Part 47

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.

Ores and minerals afford an immense variety of very beautiful and splendid objects. From amongst these the observer may select the peacock or coloured copper ore, green crystallized ditto, lead ore, crystallized ditto, crystals of lead, small grained marcasites, coloured mundic, cinnabar, native sulphur, needle and other antimony, moss copper, &c. A mixture of small pieces of ores, &c. of different kinds, produces a pleasing effect. Sands in general exhibit something not discoverable with the naked eye. Sand from the sea-shore is often intermixed with minute shells, particularly that from Rimini, in Italy. Mr. Walker has published a specimen of the small microscopic shells which are found on our own coast. From this work we learn, that there are shell-fish as small as the minutest insects, and possessed of beauties of which we can form no conception till we have seen them. Mr. Walker’s work is entitled, “A Collection of the minute and rare Shells lately discovered in the Sand on the Sea-shore near Sandwich.”[147] There is a sand from Africa full of small garnets. The ketton, or kettering stone, is a pleasing object; when examined by the microscope, we find the grain of it very different from that of other stones, being composed of innumerable minute balls, which barely touch each other, and yet form a substance much harder than free-stone; the grains are, in general, so firmly united together at the points of contact, that it is hardly possible to separate them without breaking one or both of the grains. See Hooke’s Micrographia.

[147] This publication will be more particularly noticed in the ensuing chapter. EDIT.

Insects of all kinds, both foreign and domestic, are pleasing objects; but as the foreign ones are not so easily met with, I shall mention but a few of them, confining myself principally to those of this country. Among the exotic insects, none appear more beautiful in the microscope than the curculio imperialis, Brazil or diamond beetle; the buprestis ignita, or large beetle from China; the meloe vesicatorius, Linn. the cantharis or Spanish fly of the shops; several species of locusts, grasshoppers, &c. Among the English beetles, we may reckon the scarabæus auratus or rose chaffer, scarabæus nobilis, scarabæus horticola, silpha aquatica, cassida nobilis and nebulosa. Coccinella or lady-cow; of these there are great varieties both in size and colour, some red and black, others black and red, and some yellow and black. Chrysomela graminis, chrysomela fastuosa, chrysomela nitidula, chrysomela sericea, chrysomela melanopa, chrysomela asparagi, see Plate XX. Fig. 2. Curculio frumentarius, lapathi, betula, nucum, scrophularia, argenteus, a beautiful little insect resembling the diamond beetle, but in miniature; curculio albinus, very beautiful, but scarce in this country. Leptura aquatica, these are of various colours, as blue, purple, bronze, and crimson. Arcuata arietis, very common, and is often called the wasp beetle. Cicindela campestris, on dry banks. Carabus nitens, found in Yorkshire, a beautiful insect; many small carabi. Gryllus, gryllo-talpa or mole cricket, this insect, and the grasshoppers, are many of them too large to be observed at one view, but the head, fore and hind feet, elytra, &c. viewed separately, are fine objects. Cicada sanguinolenta, nervosa, interrupta, notonecta striata, minutissima, head and claws of the nepa cinerea or water-scorpion, and the whole variety of cimices or field bugs. The wings of butterflies and moths; the chrysalis of the common white butterfly is extremely fine.

I wish it were in my power to invite the reader to consider the pupa state of these insects, as he would find them interesting in various points of view. Perhaps the following passage from an ingenious writer may have this effect.