hexagonal cells or bladders, containing for the most part a colored juice, and formed apparently of the foldings and doublings of a fine and delicate membrane, in which no traces of organization are to be distinguished. 1545. The path is a soft and spongy, but often succulent substance, occupying the centre of the root, stem, and branches, and extending in the direction of their longitudinal axis, in which it is enclosed as in a tube. The structure of the pith is precisely similar to that of the pulp, being composed of an assemblage of hexagonal cells containing a watery and colorless juice, or of cellular tissues and a parenchyma. 228 227 1346. The cortical layers, or interior and concentric layers, constituting the mass of the bark, are situated immediately under the cellular integument, where such integument exists, and where not, immediately under the epidermis; or they are themselves external. They are distinguishable chiefly in the bark of woody plants, but particularly in that of the lime-tree. They are composed of two elementary parts-bundles of longitudinal fibres constituting a network (fig. 227.), and a mass of pulp more or less indurated, filling up the meshes. The innermost of the layers is denominated the liber, and was used by the ancients to write on before the invention of paper. It is the finest and most delicate of them all, and often most beautifully reticulated (fig. 228 a), and varied by bundles of longitudinal fibre (b). But the liber of daphne lagetto is remarkable beyond that of all other plants for the beauty and delicacy of its network, which is not inferior to that of the finest lace, and at the same time so very soft and flexible that in countries of which the tree is a native the lace of the liber is often made to supply the place of a neckcloth. If the cortical layers are injured or destroyed by accident, the part destroyed is again regenerated, and the wound healed up without a scar; but if the wound penetrates beyond the liber, the part destroyed is no longer regenerated. Or if a tree is bent so as to break part of the cortical fibres, and then propped up in its former position, the fractured fibres will again unite. Or if a portion of the stem is entirely decorticated and covered with a piece of bark, even from another tree, the two different barks will unite. Hence the practicability of ascertaining how far the liber extends. And hence also the origin of grafting, which is always effected by a union of the liber of the graft and stock. 1947. The ligneous layers, or layers constituting the wood, occupy the intermediate portion of the stem between the bark and pith; and are distinguishable into two different sorts,-concentric layers and divergent layers. (Fig. 297.) ab a b a b 1348. The concentric layers, which constitute by far the greater part of the mass of the wood, are sufficiently conspicuous for the purpose of exemplification on the surface of a horizontal section of most trunks or branches, as on that of the oak and elm. But though they are generally described as being concentric, they are not always strictly so. For they are often found to extend more on the one side of the axis of the stem or branch, than on the other. Some authors say the excess is on the north side, but others say it is on the south side. The former account for it by telling us it is because the north side is sheltered from the sun; and the latter by telling us it is because the south side is sheltered from the cold; and thus from the operation of contrary causes alleging the same effect, which has been also thought to be sufficiently striking and uniform to serve as a sort of compass, by which the bewildered traveller might safely steer his course, even in the recesses of the most extensive forest. But Du Hamel has exposed the futility of this notion, by showing that the excess is sometimes on the one side of the axis, and sometimes on the other, according to the accidental situation of the great roots and branches; a thick root or branch producing a proportionably thick layer of wood on the side of the stem from which it issues. The layers are indeed sometimes more in number on the one side than on the other, as well as thicker. But this is the exception, and not the rule. They are thickest, however, on the side on which they are fewest, though not of the same thickness throughout. Du Hamel, after counting twenty layers on the one side of the transverse section of the trunk of an oak, found only fourteen on the other. But the fourteenth exceeded the twenty in thickness by one fourth part. But the layers thus discoverable on the horizontal section of the trunk are not all of an equal consistency throughout, there being an evident diminution in their degree of solidity from the centre, where they are hardest, to the circumference, where they are softest. The outermost layer, which is the softest of all, is denominated the alburnum, perhaps from its being of a brighter white than any of the other layers, either of wood or bark; from which character, as well as from its softer texture, it is also easily distinguished. It does not acquire its utmost degree of solidity till after a number of years; but if a tree is barked a year before it is cut down, then the alburnum is converted into wood in the course of that year. 1349. The divergent layers which intersect the concentric layers in a transverse direction, constitute also a considerable proportion of the wood, as may be seen in a horizontal section of the fir or birch, or ot almost any woody plant, on the surface of which they present an appearance like that of the radii of a circle. 1350. The structure of the concentric layers will be found to consist of several smaller and component layers, which are themselves composed of layers smaller still, till at last they are incapable of farther division. The concentric layers are composed of longitudinal fibres, generally forming a network; and the divergent layers, of parallel threads or fibres of cellular tissue, extending in a transverse direction, and filling up the interstices of the network. 1561 The structure of the stem in plants that are purely herbaceous, and in the herbaceous parts of woody plants, is distinguished by a number of notable and often insulated fibres passing longitudinally throughout its whole extent, as in the stipe of apsidium filix mass, or leaf-stalk of the alder. These fibres, when viewed superficially, appear to be merely individuals, but when inspected minutely, and under the microscope, they prove to be groups or bundles of fibres smaller and minuter still, firmly cemented together, and forming in the aggregate a strong and elastic thread, but capable of being split into a number of component fibres, till at last you can divide them no longer. If the fibres of the bark are separated by the destruction of a part, the part is again regenerated, and the fibres are again united, without leaving behind them any traces of a wound. But if the fibres of the wood are separated by the destruction of a part, the part is never regenerated, and the fibres are never united. SUBSECT. 3. Elementary or Vascular Organs. 1352. Fibre, cellular tissue with or without parenchyma, and reticulated membrane, are the ultimate and elementary organs of which the whole mass of the plant is composed. If it is asked of what the elementary organs are themselves composed, the reply is, they are composed, as appears from the same analysis, of a fine, colorless, and transparent membrane, in which the eye, aided by the assistance even of the best glasses, can discover no traces whatever of organisation; which membrane we must also regard as constituting the ultimate and fundamental fabric of the elementary organs themselves, and, by consequence, of the whole of the vegetable body. It has been asked by some phytologists whether or not plants are furnished with vessels analogous to the blood-vessels of the animal system. But if it is admitted that plants contain fluids in motion, which cannot possibly be denied, it will follow, as an unavoidable consequence, that they are furnished with vessels conducting or containing such fluids. If the stem of a plant of marigold is divided by means of a transverse section, the divided extremities of the longitudinal fibres, arranged in a circular row immediately within the bark, will be distinctly perceived, and their tubular structure demonstrated by means of the orifices which they present, particularly when the stem has begun to wither. Regarding it, therefore, as certain that plants are furnished with longitudinal tubes, as well as with cells or utricles for the purpose of conveying or containing their alimentary juices, we proceed to the specific illustration of both, together with their peculiarities and appendages. 1353. The utricles are the fine and membranous vessels constituting the cellular tissue of the pith and pulp already described, whether of the plant, flower, or fruit. Individually they resemble oblong bladders inflated in the middle, as in the case of some plants; or circular or hexagonal cells, as in the case of others. Collectively they have been compared to an assemblage of threads of contiguous bladders, or vesicles, or to the bubbles that are found on the surface of liquor in a state of fermentation. 1354. The tubes are the vessels formed by the cavities of the longitudinal fibres, whether as occurring in the stem of herbaceous plants, or in the foot-stalk of the leaf and flower, or in the composition of the cortical and ligneous layers, or by longitudinal openings pervading the pulp itself, as in the case of the vine. 1355. The large tubes are tubes distinguishable by the superior width of the diameter which they present on the horizontal section of the several parts of the plant. 1356. Simple tubes (fig.229.) are the largest of all the large tubes, and are formed of a thin and entire membrane, without any perceptible disruption of continuity, and are found chiefly in the bark, though not confined to it, as they are to be met with also in the alburnum and matured wood, as well as in the fibres of berbaceous plants. 1357. Porous tubes resemble the simple tubes in their general aspect; but differ from them in being pierced with small holes or pores, which are often distributed in regular and parallel rows. They are found in most abundance in woody plants, and particularly in wood that is firm and compact, like that of the oak: but they do not, like the simple tubes, seem destined to contain any oily or resinous juice. 1358. Spiral tubes are fine, transparent, and threadlike substances, occasionally interspersed with the other tubes of the plant, but distinguished from them by being 229 twisted from right to left, or from left to right, in the form of a cork screw. They occur in most abundance in herbaceous plants, particularly in aquatics. 1359. False spiral tubes are tubes apparently spiral on a slight inspection, but which, upon minute examination, are found to derive their appearance merely from their being cut transversely by parallel fissures. 1360. Mixed tubes are tubes combining in one individual two or more of the foregoing varieties. Mirbel exemplifies them in the case of the butomus umbellatus, in which the porous tubes, spiral tubes and false spiral tubes, are often to be met with united in one. 1361. The small tubes are tubes composed of a succes sion of elongated cells united, like those of the cellular tissue. Individually they may be compared to the stem of the grasses, which is formed of several internodia, separated by transverse diaphragms; and collectively to a united assemblage of parallel and collateral reeds. 1362. Pores are small and minute openings of various shapes and dimensions, that seem to be destined to the absorption, transmission, or exaltation of fluids. They are distinguishable into perceptible pores and imperceptible pores. 1363. Gaps, according to Mirbel, are empty, but often regular and symmetrical spaces formed in the interior of the plant by means of a partial disruption of the membrane constituting the tubes or utricles. In the leaves of herbaceous plants the gaps are often interrupted by transverse diaphragms formed of a portion of the cellular tissue which still remains entire, as may be seen in the transparent structure of the leaves of typha and many other plants. Transverse gaps are said to be observable also in the bark of some plants, though very rarely. 1364. There are various appendages connected with the elementary organs, such as internal glands, internal pubescence, &c. : the latter occurs in dissecting the leaf or flower-stalk of nymphæa lutea. CHAP. III. Vegetable Chemistry, or Primary Principles of Plants. 1365. As plants are not merely organized beings, but beings endowed with a species of life, absorbing nourishment from the soil in which they grow, and assimilating it to their own substance by means of the functions and operations of their different organs, it is plain that no progress can be made in the explication of the phenomena of vegetable life, and no distinct conception formed of the rationale of vegetation, without some specific knowledge of the primary principles of vegetables, and of their mutual action upon one another. The latter requisite presupposes a competent acquaintance with the elements of chemistry; and the former points out the necessity of a strict and scrupulous analysis of the several compound ingredients constituting the fabric of the plant, or contained within it. If the object of the experimenter is merely that of extracting such compound ingredients as may be known to exist in the plant, the necessary apparatus is simple, and the process easy. But if it is that of ascertaining the primary and radical principles of which the compound ingredients are themselves composed, the apparatus is then complicated, and the process extremely difficult, requiring much time and labor, and much previous practice in analytical research. But whatever may be the object of analysis, or particular view of the experimenter, the processes which he employs are either mechanical or chemical. 1866. The mechanical processes are such as are affected by the agency of mechanical powers, and are often indeed the operation of natural causes; hence the origin of gums and other spontaneous exudations. But the substances thus obtained do not always flow sufficiently fast to satisfy the wants or necessities of man. And men have consequently contrived to accelerate the operations of nature by means of artificial aid in the application of the wimble or axe, widening the passages which the extravasated fluid has forced, or opening up new ones. But it more frequently happens that the process employed is wholly artificial, and altogether effected without the operation of natural causes. When the juices are enclosed in vesicles lodged in parts that are isolated, or may easily be isolated, the vesicles may be opened by means of rasps or graters, and the juices expressed by the hand, or by some other fit instrument. Thus the volatile oil may be obtained that is lodged in the rind of the lemon. When the substance to be extracted hes more deeply concealed in the plant, or in parts which cannot be easily detached from the rest, it may then become necessary to pound or bruise the whole, or a great part of the plant, and to subject it, thus modified, to the action of the press. Thus seeds are sometimes treated to express their essential oils. And if by the action of bruising or pressing heterogeneous ingredients have been mixed together, they may generally be separated with considerable accuracy by means of decantation, when the substances real in suspension have been precipitated. Thus the acid of lemons, oranges, gooseberries, and other fruits, may be obtained in considerable purity, when the mucilage that was mixed with them has subsided 15. The chemical processes are such as are affected by the agency of chemical powers, and may be reduced to the following: distillation, combustion, the action of water, the action of acids and alkalies, the action of oils and alcohols, and lastly fermentation. They are much more intricate in their nature than the mechanical processes, as well as more difficult in their application. 18. Of the products of vegetable analysis, as obtained by the foregoing processes, some consist of several heterogeneous substances, and are consequently compound, as being capable of further decomposition; and some consist of one individual substance only, and are consequently simple, as being incapable of further decomposition. SECT. I. Compound Products. 1969. The compound products of analysis are very numerous in themselves, and much diversified in their qualities. They are gum, sugar, starch, gluten, albumen, fibrina, extract, tannin, coloring matter, bitter principle, narcotic principle, acids, oils, wax, resins, gum resins, balsams, camphor, caoutchouc, cork, woody fibre, sap, proper juice, charcoal, ashes, alkalies, earths, metallic oxides. 1570. Gum is an exudation that issues spontaneously from the surface of a variety of plants in the state of a clear, viscid, and tasteless fluid, that gradually hardens upon being exposed to the action of the atmosphere, and condenses into a solid mass. It issues copiously from many fruit-trees, but especially from such as produce stone-fruit, as the plum and cherry-tree. From plants or parts of plants containing it, but not discharging it by spontaneous exudation, it may be obtained by the process of maceration in Water. 1571. The uses of gum are considerable. In all its varieties it is capable of being used as an article of food, and is highly nutritive, though not very palatable. It is also employed in the arts, particularly in calico-printing, in which the printer makes choice of it to give consistency to his colors, and to prevent them from spreading. The botanist often uses it to fix his specimens upon paper, for which purpose it is very well adapted. It forms likewise an ingredient in ink; and in medicine it forms the basis of many mixtures, in which its influence is sedative and emollient. 1572 Sugar is the produce of the saccharum officinarum. The canes or stems of the plant, when ripe, are bruised between the rollers of a mill, and the expressed juice is collected and put into large boilers, in which it is mixed with a small quantity of quicklime, or strong ley of ashes, to neutralise its acid, and is then made to boil. The scum which gathers on the top during the process of boiling is carefully cleared away; and when the juice has been boiled down to the consistence of a syrup, it is drawn off and allowed to cool in vessels which are placed above a cistern, and perforated with small holes, through which the impure and liquid part, known by the name of molasses, escapes; while the remaining part is converted into a mass of small and hard granules of a brownish or whitish color, known by the designation of raw sugar, which when imported into Europe is further purified by an additional process, and converted by filtration or crystallization into what is called loaf sugar, or refined sugar, or candied sugar. The juice of the acer saccharinum, or American maple, yields sugar in such considerable abundance as to make it an object with the North American farmer to manufacture it for his own use. A hole is bored in the trunk of the vegetating tree early in the spring, for the purpose of extracting the sap; of which a tree of ordimary size, that is, of from two to three feet in diameter, will yield from one hundred and fifty to two bundred pints and upwards, in a good season. The sap, when thus obtained and neutralised by lime, deposits, by evaporation, crystals of sugar in the proportion of about a pound of sugar to forty pints of sp. It is not materially different in its properties from that of the sugar-cane. The juice of the grape, when ripe, yields also a sugar by evaporation and the action of pot-ashes, which is known by the appellation of the sugar of grapes, and has been lately employed in France as a substitute for colonial sugar, though it is not so sweet or agreeable to the taste. The root of beta vulgaris, or common beet, yields alo, by boiling and evaporation, a sugar which is distinguished by a peculiar and slightly bitter taste, owing perhaps to the presence of a bitter extractive matter which has been found to be one of the constituents of the beet. Sugar has been extracted from the following vegetables also, or from their productions: from the sap of the birch, sycamore, bamboo, maize, parsnep, cow-parsnep, American aloe, dulse, walnut-tree, and cocoa-nut-tree; from the fruit of the common arbutus, and other sweet-tasted fruits; from the roots of the turnip, carrot, and parsley; from the flower of the euxine rhododendron; and from the nectary of most other flowers. 1573. The utility of sugar, as an aliment, is well known; and it is as much relished by many animals as by man. By bees it is sipped from the flowers of plants, under the modification of nectar, and converted into honey; and also seems to be relished by many insects, even in its concrete state; as it is also by many birds. By man it is now regarded as being altogether indispensable, and though used chiefly to give a relish or seasoning to food, is itself highly nutritive. It is also of much utility in medicine, and celebrated for its anodyne and antiseptic qualities, as well as thought to be peculiarly efficacious in preTeating diseases by worms. 154. Starch. If a quantity of wheaten flour is made into a paste with water, and kneaded and washed under the action of a jet, till the water runs off colorless, part of it will be found to have been taken up and to be still held in suspension by the water, which will, by-and-by, deposit a sediment that may be separated by decantation. This sediment is starch, which may be obtained also immediately from the gram itself, by means of a process well known to the manufacturer, who renders it finally fit for the market by washing and edulcorating it with water, and afterwards drying it by a moderate heat. Starch, when thrown upon red-hot iron, burns with a kind of explosion, and leaves scarcely any residuum behind. It has been found by the analysis of Gay Lussac and Thenard, to be composed of carbon 43:55; oxygen 996; hydrogen 677; total 100. This result is not very widely different from that of the analysis of sugar, into which, it seems, starch may be converted by diminishing the proportion of its carbon, and increasing that of its oxygen and hydrogen. This change is exemplified in the case of the malting of barley, which contains a great proportion of starch, and which absorbs during the process a quantity of Lxygen, and evolves a quantity of carbonic acid; and accordingly part of it is converted into sugar Perhaps it is exemplified also in the case of the freezing of potatoes, which acquire in consequence a sweet and sugary taste, and are known to contain a great deal of starch, which may be obtained as follows: let the potatoes be taken and grated down to a pulp, and the pulp placed upon a fine sieve, and water made to pass through it; the water will be found to have carried off with it an infinite number of particles, which it will afterwards deposit in the form of a fine powder, separable by decantation; which powder is starch, possessing all the essential properties of wheaten starch. It may be obtained from the pith of several species of palms growing in the Moluccas and several other East India islands, by the following process: the stem, being first cut into pieces of five or six feet in length, is split longitudinally so as to expose the pith, which is now taken out and pounded, and mixed with cold water, which after being well stirred up, deposits at length a sediment that is separated by decantation, and is the starch which the pith contained, or the sago of the shops. 1375. Salop is also a species of starch that is prepared, in the countries of the East, from the root of the orchis morio, mascula, bifolio, and pyramidalis, and in the isle of Portland, from the arum maculatum. So also is cassava, which is prepared from the root of jatropha manihot, a native of America, the expressed juice of which is a deadly poison, used by the Indians to poison their arrows; but the sediment which it deposits is a starch that is manufactured into bread, retaining nothing of the deleterious property of the juice; and so also is sowans, which is prepared from the husk of oats, as obtained in the process of grinding. 1376. Starch may be extracted from a number of plants; as arctium lappa, atropa belladonna, polygonum bistorta, bryonia alba, colchicum autumnale, spirea filipendula, ranunculus bulbosus, scrophularia nodosa, sambucus ebulus and nigra, orchis morio and mascula, imperatoria ostruthium, hyoscyamus niger, rumex obtusifolius, acutus, and aquaticus, arum maculatum, iris pseudacorus and fætidissima, orobus tuberosus, bunium bulbocastanum. It is found also in the following seeds: wheat, barley, oats, rice, maize, millet-seed, chestnut, horse-chestnut, peas, beans, acorns. 1577. Starch is an extremely nutritive substance, and forms one of the principal ingredients in almost all articles of vegetable food used, whether by man or the inferior animals. The latter feed upon it in the state in which nature presents it; but man prepares and purifies it so as to render it pleasing to his taste, and uses it under the various modifications of bread, pastry, or confectionery. Its utility is also considerable in medicine and in the arts; in the preparation of anodyne and strengthening medicaments, and in the composition of cements; in the clearing and stiffening of linen; and in the manufacture of hairpowder. 1378. Gluten is that part of the paste formed from the flour of wheat that remains unaffected by the water after all the starch contained in it has been washed off. It is a tough and elastic substance, of a dull white color, without taste, but of a very peculiar smell. It is soluble in the acids and alkalies, but insoluble in water and in alcohol. Gluten has been detected, under one modification or other, in a very considerable number of vegetables or vegetable substances, as well as in the flour of wheat. 1379. Gluten is one of the most important of all vegetable substances, as being the principle that renders the flour of wheat so fit for forming bread, by its occasioning the panary fermentation, and making the bread light and porous. It is used also as a cement, and capable of being used as a varnish, and a ground for paint. 1380. Albumen, which is a thick, glary, and tasteless fluid, resembling the white of an unboiled egg, is a substance that has been but lately proved to exist in the vegetable kingdom. Its existence was first announced by Fourcroy, and finally demonstrated by the experiments of Vauquelin on the dried juice of the papaw-tree. It is nearly related to animal gluten. 1381. Fibrina is a peculiar substance which chemists extract from the blood and muscles of animals. This substance constitutes the fibrous parts of the muscles, and resembles gluten in its appearance and elasticity. A substance possessing the same properties has been detected by Vauquelin in the juice of the papawtree, which is called vegetable fibrina. 1382. Extract. When vegetable substances are macerated in water, a considerable portion of them is dissolved; and if the water is again evaporated, the substance held in solution may be obtained in a sepa rate state. This substance is denominated extract. But it is evident that extract thus obtained will not be precisely the same principle in every different plant, but will vary in its character according to the species producing it, or the soil in which the plant has grown, or some other accidental cause. Its distinguishing properties are the following:- it is soluble in water as it is obtained from the vegetable, but becomes afterwards insoluble in consequence of the absorption of oxygen from the atmosphere. It is solu ble in alcohol; and it unites with alkalies, and forms compounds which are soluble in water. When distilled it yields an acid fluid impregnated with ammonia, and seems to be composed principally of hydrogen, oxygen, carbon, and a little nitrogen. Extract, or the extractive principle, is found in a greater or less proportion in almost all plants whatever, and is very generally an ingredient of the sap and bark, particularly in barks of an astringent taste. But still it is not exactly the same in all individual plants, even when separated as much as possible from extraneous substances. It may therefore, be regarded as constituting several different species, of which the following are the most remarkable: 1383. Extract of catechu. This extract is obtained from an infusion of the wood or powder of catechu in cold water. Its color is pale brown; and its taste slightly astringent. It is precipitated from its solution by nitrate of lead, and yields by distillation carbonic and carburetted hydrogen gas, leaving a porous charcoal. 1384. Extract of senna. This extract is obtained from an infusion of the dried leaves of cassia senna in alcohol. The color of the infusion is brownish, the taste slightly bitter, and the smell aromatic. It is precipitated from its solution by the muriatic and oxymuriatic acids; and when thrown on burning coals consumes, with a thick smoke and aromatic odor, leaving behind a spongy charcoal. 1385. Extract of quinquina. This extract was obtained by Fourcroy, by evaporating a decoction of the hark of the quinquina of St. Domingo in water, and again dissolving it in alcohol, which finally deposited by evaporation the peculiar extractive. It is insoluble in cold water, but very soluble in boiling water; its color is brown, and its taste bitter. It is precipitated from its solution by lime water, in the form of a red powder; and when dry it is black and brittle, breaking with a polished fracture. 1386. Extract of saffron. This extract is obtained in great abundance from the summits of the pistils of crocus sativus, which are almost wholly soluble in water. 1587. Extracts were formerly much employed in medicine; though their efficacy seems to have been overrated. But a circumstance of much more importance to society is that of their utility in the art of dycing. By far the greater part of colors used in dyeing are obtained from vegetable extracts, which have a strong affinity to the fibres of cotton or linen, with which they enter into a combination that is rendered still stronger by the intervention of mordants. 1388. Coloring matter. The beauty and variety of the coloring of vegetables, chemists have ascribed to the modifications of a peculiar substance which they denominate the coloring principle, and which they have accordingly endeavored to isolate and extract; first, by means of maceration or boiling in water, and then by precipitating it from its solution. The chemical properties of coloring matter seem to be as yet but imperfectly known, though they have been considerably elucidated by the investigations of Bertholet, Chaptal, and others, Its affinities to oxygen, alkalies, earths, metallic oxides, and cloths fabricated, whether of animal or vegetable substances, such as wool or flax, seem to be among its most striking characteristics. But its affinity to animal substances is stronger than its affinity to vegetable substances; and hence wool and silk assume a deeper dye, and retain it longer than cotton or linen. Coloring matter exhibits a great variety of different tints, as it occurs in different species of plants; and as it combines with oxygen, which it absorbs from the atmosphere, it assumes a deeper shade; but it loses at the same time a portion of its hydrogen, and becomes insoluble in water; and thus it indicates its relation to extract. Fourcroy reduced colors to the four following sorts; extractive colors, oxygenated colors, carbo nated colors, and hydrogenated colors; the first being soluble in water, and requiring the aid of saline or metallic mordants to fix them upon cloth; the second being insoluble in water, as altered by the absorp tion of oxygen, and requiring no mordant to fix them upon cloth; the third containing in their compo sition a great proportion of carbon, but soluble in alkalies; and the fourth containing a great proportion of resin, but soluble in oils and alcohol. But the simplest inode of arrangement is that by which the dif ferent species of coloring matter are classed according to their effect in the art of dyeing. The principal and fundamental colors in this art are the blue, the red, the yellow, and the brown. 1389. The finest of all vegetable blues la that which is known by the name of indigo. It is the produce of the indigofera tinctoria, Lin, a shrub which is cultivated for the sake of the dye it affurds, in Mexico and the East Indies. The plant reaches maturity in about six months, when its leaves are gathered and immersed in vessels tilled with water till fermentation takes place. The water then becomes opaque and green, exhairng an odor like that of volatile alkali, and evolving bubbles of carbonic acid gas. When the fermentation has been continued long enough, the liquid is decanted and put into other ressels, where it is agitated till blue flakes begin to appear. Water is now poured in, and flakes are precipitated in the form of a blue powdery sediment, which is obtained by de. cantation; and which, after being made up into small lumps and dried in the shade, is the indigo of the shops. It is insolu ble in water, though slightly soluble in alcohol. But its true solvent is sulphuric acid, with which it forms a fine blue dye, known by the name of liquid blue. It affords by distillation carbonic acid gas, water, ammonia, some oily and acid matter, and much charcoal; whence its constituent principles are most probably carbon, hydrogen, oxygen, and nitrogen. Ledige may be procured also froin several other plants besides indigofera tinctoria, and particularly from isatis tinctoria or woad, a plant indigenous to Britain, and thought to be the plant with the juice of which the ancient Britons stained their naked bodies, to make them look terrible to their enemies. If this plant is digested in alcohol, and the solution evaporated, white crystalline grains, somewhat resembling starch, will be left behind; which grains are indigo, becoming gradually blue by the action of the atmosphere. The blue color of tdigo, therefore, is owing to its combination with oxygen. 1390. The principal red colors are such as are found to exist in the root, stem, or flower, of the five following plants: rubia tinctorum, lichen, roccella and parellus, carthamus tinctorius, Caesalpinia crista, and hæmatoxylon campechianum. 1391. Yellow, which is a color of very frequent occurrence among vegetables, and the most permanent among flowers, is extracted for the purpose of dyeing, from a variety of plants. It is extracted from the reseda luteola, Lin., by the decoction of its dried stems. The coloring matter is precipitated by means of alum, and is much used in dyeing wool, silk, and cotton. It is also obtained from the morus tinctoria, bixa orellana or arnotta, serratula tinctoria, genista tinctoria, rhus cotinus, rhamnus infectorius, and quercus tinctoria, or quercitron, the bark of which last affords a rich and permanent yellow that is at present much in use. 1392. The brown coloring matter of vegetables is very abundant, particularly in astringent plants. It is obtained from the root of the walnut-tree, and rind of the walnut; as also from the sumac and alder, but chiefly from nut-galls, which are excrescences formed upon the leaves of a species of quercus, indigenous to the south of Europe, in consequence of the puncture of insects. The best in quality are brought from the Levant. They are sharp and bitter to the taste, and extremely astringent; and soluble in water by decoction when ground or grated to a powder. The decoction strikes, with the solution of iron, a deep black, that forms the basis of ink, and of most dark colors used in dyeing cloths. 183. Tannin. If a quantity of pounded nut-galls, or bruised seeds of the grape, is taken and dissolved in cold water, and the solution evaporated to dryness, there will be left behind a brittle and yellowish substance of a highly astringent taste, which substance is tannin, or the tanning principle. It is soluble both in water and alcohol, but insoluble in ether. With the salts of iron it strikes a black. And when a solution of gelatine is mixed with an aqueous solution of tannin, the tannin and gelatine fall down in combination, and form an insoluble precipitate. When tannin is subjected to the process of distillation, it yields charcoal, carbonic acid, and inflammable gases, with a minute quantity of volatile alkali, and seems accordingly to consist of the same elements with extract, from which, however, it is distinguished by the peculiar property of its action upon gelatine. Tannin may be obtained from a great variety of other vegetables also, as well as those already enumerated, but chiefly from their bark; and of barks, chiefly from those that are astringent to the taste. The following table exhibits a general view of the relative value of different species of bark, as ascertained by Sir Humphry Davy. It gives the average obtained from 44b. of the entire bark of a middle-sized tree of the several different species, taken in the spring, when the quantity of tannin is the largest. 124. Tannin is of the very first utility in its application to medicine and the arts; being regarded by chemists as the general principle of astringency. The medical virtues of Peruvian bark, so celebrated as a febrifuge and antiseptic, are supposed to depend upon the quantity and quality of its tannin. In consequence of its peculiar property of forming an insoluble compound with gelatine, the hides of animals are converted into leather, by the important art of tanning. The bark of the oak-tree, which contains tannin in great abundance, is that which is most generally used by the tanner. The hides to be tanned are prepared for the process by steeping them in lime-water, and scraping off the hair and cuticle. They are then soaked, first in weaker and afterwards in stronger infusions of the bark, till at last they are completely impregnated. This process requires a period of from ten to eighteen months, if the hides are thick; and four or five pounds of bark are necessary on an average to form one pound of leather. 1395. Bitter principle. The taste of many vegetables, such as those employed in medicine, is extremely bitter. The quassia of the shops, the roots of the common gentian, the bark and wood of common broom, the calyx and floral leaves of the hop, and the leaves and flowers of chamomile, may be quoted as examples. This bitter taste has been thought to be owing to the presence of a peculiar substance, different from every other vegetable substance, and has been distinguished by the name of the bitter principle. When water has been digested for some time over quassia, its color becomes yellow, and its taste intensely bitter; and if it is evaporated to dryness, it leaves behind a substance of a brownish yellow, with a slight degree of transparency, that continues for a time ductile, but becomes afterwards brittle. This substance Dr. Thomson regards as the bitter principle in a state of purity. It is soluble in water and in alcohol; but the solution is not much affected by re-agents. Nitrate of silver and acetate of lead are the only two that occasion a precipitate. The bitter principle is of great importance, not only in the practice of medicine, but also in the art of brewing; its influence being that of checking fermentation, preserving the fermented liquor, and when the bitter of the hop is used, communicating a peculiar and agreeable flavor. The bitter principle appears to consist principally of carbon, hydrogen, and oxygen, with a little nitrogen. 12 Narcotic principle. There is a species of medical preparations known by the name of narcotics, which have the property of inducing sleep; and if administered in large doses, of occasioning death. They are obtained from the milky and proper juices of some vegetables, and from the infusion of the leaves or stem of others, all which have been supposed to contain in their composition some common ingredient, which chemists have agreed to designate by the name of the narcotic principle. It exists in great abundance in opium, which is the concrete juice of papaver album, or the white poppy, from which it is obtained pure, in the form of white crystals. It is soluble in boiling water and in alcohol, as well as in all acid menstrua; and it appears that the action of opium on the animal subject depends on this printiple. When distilled it emits white vapors, which are condensed into a yellow oil. Some water and carbonate of ammonia pass into a receiver; and at last carbonic acid gas, ammonia, and carburetted hydrogen, are disengaged, and a bulky charcoal left behind. Many other vegetable substances besides pium possess narcotic qualities though they have not yet been minutely analysed. The following are the most remarkable: the inspissated juice of lettuce, which resembles opium much in its appearance, is obtained by the same means, and possesses the same medical virtues; the leaves of atropa belladonna, or deadly nightshade, and indeed the whole plant; the leaves of digitalis purpurea, or foxglove; and lastly, the following plants, hyoscyamus niger, conium maculatum, datura stramonium, and sedum palustre, with many others belonging to the Linnæan natural order of Lurida. 1397. Acids. Acids are a class of substances that may be distinguished by their exciting on the palate the sensation of sourness. They exist, not only in the animal and mineral, but also in the vegetable kingdom; and such of them as are peculiar to vegetables have been denominated vegetable acids. Of ards peculiar to vegetables chemists enumerate the following:-the oxalic, acetic, citric, malic, gallic, tartarie, benzoic, and prussic, which exist ready formed in the juices or organs of the plant, and are ac |