some measure, tanned, or hardened by the constant use of this liquor;
but you must remember that where a number of other chemical agents are
concerned, and, above all, where life exists, no certain chemical
inference can be drawn.
I must not dismiss this subject, without mentioning a recent discovery
of Mr. Hatchett, which relates to it. This gentleman found that a
substance very similar to tannin, possessing all its leading properties,
and actually capable of tanning leather, may be produced by exposing
carbon, or any substance containing carbonaceous matter, whether
vegetable, animal, or mineral, to the action of nitric acid.
CAROLINE.
And is not this discovery very likely to be of use to manufactures?
MRS. B.
That is very doubtful, because tannin, thus artificially prepared, must
probably always be more expensive than that which is obtained from bark.
But the fact is extremely curious, as it affords one of those very rare
instances of chemistry being able to imitate the proximate principles of
organised bodies.
The last of the vegetable materials is _woody fibre_; it is the hardest
part of plants. The chief source from which this substance is derived is
wood, but it is also contained, more or less, in every solid part of
that plant. It forms a kind of skeleton of the part to which it belongs,
and retains its shape after all the other materials have disappeared. It
consists chiefly of carbon, united with a small proportion of salts, and
the other constituents common to all vegetables.
EMILY.
It is of woody fibre, then, that the common charcoal is made?
MRS. B.
Yes. Charcoal, as you may recollect, is obtained from wood, by the
separation of all its evaporable parts.
Before we take leave of the vegetable materials, it will be proper, at
least, to enumerate the several vegetable acids which we either have
had, or may have occasion to mention. I believe I formerly told you that
their basis, or radical, was uniformly composed of hydrogen and carbon,
and that their difference consisted only in the various proportions of
oxygen which they contained.
The following are the names of the vegetable acids:
The _Mucous Acid_, obtained from gum or mucilage;
_Suberic_ - - - from cork;
_Camphoric_ - - - from camphor;
_Benzoic_ - - - from balsams;
_Gallic_ - - - from galls, bark, &c.
_Malic_ - - - from ripe fruits;
_Citric_ - - - from lemon juice;
_Oxalic_ - - - from sorrel;
_Succinic_ - - - from amber;
_Tartarous_ - - - from tartrit of potash:
_Acetic_ - - - from vinegar.
They are all decomposable by heat, soluble in water, and turn vegetable
blue colours red. The _succinic_, the _tartarous_, and the _acetous
acids_, are the products of the decomposition of vegetables; we shall,
therefore, reserve their examination for a future period.
The _oxalic acid_, distilled from sorrel, is the highest term of
vegetable acidification; for, if more oxygen be added to it, it loses
its vegetable nature, and is resolved into carbonic acid and water;
therefore, though all the other acids may be converted into the oxalic
by an addition of oxygen, the oxalic itself is not susceptible of a
further degree of oxygenation; nor can it be made, by any chemical
processes, to return to a state of lower acidification.
To conclude this subject, I have only to add a few words on the _gallic
acid_. . . . .
CAROLINE.
Is not this the same acid before mentioned, which forms ink, by
precipitating sulphat of iron from its solution?
MRS. B.
Yes. Though it is usually extracted from galls, on account of its being
most abundant in that vegetable substance, it may also be obtained from
a great variety of plants. It constitutes what is called the _astringent
principle_ of vegetables; it is generally combined with tannin, and you
will find that an infusion of tea, coffee, bark, red-wine, or any
vegetable substance that contains the astringent principle, will make a
black precipitate with a solution of sulphat of iron.
CAROLINE.
But pray what are galls?
MRS. B.
They are excrescences which grow on the bark of young oaks, and are
occasioned by an insect which wounds the bark of trees, and lays its
eggs in the aperture. The lacerated vessels of the tree then discharge
their contents, and form an excrescence, which affords a defensive
covering for these eggs. The insect, when come to life, first feeds on
this excrescence, and some time afterward eats its way out, as it
appears from a hole which is formed in all gall-nuts that no longer
contain an insect. It is in hot climates only that strongly astringent
gall-nuts are found; those which are used for the purpose of making ink
are brought from Aleppo.
EMILY.
But are not the oak-apples, which grow on the leaves of the oak in this
country, of a similar nature?
MRS. B.
Yes; only they are an inferior species of galls, containing less of the
astringent principle, and therefore less applicable to useful purposes.
CAROLINE.
Are the vegetable acids never found but in their pure uncombined state?
MRS. B.
By no means; on the contrary, they are frequently met with in the state
of compound salts; these, however, are in general not fully saturated
with the salifiable bases, so that the acid predominates; and, in this
state, they are called _acidulous_ salts. Of this kind is the salt
called cream of tartar.
CAROLINE.
Is not the salt of lemon, commonly used to take out ink-spots and
stains, of this nature?
MRS. B.
No; that salt consists of the oxalic acid, combined with a little
potash. It is found in that state in sorrel.
CAROLINE.
And pray how does it take out ink-spots?
MRS. B.
By uniting with the iron, and rendering it soluble in water.
Besides the vegetable materials which we have enumerated, a variety of
other substances, common to the three kingdoms, are found in vegetables,
such as potash, which was formerly supposed to belong exclusively to
plants, and was, in consequence, called the vegetable alkali.
Sulphur, phosphorus, earths, and a variety of metallic oxyds, are also
found in vegetables, but only in small quantities. And we meet sometimes
with neutral salts, formed by the combination of these ingredients.
CONVERSATION XXI.
ON THE DECOMPOSITION OF VEGETABLES.
CAROLINE.
The account which you have given us, Mrs. B., of the materials of
vegetables, is, doubtless, very instructive; but it does not completely
satisfy my curiosity. I wish to know how plants obtain the principles
from which their various materials are formed; by what means these are
converted into vegetable matter, and how they are connected with the
life of the plant?
MRS. B.
This implies nothing less than a complete history of the chemistry and
physiology of vegetation, subjects on which we have yet but very
imperfect notions. Still I hope that I shall be able, in some measure,
to satisfy your curiosity. But, in order to render the subject more
intelligible, I must first make you acquainted with the various changes
which vegetables undergo, when the vital power no longer enables them to
resist the common laws of chemical attraction.
The composition of vegetables being more complicated than that of
minerals, the former more readily undergo chemical changes than the
latter: for the greater the variety of attractions, the more easily is
the equilibrium destroyed, and a new order of combinations introduced.
EMILY.
I am surprised that vegetables should be so easily susceptible of
decomposition; for the preservation of the vegetable kingdom is
certainly far more important than that of minerals.
MRS. B.
You must consider, on the other hand, how much more easily the former is
renewed than the latter. The decomposition of the vegetable takes place
only after the death of the plant, which, in the common course of
nature, happens when it has yielded fruit and seeds to propagate its
species. If, instead of thus finishing its career, each plant was to
retain its form and vegetable state, it would become an useless burden
to the earth and its inhabitants. When vegetables, therefore, cease to
be productive, they cease to live, and nature then begins her process of
decomposition, in order to resolve them into their chemical
constituents, hydrogen, carbon, and oxygen; those simple and primitive
ingredients, which she keeps in store for all her combinations.
EMILY.
But since no system of combination can be destroyed, except by the
establishment of another order of attractions, how can the decomposition
of vegetables reduce them to their simple elements?
MRS. B.
It is a very long process, during which a variety of new combinations
are successively established and successively destroyed: but, in each of
these changes, the ingredients of vegetable matter tend to unite in a
more simple order of compounds, till they are at length brought to their
elementary state, or, at least, to their most simple order of
combinations. Thus you will find that vegetables are in the end almost
entirely reduced to water and carbonic acid; the hydrogen and carbon
dividing the oxygen between them, so as to form with it these two
substances. But the variety of intermediate combinations that take place
during the several stages of the decomposition of vegetables, present us
with a new set of compounds, well worthy of our examination.
CAROLINE.
How is it possible that vegetables, while putrefying, should produce any
thing worthy of observation?
MRS. B.
They are susceptible of undergoing certain changes before they arrive at
the state of putrefaction, which is the final term of decomposition; and
of these changes we avail ourselves for particular and important
purposes. But, in order to make you understand this subject, which is of
considerable importance, I must explain it more in detail.
The decomposition of vegetables is always attended by a violent internal
motion, produced by the disunion of one order of particles, and the
combination of another. This is called FERMENTATION. There are several
periods at which this process stops, so that a state of rest appears to
be restored, and the new order of compounds fairly established. But,
unless means be used to secure these new combinations in their actual
state, their duration will be but transient, and a new fermentation will
take place, by which the compound last formed will be destroyed; and
another, and less complex order, will succeed.
EMILY.
The fermentations, then, appear to be only the successive steps by which
a vegetable descends to its final dissolution.
MRS. B.
Precisely so. Your definition is perfectly correct.
CAROLINE.
And how many fermentations, or new arrangements, does a vegetable
undergo before it is reduced to its simple ingredients?
MRS. B.
Chemists do not exactly agree in this point; but there are, I think,
four distinct fermentations, or periods, at which the decomposition of
vegetable matter stops and changes its course. But every kind of
vegetable matter is not equally susceptible of undergoing all these
fermentations.
There are likewise several circumstances required to produce
fermentation. Water and a certain degree of heat are both essential to
this process, in order to separate the particles, and thus weaken their
force of cohesion, that the new chemical affinities may be brought into
action.
CAROLINE.
In frozen climates, then, how can the spontaneous decomposition of
vegetables take place?
MRS. B.
It certainly cannot; and, accordingly, we find scarcely any vestiges of
vegetation where a constant frost prevails.
CAROLINE.
One would imagine that, on the contrary, such spots would be covered
with vegetables; for, since they cannot be decomposed, their number must
always increase.
MRS. B.
But, my dear, heat and water are quite as essential to the formation of
vegetables, as they are to their decomposition. Besides, it is from the
dead vegetables, reduced to their elementary principles, that the rising
generation is supplied with sustenance. No young plant, therefore, can
grow unless its predecessors contribute both to its formation and
support; and these not only furnish the seed from which the new plant
springs, but likewise the food by which it is nourished.
CAROLINE.
Under the torrid zone, therefore, where water is never frozen, and the
heat is very great, both the processes of vegetation and of fermentation
must, I suppose, be extremely rapid?
MRS. B.
Not so much as you imagine: for in such climates great part of the water
which it requires for these processes is in an aëriform state, which is
scarcely more conducive either to the growth or formation of vegetables
than that of ice. In those latitudes, therefore, it is only in low damp
situations, sheltered by woods from the sun’s rays, that the smaller
tribes of vegetables can grow and thrive during the dry season, as dead
vegetables seldom retain water enough to produce fermentation, but are,
on the contrary, soon dried up by the heat of the sun, which enables
them to resist that process; so that it is not till the fall of the
autumnal rains (which are very violent in such climates), that
spontaneous fermentation can take place.
The several fermentations derive their names from their principal
products. The first is called the _saccharine fermentation_, because its
product is _sugar_.
CAROLINE.
But sugar, you have told us, is found in all vegetables; it cannot,
therefore, be the product of their decomposition.
MRS. B.
It is true that this fermentation is not confined to the decomposition
of vegetables, as it continually takes place during their life; and,
indeed, this circumstance has, till lately, prevented it from being
considered as one of the fermentations. But the process appears so
analogous to the other fermentations, and the formation of sugar,
whether in living or dead vegetable matter is so evidently a new
compound, proceeding from the destruction of the previous order of
combinations, and essential to the subsequent fermentations, that it is
now, I believe, generally esteemed the first step, or necessary
preliminary, to decomposition, if not an actual commencement of that
process.
CAROLINE.
I recollect your hinting to us that sugar was supposed not to be
secreted from the sap, in the same manner as mucilage, fecula, oil, and
the other ingredients of vegetables.
MRS. B.
It is rather from these materials, than from the sap itself, that sugar
is formed; and it is developed at particular periods, as you may observe
in fruits, which become sweet in ripening, sometimes even after they
have been gathered. Life, therefore, is not essential to the formation
of sugar, whilst on the contrary, mucilage, fecula, and the other
vegetable materials that are secreted from the sap by appropriate
organs, whose powers immediately depend on the vital principle, cannot
be produced but during the existence of that principle.
EMILY.
The ripening of fruits is, then, their first step to destruction, as
well as their last towards perfection?
MRS. B.
Exactly. --A process analogous to the saccharine fermentation takes
place also during the cooking of certain vegetables. This is the case
with parsnips, carrots, potatoes, &c. in which sweetness is developed by
heat and moisture; and we know that if we carried the process a little
farther, a more complete decomposition would ensue. The same process
takes place also in seeds previous to their sprouting.
CAROLINE.
How do you reconcile this to your theory, Mrs. B.? Can you suppose that
a decomposition is the necessary precursor of life?
MRS. B.
That is indeed the case. The materials of the seed must be decomposed,
and the seed disorganized, before a plant can sprout from it. Seeds,
besides the embrio plant, contain (as we have already observed) fecula,
oil, and a little mucilage. These substances are destined for the
nourishment of the future plant; but they undergo some change before
they can be fit for this function. The seeds, when buried in the earth,
with a certain degree of moisture and of temperature, absorb water,
which dilates them, separates their particles, and introduces a new
order of attractions, of which sugar is the product. The substance of
the seed is thus softened, sweetened, and converted into a sort of white
milky pulp, fit for the nourishment of the embrio plant.
The saccharine fermentation of seeds is artificially produced, for the
purpose of making _malt_, by the following process:-- A quantity of
barley is first soaked in water for two or three days: the water being
afterwards drained off, the grain heats spontaneously, swells, bursts,
sweetens, shows a disposition to germinate, and actually sprouts to the
length of an inch, when the process is stopped by putting it into a
kiln, where it is well dried at a gentle heat. In this state it is crisp
and friable, and constitutes the substance called _malt_, which is the
principal ingredient of beer.
EMILY.
But I hope you will tell us how malt is made into beer?
MRS. B.
Certainly; but I must first explain to you the nature of the second
fermentation, which is essential to that operation. This is called the
_vinous fermentation_, because its product is _wine_.
EMILY.
How very different the decomposition of vegetables is from what I had
imagined! The products of their disorganisation appear almost superior
to those which they yield during their state of life and perfection.
MRS. B.
And do you not, at the same time, admire the beautiful economy of
Nature, which, whether she creates, or whether she destroys, directs all
her operations to some useful and benevolent purpose? --It appears that
the saccharine fermentation is extremely favourable, if not absolutely
essential, as a previous step, to the vinous fermentation; so that if
sugar be not developed during the life of the plant, the saccharine
fermentation must be artificially produced before the vinous
fermentation can take place. This is the case with barley, which does
not yield any sugar until it is made into malt; and it is in that state
only that it is susceptible of undergoing the vinous fermentation by
which it is converted into beer.
CAROLINE.
But if the product of the vinous fermentation is always wine, beer
cannot have undergone that process, for beer is certainly not wine.
MRS. B.
Chemically speaking, beer may be considered as the wine of grain. For it
is the product of the fermentation of malt, just as wine is that of the
fermentation of grapes, or other fruits.
The consequence of the vinous fermentation is the decomposition of the
saccharine matter, and the formation of a spirituous liquor from the
constituents of the sugar. But, in order to promote this fermentation,
not only water and a certain degree of heat are necessary, but also some
other vegetable ingredients, besides the sugar, as fecula, mucilage,
acids, salts, extractive matter, &c. all of which seem to contribute to
this process; and give to the liquor its peculiar taste.
EMILY.
It is, perhaps, for this reason that wine is not obtained from the
fermentation of pure sugar; but that fruits are chosen for that purpose,
as they contain not only sugar, but likewise the other vegetable
ingredients which promote the vinous fermentation, and give the peculiar
flavour.
MRS. B.
Certainly. And you must observe also, that the relative quantity of
sugar is not the only circumstance to be considered in the choice of
vegetable juices for the formation of wine; otherwise the sugar-cane
would be best adapted for that purpose. It is rather the manner and
proportion in which the sugar is mixed with other vegetable ingredients
that influences the production and qualities of wine. And it is found
that the juice of the grape not only yields the most considerable
proportion of wine, but that it likewise affords it of the most grateful
flavour.
EMILY.
I have seen a vintage in Switzerland, and I do not recollect that heat
was applied, or water added, to produce the fermentation of the grapes.
MRS. B.
The common temperature of the atmosphere in the cellars in which the
juice of the grape is fermented is sufficiently warm for this purpose;
and as the juice contains an ample supply of water, there is no occasion
for any addition of it. But when fermentation is produced in dry malt,
a quantity of water must necessarily be added.
EMILY.
But what are precisely the changes that happen during the vinous
fermentation?
MRS. B.
The sugar is decomposed, and its constituents are recombined into two
new substances; the one a peculiar liquid substance, called _alcohol_ or
_spirit of wine_, which remains in the fluid; the other, carbonic acid
gas, which escapes during the fermentation. Wine, therefore, as I before
observed, in a general point of view, may be considered as a liquid of
which alcohol constitutes the essential part. And the varieties of
strength and flavour of the different kinds of wine are to be attributed
to the different qualities of the fruits from which they are obtained,
independently of the sugar.
CAROLINE.
I am astonished to hear that so powerful a liquid as spirit of wine
should be obtained from so mild a substance as sugar.
MRS. B.
Can you tell me in what the principal difference consists between
alcohol and sugar?
CAROLINE.
Let me reflect . . . . . Sugar consists of carbon, hydrogen, and oxygen.
If carbonic acid be subtracted from it, during the formation of alcohol,
the latter will contain less carbon and oxygen than sugar does;
therefore hydrogen must be the prevailing principle of alcohol.
MRS. B.
It is exactly so. And this very large proportion of hydrogen accounts
for the lightness and combustible property of alcohol, and of spirits in
general, all of which consist of alcohol variously modified.
EMILY.
And can sugar be recomposed from the combination of alcohol and carbonic
acid?
MRS. B.
Chemists have never been able to succeed in effecting this; but from
analogy, I should suppose such a recomposition possible. Let us now
observe more particularly the phenomena that take place during the
vinous fermentation. At the commencement of this process, heat is
evolved, and the liquor swells considerably from the formation of the
carbonic acid, which is disengaged in such prodigious quantities as
would be fatal to any person who should unawares inspire it; an accident
which has sometimes happened. If the fermentation be stopped by putting
the liquor into barrels, before the whole of the carbonic acid is
evolved, the wine is brisk, like Champagne, from the carbonic acid
imprisoned in it, and it tastes sweet, like cyder, from the sugar not
being completely decomposed.
EMILY.
But I do not understand why heat should be evolved during this
operation. For, as there is a considerable formation of gas, in which a
proportionable quantity of heat must become insensible, I should have
imagined that cold, rather than heat, would have been produced.
MRS. B.
It appears so on first consideration; but you must recollect that
fermentation is a complicated chemical process; and that, during the
decompositions and recompositions attending it, a quantity of chemical
heat may be disengaged, sufficient both to develope the gas, and to
effect an increase of temperature. When the fermentation is completed,
the liquid cools and subsides, the effervescence ceases, and the thick,
sweet, sticky juice of the fruit is converted into a clear, transparent,
spirituous liquor, called wine.
EMILY.
How much I regret not having been acquainted with the nature of the
vinous fermentation, when I had an opportunity of seeing the process!
MRS. B.
You have an easy method of satisfying yourself in that respect by
observing the process of brewing, which, in every essential
circumstance, is similar to that of making wine, and is really a very
curious chemical operation.