the wood, which are perceived very distinctly in a transverse section,
in which the ends of the vessels are seen as cut through by the knife.
The scarlet oak of America is recommended as a proper object for
exhibiting them. If a short cylinder of a three years branch of this
oak, a little macerated, be hollowed away with a chissel, we shall see
what a large portion of the wood is occupied by these vessels; they are
thick and strong, and it is easy, with some care and attention, to
loosen several of them.
If a number of these thus separated be put into a vial of rain water,
and frequently shook for several days, some will at length be found
perfectly clean; these are then to be put into spirit of wine, and when
that has been two or three times changed, they will be in a condition to
be viewed for understanding their structure; another method of
preparation has already been shewn in page 162.
These are the vessels which have been called by some writers air, by
others, tracheal vessels. It is, however, to be remarked, that most of
those who have considered them as air vessels, refer us to the tree
while in a more herbaceous state; in this case they say, that we shall
find these parts filled with a fine spiral filament. As these vessels
are often to be found empty, they have been supposed to answer the
purposes of lungs to the plant. Malpighi asserts, that if they be
examined in winter, they often exhibit a vermicular motion, which
astonishes the spectator.
Those who suppose the corona to contain the whole structure of the tree
in miniature, and that it is the embryo of future shoots, suppose it to
contain the vessels proper for each part, a subject that must be left to
the decision of future observers.
OF THE VASA PROPRIA INTIMA.
These are the only vessels which remain to be spoken of. They are large,
conspicuous, and important; their natural place is in the blea, though
they are sometimes repeated in the wood and the corona. Their coats are
thicker than those of any other vessels.[141] It is not difficult, after
a successful maceration, to separate some of these vessels from the
blea; in this state they appear perfect cylinders, with thick white
coats, of a firm, solid, and uniform texture.
[141] Hill’s Construction of Timber, p. 83 and 85.
It has generally been supposed, that each of those concentric circles,
which are to be observed in the transverse section of almost every tree,
was the product of one year, or the quantity of wood added to the tree
in that space; here, however, Dr. Hill differs again from the general
opinion.
From what has been said, we may deduce the following general ideas
relative to the organization of trees. The most obvious and remarkable
parts of a plant, or tree, are the root, the stem, the branches, the
leaves, the flower, and the fruit. The component parts of these
divisions are not complicated; they are simple when compared with those
of an animal, and this because the offices of the vegetable are fewer
than those of the animal.
The interior part may be considered as consisting of ligneous fibres,
interspersed with a vast number of bladders, which are here named the
cellular tissue, the vasa propria, and the sap vessels; though these are
considered by some writers as mere air vessels.
The ligneous fibres are very fine tubes, proceeding nearly in a vertical
direction from the top to the bottom of the tree; they are sometimes
parallel to each other, sometimes they divaricate, and often leave
oblong intervals or spaces. There is great reason for supposing them to
be a species of lymphatic vessels. The vacant spaces between these
fibres are filled up by a vesicular membrane, lying in an horizontal
direction, and which is called in this chapter the cellular tissue.
The vasa propria are formed of ligneous fibres, but differ from the
foregoing in their size, and in the juices which they contain. In the
part properly called the wood, we meet with the sap vessels; but as in
some states they seem as if they were formed of a silver-coloured spiral
membrane, and are found without any juices, they have been supposed to
be air vessels, and called the trachea, making up an arterial system,
and supplying the place of the heart in animals.
The interior part of the tree may be further considered as divided into
four principal concentric strata, the bark, the blea, the wood, and the
pith; to these Dr. Hill has added the corona. Whatever part of a plant
is examined, we find these and no more. The root, its ascending stalk,
and descending fibre, are formed of one, and not three different
substances. Thus the whole vegetable is reduced to one entire body. And
what appears in the flower to be formed of altogether distinct parts,
will be found to originate in these.
The bark, which is the exterior covering of the tree, is divided into
two parts, a thin outer rind, and a much thicker inner one. The exterior
one seems to be little more than a fine film of irregular meshes, the
inner one composed of large blebs, leaving in some subjects large vacant
spaces, which form its vasa propria. It is made up of several strata
lying one over the other.
Next to this is the blea, which is of an uniform structure. It is an
imperfect wood, waiting only for the hand of time to be brought to
perfection. The duration of the blea in this middle state depends on the
internal powers and strength of the tree, being so much shorter as this
is more vigorous.
The wood, including the corona, comes next; it differs in density and
duration both from the blea, the bark, and the wood. It is made up of
strong fibres. The life of the vegetable seems to reside in it; from it
all the other parts are produced. It shoots a pith inwards, and a blea
and a bark outwards.
Every tree may be considered as consisting of numerous concentric strata
or flakes, forming so many cones, inscribed one within the other, and
whose number is almost indefinite. The most exterior contain the
rudiments of the bark; the more interior, those of the wood. In the
germ they are gelatinous, by degrees they become herbaceous, and in
process of time assume the consistence of wood. Thus the stem, the root
and the branch, may be considered as formed of a prodigious number of
concentric vertical strata, each composed of different fascicles of
fibres; which fibres are again formed of smaller ones. The spaces
between these, and among the fibres, are filled up, interwoven with, and
connected by the cellular tissue, of which the radial insertions are
formed.
The strata harden successively one after the other; the most interior
stratum is that which hardens first; this is then covered by another
which is more ductile and herbaceous, and so on; so that the bulk of the
tree is increased every year by the accession of an hollow cylinder of
wood derived from the internal bark. From the extension in breadth, the
tree acquires bulk; from that in length it gains its height. The strata
gradually diminish in size as they gain in length; from hence the
conical figure of the root, stem, and branch. All the parts of the plant
are the same, differing in nothing more than in shape and size. The
roots are sharp and pointed, that they may make their way more readily
through the earth. The leaves are broad, that they may more effectually
catch the moisture from the atmosphere, &c. When the root of a tree is
elevated above, instead of being retained under the earth, it assumes
the appearance of a perfect plant, with leaves and branches. Experiment
shews that a young tree may have its branches placed in the earth, and
its roots elevated in the air, and in that inverted state it will
continue to live and grow. The principal source of the phænomena of
vegetation is the simplicity and uniformity of their organization.
The figures in Plates XXVIII. XXIX. and XXX. are portions of transverse
sections of trees and herbs. The sections were cut by Mr.
Custance,[142] who first brought this art to perfection, and remains
hitherto unrivalled in these performances.
[142] For a collection of Mr. Custance’s vegetable cuttings, and
which, in sets, usually accompany the best sort of microscopes, made
by Messrs. Jones, see the list of microscopical objects now annexed to
this work by the editor.
Plate XXVIII. Fig. 1, exhibits a piece of an herb growing on rubbish,
and known by the name of fat-hen:[143] Fig. 2, a microscopic view of the
same. Fig. 3, a magnified representation of a section of a reed that
comes from Portugal: Fig. 4, the real size of the section.
[143] Chenopedium bonus Henricus.
Plate XXIX. Fig. 1, is a magnified view of a section of the althea
frutex: Fig. 2, the natural size of the section. Fig. 3, a magnified
view of a section of the hazel: Fig. 4, its natural size. Fig. 5, a
microscopic view of a section of a branch of the lime-tree: Fig. 6
represents its natural size.
Plate XXX. Fig. 1, a magnified view of a section of the sugarcane: Fig.
2, its natural size. Fig. 3, a magnified view of a section of the bamboo
cane: Fig. 4, the natural size. Fig. 5, a magnified view of a section of
the common cane: Fig. 6, the real size.
CHAP. X.
OF THE CRYSTALLIZATION OF SALTS, AS SEEN BY THE MICROSCOPE; TOGETHER
WITH A CONCISE LIST OF OBJECTS.
Crystallization, in general, signifies the natural formation of any
substance into a regular figure, resembling that of a natural crystal.
Hence the phrases of the crystallized ores, crystallized salts, &c. and
even the basaltic rocks are now generally reckoned to be effects of this
operation; the term, however, is most commonly applied to bodies of the
saline kind; and their separation in regular figures from the water, or
other fluid in which they were dissolved, is called their
crystallization. If the word crystallization were to be confined to its
most proper sense, as it seems to have been formerly, it could only be
applied to operations by which certain substances are disposed to pass
from a fluid to a solid state, by the union of their parts, which so
arrange themselves, that they form transparent and regularly-figured
masses, like native crystal; from which resemblance the word
crystallization has evidently been taken.[144]
[144] Macquer’s Dictionary of Chemistry, Art. Crystallization.
But modern chemists and naturalists have much extended this expression,
and it now signifies a regular arrangement of the parts of any body
which is capable of it, whether the masses so arranged be transparent or
not. Thus opake stones, pyrites, and minerals when regularly formed, are
said to be crystallized, as well as transparent stones and salts.
The opacity and transparency of substances are justly disregarded, in
considering whether they be crystallized or not; for these qualities are
perfectly indifferent to the regular arrangement of the integrant parts
of substances, which is the essential object of crystallization.
This being established, crystallization may be defined, an operation by
which the integrant parts of a body, separated from each other by the
interposition of a fluid, are disposed to unite again, and to form
solid, regular, and uniform masses.
To understand as much as we can of the mechanism of crystallization, we
must remark,
1. That the integrant parts of all bodies have a tendency to each other,
by which they approach, unite, and adhere together, when not prevented
by an obstacle.
2. That in bodies simple or little-compounded, this tendency of
integrant parts is more obvious and sensible than in others more
compounded; hence the former are much more disposed to crystallize.
3. That although we do not know the figure of the primitive integrant
molecules of any body, we cannot doubt but that those of every different
body have a constantly uniform and peculiar figure.
4. That these integrant parts cannot have an equal tendency to unite
indiscriminately by any of their sides, but by some preferably to
others, excepting all the sides of an integrant part of a body be equal
and similar; and probably the sides, by which they tend to unite, are
those by which they can touch most extensively and immediately.
The most general phænomena of crystallization may be conceived in the
following manner:
Let a body be supposed to have its integrant parts separated from each
other by some fluid; if a part of this fluid be taken away, these
integrant parts will approach together: and, as the quantity of
intervening fluid diminishes, they will at last touch and unite. They
may also unite when they come so near to each other, that their mutual
tendency shall be capable of overcoming the distance betwixt them. If,
besides, they have time and liberty to unite with each other by the
sides most disposed to this union, they will form masses of a figure
constantly uniform and similar. For the same reason, when the interposed
fluid is hastily taken away, so that the integrant parts shall be
approximated, and be brought into contact before they have taken the
position of their natural tendency, then they will join confusedly by
such sides as chance presents to them; they will, in such circumstances,
form solid masses, whose figures will not be determinate, but irregular
and various.
Different salts assume different figures in crystallization, and are, by
these means, easily distinguished from one another. But besides the
large crystals produced in this way, each salt is capable of producing a
very different appearance of the crystalline kind, when only a drop of
the saline solution is made use of, and the crystallization viewed
through a microscope. For our knowledge of this species of
crystallization, we are indebted to Mr. Henry Baker, who was presented
by the Royal Society with a gold medal for the discovery, in the year
1744. These microscopical crystals he distinguishes from the larger ones
by the name of configurations; but this term seems inaccurate, and the
distinction may be properly preserved by calling the large ones the
COMMON, and the small ones the MICROSCOPICAL, crystals of the salt.
It has not yet been shewn by any writer on the subject, why salts should
assume any regular figure, much less why every one should have a form
peculiar to itself. Sir Isaac Newton endeavoured to account for this, by
supposing the particles of salt to be diffused through the solvent
fluid, at equal distances from each other; and that then the power of
the attraction between the saline particles could not fail to bring them
together in regular figures, as soon as the diminution of heat suffered
them to act on each other. But it is certain some other agent must be
concerned in this operation, besides mere attraction, otherwise all
salts would crystallize in the same manner. Others have, therefore, had
recourse to some kind of polarity in the particles of each salt, which
determined them to arrange themselves in such a certain form; but unless
we give a reason for this polarity, we only explain crystallization by
itself. One thing seems to have been overlooked by those who have
endeavoured to investigate this subject, namely, that the saline
particles do not only attract one another, but they also attract some
part of the water which dissolves them.
Did they only attract each other, the salt, instead of crystallizing,
would fall to the bottom as a powder; whereas, a saline crystal is
composed of salt and water, as certainly as the body of an animal is
composed of flesh and blood, or a vegetable of solid matter and sap; if
a saline crystal be deprived of its aqueous part, it will as certainly
lose its crystalline form, as if it were deprived of the saline part. It
is, therefore, not improbable, that crystallization is a species of
vegetation, and is accomplished by the same powers to which the growth
of plants and animals is to be ascribed. Some kinds of crystallization
resemble vegetation so much, that we can scarce avoid attributing them
to the same cause.
It has been imagined, that all the great operations in nature may be
reduced to two principles, those of crystallization and organization;
but that often they are so concealed, as to be invisible. Hence
crystallized substances have been frequently mistaken for organized
ones, and vice versa. They differ, however, essentially in their growth
and origin. Organized beings spring from a germ, in which all the
essential parts are concentrated, and they grow by intusception; whereas
crystallized substances increase by the successive apposition of certain
molecules of a determined figure, which unite in one common mass. Thus
crystallized beings do not grow, properly speaking, though their
substance is augmented, they are not preformed, but formed daily.
The phænomena of crystallization have much engaged the attention of
modern chemists, and a vast number of experiments has been made with a
view to determine exactly the different figures assumed by salts in
passing from a fluid to a solid form. It does not, however, appear, from
all that has yet been done, that any certain rule can be laid down in
these cases, as the figure of saline crystals may be varied by the
slightest circumstances. Thus, sal ammoniac, when prepared by a mixture
of pure volatile alkali with spirit of salt, shoots into crystals
resembling feathers; but if, instead of a pure alkali, we make use of
one just distilled from bones, and containing a great quantity of animal
oil, we shall, after some crystallizations of the feathery kind, obtain
the very same salt in the form of cubes.
Such salts as are sublimeable crystallize not only in the aqueous way by
solution and evaporation, but also by sublimation; and the difference
betwixt the figures of these crystals is often very remarkable. Thus,
sal ammoniac, by sublimation never exhibits any appearance of feathery
crystals, but always forms cubes or parallelopipeds. This method of
crystallizing salts by sublimation has not as yet been investigated by
chemists; nor indeed does the subject seem capable of investigation
without much trouble, as the least augmentation of the heat beyond the
proper degree would make the crystals run into a solid cake, while a
diminution of it would cause them to fall into powder. In aqueous
solutions, too, the circumstances which determine the shapes of the
crystals are innumerable; and the degree of heat, the quantity of salt
contained in the liquor, nay, the quantity of the liquor itself, and the
various constitutions of the atmosphere at the time of crystallization,
often occasion such differences as seem quite unaccountable and
surprizing.
Mr. Bergman has given a dissertation on the various forms of crystals;
which, he observes, always resemble geometrical figures more or less
regular. Their variety at first appears infinite; but by a careful
examination it will be found, that a great number of crystals, seemingly
very different from each other, may be produced by the combination of a
small number of original figures, which therefore he thinks may be
called primitive. On this principle he explains the formation of the
crystalline gems, as well as salts.[145]
[145] Encycl. Britan. Vol. V. p. 583.
It has been already shewn, page 163, how to prepare the various salts
for microscopical observations. The beautiful crystallizations
represented in Plates XXXI. and XXXII. were produced in the manner there
described.
Plate XXXI. Fig. 2, exhibits a view of the microscopical crystals of
nitre. These shoot from the edges with very little heat, in flattish
figures, of various lengths, and exceedingly transparent, the sides
nearly parallel, though rather jagged, and tapering to a point; after a
number of these are formed, they often dissolve under the eye, and
disappear entirely; but in a little time new shoots will push out, and
the process go on afresh. Beautiful ramifications are formed round the
edge, and many regular figures are to be observed in different parts of
the drop. Fig. 1 is the real size of the drop.
Fig. 4 is a drop of distilled verdigrise, as it appeared when viewed by
the microscope. There is a difference in the appearance from this
substance, according as the time of the application is nearer to, or
more distant from that in which the solution was made. Fig. 3, the size
of the drop.
If a drop of distilled verdigrise upon glass be viewed through the
microscope, after the crystallization is completed and the water
evaporated, there remains a substance round the crystallization, which
preserves the original size and shape of the drop when a liquid; betwixt
this verge of the drop and the crystals fine lines are discernible
running from the crystals to the circumference of the drop, at various
angles with the crystals; whatever direction they take, they are always
perfectly straight, and of an equal thickness throughout. When the drop
is viewed through a light ground, these lines appear dark; but when
viewed through a dark ground, they then shine and appear of the
beautiful green colour natural to the crystals of verdigrise.
Plate XXXII. Fig. 1, represents the microscopical appearance of the
crystals of salt of wormwood. The shootings from the edges of this
solution are often very thick in proportion to their length, their sides
full of notches, the ends generally acute; many spear-like forms are
also to be observed, as well as little crystals of a variety of figures.
Fig. 2. Salt of amber. The shootings of this salt are highly
entertaining, though the process is very slow; many spiculæ shoot from
the edge towards the middle of the solution, and from the pointed ends
of the spiculæ a great variety of diversified branches may be observed,
variously divided and subdivided, and forming at last, says Baker, a
winter scene of trees without leaves.
Fig. 3. Salt of hartshorn. This salt shoots out from the edge of the
drop into solid, thick, and rather opake figures; from these it often
shoots into branches of a rugged appearance, similar to those of some
species of coral.
Fig. 4 represents the microscopical crystals of sal ammoniac. These form
a most beautiful object in the microscope; a general idea may be more
easily acquired by attentively viewing the figure here exhibited, than
by any verbal description.[146]
[146] A collection of salts, as recommended by Mr. Baker, properly
prepared and packed in portable boxes by Messrs. Jones, the reader
will see in the extensive list of microscopic objects now annexed to
this work by the editor.
A
CONCISE LIST OF OBJECTS
FOR
_THE MICROSCOPE_.
The short list here presented to the reader must, from the nature of the
subject, be very imperfect; for the whole of the animal, vegetable, and
mineral kingdoms, with all their numerous subdivisions, furnish objects
for the microscope; and there is not one of them, that, when properly
examined, will not afford instruction and entertainment to the rational
investigator of the works of creation. The Systema Naturæ of Linnæus may
therefore be regarded as a catalogue of universals for microscopic
observation, each of which comprehends a variety of particulars. The
list here given can be considered as little more than a directory, to
point out to those who have only begun to study this part of natural
history a few of those objects which merit their attention, and which,
from their beauties, may incite them to pursue the study with greater
ardor.
OF OPAKE OBJECTS.