basis not only of the sap, but of all the vegetable juices. Water is the
vehicle which carries into the plant the various salts and other
ingredients required for the formation and support of the vegetable
system. Nor is this all; part of the water itself is decomposed by the
organs of the plant; the hydrogen becomes a constituent part of oil, of
extract, of colouring matter, &c. whilst a portion of the oxygen enters
into the formation of mucilage, of fecula, of sugar, and of vegetable
acids. But the greater part of the oxygen, proceeding from the
decomposition of the water, is converted into a gaseous state by the
caloric disengaged from the hydrogen during its condensation in the
formation of the vegetable materials. In this state the oxygen is
transpired by the leaves of plants when exposed to the sun’s rays. Thus
you find that the decomposition of water, by the organs of the plant, is
not only a means of supplying it with its chief ingredient, hydrogen,
but at the same time of replenishing the atmosphere with oxygen,
a principle which requires continual renovation, to make up for the
great consumption of it occasioned by the numerous oxygenations,
combustions, and respirations, that are constantly taking place on the
surface of the globe.
EMILY.
What a striking instance of the harmony of nature.
MRS. B.
And how admirable the design of Providence, who makes every different
part of the creation thus contribute to the support and renovation of
each other!
But the intercourse of the vegetable and animal kingdoms through the
medium of the atmosphere extends still further. Animals, in breathing,
not only consume the oxygen of the air, but load it with carbonic acid,
which, if accumulated in the atmosphere, would, in a short time, render
it totally unfit for respiration. Here the vegetable kingdom again
interferes; it attracts and decomposes the carbonic acid, retains the
carbon for its own purposes, and returns the oxygen for ours.
CAROLINE.
How interesting this is! I do not know a more beautiful illustration of
the wisdom which is displayed in the laws of nature.
MRS. B.
Faint and imperfect as are the ideas which our limited perceptions
enable us to form of divine wisdom, still they cannot fail to inspire us
with awe and admiration. What, then, would be our feelings, were the
complete system of nature at once displayed before us! So magnificent a
scene would probably be too great for our limited and imperfect
comprehension, and it is no doubt among the wise dispensations of
Providence, to veil the splendour of a glory with which we should be
overpowered. But it is well suited to the nature of a rational being to
explore, step by step, the works of the creation, to endeavour to
connect them into harmonious systems; and, in a word, to trace in the
chain of beings, the kindred ties and benevolent design which unites its
various links, and secure its preservation.
CAROLINE.
But of what nature are the organs of plants which are endued with such
wonderful powers?
MRS. B.
They are so minute that their structure, as well as the mode in which
they perform their functions, generally elude our examination; but we
may consider them as so many vessels or apparatus appropriated to
perform, with the assistance of the principle of life, certain chemical
processes, by means of which these vegetable compounds are generated. We
may, however, trace the tannin, resins, gum, mucilage, and some other
vegetable materials, in the organised arrangement of plants, in which
they form the bark, the wood, the leaves, flowers, and seeds.
The _bark_ is composed of the _epidermis_, the _parenchyma_, and the
_cortical layers_.
The epidermis is the external covering of the plant. It is a thin
transparent membrane, consisting of a number of slender fibres, crossing
each other, and forming a kind of net-work. When of a white glossy
nature, as in several species of trees, in the stems of corn and of
seeds, it is composed of a thin coating of siliceous earth, which
accounts for the strength and hardness of those long and slender stems.
Sir H. Davy was led to the discovery of the siliceous nature of the
epidermis of such plants, by observing the singular phenomenon of sparks
of fire emitted by the collision of ratan canes with which two boys were
fighting in a dark room. On analysing the epidermis of the cane, he
found it to be almost entirely siliceous.
CAROLINE.
With iron then, a cane, I suppose, will strike fire very easily?
MRS. B.
I understand that it will. --In ever-greens the epidermis is mostly
resinous, and in some few plants is formed of wax. The resin, from its
want of affinity for water, tends to preserve the plant from the
destructive effects of violent rains, severe climates, or inclement
seasons, to which this species of vegetables is peculiarly exposed.
EMILY.
Resin must preserve wood just like a varnish, as it is the essential
ingredient of varnishes?
MRS. B.
Yes; and by this means it prevents likewise all unnecessary expenditure
of moisture.
The parenchyma is immediately beneath the epidermis; it is that green
rind which appears when you strip a branch of any tree or shrub of its
external coat of bark. The parenchyma is not confined to the stem or
branches, but extends over every part of the plant. It forms the green
matter of the leaves, and is composed of tubes filled with a peculiar
juice.
The cortical layers are immediately in contact with the wood; they
abound with tannin and gallic acid, and consist of small vessels through
which the sap descends after being elaborated in the leaves. The
cortical layers are annually renewed, the old bark being converted into
wood.
EMILY.
But through what vessels does the sap ascend?
MRS. B.
That function is performed by the tubes of the alburnum, or wood, which
is immediately beneath the cortical layers. The wood is composed of
woody fibre, mucilage, and resin. The fibres are disposed in two ways;
some of them longitudinally, and these form what is called the silver
grain of the wood. The others, which are concentric, are called the
spurious grain. These last are disposed in layers, from the number of
which the age of the tree may be computed, a new one being produced
annually by the conversion of the bark into wood. The oldest, and
consequently most internal part of the alburnum, is called heart-wood;
it appears to be dead, at least no vital functions are discernible in
it. It is through the tubes of the living alburnum that the sap rises.
These, therefore, spread into the leaves, and there communicate with the
extremities of the vessels of the cortical layers, into which they pour
their contents.
CAROLINE.
Of what use, then, are the tubes of the parenchyma, since neither the
ascending nor descending sap passes through them?
MRS. B.
They are supposed to perform the important function of secreting from
the sap the peculiar juices from which the plant more immediately
derives its nourishment. These juices are very conspicuous, as the
vessels which contain them are much larger than those through which the
sap circulates. The peculiar juices of plants differ much in their
nature, not only in different species of vegetables, but frequently in
different parts of the same individual plant: they are sometimes
saccharine, as in the sugar-cane, sometimes resinous, as in firs and
evergreens, sometimes of a milky appearance, as in the laurel.
EMILY.
I have often observed, that in breaking a young shoot, or in bruising a
leaf of laurel, a milky juice will ooze out in great abundance.
MRS. B.
And it is by making incisions in the bark that pitch, tar, and
turpentine are obtained from fir-trees. The durability of this species
of wood is chiefly owing to the resinous nature of its peculiar juices.
The volatile oils have, in a great measure, the same preservative
effects, as they defend the parts, with which they are connected, from
the attack of insects. This tribe seems to have as great an aversion to
perfumes, as the human species have delight in them. They scarcely ever
attack any odoriferous parts of plants, and it is not uncommon to see
every leaf of a tree destroyed by a blight, whilst the blossoms remain
untouched. Cedar, sandal, and all aromatic woods, are on this account of
great durability.
EMILY.
But the wood of the oak, which is so much esteemed for its durability,
has, I believe, no smell. Does it derive this quality from its hardness
alone?
MRS. B.
Not entirely; for the chesnut, though considerably harder and firmer
than the oak, is not so lasting. The durability of the oak is,
I believe, in a great measure owing to its having very little
heart-wood, the alburnum preserving its vital functions longer than in
other trees.
CAROLINE.
If incisions are made into the alburnum and cortical layers, may not the
ascending and descending sap be procured in the same manner as the
peculiar juice is from the vessels of the parenchyma?
MRS. B.
Yes; but in order to obtain specimens of these fluids, in any quantity,
the experiment must be made in the spring, when the sap circulates with
the greatest energy. For this purpose a small bent glass tube should be
introduced into the incision, through which the sap may flow without
mixing with any of the other juices of the tree. From the bark the sap
will flow much more plentifully than from the wood, as the ascending sap
is much more liquid, more abundant, and more rapid in its motion than
that which descends; for the latter having been deprived by the
operation of the leaves of a considerable part of its moisture, contains
a much greater proportion of solid matter, which retards its motion. It
does not appear that there is any excess of descending sap, as none ever
exudes from the roots of plants; this process, therefore, seems to be
carried on only in proportion to the wants of the plant, and the sap
descends no further, and in no greater quantity, than is required to
nourish the several organs. Therefore, though the sap rises and descends
in the plant, it does not appear to undergo a real circulation.
The last of the organs of plants is the _flower_, or _blossom_, which
produces the _fruits_ and _seed_. These may be considered as the
ultimate purpose of nature in the vegetable creation. From fruits and
seeds animals derive both a plentiful source of immediate nourishment,
and an ample provision for the reproduction of the same means of
subsistence.
The seed which forms the final product of mature plants, we have already
examined as constituting the first rudiments of future vegetation.
These are the principal organs of vegetation, by means of which the
several chemical processes which are carried on during the life of the
plant are performed.
EMILY.
But how are the several principles which enter into the composition of
vegetables so combined by the organs of the plant as to be converted
into vegetable matter?
MRS. B.
By chemical processes, no doubt; but the apparatus in which they are
performed is so extremely minute as completely to elude our examination.
We can form an opinion, therefore, only by the result of these
operations. The sap is evidently composed of water, absorbed by the
roots, and holding in solution the various principles which it derives
from the soil. From the roots the sap ascends through the tubes of the
alburnum into the stem, and thence branches out to every extremity of
the plant. Together with the sap circulates a certain quantity of
carbonic acid, which is gradually disengaged from the former by the
internal heat of the plant.
CAROLINE.
What! have vegetables a peculiar heat, analogous to animal heat?
MRS. B.
It is a circumstance that has long been suspected; but late experiments
have decided beyond a doubt that vegetable heat is considerably above
that of unorganised matter in winter, and below it in summer. The wood
of a tree is about sixty degrees, when the thermometer is seventy or
eighty degrees. And the bark, though so much exposed, is seldom below
forty in winter.
It is from the sap, after it has been elaborated by the leaves, that
vegetables derive their nourishment; in its progress through the plant
from the leaves to the roots, it deposits in the several sets of vessels
with which it communicates, the materials on which the growth and
nourishment of each plant depends. It is thus that the various peculiar
juices, saccharine, oily, mucous, acid, and colouring, are formed; as
also the more solid parts, fecula, woody fibre, tannin, resins, concrete
salts; in a word, all the immediate materials of vegetables, as well as
the organised parts of plants, which latter, besides the power of
secreting these from the sap for the general purpose of the plant, have
also that of applying them to their own particular nourishment.
EMILY.
But why should the process of vegetation take place only at one season
of the year, whilst a total inaction prevails during the other?
MRS. B.
Heat is such an important chemical agent, that its effect, as such,
might perhaps alone account for the impulse which the spring gives to
vegetation. But, in order to explain the mechanism of that operation, it
has been supposed that the warmth of the spring dilates the vessels of
plants, and produces a kind of vacuum, into which the sap (which had
remained in a state of inaction in the trunk during the winter) rises:
this is followed by the ascent of the sap contained in the roots, and
room is thus made for fresh sap, which the roots, in their turn, pump up
from the soil. This process goes on till the plant blossoms and bears
fruit, which terminates its summer career: but when the cold weather
sets in, the fibres and vessels contract, the leaves wither, and are no
longer able to perform their office of transpiration; and, as this
secretion stops, the roots cease to absorb sap from the soil. If the
plant be an annual, its life then terminates; if not, it remains in a
state of torpid inaction during the winter; or the only internal motion
that takes place is that of a small quantity of resinous juice, which
slowly rises from the stem into the branches, and enlarges their buds
during the winter.
CAROLINE.
Yet, in evergreens, vegetation must continue throughout the year.
MRS. B.
Yes; but in winter it goes on in a very imperfect manner, compared to
the vegetation of spring and summer.
We have dwelt much longer on the history of vegetable chemistry than I
had intended; but we have at length, I think, brought the subject to a
conclusion.
CAROLINE.
I rather wonder that you did not reserve the account of the
fermentations for the conclusion; for the decomposition of vegetables
naturally follows their death, and can hardly, it seems, be introduced
with so much propriety at any other period.
MRS. B.
It is difficult to determine at what point precisely it may be most
eligible to enter on the history of vegetation; every part of the
subject is so closely connected, and forms such an uninterrupted chain,
that it is by no means easy to divide it. Had I begun with the
germination of the seed, which, at first view, seems to be the most
proper arrangement, I could not have explained the nature and
fermentation of the seed, or have described the changes which manure
must undergo, in order to yield the vegetable elements. To understand
the nature of germination, it is necessary, I think, previously to
decompose the parent plant, in order to become acquainted with the
materials required for that purpose. I hope, therefore, that, upon
second consideration, you will find that the order which I have adopted,
though apparently less correct, is in fact the best calculated for the
elucidation of the subject.
CONVERSATION XXIII.
ON THE COMPOSITION OF ANIMALS.
MRS. B.
We are now come to the last branch of chemistry, which comprehends the
most complicated order of compound beings. This is the animal creation,
the history of which cannot but excite the highest degree of curiosity
and interest, though we often fail in attempting to explain the laws by
which it is governed.
EMILY.
But since all animals ultimately derive their nourishment from
vegetables, the chemistry of this order of beings must consist merely in
the conversion of vegetable into animal matter.
MRS. B.
Very true; but the manner in which this is effected is, in a great
measure, concealed from our observation. This process is called
_animalisation_, and is performed by peculiar organs. The difference of
the animal and vegetable kingdoms does not however depend merely on a
different arrangement of combinations. A new principle abounds in the
animal kingdom, which is but rarely and in very small quantities found
in vegetables; this is nitrogen. There is likewise in animal substances
a greater and more constant proportion of phosphoric acid, and other
saline matters. But these are not essential to the formation of animal
matter.
CAROLINE.
Animal compounds contain, then, four fundamental principles; oxygen,
hydrogen, carbon, and nitrogen?
MRS. B.
Yes; and these form the immediate materials of animals, which are
_gelatine_, _albumen_, and _fibrine_.
EMILY.
Are those all? I am surprised that animals should be composed of fewer
kinds of materials than vegetables; for they appear much more
complicated in their organisation.
MRS. B.
Their organisation is certainly more perfect and intricate, and the
ingredients that occasionally enter into their composition are more
numerous. But notwithstanding the wonderful variety observable in the
texture of the animal organs, we find that the original compounds, from
which all the varieties of animal matter are derived, may be reduced to
the three heads just mentioned. Animal substances being the most
complicated of all natural compounds, are most easily susceptible of
decomposition, as the scale of attractions increases in proportion to
the number of constituent principles. Their analysis is, however, both
difficult and imperfect; for as they cannot be examined in their living
state, and are liable to alteration immediately after death, it is
probable that, when submitted to the investigation of a chemist, they
are always more or less altered in their combinations and properties,
from what they were, whilst they made part of the living animal.
EMILY.
The mere diminution of temperature, which they experience by the
privation of animal heat, must, I should suppose, be sufficient to
derange the order of attractions that existed during life.
MRS. B.
That is one of the causes, no doubt: but there are many other
circumstances which prevent us from studying the nature of living animal
substances. We must therefore, in a considerable degree, confine our
researches to the phenomena of these compounds in their inanimate state.
These three kinds of animal matter, gelatine, albumen, and fibrine, form
the basis of all the various parts of the animal system; either solid,
as the _skin_, _flesh_, _nerves_, _membranes_, _cartilages_, and
_bones_; or fluid, as _blood_, _chyle_, _milk_, _mucus_, the _gastric_
and _pancreatic juices_, _bile_, _perspiration_, _saliva_, _tears_, &c.
CAROLINE.
Is it not surprising that so great a variety of substances, and so
different in their nature, should yet all arise from so few materials,
and from the same original elements?
MRS. B.
The difference in the nature of various bodies depends, as I have often
observed to you, rather on their state of combination, than on the
materials of which they are composed. Thus, in considering the chemical
nature of the creation in a general point of view, we observe that it is
throughout composed of a very small number of elements. But when we
divide it into the three kingdoms, we find that, in the mineral, the
combinations seem to result from the union of elements casually brought
together; whilst in the vegetable and animal kingdoms, the attractions
are peculiarly and regularly produced by appropriate organs, whose
action depends on the vital principle. And we may further observe, that
by means of certain spontaneous changes and decompositions, the elements
of one kind of matter become subservient to the reproduction of another;
so that the three kingdoms are intimately connected, and constantly
contributing to the preservation of each other.
EMILY.
There is, however, one very considerable class of elements, which seems
to be confined to the mineral kingdom: I mean metals.
MRS. B.
Not entirely; they are found, though in very minute quantities, both in
the vegetable and animal kingdoms. A small portion of earths and sulphur
enters also into the composition of organised bodies. Phosphorus,
however, is almost entirely confined to the animal kingdom; and
nitrogen, but with few exceptions, is extremely scarce in vegetables.
Let us now proceed to examine the nature of the three principal
materials of the animal system.
_Gelatine_, or _jelly_, is the chief ingredient of skin, and of all the
membranous parts of animals. It may be obtained from these substances,
by means of boiling water, under the forms of glue, size, isinglass, and
transparent jelly.
CAROLINE.
But these are of a very different nature; they cannot therefore be all
pure gelatine.
MRS. B.
Not entirely, but very nearly so. Glue is extracted from the skin of
animals. Size is obtained either from skin in its natural state, or from
leather. Isinglass is gelatine procured from a particular species of
fish; it is, you know, of this substance that the finest jelly is made,
and this is done by merely dissolving the isinglass in boiling water,
and allowing the solution to congeal.
EMILY.
The wine, lemon, and spices, are, I suppose, added only to flavour the
jelly?
MRS. B.
Exactly so.
CAROLINE.
But jelly is often made of hartshorn shavings, and of calves’ feet; do
these substances contain gelatine?
MRS. B.
Yes. Gelatine may be obtained from almost any animal substance, as it
enters more or less into the composition of all of them. The process for
obtaining it is extremely simple, as it consists merely in boiling the
substance that contains it with water. The gelatine dissolves in water,
and may be attained of any degree of consistence or strength, by
evaporating this solution. Bones in particular produce it very
plentifully, as they consist of phosphat of lime combined or cemented by
gelatine. Horns, which are a species of bone, will yield abundance of
gelatine. The horns of the hart are reckoned to produce gelatine of the
finest quality; they are reduced to the state of shavings in order that
the jelly may be more easily extracted by the water. It is of hartshorn
shavings that the jellies for invalids are usually made, as they are of
very easy digestion.
CAROLINE.
It appears singular that hartshorn, which yields such a powerful
ingredient as ammonia, should at the same time produce so mild and
insipid a substance as jelly?
MRS. B.
And (what is more surprising) it is from the gelatine of bones that
ammonia is produced. You must observe, however, that the processes by
which these two substances are obtained from bones are very different.
By the simple action of water and heat, the gelatine is separated; but
in order to procure the ammonia, or what is commonly called hartshorn,
the bones must be distilled, by which means the gelatine is decomposed,
and hydrogen and nitrogen combined in the form of ammonia. So that the
first operation is a mere separation of ingredients, whilst the second
requires a chemical decomposition.
CAROLINE.
But when jelly is made from hartshorn shavings, what becomes of the
phosphat of lime which constitutes the other part of bones?
MRS. B.
It is easily separated by straining. But the jelly is afterwards more
perfectly purified, and rendered transparent, by adding white of egg,
which being coagulated by heat, rises to the surface along with any
impurities.
EMILY.
I wonder that bones are not used by the common people to make jelly;
a great deal of wholesome nourishment, might, I should suppose, be
procured from them, though the jelly would perhaps not be quite so good
as if made from hartshorn shavings?
MRS. B.