an emulsion; and cannot unite
together but in small quantities, which hinders their being easily
separated from the water.
This inconvenience doth not happen, or at least is considerably less,
when the greatest part of the humidity of the plant is evaporated by
desiccation: for the oily particles, being thus delivered from the
intervening aqueous parts, which kept them separated from each other,
are brought nearer together, unite, and form little visible globules of
Oil, which easily emerge from the water employed in the distillation.
But, in drying plants from which the Essential Oil is to be extracted,
great care must be taken that they be neither exposed to the sun, nor
laid in a warm place; because the heat would carry off part of their
odour, and even, from some plants, a pretty considerable quantity of
their Essential Oil.
Plants of a loose texture, that easily give out their Essential Oils,
need not be comminuted, or macerated in water with Salt. But this
method must unavoidably be taken with such as are hard, and do not
readily part with their Oil. Woods, barks, roots, for instance, must be
first rasped, then set to macerate in water impregnated with Salt, as
before directed; and this sometimes for several weeks before they be
distilled.
On this occasion Salt procures three different advantages. In the
first place, it prevents the matters, that must stand in maceration
for some time, from running into fermentation: an inconvenience that
would considerably diminish the quantity of Essential Oil, or perhaps
rob us of the whole, by converting it into an Ardent Spirit, if the
fermentation were spirituous; or into a Volatile Alkali, if it went
on to the last stage, and as far as putrefaction. In the next place,
it acuates the water, and renders it more capable of penetrating and
properly dividing, during the maceration, the texture of the plant
which requires to be thus prepared. Lastly, it adds a little to the
heat of the boiling water, and so promotes the ascent of the heaviest
Oils.
Nevertheless, when you find it necessary, for the reasons assigned
above, to mix Salt with the water to be employed in distilling your
Essential Oil, you must be cautious of putting in too much. You will
indeed obtain, by means thereof, much more Oil than if you distilled
it without Salt: but, as a great quantity of Salt will make the water
acquire a much greater degree of heat than that of pure boiling water,
a good deal of the heavy Oil of the vegetable will be raised by such a
heat, mix with the Essential Oil, deprave it, and make it like those
that are adulterated with a mixture of some heterogeneous Oil, as will
be afterwards shewn.
When every thing is prepared for distillation, it is proper, as
directed in the process, to apply at once a flaming fire, brisk enough
to make the liquor boil immediately: for, if the water be kept long
heating before it be made to boil, the Essential Oil, which cannot rise
without the heat of boiling water, will, by a less degree of heat, be
only agitated, dashed about every way, and churned as it were; by which
means it will be divided into very minute particles, and dispersed in
the water, which will thence acquire a milky colour: and consequently
we shall fall into the inconvenience that was pointed out above, as
happening when we distil plants without having dried them, and while
they are loaded with all the moisture and sap that was in them when
fresh gathered.
When the water in the cucurbit boils, it will be known by the noise
that boiling water usually makes, which is produced by the numerous
bubbles that rise and burst on its surface. The spout of the alembic
is then so hot, that a man cannot lay his finger on it, without such a
sensation of burning heat as is not to be endured. With this degree of
heat the water distils in drops, which succeed each other so fast, that
they seem to form a continued small stream; and this water is replete
with much Essential Oil.
And now it is proper to weaken the fire considerably, so as to leave it
but just strong enough to keep the liquor gently boiling: for if the
distillation be urged too precipitately, the aqueous and oily vapours,
being forcibly hurried up by too great a heat, may carry along with
them some parts of the plant, which may stick in the spout, stop it
up, and endanger the bursting of the vessel, or at least the forcing
off its head, by the exceedingly rarefied particles of water, oil, and
air, all striving to escape at the same time; and these burning hot
vapours, being discharged with impetuosity, may not only scald the
operator, but injure his lungs.
In such distillations it is of consequence to keep constantly cooling
the head of the alembic, by frequency renewing the water in the
refrigeratory, in order to facilitate the condensation of the oily
particles. The water in the cooler ought to be renewed when it begins
to smoke very perceptibly.
Whatever care be taken to save as much of the Oil as possible, and
to prevent its being left dispersed in the water, yet some loss of
this kind cannot be totally avoided: and thus the water that rises
in distilling the Oil is always more or less milky, and strongly
scented, even after it is separated from the Essential Oil. Yet this
portion of the Oil and of the odorous principle, which is retained by
the water employed in such distillation, is not therefore lost: the
water impregnated with these principles partakes of the properties
of the plant from which the Essential Oil was drawn, and may be used
medicinally: it is known in Pharmacy by the title of the _Distilled
Water_ of the plant.
The same water may be used again, with advantage, in distilling the
Essential Oil of a fresh plant of the same sort; because the oily and
odorous particles, with which it is impregnated, joining with those
afforded by the fresh plant, form larger _moleculæ_, capable of uniting
more easily, and emerging better from the water; and consequently
they increase the quantity of Oil. Thus the same water may be always
employed in new distillations; and, the oftener it is used, with the
greater advantage may it be used again.
After all the Essential Oil is risen, if the distillation be continued,
and the receiver changed, the liquor that will then come off will not
be milky, but limpid. It will have no odour at all of the plant, but
a kind of sourish smell; and indeed it is a part of the Acid of the
vegetable in the still, which is elevated by the heat of boiling water,
after all the Essential Oil is come over.
If you intend to keep the distilled water which hath served as a
vehicle to the Essential Oil, and design it for medicinal use, great
care must be taken to stop the distillation before this acid phlegm
begin to rise: for, if it should mix with the distilled water, it would
spoil it, and hinder it from keeping; probably because it contains some
mucilaginous parts, which are apt to putrify.
PROCESS IV.
_To extract the Essential Oils of Plants by Distillation_ per Descensum.
Reduce to a powder, or a paste, the vegetable substances from which you
intend to extract the Essential Oil by the method proposed. Lay this
matter about half an inch thick on a fine, close, linen cloth. If it be
dry and hard, expose the cloth containing it to the steam of boiling
water, till the matter become moist and soft. Then lay the cloth, with
its contents, over the mouth of a very tall cylindrical glass vessel,
which is to do the office of a receiver in this distillation; and, by
means of a piece of small pack-thread, fasten down the extremities of
the cloth, by winding the thread several times over them and round the
vessel; in such a manner, however, that the cloth be not tight, but
may yield to a small weight, and sink about five or six lines deep
into the vessel over which it is fastened. Set this recipient in a
larger vessel, containing so much cold water as will reach half way up
the cylindrical vessel; which, having little in it but air, must be
ballasted with as much lead as will sink it to the bottom of the water.
On the cloth containing the substance to be distilled set a flat pan
of iron or copper, about five or six lines deep, that may just fit
the mouth of the glass vessel over which the cloth is fastened, so as
to shut it quite close. Fill this pan with hot ashes, and on these
lay some live coals. Soon after this, you will see vapours descend
from the cloth, which will fill the recipient, and drops of liquor
will be formed on the under side of the cloth, from whence they will
fall into the vessel. Keep up an equal gentle heat till you perceive
nothing more discharged. Then uncover the recipient: you will find in
it two distinct liquors; one of which is the phlegm, and the other the
Essential Oil of the substance distilled.
_OBSERVATIONS._
The apparatus for distilling above described is very convenient, when
we have not the vessels necessary for distilling with water, or when
we want to obtain the Essential Oil of any vegetable substance in much
less time. The aqueous and oily parts of the substances distilled in
this manner, being rarefied by the heat of the fire placed over them,
cannot ascend upwards, because they are close confined on that side;
and, moreover, the fire which rarefies them possessing all the upper
part of the vessel in which they are contained, they are forced to fly
from it to the place which most favours their condensation: and this
determines them to descend in the recipient, where they meet with a
coolness that condenses and fixes them. It was with a view to promote
this condensation, that we ordered the lower part of the recipient to
be sunk in cold water.
Cloves are one of those substances whose Essential Oil is best obtained
by this method. In the same way also may be drawn the Essential Oil
of Lemon-peel, Citron-peel, Orange-peel, Nutmegs, and several other
vegetable substances: but you must be cautious of applying too strong
a heat; for in that case the Oil, instead of being white and limpid,
acquires a red, dark-brown, blackish colour, is burnt, and smells of
empyreuma: and, on the other hand, if you do not apply a proper degree
of heat, you will scarce get any Oil at all. It is the surest, and
therefore the best, way to distil these Oils with water in an alembic.
And indeed the distillation _per descensum_ is seldom used, but out of
curiosity to try its effect, or on such pressing occasions as allow no
choice.
PROCESS V.
_Infusions, Decoctions, and Extracts of Plants._
Make some water boiling-hot, and then take it off the fire. When it
ceases to boil, pour it on the plant of which you desire to have
the Infusion; taking care there be enough of it to cover the plant
entirely. Cover the vessel, and let your plant lie in the hot water for
the space of half an hour, or longer, if it be of a firm close texture.
Then pour off the water by inclination: it will have partly acquired
the colour, the smell, the taste, and the virtues of the plant. This
liquor is called an _Infusion_.
To make the Decoction of a vegetable substance, put it into an
earthen pan, or into a tinned copper vessel, with a quantity of water
sufficient to bear being boiled for several hours, without leaving
any part of the plant dry. Boil your plant more or less according to
its nature; and then pour off the water by inclination. This water is
impregnated with several of the principles of the plant, of which we
shall take notice in the following observations.
_OBSERVATIONS._
Water, especially when boiling hot, is capable of dissolving not only
all that is purely saline in vegetables, but also a pretty considerable
quantity of their Oil and of their earth, which, by contracting an
union with the saline parts, have formed saponaceous, gummy, and
mucilaginous compounds, that are soluble in water. After violent and
long-continued boiling, therefore, there remains nothing in the plant
but the purest oily part, and such as is the most fixed, that is, the
most closely united with the earth of the plant. I say, the most fixed:
for some part of the oily matters, though not soluble in water, may be
separated by the action of boiling water, when those matters abound
greatly in the vegetable decocted; as we have seen happen to the Fat
Oils of certain vegetable matters; but in that case these oily matters
float upon the Decoction, and do not constitute a part of it.
From what we have already said, touching the analysis of plants, it
seems evident, that, if those decocted be odoriferous and contain an
Essential Oil, the Decoction will contain none, or at most but very
little, of their Essential Oil, or their odorous principle; seeing
we know that these substances cannot bear the heat of boiling water,
without being carried off and entirely dissipated by it. Therefore,
when we make a decoction of an aromatic plant, containing an Essential
Oil, we may be assured that it will not possess the virtues, either
of the odorous part, or of the Essential Oil, and that it will have
none but those of the other more fixed principles of the plant, with
which it may be impregnated. The Decoction of such a plant perfectly
resembles the water left in the cucurbit, after distilling its
Essential Oil. But for those plants in which there are no such volatile
parts, or whose virtue doth not reside in those principles, such as
astringent and emollient plants, for example, that owe their properties
wholly to an earthy Salt, or to a mucilage, they are capable of
communicating their whole virtue to the water in which they are infused
or decocted.
If, on one hand, the Salts of plants render some portion of the
principles of those plants soluble in water, such as part of their
Oil and their earth, which if they were pure would not dissolve
therein; on the other hand, these principles, being of their own
nature indissoluble in water, hinder the Salts, by the union they have
contracted together, from dissolving in it so easily, so soon, and in
such quantities, as if they were pure. This is so true, that water,
though boiled long and violently, is far from extracting out of plants
all those parts that it is capable of dissolving. If, after boiling a
plant in water, as directed in the process, this water be poured off,
fresh water added, and a second decoction made in the same manner as
the first, the water of this latter decoction will, by that means, be
almost as strongly impregnated with the principles of the plant as
the former was. Mr. Boerhaave was obliged to make twenty successive
decoctions of the same plant, to wit, Rosemary, before the water came
off the plant colourless and insipid; in a word, just as it was before
the plant was boiled in it.
Mr. Boerhaave observes, that a plant, after having thus given out all
that water can dissolve, still retains exactly the same form that it
had before it underwent any of the many boilings necessary to exhaust
it; that its colour, from being green at first, becomes brown; and
that the plant, which when green is lighter than water, or at least
doth not sink in it, is heavier after this operation, and falls to the
bottom. This is a proof that the water hath extracted out of the plant
its lightest substances, assuming their places itself, and that it hath
left nothing but its heaviest principles, namely, its fixed oil and its
earth. We shall afterwards examine more particularly these remains of
plants exhausted by water.
If the Infusions and Decoctions of plants be filtered, and evaporated
in a gentle heat, they become Extracts, that may be kept for whole
years, especially if they be evaporated to a thick consistence; and
better still if they be evaporated to dryness.
From what hath been said concerning the Infusions, Decoctions, and
Extracts of plants, it follows, 1. That Infusions and Decoctions of
aromatic plants do not furnish a complete Extract of those plants;
because they do not contain the volatile and odorous parts, in which
the principal virtue of such plants usually resides. If therefore you
desire to make Extracts of such vegetables, that shall have no defect,
you must employ their juices drawn by expression, or water impregnated
with their principles by the means of trituration, and evaporate
the liquor by spreading it over a great number of plates, in order
to enlarge its surface, and quicken the evaporation, which must be
effected by the heat of the sun alone, or the well-tempered warmth of a
stove.
2. It may also be inferred, that water alone, aided by the degree
of heat it is capable of acquiring by being made to boil, is not
sufficient to effect the complete analysis of a plant; since not
only some of its principles are still left combined in it, though
exhausted as much as it can be by boiling water; but also several of
the substances extracted from it by water are compounds of some of
the principles of the plant, and susceptible of a much more accurate
analysis; as we shall be convinced when we come to examine the effects
which a degree of heat superior to that of boiling water is able to
produce on entire plants, on their Extracts, and on their remains
exhausted as much as they can be by boiling water.
But before we enter on that part of the analysis, it is proper to
consider the experiments and combinations that may be made with the
principles we have already obtained; in order to discover their nature,
and in some measure analyze even them. Essential Oils in particular
deserve to be thus examined.
We also obtain from certain plants, with a degree of heat less than
that of boiling water, a Volatile Alkali, which exists formally in
them: but as these plants, when analyzed, yield principles different
from these we obtain out of all other vegetable substances, and as they
resemble animal matters, we shall refer their analysis to a distinct
chapter.
CHAP. V.
_Of_ OPERATIONS _on_ ESSENTIAL OILS.
PROCESS I.
_The Rectification of Essential Oils._
Put into a cucurbit the Essential Oil you propose to rectify. Set the
cucurbit in a _balneum mariæ_; fit to it a head of tin, or of copper
tinned, together with its refrigeratory; and lute on a receiver.
Make the water in the bath boil, and keep up this degree of heat
till nothing more will come over. When the distillation is finished,
you will find in the receiver a rectified Essential Oil, which will
be clearer, thinner, and better scented, than before it was thus
re-distilled; and in the bottom of the cucurbit will be left a matter
of a deeper colour, more tenacious, more resinous, and of a less
grateful smell.
_OBSERVATIONS._
Essential Oils, even the purest, the best prepared, and the thinnest,
suffer great changes, and are much impaired by growing old: they
gradually turn thick and resinous; their sweet grateful scent is lost,
and succeeded by a more disagreeable smell, somewhat like that of
Turpentine. The cause of these changes is, that their finest and most
volatile part, that which contains most of the odorous principle, is
dissipated and separated from that which contains least of it; which
therefore grows thicker, and comes so much the nearer to the nature
of a resin, as the quantity of Acid, that was distributed through the
whole Oil before the dissipation of the more volatile part is, after
such dissipation, united and concentrated in the heaviest part; the
Acid in Oils being much less volatile than the odorous part, to which
alone they owe their levity.
Hence it appears what precautions are to be used for preserving
Essential Oils, as long as possible, without spoiling. They must be
kept in a bottle perfectly well stopped, and always in a cool place,
because heat quickly dissipates the volatile parts. Some authors direct
the bottle to be kept under water.
If these Oils should grow thick and resinous by age, yet they are
not to be thrown away. We shall shew, in the analysis of Balsams and
Resins, that, from these thick and even solid substances, Essential
Oils may be drawn, as thin and as limpid as from plants. Essential
Oils, thickened by time, may therefore be treated like Balsams, and
actually analyzed, by separating all the subtile odorous matter they
contain from their thick acid parts. For this purpose they need only
be distilled with a degree of heat just sufficient to elevate the thin
odorous parts, without raising the thick matter.
The residue left at the bottom of the vessel, because it could not
rise in distillation, is much thicker and less odorous than the Oil
was before rectification. The reason of this is evident, and follows
from what hath just been said. This remainder dissolves in Spirit of
Wine more readily, and in greater quantity, than the light Oil drawn
from it; because it contains more Acid, and because Oils owe their
solubility in this menstruum to their Acid part, as is proved in our
Memoir on Oils already quoted.
When we come to treat of Resins, we shall inquire more particularly
what this remainder is, and what principles it yields when analyzed: in
this place it is sufficient to take notice, that though all the Oil of
which it made a part came over at first with the heat of boiling water,
yet it cannot now be raised by the same degree of heat in distillation;
because it is not now combined with the principle of odour which gives
the Oil its volatility, and because it is rendered sluggish by being
clogged with too great a proportion of Acid.
From what hath been already said, it must be concluded, that Essential
Oils suffer great diminution by being rectified; and that in proportion
to the quantity of resinous matter left behind. All this resinous
matter, while combined with a proper quantity of the odorous principle
of the plant, (that is, at the time of its being distilled, and a
little while after), was really an Essential Oil: the change of its
nature, therefore, is entirely owing to its having left that principle.
An Essential Oil, though rectified, is still as apt to change and be
spoiled as before, because it still continues to lose its odorous
principle by degrees. After some time, therefore, it requires a second
rectification, which again lessens its quantity. In short, it is plain
that Oils will, in a number of years, greater or smaller according to
their nature, and the manner in which they are kept, be wholly changed,
and metamorphosed into a resinous matter, from which no thin Oil can
be drawn with the heat of boiling water: and this is a proof of the
fugacity of that odorous principle, or _Spiritus Rector_, of plants,
which, when united with their lightest Oil, gives it the character of
an Essential Oil.
This resinous matter, to which Essential Oils are finally reduced,
being subjected to repeated distillations, with a degree of heat
superior to that of boiling water, is still capable of yielding a
certain portion of a thin, limpid, sweet-scented Oil, which is as light
as an Essential Oil; as we observed before is the case with Fat Oils
drawn by expression: but the thin Oil obtained by this means, though it
possesses almost all the properties of an Essential Oil, is not for all
that a genuine one; seeing it hath not the same odour with the plant
from which it was originally drawn.
Essential Oils must be rectified in the _balneum mariæ_, as ordered in
the process: for, as some of the Oil touches the sides of the vessel in
the operation, if that vessel be made hotter than boiling water, the
thick matter will rise with the thin Oil, which therefore will not be
rectified.
Rectification is of use not only for procuring to Essential Oils the
tenuity and levity they may have lost by age, but also to separate
them from other oily matters with which they may be adulterated. If,
for instance, an Essential Oil be not properly distilled; if, by
the addition of too much Salt, the water have acquired a degree of
heat greater than that of pure boiling water, and if, in consequence
thereof, some of the heavy Oil of the plant have risen with the
Essential Oil, and mixed therewith, the Essential Oil may, by
rectification, be separated from this heterogeneous Oil; which, being
heavier and incapable of rising with the heat of pure boiling water,
will remain at the bottom of the vessel.
The effect will be the same, if your Essential Oil be falsified with a
mixture of any Fat Oil, as is often the case: for, some of them being
extremely dear, the vender frequently adds a portion of Fat Oil to
increase the quantity. For this purpose Oil of Ben is generally used.
When an Essential Oil is thus falsified with a mixture of any Fat Oil,
it may be discovered by letting a few drops of it fall into rectified
Spirit of Wine; which will dissolve the Essential Oil only, leaving the
Fat Oil quite untouched.
Essential Oils are sometimes falsified by mixing them with a certain
quantity of Spirit of Wine. This fraud doth not render their smell
less fragrant: on the contrary, it becomes rather more agreeable
and quicker. In order to try an Oil suspected of being falsified in
this manner, drop a little of it into very clear water. If a milky
cloud appear in the water, be assured the Oil is mixed with Spirit
of Wine: for as this liquor unites more readily with water than with
Oil, it quits the Oil with which it was mixed to incorporate with the
water: mean time a good deal of the Oil that was dissolved by the
Spirit of Wine, and is now separated from it by the intervention of
water, necessarily remains dispersed through this water in very small
particles; and these form the milky cloud produced on this occasion.
An Essential Oil may also be adulterated with another Essential Oil
that is much more common, and of much less value. Those who practise
this fraud generally employ Oil of Turpentine for that purpose, on
account of its cheapness and tenuity. The cheat is easily discovered,
by moistening a linen rag with the Oil supposed to be thus falsified,
and then holding the rag a little before the fire, which presently
dissipates the odorous part of the falsified Oil. This odour, which
prevented our distinguishing that of the Oil of Turpentine, being
vanished, the peculiar smell of the Turpentine, which is much more
permanent, remains alone; and is so perceptible that it cannot easily
be mistaken.
Those who are much accustomed to see and examine Essential Oils,
have seldom occasion to make the experiments here proposed for
discovering their qualities. A certain degree of thickness, partaking
of unctuosity, in an Essential Oil, convinces them that it is falsified
with a Fat Oil: on the other hand, a greater degree of tenuity,
together with a quicker smell, than a pure Essential Oil ought to have,
discovers the admixture of Spirit of Wine. Lastly, any one, whose sense
of smelling is not very dull, will easily discover the odour of the Oil
of Turpentine, though disguised by that of the Essential Oil with which
it is mixed.
PROCESS II.
_To fire Oils by combining them with highly concentrated Acids:
instanced in Oil of Turpentine._
Mix together, in a glass, equal parts of concentrated Oil of Vitriol,
and highly smoking fresh-drawn Spirit of Nitre: pour this mixture at
several times, but suddenly, on three parts of Oil of Turpentine, set
for that purpose in a glass bason. By a part here must be understood a
dram at least. A most violent commotion, accompanied with smoke, will
immediately be raised in the liquors, and the whole will take fire in
an instant, flame, and be consumed.
_OBSERVATIONS._
There is not in Chymistry a phenomenon more extraordinary, and more
surprising, than the firing of Oils by mixing them with Acids. It
could never have been suspected that a mixture of two cold liquors
would produce a sudden, violent, bright, and lasting flame, like that
we are at present considering. Beccher gave notice, in his _Physica
subterranea_, that highly rectified Spirit of Wine would be set on fire
by mixing it with highly concentrated Oil of Vitriol.
Afterwards Borrichius, a Danish Chymist, published a process for
kindling Oil of Turpentine, by mixing it with the Nitrous Acid, as
we find in the Philosophical Transactions of Copenhagen for the year
1671. Most Chymists have since tried to repeat those experiments, and
particularly to fire the Oil of Turpentine by mixing it with Oil of
Vitriol, or Spirit of Nitre; but to no purpose, when they made use of
the Oil of Vitriol, till Mr. Homberg told us, in the Memoirs of the
Academy of Sciences for 1701, that he had fired Oil of Turpentine by
mixing it with Oil of Vitriol.
To make the experiment succeed he requires, "That the Oil of Vitriol
be dephlegmated as much as possible, and that the Oil of Turpentine be
the last that comes over in distillation, which is thick like a syrop,
and of a dark-brown colour; for that which is white, and rises at the
beginning of the distillation, never takes fire." These are his own
words: but no body else hath ever succeeded in making the experiment.
Tournefort had succeeded, a little before Homberg, in firing, not
Oil of Turpentine indeed, in which he always failed, but the Oil of
Sassafras, by mixing it with an equal quantity of well dephlegmated
Spirit of Nitre. Homberg came afterwards, as appears by the Memoirs
of the Academy for the year 1702, to fire with Spirit of Nitre the
Essential Oils of the aromatic plants of India; and in 1706 Mr.
Rouviere fired, with Spirit of Nitre, the empyreumatic Oil of Guaiacum.
While this Oil of Guaiacum is burning, a porous spongy body rises from
the midst of the flame, to the height of about two feet above the
vessel.
Lastly, several years after all these discoveries, Messrs. Geoffroy and
Hoffman, the one at Paris, and the other at Hall in Saxony, found a way
to fire the Æthereal Oil of Turpentine, each by a different process;
yet agreeing in this, that they both combined the Vitriolic Acid with
the Nitrous, and with this compound Acid fired that Æthereal Essential
Oil, which is one of the thinnest, and, probably for that very reason,
the most unfit to produce a flame with Acids.
The most celebrated Chymists, as appears from this short account, have
employed themselves in firing Essential Oils; but no body attempted the
experiment on Fat Oils. It was not so much as suspected that they were
capable of taking fire after this manner, till in 1745 I read before
the Academy a Memoir on Oils, which I have already mentioned, and in
which I express myself thus:
"I put two ounces and a half of Walnut Oil into the bottom part of a
broken retort, having the figure of a cap, or concave hemisphere; and
poured thereon two ounces of smoking Spirit of Nitre. It was scarce
put in when a considerable ebullition arose, with a very thick smoke.
As I found it continually increasing, and very fast too, I retired a
little, that I might observe the event without danger. This caution was
not unnecessary: for immediately the whole mixture blew up as high as
the ceiling, with a noise like the discharge of a musket. Nothing was
left in the vessel but a black matter, which still continued to boil
a little and run over, and at last remained very rare, spungy, and as
full of holes as a honeycomb: its consistence also was such that it did
not stick to my fingers when I handled it.
"As Mr. Geoffroy, who first found the means of firing the natural
Balsams, observed in them a similar explosion on that occasion, it
appears that my Oil was very near taking fire in this experiment:
which makes me presume that we may at last succeed in firing Fat Oils
likewise, and consequently all others; seeing these have always been
looked upon as the most unlikely to produce that phenomenon. I imagine
that, to accomplish this, nothing more is necessary than to make use of
sufficiently great quantities, and to order it so that the surfaces of
the liquors, where they come into contact, may be of a large extent."
Afterwards, in 1747, Mr. Rouelle read before the Academy a Memoir on
the accension of Oils by Acids. That Memoir contains a great number
of curious experiments, and peculiar manual operations described very
distinctly, from which there results a general method of firing without
fail, not only Essential Oils, but even any Fat Oil whatever: so that
my conjecture, concerning the possibility of firing these latter Oils,
mentioned in my above-cited Memoir of 1745, is now changed into a
certainty. I shall proceed to explain how I conceive these accensions
are brought about, and endeavour to account for the phenomenon from
such causes as to me seem the most probable.
A due attention to the phenomena produced by mixing Oils with Acids
will enable us, I imagine, to discover the natural cause why the
Oils take fire. It is certain, and demonstrated by the most decisive
experiments, that the friction of several bodies rubbing against each
other produces heat; and that when these bodies are combustible, and
the heat produced by their friction rises to a certain degree, they
take fire. This, in my opinion, is what happens to Oils when mixed
with concentrated Acids. When these two sorts of substances rush into
union with rapidity, as in the experiments under consideration, there
must necessarily be a great friction among their parts. This friction
produces the heat observed at the time of their union. The more
concentrated the Acids are, with the greater violence and rapidity do
they act upon the Oils, and the greater is the heat raised. If the
Acids be concentrated to such a degree as to produce, by uniting with
the Oils, a heat equal to that of an ignited body, the combustible
substances that are exposed to it, which in this case are Oils, must
needs take fire and flame.
The heat produced on this occasion is so great, that, even when the
inflammation doth not take place, if you touch the surface of the Oil
with your finger, as soon as the Acid hath had its effect, you will
find it burn you like a live coal.
Two pieces of wood, rapidly and violently rubbed against each other,
take fire. What is it that is kindled in this case? It can be nothing
but their Oil: for they contain no other combustible principle. Why
doth this Oil take fire? I do not think it possible to assign any
reason for it, but the heat produced by the friction of the pieces of
wood containing the Oil. If, when Oil is dispersed in a body, of which
it is only one component principle, and consequently mixed with many
saline, aqueous, and earthy parts, that are not inflammable, but, on
the contrary, make the Oil less so, the Oil nevertheless takes fire,
and burns when agitated by a sufficient degree of heat; why shall not
this very Oil, when separated from the mixt of which it made a part,
when united into one distinct mass, and entirely, or almost entirely,
freed from the heterogeneous, incombustible parts with which it was
combined, and consequently now more inflammable than before; why, I
say, shall it not take fire, when exposed to a degree of heat equal,
or rather superior, to that which is produced by rubbing two pieces of
wood together?
Let us now examine the phenomena produced when Oils are fired by Acids,
all the circumstances that favour or hinder their accension, and see if
they agree with the explanation here offered.
First, no sort of Oil will take fire with any Acid whatever that
is not highly concentrated; for weak Acids act but feebly on Oils,
and dissolve them slowly; so that the friction is neither quick nor
violent, and consequently produces too faint a heat, far below the
degree of ignition.
Secondly, no inflammation is produced when Acids and Oils are mixed
in too small quantities; but the more Acid and Oil you mix together,
the greater is the certainty of succeeding: for the heat is exactly in
proportion to the friction that produces it; and the total quantity,
or amount, of this friction is so much the greater, as there are more
particles rubbing against each other at the same time. So that if a
very small quantity of Acid and Oil be mixed together, there will be
but a very small quantity of friction, and consequently a very small
quantity of heat; and in that case no inflammation. It was with a view
to avoid these inconveniencies, and to procure the opposite advantages
in as great a degree as possible, that, in the passage above quoted
from my Memoir of Oils, I proposed mixing together large doses of
Acid and of Oil, as one of the means by which we might succeed in the
accension of Fat Oils.
Thirdly, the figure of the vessel, in which the two liquors are mixed
together, is not a matter of indifference. A wide-spreading vessel,
of a large diameter with respect to the quantity of liquor it is
to contain, favours the inflammation much more than one of a small
diameter. Nay, it may not succeed at all in too narrow a vessel, though
all other circumstances be properly attended to.
The reason of this is, that the activity of heat produced by friction
is not in proportion to the successive, but to the simultaneous
frictions: for the heat actually produced by the frictions of an
hundred particles, rubbing successively against each other, with
intervals sufficient to let the heat go off, almost as fast as it is
generated, would be equal to the friction of a single particle only;
whereas the heat actually produced by the friction of the same number
of particles, all rubbing against each other at the same instant,
would be equal to the frictions of all the particles taken together,
and consequently an hundred times more active than the other[11]. This
being laid down, it is easy to conceive how a large vessel favours the
accension more than a small one. It is certain that two liquors which
mutually present large surfaces to each other, at the instant of their
being mixed together, touch each other at one and the same time in a
much greater number of points, than if each had but a small surface;
and consequently that they must unite much sooner, and with greater
rapidity, in the former case than in the latter.
[11] I believe this proposition is not strictly true: for it appears
to me, that, in order to make the heat, produced by the simultaneous
frictions of an hundred particles, an hundred times more active than
that produced by the successive frictions of the same number of
particles, it is necessary that the simultaneous frictions should act
all together in one point or center; which is impossible. But, as the
particles that rub against each other, in the present case, are very
near and contiguous, it is still true that the heat, resulting from
their simultaneous frictions, is much more active than that produced
by successive frictions only: which is sufficient for our present
purpose.
With these views, and in order to give the liquors this advantageous
disposition, I recommended it as what would greatly promote the
inflammation of Fat Oils, to order the liquors so, that, at the moment
of their mixture, a large surface of each might come into contact with
the other.
Fourthly, if we reflect on the experiments hitherto made for kindling
Oils by Acids, we shall easily be convinced that all Oils are not
equally apt to be fired; and that light, æthereal, very thin, Essential
Oils do not produce this phenomenon so readily and so surely, as those
of the same kind that are heavy and thick, or at least soon grow thick
upon being mixed with Acids.
Mr. Homberg says positively in the above-cited passage of his Memoir,
that he never could succeed in setting fire with the Acid of Vitriol to
the white, æthereal Oil of Turpentine; that is, to the lightest which
comes over first in distillation; but that the very same Acid set fire
to "that which comes over last in distillation, which is thick like a
syrop, and of a dark-brown colour."
All the experiments by which Oils have been fired, from those of
Beccher and Borrichius down to those of Geoffroy and Hoffman, were
made on the Essential Oils of the aromatic plants of India, which are
the heaviest we know, and on the empyreumatic Oil of Guaiacum, which,
besides being very ponderous, is also very thick.
Now these singular effects likewise agree perfectly well with our
explanation. It is certain that the parts of a heavy fluid do not yield
to any impulse or shock, so easily as those of a lighter fluid; just
as the parts of a thick, viscous fluid undoubtedly resist any attempt
to separate them, so much the more the nearer the consistence of that
fluid is to solidity, or the further it is removed from the state of
fluidity. Now, the more resistance the Acid meets with in separating
and dividing the parts of the Oil, as it must do to dissolve them,
the more considerable will be the force and motion with which it must
necessarily act to surmount those obstacles; besides, as experience
teaches us that the density and viscidity of the Oils do not, at least
to sense, diminish the quickness and activity which the Acid exerts
in uniting with them; the greater therefore must be the collisions,
frictions, and heat produced: and this plainly shews why heavy, thick
Oils take fire, in this case, more readily than those which are fluid
and light.
It may here be objected, that Fat Oils, which are thicker and heavier
than the light Essential Oils, take fire nevertheless with greater
difficulty. This objection is easily answered, by observing, that when
we say Acids fire heavy thick Oils with more ease than thin light Oils,
this position must be restricted to Oils of the same kind, on which
Acids have an equal, or nearly equal, action; that is, to such Oils
as differ from each other in no other respect but their thickness and
weight.
For example, Mr. Homberg, who could by no means set fire, with Oil of
Vitriol, to the Oil that rises first in the distillation of Turpentine,
found that the same Acid would fire the Oil that comes last over: and
therefore it is reasonable to attribute his success, in firing this
last Oil, to its being thicker and heavier than the former; seeing
these two Oils are in other respects of the same nature; that Acids
have an equal action on both; and that they differ from each other only
in the qualities specified above.
But it is evident, that, if the Oils compared together be of different
kinds, and differ from each other, not only in weight and thickness,
but also by containing different principles, or, at least, the same
principles combined differently, and in different proportions,
the action of any Acid on those Oils must also be different; and
that regard must be had thereto in determining their degrees of
inflammability.
Now all this is applicable to Fat Oils, when compared with light
Essential Oils, in point of inflammability. If all these Oils were of
the same nature, and differed from each other in weight and thickness
only, the objection drawn from Fat Oils, which though thicker than
Essential Oils do not take fire so easily, would be a very good one,
and fact would be against our reasoning. But this is far from being
the case: the properties, as well as the analysis, of Fat Oils shew
their nature to be very different from that of Essential Oils; that
there is more water in their composition; and that they are full of
a mucilaginous or gummy principle, which must greatly obstruct their
inflammability, and the action of Acids upon them.
None of the effects, therefore, that attend the firing of Oils with
Acids, is repugnant to our way of accounting for the phenomenon, which
is one of the most beautiful in all Natural Philosophy. To conclude
this important subject, nothing now remains but to consider the effects
produced by the Vitriolic Acid in these accensions.
This Acid, though of a stronger nature, and capable of being more
highly concentrated than the Nitrous Acid, seems however less qualified
to produce a flame with Oils. Indeed Mr. Homberg fired Oil of
Turpentine by mixing it with Oil of Vitriol: but I do not know that the
experiment hath succeeded with any other Chymist; on the contrary, most
of those who have tried it affirm, that they never could fire any Oil
with that Acid alone.
Oils are probably in the same case as metallic substances, with regard
to these two Acids. We know that the Nitrous Acid dissolves those
substances with vastly more activity and violence than the Vitriolic
Acid exerts upon them; which may depend, either on the disposition and
configuration of their parts, or on the portion of phlogiston which,
according to the opinion of most Chymists, is united with the Nitrous
Acid, is its peculiar characteristic, and the cause of the great
vivacity with which it dissolves almost all matters that contain the
phlogiston.
I say _almost_ all matters that contain the phlogiston; because there
are some substances that contain a great deal thereof, and yet are
not at all acted on by the pure Nitrous Acid. These substances are
matters perfectly charred: that is, such as are capable of enduring the
greatest violence of fire in close vessels, without yielding a single
atom of Oil; that burn almost quite away, yet only grow red hot without
flaming; or at least produce but a very small, slight flame, from which
it is impossible to obtain the least particle of soot or fuliginosity;
in a word, that contain an inflammable matter, but such as is fit to be
an ingredient in the composition of metallic substances, to which the
peculiar title of the Phlogiston is appropriated.
I say, then, that if the Nitrous Acid be poured on a mere coal,
perfectly charred, it is impossible for the Acid, be it ever so highly
concentrated, to set the coal on fire, though heated before to the
greatest degree that it can possibly admit of without kindling; and,
which is still more remarkable, if a live coal be plunged into the most
highly smoking Spirit of Nitre, it will be extinguished as if dipt in
pure water.
But to return to the Vitriolic Acid: it is singular enough that this
Acid, which attacks Oils with less activity, and for that reason seems
less fit to set them on fire, than the Nitrous Acid, yet greatly
promotes their accension, when mixed with that very Acid. This may
be owing to its rendering the Oils with which it mixes heavier and
thicker; or else, as Mr. Rouelle conjectures with great probability,
being more concentrated than the Nitrous Acid, and having a greater
affinity with water, it dephlegmates the other, and thereby increases
its activity; or, lastly, this may arise from some other cause yet
unknown to us, and perhaps from that by which the Acids of Nitre and of
Sea-salt, which, when separate and perfectly pure, can neither of them
dissolve Gold, are enabled, when combined together, to make a perfect
solution of that metal.
PROCESS III.
_To combine Essential Oils with Mineral Sulphur. Balsam of Sulphur.
This Composition decompounded._
Put into a matrass one part of Flowers of Sulphur; pour on them six
parts of the Essential Oil of Turpentine, for instance; set the matrass
in a sand-bath, and heat it gradually till the Oil boil. The Sulphur,
which at first lay at the bottom of the matrass, will begin to melt,
and appear to dissolve in the Oil. When it hath boiled in this manner
for about an hour, take the matrass from the fire, and let the liquor
cool. A great deal of the Sulphur that was dissolved therein will
separate from it as it cools, and fall to the bottom of the vessel in
the form of needles, much like a Salt shooting in water.
When the liquor is perfectly cold, decant it from the Sulphur that
lies at the bottom of the vessel: to that Sulphur put fresh Oil of
Turpentine, and proceed as before: the Sulphur will again disappear,
and be dissolved in the Oil: but when the mixture is cold, you will
find new crystals of Sulphur deposited at the bottom. Decant once more
this Oil from the crystals, and pour on fresh Oil to dissolve them:
continue the same method, and you will find that about sixteen parts of
Essential Oil are required to keep one part of Sulphur dissolved when
cold. This combination is called _Balsamum Sulphuris Terebinthinatum_,
if made with Oil of Turpentine; _Anisatum_, if with Oil of Anise-seeds;
and so of others.
_OBSERVATIONS._
Essential Oils do not dissolve Sulphur, in such quantities, and with
so much ease, as Fat Oils do. It was shewn above, that a Fat Oil is
capable of keeping a considerable quantity of Sulphur in solution;
whereas no less than sixteen parts of Essential Oil are required to
dissolve one part only of Sulphur, as in this process.
The property which Sulphur hath of separating, in part, from the
Essential Oil in which it is dissolved, and falling to the bottom of
the vessel in the form of crystals, as the Oil cools, proves that it is
a kind of Neutral Salt, which, being insoluble in water, because of the
great quantity of inflammable matter that serves it for a basis, is not
to be dissolved but by substances that actually contain themselves a
great deal of inflammable matter; such as Oils and Metallic substances.
Though the latter are almost always solid, it nevertheless unites with
several of them into regular forms, resembling saline crystals in every
thing but pellucidity; as appears, for example, in several Pyrites,
Antimony, and some other sulphureous minerals. But when it is dissolved
in Oils, especially in such as are capable of keeping but a small
quantity thereof in solution, and consequently drop a good deal of it
as they cool, it is precisely in the case of one of those Salts whereof
hot water dissolves more than cold; that is, the Oil, that is saturated
with as much Sulphur as it can possibly take up when boiling hot, lets
some part thereof precipitate as it cools; while the Sulphur thus
separated from the Oil unites into little glebes of a regular figure,
and actually crystallizes; in the same manner as Nitre, when boiling
water hath dissolved as much thereof as it can possibly take up, partly
separates from it when it cools, and falls to the bottom of the vessel
in small crystalline _moleculæ_, of the form peculiar to that Salt.
Mr. Homberg made some very curious experiments on this combination of
Sulphur with an Essential Oil. In the Memoirs of the Academy he gives
the following analysis thereof.
"Put your Sulphur dissolved by Oil of Turpentine into a pretty large
retort, because the matter puffs up towards the end, and distil with
a very gentle heat for twelve or fifteen days and nights. There will
come over about two thirds of the quantity of a colourless Oil of
Turpentine, and at the same time _a pretty considerable quantity_ of
a whitish ponderous water, as acid as good Spirit of Vitriol. After
this, the drops of Oil that come off will begin to be red. Then change
your receiver, and increase the fire gradually; and in seven or eight
hours time, with a very great heat, force off all that will rise, using
a glass retort for your recipient. At last, most of the Oil will come
over into the receiver very thick and high-coloured, still accompanied
with a whitish and very acid water. In the retort will be left a
black _caput mortuum_, spongy, or foliated, shining, and insipid....
This _caput mortuum_ neither grows white, nor flames, nor wastes
considerably in a strong fire.
"The matter that comes over into the receiver must be distilled again,
with a very gentle heat continued for several days and nights, in order
to separate once more the colourless Oil and the remaining acid water,
till the Oil begin to come off red. Then take the retort from the fire,
and on the black gummy matter left in it pour good Spirit of Wine; mix
the whole well together, and distil with a very gentle heat. When this
Spirit of Wine is come off, pour some fresh on the black gum left in
the retort, and distil as before. Repeat this till the Spirit of Wine
cease to have a bad smell."
There is great reason to believe, that, by the union which the Sulphur
contracts with the Oil, the cohesion of the Acid and the Phlogiston,
which constitute that mineral, is considerably weakened; and that
this is what occasions the decomposition of the Sulphur so manifest
in Mr. Homberg's analysis. The inflammable matter of the Sulphur is
so incorporated with that of the Oil in the solution, that they form
together one homogeneous whole; by which means the Acid of the Sulphur,
which is of course dispersed through the whole liquor, is not now
combined with the Phlogiston, as it was in the Sulphur before it was
blended with the Oil; that is, with the pure Phlogiston; but with that
Phlogiston which constitutes the oily mixture, or, which is the same
thing, with actual Oil. And this is the reason that a composition of
Oil and Sulphur yields, in distillation, nearly the same principles
that a combination of the same Oil with the Vitriolic Acid would yield.
We have already seen, under the head of Fat Oils, that when Oils are
combined with Acids, if this combination be again decompounded by
distillation, those two substances cannot be obtained in their original
state; but that they are changed and partly decomposed. The case is
the same in the experiment before us. We first get, by distillation, a
pretty considerable quantity of Oil of Turpentine, that seems to have
suffered no change at all. This first Oil is that which the action of
fire separates from the Acid; and this it effects with so much the
more ease, that, a great quantity thereof having been necessarily
used to dissolve a little Sulphur, it greatly exceeds the quantity of
Acid in the mixture, and that the distillation is ordered to be made
with a very weak degree of heat: for M. Homberg says, it ought to
be continued twelve or fifteen days and nights. Now this manner of
distilling, with a very gentle heat, is the most effectual means of
separating Oils, especially light Essential Oils, from Acids; because
these Oils rise in distillation with very little heat; whereas the
Acids, being much more ponderous, require a great deal more.
The Oil that rises first in distillation, appears indeed to be the same
with that which was originally used in the mixture; but the quantity is
much smaller: first, because some part of it, being combined with the
Acid of the Sulphur, is thereby rendered thick and heavy, which hinders
it from rising in this first distillation with a very gentle heat, and
is the reason that it cannot be elevated without a much stronger degree
of fire. It is this part that afterward comes over in the form of a red
liquor upon increasing the fire.
The second cause why the quantity of Oil is lessened, is, that part
of it is decomposed in the operation. This decomposed part of the Oil
furnishes that considerable quantity of water which ascends at the same
time with the Oil, or a little after it, and serves for a vehicle to
the Acid that rises with it in this first distillation; which Acid,
though pretty strong, is now much more loaded with water than when it
was an ingredient in the combination of Sulphur. This acid water is of
a milky white colour, because many oily particles are suspended and
diffused in it, but not perfectly dissolved.
The _caput mortuum_ that is left in the retort, after all the red thick
Oil is driven up by a very strong degree of fire, is a sort of charred
matter, consisting of some of the earth of the Sulphur, and of the
decomposed Oil, united with a phlogiston, which is probably furnished
by both these substances. This matter contains also a little Acid fixed
with it. This Acid reproduces Sulphur, or at least becomes sulphureous,
and flies off in vapours, when the coal is urged by a violent
forge-heat: for Mr. Homberg observed, that by this means it exhaled an
odour of Sulphur, and lost in weight.
This charred matter is of a singular nature: for, by being exposed to
a forge-heat, and even to the heat in the focus of a burning glass, it
seemed to suffer no other change than some loss of weight, occasioned
by the evaporation of the acid effluvia carried off by the heat; for
it still retained its black colour, and was neither consumed nor
vitrified. In order to melt it, Mr. Homberg was forced to mix it with
Borax. This Salt converted it into a glass of a dark-grey colour: and,
as there appeared a little verdegris on the surface of this glass
after keeping it in a moist place, he thereby found that the Sulphur he
had used contained a little Copper.
We know that the earth of Copper is refractory, and that it
communicates a dark colour to matters vitrified along with it: and
perhaps it was the cause why the fixed matter in question retained its
blackish colour so obstinately, notwithstanding the phlogiston that
must have been in it at first was, in all probability, consumed by the
violent ignitions it underwent.
As to the thick oily matter, called _gummy_ by Mr. Homberg, from which
he directs Spirit of Wine to be repeatedly distilled, till it cease to
have a disagreeable smell, there is great reason for thinking it to be,
as we said before, a portion of the Oil which the Acid hath rendered
thick and heavy. The Spirit of Wine dissolves and carries up the most
acid part, which always hath a disagreeable smell.
Mr. Homberg says, that "the part remaining after this, which he calls
the _Gum of common Sulphur_, hath a pleasant balsamic odour; that it
partly dissolves in Spirit of Wine, a hard resinous matter being left,
which will not dissolve, either in Spirit of Wine, or in the strongest
lixivium." Of consequence, therefore, it is neither a resinous matter
nor a sulphur; "yet it dissolves perfectly in distilled Oils." What
then is this singular body? It is certainly a subject for very
curious inquiries. In general, Mr. Homberg's whole process is full of
interesting facts, and well deserves to be repeated, carried further,
and carefully attended to.
PROCESS IV.
_To combine Essential Oils with Fixed Alkalis. Starkey's Soap._
Take Salt of Tartar, or any other Alkali, thoroughly calcined. Heat it
in a crucible till it be red, and in that condition throw it into a hot
iron mortar: rub it quickly with a very hot iron pestle; and as soon as
it is powdered pour on it, little by little, nearly an equal quantity
of Oil of Turpentine. The Oil will enter into the Salt, and unite
intimately with it, so as to form a hard paste. Continue rubbing this
composition with the pestle, in order to complete the union of the two
substances; and, as your Oil of Turpentine disappears, add more, which
will unite in the same manner, and give a softer consistence to the
soapy mass. You may add still more Oil, according to the consistence
you intend to give your Soap.
_OBSERVATIONS._
Essential Oils do not unite near so easily as Fat Oils with Alkalis.
For this reason, to make a Soap with an Essential Oil, we must take
a method different from that used in common soaperies. For if an
Essential Oil be substituted for the Fat Oil, in the ordinary way of
making Soap, far from combining with the alkaline lixivium, though ever
so strong, it will be wholly dissipated and vanish: so that, after
boiling some time, you will find nothing but the lye, just as when
first put in, only a little more concentrated.
The water, in which the Alkali is dissolved when in the form of a
lye, is the principal thing that hinders the Salt from uniting with
the Essential Oil. Water is such an enemy to this union, that, if the
Alkali be ever so little moist, the operation will not succeed; even
though all the other precautions mentioned in the process should be
exactly observed.
In order, therefore, to free the Alkali from all humidity, it is
necessary to begin with making it red-hot; and then, that this Salt,
which is very greedy of moisture, may not imbibe any from the air,
before it be mixed with the Essential Oil, it must not be suffered to
cool; but the mixture must be made in a hot vessel, as soon as the Salt
is reduced to powder. When every particle of the Salt is once covered
with Oil, you need not fear its attracting any moisture, at least very
quickly, because the Oil opposes its admission.
Starkey, the first Chymist who found the means of making Soap with
an Essential Oil, and by whose name this kind of Soap is therefore
called, made use of a much more tedious method than that proposed in
our process. He began with mixing a very small quantity of Oil with
this Salt, and waited till all the Oil united therewith of its own
accord, so as to disappear entirely, before he added any more; and thus
protracted his operation exceedingly, though in the main it was the
same with ours. The method here proposed is more expeditious, and was
invented by Dr. Geoffroy.
Starkey's Soap dissolves in water much as common Soap does, without
any separation of the Oil: and by this mark it is known to be well
made. It may also be decompounded, either by distillation, or by mixing
it with an Acid: and its decomposition, in either of these ways, is
attended with nearly the same phenomena as the decomposition of common
Soap.
CHAP. VI.
_Of the_ SUBSTANCES _obtained from_ VEGETABLES _by Means of a_
GRADUATED HEAT, _from that of boiling Water, to the strongest that can
be applied to them in close Vessels_.
PROCESS I.
_To analyze Vegetable Substances that yield neither a Fat nor an
Essential Oil. Instanced in Guaiacum-Wood._
Take thin shavings of Guaiacum-wood, and put them into a glass or
stone retort, leaving one half thereof empty. Set your retort in a
reverberating furnace, and lute on a large glass receiver having a
small hole drilled in it; such as is used for distilling the Mineral
Acids. Put a live coal or two in the furnace, to warm the vessels
gently and slowly.
With a degree of heat below that of boiling water, you will see drops
of a clear insipid phlegm fall into the receiver. If you raise the
fire a little, this water will come slightly acid, and begin to have a
pungent smell. With a degree of fire somewhat stronger, a water will
continue to rise which will be still more acid, smell stronger, and
become yellowish. When the heat comes to exceed that of boiling water,
the phlegm that rises will be very acid, high coloured, have a strong
pungent smell, like that of matters long smoked with wood in a chimney,
and will be accompanied with a red, light Oil, that will float on the
liquor in the receiver.
And now it is necessary that the operation be carried on very
cautiously, and vent frequently given to the rarefied air by opening
the small hole in the receiver: such an incredible quantity thereof
rushing out of the Wood, with this degree of heat, as may burst the
vessels to pieces, if not discharged from time to time.
When this red, light Oil is come over, and the air ceases to rush out
with impetuosity, raise your fire gradually, till the retort begin to
redden. The receiver will be filled with dense vapours; and, together
with the watery liquor, which will then be extremely acid, there will
rise a black, thick, ponderous Oil, which will fall to the bottom of
the receiver, and lye under the liquor.
Then give the utmost degree of heat; that is, the greatest your furnace
will allow, and your vessels bear. With this excessive heat a little
more Oil will rise, which will be very ponderous, as thick and black
as pitch; and the vessels will continue full of vapours that will not
condense.
At last, when you have kept the retort exceeding red for a long time in
this extremity of heat, so that it begins to melt, if it be of glass,
and you perceive nothing more come over, let the fire go out and the
vessel cool. Then take off your receiver: from the black oil at bottom
decant the acid liquor with the red Oil floating on it, and pour them
both into a glass funnel, lined with brown filtering paper, and placed
over a bottle. The acid liquor will pass through the filter into the
bottle, and the Oil will be left behind, which must be kept by itself
in a separate bottle. Lastly, into another funnel, prepared as the
former, pour the thick Oil remaining with a little of the acid liquor
at the bottom of the receiver. This liquor will filter off in the same
manner, and thus be separated from the heavy Oil.
In the retort you will find your Guaiacum-shavings, not in the least
altered as to their figure, but light, friable, very black, scentless,
and tasteless, easily taking fire, and consuming without flame or
smoke; in short, you will find them charred to a perfect coal.
_OBSERVATIONS._
Hitherto we have examined the substances that may be obtained from
vegetables, either without the help of fire, or with a degree of heat
not exceeding that of boiling water. The analysis of plants can be
carried no further without a greater degree of heat: for, when the
principle of odour, and the essential oil of an aromatic plant, are
wholly extracted by the preceding processes, if the distillation be
afterward continued without increasing the heat, nothing more will be
obtained but a little Acid; which will soon cease, as a small part only
of the quantity contained in the plant will be elevated; the rest being
either too ponderous, or too much entangled with the other principles
of the body, to rise with so small a degree of heat.
In order, therefore, to carry on the decomposition of a plant, from
which you have, by the methods before proposed, extracted all the
principles it is capable of yielding when so treated; or, which comes
to the same thing, in order to analyze a vegetable matter, which
affords neither an expressed nor an essential oil, it must be distilled
in a retort with a naked fire, as directed in the process, and be made
to undergo all the degrees of heat successively, from that of boiling
water, to the highest that can be raised in a reverberating furnace.
A heat inferior to that of boiling water, with which we must begin in
order to warm the vessel gradually, brings nothing over, as hath been
said, but an insipid water, destitute of all acidity. By increasing it
nearly to the degree of boiling water, the distilled water comes to be
slightly acid.
When the heat is made a little stronger than that which is necessary
for the elevation of an Essential Oil, the acidity of the water that
comes off is much more considerable. It hath now both colour and smell,
and there rises with it a red, light Oil, that floats on the liquor in
the receiver. This is not an Essential Oil; it hath none of the odour
of the plant. Though so light as to float on water, yet it will not
rise with the degree of heat that raises Essential Oils; even those
that much surpass it in gravity, and will not swim on water as this
does. This proves that the ease or difficulty, with which a particular
degree of heat raises any substance in distillation, doth not depend
altogether on its gravity: its dilatability, or the volatile nature of
the matters, with which it is so closely united as not to be separated
from them by distillation, may probably contribute greatly to produce
this effect.
It is very surprising that a substance so hard, so compact, so dry,
in appearance, as Guaiacum-wood, should yield such a large quantity
of water by distillation; and it is equally so, that it should
discharge so much air, and with so much impetuosity, as nothing but
experience could render credible. We have, in the process, directed
the precautions to be taken when this air, from being prodigiously
condensed in the body of which it made a part, is set at large, rushes
out of confinement, and expands with all its natural elasticity. From
this air arises the greatest danger attending the operation.
It hath been remarked, that the heaviest and most compact woods yield
the most air in distillation: and accordingly Guaiacum-wood, which we
have chosen for an instance, as exceeding almost all others in hardness
and weight, discharges a vast quantity of air when analyzed.
The thick, burnt, empyreumatic Oil, that comes over last in this
distillation, is heavier than water; on account, probably, of the
great quantity of Acid with which it is replete. The two kinds of Oil
obtained in this analysis may be rectified, by distilling them a second
time, or rather several times; by which means they will become lighter
and more fluid, as we have seen happen to Fat and Essential Oils. In
general, all thick, heavy Oils constantly owe these qualities to an
Acid united with them; and it is by being freed from some of that Acid
in distillation, that they always acquire a greater degree of lightness
and fluidity from that operation. To these laws all vegetable Oils are
subject, of what nature soever they be.
The analysis of a vegetable substance, exhibited above, shews what
may be obtained from them, when distilled in close vessels, with a
graduated heat, from that of boiling water, to that which converts the
mixt to a perfect coal; viz. Phlegm, an Acid, a light Oil, much Air,
and a thick Oil. But this analysis is far from being a complete one: it
may be carried much farther, and made more perfect.
None of the principles obtained by this analysis are pure, simple, and
thoroughly separated from the rest. They are still in some measure
blended all together: their separation is but begun; and each requires
a second and more accurate analysis, to reduce it to the greatest
degree of purity of which it is capable. The Oil and the Acid chiefly
merit so much pains.
A great deal of the Acid of the plant remains, as was said, combined
with the two sorts of Oil here obtained; which we have reason to
think differ no otherwise from one another, than as there is more
or less Acid united with each. The best way of freeing these Oils
from their redundant Acid is to distil them frequently from Alkalis
and Absorbents. Some of our best Chymists have taken this pains with
several sorts of Oils; but the method might be still extended, and the
operation carried further than hath yet been done.
The Acid is in the same circumstances nearly as the Oil. The first that
rises is mortified with much water, to which it owes a good deal of its
volatility. That which comes over last is much more concentrated, and
consequently heavier; yet it is still very aqueous. It might be freed
in a great measure from this adventitious water, and so rendered much
stronger; which would give us a better opportunity to discover its
nature and properties, of which we know but very little.
Water is not the only heterogeneous substance that disguises the
vegetable Acid: a pretty considerable quantity of the Oil of the plant
is also combined with it, and contaminates its purity. The proof of
this is, that, when these Acids are kept, in the same condition in
which they first come over, for any length of time, in a glass vessel,
they gradually deposite, on the bottom and sides of the vessel, an oily
incrustation, which grows thicker and thicker the longer it stands;
and, as this oily matter separates from it, the Acid liquor appears
less unctuous and saponaceous.
A very good way to separate this Oil more effectually from the Acid
is to combine the whole with absorbents, and abstract the Oil again
by distillation. By this means a very sensible quantity of Oil may
be separated that was not perceived before. On this occasion it is
proper to remark, that the Oil thus united with the vegetable Acid is
perfectly dissolved by it; seeing it is thereby rendered miscible with
water, so that it doth not, like Alkaline soaps, in the least obscure
its limpidity, or give it a milky cast: for these aqueous, oily Acids
are very transparent, especially after they have stood for some time.
The air that is discharged with impetuosity in the operation, and must
be let out, is loaded with many particles of Acid and Oil reduced to
vapours, which it carries off; and by this means the quantity of the
principles extracted from the mixt cannot be accurately determined: nor
are the vapours, of which the vessels remain full after the operation,
any other than particles of Acid and Oil, which the violence of the
fire hath rarefied exceedingly, and which do not easily condense.
If we distil in this manner a vegetable aromatic substance, which of
course contains an Essential Oil, provided it hath not been previously
extracted by the appropriated process, this Essential Oil will rise
first, as soon as the distilling vessel acquires the heat of boiling
water: but its scent will not be near so sweet or grateful, as if it
were distilled in the manner before directed as properest for it. On
the contrary, it will have an empyreumatic smell: because in this way
it is impossible to avoid scorching and half-burning some of the matter
distilled; especially that part of it which touches the sides of the
retort. Moreover, the very same equable degree of heat can hardly be
kept up with a naked fire. The Essential Oil, therefore, though it
rises first, will not be pure, but contaminated with a mixture of
the empyreumatic Oil that first comes over, and will be confounded
therewith.
If a substance abounding with Fat Oil, that hath not been expressed
from it, be distilled according to the present process, it will yield
no Fat Oil by distillation; but only much more of the first clear
Oil, and of the second thick Oil, than if all the Fat Oil it would
have afforded had been first drawn off by expression: for as the Fat
Oil will not rise in distillation, without a degree of heat greater
than that of boiling water, neither can it endure such a degree of
heat without changing its nature, without losing that mildness, and,
in a great measure, that unctuosity which is natural to it. It will
therefore be confounded with the other empyreumatic Oil, which, in
all probability, would itself be no other than a Fat Oil, if it could
be wholly extracted, without the aid of fire, from the vegetable
substances containing it.
Most vegetable substances, when distilled with a strong fire, yield
the same principles with that which we have chosen for an instance.
Entire plants of this kind, those from which the odorous principle, the
Essential Oil, or the Fat Oil, hath been drawn, those of which extracts
have been made by infusion or decoction, or the extracts themselves;
all such matters being distilled yield a Phlegm, an Acid, a thin Oil,
Air, and a thick Oil, and the products of their several analyses
differ from each other, only on account of the different quantity or
proportion that each contains of the principles here enumerated.
But there are many other plants, which, besides these substances, yield
also a considerable quantity of a Volatile Alkaline Salt. This property
is possessed chiefly by that tribe of plants which is distinguished
by having cruciform flowers; among which there are some that being
analyzed greatly resemble animal matters. We shall now analyze one of
these; Mustard-seed, for instance.
PROCESS II.
_To analyze a vegetable Substance which yields the same Principles as
are obtained from Animal Matters; instanced in Mustard-seed._
With an apparatus like that of the preceding process, and with the
same fire, distil Mustard-seed. With a degree of heat inferior to that
of boiling water, there will come over a phlegm somewhat coloured,
and impregnated with a Volatile Alkaline Salt. With a degree of
heat greater than that of boiling water, the same kind of phlegm,
impregnated with the same Salt, will continue to come over; but it will
be much higher coloured, and will be accompanied with a light Oil. At
this time a considerable quantity of air is discharged; with regard to
which the same precautions must be taken as in distilling Guaiacum.
If the fire be gradually raised, there will come over a black thick
Oil, lighter however than water; and at the same time vapours will
rise, and, condensing on the sides of the receiver, form into sprigs or
ramifications. This is a Volatile Alkaline Salt, in a concrete form,
like that of animals, as we shall hereafter see. These vapours are much
whiter than those of Guaiacum.
When you have thus drawn off, with a very strong fire, all the Volatile
Alkali and thick Oil contained in the subject, there will be nothing
left in the retort but a sort of coal, from which a small quantity of
phosphorus may be obtained, provided the retort you employ for that
purpose be good enough to stand a very violent heat.
_OBSERVATIONS._
Mustard-seed furnishes us with an instance of a vegetable, from which
we obtain, by analyzing it, the very same principles that animal
matters yield. Instead of getting an Acid from it, we obtain only a
Volatile Alkali; probably because the Acid, which originally enters
into the composition of this kind of vegetables, as well as of all
others, undergoes in passing through their strainers, and mixing with
their juices, such alterations as it suffers when it enters into the
composition of animals: that is, it combines with some of their Earth
and of their Oil, in such a manner as to be changed into a Volatile
Alkali, or at least disposed to be converted into one with the aid of
fire.
We shall not here speak of the manner of separating and depurating the
principles obtained by this process; but reserve it for the analysis of
animals, which is absolutely the same. We shall content ourselves with
observing, that the first Volatile Alkali which rises at the beginning
of the operation together with the phlegm, in a degree of heat below
that of boiling water, differs from that which doth not come over till
towards the end of the distillation, when the last thick Oil ascends.
The different times, and different degrees of heat, in which these two
Alkalis rise, shew that the former exists actually and perfectly in the
plant; but that the latter is generated during the distillation, and is
the product of the fire, which combines together the materials whereof
it is composed.
Vegetables that thus yield a Volatile Alkali with a heat less than
that of boiling water, irritate the organ of smelling, affecting it
with a sensation of acrimony; and the effluvia, which rise from them
when bruised, make the eyes smart so as to draw tears from them in
abundance. Several of these matters, being only bruised, effervesce
with Acids: effects producible only by a very Volatile Alkaline
principle.
This is that Alkali, the lightest of all the principles that can be
extracted from bodies, which rises first in our distillation along
with the phlegm, and with a degree of heat much inferior to that of
boiling water. As the phlegm with which it rises is very copious, it is
dissolved thereby; which is the reason it doth not appear in a concrete
form. To this water it gives a slight yellowish tinge, because it is
impure and oily. The saline Alkaline properties of this liquor have
procured it the title of a Volatile Spirit. This Volatile Alkali, which
exists naturally and perfectly formed in Mustard-seed, Onions, Garlic,
Cresses, and other such vegetables, constitutes a difference between
them and animal substances, which contain only the materials requisite
to form a Volatile Alkali, but none ready formed, unless they have
undergone the putrid fermentation.
The second Volatile Alkali which rises in our distillation, but not
without a very strong degree of fire, and at the same time with the
last thick Oil, seems to be a production of the fire; for if it were
already formed in the mixt, as the other is, it would rise with the
same heat, and at the same time, being equally volatile. It is not
impossible, however, that it may exist perfectly formed in the plant;
but, having contracted an union with some Acid, and therewith composing
an Ammoniacal Salt, it may by that means be hindered from rising so
readily as is agreeable to its natural volatility.
The Phosphorus obtained by a violent fire, from the _caput mortuum_
of this distillation, seems to throw a light of probability on this
conjecture. There is certainly a great deal of Acid in the composition
of Phosphorus. Perhaps this Acid was originally combined with our
second Volatile Alkali, and formed therewith, as was said, a sort of
Sal Ammoniac. Moreover, almost all the plants that yield a Volatile
Alkali by distillation, yield also a considerable quantity of Acid:
which may perhaps be the remains of such a Sal Ammoniac decomposed by
the operation. This is a subject for curious and useful inquiries. This
second Volatile Alkali appears in a concrete form, because very little
phlegm comes over along with it; so that the vapours thereof are not
sufficient to dissolve it, as they did the first.
CHAP. VII.
_Of the_ SUBSTANCES _obtained from_ VEGETABLES _by_ COMBUSTION.
PROCESS I.
_To procure a Fixed Caustic Alkaline Salt from a Vegetable Substance,
by burning it in the open Air._
Take any vegetable matter whatever; set it on fire, and let it burn
in the open air till it be wholly reduced to ashes. On these ashes
pour a quantity of boiling water sufficient to drench them thoroughly.
Filter the liquor in order to separate the earthy parts; and evaporate
your lye to dryness, stirring it incessantly; and you will have a
yellowish-white Salt.
Put this Salt in a crucible; set it in a melting furnace, and make a
moderate fire, so as not to fuse the Salt. It will turn first of a
blue-grey colour, afterwards of a blue-green, and at last reddish. Put
on the dome of the furnace; fill it with coals; make your fire strong
enough to melt the Salt, and keep it in fusion for an hour, or an hour
and half. Then pour it into a heated metal mortar; pound it while it
is red-hot; put it, as soon as possible, into a glass bottle, first
made very hot and dry, and shut it up close with a glass stopple rubbed
with emery. By this means you will have the pure Fixed Alkali of the
vegetable substance you burnt.
_OBSERVATIONS._
Burning a vegetable substance in the open air is a kind of violent and
rapid analysis made by fire, which separates, resolves, and decomposes,
several of its principles.
When any wood or plant is laid on a quick fire, there ascends from
it immediately an aqueous smoke, which consists of little more than
phlegm; but this smoke soon becomes thicker and blacker: it is then
pungent, draws tears from one's eyes, and excites a cough if drawn into
the lungs with the breath. These effects arise from its being replete
with the Acid, and some of the Oil, of the vegetable converted into
vapours. Soon after this the smoke grows exceeding black and thick: it
is now still more acrid, and the plant turns black. Its strongest Acid
and last thick Oil are now discharged with impetuosity.
This rarefied Oil being heated red-hot suddenly takes fire and flames.
The vegetable burns and deflagrates rapidly, till all its Oil is
consumed. Then the flame ceases; and nothing remains but a coal,
like that found in a retort after all the principles of a plant have
been extracted by the force of fire. But this coal having a free
communication with the air, which is absolutely necessary to keep a
combustible burning, continues to be red, sparkles, and wastes, till
all its phlogiston is dissipated and destroyed. After this nothing
remains but the Earth and Fixed Salt of the vegetable; which, mixed
together, form what we call the Ashes. Water, which is the natural
solvent of Salts, takes up every thing of that kind that is contained
in the ashes; so that, by lixiviating them, as directed, all the Salt
is extracted, and nothing left but the pure earth of the mixt which is
thus decomposed.
The phenomena observed in the burning of a vegetable substance, and the
production thereby of a Fixed Alkali, seem to prove that this salt is
the work of the fire; that it did not exist in the plant before it was
burnt; that the plant only contained materials adapted to form this
Salt; and that this Salt is no other than a combination of some of the
Acid, united with a portion of Earth, by means of the igneous motion.
In the first place; a Fixed Alkali may be obtained by lixiviation from
the ashes of all vegetable matters that contain an Acid, Earth, and
Phlogiston, in due proportion. Thus Essential Salts; the substance
of extracts made by trituration, infusion, or decoction; wood coals
burnt to ashes; all yield a quantity of this Salt in proportion to the
quantity of Acid and Earth contained in them.
Secondly; Fat, Essential, and Empyreumatic Oils afford, when burnt,
such a small quantity of Fixed Alkali as is scarce perceptible; because
they contain but a little Acid, and still less Earth: and these same
Oils, when rectified by repeated distillations, and then burnt, leave
still less of this Salt; because they are separated by rectification
from most of the Acid, together with, the small matter of Earth
contained in them.
Thirdly; those vegetable matters which, being analyzed, furnish a great
deal of Volatile Alkali, yield but very little Fixed Alkali; because a
great deal of their Acid is employed in forming the Volatile Alkali,
which is dissipated by burning the plant: and, for the same reason,
those which in distillation afford only a Volatile Alkali, and no Acid,
leave in their ashes little or no Fixed Alkali, as is also the case
with animal matters.
Fourthly, and lastly; the ashes of such plants as have been long
steeped in water, and from which infusions and decoctions have been
made, always contain the less Alkali the longer they have been infused
or boiled, and the more water they were infused or boiled in; because
water dissolves and carries off their Acid. It is for this reason that
the ashes of float-wood are much less saline than those of green wood.
Boerhaave assures us, in his Chymistry, that having exhausted Rosemary
by repeated decoctions, and having afterwards boiled the plant thus
treated, the ashes produced by it shewed not the least sign of a Fixed
Alkali. He says, that, in order to exhaust thoroughly all the saline
matters contained in Rosemary, he was obliged to decoct it no less
than twenty times successively, with fresh water every time, and never
ceased boiling it in this manner, till he was sure that the water, by
boiling the plant in it for a long time, took up from it no kind of
matter whatever that in the least affected its purity: so that the
water of his last decoction had absolutely no smell, taste, or colour;
but was in short precisely the same as before he used it for the
decoction. The same author observes, that his plant, after having been
exhausted in this manner, and having suffered such continued boiling,
retained nevertheless its perfect external form; that from being green
at first it became brown, and sunk to the bottom of the water, instead
of floating thereon as it did before decoction.
If, in reiterating this beautiful experiment of Mr. Boerhaave's, you
should not succeed as you expect, you must not therefore accuse this
great man of having been mistaken on this occasion; seeing it is very
difficult, not to say impossible, to ascertain exactly, from the
account he hath given of his experiment, all that is necessary to its
perfect success: for he hath not specified either the duration of the
coctions which he made the Rosemary undergo, or the quantity of water
he employed in each; whereas a difference in either of these may
occasion a vast difference in the result. It is evident, that if five
or six pounds of water be used for each coction of a pound of Rosemary,
and be kept boiling for two or three hours, the plant will not be near
so much exhausted by being so treated, as if the same quantity thereof
were kept boiling for several days, in forty or fifty quarts of water.
Indeed, these points seem, in some measure, to be determined, by
what he says of the quality which the water of the last decoction
ought to have. But the same objections occur here also; nay, the two
circumstances of the quantity of water and the duration of the boiling,
have the greatest influence here: for the more a plant is exhausted
of its Salts, the more difficult it becomes for the water to dissolve
and separate the small quantity thereof that remains united with the
tenacious Oil; and consequently it may happen, that this last water,
after the plant hath boiled in it five or six hours, shall appear
insipid, scentless, colourless; and yet that a much greater quantity
of water, but reduced by longer boiling to the same quantity with that
which hath been boiled but five or six hours, shall have acquired both
taste and colour; in a word, shew that it hath taken up some of the
principles of the plant. It may also happen, that, a small portion of
saline matter being diffused through a large quantity of water, after
long continued coction, shall not be perceptible either to the taste
or to the eye; but that the very same portion of saline matter shall
become very sensible, when the quantity of water in which it is lost,
as it were, is sufficiently lessened by evaporation.
Hence, if we would make sure of fulfilling the conditions required
by Mr. Boerhaave, the last decoction of the plant must be made in a
much greater quantity of water, and continued for a much longer time,
than may perhaps be imagined, or perhaps easily determined; and this
decoction being evaporated to any degree you please, must have neither
taste, smell, nor colour: in short, it must from first to last remain
perfectly like pure water. In other words, it is very difficult to
attain to any certainty in this matter.
Though what hath hitherto been said, about procuring the Fixed Alkali
of plants by combustion, seems to prove that this Salt is wholly the
production of the fire, yet it must not be asserted that no part
thereof pre-existed formally in the plant before it was burnt. On the
contrary, it is certain that, amongst the saline matters found in the
composition of plants, there are true Neutral Salts whose basis is a
Fixed Alkali; but this Alkali being combined with an Acid discovers
none of its properties, and never appears in its true form till the
Neutral Salt, of which it makes a part, is decomposed by combustion.
The case of Sea-plants, all of which contain Sea-salt, and when burnt
yield an Alkaline Salt perfectly resembling the basis of Sea-salt,
seems to decide this point.
If, in lixiviating the ashes of a plant, to dissolve and wash out
its Alkali, you intend that nothing should be left but an absolutely
pure earth, fit for making cupels, you must not be contented with one
ablution only, even with a large quantity of water; because the ashes
continue drenched with the water in which the Salts are dissolved, and
consequently, when this water is evaporated, some of the Salts will be
left with the earth. Therefore, if this be your view, you must wash it
three or four several times, using fresh water every time.
The water impregnated with the Alkali cannot be evaporated without
a considerable loss of Salt, especially if it be violently boiled;
because the water, with which it is closely united, carries off part of
it. In consequence of this intimate union, it is very difficult, when
the evaporation is near finished, and but a little water left, to dry
the Salt perfectly, because it pertinaciously retains this last portion
of humidity.
The Alkali obtained from the ashes of a burnt plant is not perfectly
pure: it is contaminated with a small mixture of fatty matters, which
were probably defended thereby against the action of the fire, and
which render it somewhat saponaceous. In order to free it from this
extraneous matter, and to render it very caustic, it must be calcined a
long time in a crucible, but without melting it at first: because it is
with this Salt as with most metallic matters, which are sooner and more
easily deprived of their phlogiston by being calcined without melting,
provided they be comminuted into small particles, than when they are in
fusion; all melted matters having but a small surface exposed to the
air, by the contact of which the evaporation or anything whatever is
exceedingly promoted. It was for this reason we directed the Salt to be
calcined for a long time in a crucible before melting it.
Mr. Boerhaave was very sensible of the utility of this calcination
of the Alkali previous to its being melted, when in his Chymistry he
ordered the ashes containing this Salt to be put into a large earthen
vessel, kept red-hot for a considerable time, taking great care that
the Salt do not melt. He takes notice, that, the longer the ashes are
calcined in this manner, the stronger is the Alkali obtained from
them. This method is, in the main, the very same with that here
prescribed, and produces the same effect; because the Alkali is equally
well freed of the extraneous fatty matter, whether it be calcined
before or after its separation, provided it be not suffered to melt.
Mr. Boerhaave gives another reason for recommending care to be taken
that the Fixed Alkali do not melt, while the ashes are calcining to
render it stronger and more caustic: for, if that should happen, the
melted mixture of the Salt and ashes would produce a vitrified mass,
which would have none of the properties of the Salt.
PROCESS II.
_To procure the Fixed Salt of a Plant by burning it after the manner of
Tachenius._