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these bodies as forming a single group,
under the name of the metallic class.

Still (as Dr. Ure justly remarks,) what-
ever may be the revolutions of chemical
nomenclature, mankind will never cease
to consider as earths, those solid bodies
composing the mineral strata, which are
incombustible, colorless, not convertible
into metals by all the ordinary methods of
reduction, or, when reduced by scientific
refinements, possessing but an evanescent
metallic existence. — Encyc. Amer.



SIR H. DAVY S AGRICULTURALCHEMISTRY.

(Coiitiiim.d from page 9fi, No. 6.)

It has been shown by the experiments
of Mr. Knight, and those made by other
physiologists, that the sap descending
through the bark, after being modified in
the leaves, is the principal cause of the
growth of the tree; thus, if the bark is
wounded, the principal formation of new
hark is on the upper edge of the wound,
and when the wood has been removed,
the formation of new wood takes place
immediately beneath the bark : yet it
would appear from the late observations
of M. Palisot de Beauvois, that the sap
may be transferred to the bark, so as to
exert its nutritive functions, independent
of any general system of circulation. That
gentleman separated different portions of



bark from the rest of the bark in several
trees, and found that in most instances
the separated bark grew in the same
manner as the bark in its natural state.
The experiment was tried with most suc-
cess on the lime tree, the maple and the
lilac; the layers of bark were removed
in August, ISIO, and in the spring of the
next year, in the case of the maple and
the lilac, small annual shoots were pro-
duced in the parts where the bark was
insulated.

The wood of trees is composed of an
external or living part, called alburnum
or sap-wood, and of an internal and dead
part, the heart-wood. The alburnum iS
white, and full of moisture, and in young
trees and annual shoots it reaches even to
the pith. The alburnum is the great
vascular system of the vegetable through
which the sap rises, and the vessels in it
extend from the leaves to the minutest
filaments in the roots.

There is in the alburnum a membranous
substance composed of cells which are
constantly filled with the sap of the plant,
and there are in the vascular system
several different kinds oi tubes. Mirbel
has distinguished four species, the 5/my;/e
tubes, the porous tubes, the tracheas, and
the false tracheae. — The tubes, which he
has called simple tubes, seem to contain
the resinous or oily fluids peculiar to
different plants.

The porous tubes likewise contain these
fluids; and their use is probably that of
conveying them into the sap for the pro-
duction of new arrangements.

The tracheae contain fluid matter, which
is always thin, watery, and pelucid, and
these organs, as well as the false tracheae,
probably carry off water from the denser
juices, which are thus enabled to con-
solidate for the production of new woods.

In the arrangement of the fibres of the
wood, there are two distinct appearances.
There are series of white and shining
laminte which shoot from the centre
towards the circumference, and these con-
stitute what is called the silver grain of
the wood.

There are likewise numerous series of
concentric layers which are usually called
the spurious grain, and their number
denotes the age of the tree.



SIR H. DAVY S AGRICULTURAL CHEMISTRY.



105



The silver grain is elastic and con-
tractile, and it has been supposed by Mr.
Knight, that the change of volume pro-
duced in it by change of temperature is
one of the principal causes of the ascent
of the sap. The fibres of it seem always
to expand in the morning and contract
at night ; and the ascent of the juices, as
was stated in the last lecture, depends
principally on the agency of heat. The
silver grain is most distinct in forest trees;
but even annual shrubs have a system of
fibres similar to it. The analogy of
nature is constant and uniform, and
similar effects are usually produced by
similar organs.

The pith occupies the centre of the
wood, its texture is membranous ; it is
composed of cells, which are circular
towards the extremity', and hexagonal in
the centre of the substance. In the first
infancy of the vegetable, the pith occupies
but a small space. It gradually dilates,
and in annual shoots and young trees
offers a considerable diameter. In the
more advanced age of the tree, acted on
by the heart-wood, pressed by the new
layers of the alburnum, it begins to
diminish, and in very old forest trees
disappears altogether.

Many different opinions have prevailed
with regard to the use of pith. Dr. Hales
supposed, that it was the great cause of
the expansion and development of the
other parts of the plants, that being the
most interior, it was likewise most acted
upon of all the organs, and that from its
reaction the phenomena of their growth
resulted.

Linnaeus, whose lively imagination was
continually employed in endeavors to
discover analogies between the animal
and vegetable systems, conceived " that
the pith performed for the plant the same
functions as the brain and nerves in an-
imated beings." He considered it as the
organ of irritability and the seat of life.

The latest discoveries have proved, that
these two opinions are equally erroneous.
Mr. Knight has removed the pith in
several young trees and they continued
to live and to increase.

It is evidently then only an organ of
secondary importance. In early shoots,
in vigorous growth it is filled with mois-



ture, and it is a reservoir, perhaps, of
fluid nourishment, at the time it is most
wanted. As the heart-wood forms, it is
more and more separated from the living
part, the alburnum; its functions become
extinct, it diminishes, dies, and at last dis-
appears.

The tendrils, the spines, and other
similar parts of plants are analogous in
their organization to the branches, and
offer a similar cortical and alburnous or-
ganization. It has been shown, by the
late observations of Mr. Knight, that the
directions of tendrils, and the spinal form
they assume depend upon the unequal
action of light upon them, and a similar
reason has been assigned by Mr. Decan-
dolle to account for the turning of plants
towards the sun ; that ingenious physio-
logist supposes that the fibres are short-
ened by the chemical agency of the solar
rays upon them, and that consequently,
the parts will move towards the light.

The leaves, the great sources of the
permanent beauty of vegetation, though
infinitely diversified in their forms are in
all cases similar in interior organization,
and perform the same functions.

The alburnum spreads itself from the
foot stalks, into the very extremity of the
leaf; it retains its vascular system and
its living powers; and its peculiar tubes,
particularly the tracheas, may be distinctly
seen in the leaf.

The green membranous substance may
be considered as an extension of the
parenchyma, and the fine and thin cover-
ing as the epidermis. Thus the organiza-
tion of the roots and branches may be
traced into the leaves, which present,
however, a more perfect, refined and
minute structure.

The great use of the leaves is for the
exposure of the sap to the influence of ihe
air, heat, and light. Their surface is ex-
tensive, the tubes and cells very delicate,
and their texture porous and transparent.
In the leaves much of the water of the
sap is evaporated; it is combined with new
principles and fitted for its organizing
functions, and probably passes in its pre-
pared state, from the extreme tubes of the
alburnum into the ramifications of the
cortical tubes, and then descends through
the bark.



106



SIR H. DAVY S AGRICULTURAL CHEMISTRY.



On the u])per surface of the leaves,
which is ex])osed to the sun, the epidermis
is thick but transparent, and is composed
of matter possessed of little organization
which is either principally earthy, or
consists of some homogeneous chemical
substance. In the grasses it is partly
siliceous, in the laurel resinous, and in the
inaple and thorn it is principally con-
stituted by a substance analogous to wax.
By these arrangements any evaporation,
except from the appropriated tubes, is
prevented.

On the lower surface the epidermis
is a thin transparent membrane full of
cavities, and it is probably altogether by
this surface that moisture and the princi-
ples of the atmosphere necessary to vege-
tation are absorbed. If a leaf be turned,
so as to present its lower surface to the
sun, its fibres will twist so as to bring it
as much as possible into its original posi-
tion; and all leaves elevate themselves on
the foot stalk during their exposure to the
solar light, and, as it were, move toward
the sun.

This effect seems in a great measure
dependant upon the mechanical and
chemical agency of light and heat. Bonnet
made artificial leaves, which when a moist
sponge was held under the lower surface
and a heated iron above the upper surface,
turned exactly in the same manner as the
natural leaves.

This, however, can be considered only
as a very rude imitation of the natural
process.

What Linnaeus has called the sleep of
the leaves, appears to depend wholly
upon the defect of the action of light and
heat, and the excess of the operation of
moisture.

This singular but constant phenomenon
had never been scientifically observed, till
the attention of the botanist of Upsal was
fortunately directed to it. He was ex-
amining particularly a species of lotus, in
which four flowers had appeared during
the day, and he missed two in the even-
ing ; by accurate inspection he soon dis-
covered that these two were hidden by
the leaves which had closed round them.
Such a circumstance could not be lost
upon so acute an observer.

He immediately took a lantern, went



into the garden and witnessed a series of
curious facts before unknown. All the
simple leaves of the plants he examined,
had an arrangement totally different from
their arrangement in the day; and the
greater number of them were seen closed
or folded together.

The sleep of leaves is in some cases
capable of being produced artificially.
DecandoUe made this experiment on the
sensitive plant. By confining it in a dark
place in the day time, the leaves soon
closed; but on illuminating the chamber
with many lamps, they again expanded.
So sensible were they to the effects of
light and radiant heat.

In the greater number of plants the
leaves annually decay, and are reproduced;
their decay takes place either at the con-
clusion of summer, as in very hot climates,
when they are no longer supplied with
sap, in consequence of the dryness of soil
and the evaporating powers of heat; or
in the autumn, as in the northern climates
at the commencement of the frosts. The
leaves preserve their functions in common
cases no longer than there is a circulation
of fluids through them. In the decay of
the leaf, the color assumed seems to de-
pend upon the nature of the chemical
change, and the acids are generally de-
veloped, it is usually either reddish brown
or yellow; yet there are great varieties.
Thus in the oak, it is bright brown ; in
the beech, orange; in the elm, yellow; in
the vine, red; in the sycamore, dark
brown; in the cornel tree, purple; and in
the woodbine, blue.

The cause of the preservation of the
leaves of evergreens through the winter
is not accurately known.

From the experiments of Hales it ap-
pears that the force of the sap is much
less in the plants of this species, and
probably there is a certain degree of
circulation throughout the winter; their
juices are less watery than those of other
plants, and probably less liable to be con-
gealed by cold, and they are defended by
stronger coatings from the action of the
elements.

The production of the other parts of
the plant takes place at the time the leaves
are most vigorously performing their
functions. If the leaves are stripped off



SIR H. DAVYS AGRTCULTITRAL CHEMISTRY.



107



from a tree in the spring it uniformly
dies, and when many of the leaves of
forest trees are injured by blasts, the
trees always become stagheaded and un-
healthy.

The leaves are necessary for the exist-
ence of the individual tree, the flowers
for the continuance of the species. Of all
tile parts of the plants they are the most
refined, the most beautiful in their struc-
ture and appear as the master-work of
nature in the vegetable kingdom. The
elegance of their lints, the variety of their
forms, the delicacy of their organization,
and the adaptation of their parts, are all
calculated to awaken our curiosity, and
excite our admiration.

In the flower there are to be observed,
1st, the calyx, ov green membranous part
forming tlie support of the colored floral
leaves. This is vascular, and agrees with
the common leaf in its texture and organi-
zation; it defends, supports, and nourishes
the more perfect parts, 2d, the corolla,
which consists either of a single piece,
when it is called monopetalous, or of
jrtany pieces, when it is called polypetal-
ous. It is usually very vivid in its color,
is filled with an almost infinite variety of
small tubes of the porous kind; it incloses
and defends the essential parts in the in-
terior, and supplies the juices of the sap
to them. These parts are, 3d, the stamens
and the pistils.

The essential part of the stamens are
the summit or anthers, which are usually
circular and of a highly vascular texture,
and covered with a fine dust called the
pollen.

The pistil is cylindrical, and surmount-
ed by the style; the top of which is
generally round and protuberant.

In the pistil, when it is examined by
the microscope, congeries of spherical
forms may usually be perceived, which
seem to be the bases of future seeds.

It is upon the arrangement of the
stamens and the pistils, that the Linnaean
classification is founded. The numbers
of the stamens and pistils in the same
flower, their arrangements, or their divi-
sion in different flowers, are the circum-
stances which guided the Swedish philo-
sopher, and enabled him to form a system
admirably adapted to assist the memory,



and render botany of easy acquisition; and
which, though it does not always associate
together the plants most analogous to each
other in their general characters, is yet so
ingeniously contrived as to denote all the
analogies of their most essential parts.

The pistil is the organ which contains
the rudiments of the seed, but the seed
is never formed as a reproductive germ,
without the influence of the pollen, or
dust on the anthers.

This mysterious impression is neces-
sary to the continued succession of the
different vegetable tiibes. It is a feature
which extends the resemblances of the
different orders of beings, and establishes,
on a great scale, the beautiful analogy of
nature.

The ancients had observed, that different
date trees, bore different flowers, and that
those trees producing flowers which con-
tained pistils bore no fruit, unless in the
immediate vicinity of such trees as pro-
duced flowers containing stamens. This
long established fact strongly impressed
the mind of Malpighi, who ascertained
several analogous facts with regard to
other vegetables. Grew, however, was
the first person who attempted to general-
ize upon them, and much just reasoning
upon the subject may be found in his
works. Linnaeus gave a scientific and
distinct form to that which Grew had
only generally observed, and has the
glory of establishing what has been called
the sexual system, upon the basis of
minute observations and accurate ex-
periments.

The seed, the last production of vigor-
ous vegetation, is wonderfully diversified
in form. Being of the highest importance
to the resources of nature, it is defended
above all other jiarts of the plant; by soft
pulpy substances, as in the esculent fruits,
by thick membranes, as in the lugumin-
ous vegetables, and by hard shells, or a
thick epidermis, as in the palms and
grasses. In every seed there is to be
distinguished, 1st, the organ of nourish-
ment; 2d, the nascent plant, or the plume;
3d, the nascent root, or the radicle.

In the common garden bean, the organ
of nourishment is divided into two lobes
called cotyledons; the plume is the small
white point between the upper part of the



108



SIR H. DAW S AGRICULTURAL CHEMISTRY.



lobes, and radicle is the small curved cone
at their base.

In wheat, and in many of the grasses,
the organ of nourishment is a single part,
and these plants are called monocotyle-
dunous. In other cases it consists of
more than two parts, when the plants are
called phycotyledonous. In the greater
number of instances, it is, however, simply
divided into two, and is dicotyledonous.

The matter of the seed, when examined
in its common state, appears dead and
inert; it exhibits neither the forms nor
the functions of life. But let it be acted
upon by moisture, heat and air, and its or-
ganized powers are soon distinctly deve-
loped. The cotyledons expand, the mem-
branes burst, the radicle acquires new mat-
ter, descends into the soil, and the plume
rises towards the free air. By degrees, the
organs of nourishment of dicotj.'ledonous
plants become vascular, and are converted
into seed leaves, and the perfect plant ap-
pears above the soil. Nature has pro-
vided the elements of germination on
every part of the surface; water and pure
air and heat are universally active, and
the means for the preservation and multi-
plication of life, are at once simple and
grand.

To enter into more minute details on
the vegetable physiology would be in-
compatible with the objects of these lec-
tures. I have attempted only to give
such general ideas on the subject, as may
enable the philosophical agricultui'ist to
understand the functions of plants; those
who wish to study the anatomy of vege-
tables, as a distinct science, will find
abundant materials in the works of the
authors I have quoted, page 9, and like-
wise in the writings of Linnseus, Desfon-
taines, Decandolle, de Saussure, Bonnet,
and Smith.

The history of the peculiarities of
structure in the different vegetable classes,
rather belong to botanical than agricul-
tural knowledge. As I mentioned in the
commencement of this lecture, their or-
gans are possessed of the most distinct
analogies, and are governed by the same
laws. In the grasses and palms, the cor-
tical layers are larger in proportion than
the other parts; but their uses seem to be
the same as in forest trees.



In bulbous roots, the alburnous sub-
stance forms the largest part of the vege-
table; but in all cases it seems to contain
sap, or solid materials deposited from the
sap. The slender and comparatively dry
leaves of the pine and the cedar perform
the same functions as the large and juicy
leaves of the fig tree, or the walnut.

Even in the cryptogamia, where no
flowers are distinct, still there is every
reason to believe that the production of
seed is effected in the same way as in the
more perfect plants. The mosses, and
lichens, which belong to this family, have
no distinct leaves, or roots, but they are
furnished with filaments which perform
the same functions; and even in the fungus
and the mushroom there is a system for
the absorption and aeration of the sap.

It was stated in the last lecture, that all
the different parts of plants are capable of
being decomposed into a few elements.
Their uses as food, or for the purposes
of the arts, depend upon compound ar-
rangements of those elements which are
capable of being produced either from
their organized parts, or from the juices
they contain; and the examination of the
nature of these substances, is an essential
part of agricultural chemistry.

Oils are expressed from the fruits of
many plants, resinous fluids exude from
the wood; sacharine matters are afforded
by the sap ; and dying materials are
furnished by the leaves, or petals of
flowers ; but particular processes are ne-
cessary to separate the different compound
vegetable substances from each other, such
as maceration, infusion, or digestion in
water, or spirits of wine; but the applica-
tion and the nature of these processes will
be better understood, when the chemical
nature of the substances is known ; the
consideration of them will, therefore, be
reserved for another place in this lecture.

The compound substances found in ve-
getables are, 1, gum or mucilage, and its
different modifications; 2, starch; 3, sugar;
4, albumen ; 5, gluten; 6, gum elastic;
7, extract; 8, tannin ; 9, indigo; 10, nar-
cotic principle; 11, bitter principle; 12,
wax; 13, resins; 14, camphor; 15, fixed
oils; 16, volatile oils; 17, woody-fibre;
IS, acids ; 19, alkalies ; earths, metallic
oxides and saline compounds.



SIR H. Davy's agricultural chemistry.



109



I shall describe generally the properties
and composition of these bodies, and the
manner in which they are procured.

1. Gum is a substance which exudes
from certain trees; it appears in the form
of a thick fluid, but soon hardens in the
air, and becomes solid; when it is white,
or yellowish, white more or less trans-
parent, and somewhat brittle, its specific
gravity varies from 1300 to 1490.

There is a great variety of gums, but
the best known are gum arabic, gum
Senegal, gum tragacanth, and the gum of
the plum or cherry tree. Gum is soluble
in water, but not soluble in spirits of wine.

If a solution of gum be made in water,
and spirits of wine or alcohol be added
to it, the gum separates in the form of
white flakes. Gum can be made to in-
flame only with difficulty ; much mois-
ture is given off" in the process, which
takes place with a dark smoke and feeble
blue flame, and a coal remains.

The characteristic poperties of gum are
its easy solubility in water, and its inso-
lubility in alcohol. Diflferent chemical
substances have been proposed for ascer-
taining the presence of gum, but there is
reason to believe that few of them afford
accurate results ; and most of them (par-
ticularly the metallic salts,) which pro-
duce changes in solutions of gum, may
be conceived to act rather upon some sa-
line compounds existing in the gum, than
upon the pure vegetable principle. Dr.
Thomson has proposed an aqueous solu-
tion of silica in potassa as a test of the
presence of gum ; in solutions he states
that the gum and silica are precipitated
together : — this test, however, cannot be
applied with correct results in cases when
acids are present.

Mucilage must be considered as a va-
riety of gum ; it agrees with it in its most
important properties, but seems to have
less attraction for water.

According to Hermbstadt, when gum
and mucilage are dissolved together in
water, the mucilage may be separated by
means of sulphuric acid ; — mucilage may
be procured from linseed, from the bulbs
of the hyacinth, from the leaves of the
marsh-mallows, from several of the lich-
ens, and from many other vegetable sub-
stances.



From the analysis of M.M. Gay Lus-
sac and Thenard, it appears that gum
arabic contains in 100 parts of carbon
42.23, of oxygen 50.84, and of hydrogen
6.93, with a small qantity of saline and
earthy matter, or of carbon 42.23, oxy-
gen and hydrogen in the proportions
necessary to form water — 57.77. This
estimation agrees very nearly with the
definite proportions of 11 of carbon, 10
of oxygen, and 20 of hydrogen.

All the varieties of gum and mucilage
are nutritious as food. They either par-
tially or wholly lose their solubility in
water by being exposed to a heat of 500°
or 600° Fahrenheit, but their nutritive
powers are not destroyed unless they
are decomposed. Gum and mucilage are
employed in some of the arts, particu-
larly in calico printing ; till lately, in this
country the calico printers used gum ara-
bic, but many of them at the suggestion of
Lord Dundonald, now employ the mu-
cilaoe from lichens.

2. Starch is procured from different
vegetables, but particularly from wheat
or potatoes. To make starch from wheat,
the grain is steeped in cold water till it
becomes soft, and yields a milky juice
by pressure ; it is then put into sacks of
linen, and pressed in a vat filled with
water : as long as any milky juice ex-
udes, the pressure is continued ; the fluid
gradually becomes clear and a white pow-
der subsides, which is starch.

Starch is soluble in boiling water, but
not in cold water, nor in spirits of wine.



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