(Sir James) was born at Edinburgh on the 10th of October, O.S. in the year 1713. His father was Sir James Stewart of Goodtrees, Bart. Solicitor-general for Scotland; and his mother ther was Anne, daughter of Sir Hugh Dalrymple of North Berwick, Bart. president of the college of justice.

The first rudiments of his education he received at the grammar-school of North-Berwick, which at the time of his father's death he quitted at the age of fourteen, with the reputation of being a good scholar, but without any extraordinary advancement in knowledge.

It is remarkable, that many men who have been singularly useful to society have not shown early symptoms of the greatness of their intellectual powers. A great understanding must be the offspring of happy organization in a healthy body, with co-operation of time, of circumstance, and of inclination, without being forced into prematurity by excessive cultivation. This holds with respect to the growth and perfection of every creature; and the truth appears remarkable with respect to our own species, because we are apt to mistake the flimsy attainments of artificial education for the steady and permanent foundations of progressive knowledge.

From the school of North-Berwick Sir James was sent to the university of Edinburgh, where he continued until the year 1735, when he passed advocate before the Court of Session, and immediately afterwards went abroad to visit foreign countries. He was then in the 23rd year of his age, had made himself well acquainted with the Roman law and history, and the municipal law of Scotland. He had likewise maturely studied the elements of jurisprudence; was versed in the general, as well as the particular, politics of Europe; and was bent upon applying his knowledge to the investigation of the state of men and of manners in other nations, with a view to promote the benefit of his own, and to confirm himself in the love of a free constitution of government, by contemplating the beneficial effects of unlimited monarchy in Germany, Italy, and Spain, and of extravagant attachment to a king and nobility, to war, and to pernicious splendour in France.

He travelled first, however, into Holland, with a view to study the constitution of the empire before he should visit Germany, and to attend some of the lectures of the most eminent professors at Utrecht and Leyden, on public law and politics. From thence he passed into Germany, resided about a year in France, travelled through some part of Spain, where he had a fever, that obliged him, for his perfect recovery from its effects, to go by the advice of his friends to the sea-coast of the lovely province of Valencia; thence returning, he crossed the Alps, and by Turin made the tour of Italy, where chiefly at Rome and Florence he resided till the beginning of the year 1740; when, having spent five years on his travels, he returned to Scotland, and married the Lady Frances Wemyss, eldest daughter of the Earl of Wemyss, about two years after his return.

A few months after his marriage the representation of the county of Mid-Lothian became vacant, by the member being made a lord of trade and plantation. The candidates were the late member and Sir John Baird of Newbyth. On the day of election Mr Dundas of Arniston, one of the senators of the college of justice, was chosen preses of the meeting; and somehow or other omitted to cause the name of Sir James Stewart to be called on the roll of freeholders. For this illegal use of his temporary power, Sir James commenced a suit against the president; and refusing the gown as an advocate, pleaded his own cause with great energy and eloquence, and with the applause of the bench, the bar, and the public. This called Lord Arniston from the bench to plead in his own defence at the bar; and Sir James could not have been opposed to an antagonist better qualified to call forth all his powers; for that judge is talked of at this day in Edinburgh as the profoundest lawyer and the ablest pleader that ever graced the Scottish bench or the Scottish bar.

With the issue of this contest we are not acquainted; but it drew upon Sir James Stewart very general attention, and convinced the public, that had he continued at the bar, he must have risen rapidly to the head of his profession. On his travels, however, he had contracted friendships with Lord Marshal, and other eminent men, attached to the pretensions of the royal family of Stuart, and had received flattering attentions from the Pretender to the British throne; the impression arising from which, added to the irritations of his controversy with the powerful party in Scotland attached to the court, led him, unwisely, into connections with the movers of the rebellion in 1745.

As he was by far the ablest man of their party, the Jacobites engaged him to write the Prince Regent's manifesto, and to assist in his councils. Information having been given of his participation in these affairs, he thought it prudent, on the abortion of this unhappy attempt, to leave Britain; and by the zeal, it is said, of Arniston, he was excepted afterwards from the bill of indemnity, and rendered an exile from his country.

He chose France for his residence during the first years of his banishment; and was chiefly at Angoulême, where he superintended the education of his son; from thence he went to Tubingen in Swabia, for the benefit of his university, in prosecution of the laudable and laudable design; but in the end of the war 1756, having been suspected by the court of Versailles of communicating intelligence to the court of London, he was seized at Spa, and kept some time in confinement; from which being liberated, after the accession of the present king of Great Britain, he came, by toleration, to England, and resided at London, where he put the last hand to his System of Political Economy, the copy right of which he sold to Andrew Millar; and being permitted to dedicate this work to the king, he applied for a non profugum, which, after some malicious objections, he obtained, and had the comfort of returning to his family estate in Scotland.

Having nothing professional to do during his long residence in France, the active mind of Sir James was occupied in study. His book on the Principles of Political Economy contains most of the fruits of it. He turned himself, in the intervals of leisure, to consider the resources of France, that he might the better compile that part of his great work which was to treat of revenue and expenditure. It was by studying the language of the finances, without which nobody can ask a proper question concerning them, so as to be understood, that he attained his great purpose.

As soon as he could ask questions properly, he applied in familiar conversation to the intendants and their substitutes in the provinces where he resided, whom he found extremely desirous to learn the state of the British finances, under the branches of the land-tax, customs, excise, and other inland duties. This led him to compare... compare the state of the two nations. The information he gave was an equivalent for the information he received; curiosity balanced curiosity, each was satisfied and instructed. The department of the intendants in France was confined to the taxes which composed the recettes générales, namely, the taille, the capitulation, and the rentes, or signes. All the intendants had been Maîtres des Requêtes, bred at Paris, and could not fail to have much knowledge of the general finances and other branches of the revenue. He carefully noted down at all times the answers he got; and when he came to reside at Paris, he obtained more ample information, both from the gentlemen of the revenue, and from persons of the parliament of Paris, who to the number of 25 had been for 15 months exiled in the province where he had so long resided at Angoulême.

With these advantages, with much study and attention to arrangement, he was enabled to compose the sixth chapter of the fourth part of the fourth book of his System of Political Economy; a portion of that great work well worthy the attention of those who wish to know the state of France in respect of revenue under the old government.

Although Sir James Stewart's leisure, during the first ten years of his exile, was chiefly employed in social intercourse with the most learned, elegant, and polished characters in France, who delighted in the conversation and friendship of a man who possessed at once immense information, on almost every subject, important or agreeable to society, and the talent of clearly and beautifully expressing his sentiments in flowing and animated conversation; yet he did not allow the pleasures of the circle and of the table to blunt the fine feelings of a man of genius and science. The labour of collecting materials for his great political work was oppressive, and he relieved himself with various enquiries, suited to the exalted ambition of his cultivated understanding, while he turned the charms of conversation to the permanent delight of his associates and of posterity. The motto of Apelles, "Nulla dies sine linea," was the emblem of his employment; and it is amazing what may be done by daily attention for improvement, without appearing to abridge any extraordinary time from the common offices and rational pleasures of society.

In the beginning of the year 1755, Sir James wrote his Apology, or Defence of Sir Isaac Newton's Chronology, which at that time he intended to publish, but was prevented by other engagements. It was communicated to several persons of eminence in France and Germany in MS. and produced, in the month of December that year in the "Mercure de France," an answer from M. Deshoulieres, to which Sir James soon after replied.

The great Newton, applying astronomical and statistical principles to the ancient chronology of Greece, had chastised the vanity of nations, and arrested the progress of infidelity in delineating the history of the world. Lost in the confusion of excessive pretensions to antiquity beyond all measure, and disgusted by the superstitious aids that were assumed to support these pretensions among ancient nations, the revivers of learning in Europe, during the last and the preceding century, tormented themselves with controversies between the comparative merits of the ancients and moderns; and

the abettors of the latter, entrenching themselves behind the falsehoods of the ancients, on the scope of their remote history, gave the lie to all antiquity, and in despair plunged themselves into the ocean of skepticism.

Happy had it been for society if this skepticism had confined itself to the history of ancient nations in general; but the same spirit, taking disgust at the horrors of Christian ambition and bigotry, and contemplating with derision the ridiculous legends of modern miracles, gave the lie to all religious scripture of the Jews and Christians, and attempted to banish divine intelligence, the superintending providence of Deity, and the true dignity of the human species, from the face of the earth!

It was a noble undertaking, therefore, in Sir James, to attempt to dispel this mist of error, by diffusively and scientifically explaining and supporting the chronology of Sir Isaac Newton. He has done it with great precision and effect; and it is a book well worth the perusal of those who wish to read ancient history with improvement, or to prevent themselves from being bewildered in the mazes of modern conjecture. It was printed in 4to at Frankfort on the Main, for John Bernard Eichemberg the Elder, in 1757.

In the year 1758, and the following, the British House of Commons took up the consideration of a statute to regulate a general uniformity of weights and measures throughout the united kingdoms, which had been so often unsuccessfully attempted.

This called the attention of Sir James, not only to the investigation of the particular subject that engaged that of the House of Commons, but to devise a method of rendering an uniformity of weights and measures universal. He thought the cause of former disappointments in this useful pursuit had been the mistaken notion that one or other of our present measures should be adopted for the new standard. After the plan had been relinquished by the parliament of England, he digested his notes and observations on this important disquisition into the form of an epistolary dissertation, which he transmitted to his friend Lord Barrington, and resolved, if there had been a congress assembled, as was once proposed, to adjust the preliminaries of the general peace in 1763, to have laid his plan before the ministers of the different nations, who were to prepare that salutary pacification of the contending powers.

This epistolary dissertation Sir James afterwards reduced at Colneth, in the year 1777, into a form more proper for the public eye, and sent a corrected copy to a friend, referring another for the press, which was printed 1790 for Stockdale in Piccadilly.

In this tract the author shows, from the ineffectual attempts that have been made to alter partially, by innovation, the standards of measures or weights, that the effectual plan to be adopted, is to depart entirely from every measure whatsoever now known, and to take, ad libitum, some new mass instead of our pound, some new length instead of our ell, some new space instead of our acre, and some new solid instead of our gallon and buttell.

For this purpose Sir James proposes as the unit a mass to be verified with the greatest possible accuracy, equal in weight to ten thousand Troy grains. The pendulum, as it swings at London, to beat seconds of time, ANIMAL AND VEGETABLE SUBSTANCES.

The reader will recollect, that the article Chemistry, in this Supplement, was divided into four parts; of which only the first three, comprehending the elements of the science, were given under the word Chemistry. The fourth part, which was entitled an examination of bodies as they are presented to us by nature in the mineral, vegetable, and animal kingdoms, naturally subdivides itself into three parts, comprehending respectively, 1. Minerals; 2. Vegetables; 3. Animals.

The first of these subdivisions, which has been distinguished by the name of Mineralogy, we have treated of already in a former part of this Volume. As the other two subdivisions have not hitherto received any appropriate name, we have satisfied ourselves with the word Substance, by which chemists have agreed to denote the objects which belong to these subdivisions. This name, it must be acknowledged, is not unexceptionable; but we did not consider ourselves as at liberty to invent a new one.

The present article, then, seems to divide itself into two parts: the first part comprehending vegetable; the Division of second animal substances. But there are certain animal and vegetable substances distinguished from all others by being used as articles of clothing. It is usual to tinge these of various colours, by combining with them different colouring matters for which they have an affinity. This process, well known by the name of dyeing, is purely chemical; and as it belongs exclusively to animal and vegetable substances, it comes naturally to be examined here. We shall therefore add a third part, in which we shall give a view of the present state of dyeing, as far, at least, as is consistent with the nature of a supplementary article. Part I. Of Vegetable Substances.

Vegetables, or plants, as they are also called, are too well known to require any definition. Their number is prodigious, and their variety, regularity, and beauty, are wonderful. But it is not our intention in this place either to enumerate, to describe, or to classify plants. These tasks belong to the botanist, and have been successfully accomplished by the zeal, the singular address, and the indefatigable labour of Linnaeus and his followers.

It is the business of the chemist to analyse vegetables, to discover the substances of which they are composed, to examine the nature of these substances, to investigate the manner in which they are combined, to detect the processes by which they are formed, and to ascertain the chemical changes to which plants, after they have ceased to vegetate, are subject. Hence it is evident, that a chemical investigation of plants comprehends three particulars:

1. An account of the substances of which plants are composed. 2. An account of the vegetation of plants, as far as it can be illustrated by chemistry. 3. An account of the changes which plants undergo after they cease to vegetate.

We therefore divide this part into three chapters, assigning a chapter to each of these particulars.

Chap. I. Of the Ingredients of Plants.

The substances hitherto found in the vegetable kingdom, all of them at least which have been examined with any degree of accuracy, may be reduced to the following heads:

1. Sugar, 2. Starch, 3. Gluten, 4. Albumen, 5. Gum, 6. Jelly, 7. Extract, 8. Tan, 9. Oils, 10. Camphor, 11. Resin, 12. Caoutchouc, 13. Wax, 14. Wood, 15. Acids, 16. Alkalies, 17. Earths, 18. Metals.

These shall form the subject of the following sections:

Sect. I. Of Sugar.

Sugar, which at present forms so important an article in our food, seems to have been known at a very early period to the inhabitants of India and China. But Europe probably owes its acquaintance with it to the conquests of Alexander the Great. For ages after its introduction into the west, it was used only as a medicine; but its consumption gradually increased, and during the time of the Crusades, the Venetians, who brought it from the east, and distributed it to the northern parts of Europe, carried on a lucrative commerce with sugar. It was not till after the discovery of America, and the extensive cultivation of sugar in the West Indies, that its use in Europe, as an article of food, became general.

Sugar is obtained from the arundo saccharifera, or sugar cane. The juice of this plant is pressed out and boiled in as low a temperature as possible, till the sugar precipitates in the form of confused crystals. These crystals, known by the name of raw sugar, are again dissolved in water, the solution is clarified, and pure refined crystals are obtained by a subsequent evaporation. But for the particulars of the art of manufacturing sugar, we refer the reader to the article Sugar in the Encyclopaedia.

Sugar, after it has been purified, or refined as the manufacturers term it, is usually sold in Europe in the form of a white opaque mass, well known by the name of loaf sugar. Sometimes also it is crystallized, and then it is called sugar candy.

Sugar has a very strong sweet taste; when pure it has no smell; its colour is white, and when crystallized it is somewhat transparent. It has often a considerable degree of hardness; but it is always so brittle that it can be reduced without difficulty to a very fine powder. It is not altered by exposure to the atmosphere.

It is exceedingly soluble in water. At the temperature of 48° water, according to Mr Wenzel, dissolves its own weight of sugar. The solvent power of water increases with its temperature; when nearly at the boiling point, it is capable of dissolving any quantity of sugar whatever. Water thus saturated with sugar is known by the name of syrup.

Syrup is thick, syrupy, and very adhesive; when spread thin upon paper, it soon dries, and forms a kind of varnish, which is easily removed by water. Its specific gravity, according to the experiments of Dr Crawford, is 1.086. When syrup is sufficiently concentrated, the sugar which it contains precipitates in crystals. The primitive form of these crystals is a four-sided prism, whose base is a rhomb, the length of which is to its breadth as 10 to 7; and whose height is a mean proportion between the length and breadth of the base. The crystals are usually four or six-sided prisms, terminated by two sided, and sometimes by three-sided funnels.

Sugar is insoluble in alcohol, but not in so large a proportion as in water. According to Wenzel, four parts of boiling alcohol dissolve one of sugar. It unites readily with oils, and renders them miscible with water. A moderate quantity of it prevents, or at least retards, the coagulation of milk; but Scheele discovered that a very large quantity of sugar causes milk to coagulate.

Sugar absorbs muriatic acid gas slowly, and affords a brown colour and very strong smell.

Sulphuric acid, when concentrated, readily decomposes sugar; water is formed, and perhaps also acetic acid; while charcoal is evolved in great abundance, and gives the mixture a black colour, and a considerable degree of consistency. The charcoal may be easily separated by dilution and filtration. When heat is applied the sulphuric acid is rapidly converted into sulphurous acid.

When sugar is mixed with potash, the mixture acquires a bitter and astringent taste, and is insoluble in alcohol, though each of the ingredients is very soluble in that liquid. When the alkali is saturated with sulphuric acid... ric acid; and precipitated by means of alcohol, the sweet taste of the sugar is restored; a proof that it had undergone no decomposition from the action of the potash, but had combined with it in the state of sugar.

Lime boiled with sugar produces nearly the same effect as potash; when an alkali is added to the compound, a substance precipitates in white flakes. This substance is sugar combined with lime. Sugar and chalk compose, as Leonardi informs us, a kind of cement.

Sugar, when thrown upon a hot iron, melts, swells, becomes brownish black, emits air bubbles, and exhales a peculiar smell, known in French by the name of caramel. At a red heat it instantly bursts into flames with a kind of explosion. The colour of the flame is white with blue edges.

When sugar is distilled in a retort, there comes over a fluid which, at first, scarcely differs from pure water; by and bye it is mixed with pyromucous acid, afterwards some empyreumatic oil makes its appearance; and a bulky charcoal remains in the retort. This charcoal very frequently contains lime, because lime is used in refining sugar; but if the sugar, before being submitted to distillation, be dissolved in water, and made to crystallize by evaporation in a temperature scarcely higher than that of the atmosphere, no lime whatever, nor any thing else, except pure charcoal, will be found in the retort. During the distillation, there comes over a considerable quantity of carbonic acid, and carbonated-hydrogen gas. Sugar therefore is decomposed by the action of heat; and the following compounds are formed from it: Water, pyromucous acid, oil, charcoal, carbonic acid, carbonated hydrogen gas. The quantity of oil is inconsiderable; by far the most abundant product is pyromucous acid. Sugar indeed is very readily converted into pyromucous acid; for it makes its appearance always whenever syrup is raised to the boiling temperature. Hence the smell of caramel, which syrup at that temperature emits. Hence also the reason that, when we attempt to crystallize syrup by heat, there always remains behind a quantity of incrustable matter, known by the name of molasses; whereas if the syrup be crystallized without artificial heat, every particle of sugar may be obtained from it in a crystalline form.

Hence we see the importance of properly regulating the fire during the crystallization of sugar, and the immense saving that would result from conducting the operation at a low heat.

It follows from these facts, and from various other methods of decomposing sugar, that it is composed of oxygen, hydrogen, and carbon; for all the substances obtained from sugar by distillation may be resolved into these elements. Lavoisier has made it probable, by a series of very delicate experiments, that these substances enter into the composition of sugar in the following proportions:

- 64 oxygen, - 28 carbon, - 8 hydrogen.

Of the way in which these ingredients are combined in sugar, we are still entirely ignorant. Lavoisier's conclusions can only be considered as approximations to the truth.

Sugar is considered as a very nourishing article of food. It is found most abundantly in the juice of the sugar cane, but many other plants also contain it. The juice of the acer saccharinum, or sugar maple, contains sugars containing much of it, that in North America sugar is often named it extracted from that tree. Sugar is also found in the roots of carrot, parsnip, beet, &c. Mr Achard has lately pointed out a method of increasing the quantity of sugar in beet to much, that, according to his own account, it is at present cultivated in large quantities in Prussia, and sugar extracted from it with advantage. Parmentier has also ascertained that the grains of wheat, barley, &c. and all the other similar seeds which are used as food, contain at first a large quantity of sugar, which gradually disappears as they approach to a state of maturity. This is the case also with peas and beans, and all leguminous seeds, and is one reason why the flavour of young peas is so much superior to that of old ones.

Sect. II. Of Starch.

When a quantity of wheat flour is formed into a paste, and water poured upon it till it runs off colour-obtaining lees, this water soon deposits a very fine whitish powdery substance, which, when properly washed and dried, is known by the name of starch. When first prepared, it is of a grey colour; but the starchmakers render it white by steeping it in water slightly acidulated. The acid seems to dissolve and carry off the impurities.

Starch was well known to the ancients. Pliny informs us, that the method of obtaining it was first invented by the inhabitants of the island of Chio.

Starch has a fine white colour, and is usually concreted in longish masses; it has scarcely any smell, and has very little taste. When kept dry, it continues for a long time unjured though exposed to the air.

Starch does not dissolve in cold water, but very soon falls to powder. It combines with boiling water, and on by forms with it a thick paste. Linen dipped into this paste, water, and afterwards dried suddenly, acquires, as is well known, a great degree of stiffness. When this paste is left exposed to damp air it soon loses its consistency, acquires an acid taste, and its surface is covered with mould.

Starch is so far from dissolving in alcohol, even when Alcohol, assisted by heat, that it does not even fall to powder.

When starch is thrown into any of the mineral acids, at first no apparent change is visible. But if an attempt is made to break the larger pieces while in acids to powder, they resist it, and feel exceedingly tough and adhesive. Sulphuric acid dissolves it slowly, and at the same time a smell of sulphurous acid is emitted, and such a quantity of charcoal is evolved, that the dish containing the mixture may be inverted without foiling any of it. Indeed if the quantity of starch be sufficient, the mixture becomes perfectly solid. The charcoal may be separated by dilution and filtration. In muriatic acid starch dissolves still more slowly. The solution resembles mucilage of gum arabic, and still retains the peculiar odour of muriatic acid. When allowed to stand for some time, the solution gradually separates into two parts; a perfectly transparent straw-coloured liquid below, and a thick, muddy, oily, or rather mucilaginous, substance above. When water is poured in, the muriatic smell instantly disappears, and a strong smell is exhaled, precisely similar to that which is felt in corn-mills. Ammonia occasions a slight precipitate, but too small to be examined.

Nitric acid dissolves starch more rapidly than the other two acids; it acquires a green colour, and emits nitrous gas. The solution is never complete, nor do any crystals of oxalic acid appear unless heat be applied. In this respect starch differs from sugar, which yields oxalic acid with nitric acid, even at the temperature of the atmosphere. When heat is applied to the solution of starch in nitric acid, both oxalic and malic acid is formed, but the undissolved substance still remains. When separated by filtration, and afterwards edulcorated, this substance has the appearance of a thick oil, not unlike tallow; but it dissolves readily in alcohol. When distilled, it yields acetic acid, and an oil having the smell and the consistence of tallow.

When starch is thrown upon a hot iron, it melts, blackens, froths, swells, and burns with a bright flame like sugar, emitting, at the same time, a great deal of smoke; but it does not explode, nor has it the caramel smell which distinguishes burning sugar. When distilled, it yields water impregnated with an acid, supposed to be the pyromucous, and mixed with a little empyreumatic oil. The charcoal which remains is easily dissipated when set on fire in the open air; a proof that it contains very little earth.

Barley grain consists almost entirely of starch, not however in a state of perfect purity. In the process of malting, which is nothing else than causing the barley to begin to vegetate, a great part of the starch is converted into sugar. During this process oxygen gas is absorbed, and carbonic acid gas is emitted. Water, too, is absolutely necessary; hence it is probable, that it is decomposed, and its hydrogen retained. Starch, then, seems to be converted into sugar by diminishing the proportion of its carbon, and increasing that of its hydrogen and oxygen. Its distillation shows us that it contains no other ingredients than these three.

Starch is contained in a great variety of vegetable substances; most commonly in their seeds or bulbous roots; but sometimes also in other parts. Mr Parentier, whose experiments have greatly contributed towards an accurate knowledge of starch, has given us the following list of the plants from the roots of which it may be extracted:

- Arctium lappa, - Atropa belladonna, - Polygonum bistorta, - Bryonia alba, - Colchicum autumnale, - Spira filipendula, - Ranunculus bulbosus, - Scrophularia nodosa, - Sambucus ebulus, - nigra, - Orchis morio,

It is found also nearly pure in the following seeds:

- Oats, - Rice, - Maize, - Millet, - Chefsnut, - Horsechefsnut, - Peas, - Beans,

Sect. III. Of Gluten.

When wheat flour is washed in the manner described in the last section, in order to obtain starch from it, the substance which remains, after every thing has been washed away which cold water can separate, is called gluten. It was discovered by Beccaria an Italian philosopher, to whom we are indebted for the first analysis of wheat flour.

Gluten, when thus obtained, is of a grey colour, exceedingly tenacious, ductile, and elastic, and may be extended to twenty times its original length. When very thin, it is of a whitish colour, and has a good deal of resemblance to animal tendon or membrane. In this state it adheres very tenaciously to other bodies, and has often been used to cement together broken pieces of porcelain. Its smell is agreeable. It has scarcely any taste, and does not lose its tenacity in the mouth.

When exposed to the air, it gradually dries; and, when completely dry, it is pretty hard, brittle, slightly transparent, of a dark brown colour, and has some resemblance to glue. It breaks like a piece of glass, and the edges of the fracture resemble in smoothness those of broken glass; that is to say, it breaks with a conical fracture.

When exposed to the air, and kept moist, it soon putrefies; but when dry, it may be kept any length of time without alteration. It is insoluble in water; though it imbibes and retains a certain quantity of it with great obstinacy. To this water it owes its elasticity and tenacity. When boiled in water, it loses both these properties. It is soluble in alcohol, as Mr Vauquelin informs us, and precipitated again, as Mr Fourcroy has observed, by pouring into the alcohol two parts of water.

Gluten is soluble in the three mineral acids. When nitric acid is poured on it, and heat applied, there is a quantity of azotic gas emitted, as Berthollet discovered, and, by continuing the heat, a quantity of oxalic acid is formed.

Alkalies dissolve gluten when they are assisted by heat. The solution is never perfectly transparent. Acids precipitate the gluten from alkalies, but it is destroyed by their elasticity.

When moist gluten is suddenly dried, it swells amazingly. Dry gluten, when exposed to heat, cracks, swells, melts, blackens, exhales a fetid odour, and burns precisely like feathers or horn. When distilled, there comes over water impregnated with ammonia and an empyreumatic oil; the charcoal which remains is with difficulty reduced to ashes. From these phenomena, it is evident that gluten is composed of carbon, hydrogen, azot, and oxygen; perhaps also it contains a little lime. In what manner these substances are combined is unknown.

The only vegetable substance which has been hitherto found to contain it abundantly, is wheat flour. Vauquelin also found it in the fruit of the castor-seed, and Fourcroy in the bark of a species of quinquina from St Domingo. It probably exists in many other plants.

Sect. IV. Of Albumen.

If the water in which wheat flour has been washed in order to obtain starch and gluten, according to the directions laid down in the two last sections, be filtered, and afterwards boiled, a substance precipitates in white flakes; to which Mr Fourcroy, who first pointed it out, gave the name of albumen. vegetable substances.

It is evident, from the method of obtaining it, that albumen, in its natural state, is soluble in water, and that heat precipitates it from that fluid in a concrete state. While dissolved in water, it has scarcely any taste; but it has the property of changing vegetable blues, especially that which is obtained from the flowers of the mallow (malva sylvestris), into a green. When allowed to remain dissolved in water, it passes without becoming previously acid.

After it has been precipitated from water in a concrete state by boiling, it is no longer soluble in water as before. Alcohol also precipitates it from water precisely in the same state as when it is precipitated by heat.

When concrete albumen is dried it becomes somewhat transparent, and very like glue. In that state it is soluble in alkalies, especially ammonia.

When distilled it gives out carbonic acid, a red fetid oil, and carbonated hydrogen gas; and a spongy charcoal remains behind. From this, it is evident that albumen, like gluten, is composed of carbon, azot, hydrogen, and oxygen; but the proportions and combinations of these substances are altogether unknown.

Mr Fourcroy found albumen in the expressed juice of leavy grasses, cereals, cabbage, and almost all cruciform plants. He found it, too, in a great many young and succulent plants; but never a particle in those parts of vegetables which contain an acid. He observed also that the quantity decreased constantly with the age of the plant.

Sect. V. Of Jelly.

If we press out the juice of ripe blackberries, currants, and many other fruits, and allow it to remain for some time in a state of rest, it partly coagulates into a tremulous soft substance, well known by the name of jelly. If we pour off the uncoagulated part, and wash the coagulum with a small quantity of water, we obtain jelly approaching to a state of purity.

In this state, it is nearly colourless, unless tinged by the peculiar colouring matter of the fruit; it has a pleasant taste, and a tremulous consistence. It is scarcely soluble in cold water, but very soluble in hot water; and, when the solution cools, it again coagulates into the form of a jelly. When long boiled, it loses the property of gelatinizing by cooling, and becomes analogous to mucilage. This is the reason that in making currant jelly, or any other jelly, when the quantity of sugar added is not sufficient to absorb all the watery parts of the fruit, and consequently it is necessary to concentrate the liquid by long boiling, the mixture often loses the property of coagulating, and the jelly, of course, is spoiled.

Jelly combines readily with alkalies; nitric acid converts it into oxalic acid, without separating any azotic gas. When dried it becomes transparent. When distilled, it affords a great deal of pyromucous acid, a small quantity of oil, and scarcely any ammonia.

Jelly exists in all acid fruits, as oranges, lemons, gooseberries, &c., and no albumen is ever found in those parts of vegetables which contain an acid. This circumstance has induced Fourcroy to suppose that jelly is albumen combined with an acid; but this conjecture has not been verified by experiment; nor indeed is it probable that it ever shall; as albumen evidently contains a quantity of azot, and jelly scarcely any. The products of jelly by distillation show that it approaches nearer than any other vegetable substance to the nature of sugar.

Sect. VI. Of Gum.

There is a thick transparent tallow-like fluid which sometimes exudes from certain species of trees. It is very adhesive, and gradually hardens without losing its transparency; but easily softens again when moistened with water. This exudation is known by the name of gum. The gum most commonly used is that which exudes from different species of the mimosa, particularly the niloticum. It is known by the name of gum arabicus. Gum likewise exudes abundantly from the prunus avium, Phil. or common wild cherry tree of this country.

Gum is usually obtained in small pieces like tears, moderately hard, and somewhat brittle while cold, so that it can be reduced by pounding to a fine powder. Its colour is usually yellowish, and it is not destitute of fat. It has no smell; its taste is insipid.

Gum undergoes no change from being exposed to the atmosphere; but the light of the sun makes it assume a white colour. Water dissolves it in large quantities. The solution which is known by the name of mucilage is thick and adhesive; it is often used as a paste, and to give thickness and lustre to linen. When spread out thin it soon dries, and has the appearance of a varnish; but it readily attracts moisture, and becomes glutinous. Water washes it away entirely. When mucilage is evaporated the gum is obtained unaltered.

Gum is insoluble in alcohol. When alcohol is poured into mucilage, the gum immediately precipitates; because the affinity between water and alcohol is greater than that between water and gum.

The action of alkalies and earths upon gum has not been

(a) The existence of albumen in vegetables was known to Scheele. He mentions it particularly in his paper on Milk, first published in the year 1780. See Scheele's Works, II. 55. Dijon edition.

(b) Hermitadt uses this word in a different sense. He makes a distinction between gum and mucilage. The solution of gum in water is transparent and glutinous, and can be drawn out into threads; whereas that of mucilage is opaque, does not feel glutinous, but slippery, and cannot be drawn into threads. Gum may be separated from mucilage by the following process:

Let the gum which is supposed to be mixed with mucilage, previously reduced to a dry mass, be dissolved in as small a quantity of water as possible, and into the solution drop at intervals diluted sulphuric acid. The mucilage coagulates while the gum remains dissolved. When no more coagulation takes place, let the mixture remain at rest for some time, and the mucilage will precipitate to the bottom, and assume the consistence of jelly. Decant off the liquid part, and evaporate the mucilage to dryness by a gentle heat till it acquires the consistence of horn. Med. and Phys. Journ. iii. 370. Extract is soluble in acids. Heat softens but does not melt it.

It is found in a great variety of plants; but no method of obtaining it perfectly pure has hitherto been discovered, the extracts of different plants differ somewhat from each other both in their colour and smell.

**Secr. VIII. Of Tan.**

If a quantity of nut galls, coarsely powdered, be kept for some time infused in cold water, if the water be filtered, and a solution of muriate of tin be dropped into it, a copious white precipitate falls to the bottom. This precipitate is to be carefully washed and diffused (for it will not dissolve) through a large quantity of water, and this water is to be saturated with sulphurated hydrogen gas to completely that it will not absorb any more. By this treatment the white precipitate will gradually disappear, and a brown precipitate will take its place. This brown precipitate must be separated by filtration; and the water, which has now acquired the colour and the taste of the infusion of nut galls, must be evaporated to dryness. A substance remains behind, known by the name of tan or tannin.

It was first discovered by Seguin, who pointed out some of its properties, and the method of detecting it in plants. The above method of obtaining it in a state of purity was contrived by Mr Proust. An extract is obtained in the solution of nut galls combined with gallic acid.

The oxyd of tin has a strong affinity for it. When muriate of tin is poured in, the tan combines with the oxyd, and the compound, being insoluble, falls to the bottom. Sulphur has a stronger affinity for the oxyd than tan has. Hence when sulphurated hydrogen gas is thrown upon this compound, the sulphur leaves the gas and combines with the tin; and the compound, being insoluble, falls to the bottom. The hydrogen gas escapes, and nothing remains in the water except the tan.

Tan is a brittle substance, of a brown colour. It breaks with a vitreous fracture, and does not attract moisture from the air. Its taste is exceedingly astringent. It is very soluble in water. The solution is of a deep brown colour, a very astringent and bitter taste, and has the odour which distinguishes a solution of nut galls. It froths, when agitated, like a solution of soap; but does not feel unctuous. Acids precipitate the tan from this solution.

Tan is still more soluble in alcohol than in water.

When the solution of tan is poured into a solution of the brown sulphate of iron, a deep blue coloured precipitate immediately appears, consisting of the tan combined with the oxyd. This precipitate, when dried, assumes a black colour. It is decomposed by acids. The green sulphate of iron is not altered by tan.

When too great a proportion of brown sulphate of iron is poured into a solution of tan, the sulphuric acid, set at liberty by the combination of the iron and tan, is sufficient to redissolve the precipitate as it appears; but the precipitate may easily be obtained by cautiously saturating this excess of acid with potash. When the experiment is performed in this manner, all the red sulphate of iron which remains in the solution undecomposed is converted into green sulphate. Mr Proust, to whom we are indebted for almost every thing yet known concerning the properties of tan, supposes that this change is produced... produced by the tan absorbing oxygen from the iron. This may very possibly be the case; but his experiments are insufficient to prove that it is. The same change takes place if red oxide be mixed with a considerable excess of sulphuric acid, and diluted with water.

Tan combines readily with oxygen. When oxymuriatic acid is poured upon it, its colour deepens, and it loses all its peculiar characters.

Tan exists in almost all those vegetable substances which have an astringent taste. It is almost constantly combined with gallic acid. The following table, drawn up by Mr Biggin, though the rule which the author followed in making his experiments precluded rigid accuracy, will serve to give some idea of the proportions of tan which exist in different plants:

| Prop. of Tan | Prop. of Tan | |--------------|-------------| | Elm | 2½ | | Oak cut in winter | 2½ | | Horse chestnut | 2½ | | Beech | 2 | | Willow (boughs) | 2¼ | | Elder | 3 | | Plum tree | 4 | | Willow (trunk) | 4 | | Sycamore | 4 | | Birch | 4 | | Cherry tree | 4½ | | Sallow | 4½ | | Mountain ash | 4½ | | Poplar | 6 | | Hazel | 6 | | Ash | 6½ | | Spanish chestnut | 9½ | | Smooth oak | 9 | | Oak cut in spring | 9½ | | Huntingdon or Lea willow | 10½ | | Sumach | 16½ |

Sect. IX. Of Oils.

There are two species of oils; namely, fixed and volatile; both of which are found abundantly in plants.

1. Fixed oil is found in the seeds of many plants, especially of the olive, beech, flax, almond, rape, &c.

2. Volatile oil is obtained by distillation from the leaves, flowers, or roots of aromatic plants, as lavender, roses, rosemary, &c.

As an account of the properties of oils has been given already in the article Chemistry, Suppl., it would be superfluous to repeat it here.

Sect. X. Of Camphor.

The laurem camphorata is a tree which grows in China, Japan, and several parts of India. When the roots of this tree are put into an iron pot furnished with a capital, and a sufficient heat is applied, a particular substance sublimes into the capital, which is known by the name of camphor. The Dutch afterwards purify this camphor by a second sublimation.

Camphor is a white brittle substance, having a peculiar aromatic odour and a strong taste.

It is not altered by atmospheric air; but it is so volatile that if it be exposed during warm weather in an open vessel, it evaporates completely. When sublimed in close vessels it crystallizes in hexagonal plates or pyramids.

It is insoluble in water; but it communicates to that liquid a certain portion of its peculiar odour.

It dissolves readily in alcohol, and is precipitated again by water. If the alcohol be diluted with water as much as possible, without causing the camphor to precipitate, small crystals of camphor resembling feathers gradually form.

Camphor is soluble also in hot oils, both fixed and volatile; but as the solution cools, the camphor precipitates, and assumes the form of plumose, or feather-like camphor crystals.

Camphor is not acted on by alkalis, either pure or in the state of carbonates. Pure alkalis indeed seem to dissolve a little camphor; but the quantity is too small to be perceptible by any other quality than its odour. Neither is it acted upon by any of the neutral salts which have hitherto been tried.

Acids dissolve camphor, but it is precipitated again, unaltered, by alkalis, and even by water. The solution of camphor in sulphuric acid is red; that in the nitric acid is yellow. This last solution has obtained the absurd name of oil of camphor. When nitric acid is distilled repeatedly off camphor, it converts it into camphoric acid.

Muriatic, fulphurous, and fluoric acids, in the state of gas, dissolve camphor. When water is added, the camphor appears unaltered in flakes, which float on the surface of the water.

When heat is applied to camphor it is volatilized. If the heat be sudden and strong, the camphor melts before it evaporates. It catches flame very readily, and emits a great deal of smoke as it burns, but it leaves no residuum. It is so inflammable that it continues to burn even on the surface of water. When camphor is set on fire in a large glass globe filled with oxygen gas, and containing a little water, it burns with a very bright flame, and produces a great deal of heat. The inner surface of the glass is soon covered with a black powder, which has all the properties of charcoal; a quantity of carbuncle acid gas is evolved, the water in the globe acquires a strong smell, and is impregnated with carbuncle acid and camphoric acid.

If two parts of alumina and one of camphor be formed into a paste with water, and distilled in a glass retort, there comes over into the receiver (which should be Grange, contain a little water, and communicate with a pneumatic apparatus) a volatile oil of a golden yellow colour, a little camphoric acid which dissolves in the water, and a quantity of carbuncle acid gas, and carbonated hydrogen gas, which may be collected by means of a pneumatic apparatus. There remains in the retort a residuum of a deep black colour, composed of alumina and charcoal. By this process, from 122.284 parts of camphor, Mr Bouillon la Grange, to whom we are indebted for the whole of the analysis of camphor, obtained 45.856 parts of volatile oil, and 30.571 parts of charcoal. The proportion of the other products was not ascertained.

From this analysis, Mr Bouillon la Grange concludes, that camphor is composed of volatile oil, and charcoal or carbon, combined together. We learn, from his experiments, that the ultimate ingredients of camphor are carbon and hydrogen; and that the proportion of carbon is much greater than in oils.

Camphor exists in a great many plants. Neumann, Geoffroy, and Cartheuler, extracted it from the roots containing it, of zedoary, thyme, sage, &c. and rendered it probable that it is contained in almost all the labiate plants. It has been supposed to exist in these plants combined with volatile oil. Proust has shown how it may be extracted, in considerable quantity, from many volatile oils.

Camphor, which was unknown to the ancient Greeks and Romans, was introduced into Europe by the Arabs. Sect. XI. Of Resins.

There is a yellowish white coloured substance which often exudes from the Abies Montana, or common Scotch fir, and likewise from other fir trees. It is somewhat transparent, is hard and brittle, of disagreeable taste, and may be collected in considerable quantities. This substance is known by the name of resin; and the same name is also applied to all substances which possess nearly the same properties with it. Resin may be distinguished from every other substance by the following properties:

1. It is more or less concrete, and has an acid and hot taste. 2. It is totally insoluble in water. By this property it may easily be separated from gum, if they happen to be mixed together. 3. It is soluble in alcohol, and in sulphuric ether. By the first of these properties we may separate it from gum, and by the last from extract; for extract is insoluble in sulphuric ether. When these solutions are evaporated the resin is obtained unaltered. If the solution be spread thin upon any body, it soon dries by the evaporation of the alcohol; the resin remains behind, and covers the body with a smooth shining transparent coat, which cannot be washed off by water. This process is called varnishing.

Resin is soluble also in volatile oils; and these solutions are often used likewise in varnishing.

Resin is scarcely acted upon by acids. Alkalies combine with it, but the combination is not easily effected.

When resin is heated it readily melts; and if the heat be increased it is volatilized, and burns with a white flame and strong smell. When distilled it yields much volatile oil, but scarcely any acid.

When volatile oils are exposed for some time to the action of the atmosphere they acquire constancy, and assume the properties of resins. During this change they absorb a quantity of oxygen from the air. Wellrum put 30 grains of oil of turpentine into 40 cubic inches of oxy-muriatic acid gas. Heat was evolved, the oil gradually evaporated, and assumed the form of yellow resin. These facts render it probable that resin is merely volatile oil combined with a quantity of oxygen.

To know whether any vegetable substance contains resin, we have only to pour some sulphuric ether upon it in powder, and expose the infusion to the light. If any resin be present the ether will assume a brown colour.

The number of resins is considerable. They differ from each other chiefly in colour, taste, smell, and consistence. Whether these resins be really different combinations, or, as is most likely, owe these differences to foreign ingredients, either combined with the resin, or mechanically mixed with it, is not at present known. To describe each resin separately would be to little purpose, as scarcely anything is known of them except their general properties as resins. The following is a list of the principal. The reader will find an account of the manner of obtaining them, and of their uses, by consulting the name of each in the Encyclopedia.

1. Common resin, 7. Sandarac, 2. Turpentine, 8. Guaiacum, 3. Pitch, 9. Labdanum, 4. Galipot, 10. Dragon's blood, 5. Elemi, 11. Copalba.

There are three vegetable substances which have been denominated balsams by some of the later French writers. They appear to consist of resin, or volatile oil combined with benzoic acid. These substances are, benzoin, balsam of Tolu, and storax. For an account of them we refer to the Encyclopedia.

Many vegetable substances occur in medicine which consist chiefly of a mixture of gum and resin. These substances, of course, have a number of the properties both of gums and resins. For this reason they have been denominated gum resins. The following are the most important of these substances:

Olibanum, Aloe, Galbanum, Myrrh, Scammony, Ammoniac, Asafoetida, Opium.

For an account of them we refer to the Encyclopedia.

Sect. XII. Of Caoutchouc.

About the beginning of the 18th century a substance, called caoutchouc, was brought as a curiosity from America. It was soft, wonderfully elastic, and very combustible. The pieces of it that came to Europe were usually in the shape of bottles, birds, &c. This substance is very much used in rubbing out the marks made upon paper by a black lead pencil; and therefore in this country it is often called Indian rubber. Nothing was known of its production, except that it was obtained from a tree, till the French academicians went to South America in 1735 to measure a degree of the meridian. Mr de la Condamine sent an account of it to the French Academy in the year 1736. He told them, that there grew in the province of Esmeraldas, in Brazil, a tree, called by the natives Hveo; that from this tree there flowed a milky juice, which, when jaspillated, was caoutchouc. Don Pedro Maldonado, who accompanied the French academicians, found the same tree on the banks of the Maranon; but he died soon after, and his papers were never published. Mr Trefus, after a very laborious search, discovered the same tree in Cayenne. His account of it was read to the French Academy in 1751.

It is now known that there are at least two trees in South America from which caoutchouc may be obtained, the Hvea Caoutchouc and the Jatropha Elattica; and it is exceedingly probable that it is extracted also from other species of Hvea and Jatropha. Several trees likewise which grow in the East Indies yield caoutchouc; the principal of these are, the Ficus Indica, the Artocarpus Integrifolia, and the Urecoa Elattica; a plant discovered by Mr Howison, and first described vi. 167, and named by Dr Roxburgh. When any of these plants is punctured, there exudes from it a milky juice, which, when exposed to the air, gradually lets fall a concrete substance, which is caoutchouc.

If oxy-muriatic acid be poured into the milky juice, the caoutchouc precipitates immediately, and, at the same time, the acid loses its peculiar odour. This renders it probable that the formation of the caoutchouc is owing to its basis absorbing oxygen*. If the milky juice be confined in a glass vessel containing common air, it gradually absorbs oxygen, and a pellicle of caoutchouc appears on its surface†.

Caoutchouc was no sooner known than it drew the attention of philosophers. Its singular properties promised that it would be exceedingly useful in the arts, provided any method could be fallen upon to mould it into various instruments for which it seemed peculiarly adapted. Messrs de la Condamine and Freynau had mentioned some of its properties; but Macquer was the first person who undertook to examine it with attention. His experiments were published in the memoirs of the French Academy for the year 1768. They threw a good deal of light on the subject; but Macquer fell into some mistakes, which were pointed out by Mr Berniard, who published an admirable paper on caoutchouc in the 17th volume of the Journal de Physique. To this paper we are indebted for the greater number of facts at present known respecting caoutchouc. Mr Groffart and Mr Fourcroy have likewise added considerably to our knowledge of this singular substance; both of their treatises have been published in the 11th volume of the Annales de Chimie.

Caoutchouc, when pure, is of a white colour (c), and without either taste or smell†. The blackish colour of the caoutchouc of commerce is owing to the method employed in drying it after it has been spread upon moulds. The usual way is to spread a thin coat of the milky juice upon the mould, and then to dry it by exposing it to smoke; afterwards another coat is spread on, which is dried in the same way. Thus the caoutchouc of commerce consists of numerous layers of pure caoutchouc alternating with as many layers of soil.

Caoutchouc is soft and pliable like leather. It is exceedingly elastic and adhesive; so that it may be forcibly stretched out much beyond its usual length, and instantly recover its former bulk when the force is withdrawn. It cannot be broken without very considerable force.

It is not altered by exposure to the air; it is perfectly insoluble in water; but if boiled for some time its edges become somewhat transparent, owing undoubtedly to the water carrying off the fat; and so soft, that when two of them are pressed and kept together for some time, they adhere as closely as if they formed one piece. By this contrivance pieces of caoutchouc may be foldered together, and thus made to assume whatever shape we please §.

Caoutchouc is insoluble in alcohol. This property was discovered very early, and fully confirmed by the experiments of Mr Macquer. The alcohol, however, renders it colourless.

Caoutchouc is soluble in ether. This property was first pointed out by Macquer. Berniard, on the contrary, found that caoutchouc was scarcely soluble at all in sulphuric ether, which was the ether used by Macquer, and that even nitric ether was but an imperfect solvent. The difference in the results of these two chemists was very singular; both were remarkable for their accuracy, and both were too well acquainted with the subject to be easily misled. The matter was first cleared up by Mr Cavallo. He found that ether, when newly prepared, seldom or never dissolved caoutchouc completely; but if the precaution was taken to wash the ether previously in water, it afterwards dissolved caoutchouc with facility. Mr Groffart tried this experiment, and found it accurate ||. It is evident from this that these chemists had employed ether in different states. The washing of ether has two effects. It deprives it of a little acid with which it is often impregnated, and it adds to it about one-tenth of water, which remains combined with it.

When the ether is evaporated, the caoutchouc is obtained unaltered. Caoutchouc, therefore, dissolved in ether, may be employed to make instruments of different kinds, just as the milky juice of the hevea; but this method would be a great deal too expensive for common use.

Caoutchouc is soluble in volatile oils*; but, in general, when these oils are evaporated, it remains somewhat glutinous, and therefore is scarcely proper for those uses to which, before its solution, it was so admirably adapted.

It is insoluble in alkalies†. The acids act upon it with more or less violence according to their nature. Sulphuric acid decomposes it completely, charcoal precipitates, and part of the acid is converted into sulphurous acid. Nitric acid converts it into a yellow substance, analogous to fulvic acid. Muriatic acid does not affect it ‡. The other acids have not been tried.

Fabroni has discovered, that rectified petroleum dissolves it, and leaves it unaltered when evaporated §.

When exposed to heat it readily melts; but it never afterwards recovers its properties, but continues always of the consistence of tar. It burns very readily with a bright white flame, and diffuses a fetid odour. In those countries where it is produced, it is often used by way of candle.

When distilled, it gives out ammonia $. It is evident from this, and from the effect of sulphuric acid and nitric acid upon it, that it is composed of carbon, hydrogen, azot, and oxygen; but the manner in which they are combined is unknown.

When treated with nitric acid, there came over azotic gas, carbonic acid gas, prussic acid gas; and oxalic acid was formed ||.

It seems to exist in a great variety of plants; but is usually confounded with the other ingredients. It may be separated from resins by means of alcohol. It may also be extracted from the different species of milk by water, with which, in the fluid state in which it exits in these plants, it readily combines. When mixed with gum or extract, it may be separated by the following process: Digest a part of the plant containing it first in water and then in alcohol, till all the substances soluble

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(c) Mr De Fourcroy says, that blackish brown is the natural colour of caoutchouc. But we have seen some pieces of it from the East Indies, which had been allowed to infuse in the open air; They were white, with a slight cast of yellow, and had very much the appearance and feel of white soap. VEGETABLE SUBSTANCES.

Sect. XIII. Of Wax.

The upper surface of the leaves of many trees is covered with a varnish of wax. This varnish may be separated and obtained in a state of purity by the following process:

Digest the bruised leaves, first in water and then in alcohol, till every part of them which is soluble in these liquids be extracted. Then mix the residuum with five times its weight of a solution of pure ammonia, and, after sufficient maceration, decant off the solution, filter it, and drop into it, while it is incessantly filtered, diluted sulphuric acid, till more be added than is sufficient to saturate the alkali. The wax precipitates in the form of a yellow powder. It should be carefully washed with water, and then melted over a gentle fire.

Mr Tingry first discovered that this varnish possessed all the properties of beeswax. Wax then is a vegetable product. The bees extract it unaltered from the leaves of trees and other vegetable substances which contain it. They seem, however, to mix it with some of the pollen of flowers.

Wax, when pure, is of a whitish colour, it is destitute of taste, and has scarcely any smell. Beeswax indeed has a pretty strong aromatic smell; but this seems chiefly owing to some substance with which it is mixed; for it disappears almost completely by exposing the wax, drawn out into thin ribands, for some time to the atmosphere. By this process also, which is called bleaching, the yellow colour of the wax disappears, and it becomes very white. Bleached wax is not affected by the air.

Wax is insoluble in water and in alcohol. It combines readily with alkalies, and forms with them a soap which is soluble in water.

Punic wax, which the ancients employed in painting in escutcheons, is a soap composed of twenty parts of wax and one of soda. Its composition was ascertained by

Sulphuric and nitric acids decompose wax completely; oxy-muriatic acid bleaches it instantaneously.

Wax combines readily with oils, and forms with them a substance of greater or less consistency according to the quantity of oil. This composition, which is known by the name of terne, is much employed by surgeons.

When heat is applied to wax it becomes soft; and at the temperature of 142°, if unbleached, or of 155° if bleached, it melts into a colourless transparent fluid, which concretes again, and resumes its former appearance as the temperature diminishes. If the heat be still farther increased, the wax boils and evaporates; and if a red heat be applied to the vapour, it takes fire and burns with a bright flame. It is this property which renders wax so useful for making candles.

Mr Lavoisier, by means of the apparatus described in the article Chemistry, Suppl. n° 353, contrived to burn wax in oxygen gas. The quantity of wax consumed was 21.9 grams. The oxygen gas employed in consuming that quantity amounted to 66.55 grams. Consequently the substances consumed amounted to 88.45 grams. After the combustion, there were found in the glass vessel 62.58 grams of carbonic acid, and a quantity of water, which was supposed to amount to 25.87 grams. These were the only products.

Now 62.58 grams of carbonic acid gas contain 44.50 of oxy., and 18.02 of carb.; and 25.87 gr. of water contain 21.90 of oxy., and 3.98 of hydro.

Consequently 21.9 parts of wax are composed of 18.02 of carbon, and 3.88 of hydrogen. And 100 parts of wax are composed of 82.28 carbon, 17.72 hydrogen.

If wax be distilled with a heat greater than 212°, there comes over a little water, some sebaceous acid, a little very fluid and odorous oil; the oil, as the distillation advances, becomes thicker and thicker, till at last it is of the consistence of butter, and for this reason has been called butter of wax. There remains in the retort a small quantity of coal, which is not easily reduced to ashes. When the butter of wax is repeatedly distilled it becomes very fluid, and assumes the properties of volatile oil.

Sect. XIV. Of the Woody Fibre.

All trees, and most other plants, contain a particular substance, well known by the name of wood. If a piece of wood be well dried, and digested, first in a sufficient quantity of water, and then of alcohol, to extract from it all the substances soluble in these liquids, there remains behind only the woody fibre.

This substance, which constitutes the basis of wood, is composed of longitudinal fibres, easily subdivided into a number of smaller fibres. It is somewhat transparent; is perfectly tasteless; has no smell; and is not altered by exposure to the atmosphere.

It is insoluble in water and in alcohol; but soluble in alkalies. The mineral acids decompose it. When distilled it yields, in all probability, pyrolignous acid. When burnt with a smothered fire it leaves behind it a considerable quantity of charcoal.

It is precipitated from alkalies unaltered by acids.

By nitric acid Fourcroy converted the residuum of quinquina, which does not seem to differ from the woody fibre, into oxalic acid; at the same time there was a little citric acid formed, and a very small quantity of malic and acetic acids. Some azotic gas also was disengaged. By this process he obtained from 100 parts of woody fibre

- 56.250 oxalic acid, - 3.925 citric acid, - 0.488 malic acid, - 0.486 acetic acid, - 0.867 azotic gas, - 8.330 carbonat of lime,

Total 70.216

There was likewise a quantity of carbonic acid gas disengaged, the weight of which was unknown. This increase increase of weight in the product was evidently owing to the oxygen derived from the nitric acid.

When distilled in a retort, 100 parts yield the following products:

- 26.62 of a yellow liquid, containing alcohol, and acid which had the smell of pyromusous. - 6.977 of concrete oil, mostly soluble in alcohol. - 22.905 charcoal - 3.567 carbonat of lime

in the retort.

60.159 gas; half carbonic acid, half carbonated hydrogen.

These facts show us, that the woody fibre is composed of oxygen, carbon, hydrogen, azot, and lime. Mr Chaptal supposes that mucilage differs from woody fibre merely in containing less oxygen. We are certain at least that mucilage or gum is composed of the same ingredients; and Mr Chaptal has shown, that the juices of plants are partly converted into woody fibre by oxy-muriatic acid, which imparts to them oxygen. These juices contain both gum and resin; after the formation of the woody fibre the resin is still unaltered. This gives a good deal of probability to his opinion.

**Sect. XV. Of Acids.**

The acids found ready formed in vegetables are the following:

1. Oxalic, 2. Tartarous, 3. Citric, 4. Malic, 5. Gallic, 6. Benzoic, 7. Phosphoric.

Sometimes also the sulphuric, nitric, and muriatic acids occur in vegetables, combined with alkalies or earths, but never except in very minute quantities.

1. Oxalic acid is easily detected and distinguished by the following properties: It decomposes all calcareous salts, and forms with lime a salt insoluble in water. It readily crystallizes. Its crystals are quadrilateral prisms. It is totally destroyed by heat.

Oxalic acid was first detected in vegetables by Mr Scheele. It has been discovered in the following plants:

- The leaves of the oxalis acetofella. - Oxalis corniculata. - The root of rhubarb. - The leaves of the geranium acidum. - The pulp of the tamarind. - The juice of grapes. - Mulberries. - Rumex acetofa, sorrel. - Rhus coriaria, sumach. - Rheum rhaboticum. - Agave Americana. - The roots of triticum repens. - Lactodon taraxicum.

3. Citric acid is distinguished by the following properties: It does not form tartar when potash is added to it. With lime it forms a salt insoluble in water. Citric acid, which is decomposed by sulphuric, nitric, and muriatic acids. It readily crystallizes. It is destroyed by heat.

Citric acid has been found unmixed with other acids in the following vegetable substances:

- The juice of oranges and lemons. - The berries of vaccinium oxycoccus, cranberry. - Vitex idaea, red whortle berry. - Prunus padus, bird cherry. - Solanum dulcamara, nightshade. - Rosa canina, hip.

It occurs mixed with other acids in many other fruits.

4. Malic acid is known by the following properties: Malic acid, it forms with lime a salt soluble in water, which is decomposed by citric acid. It does not form tartar with potash. It is incrustable. Heat destroys it.

Malic acid has been found, by Scheele, in the fruits of the following plants, which contain no other acid:

- Apples. - Berberis vulgaris, barberry. - Prunus domestica, plum. - Spinacia, spinach. - Sambucus nigra, elder. - Sorbus aucuparia, rowan or service.

In the following fruits he found nearly an equal quantity of malic and citric acids:

- Ribes grossularia, gooseberry. - Rubus rubrum, currant. - Vaccinium myrtillus, blueberry. - Crataegus aria, hawthorn. - Prunus cerasus, cherry. - Fragaria vesca, strawberry. - Rubus chamaemorus, cloudberry, cowberry. - Ilex aquifolium, holly. - Malus sylvestris, apple.

Malic acid has also been found in the agave americana, and in the pulp of tamarinds. In the first of these it is mixed with tartarous acid; in the second with tartarous and citric acids.

5. Gallic acid is known by the following properties: With the brown oxide of iron it produces a black colour. It is crystallizable. Heat destroys it. It has been found in a great number of plants, chiefly in the bark. The following table, drawn up by Mr Biggin, will serve to show the relative proportions of this acid in different plants:

| Plant | Proportion | |----------------|------------| | Elm | 7 | | Oak cut in winter | 8 | | Horse chestnut | 6 | | Beech | 7 | | Willow (boughs) | 8 | | Elder | 4 | | Plum tree | 8 | | Willow (trunk) | 9 | | Sycamore | 6 | | Birch | 4 | | Cherry tree | 8 |

6. Benzoic acid is distinguished by its aromatic odour and its volatility on the application of a very moderate heat. It has been found hitherto only in three vegetable substances, to which the French chemists have confined the term balsam. These three are, benzoin, balsam of iolus, and florax. In these substances it seems to be combined with a resin, or something which has nearly the properties of a resin. Phosphoric acid is easily distinguished from the former fix, for it is very fixed, and a violent heat does not destroy it as it does the others.

Phosphoric acid has been found in different plants, but only in very small quantities; it is almost constantly combined with lime. Meyer found it in the leaves of many trees*; Thuren found phosphat of lime in the Physalis, Aconites, Napolus†; and Bergmann found it in all kinds of grain‡.

Sect. XVI. Of Alkalies.

The only alkalies found in plants are potasfs and soda. Ammonia may indeed be obtained by distilling many vegetable substances, but it is produced during the operation. One or other of these alkalies is found in every plant which has hitherto been examined. The quantity indeed is usually very small.

Potasfs is found in almost all plants which grow at a distance from the sea. It may be extracted by burning the vegetables, washing the ashes in water, filtrating the water, and evaporating it to dryness. It is in this manner that all the potasfs of commerce is procured.

The following table exhibits the quantity of ashes and potasfs which may be extracted from 100 parts of various plants:

| Plant | Ashes | Potasfs | |----------------|-------|---------| | Sallow | 2.8 | 0.285 | | Elm | 3.6727| 0.39 | | Oak | 3.5185| 0.5343 | | Poplar | 3.4746| 0.07481 | | Hornbeam | 1.283 | 0.1254 | | Beech | 0.58432| 0.14572 | | Fir | 0.34133| | | Vine branches | 3.379 | 0.55 | | Common nettle | 6.7186| 2.5033 | | Common thistle | 4.04265| 0.53734 | | Fern | 5.0781| 0.6239 | | Cow thistle | 10.5 | 1.96603 | | Great river rush | 3.85395| 0.72234 | | Feathered rush | 4.3393| 0.50811 | | Stalks of turkey wheat | 8.86 | 1.75 | | Wormwood | 9.744 | 7.3 | | Fumitory | 21.9 | 7.9 | | Trifolium pratense | 0.078 | | | Vetches | 2.75 | | | Beans with their stalks | 2.0 | |

In general, three times as much ashes are obtained from shrubs, and five times as much from herbs, as from trees. Equal weights of the branches of trees produce more ashes than the trunk, and the leaves more than the branches. Herbs arrived at maturity produce more ashes than at any other time. Green vegetables produce more ashes than dry†.

The salt which is obtained from plants does not consist wholly of potasfs, there are other salts mixed with it; these usually are sulphat of potasfs, muriat of potasfs, sulphat of lime, phophat of lime, &c.; but these bear, in general, but a small proportion to the potasfs. The ashes consist of potasfs mixed with earths.

Some judgment may be formed of the quantity of potasfs which a plant contains from the quantity of ashes which it yields; but the above table is sufficient to shew us, that were we to trust to that we would often be misled.

Soda is found in almost all the plants which grow in the sea, and in many of those which grow on the shore. In general, the quantity of soda which plants contain bears a much greater proportion to their weight than the potasfs does which is found in inland vegetables. 100 parts of the saltpota soda, for instance, yield 19.221 of ashes; and these contain 1.992 parts of soda, some of which, however, is combined with muriatic acid*. The plants from which the greater part of the potasfs soda, or barilla, as it is called, which is imported from Spain, is extracted, are the saltpota falcata, and wormwood.

Sect. XVII. Of Earths.

The only earths hitherto found in plants are the four following: lime, silica, magnesia, alumina.

1. Lime is usually the most abundant of the earths of plants, and the most generally diffused over the vegetable kingdom. Indeed, it is a very uncommon thing to find a plant entirely destitute of lime: lalola soda is almost the only one in which we know for certain that this earth does not exist*.

2. Silica exists also in many plants, particularly grasses and equisetums. Mr Davy has ascertained, that it forms a part of the epidermis, or outermost bark of these plants; and that in some of them almost the whole epidermis is silica.

The concretions which are sometimes found in the bamboo cane have been ascertained by Mr Macie to be composed of pure silica.

3. Magnesia does not exist so generally in the vegetable kingdom as the two preceding earths. It has been found, however, in considerable quantities in several sea plants, especially fuci†. But the saltpota soda contains a greater proportion of magnesia than any plant hitherto examined. Mr Vauquelin found that 100 parts of it contained 17.929 of that earth‡.

4. Alumina has only been found in very small quantities in plants.

The following table will shew the quantity of these four earths which exist in several vegetables.

| Plant | Earths | |----------------|--------| | Beech | 0.453† | | Fir | 0.003† | | Turkey wheat | 7.11† | | Sunflower | 3.72† | | Vine branches | 2.85† | | Box | 2.67† | | Willow | 2.51† | | Elm | 1.96† | | Aspin | 1.14† | | Fern | 3.22† | | Wormwood | 2.44† | | Fumitory | 14.000†|

(c) Those marked * are from Kirwan, Irish Trans. v. 164. The rest from Pertuis, Ann. de Chim. 19. 178. This table shows us, that the quantity of earth is greater in herbs than in trees.

Bergman found all the four earths in every kind of grain which he analysed.

Vauquelin found, that 100 parts of oat grain left 3.1591 of residuum. This residuum is composed of 60.7 silica, 39.3 phosphat.

When the whole of the avena sativa, however, stalk and seed together, are burnt, they leave a residuum composed of 55 silica, 15 phosphat of lime, 20 potash, 5 carbonat of lime.

This shows us that the stalk contains several substances not to be found in the grain.

Sect. XVIII. Of Metals.

Several metallic substances have also been found in vegetables, but their quantity is exceedingly small; so small, indeed, that without very delicate experiments their presence cannot even be detected.

The metals hitherto discovered are iron, which is by far the most common, manganese, and gold.

We have now taken a survey of all the substances which have hitherto been obtained from vegetables: by analysing each of these, we come at last to those bodies which we are at present obliged to consider as simple, because they have not yet been decomposed, and of which accordingly we must suppose that vegetables are ultimately composed. These bodies amount to 16, namely:

1. Oxygen, 2. Sulphur, 3. Phosphorus, 4. Carbon, 5. Hydrogen, 6. Azot, 7. Iron, 8. Manganese, 9. Gold, 10. Lime, 11. Magnesia, 12. Silica, 13. Alumina, 14. Potash, 15. Soda, 16. Muriatic acid.

But of these substances there are twelve which compose but a very small proportion indeed of vegetables. Almost the whole of vegetable substances are composed of four ingredients, namely,

Carbon, Oxygen, Hydrogen, Azot.

Of these the last, namely azot, forms but a small proportion even of those vegetable substances of which it is a constituent part, while into many it does not enter at all: So that, upon the whole, by far the greater part of vegetable substances is composed of carbon, hydrogen, and oxygen. We do not mention caloric and light, concerning the nature of which too little is known to enable us to determine with certainty into what substances they enter.

The substances at present known to chemists, which they have not been hitherto able to decompose, amount (omitting caloric and light) to 40. Sixteen of these exist in plants; the other 24 belong exclusively to the mineral kingdom: for it is a fact, that no substance (we mean simple substance) has been hitherto found in the animal kingdom which does not exist also in vegetables.

On the contrary, all the simple substances at present known may be found in minerals. This indeed ought not to surprise us, if we recollect, that the spoils of animals and vegetables, after they have undergone decomposition, are ultimately confounded with minerals, and consequently arranged under the mineral kingdom. Besides, as vegetables draw all their food from the mineral kingdom, it would be absurd to suppose that they contain substances which they could not have procured from minerals. It must follow, therefore, of necessity, that minerals contain all the simple substances which exist in this globe of ours; and that plants owe their diversity merely to different modifications of those principles which they imbibe from the soil. But it is impossible to have any precise notions about a subject so intricate, without considering with some attention the structure of vegetables, the food which they imbibe, and the changes which they produce on that food. These enquiries shall form the subject of the next chapter; in which we propose to take a view of those phenomena of vegetation which are connected with chemistry, or which may be elucidated by the application of the principles of that science.

Chap. II. Of Vegetation.

We have now seen the different substances which are contained in plants; but we have still to examine the manner in which these substances are produced, and to endeavour to trace the different processes which constitute vegetation. We must warn our readers not to expect complete information in this chapter. The wonders of the vegetable kingdom are still but very imperfectly explored; many of the organs of plants are too minute for our senses; and scarcely a single process can be completely traced.

The multiplicity of operations continually going on in vegetables at the same time, and the variety of different substances, and even opposite substances, formed out of the same ingredients, and almost in the same place, astonish and confound us. The order, too, and the skill with which every thing is conducted, are no less surprising. No two operations clash; there is no discord, no irregularity, no disturbance; every object is gained, and everything is ready for its intended purpose. This is too wonderful to escape our observation, and of too much importance not to claim our attention. Many philosophers, accordingly, distinguished equally by their industry and sagacity, have dedicated a great part of their lives to the study of vegetation. But hitherto their success has not been equal to their exertions. No person has been able to detect this agent, always so busy, and performing such wonders, or to discover him at his work; nor have philosophers been much more fortunate. Vegetable Substances.

Plants arise from seeds. Natural historians have proved, by a very complete induction of facts, that all plants arise from seeds. The pretended exceptions have disappeared, one after another, as our knowledge of vegetables increased; and now there remains scarcely a single objection entitled to the smallest regard. The late attempt of Girtanner* to revive the doctrine of equivocal generation, deserves no attention whatever; because his conclusions are absolutely incompatible with the experiments of Mr Senebier upon the very substance on which his theory is founded.

A seed consists of three parts; namely, the cotyledons, the radicle, and the plumula, which are usually inclosed in a cover.

If we take a garden bean, we may perceive each of these three parts with great ease; for this seed is of so large a size, that all its organs are exceedingly distinct.

When we strip off the external coats of the bean, which are two, and of different degrees of thickness in different parts, we find that it easily divides into two lobes, pretty nearly of the same size and figure. Each of these lobes is called a cotyledon (fig. r. a). The cotyledons of the bean, then, are two in number.

Near that part of the lobes which is contiguous to what is called the eye of the bean, there is a small round white body (b), which comes out between the two lobes. This body is called the radicle.

Attached to the radicle, there is another small round body (c), which lies between the cotyledons and wholly within them, so that it cannot be seen till they are separated from each other. This body is called the plumula.

The appearance and shape of these three parts differ very much in different seeds, but there is no seed which wants them. The figure and size of the seed depend chiefly upon the cotyledons. This is evidently the case with the bean, and it is so with all other seeds. The number of cotyledons is different in different seeds. Some seeds have only one cotyledon, as the seeds of wheat, oats, barley, and the whole tribe of grasses; some have three; others six, as the seeds of the garden peas; but most seeds, like the bean, have two cotyledons.

2. When a seed is placed in a situation favourable to vegetation, it very soon changes its appearance. The radicle is converted into a root, and sinks into the earth; the plumula, on the other hand, rises above the earth, and becomes the trunk or stem. When these changes take place, the seed is said to germinate; the process itself has been called germination. Seeds do not germinate equally, and indifferently in all places and seasons. Germination, therefore, is a process which does not depend upon the seed alone; something external must also affect it.

3. It is a well-known fact, that seeds will not germinate unless moisture have access to them; for seeds, if they are kept perfectly dry, never vegetate at all, and yet their power of vegetating is not destroyed. There are indeed some apparent objections to this: potatoes, for instance, and other bulbous bodies, germinate, though kept ever so dry. But the reason of this is, that these bodies (which are not seeds, though they resemble them in some particulars) have a sufficient quantity of water within themselves to give a beginning to germination. We may conclude, then, that no seed will germinate unless water has access to it. Water, then, is essential to germination. Too much water, however, is no less prejudicial to most seeds than none at all. The seeds of water plants, indeed, germinate and vegetate extremely well in water; but most other seeds, if they are kept in water beyond a certain time, are rotted and destroyed altogether.

4. It is well known also, that seeds will not germinate, even though supplied with water, provided the temperature be below a certain degree. No seed, for instance, on which the experiment has been tried, can be made to vegetate at or below the freezing point; yet this degree of cold does not injure the vegetating power of seeds; for many seeds will vegetate as well as ever after having been frozen, or after having been kept in frozen water. We may conclude, then, that a certain degree of heat is necessary for the germination of seeds. And every species of plants seems to have a degree peculiar to itself, at which its seeds begin to germinate; for we find that almost every seed has a peculiar season at which it begins to germinate, and this season varies always according to the temperature of the air. Mr Adanson found that seeds, when sown at the same time in France and in Senegal, always appeared sooner above ground in the latter country, where the climate is hotter, than in France.

5. Seeds, although supplied with moisture, and placed in a proper temperature, will not germinate, provided atmospherical air be completely excluded from them. Mr Ray found that grains of lettuce did not germinate in the vacuum of an air-pump, but they began to grow as soon as air was admitted to them. Mr Homberg made a number of experiments on the same subject, which were published in the Memoirs of the French Academy for the year 1693. He found, that the greater number of seeds which he tried refused to vegetate in the vacuum of an air-pump. Some, however, did germinate; but Boyle, Mutenbroek, and Boerhaave, who made experiments on the same subject in succession, proved beyond a doubt, that no plant vegetates in the vacuum of an air-pump; and that in those cases in which Homberg's seeds germinated, the vacuum was far from perfect, a quantity of air still remaining in the receiver. It follows, therefore, that no seed will germinate unless atmospherical air, or some air having the same properties, have access to it. It is for this reason that seeds will not germinate at a certain depth below the surface of the earth.

Mr Scheele found that beans would not germinate except oxygen gas were present; Mr Achard afterwards proved, that oxygen gas is absolutely necessary for the germination of all seeds, and that no seed will germinate in azotic gas, or hydrogen gas, or carbonic acid gas, unless these gases contain a mixture of oxygen gas. These experiments have been confirmed by Mr Mr Gough, Mr Cruickshank, and many other philosophers. It follows, therefore, that it is not the whole atmospheric air, but merely the oxygen gas which it contains, that is necessary for the germination of seeds.

6. Seeds do not germinate equally well when they are exposed to the light, and when they are kept in a dark place; light therefore has some effect on germination.

Mr Ingenhouz found, that seeds always germinate faster in the dark than when exposed to the light. His experiments were repeated by Mr Senebier with equal success; and it was concluded, in consequence of their experiments, that light is injurious to germination. But the Abbé Bertholin, who distinguished himself so much by his labours to demonstrate the effect of electricity on vegetation, objected to the conclusions of these philosophers, and affirmed, that the difference in the germination of seeds in the shade and in the light was owing, not to the light itself, but to the difference of the moisture in the two situations; the moisture evaporating much faster from the seeds in the light than from those in the shade; and he affirmed, that when precautions were taken to keep the seeds equally moist, those in the sun germinated sooner than those in the shade. But when Mr Senebier repeated his former experiments, and employed every possible precaution to ensure the equality of moisture in both situations, he constantly found the seeds in the shade germinate sooner than those in the light. We may conclude, therefore, that light is injurious to germination; and hence one reason for covering seeds with the soil in which they are to grow.

7. Thus we have seen that seeds will not germinate unless moisture, heat, and oxygen gas, be present; and that they do not germinate well if they are exposed to the action of light. Now, in what manner do these substances affect the seed? What are the changes which they produce?

We observed before, that all seeds have one or more cotyledons. These cotyledons contain a quantity of farinaceous matter, laid up on purpose to supply the embryo plant with food as soon as it begins to require it. This food, however, must undergo some previous preparation, before it can be applied by the plant to the formation or completion of its organs. Now all the phenomena of germination which we can perceive consist in the chemical changes which are produced in that food, and the consequent development of the organs of the plant.

When a seed is placed in favourable circumstances, it gradually imbibes moisture, and very soon after emits a quantity of carbonic acid gas, even though no oxygen gas be present. This seems to prove, as Mr Cruickshank has supposed, that some of the water imbibed by the seed is decomposed, that its oxygen combines with part of the carbon of the farina, and goes off in the form of carbonic acid gas, while the hydrogen remains behind, and combines with the ingredients contained in the cotyledon. The first part of germination, then, consists in diminishing the quantity of carbon, and increasing the hydrogen of the farina. If no oxygen gas be present, the process stops here, and no germination takes place.

But if oxygen gas be present, it is gradually absorbed and retained by the seed; and at the same time, the farina of the cotyledons assumes a sweet taste resembling sugar; it is therefore converted into sugar, or some substance analogous to it. Farina, then, is changed into sugar, by diminishing its carbon, and augmenting the proportion of its hydrogen and oxygen. This is precisely the process of malting, or of converting grain into malt; during which it is well known that there is a considerable heat evolved; so much indeed, that in certain circumstances grain improperly kept has even taken fire. We may conclude from this, that during the germination of seeds in the earth there is also an evolution of a considerable portion of heat. This indeed might have been expected, as it usually happens when oxygen gas is absorbed.

So far seems to be the work of chemistry alone; at least we have no right to conclude that any other agent interferes; since hay, when it happens to imbibe moisture, exhibits nearly the same processes. Carbonic acid gas is evolved, oxygen gas is absorbed, heat is produced so abundantly, that the hay often takes fire; at the same time a quantity of sugar is formed. It is owing to a partial change of the same kind that old hay generally tastes much sweeter than new hay. Now we have no reason to suppose that any agents peculiar to the vegetable kingdom reside in hay; as all vegetation, and all power of vegetating, are evidently destroyed.

But when the farina in the seeds of vegetables is converted into sugar, a number of vessels make their appearance in the cotyledon. The reader will have a pretty distinct notion of their distribution, by inspecting fig. 2. These vessels may indeed be detected in many seeds before germination commences, but they become much more distinct after it has made some progress. Branches from them have been demonstrated by Grew, Malpighi, and Hedwig, passing into the radicle, and distributed through every part of it. They evidently carry the nourishment prepared in the cotyledons to the radicle; for if the cotyledons be cut off even after the processes above described are completed, germination, as Bonnet and Senebier ascertained by experiment, immediately stops. The food therefore is immediately carried from the cotyledons into the radicle, the radicle increases in size, assumes the form of a root, sinks down into the earth, and soon becomes capable of extracting the nourishment necessary for the future growth of the plant. Even at this period, after the radicle has become a perfect root, the plant, as Senebier ascertained by experiment, ceases to vegetate if the cotyledons be cut off. They are still then absolutely necessary for the vegetation of the plant.

The cotyledons now assume the appearance of leaves, and appear above the ground, forming what are called the seminal leaves of the plant. After this the plumula gradually increases in size, rises out of the earth, and expands itself into branches and leaves. The seminal leaves, soon after this, decay and drop off, and the plant carries on all the processes of vegetation without their assistance.

Mr Eller attempted to shew, that there is a vessel in seeds which passes from the cotyledons to the plumula; but later anatomists have not been able to perceive any such vessel. Even Mr Hedwig, one of the most patient, acute, and successful philosophers that ever turned their attention to the structure of vegetables, could never... vegetable substances.

never discover any such vessel, although he traced the vessels of the cotyledons even through the radicle. As it does not appear, then, that there is any communication between the cotyledons and the plumula, it must follow that the nourishment passes into the plumula from the radicle; and accordingly we see, that the plumula does not begin to vegetate till the radicle has made some progress. Since the plant ceases to vegetate, even after the radicle has been converted into a root, if the cotyledons be removed before the plumula is developed, it follows, that the radicle is insufficient of itself to carry on the processes of vegetation, and that the cotyledons still continue to perform a part. Now we have seen already what that part is: they prepare food for the nourishment of the plant. The root, then, is of itself insufficient for this purpose. When the cotyledons assume the form of seminal leaves, it is evident that the nourishment which was originally laid up in them for the support of the embryo plant is exhausted, yet they still continue as necessary as ever. They must therefore receive the nourishment which is imbibed by the root; they must produce some changes on it, render it suitable for the purposes of vegetation, and then send it back again to be transmitted to the plumula.

After the plumula has acquired a certain size, which must be at least a line, if the cotyledons be cut off, the plant, as Mr Bonnet ascertained by a number of experiments, afterwards repeated with equal success by Mr Senebier, does not cease to vegetate, but it continues always a mere pigmy in its size, when compared with that of a plant whose cotyledons are allowed to remain, being only as 2 to 7.

When the plumula has expanded completely into leaves, the cotyledons may be removed without injuring the plant, and they very soon decay of themselves. It appears, then, that this new office of the cotyledons is afterwards performed by that part of the plant which is above ground.

Thus we have traced the phenomena of germination as far as they have been detected. The facts are obvious; but the manner in which they are produced is a profound secret. We can neither explain how the food enters into the vessels, how it is conveyed to the different parts of the plant, how it is deposited in every organ, nor how it is employed to increase the size of the old parts, or to form new parts. These phenomena are analogous to nothing in mechanics or chemistry. He that attempts to explain them on the principles of these sciences, merely substitutes new meanings of words instead of old ones, and gives us no assistance whatever in conceiving the processes themselves. As the substances employed in vegetation are all material, it is evident that they possess the properties of matter, and that they are arranged in the plant according to these laws. It follows, therefore, that all the changes which take place in the plant are produced according to the known laws of mechanics and chemistry. This cannot be disputed; but it explains nothing; for what we want to know is the agent that brings every particle of matter to its proper place, and enables the laws of chemistry and mechanics to act only in order to accomplish a certain end. Who is the agent that acts according to this end? To say, that it is chemistry or mechanics is to pervert the use of words. For what are the laws of chemistry and mechanics? Are they not certain fixed and unalterable properties of matter? Now, to say that a property of matter has an end in view, or that it acts in order to accomplish some design, is a downright absurdity. There must therefore be some agent in all cases of germination, which regulates and directs the mechanical and chemical processes, and which therefore is neither a mechanical nor chemical property.

8. When the process of germination is accomplished, the plant is complete in all its parts, and capable of vegetating in a proper soil, for a time and with a vigour proportional to its nature.

Plants, as everybody knows, are very various, and of course the structure of each species must have many peculiarities. Trees have principally engaged the attention of anatomists, on account of their size and the distinctness which they expected to find in their parts. We shall therefore take a tree as an instance of the structure of plants; and we shall do it the more readily, as the greater number of vegetables are provided with analogous organs, dedicated to similar uses.

A tree is composed of a root, a trunk, and branches; the structure of each of which is so familiar, that a general description of their component parts will be sufficient. Each of them consists of three parts, the bark, the wood, and the pith.

The bark is the outermost part of the tree. It covers the whole plant from the extremity of the roots to the extremity of the branches. It is usually of a green colour: if a branch of a tree be cut across, the bark is easily distinguished from the rest of the branch by this colour. If we inspect such a horizontal section with attention, we shall perceive that the bark itself is composed of three distinct bodies, which, with a little care, may be separated from each other. The outermost of these bodies is called the epidermis, the middlemost is called the parenchyma, and the innermost, or that next the wood, is called the cortical layers.

The epidermis is a thin transparent membrane, which covers all the outside of the bark. It is pretty tough, of epithelial composition, when inspected with a microscope, it appears to be composed of a number of slender fibres crossing each other, and forming a kind of network. It seems even to consist of different thin retiform membranes, adhering closely together. This, at least, is the case with the epidermis of the birch, which Mr Dulamel separated into six layers. The epidermis, when rubbed off, is reproduced. In old trees it cracks and decays, and new epidermes are successively formed. This is the reason that the trunks of many old trees have a rough surface.

The parenchyma lies immediately below the epidermis; it is of a deep green colour, very tender, and succulent. When viewed with a microscope, it seems to be composed of fibres which cross each other in every direction, like the fibres which compose a hat. Both in it and the epidermis there are numberless interstices, which have been compared to many small bladders.

The cortical layers form the innermost part of the bark, or that which is next to the wood. They consist of several thin membranes, lying one above the other; and their number appears to increase with the age of the plant. Each of these layers is composed of longitudinal fibres, which separate and approach each other alternately, so as to form a kind of network. The meshes of this network correspond in each of the layers; vegetables; and they become smaller and smaller in every layer as it approaches the wood. These meshes are filled with a green coloured cellular substance, which has been compared by anatomists to a number of bladders adhering together, and communicating with each other.

The wood lies immediately under the bark, and forms by far the greatest part of the trunk and large branches of trees. It consists of concentric layers, the number of which increases with the age of the part. Each of these layers, as Mr Du Hamel ascertained, may be separated into several thinner layers, and these are composed chiefly of longitudinal fibres. Hence the reason that wood may be much more easily split asunder than cut across.

The wood, when we inspect it with attention, is not, through its whole extent, the same; the part of it next the bark is much softer and whiter, and more juicy than the rest, and for that reason obtained a particular name: it has been called the alburnum or ambrosia. The perfect wood is browner, and harder, and denser, than the alburnum, and the layers increase in density the nearer they are to the centre. Sir John Hill gave to the innermost layer of wood the name of corona, or rather he gave this name to a thin zone which, according to him, lies between the wood and the pith.

The pith occupies the centre of the wood. It is a very spongy body, containing a prodigious number of cells, which anatomists have compared to bladders. In young shoots it is very succulent; but it becomes dry as the plant advances, and at last in the large trunks of many trees disappears altogether.

The leaves are attached to the branches of plants by short footstalks. From these footstalks a number of fibres issue, which ramify and communicate with each other in every part of the leaf, and form a very curious network. These fibres may be obtained separately, by keeping the leaf long in moisture. Every other part of it putrefies and falls off, or may easily be rubbed off, and only the fibres remain, constituting a skeleton of the leaf. In every leaf there are two layers of these fibres, forming two distinct skeletons, which had constituted the upper and under surface of the leaf.

The whole leaf is covered with the epidermis of the plant; and this epidermis, as Sauflure has shown, contains in it a great number of glands. The other parts of the bark may also be traced on many leaves; at least Sauflure has shown, that the bark of leaves is composed of two different layers. The interfaces between the fibres of the leaf are filled up by a pulpy-like substance, to which the green colour of the leaf is owing.

Such is a short description of the most conspicuous parts of plants. A more minute account would have been foreign to the subject of the present article.

9. Plants, after they have germinated, do not remain stationary, but are continually increasing in size. A tree, for instance, every season, adds considerably to its former bulk. The root sends forth new shoots, and the old ones become larger and thicker. The same increment takes place in the branches and the trunk. When we examine this increase more minutely, we find that a new layer of wood, or rather of alburnum, has been added to the tree in every part, and this addition has been made just under the bark. We find, too, that a layer of alburnum has assumed the appearance of perfect wood. Besides this addition of vegetable fibre, a great number of leaves have been produced; and the tree puts forth flowers, and forms seeds.

It is evident from all this, that a great deal of new matter is continually making its appearance in plants. Therefore hence, since it would be absurd to suppose that they require create new matter, it must follow that they receive it by some channel or other. Plants, then, require food as well as animals. Now, what is this food, and whence do they derive it? These questions can only be answered by an attentive survey of the substances which are contained in vegetables, and an examination of those substances which are necessary for their vegetation. If we could succeed completely, it would throw a great deal of light upon the nature of soils and of manures, and on some of the most important questions in agriculture. But we are far indeed at present from being able to examine the subject to the bottom.

10. In the first place, it is certain that plants will not vegetate without water; for whenever they are deprived of it, they wither and die. Hence the well-known use of rains and dews, and the artificial watering of ground. We may conclude, then, that water is at least an essential part of the food of plants.

But many plants grow in pure water; and therefore it may be questioned whether water is not the only food of plants. This opinion was adopted very long ago, and numerous experiments have been made in order to demonstrate it. Indeed, it was the general opinion of the 17th century; and some of the most successful improvers of the physiology of plants, in the 18th century, have embraced it. The most zealous advocates for it were, Van Helmont, Boyle, Bonnet, Duhamel, and Tillet.

Van Helmont planted a willow which weighed five pounds, in an earthen vessel filled with soil previously dried in an oven, and moistened with rain water. This vessel he sunk into the earth, and he watered his willow, sometimes with rain, and sometimes with distilled water. After five years it weighed 169½ lbs. and the earth in which it was planted, when again dried, was found to have lost only two ounces of its original weight. Here, it has been said, was an increase of 164 lbs. and yet the only food of the willow was pure water; therefore it follows that pure water is sufficient to afford nourishment to plants. The insufficiency of this experiment to decide the question was first pointed out by Bergman in 1772. He showed, from the experiments of Margraff, that the rain water employed by Van Helmont contained in it as much earth as could exist in the willow at the end of five years. For, according to the experiments of Margraff, 1 lb. of rain water contains 1 gr. of earth. The growth of the willow, therefore, by no means proves that the earth which plants contain has been formed out of water. Besides, as Mr Kirwan has remarked, the earthen vessel must have often absorbed moisture, from the surrounding earth, impregnated with whatever substance that earth contained; for unglazed earthen vessels, as Hales and Tillet have shown, readily transmit moisture.

Hence it is evident that no conclusion whatever can be drawn from this experiment; for all the substances which the willow contained, except water, may have been derived from the rain water, the earth in the pot, and the moisture imbibed from the surrounding soil. The experiments of Duhamel and Tillet are equally inconclusive; so that it is impossible from them to decide the question, Whether water be the sole nourishment of plants or not? We owe the solution of this difficulty to the experiments of Mr Hassenfratz, who pointed out the fallacy of those just mentioned.

He analyzed the bulbous roots of hyacinths, in order to discover the quantity of water, carbon, and hydrogen, which they contained; and by repeating the analysis on a number of bulbs, he discovered how much of these ingredients was contained in a given weight of the bulb. He analyzed also kidney beans and cereals seeds in the same manner. Then he made a number of each of these vegetate in pure water, taking the precaution to weigh them beforehand, in order to ascertain the precise quantity of carbon which they contained. The plants being then placed, some within doors, and others in the open air, grew and flowered, but produced no seed. He afterwards dried them, collecting with care all their leaves and every other part which had dropped off during the course of the vegetation. On submitting each plant to a chemical analysis, he found that the quantity of carbon, which it contained, was somewhat less than the quantity which existed in the bulb or the seed from which the plant had sprung.

Hence it follows irresistibly, that plants growing in pure water do not receive any increase of carbon; that the water merely serves as a vehicle for the carbonaceous matter already present, and diffuses it thro' the plant. Water, then, is not the sole food of plants; for all plants during vegetation receive an increase of carbonaceous matter, without which they cannot produce perfect seeds, nor even continue to vegetate beyond a certain time; and that time seems to be limited by the quantity of carbonaceous matter contained in the bulb or the seed from which they grow. For Duhamel found, that an oak which he had raised by water from an acorn, made less and less progress every year. We see, too, that those bulbous roots, such as hyacinths, tulips, &c., which are made to grow in water, unless they be planted in the earth every other year, refuse at last to flower, and even to vegetate; especially if they produce new bulbous roots annually, and the old ones decay.

So far, indeed, is water from being the sole food of plants, that in general only a certain proportion of it is serviceable, too much being equally prejudicial to them as too little. Some plants, it is true, grow constantly in water, and will not vegetate in any other situation; but the rest are entirely destroyed when kept immersed in that fluid beyond a certain time. Most plants require a certain degree of moisture, in order to vegetate well. This is one reason why different soils are required for different plants. Rice, for instance, requires a very wet soil; were we to sow it in the ground on which wheat grows luxuriously, it would not succeed; and wheat, on the contrary, would rot in the rice ground.

We should, therefore, in choosing a soil proper for the plants which we mean to raise, consider the quantity of moisture which is best adapted for them; and choose our soil accordingly. Now, the dryness or moisture of a soil depends upon two things; the nature and proportions of the earths which compose it, and the quantity of rain which falls upon it. Every soil contains at least three earths, silica, lime, and alumina, and sometimes also magnesia. The silica is always in the state of sand. Now soils retain moisture longer or shorter according to the proportions of these earths. Those which contain the greatest quantity of sand retain it the shortest, and those which contain the greatest quantity of alumina retain it longest. The first is a dry, the second a wet soil. Lime and magnesia are intermediate between these two extremes: they render a sandy soil more retentive of moisture, and diminish the wetness of a clayey soil. It is evident, therefore, that, by mixing together proper proportions of these four earths, we may form a soil of any degree of dryness and moisture that we please.

But whatever be the nature of the soil, its moisture must depend in general upon the quantity of rain which falls. If no rain at all fell, a soil, however retentive of moisture it be, must remain dry; and if rain were very frequently falling, the soil must be open indeed, if it be not constantly wet. The proportion of the different earths in a soil, therefore, must depend upon the quantity of rain which falls. In a rainy country, the soil ought to be open; in a dry country, it ought to be retentive of moisture. In the first, there ought to be a greater proportion of sand; in the second, of clay.

Almost all plants grow in the earth, and every Earth soil contains at least silica, lime, alumina, and often clay, magnesia. We have seen already, that one use of these earths is to administer the proper quantity of water to the vegetables which grow in the soil. But as all plants contain earths as a part of their ingredients, is it not probable that earths also serve as a food for plants? It has not yet indeed been shewn, that those plants which vegetate in pure water do not contain the usual quantity of earth; but as earths are absolutely necessary for the perfect vegetation of plants, as they are contained in all plants, and are even found in their juices, we can scarcely doubt that they are actually imbibed, though only in small quantities.

We have seen in the last chapter, that all plants contain various saline substances; and if we analyze the most fertile soils, and the richest manures, we never find them destitute of these substances. Hence it is probable that different salts enter as ingredients into the food of plants. It is probable also, that every plant absorbs particular kinds of salts. Thus sea plants yield soda by analysis, while inland plants furnish potash. The potash contained in plants has indeed been supposed to be the produce of vegetation; but this has not been proved in a satisfactory manner. We find potash in the very juices of plants, even more abundantly than in the vegetable fibres themselves. But this subject is still buried in obscurity; and indeed it is extremely difficult.

(d) Mr Tennant has ascertained, that magnesia, when uncombined with carbonic acid gas, is injurious to corn when employed as a manure; and that lime, which contains a mixture of magnesia, likewise injures corn.—See Phil. Trans. 1799, p. 2. This important fact demonstrates, that earths are not mere vehicles for conveying water to plants. vegetable substances.

It is evident that the putrefied dung acted upon the soil, and was soon exhausted. It follows from this, that carbon only acts as a manure when in a particular state of combination; and this state, whatever it may be, is evidently produced by putrefaction. Another experiment of the same chemist renders this truth still more evident. He allowed shavings of wood to remain for about ten months in a moist place till they began to putrefy, and then spread them over a piece of ground by way of manure. The first two years this piece of ground produced nothing more than others which had not been manured at all; the third year it was better, the fourth year still better, the fifth year it reached its maximum of fertility; after which it declined constantly till the ninth, when it was quite exhausted. Here the effect of the manure evidently depended upon its progress in putrefaction.

Now what is the particular state into which carbon must be reduced before it is fit for the food of plants? This subject has never been examined with attention; the different combinations of carbon having been in a great measure overlooked. And yet it is evident, that it is only by an accurate examination of these combinations, and a thorough analysis of manures, in order to discover what particular combinations of carbon exist in them, and in what the most efficacious manures differ from the rest, that we can expect to throw complete light upon the nature and use of manures, one of the most important subjects to which the farmer can direct his attention. We know, from the experiments of Mr Haffenfritz, that all those manures which act with efficiency and celerity contain carbon in such a state of combination, that it is soluble in water; and that the efficiency of the manure is proportional to the quantity of carbon so soluble. He found that all efficacious manures gave a brown colour to water, and that the water so coloured, when evaporated, left a residuum, which consisted in a great measure of carbon*. He observed, too, that the soil which gives the deepest colour to water, or which contains the greatest quantity of carbon soluble in water, is, other things being the same, the most fertile.

This is not, however, to be understood without limitation; for it is well known that if we employ excessive quantities of manure, we injure vegetation instead of promoting it. This is the reason that plants will not, as Mr Duhamel found by experiment, vegetate in saturated solutions of dung†.

One of the combinations of carbon which is soluble in water, and with which we are best acquainted, is carbonic acid gas. It has been supposed by many philosophers, particularly by Mr Senebier, that this gas, dissolved in water, supplies plants with a great part of their carbon. But Mr Haffenfritz, on making the experiment, found, that the plants which he raised in water, impregnated with carbonic acid gas, differed in no respect from those which grew in pure water, and did not contain a particle of carbon which had not existed in the seeds from which they sprang‡. This experiment proves, that carbonic acid gas, dissolved in water, does not serve as food for plants. It appears, however, from the experiments of Ruckert, that when plants growing in foil are watered daily with water impregnated with carbonic acid gas, they vegetate faster than when this watering is omitted. He planted two beans... Vegetable Substances.

Since the food of plants must be in a fluid state, and since no plant will live if it be deprived of moisture, we may conclude that all its food is previously dissolved in water. As for the carbon, we know, that in all active manures it is in such a state of combination, that it is water-soluble in water. We know, too, that all the salts which we can suppose to make a part of the food of plants, are more or less soluble in water. Lime also is soluble in water, whether it be pure or in the state of a salt; magnesia and alumina may be rendered so by means of carbonic acid gas; and Bergman, Magie, and Klaproth, have shewn, that even silica may be dissolved in water. We can see, therefore, in general, though we have no precise notions of the very combinations which are immediately imbibed by plants, that all the substances which form essential parts of that food may be dissolved in water.

15. Since the food of plants is imbibed by their roots Therefore in a fluid state, it must exist in plants in a fluid state; and unless it undergoes alterations in its composition just when imbibed, we may expect to find it in the plant unaltered. If there were any method of obtaining this fluid food from plants before it has been altered by them, we might analyse it, and obtain by that means a much more accurate knowledge of the food of plants than we can by any other method. This plan indeed must fail, provided the food undergoes alteration just when it is absorbed by the roots; but if we consider, that when one species of tree is grafted upon another, each bears its own peculiar fruit, and produces its own peculiar substances, we can scarcely avoid thinking that the great changes, at least which the food undergoes after absorption, are produced, not in the roots, but in other parts of the plant.

If this conclusion be just, the food of plants, after being imbibed by the roots, must go directly to those parts organs where it is to receive new modifications, and to be rendered fit for being assimilated to the different parts of the plant. There ought therefore to be certain juices continually ascending from the roots of plants; and these juices, if we could get them pure and unmixed with the other juices or fluids which the plant must contain, and which have been secreted and formed from these primary juices, would be, very nearly at least, the food as it was imbibed by the plant. Now during the vegetation of plants, there actually is a juice continually ascending from their roots. This juice has been called the sap, the succus communis, the lymph of plants. We shall adopt the first of these names, because it has been most generally received.

The first step towards an accurate knowledge of the food, and of the changes which take place during vegetation, is an analysis of the sap. The sap is most abundant during the spring. At that season, if a cut be made through the bark and part of the wood of some trees, the sap flows out very profusely. The trees are then said to bleed. By this contrivance any quantity of sap we think proper may be collected. It is not probable, indeed, that by this method we obtain the ascending sap in all its purity; it is no doubt mixed with the peculiar juices of the plant; but the less progress vegetation has made, the purer we may expect to find it; both because the peculiar juices must be in much smaller quantity, and because its quantity may may be supposed to be greater. We should therefore examine the sap as early in the season as possible, and at all events before the leaves have expanded.

For the most complete set of experiments hitherto made upon the sap, we are indebted to Mr. Vauquelin. An account of his experiments has been published in the third volume of the *Annales de Chimie*. He has neglected to inform us of the state of the tree when the sap which he analysed was taken from it; so that we are left in a state of uncertainty with respect to the purity of the sap: but from the comparison which he has put it in our power to draw between the state of the sap at different succedane periods, we may in some measure obviate this uncertainty.

He found that 1039 parts of the sap of the *ulmus campestris*, or common elm, were composed of:

- 1027.567 water and volatile matter, - 9.533 acetite of potas, - 1.062 vegetable matter, - 0.818 carbonat of lime,

Besides some slight traces of sulphuric and muriatic acids.

On analysing the same sap somewhat later in the season, Mr. Vauquelin found the quantity of vegetable matter a little increased, and that of the carbonat of lime and acetite of potas diminished. Still later in the season the vegetable matter was farther increased, and the other two ingredients farther diminished. The acetite of potas, in 1039 parts of this third sap, amounted to 8.615 parts.

If these experiments warrant any consequence to be drawn from them, they would induce us to suppose that the carbonat of lime and acetite of potas were contained in the pure ascending sap, and that part at least of the vegetable matter was derived from the peculiar juices altered by the secreting organs of the plant; for the two fats diminished in quantity, and the vegetable matter increased as the vegetation of the tree advanced. Now this is precisely what ought to have taken place, on the supposition that the sap became more and more mixed with the peculiar juices of the tree, as we are supposing it to do. If these conclusions have any solidity, it follows from them, that carbonat of lime and acetite of potas are absorbed by plants as a part of their food: Now these fats, before they are absorbed, must be dissolved in water. But the carbonat of lime may be dissolved in water by the help of carbonic acid. This shows us how water saturated with carbonic acid may be useful to plants vegetating in a proper soil, while it is useless to those that vegetate in pure water. In the pure water there is no carbonat of lime to be dissolved; and therefore carbonic acid gas cannot enter into a combination which renders it proper for becoming the food of plants. Part of the vegetable matter was precipitated from the sap by alcohol. This part seems to have been gummy. Now gums we know are produced by vegetation.

The sap of the *fagus sylvatica*, or beech, contained the following ingredients:

- Water, - Acetite of lime with excess of acid, - Acetite of potas, - Gallic acid, - Tan, - A mucous and extractive matter, - Acetite of alumina.

Although Mr. Vauquelin made two different analyses of this sap at different seasons, it is impossible to draw any satisfactory conclusions from them, as he has not given us the proportions of the ingredients. It seems clear that the gallic acid and tan were combined together; for the sap tasted like the infusion of oak bark. The quantity of each of these ingredients increased as vegetation advanced; for the colour of the second sap collected later was much deeper than that of the first. This shows us that these ingredients were produced by vegetation, and that they did not form a part of the ascending sap. Probably they were derived from the bark of the tree. The presence of alumina, and the absence of carbonic acid gas, would seem to indicate that all plants do not imbibe the very same food.

The sap of the *carpinus betulus* contains water, acetite of potas, acetite of lime, sugar, mucilage, vegetable extract. It cannot be doubted that the sugar and the mucilage are the produce of vegetation.

The sap of the *betula alba*, or common birch, contains water, sugar, vegetable extract, acetite of lime, acetite of alumina, and acetite of potas.

These experiments are curious, and certainly add to the precision of our notions concerning the food of plants; but they are not decisive enough to entitle us to draw conclusions. They would seem to show, either that acetite of potas and lime are a part of the food of plants, or at least some substances which have the property of affuming these combinations.

These experiments lead to the conclusion that whether acetic acid forms a component part of the sap. Now the food is it not easy to suppose that this substance is actually absorbed by the roots in the state of acetic acid. The thing might be determined by examining the mould in which plants grow. This examination indeed has been performed; but no chemist has ever found acetic acid, at least in any sensible quantity. Is it not probable, then, that the food, after it is imbibed, is somewhat modified and altered by the roots? In what manner this is done we cannot say, as we know very little about the vascular structure of the roots. We may conclude, however, that this modification is nearly the same in most plants: for one plant may be grafted on another, and each continue to produce its own peculiar products; which could not be, unless the proper substances were conveyed to the digestive organs of all. There are several circumstances, however, which render the modifying power of the roots somewhat probable. The strongest of these is the nature of the ingredients found in the sap. It is even possible that the roots may, by some means or other, throw out again some part of the food which they have imbibed as excrementitious. This has been suspected by several physiologists; and there are several circumstances which render it probable. It is well known that some plants will not vegetate well after others; and that some again vegetate unusually well when planted in ground where certain plants had been growing. These facts, without doubt, may be accounted for on other principles. If there be any excrementitious matter emitted by the roots, it is much more probable that this happens in the last stage of vegetation. That is to say, when the food, after digestion, is applied to the purposes which the root requires. But the fact ought to be supported by experiments, otherwise it cannot be admitted. The sap, as Dr Hales has shewn us, ascends with a very considerable force. It issued during the bleeding season with such impetuosity from the cut end of a vine branch, that it supported a column of mercury 32 inches high.

Now what is the particular channel through which the sap ascends, and what is the cause of the force with which it moves? These are questions which have excited a great deal of the attention of those philosophers who have made the physiology of vegetables their particular study; but the examination of them is attended with so many difficulties that they are very far from being decided.

It is certain that the sap flows from the roots towards the summit of the tree. For if in the bleeding season a number of openings be made in the tree, the sap begins first to flow from the lowest opening, then from the lowest but one, and so on successively, till at last it makes its appearance at the highest of all. And when Duhamel and Bonnet made plants vegetate in coloured liquids, the colouring matter, which was deposited in the wood, appeared first in the lowest part of the tree, and gradually ascended higher and higher, till at last it reached the top of the tree, and tinged the very leaves.

It seems certain, too, that the sap ascends through the wood, and not through the bark of the tree: for a plant continues to grow even when stript of a great part of its bark; which could not happen if the sap ascended through the bark. When an incision, deep enough to penetrate the bark, and even part of the wood, is carried quite round a branch, provided the wound be covered up from the external air, the branch continues to vegetate as if nothing had happened; which could not be the case if the sap ascended between the bark and the wood. It is well known, too, that in the bleeding season little or no sap can be got from a tree unless our incision penetrate deeper than the bark.

If the sap ascended thro' the parenchyma of plants, as some physiologists have supposed, since there is a communication between every part of that organ, it is evident that the tree ought to bleed whenever any part of the parenchyma is wounded. But this is not the case. Consequently the sap does not ascend through the parenchyma. Besides, if the supposition were true, the sap, from the very structure of the parenchyma, must ascend in the same manner as water through a sponge; and in that case could not possibly possess the force with which we know that it ascends. But if the sap is not found in the parenchyma, as is now well known to be the case, it must, of necessity, be confined in particular vessels; for if it were not, it would undoubtedly make its appearance there. Now what are the vessels through which the sap ascends?

Grew and Malpighi, the first philosophers who examined the structure of plants, took it for granted that the woody fibres were tubes, and that the sap ascended through them. For this reason they gave these fibres the name of lymphatic vessels. But they were unable, even when assisted by the best microscopes, to detect anything in these fibres which had the appearance of a tube; and succeeding observers have been equally unsuccessful. The conjecture therefore of Malpighi and Grew, about the nature and use of these fibres, remains totally unsupported by any proof. Dubuquet has even gone far to overturn it altogether. For he found that these woody fibres are divisible into smaller fibres, and these again into still smaller; and even, by the assistance of the best microscopes, he could find no end of this subdivision. Now granting these fibres to be vessels, it is scarcely possible, after this, to suppose that the sap really moves through tubes, whose diameters are almost infinitely small. There are, however, vessels in plants which may easily be distinguished by the help of a small microscope, and even, in many cases, by the naked eye. These were seen, and distinctly described, by Grew and Malpighi. They consist of a fibre twisted round like a corkscrew. If we take a small cylinder of wood, and wrap round it a slender braided wire, so closely that all the rings of the wire touch each other, and if, after this, we pull out the wooden cylinder altogether, the braided wire thus twisted will give us a very good representation of these vessels. If we take hold of the two ends of the braided wire thus twisted, and pull them, we can easily draw out the wire to a considerable length. In the same manner, when we lay hold of the two extremities of these vessels, we can draw them out to a great length. Malpighi and Grew finding them always empty, concluded that they were intended for the circulation of the air through the plant, and therefore gave them the name of tracheae; which word is used to denote the windpipe of animals. These tracheae are not found in the bark; but Hedwig has shewn that they are much more numerous in the wood than was supposed; and that they are of very different diameters; and Reichel has demonstrated that they go to the minute branches, and spread through every leaf. He has shewn, too, that they contain sap; and Hedwig has proved that the notion which generally prevailed of their containing nothing but air, arose from this circumstance, that the larger tracheae, which alone were attended to, lost their sap as soon as they are cut; and, of course, unless they are inspected the instant they are divided, they appear empty. Is it not probable, then, or rather is it not certain, from the discoveries of that very ingenious and philosophic man, that the tracheae are, in reality, the sap vessels of plants? Indeed it seems established by the experiments both of Reichel and Hedwig, that all, or almost all the vessels of plants may, if we attend only to their structure, be denominated tracheae.

But by what powers is the sap made to ascend in these vessels? And not only to ascend, but to move abroad with very considerable force; a force, as Hales has shewn, sufficient to overcome the pressure of 43 feet perpendicular of water?

Grew ascribed this phenomenon to the levity of the sap; which, according to him, entered the plant in the state of a very light vapour. But this opinion will not bear the slightest examination. Malpighi supposed that the sap was made to ascend by the contraction and dilatation of the air contained in the air vessels. But even were we to grant that the tracheae are air vessels, the sap, according to this hypothesis, could only ascend when a change of temperature takes place; which is contrary to fact. And even if we were to waive every objection of that kind, the hypothesis would not account for the circulation of the sap, unless the sap vessels be provided with valves. Now the experiments of Hales and Duhamel shew that no valves can possibly exist in them. For branches imbibe moisture nearly equally. vegetable substances.

It has been demonstrated, that the heights to which liquids rise in capillary tubes, are inversely as the diameter of the tube. Consequently the smaller the diameter of the tube, the greater is the height to which the liquid will rise. But the particles of water are not infinitely small; therefore whenever the diameter of the tube is diminished beyond a certain size, water cannot ascend in it, because its particles are now larger than the bore of the tube. Consequently the rise of water in capillary tubes must have a limit: if they exceed a certain length, how small soever their bore may be, water will either not rise to the top of them, or it will not enter them at all. We have no method of ascertaining the precise height to which water would rise in a capillary tube, whose bore is just large enough to admit a single particle of water. Therefore we do not know the limit of the height to which water may be raised by capillary attraction. But whenever the bore is diminished beyond a certain size, the quantity of water which rises in it is too small to be sensible. We can easily ascertain the height which water cannot exceed in capillary tubes before this happens; and if any person calculate, he will find that this height is not nearly equal to the length of the sap vessels of many plants. But besides all this, we see in many plants very long sap vessels, of a diameter too large for a liquid to rise in them a single foot by capillary attraction, and yet the sap rises in them to very great heights.

If any person says that the sap vessels of plants gradually diminish in diameter as they ascend; and that, in consequence of this contrivance, they act precisely as an indefinite number of capillary tubes, one standing upon another, the inferior serving as a reservoir for the superior: we answer, that the sap may ascend by that means to a considerable height; but certainly not in any greater quantity than if the whole sap vessel had been precisely of the bore of its upper extremity. For the quantity of sap raised must depend upon the bore of the upper extremity, because it must all pass through that extremity. The quantity of sap, too, on that supposition, must diminish the farther we go from the root; because the bore of the sap vessels is constantly diminishing; the ascending force must also diminish, because it is, in all cases, proportional to the quantity of water raised. Now neither of these, as Dr Hales has demonstrated, is true.

But farther, if the sap moved only in the vessels of Androplanta by capillary attraction, it would be so far from being able to flow out at the extremity of a branch, with a force sufficient to overcome the pressure of a column of water 43 feet high, that it could not flow out at all. It would be impossible in that case for any such thing as the bleeding of trees ever to happen.

If we take a capillary tube, of such a bore that a liquid will rise in it six inches, and after the liquid has risen to its greatest height, break it short three inches from the bottom, none of the liquid in the under half flows over. The tube, thus shortened, continues indeed full, but not a single particle of liquid ever escapes from it. And how is it possible for it to escape? The film,

(c) The action of all the other films, of which the tube is composed, on the water, as far as it is measured by its effect, is nothing at all. For every particle of water in the tube (except those attracted by the undermost film) is attracted upwards and downwards by the same number of films; it is therefore precisely in the same state as if it were not attracted at all. film, at the upper extremity of the tube, must certainly have as strong an attraction for the liquid as the film at the lower extremity. As part of the liquid is within its attracting distance, and as there is no part of the tube above to counterbalance this attraction, it must of necessity attract the liquid nearest it, and with a force sufficient to counterbalance the attraction of the undermost film, how great forever we may suppose it. Of course no liquid can be forced up, and consequently none can flow out of the tube. Since then the sap flows out at the upper extremity of the sap vessels of plants, we are absolutely certain that it does not ascend in them merely by its capillary attraction, but that there is some other cause.

It is impossible therefore to account for the motion of the sap in plants by any mechanical or chemical principles whatever; and he who ascribes it to these principles has not formed to himself any clear or accurate conception of the subject. We know indeed that heat is an agent; for Dr Walker found that the ascent of the sap is much promoted by heat, and that after it had begun to flow from several incisions, cold made it give over flowing from the higher orifices while it continued to flow at the lower. But this cannot be owing to the dilating power of heat; for unless the sap vessels of plants were furnished with valves (and they have no valves), dilatation would rather retard than promote the ascent of the sap. Consequently the effect of heat can give us no assistance in explaining the ascent of the sap upon mechanical and chemical principles.

We must therefore ascribe it to some other cause: the vessels themselves must certainly act. Many philosophers have seen the necessity of this, and have accordingly ascribed the ascent of the sap to irritability. But the first person who gave a precise view of the manner in which the vessels probably act was Saussure. He supposes that the sap enters the open mouths of the vessels, at the extremity of the roots; that these mouths then contract, and by that contraction propel the sap upwards; that this contraction gradually follows the sap, pushing it up from the extremity of the root to the summit of the plant. In the mean time the mouths are receiving new sap, which, in the same manner is pushed upwards. Whether we suppose the contraction to take place precisely in this manner or not, we can scarcely deny that it must take place; but by what means it is impossible to say. The agents cannot precisely resemble the muscles of animals; because the whole tube, however cut or maimed, still retains its contracting power, and because the contraction is performed with equal readiness in every direction. It is evident, however, that they must be the same in kind. Perhaps the particular structure of the vessels may fit them for their office. Does ring after ring contract its diameter? The contracting agents, whatever they are, seem to be excited to act by some stimulus communicated to them by the sap. This capacity of being excited to action is known in physiology by the name of irritability; and there are not wanting proofs that plants are possessed of it. It is well known that different parts of plants move when certain substances act upon them. Thus the flowers of many plants open at sunrise, and close again at night. Linnaeus has given us a list of these plants. Des Fontaines has shewn that the stamens and anthers of many plants exhibit distinct motions. Dr Smith has observed, that the stamens of the barberries are thrown into motions when touched. Roth has ascertained that the leaves of the drosera longiflora and rotundiflora have the same property. Mr Coulon, too, who has adopted the opinion that the motion of the sap in plants is produced by the contraction of vessels, has even made a number of experiments in order to shew this contraction. But the fact is, that every one has it in his power to make a decisive experiment. Simply cutting a plant, the euphorbia peplus for instance, in two places, so as to separate a portion of the stem from the rest, is a complete demonstration that the vessels actually do contract. For whoever makes the experiment, will find that the milky juice of that plant flows out at both ends so completely, that if afterwards we cut the portion of the stem in the middle, no juice whatever appears. Now it is impossible that these phenomena could take place without a contraction of the vessels; for the vessels in that part of the stem which has been detached cannot have been more than full; and their diameter is so small, that if it were to continue unaltered, the capillary attraction would be more than sufficient to retain their contents, and consequently not a drop could flow out. Since, therefore, the whole liquid escapes, it must be driven out forcibly, and consequently the vessels must contract.

It seems pretty plain, too, that the vessels are excited to contract by various stimuli; the experiments of Coulon and Saussure render this probable, and an observation of Dr Smith Barton makes it pretty certain. He found that plants growing in water vegetated with much greater vigour, provided a little camphor was thrown into the water.

18. Besides the sap which ascends upwards towards the leaves, they contain also another fluid, known by the name of succus proprius, or peculiar juice. This juice differs very considerably in different plants. It seems to be the sap altered by some process or other, fitted for the various purposes of vegetation. That it flows from the leaves of the plant towards the roots, appears from this circumstance, that when we make an incision into a plant, into whatever position we put it, much more of the succus proprius flows from that side of the wound which is next the leaves and branches, than from the other side; and this happens even though the leaves and branches be held undermost. When a ligature is tied about a plant, a swelling appears above, but not below the ligature.

The vessels containing the peculiar juice are found in all the parts of the plant. Hedwig, who has examined the vessels of plants with very great care, seems to consider them as of the same structure with the trachea. The peculiar juice is easily known by its colour and its consistence. In some plants it is green, in some red, in many milky. It cannot be doubted that its motion in the vessels is performed in the same way as that of the sap.

19. It appears, then, that the sap ascends to the leaves, that there it undergoes certain alterations, and is converted into the peculiar juices; which, like the blood in animals, are afterwards employed in forming the various substances found in plants. Now the changes which the sap undergoes in the leaves, provided we can trace them, must throw a great deal of light upon the nature of vegetation. No sooner has the sap arrived at the leaves, than a great part of it is thrown off by evaporation. The quantity thus perspired bears a very great proportion to the moisture imbibed. Mr Woodward found that a sprig of mint in 7 days imbibed 2558 grains of water, and yet its weight was only increased 15 grains; therefore it must have given out 2543 grains. Another branch, which weighed 127 grains, increased in weight 128, and it had imbibed 14190 grains. Another sprig, weighing 76 grains, growing in water mixed with earth, increased in weight 168 grains, and had imbibed 10731 grains of water. These experiments demonstrate the great quantity of matter which is constantly leaving the plant. Dr Hales found that a cabbage transmitted daily a quantity of moisture equal to about half its weight; and that a sunflower, three feet high, transmitted in a day 1 lb. 14 oz. avoidupois. He showed, that the quantity of transpiration in the same plant was proportional to the surface of the leaves, and that when the leaves were taken off, the transpiration nearly ceased. By these observations, he demonstrated that the leaves are the organs of transpiration. He found, too, that the transpiration was nearly confined to the day, very little taking place during the night; that it was much promoted by heat, and flopped by rain and frost. And Millar, Guettard, and Senebier, have shown that the transpiration is also very much promoted by sunshine.

The quantity of moisture imbibed by plants depends very much upon what they transpire: the reason is evident: when the vessels are once filled with sap, if none be carried off, no more can enter; and, of course, the quantity which enters must depend upon the quantity emitted.

In order to discover the nature of the transpired matter, Hales placed plants in large glass vessels, and by that means collected a quantity of it. He found that it resembled pure water in every particular, excepting only that it sometimes had the odour of the plant. He remarked, too, as Guettard and Du Hamel did after him, that when kept for some time it putrefied, or at least acquired a flanking smell. Senebier subjected a quantity of this liquid to a chemical analysis.

He collected 13030 grains of it from a vine during the months of May and June. After filtration he gradually evaporated the whole to dryness. There remained behind two grains of residuum. These two grains consisted of nearly ½ grain of carbonat of lime, ¼ grain of sulphat of lime, ¼ grain of matter soluble in water, and having the appearance of gum, and ¼ grain of matter which was soluble in alcohol, and apparently resinous. He analyzed 60768 grains of the same liquid, collected from the vine during the months of July and August. On evaporation he obtained 2½ grains of residuum, composed of ½ grain of carbonat of lime, ½ grain of sulphat of lime, ½ grain of mucilage, and ½ grain of resin. The liquid transpired by the after new Anglia afforded precisely the same ingredients.

Senebier attempted to ascertain the proportion which the liquid transpired bore to the quantity of moisture imbibed by the plant. But it is easy to see that such experiments are liable to too great uncertainties to be depended on. His method was as follows: He plunged the thick end of the branch on which he made the experiment into a bottle of water, while the other end, containing all its leaves, was thrust into a very large glass globe. The apparatus was then exposed to the sunshine. The quantity imbibed was known exactly by the water which disappeared from the bottle, and the quantity transpired was judged by the liquid which condensed and trickled down the sides of the glass globe. The following table exhibits the result of his experiments:

| Plants | Imbibed | Perspired | Time | |--------|---------|-----------|------| | Peach | 100 gr. | 35 gr. | | | Ditto | 210 | 92 | | | Ditto | 220 | 120 | | | Mint | 200 | 90 | 2 days | | Ditto | 575 | 120 | 10 | | Raip | 725 | 560 | 2 | | Ditto | 1232 | 765 | 2 | | Peach | 710 | 295 | 1 | | Apricot| 210 | 185 | 1 |

In some of his experiments no liquid at all was condensed. Hence it is evident that the quantity of matter transpired cannot be deduced from these experiments. The mouth of the glass globe does not seem to have been accurately closed; the air within it communicated with the external air; consequently the quantity condensed must have depended entirely upon the state of the external air, the heat, &c.

The first great change, then, which takes place upon the sap after it arrives at the leaves, is the evaporation of a great part of it; consequently what remains must be very different in its proportions from the sap. The leaves seem to have particular organs adapted for throwing off part of the sap by transpiration. For the experiments of Guettard, Duhamel, and Bonnet, it was found that it is performed chiefly by the upper surfaces of leaves, and may be nearly flopped altogether by varnishing the upper surface.

The leaves of plants become gradually less and less fit for this transpiration; for Senebier found, that when all other things are equal, the transpiration is much greater in May than in September. Hence the reason that the leaves are renewed annually. Their leaves fall off, organs become gradually unfit for performing their functions, and therefore it is necessary to renew them. Those trees which retain their leaves during the winter, transpire less than others. It is now well known that these trees also renew their leaves.

20. Leaves have also the property of absorbing carbonic acid gas from the atmosphere.

We are indebted for this very singular discovery to the experiments of Dr Priestley, though he himself did not discover the truth, and though he even refused to acknowledge it when it was pointed out by others. It has been long known, that when a candle has been allowed to burn out in any quantity of air, no candle can afterwards be made to burn in it. In the year 1771 Dr Priestley made a sprig of mint vegetate for ten days in contact with a quantity of such air; after which he found that a candle would burn in it perfectly well. This experiment he repeated frequently, and found that it was always attended with the same result. According to the opinion at that time universally received, that the burning of candles rendered air impure by communicating phlogiston to it, he concluded from it, that plants, while they vegetate, absorb phlogiston.

Carbonic acid gas was at that time supposed to contain phlogiston. It was natural, therefore, to suppose that it would afford nourishment to plants, since they had the property of absorbing phlogiston from the atmosphere. Dr. Percival had published a set of experiments; by which he endeavoured to shew that this was actually the case.

These experiments induced Dr. Priestley, in 1776, to consider the subject with more attention. But as, in all the experiments which he made, the plants confined in carbonic acid gas very soon died, he concluded, that carbonic acid gas was not a food; but a poison to plants. Mr. Henry of Manchester was led, in 1784, probably by the contrary of these results, to examine the subject. His experiments, which were published in the Manchester Transactions, perfectly coincided with those of Dr. Percival. For he found, that carbonic acid gas, so far from killing plants, constantly promoted their growth and vigour. Meanwhile Mr. Senebier was occupied at Geneva with the same subject; and he published the result of his researches in his Memoirs Physico-chimique about the year 1785. His experiments shewed, in the clearest manner, that carbonic acid gas is used by plants as food. The same thing was supported by Ingenhousz in his second volume. The experiments of Sauflaire the Son, published in 1797, have at last put the subject beyond the reach of dispute. From a careful comparison of the experiments of these philosophers, it will not be difficult for us to discover the various phenomena, and to reconcile all the seeming contradictions which occur in them. The facts are as follows:

Mr. Sauflaire has shewn, that plants will not vegetate when totally deprived of carbonic acid gas. They vegetate indeed well enough in air which has been previously deprived of carbonic acid gas; but when a quantity of lime was put into the glas vessel which contained them, they no longer continued to grow, and the leaves in a few days fell off. The air, when examined, was found to contain no carbonic acid gas. The reason of this phenomenon is, that plants (as we shall see afterwards) have the power of forming and giving out carbonic acid in certain circumstances; and this quantity is sufficient to continue their vegetation for a certain time. But if this new formed gas be also withdrawn, by quicklime, for instance, which absorbs it the instant it appears, the leaves droop, and refuse to perform their functions. Carbonic acid gas, then, applied to the leaves of plants, is essential to vegetation.

Dr. Priestley, to whom we are indebted for many of the most important facts relative to vegetation, observed, in the year 1778, that plants, in certain circumstances, emitted oxygen gas; and Ingenhousz very soon after discovered that this gas is emitted by the leaves of plants, and only when they are exposed to the bright light of day. His method was to plunge the leaves of different plants into vessels full of water, and then expose them to the sun, as Bonnet, who had observed the same phenomenon, though he had given a wrong explanation of it, had done before him. Bubbles of oxygen gas very soon detached themselves from the leaves, and were collected in an inverted glass vessel. He observed, too, that it was not a matter of difference what kind of water was used. If the water, for instance, had been previously boiled, little or no oxygen gas escaped from the leaves; river water afforded but little gas; but pump water was the most productive of all.

Senebier proved, that if the water be previously deprived of all its air by boiling, the leaves do not emit a particle of air; that those kinds of water which yield most air, contain in them the greatest quantity of carbonic acid gas; that leaves do not yield any oxygen when plunged in water totally destitute of carbonic acid gas; that they emit it abundantly when the water, rendered unproductive by boiling, is impregnated with carbonic acid gas; that the quantity of oxygen emitted, and even its purity, is proportional to the quantity of carbonic acid gas which the water contains; that water impregnated with carbonic acid gas gradually loses the property of affording oxygen gas with leaves; and that whenever this happens, all the carbonic acid gas has disappeared; and on adding more carbonic acid gas the property is renewed. These experiments prove, in a most satisfactory manner, that the oxygen which the leaves of plants emit depends upon the presence of carbonic acid gas; that the leaves absorb carbonic acid gas, decompose it, give out the oxygen, and retain the carbon.

We now see why plants will not vegetate without carbonic acid gas. They absorb it and decompose it; but this process goes on only when the plants are exposed to the light of day. Therefore we may conclude, that the absorption and decomposition of carbonic acid gas is confined to the day, and that light is an essential agent in the decomposition. Probably it is by its agency, or by its entering into combination with the oxygen, that this substance is enabled to assume the gaseous form, and to separate from the carbon.

If we reason from analogy, we shall conclude, that during this process a quantity of caloric is necessary; and that therefore no increase of temperature takes place, but rather the contrary. This may be one reason why the operation takes place only during the day.

It is extremely probable that plants by this process acquire the greatest part of the carbonaceous matter which they contain; for if we compare the quantity of carbon contained in plants vegetating in the dark, where this process cannot go on, with the quantity which those plants contain which vegetate in the usual manner, we shall perceive a very conspicuous difference. Chaptal found that a hyacinth, which was vegetating in the dark, contained only \( \frac{1}{3} \) of its weight of carbonaceous matter; but the same plant, after being made to vegetate in the light for 30 days, contained \( \frac{2}{3} \) of its weight of carbonaceous matter. Hallicnratza also obtained, that plants growing in the dark contain much more water, and much less carbon and hydrogen, than plants growing in the light. Senebier analysed both with the same result. Plants growing in the dark yielded less hydrogen gas and oil; their refrinous matter was to that of plants growing in the light as 2 to 5; and their moisture as 13 to 6; they contain even one-half less of fixed matters.

It is evident, however, that this absorption and decomposition of carbonic acid gas does not depend upon the the light alone. The nature of the sap has also its influence; for Hassenfratz found, that the quantity of carbon did not increase when plants vegetated in pure water. Here the sap seems to have wanted that part which combines with and retains the carbon; and which therefore is by far the most important part of the food of plants. Upon the discovery and mode of applying this substance, whatever it is, the improvements in agriculture must in a great measure depend.

If we consider the difference in the proportion of carbonaceous matter in plants vegetating in the dark and in the usual manner, we can scarcely avoid concluding that the quantity of carbonic acid gas absorbed by plants is considerable. To form an estimate of it, would require a set of experiments performed in a very different manner from any hitherto made. The stems and branches of plants vegetating in a rich soil should be confined within a large glass globe, the inside of which ought to have no communication with the external air. A very small stream of carbonic acid gas should be made occasionally to flow into this globe, so as to supply the quantity that may appear necessary; and there should be a contrivance to carry off and examine the air within the globe when it increases beyond a certain quantity. Experiments conducted in this manner would probably throw a great deal of light upon this part of vegetation, and enable us to calculate the quantity of carbonic acid decomposed, and the quantity of oxygen emitted by plants; to compare these with the waste of oxygen by the respiration of animals and combustion, and to see whether or not they balance each other.

Senebier has ascertained, that the decomposition of the carbonic acid takes place in the parenchyma. He found, that the epidermis of a leaf would, when separated, give out no air, neither would the nerves in the same circumstances; but upon trying the parenchyma, thus separated from its epidermis and part of its nerves, it continued to give out oxygen as before. He remarked also, that everything else being equal, the quantity of oxygen emitted, and consequently of carbonic acid decomposed, is proportional to the thickness of the leaf; and this thickness depends upon the quantity of parenchyma.

That the decomposition is performed by peculiar organs, is evident from an experiment of Ingenhousz. Leaves cut into small pieces continued to give out oxygen as before; but leaves pounded in a mortar lost the property entirely. In the first state, the peculiar structure remained; in the other, it was destroyed. Certain experiments of Count Rumford, indeed, are totally incompatible with this conclusion; and they will naturally occur to the reader as an unanswerable objection. He found, that dried leaves, black poplar, fibres of raw silk, and even glass, when plunged into water, gave out oxygen gas by the light of the sun. But when Senebier repeated these experiments, not one of them would succeed; and we have attempted them with the same bad success. The Count must have been misled by something which he has not mentioned.

Thus we have seen, that when the sap arrives at the leaves, great part is thrown off by evaporation, and that the nature of the remainder is considerably altered by the addition of a quantity of carbon; but there are by no means all the alterations produced upon the sap in the leaves.

Plants will not vegetate unless atmospheric air or oxygen gas have access to their leaves. This was rendered probable by those philosophers who, about the beginning of the 17th century, turned their attention particularly towards the physical properties of the air; but Mr Ingenhousz was perhaps the first of the modern chemists who put it beyond doubt. He found that carbonic acid gas, azot, and hydrogen gas, destroyed plants altogether, unless they were mixed with atmospheric air or oxygen gas. He found also, that plants grew very well in oxygen gas and in atmospheric air. These experiments are sufficient to show, that oxygen gas is necessary to vegetation. The leaves of plants seem to absorb it; and most probably this absorption takes place only in the night. We know, at least, that in germination light is injurious to the absorption of oxygen gas; and therefore it is probable that this is the case also in vegetation.

The leaves of plants not only absorb carbonic acid gas and oxygen gas, but water also. This had been suspected in all ages; the great effect which dew, flight showers, and even wetting the leaves of plants, have in recruiting their strength, and making them vegetate with vigour, are so many proofs that the leaves imbibe moisture from the atmosphere. Hale rendered this still more probable, by observing, that plants increase considerably in weight when the atmosphere is moist; and Mr Bonnet put the matter beyond doubt in his Researches concerning the Use of the Leaves. He showed, that leaves continue to live for weeks when one of their surfaces is applied to water; and that they not only vegetate themselves, but even imbibe enough of water to support the vegetation of a whole branch, and the leaves belonging to it. He discovered also, that the two surfaces of leaves differ very considerably in their power of imbibing moisture; that in trees and shrubs, the under surface possesses almost the whole of the property, while the contrary holds in many of the other plants; the kidney bean for instance.

These facts prove, not only that the leaves of plants have the power of absorbing moisture, but also that the absorption is performed by very different organs from those which emit moisture; for these organs lie on different sides of the leaf. If we consider that it is only during the night that the leaves of plants are moistened with dew, we can scarcely avoid concluding, that, except in particular cases, it is during the night that plants imbibe almost all the moisture which they do imbibe.

During the night the leaves of plants emit carbonic acid gas. This fact was first demonstrated by Mr Ingenhousz, and it has been since confirmed by every philosopher who has attended to the subject.

Thus we have seen that the leaves of plants perform very different operations at different times. During the day they are giving out moisture, absorbing carbonic acid gas, and emitting oxygen gas; during the night, on the contrary, they are absorbing moisture, giving out carbonic acid gas, and absorbing oxygen gas.

The emission of the carbonic acid gas seems to be the consequence of the decomposition of water; either of the water which is already contained in the sap, or of that which the leaves imbibe during the night; but which of the two, it is impossible to determine, nor is it of much consequence. We may conclude that this is the case, because it takes place during the germination of the seed, where all the circumstances seem to be perfectly analogous. The water is decomposed, its oxygen is combined with part of the carbon which had been absorbed during the day, and the hydrogen enters into new combinations in the sap. It appears, also, that this decomposition of water depends in a good measure upon the quantity of oxygen gas absorbed; for Dr Ingenhousz found, that when plants are confined in oxygen gas, they emit more carbonic acid gas than when they are confined in common air.

To describe in what manner these decompositions take place, is impossible; because we neither know precisely the substances into which the sap has been converted by the operations performed during the day, nor the new substances formed by the operations of the night. We only see the elementary substances which are added and subtracted; which is far from being sufficient to give us precise notions concerning the chemical changes and the affinities by which these changes are produced. We have reason, however, to conclude, that during the day the carbon of the sap is increased, and that during the night the hydrogen and oxygen are increased; but the precise new substances formed are unknown to us. Nor let any one suppose that the increase of the hydrogen, and of the oxygen of the sap, is the same thing as the addition of a quantity of water. Far from it. The substances into which the sap is converted have been enumerated in the last chapter; almost all of them consist chiefly of carbon, hydrogen, and oxygen, and yet none of them has the smallest resemblance to water. In water, oxygen and hydrogen are already combined together in a certain proportion; and this combination must be broken before these elementary bodies can enter into those triple compounds with carbon, of which a great part of the vegetable products consist. We have not the smallest conception of the manner in which these triple combinations are formed, and as little of the manner in which the bodies which compose vegetable substances are combined together. The combination may, for any thing we know to the contrary, be very complicated, though it consists only of three ingredients; and analogy leads us to suppose, that it actually is very complicated: for in chemistry it may be considered as a truth, to which at present few or no exceptions are known, that bodies are decomposed with a facility inversely as the simplicity of their composition; that is to say, that those bodies which consist of the fewest ingredients are most difficultly decomposed, and that those which are formed of many ingredients are decomposed with the greatest facility.

Neither let any one suppose, that the absorption of carbonic acid gas, during the day, is balanced by the quantity emitted during the night, and that therefore there is no increase of carbon: for Ingenhousz has shewn, that the quantity of oxygen gas emitted during the day is much greater than the carbonic acid gas emitted during the night; and that in favourable circumstances, the quantity of oxygen gas in the air surrounding plants is very much increased, and the carbonic acid gas diminished; so much so, that both Dr Priestley and Dr Ingenhousz found, that air which had been spoiled by a lighted candle, or by animals, was rendered as good as ever by plants. Now we know, that combustion and respiration diminish the oxygen gas, and add carbonic acid gas to air; therefore vegetation, which restores the purity of air altered by these processes, must increase the oxygen, and diminish the carbonic acid gas of that air; consequently the quantity of carbonic acid gas absorbed by plants during the day is greater than the quantity emitted by them during the night, and of course the carbon of the sap is increased in the leaves.

It is true, that when plants are made to vegetate for a number of days in a given quantity of air, its ingredients are not found to be altered. Thus Haffentratz ascertained, that the air in which young chestnuts vegetated for a number of days together, was not altered in its properties, whether the chestnuts were vegetating in water or in earth*. And Sauflure the Younger proved, that peas growing ten days in water did not alter the surrounding air†. But this is precisely what ought to be the case, and what must take place, provided the conclusions which we have drawn be just. For if plants only emit oxygen gas, by absorbing and decomposing carbonic acid gas, it is evident, that unless carbonic acid gas be present, they can emit no oxygen gas; and whenever they have decomposed all the carbonic acid gas contained in a given quantity of air, we have no longer any reason to look for their emitting any more oxygen gas; and if the quantity of carbonic acid gas emitted during the night be smaller than that absorbed during the day, it is evident, that during the day the plant will constantly decompose all the acid which had been formed during the night. By these processes, the mutual changes of day and night compensate each other; and they are prevented from more than compensating each other by the forced state of the plant. It is probable, that when only part of a plant is made to vegetate in this forced state, some carbonated sap (if we may be allowed the expression) is supplied by the rest of the plant; and that therefore the quantity of carbonic acid gas emitted during the night may bear a nearer proportion to that emitted in a state of nature, than that of the absorption of fixed air can possibly do. And probably, even when the whole plant is thus confined, the nightly process goes on for a certain time at the expense of the carbon already in the sap: for Haffentratz found, that in these cases the quantity of carbon in the plant, after it had vegetated for some time in the dark, was less than it had been when it began to vegetate*. This is the reason that plants growing in the dark, when confined, absorb all the oxygen gas, and emit an equal quantity of carbonic acid gas: and whenever this has happened, they die; because then neither the daily nor nightly processes can go on.

24. Certain changes are also produced on the sap in the leaves by the action of light; and these changes seem to be in some measure independent, or at least different from the absorption and decomposition of carbonic acid gas, in which light, as we have seen, acts an important part.

The green colour of plants is owing entirely to their vegetating in the light; for when they vegetate in the dark they are white; and when exposed to the light, they acquire a green colour in a very short time, in light. whatsoever situation they are placed, even though plunged in water, provided always that oxygen be present; for Mr Gough has shown, that light without oxygen has not the power of producing the green colour.

In what manner this change is operated, cannot, in the present limited state of our knowledge, be ascertained. We know too little about the properties of light to be able even to conjecture with any plausibility. We know indeed, that part of the light is absorbed by green plants; but this will not account for the phenomenon. When dilated, it amounts to no more than this, that plants which have grown in the dark reflect all the rays of light; while those which vegetate in the light reflect the green and absorb the others. The very mention of this phenomenon is enough to show us, that we have not advanced far enough to be able to explain it.

Etiolated (e) plants want something, or possess something peculiar; and it is on this something that the phenomenon depends. But what is this something? The sudden appearance of the green colour is rather against the supposition, that it is owing to any specific change in the qualities of the sap.

Somebody has observed, that when plants are made to vegetate in the dark, their etiolation is much diminished by mixing a little hydrogen gas with the air that surrounds them. Ingenhousz had already remarked, that when a little hydrogen gas is added to the air in which plants vegetate, even in the light, it renders their verdure deeper; and he seems to think also, that he has proved by experiments, that plants absorb hydrogen gas in these circumstances. Mr Humboldt has observed, that the pois annua and composita, plantago lanceolata, trifolium arvense, cineraria cheiri, lichen verticillatus, and several other plants which grow in the galleries of mines, retain their green colour even in the dark, and that in these cases the air around them contains a quantity of hydrogen gas. These facts are sufficient to show that there is some connection between the green colour of plants and the action of hydrogen gas on them; but what that connection is, it is impossible at present to say.

25. By these different changes which go on in the leaves, the nature of the sap is altogether changed. It is now converted into what is called the peculiar juice, and is fit for being assimilated to the different parts of the plant, and for being employed in the formation of those secretions which are necessary for the purposes of the vegetable economy.

The leaves, therefore, may be considered as the digesting organs of plants, and as equivalent in some measure to the stomach and lungs of animals. The leaves consequently are not mere ornaments; they are the most important parts of the plant. Accordingly we find, that whenever we strip a plant of its leaves, we strip it entirely of its vegetating powers till new leaves are formed. It is well known, that when the leaves of plants are destroyed by insects, they vegetate no longer, and that their fruit never makes any farther progress in ripening, but decays and dries up. Even in germination no progress is made in the growth of the stem till the feed leaves appear. As much food indeed is laid up in the cotyledons as advances the plant to a certain state, the root is prepared, and made ready to perform its functions; but the sap which it imbibes must be first carried to the feed leaves, and digested there, before it be proper for forming the plumula into a stem. Accordingly if the feed leaves are cut off, the plant refuses to vegetate.

It will be very natural to ask, If this be true, how come the leaves themselves to be produced? Even if no answer could be given to this question, it could not overturn a single fact which has been formerly mentioned, nor affect a single conclusion as far as it has been fairly deduced from these facts. We know that the leaves exist long before they appear; they have been traced even five years back. They are completely formed in the bud, and fairly rolled up for evolution, many months before that spring in which they expand. We know, too, that if we take a bud, and plant it properly, it vegetates, forms itself a root, and becomes a complete plant. It will not be said, surely, that in this case the bud imbibes nourishment from the earth; for it has to form a root before it can obtain nourishment in that manner; and this root cannot be formed without nourishment. Is not this a demonstration that the bud contains, already laid up in itself, a sufficient quantity of nourishment, not only to develop its own organs, but also to form new ones. This we consider as a sufficient answer to the objection. During the summer, the plant lays up a sufficient quantity of nourishment in each bud, and this nourishment is afterwards employed in developing the leaves. This is the reason that the leaves make their appearance, and that they grow during the winter, when the plant is deprived of its organs of digestion.

Hence we see why the branch of a vine, if it be introduced into a hothouse during the winter, puts forth leaves and vegetates with vigour, while every other part of the plant gives no signs of life. Hence also the reason that the inoculation of plants succeeds (r).

If a tree be deprived of its leaves, new leaves make their appearance, because they are already prepared for that purpose; but what would be the consequence if a tree were deprived of its leaves and of all its buds for

(e) Plants of a white colour, from vegetating in the dark, are called etiolated, from a French word which signifies a star, as if they grew by starlight.

(r) Hence also the cause of another well-known phenomenon. The sap flows out of trees very readily in spring before the leaves appear, but after that the bleeding ceases altogether. It is evident that there can be scarcely any circulation of sap before the leaves appear; for as there is no outlet, when the vessels are once full, they can admit no more. It appears, however, from the bleeding, that the roots are capable of imbibing, and the vessels of circulating, the sap with vigour. Accordingly, whenever there is an outlet, they perform their functions as usual, and the tree bleeds; that is, they send up a quantity of sap to be digested as usual; but as there are no digesting organs, it flows out, and the tree receives no injury, because the sap that flows out would not have been imbibed at all, had it not been for the artificial opening. But when the digestive organs appear, the tree will not bleed; because these organs require all the sap, and it is constantly flowing to them. five years back? That plants do not vegetate without leaves, is evident from an experiment of Duhamel. He strips the bark off a tree in ringlets, so as to leave five or six rings of it at some distance from each other, with no bark in the intervals. Some of these rings had buds and leaves; these increased considerably in size, but one ring which had none of these remained for years unaltered.

26. The peculiar juice thus formed in the leaves is carried by vessels intended for that use to all the parts of the plant, in order to be employed for the purposes of vegetation—to increase the wood, the bark, the roots; to prepare the seeds, lay up nourishment for the buds, and to repair the decayed parts of the system, or form new ones.

If we had any method of obtaining this peculiar juice in a state of purity, the analysis of it would throw a great deal of light upon vegetation; but this is scarcely possible, as we cannot extract it without dividing at the same time the vessels which contain the sap. In many cases, however, the peculiar juice may be known by its colour; and then its analysis may be performed with an approach towards accuracy. The experiments made on such juices have proved, as might have been expected, that they differ very considerably from each other, and that every plant has a juice peculiar to itself. Hence it follows, that the processes which go on in the leaves of plants must differ at least in degree, and that we have no right to transfer the conclusions deduced from experiments on one species of plants to those of another species. It is even probable, that the processes in different plants are not the same in kind; for it is not reasonable to suppose, that the phenomena of vegetation in an agaric or a boletus are precisely the same as those which take place in trees and in larger vegetables, on which alone experiments have hitherto been made.

To attempt any general account of the ingredients of the peculiar juice of plants, is at present impossible. We may conclude, however, from the experiments of Chaptal, that it contains the vegetable fibre of wood, either ready formed, or very nearly so; just as the blood in animals contains a substance which bears a strong resemblance to the mucilaginous fibres.

When oxy-muriatic acid was poured into the peculiar juice of the euphorbia, which in all the species of that singular genus is of a milky colour and consistency, a very copious white precipitate fell down. This powder, when washed and dried, had the appearance of fine starch, and was not altered by keeping. It was neither affected by water nor alkalies. Alcohol, assisted by heat, dissolved two thirds of it; which were again precipitated by water, and had all the properties of resin. The remaining third part possessed the properties of the woody fibre. Mr. Chaptal tried the same experiment on the juices of a great number of other plants, and he constantly found that oxy-muriatic acid precipitated from them woody fibre. The seeds of plants exhibited exactly the same phenomenon; and a greater quantity of woody fibre was obtained from them than from an equal portion of the juices of plants. These experiments are sufficient to show, that the proper juices of plants contain their nourishment ready prepared, nearly in the state in which it exists in the seed for the use of the young embryo.

The peculiar juices of plants, then, contain more carbon, hydrogen, and oxygen, and less water, and probably lime also, than the sap. They are conveyed to every part of the plant; and all the substances which we find in plants, and even the organs themselves, by which they perform their functions, are formed from them. But the thickest veil covers the whole of these processes; and so far have philosophers hitherto been from removing this veil, that they have not even been able to approach it. All these operations, indeed, are evidently chemical decompositions and combinations; but we neither know what these decompositions and combinations are, nor the instruments in which they take place, nor the agents by which they are regulated.

27. Such, as far as we are acquainted with them, plants die are the changes produced by vegetation. But plants do not continue to vegetate for ever; sooner or later they decay, and wither, and rot, and are totally decomposed. This change indeed does not happen to all plants at the end of the same time. Some live only for a single season, or even for a shorter period; others live two seasons, others three, others a hundred or more; and there are some plants which continue to vegetate for a thousand years. But sooner or later they all cease to live; and then those very chemical and mechanical powers which had promoted vegetation combine to destroy the remains of the plant. Now, what is the cause of this change? Why do plants die?

This question can only be answered by examining with some care what it is which constitutes the life of plants; for it is evident, that if we can discover what that is which constitutes the life of a plant, it cannot be difficult to discover what constitutes its death.

Now the phenomena of vegetable life are in general phenomena of vegetation. As long as a plant continues to vegetate, we say that it lives; when it ceases to vegetate, we conclude that it is dead.

The life of vegetables, however, is not so intimately connected with the phenomena of vegetation that they cannot be separated. Many seeds may be kept for years without giving any symptom of vegetation; yet if they vegetate when put into the earth, we say that they possess life; and if we would speak accurately, we must say also, that they possessed life even before they were put into the earth; for it would be absurd to suppose that the seed obtained life merely by being put into the earth. In like manner, many plants decay, and give no symptoms of vegetation during winter; yet if they vegetate when the mild temperature of spring affects them, we consider them as having lived all winter. The life of plants, then, and the phenomena of vegetation, are not precisely the same thing; for the one may be separated from the other, and we can even suppose the one to exist without the other. Nay, what is more, we can, in many cases, decide, without hesitation, that a vegetable is not dead, even when no vegetation appears; and the proof which we have for its life is, that it remains unaltered; for we know that, when a vegetable is dead, it soon changes its appearance, and falls into decay.

Thus it appears that the life of a vegetable consists in two things: 1. In remaining unaltered, when circumstances are unfavourable to vegetation; 2. In exhibiting vegetable substances.

The phenomena of vegetation have been enumerated above. They consist in the formation or expansion of the organs of the plant, in taking in of nourishment, in carrying it to the leaves, in digesting it, in distributing it through the plant, in augmenting the bulk of the plant, in repairing decayed parts, in forming new organs when they are necessary, in producing seeds capable of being converted into plants similar to the parent. The cause of these phenomena, whatever it may be, is the cause also of vegetable life.

All the substances which have been enumerated in the first part of the article Chemistry, Suppl., together with their compounds and component parts, possess certain qualities in common; in consequence of which, a term has been invented which includes them all. This term is matter. Now these common qualities may all ultimately be resolved into certain attractions and repulsions which these substances exert. These qualities may be said, without any impropriety, to be essential to matter; because every body to which we give the name of matter possesses them; and if any body were to be deprived of these qualities, it could no longer be included under the denomination matter. In short, the word matter comprehends under it certain qualities; every substance which possesses these qualities is called matter; and no other substance except these can receive the name of matter without altering the meaning of the word.

The attractions and repulsions of matter have been examined with care; and the changes which they produce have been ascertained with considerable accuracy. They have even been reduced to general principles under the name of mechanical and chemical laws. Whenever any change is observed, if that change be a case of a mechanical or chemical law, we say that the agent is matter; but if the change cannot be reduced under these laws, or if it be incompatible with these laws, we must say, unless we would pervert the meaning of words altogether, that the agent is not matter.

Now it cannot be disputed that several of the phenomena of life in vegetables are incompatible with the laws of mechanics and chemistry. The motion of the sap, for instance, must be produced by the contraction of the vessels; and the contraction of vessels, on the application of stimuli, is incompatible with the laws of chemistry, because no decomposition takes place; and of mechanics, because a much greater force is generated than the generating body itself possesses. The evolution of the organs of vegetables, the reparation of decayed organs, the formation of new ones to supply the place of the old, the production of seeds capable of producing new plants, the constant similarity of individuals of the same species;—these, and many other well-known phenomena, cannot be reduced under mechanical and chemical laws. The cause of life, then, in plants, is a substance (for we can form no conception of an agent which is not a substance) which does not act according to the laws of mechanics and chemistry, and which consequently is not matter. We shall therefore, till a better name be chosen, denominate it the vegetative principle (a).

The nature of the vegetative principle can only be deduced from the phenomenon of vegetation. It evidently follows a fixed plan, and its actions are directed to promote the good of the plant. It has a power over matter, and is capable of directing its attractions and repulsions, in such a manner as to render them the instruments of the formation, and improvement, and preservation of the plant. It is capable also of generating substances endowed with powers similar to itself. The plan according to which it acts, displays the most consummate wisdom and foresight; and a knowledge of the properties of matter infinitely beyond what man can boast.

Metaphysicians have thought proper to divide all substances into two classes, matter and mind. If we follow this division, the vegetative principle, as it is not material, must undoubtedly be ranked under mind. But if consciousness and intelligence be considered as essential to mind, which is the case according to their definition, we cannot give the vegetative principle the name of mind, because it has not been proved that it possesses consciousness and intelligence. It acts indeed according to a fixed plan, which displays the highest degree of intelligence; but this plan may belong, not to the vegetative principle itself, but to the Being who formed that principle. We can conceive it to have been endowed by the Author of Nature with peculiar powers, which it must always exert according to certain fixed laws; and the phenomena of vegetation may be the result of this mode of acting. This, as far as we can see, is not impossible. It must be shown to be impossible by every person who wishes to prove that plants possess consciousness and intelligence; for the proofs of this consciousness can only be deduced from the design which the actions of plants manifest. Those philosophers who have ascribed consciousness and intelligence to plants, have founded their belief principally on certain actions which plants perform on the application of stimuli. But these actions prove nothing more than what cannot be denied, that there exists a vegetative principle, which is not material, and which has certain properties in common with the living principles of animals; but whether or not this vegetative principle possesses consciousness and intelligence, is a very different question, and must be decided by very different proofs. We do not say that the heart of an animal is conscious, because it continues to beat on the application of proper stimuli for some time after it has been separated from the rest of the body.

The death of plants, if we can judge from the phenomena, is owing, not to the vegetative principle leaving them, but to the organs becoming at last altogether unfit for performing their functions, and incapable of being repaired by any of the powers which that principle possesses. The changes which vegetable substances undergo after death come now to be examined. They shall form the subject of the ensuing chapter.

(a) Physiologists have usually given it the name of living principle. We would have adopted that name, if it had not been too general for our purpose. CHAP. III. OF THE DECOMPOSITION OF VEGETABLE SUBSTANCES.

Not only entire plants undergo decomposition after death, but certain vegetable substances also, whenever they are mixed together, and placed in proper circumstances, mutually decompose each other; and new compound substances are produced. These mutual decompositions, indeed, are naturally to be expected: for as all vegetable substances are composed of several ingredients, differing in the strength of their affinity for each other, it is to be supposed that, when two such substances are mixed together, the different affinities will, in many cases, prove stronger than the quietest; and therefore decomposition, and the formation of new compounds, must take place: just as happens when the acetate of lead and sulphate of potash are mixed together.

These mutual decompositions of vegetable substances are by no means so easily traced, or to readily explained, as the mutual decompositions of neutral salts; partly on account of the number of substances, whose affinities for each other are brought into action, and partly because we are ignorant of the manner in which the ingredients of vegetable substances are mutually combined.

Chemists have agreed to give these mutual decompositions which take place in vegetable substances the name of fermentation; a word first introduced into chemistry by Van Helmont*; and the new substances produced they have called the products of fermentation. All the phenomena of fermentation lay for many years concealed in the completest darkness, and no chemist was bold enough to hazard even an attempt to explain them. They were employed, however, and without hesitation too, in the explanation of other phenomena; as if giving to one process, the name of another of which we are equally ignorant, could, in reality, add anything to our knowledge. The darkness which enveloped these phenomena, has lately begun to dispel; but they are still surrounded with a very thick mist; and we must be much better acquainted with the composition of vegetable substances, and the mutual affinities of their ingredients, than we are at present, before we can explain them in a satisfactory manner.

The vegetable fermentations or decompositions may be arranged under five heads: namely, that which produces bread, that which produces wine, that which produces beer, that which produces acetic acid or vinegar, and the putrefactive fermentation, or that which produces the spontaneous decomposition of decayed vegetables. These shall be the subject of the five following sections. In order to avoid long titles, we shall give to the first three sections the name of the new substances produced by the fermentation.

Sect. I. Of Bread.

Simple as the manufacture of bread may appear to us who have been always accustomed to consider it as a common process, its discovery was probably the work of ages, and the result of the united efforts of men, whose sagacity, had they lived in a more fortunate period of society, would have rendered them the rivals of Aristotle or of Newton.

The method of making bread similar to ours was known in the East at a very early period; but neither the precise time of the discovery, nor the name of the person who published it to the world, has been preserved. We are certain that the Jews were acquainted with it in the time of Moses: for in Exodus* we find a prohibition to use leavened bread during the celebration of the passover. It does not appear, however, to have been known to Abraham; for we hear in his history of cakes frequently, but nothing of leaven. Egypt, both from the nature of the soil and the early period at which it was civilized, bids fairest for the discovery of making bread. It can scarcely be doubted, that the Jews learned the art from the Egyptians. The Greeks assure us, that they were taught the art of making bread by the god Pan. We learn from Homer that it was known during the Trojan war†. The Romans were ignorant of the method of making bread till the year 580, after the building of Rome, or 200 years before the commencement of the Christian era‡. Since that period the art has never been unknown in the fourth part of Europe; but it made its way to the north very slowly, and even at present in many northern countries fermented bread is but very seldom used.

The only substance well adapted for making bread, substance we mean loaf bread, is wheat flour, which is composed of four ingredients; namely, gluten, starch, albumen, and a sweet mucous matter, which possesses nearly the properties of sugar, and which is probably a mixture of sugar and mucilage. It is to the gluten that wheat flour owes its superiority to every other as the basis of bread. Indeed, there are only two other substances at present known of which good loaf bread can be made; these are rye and potatoes. The rye loaf is by no means so well raised as the wheat loaf; and potatoes will not make bread at all without particular management. Potatoes, previously boiled and reduced to a very fine tough paste by a rolling pin, must be mixed with an equal weight of potato starch. This mixture, baked in the usual way, makes a very white, well raised, pleasant bread. We are indebted for the process to Mr Permentier. Barley-meal perhaps might be substituted for starch.

The baking of bread consists in mixing wheat flour baking with water, and forming it into a paste. The average proportion of these is two parts of water to three of flour. But this proportion varies considerably, according to the age and the quality of the flour. In general, the older and the better the flour is, the greater is the quantity of water required. If the paste, after being thus formed, be allowed to remain for some time, its ingredients gradually act upon each other, and the paste acquires new properties. It gets a disagreeable sour taste, and a quantity of gas (probably carbonic acid gas) is evolved. In short, the paste ferments (ii). These changes do not take place without water; that liquid, therefore, is a necessary agent. Possibly it is decomposed by the action of the starch upon it; for when starch is diluted with water, it gradually becomes sour. The gluten, too, is altered, either by the action of the water on it, or of the starch; for if we examine the paste after

(n) It was from this process that Van Helmont transferred the word fermentation into chemistry. after it has undergone fermentation, the gluten is no longer to be found. If past, after standing for a sufficient time to ferment, be baked in the usual way, it forms a loaf full of eyes like our bread, but of a taste so sour and unpleasant that it cannot be eaten. If a small quantity of this old paste, or leaven as it is called, be mixed with new made paste, the whole begins to ferment in a short time; a quantity of gas is evolved; but the glutinous part of the flour renders the paste so tough, that the gas cannot escape; it therefore causes the paste to swell in every direction; and if it be now baked into loaves, the immense number of air bubbles imprisoned in every part renders the bread quite full of eyes, and very light. If the precise quantity of leaven necessary to produce the fermentation, and no more, has been used, the bread is sufficiently light, and has no unpleasant taste; but if too much leaven be employed, the bread has a bad taste; if too little, the fermentation does not come on, and the bread is too compact and heavy. To make good bread with leaven, therefore, is very difficult.

The ancient Gauls had another method of fermenting bread. They formed their paste in the usual way; and instead of leaven, mixed with it a little of the barm which collects on the surface of fermenting beer. This mixture produced as complete and as speedy a fermentation as leaven; and it had the great advantage of not being apt to spoil the taste of the bread. About the end of the 17th century, the bakers in Paris began to introduce this practice into their processes. The practice was discovered, and exclaimed against; the faculty of medicine, in 1688, declared it prejudicial to health; and it was not till after a long time that the bakers succeeded in convincing the public that bread baked with barm is superior to bread baked with leaven. In this country the bread has for these many years been fermented with barm.

What is this barm which produces these effects? The question is curious and important; but we are not able to answer it completely. Mr Henry of Manchester has concluded, from a number of very interesting experiments, that the only useful part of barm is carbonic acid gas, and that this gas therefore is the real fermenter of paste.

That the barm of beer, in its usual state, contains carbonic acid gas, cannot be doubted; and that carbonic acid gas acts as a ferment, the experiments of Mr Henry prove decisively. But that the only active part of barm is carbonic acid gas, and nothing but carbonic gas, is extremely doubtful, or rather we are certain that it is not true. It has been customary with the bakers of Paris to bring their barm from Flanders and Picardy in a state of dryness. When skimmed off the beer, it is put into sacks, and the moisture allowed to drop out; then these sacks are subjected to a strong pressure, and when the barm is dry it is made up into balls. Now, in this state, it is not to be supposed that bubbles of carbonic acid can remain entangled in the barm; they must have been squeezed out by the press, and by the subsequent formation of the barm into balls; yet this barm, when moistened with water, ferments the bread as well as new barm.

After the bread has fermented, and is properly raised, it is put into the oven previously heated, and allowed to remain till it is baked. The mean heat of an oven, ascertained by Mr Tillet, is 448°. The bakers do not use a thermometer; but they judge that the oven is arrived at the proper heat when flour thrown on the floor of it becomes black very soon without taking fire. We see, from Tillet's experiment, that this happens at the heat of 448°.

When the bread is taken out of the oven, it is found less likely than when put in; as might naturally have been expected, from the evaporation of moisture, which must have taken place at that temperature. Mr Tillet, and the other commissioners who were appointed to examine this subject in consequence of a petition from the bakers of Paris, found that a loaf, which weighed before it was put into the oven 4.625 lbs. after being taken out baked, weighed, at an average, only 3.813 lbs. or 0.812 lb. less than the paste. Consequently 100 parts of paste loaf, at an average, 17.34 parts, or somewhat more than 1/4 th by baking. They found, however, that this loss of weight was by no means uniform, even with respect to those loaves which were in the oven at the same time, of the same form, and in the same place, and which were put in and taken out at the same instant. The greatest difference in these circumstances amounted to .2889, or 7.5 parts in the hundred, which is about 1/17 th of the whole. This difference is very considerable, and it is not easy to say to what it is owing. It is evident, that if the paste has not all the same degree of moisture, and if the barm be not accurately mixed through the whole, if the fermentation of the whole be not precisely the same, that these differences must take place. Now it is needless to observe how difficult it is to perform all this completely. The French commissioners found, as might indeed have been expected, that other things being equal, the loss of weight sustained is proportional to the extent of surface of the loaf, and to the length of time that it remains in the oven; that is to say, the smaller the extent of the external surface, or, which is the same thing, the nearer the loaf approaches to a globular figure, the smaller is the loss of weight which it sustains; and the longer it continues in the oven, the greater is the loss of weight which it sustains. Thus a loaf which weighed exactly 4 lbs. when newly taken out of the oven, being replaced as soon as weighed, lost, in ten minutes, .125 lb. of its weight, and in ten minutes more it again lost .0625 lb.

Loaves are heaviest when just taken out of the oven; they gradually lose part of their weight, at least if not kept in a damp place, or wrapped round with a wet cloth (k). Thus Mr Tillet found that a loaf of 4 lbs. after being kept for a week, wanted .3125, or nearly 1/17 th of its original weight.

When bread is newly taken out of the oven, it has a peculiar, and rather pleasant smell, which it loses by keeping; as it does also the peculiar taste by which new bread is distinguished. This shows us, that the bread undergoes chemical changes; but what these changes are, or what the peculiar substance is to which the odour of bread is owing, is not known.

(k) This is an excellent method of preserving bread fresh, and free from mould, for a long time. Bread differs very completely from the flour of which it is made; for none of the ingredients of the flour can now be discovered in it. The only chemist who has attempted an analysis of bread is Mr. Geoffroy. He found that 100 parts of bread contained the following ingredients:

- 24.735 water. - 32.030 gelatinous matter, extracted by boiling water. - 39.843 residuum insoluble in water.

Total: 96.608

But this analysis, which was published in the Memoirs of the French Academy for the year 1733, was made at a time when the infant state of the science of chemistry did not admit of any thing like accuracy.

**Sect. II. Of Wine.**

There is a considerable number of ripe fruits from which a sweet liquor may be expressed, having at the same time a certain degree of acidity. Of such fruits we have in this country the apple, the cherry, the gooseberry, the currant, &c. but by far the most valuable of these fruits is the grape, which grows luxuriantly in the southern parts of Europe. From grapes, fully ripe, may be expressed a liquid of a sweet taste, to which the name of wine has been given. This liquid is composed almost entirely of five ingredients: namely, water, sugar, jelly, mucilage, and tartaric acid partly saturated with potash. The quantity of sugar which grapes fully ripe contain is very considerable; it may be obtained in crystals by evaporating must to the consistence of syrup, separating the tartar which precipitates during the evaporation, and then setting the must aside for some months. The crystals of sugar are gradually formed.

When must is put into the temperature of about 70°, the different ingredients begin to act upon each other, and what is called vinous fermentation commences. The phenomena of this fermentation are an intense motion in the liquid, its becoming thick and muddy, a temperature equal to 72°, and an evolution of carbonic acid gas. In a few days the fermentation coalesces, the thick part subsides to the bottom, the liquid becomes clear, it has lost much of its saccharine taste, and assumed a new one, its specific gravity is diminished; and, in short, it has become the liquid well known under the name of wine.

Now what is the cause of this fermentation; what are the substances which mutually decompose each other; and what is the nature of the new substance formed?

These changes are produced altogether by the mutual action of the substances contained in must; for they take place equally well, and wine is formed equally well in close vessels as in the open air.

If the must be evaporated to the consistence of a thick syrup, or to a red, as the elder chemists termed it, the fermentation will not commence, though the proper temperature, and every thing else necessary to produce fermentation, be present. But if this syrup be again diluted with water, and placed in favourable circumstances, it will ferment. Therefore the presence of water is absolutely necessary for the existence of vinous fermentation.

If the juice of those fruits which contain but little sugar, as currants, be put into a favourable situation, fermentation indeed takes place, but so slowly, that the product is not wine, but vinegar; but if a sufficient quantity of sugar be added to these very juices, wine is readily produced. No substance whatever can be made to undergo vinous fermentation, and to produce wine, unless sugar be present. Sugar therefore is absolutely necessary for the existence of vinous fermentation; and we are certain that it is decomposed during the process; for no sugar can be obtained from properly fermented wine.

All those juices of fruits which undergo the vinous fermentation, either with or without the addition of sugar, contain an acid. We have seen already in the first chapter that the vegetable acids are obtained chiefly from fruits. The apple, for instance, contains malic acid; the lemon, citric acid; the grape, tartaric acid. The Marquis de Bullion has ascertained, that milk will not ferment if all the tartaric acid which it contains be separated from it*. We may conclude from this, that the presence of a vegetable acid is absolutely necessary for the commencement of the vinous fermentation. This renders it probable that the essential part of harm is a vegetable acid, or something equivalent; for if sugar be dissolved in four times its weight of water, mixed with the yeast of beer, and placed in a proper temperature, it undergoes the vinous fermentation†.

All the juices of fruits which undergo the vinous fermentation contain a quantity of jelly, or mucilage. And jelly or mucilage, or both. These two substances resemble each other in so many particulars, and it is so difficult to separate them, that we shall suppose they have the same effect in the mixture. The presence of these substances renders it probable that they also are necessary for the vinous fermentation. Perhaps they act chiefly by their tendency to become acid.

Thus we see, that for the production of wine a certain temperature, a certain portion of water, sugar, a vegetable acid, and, in all probability, jelly also, is necessary. Mr. Lavoisier found that sugar would not ferment unless dissolved in at least four times its weight of water. This seems to indicate that the particles of sugar must be removed to a certain distance from each other before the other ingredients can decompose them. The evolution and separation of carbonic acid gas in such quantity, shows us that the proportion of the carbon and the oxygen of the sugar is diminished. It is not certain that the mucilage of the wine is decomposed so completely as the sugar; for it has been observed, that when the must abounds in mucilage, the wine is apt to become sour.

When wine is distilled by means of a low heat, there comes over a quantity of alcohol, and the remainder is a solution of acetic acid. From this fact, it has been concluded that wine is composed of acetic acid and alcohol. But that the distillation occasions a chemical change in the ingredients of wine is evident from this, that if we again mix the alcohol and acetic acid, we do not reproduce the wine.

Fourcroy has attempted to shew that alcohol existed ready formed; but his proofs are not conclusive. Fabroni During the fermentation, a quantity of carbonic acid gas is constantly discharged, not in a state of purity, but containing, combined with it, a portion of the wort; and if this gas be made to pass through water, it will deposit wort, which may be fermented in the usual manner.

When beer is distilled, alcohol is obtained, and the residuum is an acid liquor. The theory of beer is to be obviously the same with that of wine that it requires no additional explanation.

**Sect. IV. Of the Acetous Fermentation.**

If wine or beer be kept at a temperature between 70° and 90°, it gradually loses its properties, and is converted into acetous acid.

During this change, a quantity of oxygen gas is absorbed, and the whole of the spirituous part of the wine or beer disappears. Consequently its ingredients have mutually decomposed each other.

Neither pure alcohol, nor alcohol diluted with water, are capable of undergoing this change, neither do they absorb any oxygen. This absorption, then, is made by the mucilaginous matter which always exists in these liquids. No acetous acid is ever produced, unless some acid be present in the liquid. We may conclude, then, that the mucilage acquires the properties of an acid before it begins to act upon the spirituous part of the beer or the wine.

As the acetous acid has been already treated of in the article Chemistry, Suppl., it is unnecessary to dwell any longer on this subject here.

**Sect. V. Of Putrefaction.**

All vegetable substances, both complete plants and Nature of their component parts separately, when left entirely to putrefaction themselves, are gradually decomposed and destroyed, provided moisture be present, and the temperature be not much under 45°, nor too high to evaporate suddenly all the moisture. This decomposition has obtained the name of putrefaction.

It proceeds with most rapidity in the open air; but the contact of air is not absolutely necessary. Water is, in all cases, essential to the process, and therefore is most probably decomposed.

Putrefaction is constantly attended with a fetid odour, owing to the emission of certain gaseous matters, which differ according to the putrefying substance. Some vegetable substances, as gluten, and cruciform plants, emit ammonia; others, as onions, seem to emit phosphorated hydrogen gas. Carbonic acid gas, and hydrogen gas, impregnated with unknown vegetable matters, are almost constantly emitted in abundance. When the whole process is finished, scarcely anything remains but the earths, the fats, and the metals, which formed a constituent part of the vegetable. But our chemical knowledge of vegetable compounds is by far too limited to enable us to follow this very complicated process with any chance of success. Part II. Of Animal Substances.

When we compare animals and vegetables together, each in their most perfect state, nothing can be easier than to distinguish them. The plant is confined to a particular spot, and exhibits no marks of consciousness or intelligence; the animal, on the contrary, can remove at pleasure from one place to another, is possessed of consciousness, and a high degree of intelligence. But on approaching the contiguous extremes of the animal and vegetable kingdom, these striking differences gradually disappear, the objects acquire a greater degree of resemblance, and at last approach each other so nearly, that it is scarcely possible to decide whether some of those situated on the very boundary belong to the animal or vegetable kingdom.

To draw a line of distinction, then, between animals and vegetables, would be a very difficult task; but it is not necessary for us, in this place at least, to attempt it; for almost the only animals whose bodies have been hitherto examined with any degree of chemical accuracy, belong to the most perfect classes, and consequently are in no danger of being confounded with plants. Indeed the greater number of facts which we have to relate, apply only to the human body, and to those of a few domestic animals. The task of analysing all animal bodies is immense, and must be the work of ages of indefatigable industry.

We shall divide this part of the article into four chapters. In the first chapter, we shall give an account of the different ingredients hitherto found in animals, such as they at least as have been examined with any degree of accuracy; in the second, we shall treat of the different members of which animal bodies are composed; which must consist each of various combinations of the ingredients described in the first chapter; in the third, we shall treat of those animal functions which may be elucidated by chemistry; and, in the fourth, of the changes which animal bodies undergo after death.

Chapter I. Of the Ingredients of Animals.

The substances which have been hitherto detected in the animal kingdom, and of which the different parts of animals, as far as these parts have been analysed, are found to be composed, may be arranged under the following heads:

1. Fibrina, 2. Albumen, 3. Gelatin, 4. Mucilage, 5. Basis of bile, 6. Urea, 7. Sugar, 8. Sulphur, 9. Oils, 10. Acids, 11. Alkalies, 12. Earths, 13. Metals.

These shall form the subject of the following sections:

Sect. I. Of Fibrina.

If a quantity of blood, newly drawn from an animal, be allowed to remain at rest for some time, a thick red albumen clot gradually forms in it, and subsides. Separate this clot from the rest of the blood, wash it repeatedly in water till it ceases to give out any colour or taste to show the liquid; the substance which remains after this treatment, is denominated fibrina. It has been long known to physicians under the name of the fibrous part of the blood, but has not till lately been accurately described.

Fibrina is of a white colour, has no taste, and is insoluble in water and in alcohol. It is soft and ductile, and has a considerable degree of elasticity, and resembles very much the gluten of vegetables.

Pure fixed alkalies do not act upon it, unless they be very much concentrated, and then they decompose it. All the acids combine with it readily, and dissolve it. Water and alkalies separate it again; but it has lost entirely its former properties. With muriatic acid it forms a green coloured jelly.

When nitric acid is poured upon fibrina, azotic gas is disengaged, as Berthollet first discovered. The quantity of this gas is greater than can be obtained from the same quantity of other animal substances by the same process. After this, prussic acid and carbonic acid gas are exhausted. By the influence of heat the fibrina is dissolved; much nitrous gas is disengaged; the liquid, when concentrated, yields oxalic and malic acids; and white flakes are deposited, consisting of an oily substance, and of phosphate of lime.

When fibrina is distilled, it yields a very large quantity of ammonia.

These properties are sufficient to show us that this substance is composed of azotic hydrogen, and carbon; but neither the precise proportion of these ingredients, nor the manner of their combination, are at present known.

Sect. II. Of Albumen.

The eggs of fowls contain two very different substances: a yellow oily-like matter, called the yolk; and a colourless glossy viscid liquid, distinguished by the name of white. This last is the substance which chemists have agreed to denominate albumen (1). The white of an egg, however, is not pure albumen. It contains, mixed with it, some carbonat of soda, and some sulphur; but the quantity of these substances is so small that they do not much influence its properties. We shall therefore consider it as albumen.

On the application of a heat of 165°, it coagulates, as is well known, into a white solid mass; the consistency of which, when other things are equal, depends, in some measure, on the time during which the heat was applied. The coagulated mass has precisely the same weight that it had while fluid.

The taste of coagulated albumen is quite different from that of liquid albumen: its appearance, too, and its

(1) This is merely the Latin term for the white of an egg. It was first introduced into chemistry by the physiologists. The coagulation of albumen takes place even though air be completely excluded; and even when air is present there is no absorption of it, nor does albumen in coagulating change its volume. Acids have the property of coagulating albumen, as Scheele ascertained. Alcohol also produces, in some measure, the same effect. Heat, then, acids and alcohol, are the agents which may be employed to coagulate albumen.

It is remarkable, that if albumen be diluted with a sufficient quantity of water, it can no longer be coagulated by any of these agents. Scheele mixed the white of an egg with ten times its weight of water, and then, though he even boiled the liquid, no coagulum appeared. Acids indeed, and alcohol, even then coagulated it; but they also lose their power, if the albumen be diluted with a much greater quantity of water, as has been ascertained by many experiments. Now we know, that when water is poured into albumen, not only a mechanical mixture takes place, but a chemical combination; for the albumen is equally distributed through every part of the liquid. Consequently its integrant particles must be farther separated from each other, and their distance must increase with the quantity of water with which they are diluted. We see, therefore, that albumen ceases to coagulate whenever its particles are separated from each other beyond a certain distance. That no other change is produced, appears evident from this circumstance, that whenever the watery solution of albumen is sufficiently concentrated by evaporation, coagulation takes place, upon the application of the proper agents, precisely as formerly.

It does not appear that the distance of the particles of albumen is changed by coagulation; for coagulated albumen occupies precisely the same sensible space as liquid albumen.

Thus two things seem certain respecting the coagulation of albumen: 1. That its particles must not be beyond a certain distance; 2. That the coagulation does not produce any sensible change in their distance. To what, then, is the coagulation of albumen owing? We can conceive no change to take place from a state of liquidity to that of solidity, without some change in the figure of the particles of the body which has undergone that change: for if the figure and the distance of the particles of bodies continue the same, it is impossible to conceive any change at all to take place. Since, then, the distance of the particles of albumen does not, as far as we can perceive, change, we must conclude, that the figure of the particles actually does change. Now such a change may take place three ways: 1. The figure may be changed by the addition of some new molecules to each of the molecules of the body. 2. Some molecules may be abstracted from every integrant particle of the body. 3. Or the molecules, of which the integrant particles are composed, may enter into new combinations, and form new integrant particles, whose form is different from that of the old integrant particles. Some one or other of these three things must take place during the coagulation of albumen.

1. Scheele and Fourcroy have ascribed the coagulation of albumen to the first of these causes, namely, to the addition of a new substance. According to Scheele, caloric is the substance which is added. Fourcroy, on the contrary, affirms that it is oxygen.

Scheele supported his opinion with that wonderful ingenuity which shone so eminently in every thing which he did. He mixed together one part of white of egg and four parts of water, added a little pure alkali, and then dropped in as much muriatic acid as was sufficient to saturate the alkali. The albumen coagulated; but when he repeated the experiment, and used carbonat of alkali instead of pure alkali, no coagulation ensued. In the first case, says he, there was a double decomposition; the muriatic acid separated from a quantity of caloric with which it was combined, and united with the alkali; while, at the same instant, the caloric of the acid united with the albumen, and caused it to coagulate. The same combination could not take place when the alkaline carbonat was used, because the carbonic acid gas carried off the caloric, for which it has a strong affinity.

This explanation is plausible; but it is contrary to every other known fact in chemistry, to suppose that caloric can combine with a substance without occasioning any alteration in its bulk, and cannot therefore be admitted without the most rigid proof.

Fourcroy observes, in support of his opinion, that the white of an egg is not at first capable of forming a hard coagulum, and that it only acquires that property by exposure to the atmosphere. It is well known that the white of a new laid egg is milky after boiling; and that if the shell be covered over with grease, to exclude the external air, it continues long in that state; whereas the white of an old egg, which has not been preserved in that manner, forms a very hard tough coagulum. These facts are undoubtedly; and they render it exceedingly probable, that albumen acquires the property of forming a hard coagulum only by absorbing oxygen; but they by no means prove that coagulation itself is owing to such an absorption. And since coagulation takes place without the presence of air, and since no air, even when it is present, is absorbed, this opinion cannot be maintained without inconsistency.

2. The only substance which can be supposed to leave albumen during coagulation, since it does not lose weight, is caloric. We know that in most cases where a fluid is converted into a solid, caloric is actually disengaged. It is extremely probable, then, that the same disengagement takes place here. But the opinion has not been confirmed by any proof. Fourcroy indeed says, that in an experiment made by him, the thermometer rose a great number of degrees. But as no other person has ever been able to observe any such thing, it cannot be doubted that this philosopher has been misled by some circumstance or other to which he did not attend. It is usual, in many cases, for bodies to lose bulk when they give out caloric; but that there are exceptions to this rule, is well known.

3. Even if the second opinion were true, it is scarcely possible to conceive the coagulation of albumen to take place without some change in its integrant particles. We can see how all the substances which coagulate albumen might produce such a change; and the insolubility of coagulated albumen in water, and its other different properties, render it more than probable that some such change actually takes place. But what that change is, cannot even be conjectured. The coagulation of albumen is intimately connected with one of the most important problems in chemistry, namely, the cause of fluidity and solidity. But this problem can only be resolved, with any prospect of success, by a geometrical investigation of the phenomena of heat.

Coagulated albumen is dissolved by the mineral acids, greatly diluted with water; and if a concentrated acid be added to the solution, the albumen is again precipitated*. Alkalies, however, do not precipitate it from its solution in acids†. But if a solution of tan be poured into the acid solution of albumen, a very copious precipitate appears‡.

If the solution of tan be poured into an aqueous solution of uncoagulated albumen, it forms with it a very copious precipitate, which is insoluble in water. This precipitate is a combination of tan and albumen. This property which albumen has of precipitating with tan, was discovered by Seguin§; it furnishes us with a method of detecting the presence of albumen in any liquid in which we suspect it.

Pure alkalies and lime water also dissolve albumen; at the same time ammonia is disengaged, owing to the decomposition of part of the albumen. Acids precipitate the albumen from alkalies, but its properties are changed*.

Nitric acid, when assayed by heat, disengages azotic gas from albumen†; but the quantity is not so great as may be obtained from fibrina‡. The albumen is gradually dissolved, nitrous gas is emitted, oxalic and malic acids are formed, and a thick oily matter makes its appearance on the surface§. When distilled, it furnishes the same products as fibrina, only the quantity of ammonia is not so great‖.

Hence it follows, that albumen is composed of azot, hydrogen, and carbon, as well as fibrina; but the proportion of azot is not so great in the first substance as in the second.

**Sect. III. Of Gelatine.**

If a piece of the fresh skin of an animal, an ox for instance, after the hair and every impurity is carefully separated, be washed repeatedly in cold water, till the liquid ceases to be coloured, or to abridge any thing; if the skin, thus purified, be put into a quantity of pure water, and boiled for some time, part of it will be distilled. Let the decoction be slowly evaporated till it is reduced to a small quantity, and then put aside to cool. When cold, it will be found to have assumed a solid form, and to resemble precisely that tremulous substance well known to everybody under the name of gelly. This is the substance called in chemistry gelatine. If the evaporation be still farther continued, by exposing the gelly to dry air, it becomes hard, semitransparent, breaks with a glassy fracture, and is in short the substance so much employed in different arts under the name of glue. Gelatine, then, is precisely the same with glue; only that it must be supposed always free from those impurities with which glue is so often contaminated.

Gelatine is transparent and colourless; when thrown into water, it very soon swells, and assumes a gelatinous form, and gradually dissolves completely. By evaporating the water, it may be obtained again unaltered in the form of gelly.

When an infusion of tan is dropped into a solution of gelatine in water, there is instantly formed a copious white precipitate, which has all the properties of leather. This precipitate is composed of tan and gelatine. These two substances, therefore, when combined, form leather. Albumen and gelatine are the only animal substances known which have the property of combining with tan, and forming with it an insoluble compound. They may be always easily detected, therefore, by means of tan; and they may be readily distinguished from each other, as albumen alone coagulates by heat, and gelatine alone concretizes into a gelly.

Gelatine is insoluble in alcohol, and is even precipitated from water by it; but both acids and alkalies dissolve it. Nitric acid disengages from it a small quantity of azotic gas; dissolves it, when assayed by heat, excepting an oily matter, which appears on the surface of the solution; and converts it, partly into oxalic and malic acids*.

When distilled, there comes over first water, containing some animal matter; the gelatine then swells, becomes black, emits a fetid odour, accompanied with acrid fumes; Some empyreumatic oil then comes over, and a very small quantity of carbonat of ammonia; its coaly residuum remains behind. These phenomena show, that gelatine is composed of carbon, hydrogen, and azot; but the proportion of azot is evidently much smaller than in either fibrina or albumen†.

**Sect. IV. Of Animal Mucilage.**

No word in chemistry is used with less accuracy than mucilage. It serves as a common name for almost every animal substance which cannot be referred to any other class.

None of the substances to which the name of animal mucilage has been given, have been examined with care; of course it is unknown whether these substances be the same or different.

Whenever an animal substance possesses the following properties, it is at present denominated an animal mucilage by chemists:

1. Soluble in water. 2. Insoluble in alcohol. 3. Neither coagulable by heat, nor congealing into a gelly by evaporation. 4. Not precipitated by the solution of tan.

Most of the substances called mucilage have also the property of absorbing oxygen, and of becoming by that means insoluble in water.

The mucilaginous substances shall be pointed out in the next chapter. In the present state of our knowledge, any account of them here would merely be a repetition of the properties just mentioned.

**Sect. V. Of the Basis of Bits.**

Into o.32 parts of fresh ox bile pour one part of concentrated muriatic acid. After the mixture has stood for some hours, pass it through a filter, in order to separate a white coagulated substance. Pour the filtrate liquor, which has a fine green colour, into a glass vessel, and evaporate it by moderate heat. When it has arrived at a certain degree of concentration, a green coloured substance precipitates. Decant off the clear liquid, and wash the precipitate in a small quantity of pure pure water. This precipitate is the basis of bile; or the resin of bile, as it is sometimes called.

The basis of bile is of a black colour; but when spread out upon paper or on wood, it is green: its taste is intensely bitter.

When heated to about 122°, it melts; and if the heat be still further increased, it takes fire, and burns with rapidity. It is soluble in water, both cold and hot, and still more soluble in alcohol; but water precipitates it from that liquid.

It is soluble also in alkalies, and forms with them a compound which has been compared to a soap. Acids, when sufficiently diluted, precipitate it both from water and alkalies without any change; but if they be concentrated, the precipitate is redissolved.

When distilled, it furnishes some febrifuge acid.

From these properties, it is clear that the basis of bile has a considerable resemblance to oils; but it differs from them entirely in several of its properties. The addition of oxygen, with which it combines readily, alters it somewhat, and brings it still nearer to the class of oils.

In this altered state, the basis of bile may be obtained by the following process. Pour oxy-muriatic acid cautiously into bile till that liquid loses its green colour; then pass it through a filter to separate some albumen which coagulates. Pour more oxy muriatic acid into the filtered liquid, and allow the mixture to remain for some time. The oxy-muriatic acid is gradually converted into common muriatic acid; and in the meantime the basis of bile absorbs oxygen, and acquires new properties. Pour into the liquid, after it has remained a sufficient time, a little common muriatic acid, a white precipitate immediately appears, which may be separated from the fluid. This precipitate is the basis of bile combined with oxygen.

It has the colour and the consistence of tallow, but still retains its bitter taste. It melts at the temperature of 104°. It dissolves readily in alcohol, and even in water, provided it be assisted by heat. Acids precipitate it from these solutions.

**Sect. VI. Of Urea.**

Evaporate, by a gentle heat, a quantity of human urine voided six or eight hours after a meal, till it be reduced to the consistence of a thick syrup. In this state, when put by to cool, it concretes into a crystalline mass. Pour, at different times, upon this mass four times its weight of alcohol, and apply a gentle heat; a great part of the mass will be dissolved, and there will remain only a number of saline substances. Pour the alcohol solution into a retort, and distil by the heat of a sand bath till the liquid, after boiling some time, is reduced to the consistence of a thick syrup. The whole of the alcohol is now separated, and what remains in the retort crystallizes as it cools. These crystals consist of the substance known by the name of urea.

This substance was first described by Rouelle the Younger in 1773, under the name of the Japanaceous extract of urine. He mentioned several of its properties; but very little was known concerning its nature till Fourcroy and Vauquelin published their experiments on it in 1799. These celebrated chemists have given it the name of urea, which we have adopted.

Urea, obtained in this manner, has the form of crystalline plates crossing each other in different directions. Its colour is yellowish white; it has a fetid smell, somewhat resembling that of garlic or arsenic; its taste is strong and acid, resembling that of ammonical salts; it is very viscid and difficult to cut, and has a good deal of resemblance to thick honey. When exposed to the open air, it very soon attracts moisture, and is converted into a thick brown liquid. It is extremely soluble in water; and during its solution, a considerable degree of cold is produced. Alcohol dissolves it with facility, but scarcely in so large a proportion as water. The alcoholic solution yields crystals much more readily on evaporation than the solution in water.

When nitric acid is dropped into a concentrated solution of urea in water, a great number of bright pearl coloured crystals are deposited, composed of urea and nitric acid. No other acid produces this singular effect. The concentrated solution of urea in water is brown, but becomes yellow when diluted with a large quantity of water. The infusion of nut galls gives it a yellowish-brown colour, but causes no precipitate. Neither does the infusion of tan produce any precipitate.

When heat is applied to urea, it very soon melts, swells up, and evaporates, with an insupportably fetid odour. When distilled, there comes over first benzoic acid, then carbonat of ammonia in crystals, some carbonated hydrogen gas, with traces of prussic acid and oil; and there remains behind a large residuum, composed of charcoal, muriat of ammonia, and muriat of soda. The distillation is accompanied with an almost insupportably fetid alliaceous odour. Two hundred and eighty-eight parts of urea yield by distillation two hundred parts of carbonat of ammonia, one hundred parts of carbonated hydrogen gas, seven parts of charcoal, and sixty-eight parts of benzoic acid, muriat of soda, and muriat of ammonia. These three last ingredients Fourcroy and Vauquelin consider as foreign substances, separated from the urine by the alcohol at the same time with the urea. Hence it follows, that one hundred parts of urea, when distilled, yield:

\[ \begin{align*} &92.627 \text{ carbonat of ammonia}, \\ &4.68 \text{ carbonated hydrogen gas}, \\ &3.225 \text{ charcoal}. \end{align*} \]

Now two hundred parts of carbonat of ammonia are composed of eighty-six ammonia, ninety carbonic acid gas, and twenty-four water. Hence it follows, that one hundred parts of urea are composed of:

\[ \begin{align*} &39.5 \text{ oxygen}, \\ &32.5 \text{ azot}, \\ &14.7 \text{ carbon}, \\ &13.5 \text{ hydrogen}. \end{align*} \]

But it can scarcely be doubted, that the water which was found in the carbonat of ammonia existed ready formed in the urea before the distillation.

When the solution of urea in water is kept in a boiling heat, and new water is added as it evaporates, the urea is gradually decomposed, a very great quantity of carbonat of ammonia is disengaged, and at the same time acetic acid is formed, and some charcoal precipitates.

When a solution of urea in water is left to itself for some time, it is gradually decomposed. A froth occurs due to the formation of... leaves on its surface; air bubbles are emitted which have a strong disagreeable smell, in which ammonia and acetic acid are distinguishable. The liquid contains a quantity of acetic acid. The decomposition is much more rapid if a little gelatine be added to the solution. In that case more ammonia is disengaged, and the proportion of acetic acid is not so great.

When the solution of urea is mixed with one-fourth of its weight of diluted sulphuric acid, no effervescence takes place; but, on the application of heat, a quantity of oil appears on the surface, which concretes upon cooling; the liquid, which comes over into the receiver, contains acetic acid, and a quantity of sulphate of ammonia remained in the retort dissolved in the undistilled mass. By repeated distillations, the whole of the urea is converted into acetic acid and ammonia.

When nitric acid is poured upon crystallized urea, a violent effervescence takes place, the mixture froths, assumes the form of a dark red liquid, great quantities of nitrous gas, azotic gas, and carbonic acid gas, are disengaged. When the effervescence is over, there remains only a concrete white matter, with some drops of reddish liquid. When heat is applied to this residuum, it detonates like nitrat of ammonia. Into a solution of urea, formed by its attracting moisture from the atmosphere, an equal quantity of nitric acid, of the specific gravity 1.462, diluted with twice its weight of water, was added; a gentle effervescence ensued; very gentle heat was applied, which supported the effervescence for two days. There was disengaged the first day a great quantity of azotic gas and carbonic acid gas; the second day, carbonic acid gas, and at last nitrous gas. At the same time with the nitrous gas an odour was perceptible of the oxygenated prussic acid of Berthollet. At the end of the second day, the matter in the retort, which was become thick, took fire, and burnt with a violent explosion. The residuum contained traces of prussic acid and ammonia. The receiver contained a yellowish acid liquor, on the surface of which some drops of oil swam.

Muriatic acid dissolves urea, but does not alter it. Oxy-muriatic acid gas is absorbed very rapidly by a diluted solution of urea; small whitish flakes appear, which soon become brown, and adhere to the sides of the vessel like a concrete oil. After a considerable quantity of oxy-muriatic acid had been absorbed, the solution, left to itself, continued to effervescence exceeding slowly, and to emit carbonic acid and azotic gas. After this effervescence was over, the liquid contained muriat and carbonat of ammonia.

Urea is dissolved very rapidly by a solution of potash or soda; and at the same time a quantity of ammonia is disengaged, the same substance is disengaged when urea is treated with barites, lime, or even magnesia. Hence it is evident, that this appearance must be ascribed to the muriat of ammonia, with which it is constantly mixed. When pure solid potash is triturated with urea, heat is produced, a great quantity of ammonia is disengaged. The mixture becomes brown, and a substance is deposited, having the appearance of an empyreumatic oil. One part of urea and two of potash, dissolved in four times its weight of water, when distilled give out a great quantity of ammoniacal water; the residuum contained acetite and carbonat of potash.

When muriat of soda is dissolved in a solution of urea in water, it is obtained by evaporation, not in cubic crystals, its usual form, but in regular octahedrons. Muriat of ammonia, on the contrary, which crystallizes naturally in octahedrons, is converted into cubes, by dissolving and crystallizing it in the solution of urea.

Such are the properties of this singular substance, as far as they have been ascertained by the experiments of Fourcroy and Vanquelin. It differs from all animal substances hitherto examined, in the great proportion of azot which enters into its composition, and in the facility with which it is decomposed, even by the heat of boiling water.

Sect. VII. Of Sugar.

Sugar has been already described in the former part of this article as a vegetable substance; nothing therefore is necessary here but to point out the different states in which it is found in animals. It has never indeed been found in animals in every respect similar to the sugar of vegetables; but there are certain animal substances which have so many properties in common with sugar, that they can scarcely be arranged under any other name. These substances are,

1. Sugar of milk, 2. Honey, 3. Sugar of diabetic urine.

The method of obtaining sugar of milk has been already detailed in the article Chemistry, n° 488. To milk, which we refer the reader. For an account of its properties, we are indebted to the observations of Mr. Lichtenstein.

When pure, it has a white colour, a sweetish taste, and no smell. Its crystals are semitransparent regular parallelopipeds, terminated by four-sided pyramids. Its specific gravity, at the temperature of 55°, is 1.543. At that temperature, it is soluble in seven times its weight of water; but is perfectly insoluble in alcohol. When burnt, it emits the odour of caramel, and exhibits precisely the appearance of burning sugar. When distilled, it yields the same products as sugar, only the empyreumatic oil obtained has the odour of benzoic acid.

Honey is prepared by bees, and perhaps rather belongs to the vegetable than the animal kingdom. It has a white or yellowish colour, a soft and grained consistence, a faeccharine and aromatic smell; by means of alcohol, and even by water, with peculiar management, a true sugar is obtained; by distillation it affords an acid phlegm and an oil, and its coal is light and spongy like that of the mucilages of plants. Nitric acid extracts the oxalic acid, which is entirely similar to that of sugar; it is very soluble in water, with which it forms a syrup, and like sugar passes to the vinous fermentation.

The urine of persons labouring under the disease known to physicians by the name of diabetes, yields, when evaporated, a considerable quantity of matter, which possesses the properties of sugar.

Sect. VIII. Of Oils.

The oily substances found in animals may be arranged under three heads: 1. Fixed oils; 2. Fat; 3. Spermaceti.

1. The fixed oils are obtained chiefly from different kinds of fish, as the whale, &c.; and they are distinguished Animal Substances.

1. Sulphuric, 2. Muriatic, 3. Phosphoric, 4. Carbonic, 5. Benzoic, 6. Sebacic, 7. Formic, 8. Bombyc, 9. Uric.

The first eight of these have been already described in the article Chemistry, Suppl., it is unnecessary therefore to describe them here.

Few persons are ignorant that concretions sometimes form in the human urinary bladder, and produce that very formidable disease known by the names of the stone and the gravel. These concretions are often extracted by a surgical operation; they are called urinary calculi.

The most common of these calculi is of a brown colour, and very soluble in pure potash or soda lye.

If into an alkaline solution of one of these calculi a quantity of acetic acid be poured, a copious brown coloured precipitate immediately appears, which may be separated and edulcorated in a small quantity of water. This substance is uric acid.

It was discovered by Scheele in 1776, and the French chemists afterwards called it lithic acid; but this name, in consequence chiefly of some remarks of Dr Pearson on its impropriety, has been lately given up, and that of uric acid substituted in its place. We have adopted the new name, because we think it preferable to the old; which indeed conveyed a kind of inconsistency to those who attended to the etymological meaning of the word.

Uric acid possesses the following properties: it crystallizes in thin plates; has a brown colour, and scarcely any taste. Cold water scarcely dissolves any part of it; but it is soluble in 560 parts of boiling water. The solution reddens vegetable blues, especially the tincture of turmeric. A great part of the acid precipitates again as the water cools. It combines readily with alkalies and earths; but the compound is decomposed by every other acid. Sulphuric acid, when concentrated, decomposes it entirely. Nitric acid dissolves it readily; the solution is of a pink colour, and has the property of tingling animal substances, the skin for instance, of the same colour. When this solution is boiled, a quantity of azotic gas, carbonic acid gas, and of prufic acid, is disengaged. Oxymuriatic acid converts it in a few minutes into oxalic acid.

When distilled, about a fourth of the acid passes over little altered, and is found in the receiver crystallized in plates; a few drops of thick oil make their appearance; ¼th of the acid of concrete carbonat of ammonia, some prussiat of ammonia, some water, and carbonic acid; and there remains in the retort charcoal, amounting to about ¼th of the weight of the acid distilled.

These facts are sufficient to show us, that uric acid is composed of carbon, azot, hydrogen, and oxygen; and that the proportion of the two last ingredients is much smaller than of the other two.

The different salts which uric acid forms with alkaline and earthy bases have not been examined with attention; but urat of potash, of soda, and of lime, have been formed both by Scheele and Fourcroy; and urat

---

(1) From urine; because this acid is always found in human urine. Alkalies of ammonia is not unfrequently found crystallized in Earths and urinary calculi.

The order of the affinities of the different bases for uric acids is entirely unknown; but it has been ascertained, that its affinity for these bases is much weaker than that of any other acid. Its salts are decomposed even by prussic and carbonic acid.

Sect. X. Of Alkalies, Earths, and Metals.

1. All the three alkalies have been found in the animal kingdom, as we shall show in the next chapter. 2. The only earths which have been found in animals are, 1. Lime, 2. Magnesia, 3. Silica.

The first in great abundance, almost in every large animal; the other two very rarely, and only as it were by accident. 3. The metals hitherto found in animals are, 1. Iron, 2. Manganese.

The first exists in all the larger animals in some considerable quantity; the second has scarcely ever been found in any quantity so great as to admit of being weighed.

Such are the substances hitherto found in animals. The simple bodies of which all of them consist are the following: 1. Azot, 2. Carbon, 3. Hydrogen, 4. Oxygen, 5. Lime, 6. Phosphorus, 7. Muriatic acid, 8. Potas, 9. Soda, 10. Magnesia, 11. Silica, 12. Iron, 13. Manganese, 14. Sulphur.

Of these, magnesia and silica may in a great measure be considered as foreign bodies; for they are only found in exceedingly minute quantities, and the last not unless in cases of disease. The principal elementary ingredients are the first five; animal substances may be considered as in a great measure composed of them. The first four constitute almost entirely the soft parts, and the other two form the basis of the hard parts. But we will be able to judge of this much better, after we have taken a view of the various parts of animals as they exist ready formed in the body. This shall be the subject of the next chapter.

Chap. II. Of the Parts of Animals.

The different substances which compose the bodies of animals have been described with sufficient minuteness in the article Anatomy, Encyc., to which we beg leave to refer the reader. Any repetition in this place would be improper. These substances are the following: 1. Bones and shells, 2. Muscles, 3. Tendons, 4. Ligaments, 5. Membranes, 6. Cartilages, 7. Skin, 8. Brain and nerves, 9. Horns and nails, 10. Hair and feathers.

Besides these substances which constitute the solid part of the bodies of animals, there are a number of fluids, the most important of which is the blood, which pervades every part of the system in all the larger animals: the rest are known by the name of secretions, because they are formed or secreted, as the anatomists term it, from the blood. The principal animal secretions are the following: 1. Milk, 2. Saliva, 3. Pancreatic juice, 4. Bile and biliary calculi, 5. Tears, 6. Mucus of the nose, 7. Sinovia, 8. Semen, 9. Liquefied of the amnion, 10. Urine and urinary calculi.

These substances shall form the subject of the following sections.

Sect. I. Of Bones.

By bones, we mean those hard, solid, well-known substances, to which the firmness, shape, and strength of animal bodies, are owing; which, in the larger animals, form, as it were, the ground-work upon which all the rest is built. In man, in quadrupeds, and many other animals, the bones are situated below the other parts, and scarcely any of them are exposed to view; but shell-fish and snails have a hard covering on the outside of their bodies, evidently intended for defense. As these coverings, though known by the name of shells, are undoubtedly of a bony nature, we shall include them also in this section. For the very same reasons, it would be improper to exclude egg-shells, and those coverings of certain animals, the tortoise for instance, known by the name of carapace.

It had been long known, that bones may be rendered soft and cartilaginous by keeping them in diluted acid solutions, and that some acids even dissolve them altogether; that when exposed to a violent heat, they become white, opaque, and brittle; and Dr Lewis had observed, that a sudden and violent heat rendered them hard, semitransparent, and fomorous. But their component parts remained unknown till Scheele mentioned, in his dissertation on Flor Spar, published in the Stockholm Transactions for 1771, that the earthy part of bones is phosphat of lime (m). Since that time considerable additions have been made to the chemical analysis of these substances by Bernard, Bouillon, and Rouelle. Mr Hatchett has published a very valuable paper on the subject in the Philosophical Transactions for 1799; and in the 24th volume of the Annales de Chymie, Mr Merat Guillot has given us a table of the component parts of the bones of a considerable number of animals.

The bony parts of animals may be divided into three classes; namely, bones, crusts, and shells.

1. Bones have a considerable degree of hardness; when recent, they contain a quantity of marrow, which of bones may be partly separated from them. When the water in which bones have been for some time boiled is evaporated to a proper consistence, it afflatus the form of a gelatin; bones therefore contain gelatin.

If a piece of bone be kept for some time in diluted muriatic, or even acetic acid, it gradually loses a considerable part of its weight, becomes soft, and acquires... a certain degree of transparency; and, in short, acquires all the properties of cartilage. Bone therefore consists of cartilage, combined with some substance which these acids are capable of dissolving and carrying off.

If pure ammonia be dropped into the acid which has reduced the bone to this state, a quantity of white powder precipitates, which possesses all the properties of phosphat of lime. The substance, then, which was combined with the cartilage is phosphat of lime.

After the phosphat of lime has precipitated, the addition of carbonat of ammonia occasions a farther precipitate, which consists of carbonat of lime; but the quantity of this precipitate is inconsiderable. When concentrated acids are poured on bones, whether recent or calcined, an effervescence is perceptible; the gas which escapes renders lime water turbid, and is therefore carbonic acid. Now since bones contain carbonic acid, and since they contain lime also uncombined with any acid stronger than carbonic—it is evident that they contain a little carbonat of lime. Mr Hatchett found this substance in all the bones of quadrupeds and of fish which he examined.

When bones are calcined, and the residuum is dissolved in nitric acid, nitrat of barytes causes a small precipitate, which is insoluble in muriatic acid, and is therefore sulphat of barytes. Consequently bones contain sulphuric acid. It has been ascertained, that this acid is combined with lime. The proportion of sulphat of lime in bones is very inconsiderable.

Thus we have seen, that bones are composed of cartilage, which consists almost entirely of gelatin, of phosphat of lime, carbonat of lime, and sulphat of lime. The following table, drawn up by Merat-Guillet, exhibits a comparative view of the relative proportion of these ingredients in a variety of bones. The sulphat of lime, which occurs only in a very small quantity, has been confounded with phosphat of lime.

| One hundred parts contain | Gelatine | Phosphat | Carbonat of lime | Loss | |--------------------------|---------|----------|-----------------|-----| | Human bones from a burying ground | 16 | 67 | 1.5 | 15.5| | Do. dry, but not from under the earth | 23 | 63 | 2 | 2 | | Bone of ox | 9 | 93 | 2 | 2 | | calf | 25 | 54 | trace | 21 | | horse | 9 | 67.5 | 1.25 | 22.25| | sheep | 16 | 70 | 0.5 | 13.5| | elk | 1.5 | 90 | 1 | 7.5 | | hog | 17 | 52 | 1 | 30 | | hare | 9 | 85 | 1 | 5 | | pullet | 6 | 72 | 1.5 | 20.5| | pike | 12 | 64 | 1 | 23 | | carp | 6 | 45 | 0.5 | 48.5| | Horse tooth | 12 | 85.5 | 0.25 | 2.25| | Ivory | 24 | 64 | 0.1 | 11.15| | Hartshorn | 27 | 57.5 | 1 | 14.5|

The enamel of the teeth is composed of the same earthly ingredients as other bones; but it is totally deficient in the substance of cartilage.

2. The crustaceous coverings of animals, as of echinids, crabs, lobsters, prawns, and cray-fish, and also the shell of eggs, are composed of the same ingredients as

bones; but in them the proportion of carbonat of lime far exceeds that of phosphat.

Thus 100 parts of lobster crust contain: - 60 carbonat of lime, - 14 phosphat, - 26 cartilage.

One hundred parts of crawfish crust contain: - 60 carbonat of lime, - 12 phosphat of lime, - 28 cartilage.

Mr Hatchett found traces of phosphat of lime also in the shells of snails.

3. The shells of sea animals may be divided into two Component classes: The first has the appearance of porcelain; their parts of surface is enamelled, and their texture is often slightly fibrous. Mr Hatchett has given them the name of porcellaneous shells. The second kind of shell is known by the name of mother of pearl. It is covered with a strong epidermis, and below it lies the fleshy matter in layers. The shell of the fresh water musicle, mother of pearl, heliotis iris, and turbo olearius, are instances of these shells.

Porcellaneous shells are composed of carbonat of lime cemented together by a very small quantity of animal matter.

Mother of pearl shells are composed of alternate layers of carbonat of lime and a thin membranous or cartilaginous substance. This cartilage still retains the figure of the shell, after all the carbonat of lime has been separated by acids.

Mother of pearl contains 66 carbonat of lime, 34 cartilage.

Coral, which is a bony substance formed by certain sea insects, has a nearer relation to mother of pearl shells in its structure than to any other bony substance, as the following table will show.

| White coral | Red coral | Articulated coraline | |-------------|-----------|----------------------| | Carbonat of lime | 50 | 53.5 | 49 | | Animal matter | 50 | 46.5 | 51 |

Sect. II. Of the Muscles of Animals.

The muscular parts of animals are known in common language by the name of flesh. They constitute a considerable portion of the food of man.

Muscular flesh is composed of a great number of fibres or threads, commonly of a reddish or whitish colour; but its appearance is too well known to require any description. Hitherto it has not been subjected to any accurate chemical analysis. Mr Thouvenel, indeed, has published a very valuable dissertation on the subject. Muscles of subject; but his analysis was made before the method of examining animal substances was so well understood as it is at present. It is to him, however, that we are indebted for almost all the facts known concerning the composition of muscle.

It is scarcely possible to separate the muscle from all the other substances with which it is mixed. A quantity of fat often adheres to it closely; blood pervades the whole of it; and every fibre is enveloped in a particular thin membranous matter, which anatomists distinguish by the name of cellular substance. The analysis of the muscle, then, cannot be supposed to exhibit an accurate view of the composition of pure muscular fibres, but only of muscular fibre not perfectly separated from other substances.

1. When a muscle is well washed in cold water, several of its parts are dissolved, and may be obtained by the usual chemical methods. When the water is evaporated slowly, it at last coagulates, and the coagulum may be separated by means of a filter. It possesses the properties of albumen.

2. The water is then to be evaporated gently to dryness, and alcohol poured upon the dry mass: part of it is dissolved by digestion, and there remains a saline substance, which has not been examined; but which Fourcroy conjectures to be a phosphat.

3. When the alcohol is evaporated to dryness, it leaves a peculiar mucous substance, soluble both in water and alcohol; and when its watery solution is very much concentrated, it assumes an acid and bitter taste. It swells upon hot coals, and melts, emitting an acid and penetrating smell. It attracts moisture from the air, and forms a saline efflorescence. In a hot atmosphere it becomes four and putrefies. All these properties render it probable that this substance of Mr Thouvenel is that which is converted into acetic acid during the roasting of meat.

4. The muscle is now to be boiled in water for some time. A quantity of fat appears on its surface in the form of oil, which may be taken off.

5. The water, when evaporated sufficiently, assumes the form of a jelly on cooling, and therefore contains a portion of gelatine. It contains also a little of the saline substance, and of the mucous substance mentioned above.

6. The residuum of the muscle is now white and insipid, of a fibrous structure, and insoluble in water, and has all the properties of fibrina.

Thus it appears that muscle is composed of:

- Albumen, - Mucous matter, - Gelatine, - Fibrina, - A salt.

The French chemists have discovered, that when a piece of muscle is allowed to remain a sufficient time in diluted sulphuric acid, it is converted into a substance resembling tallow; weak nitric acid, on the other hand, converts it into a substance resembling wax.

Sect. III. Of the Soft and White Parts of Animals.

Those parts of animals to which anatomists have given the names of cartilage, tendon, ligament, membrane, differ altogether in their appearance from the muscles. They have never been analysed. We know only that they are composed, in a great measure, of gelatine; for it is partly from them that glue is made; which does not differ from gelatine, except in not being perfectly pure.

Mr Hatchett has ascertained that they contain no phosphat of lime as a constituent part, and scarcely any saline ingredients; for when calcined they leave but a very inconsiderable residuum. Thus 250 grains of hog's bladder left only 0.02 grain of residuum.

Sect. IV. Of the Skin.

The skin is that strong, thick covering which envelopes the whole external surface of animals. It is composed chiefly of two parts: a thin white elastic layer on the outside, which is called epidermis, or cuticle; and a much thicker layer, composed of a great many fibres, closely interwoven, and disposed in different directions; this is called the cutis, or true skin. The epidermis is that part of the skin which is raised in blisters.

1. The epidermis is easily separated from the cutis by maceration in hot water. It possesses a very great degree of elasticity.

It is totally insoluble in water and in alcohol. Pure fixed alkalies dissolve it completely, as does lime likewise, though slowly. Sulphuric and muriatic acids do not dissolve it, at least they have no sensible action on it for a considerable time; but nitric acid soon deprives it of its elasticity, causes it to fall to pieces, and probably soon decomposes it.

It is well known that the living epidermis is tinged yellow almost instantaneously by nitric acid; but this effect does not take place, at least so speedily, when the dead cuticle is plunged in nitric acid altogether.

2. When a portion of cutis is macerated for some hours in water, and agitation and pressure is employed to accelerate the effect, the blood, and all the extraneous matter with which it was loaded, are separated from it, but its texture remains unaltered. On evaporating the water employed, a small quantity of gelatine may be obtained. No subsequent maceration in cold water has any farther effect; the weight of the cutis is not diminished, and its texture is not altered; but if it be boiled in a sufficient quantity of water, it may be completely dissolved, and the whole of it, by evaporating the water, obtained in the state of gelatine.

Seguin informs us that he has ascertained, by a great variety of experiments, that the cutis differs from gelatine merely in containing an additional quantity of oxygen. Hot water (he says) expels this oxygen, and thus converts cutis into gelatine. As these experiments have not been published, it is impossible to form any judgment of their weight.

It is the skin or cutis of animals of which leather is formed. The process of converting skin into leather is called tanning. This process, though practised in the earliest ages, was merely empirical, till the happy ingenuity of Mr Seguin led him to discover its real nature. After the epidermis and all the impurities of the skin have been separated, and its pores have been so far opened as to admit of being completely penetrated, it is steeped in an infusion of oak-bark, which consists of gallic acid and tan. The gallic acid (if we believe Seguin) deprives the skin gradually of oxygen, and thus converts it into gelatine, and the tan combines with this gelatine the instant it is formed; and this process goes... Sect. V. Of the Brain and Nerves.

The brain and nerves are the instruments of sensation, and even of motion; for an animal loses the power of moving a part the instant that the nerves which enter it are cut.

The brain and nerves have a strong resemblance to each other; and it is probable that they agree also in their composition. But hitherto no attempt has been made to analyze the nerves. The only chemists who have examined the nature of brain are Mr Thouret* and Mr Fourcroy †.

The brain consists of two substances, which differ from each other somewhat in colour, but which, in other respects, seem to be of the same nature. The outermost matter, having some small resemblance in colour to wood-ashes, has been called the cinerious part; the innermost part has been called the medullary part.

Brain has a soft feel, not unlike that of soap; its texture appears to be very close; its specific gravity is greater than that of water.

When brain is kept in close vessels so that the external air is excluded, it remains for a long time unaltered. Fourcroy filled a glass vessel almost completely with pieces of brain, and attached it to a pneumatic apparatus; a few bubbles of carbonic acid gas appeared at first, but it remained above a year without undergoing any further change ‡.

This is very far from being the case with brain exposed to the atmosphere. In a few days (at the temperature of 60°) it exhales a most detestable odour, becomes acid, assumes a green colour, and very soon a great quantity of ammonia makes its appearance in it.

Cold water does not dissolve any part of the brain; but by trituration in a mortar, it forms, with water, a whitish coloured emulsion, which appears homogeneous, may be passed through a filter, and the brain does not precipitate by itself. When this emulsion is heated to 145°, a white coagulum is formed. The addition of a great quantity of water also causes a coagulum to appear, which swims on the surface, but the water still retains a milky colour. When sulphuric acid is dropped into the watery emulsion of brain, white flakes separate and swim on the surface, and the liquid becomes red. Nitric acid produces the same effects, only the liquid becomes yellow. Alcohol also separates a white coagulum from the emulsion, after it has been mixed with it for some hours. When nitric acid is added to the emulsion till it becomes slightly acid, a coagulum is also separated. This coagulum is of a white colour; it is insoluble in water and in alcohol. Heat softens, but does not melt it. When dried, it becomes transparent, and breaks with a glairy fracture. It has therefore some resemblance to albumen §.

When brain is triturated in a mortar with diluted sulphuric acid, part is dissolved, the rest may be separated, by filtration, in the form of a coagulum. The acid liquor is colourless. By evaporation, the liquid becomes black, sulphurous acid is exhaled, and crystals appear; and when evaporated to dryness, a black mass remains behind. When this mass is diluted with water, a quantity of charcoal separates, and the water remains clear. The brain is completely decomposed, a quantity of ammonia combines with the acid and forms sulphate of ammonia, while charcoal is precipitated. The water, by evaporation and treatment with alcohol, yields sulphates of ammonia and lime, phosphoric acid, and phosphates of soda and ammonia. Brain therefore contains:

- Phosphate of lime, - Phosphate of soda, - Ammonia.

Traces also of sulphate of lime can be discovered in it. The quantity of these salts is very small; altogether they do not amount to 1/10th part ||.

Diluted nitric acid, when triturated with brain, likewise dissolves a part, and coagulates the rest. The solution is transparent. When evaporated till the acid becomes concentrated, carbonic acid gas and nitrous gas are disengaged; an effervescence takes place, while fumes appear; an immense quantity of ammonia is disengaged, a bulky charcoal remains mixed with a considerable quantity of oxalic acid ‡.

When brain is gradually evaporated to dryness by the heat of a water bath, a portion of transparent liquid separates at first from the rest, and the residuum, when nearly dry, acquires a brown colour; its weight amounts to about one-fourth of the fresh brain. It may still be formed into an emulsion with water, but very soon separates again spontaneously.

When alcohol is repeatedly boiled upon this dried residuum till it coalesces to have any more action, it dissolves about five-eighths of the whole. When this alcohol cools, it deposits a yellowish white substance, composed of brilliant plates. When kneaded together by the fingers, it assumes the appearance of a ductile paste; at the temperature of boiling water it becomes soft, and when the heat is increased it blackens, exhales empyreumatic and ammoniacal fumes, and leaves behind it a charry matter ‡. When the alcohol is evaporated, it deposits a yellowish black matter, which reddens paper tinged with turpentine, and readily diffuses itself through water ‡.

Pure concentrated potash dissolves brain, disengaging a great quantity of ammonia.

These facts are sufficient to show us, that, exclusive of the small proportion of saline ingredients, brain is composed of a peculiar matter, differing in many particulars from all other animal substances, but having a considerable resemblance in many of its properties to albumen. Brain has been compared to a soap; but it is plain that the resemblance is very faint, as scarcely any oily matter could be extracted from brain by Fourcroy, though he attempted it by all the contrivances which the present state of chemistry suggested; and the alkaline proportion of it is a great deal too small to merit any attention.

Sect. VI. Of Nails, Horns, Hair, Feathers.

These substances have not hitherto been analysed. We know only that they have a great resemblance to each other. They give out the same smell, and exhibit the same phenomena when burnt, and they yield the same products when distilled.

Pure fixed alkali has the property of decomposing these substances, and of converting them into ammonia and oil. The ammonia is disengaged in great abundance, and the oil combines with the alkali, and forms... When muriatic acid is poured into the solution of these substances in pure soda, a quantity of sulphurated hydrogen gas is disengaged, and a black substance, double's charcoal, precipitates. Hence it follows that these substances contain, in their composition, a quantity of sulphur. Accordingly, if a bit of silver is put into the solution, it instantly assumes a black colour.

These substances scarcely contain any earthly ingredients. One hundred grains of ox horn, after calcination, left only 0.04 grain of redishum, half of which was phosphate of lime. Seventy-eight grains of chamois horn left five grains of redishum.

Such is a very imperfect account of the solids which compose animal bodies. We proceed next to the fluid which circulates through living bodies, namely blood; and to the various secretions formed from the blood, either in order to answer some important purpose to the animal, or to be evacuated as useless, that the blood thus purified may be more proper for answering the ends for which it is destined. Many of these substances have been examined with more care by chemists than the animal solids.

**Sect. VII. Of Blood.**

Blood is a well-known fluid, which circulates in the veins and arteries of the more perfect animals. It is of a red colour, has a considerable degree of consistence, and an unctuous feel, as if it contained a quantity of soap. Its taste is slightly saline, and it has a peculiar smell.

The specific gravity of human blood is, at a medium, 1.027. Mr. Fourcroy found the specific gravity of bullock's blood, at the temperature of 60°, to be 1.036. The blood does not uniformly retain the same consistence in the same animal, and its consistence in different animals is very various. It is easy to see that its specific gravity must be equally various.

When the blood is viewed through a microscope, a great many globules, of a red colour, are seen floating in it. It is to these globules that the red colour of the blood is owing. They were first examined with attention by Leuwenhoek. Their form, their proportion, and the changes which they undergo from the addition of various substances, have been examined with the greatest care; but hitherto without adding much to our knowledge. We neither know the ingredients of which the red globules are composed, nor the changes to which they are subjected, nor the useful purposes which they serve; nor has any accurate method been discovered of separating them from the rest of the blood, and of obtaining them in a state of purity.

When blood, after being drawn from an animal, is allowed to remain for some time at rest, it very soon coagulates into a solid mass, of the consistence of curdled milk. This mass gradually separates into two parts: one of which is fluid, and is called serum; the other, the coagulum, has been called erud, because it alone retains the red colour which distinguishes blood. This separation is very similar to the separation of curdled milk into curds and whey. The erud usually sinks to the bottom of the vessel, and, of course, is covered by the serum.

The erud, or clot, as it is sometimes called, is of a red colour, and possesses considerable consistence. Its mean specific gravity is about 1.243. If we wash the erud in a sufficient quantity of water, it gradually loses its red colour, and assumes the appearance of a whitish, fibrous, elastic mass, which possesses all the properties of fibrina. The erud therefore is composed chiefly of fibrina. The water in which it has been washed assumes a red colour, but continues transparent. It is evident from this that it contains, dissolved in it, the red globules; not, however, in a state of purity, for it is impossible to separate the erud completely from the serum; consequently the water must contain both serum and red globules. We know, however, from this, that the red globules are soluble in water. The erud of the blood, then, is composed of red globules and fibrina.

If the erud of the blood be exposed to a gentle heat, it becomes gradually dry and brittle. If this dry mass be submitted to distillation, it yields water, ammonia, a thick empyreumatic oil, and much carbonate of ammonia; there remains a spongy coat of a brilliant appearance, from which sulphuric acid extracts soda and iron; there remains behind a mixture of phosphate of lime and charcoal.

When the fibrina is distilled, it yields precisely the same products; but the redishum contains neither iron nor soda. The red water, on the contrary, which had been employed to wash the erud, contains both of these substances, especially iron; which may be obtained in the state of oxyd by evaporating this water to dryness, and calcining the redishum. These facts are sufficient to demonstrate that the red globules contain iron; consequently the opinion that their colour depends upon that metal is at least possible. It is probably owing to the soda which it contains, that the presence of iron cannot be ascertained in the solution of these globules by the usual tests. The prussian alkali causes no precipitation; the infusion of nut galls gives it no blue or purplish tinge.

The serum is of a light greenish yellow colour; it has the taste, smell, and feel of the blood, but its consistence is not so great. Its mean specific gravity is about 1.0287. It converts syrup of violets to a green, and therefore contains an alkali. On examination, it is found that it owes this property to a portion of soda. When heated to the temperature of 156°, the serum coagulates, as Harvey first discovered. But if serum be mixed with six parts of cold water, it does not coagulate by heat. When thus coagulated, it has a greyish white colour, and is not unlike the boiled white of an egg. If the coagulum be cut into small pieces, a muddy fluid may be squeezed from it, which has been termed the serum. After the separation of this fluid, if the redishum be carefully washed in boiling water and examined, it will be found to possess all the properties of albumen. The serum, therefore, contains a considerable proportion of albumen. Hence its coagulation by heat, and the other phenomena which albumen usually exhibits.

If the serum be gently evaporated till it becomes concentrated, and then be allowed to cool, it assumes the form of a jelly, as was first observed by De Haen. Consequently it contains gelatine.

If serum be mixed with twice its weight of water, and, after coagulation by heat, the albumen be separated... ted by filtration, and the liquid be slowly evaporated till it is considerably concentrated, a number of crystals are deposited when the liquid is left standing in a cool place. These crystals consist of muriate of soda and carbonat of soda.

Thus it appears that the serum of the blood contains albumen, gelatin, soda, muriate of soda, and carbonat of soda, besides a portion of water.

Gelatin may be precipitated from the serum by the three mineral acids. Mr Hunter observed, that Gouard's extract, or which is the same thing, acetite of lead dissolved in acetic acid, produces with gelatin a copious precipitate. When nitric acid is distilled off serum, it converts it partly into prussic acid. Acids, alcohol, and tan, precipitate the albumen in different states; but this, after what has been said in the last chapter, requires no farther explanation.

The proportion between the erorn and serum of the blood varies much in different animals, and even in the same animal in different circumstances. The most common proportion is about one part of erorn to three parts of serum; but in many cases the erorn exceeds and falls short of this quantity: the limits of the ratios of these substances to each other appear, from a comparison of the conclusions of most of those who have written accurately on the subject, to be 1:1 and 1:4; but the first case must be very rare indeed.

When new-drawn blood is stirred briefly round with a stick, or the hand, the whole of the fibrina collects together upon the stick, and in this manner may be separated altogether from the rest of the blood. The red globules, in this case, remain behind in the serum. It is in this manner that the blood is prepared for the different purposes to which it is put; as clarifying sugar, making puddings, &c. After the fibrina is thus separated, the blood no longer coagulates when allowed to remain at rest, but a spongy flaky matter separates from it and swims on the surface.

When blood is dried by a gentle heat, water exhales from it, retaining a very small quantity of animal matter in solution, and consequently having the odour of blood. Blood dried in this manner being introduced into a retort and distilled, there comes over, first a clear watery liquor, then carbonic acid gas, and carbonat of ammonia, which crystallizes in the neck of the retort; after these products there comes over a fluid oil, carbonated hydrogen gas, and an oily substance of the consistence of butter. The watery liquor possesses the property of precipitating from sulphate of iron a green powder; muriatic acid dissolves part of this powder, and there remains behind a little prussian blue. Consequently this watery liquor contains both an alkali and prussic acid.

9216 grains of dried blood being put into a large crucible, and gradually heated, at first became nearly fluid, and swelled up considerably, emitted a great many fetid fumes of a yellowish colour, and at last took fire and burned with a white flame, evidently owing to the presence of oil. After the flame and the fumes had disappeared, a light smoke was emitted, which affected the eyes and the nose, which had the odour of prussic acid, and reddened moist papers stained with vegetable blues. At the end of six hours, when the matter had lost five-sixths of its sublimate, it melted anew, exhibited a purple flame on its surface, and emitted a thick smoke. This smoke affected the eyes and nostrils, and reddened blue paper, but it had not the smell of prussic acid. When a quantity of it was collected and examined, it was found to possess the properties of phosphoric acid. The residuum amounted to 181 grams; it had a deep black colour, and a metallic brilliancy, and its particles were attracted by the magnet. It contained no uncombined soda, though the blood itself, before combustion, contains it abundantly; but water extracted from it muriate of soda, part of the rest was dissolved by muriatic acid, and, of course, was lime; there was besides a little silica, which had evidently been separated from the crucible. The iron had been reduced during the combustion.

Such are the properties of blood, as far as they have been hitherto ascertained by experiment. We have seen that it contains the following ingredients:

1. Water, 2. Fibrina, 3. Albumen, 4. Gelatin, 5. Iron, 6. Soda, 7. Muriate of soda, 8. Phosphat of lime.

But our knowledge of this singular fluid is by no means so complete as it ought to be; a more accurate analysis would probably discover the presence of other substances, and enable us to account for many of the properties of blood which at present are inexplicable.

It would be of great consequence also to compare together the blood of different animals, and of the same animal at different ages, and to ascertain in what particulars they differ from each other. This would probably throw light on some of the obscurest parts of the animal economy. Very little progress has hitherto been made in these researches: if we except the labours of Rouelle, who obtained nearly the same ingredients, though in different proportions, from the blood of a great variety of animals, the experiments of Fourcroy on the blood of the human fetus are almost the only ones of that kind with which we are acquainted.

He found that it differs from the blood of the adult in three things: 1st, its colouring matter is darker, the fetus, and seems to be more abundant; 2nd, it contains no fibrina, but probably a greater proportion of gelatin than blood of adults; 3rd, it contains no phosphoric acid.

The examination of diseased blood, too, would be of great consequence; because the difference of its properties from the blood of people in health might throw much light on the nature of the disease. It is well known, that when a person labours under inflammation, his blood is not susceptible of coagulating so soon as healthy blood. This longer time allows the red globules to sink to the bottom, and the coagulated fibrina appears at the top, of its natural whitish colour. Hence the appearance of the buffy coat, as it is called, which characterizes blood during inflammation.

During that disease which is known by the name of diabetes, in which the urine is excessive in quantity, and contains sugar, the serum of blood often, as appears from the experiments of Dr Dobson and Dr Rollos, assumes the appearance of whey; and, like it, seems to contain sugar, or, at least, it has lost its usual salt taste.

Fourcroy mentions a case of extreme feebleness, in which all the parts of the body were in an unusual relaxed state. In that patient a quantity of blood oozed out. Milk.

out from the eye-lids, which tinged linen blue, as if it had been stained with prussian blue. Here prussic alkali seems to have been formed in the blood.

Sect. VIII. Of Milk.

Milk is a fluid secreted by the female of all those animals denominated mammalia, and intended evidently for the nourishment of her offspring.

The milk of every animal has certain peculiarities which distinguish it from every other milk. But the animal whose milk is most made use of by man as an article of food, and with which, consequently, we are best acquainted, is the cow. Chemists, therefore, have made choice of cow's milk for their experiments. We shall at first confine ourselves to the properties and analysis of cow's milk, and afterwards point out in what respect the milk of other animals differs from it, as far at least as these differences have hitherto been ascertained.

Milk is an opaque fluid, of a white colour, a slight peculiar smell, and a pleasant sweetish taste. When newly drawn from the cow, it has a taste very different from that which it acquires after it has been kept for some hours.

It is liquid, and wets all those substances which can be moistened by water; but its consistence is greater than that of water, and it is slightly unctuous. Like water, it freezes when cooled down to about 30°; but Parmentier and Deyeux, to whom we are indebted for by far the completest account of milk hitherto published, found that its freezing point varies considerably in the milk of different cows, and even of the same cow at different times*. Milk boils also when sufficiently heated; but the same variation takes place in the boiling point of different milks, though it never deviates very far from the boiling point of water. Milk is specifically heavier than water, and lighter than blood; but the precise degree cannot be ascertained, because almost every particular milk has a specific gravity peculiar to itself.

When milk is allowed to remain for some time at rest, there collects on its surface a thick unctuous yellowish coloured substance, known by the name of cream. The cream appears sooner in milk in summer than in winter, evidently owing to the difference of temperature. In summer, about four days of repose are necessary before the whole of the cream collects on the surface of the liquid; but in winter it requires at least double the time†.

After the cream is separated, the milk which remains is much thinner than before, and it has a bluish white colour. If it be heated to the temperature of 100°, and a little rennet, which is water digested with the inner coat of a calf's stomach, and preserved with salt, be poured into it, coagulation ensues; and if the coagulum be broken, the milk very soon separates into two substances: a solid white part, known by the name of curd; and a fluid part, called whey.

Thus we see that milk may be easily separated into three parts: namely, cream, curd, and whey.

Cream is of a yellow colour, and its consistence increases gradually by exposure to the atmosphere. In three or four days, it becomes so thick that the vessel which contains it may be inverted without risking any loss. In eight or ten days more its surface is covered over with mucors and byssus, and it has no longer the flavour of cream, but of very fat cheese*. This is the process for making what in this country is called a cream cheese.

Cream possesses many of the properties of an oil. It does not feel, stains clothes precisely in the manner of oil; and if it be kept fluid, it contracts at last a taste which is very analogous to the rancidity of oils†. When kept boiling for some time, a little oil makes its appearance, and floats upon its surface‡. Cream is neither soluble in alcohol nor oil §. These properties are sufficient to show us that it contains a quantity of oil; but this oil is combined with a part of the curd, and mixed with some serum. Cream, then, is composed of a peculiar oil, curd, and serum. The oil may be easily obtained separate by agitating the cream for a considerable time. This process, known to every body, is called churning. After a certain time, the cream separates into two portions: one fluid, and resembling creamed milk; the other solid, and called butter.

Butter is of a yellow colour, possesses the properties of an oil, and mixes readily with other oily bodies. When heated to the temperature of 96°, it melts, and becomes transparent; if it be kept for some time melted, some curd and water or whey separate from it, and it assumes exactly the appearance of oil §. But this process deprives it in a great measure of its peculiar flavour.

When butter is kept for a certain time, it becomes rancid, owing in a good measure to the presence of these foreign ingredients; for if butter be well washed, and a great portion of these matters separated, it does not become rancid nearly so soon as when it is not treated in this manner. It was formerly supposed that this rancidity was owing to the development of a peculiar acid; but Parmentier and Deyeux have shown, that no acid is present in rancid butter*. When butter is diluted, there comes over water, fecal acid, and oil, at first fluid, but afterwards concrete. The carbonaceous residuum is but small.

Butter may be obtained by agitating cream newly taken from milk, or even by agitating milk newly drawn from the cow. But it is usual to allow cream to remain for some time before it is churned. Now cream, by standing, acquires a sour taste; butter therefore is commonly made from sour cream. Fresh cream requires at least four times as much churning before it yields its butter as four cream does†; consequently cream acquires, by being kept for some time, new properties, in consequence of which it is more easily converted into butter. When very fresh cream is churned, every one who has paid the smallest attention must have perceived, that the butter-milk, after the churning, is not nearly so foul as the cream had been. The butter, in all cases, is perfectly sweet; consequently the acid which had been evolved has in a great measure disappeared during the process of churning. It has been ascertained, that cream may be churned, and butter obtained, though the contact of atmospheric air be excluded‡. We have, no doubt, that in all cases where such an experiment succeeded, the cream on which it was made had previously become sour. On the other hand, it has been ascertained, that when cream is churned in contact with air, it absorbs a considerable quantity of it §; and it cannot cannot be doubted, that the portion absorbed is oxygen.

These facts are sufficient to afford us a key to explain what takes place during the process of churning. There is a peculiar oil in milk, which has so strong an affinity for the other ingredients, that it will not separate from them spontaneously; but it has an affinity for oxygen, and when combined with it, forms the concrete body called butter. Agitation produces this combination of the oil with oxygen; either by causing it to absorb oxygen from the air, or, if that be impossible, by separating it from the acid which exists in sour cream. Hence the absorption of air during churning; hence also the increase of temperature of the cream, which Dr Young found to amount constantly to 4°; and hence the sweetness of the butter milk compared with the cream from which it was obtained.

The affinity of the oil of cream for the other ingredients is such, that it never separates completely from them. Not only is curd and whey always found in the cream, but some of this oil is constantly found in creamed milk and even in whey; for it has been ascertained by actual experiment, that butter may be obtained by churning whey; 27 Scotch pints of whey yield at an average about a pound of butter. This accounts for a fact well known to those who superintend dairies, that a good deal more butter may be obtained from the same quantity of milk, provided it be churned as drawn from the cow, than when the cream alone is collected and churned.

The butter-milk, as Parmentier and Deyens ascertained by experiment, possesses precisely the properties of milk deprived of cream.

Curd, which may be separated from creamed milk by rennet, has all the properties of coagulated albumen. It is white and solid; and when all the moisture is squeezed out, it has a good deal of brittleness. It is insoluble in water; but pure alkalis and lime dissolve it readily, especially when assisted by heat; and when fixed alkali is used, a great quantity of ammonia is emitted during the solution. The solution of curd in soda is of a red colour, at least if heat be employed; owing probably to the separation of charcoal from the curd by the action of the alkali. Indeed, when a strong heat has been used, charcoal precipitates as the solution cools. The matter dissolved by the alkali may be separated from it by means of any acid; but it has lost all the properties of curd. It is of a black colour, melts like tallow by the application of heat, leaves oily stains on paper, and never acquires the consistence of curd. Hence it appears that curd, by the action of a fixed alkali, is decomposed, and converted into two new substances, ammonia, and oil or rather fat.

Curd is soluble also in acids. If, over curd newly precipitated from milk, and not dried, there be poured eight parts of water, containing as much of any of the mineral acids as gives it a feebly acid taste, the whole is dissolved after a little boiling. Acetous acid and lactic acid do not dissolve curd when very much diluted. But these acids, when concentrated, dissolve it readily, and in considerable quantity. It is remarkable enough, that concentrated vegetable acids dissolve curd readily, but have very little action on it when they are very much diluted; whereas the mineral dissolve it when much diluted; but when concentrated, have

Suffl. Vol. II. Part II. remainder of the curd time to precipitate, is decanted off, almost as colourless as water, and scarcely any of the peculiar taste of milk can be distinguished in it. If it be now slowly evaporated, it deposits at last a number of white coloured crystals, which are sugar of milk. Towards the end of the evaporation, some crystals of muriat of potash and of muriat of lime make their appearance. According to Scheele, it contains also a little phosphat of lime.

After the fats have been obtained from whey, what remains concretes into a jelly on cooling. Hence it follows, that whey also contains gelatine. Whey, then, is composed of water, sugar of milk, gelatine, muriat of potash, and muriat of lime. The other fats, which are sometimes found in it, are only accidentally present.

If whey be allowed to remain for some time, it becomes sour, owing to the formation of a peculiar acid known by the name of lactic acid. It is to this property of whey that we are to ascribe the acidity which milk contracts; for neither curd nor cream, perfectly freed from serum, seem susceptible of acquiring acid properties. Hence the reason, also, that milk, after it become sour, always coagulates. Boiled milk has the property of continuing longer sweet; but it is singular enough, that it runs sooner to putrefaction than ordinary milk.

The acid of milk differs considerably from the acetous; yet vinegar may be obtained from milk by a very simple process. If to somewhat more than 8 lbs. troy of milk, fix spoonfuls of alcohol be added, and the mixture well cooked be exposed to a heat sufficient to support fermentation (provided attention be paid to allow the carbonic acid gas to escape from time to time), the whey, in about a month, will be found converted into vinegar.

Milk is almost the only animal substance which may be made to undergo the vinous fermentation, and to afford a liquor resembling wine or beer, from which alcohol may be separated by distillation. This singular fact seems to have been first discovered by the Tartars; they obtain all their spirituous liquors from mare's milk. It has been ascertained, that milk is incapable of being converted into wine till it has become sour; after this, nothing is necessary but to place it in the proper temperature, the fermentation begins of its own accord, and continues till the formation of wine be completed. Scheele had observed, that milk was capable of fermenting, and that a great quantity of carbonic acid gas was extricated from it during this fermentation. But he did not suspect, that the result of this fermentation was the formation of an intoxicating liquor similar to wine.

When milk is distilled by the heat of a water bath, there comes over water, having the peculiar odour of milk; which putrifies, and consequently contains, besides mere water, some of the other constituent parts of milk. After some time, the milk coagulates, as always happens when hot albumen acquires a certain degree of concentration. There remains behind a thick unctuous yellowish white substance, to which Hoffman gave the name of frangipani. This substance, when the fire is increased, yields at first a transparent liquid, which becomes gradually more coloured; some very fluid oil comes over, then ammonia, an acid, and at last a very thick black oil. Towards the end of the process, carbonated hydrogen gas is disengaged. There remains in the retort a coal which contains carbonat of potash, muriat of potash, and phosphat of lime, and sometimes magnesia, iron, and muriat of soda.

Thus we see, that cows milk is composed of the following ingredients:

1. Water, 2. Oil, 3. Albumen, 4. Gelatine, 5. Sugar of milk, 6. Muriat of lime, 7. Muriat of potash, 8. Sulphur.

The milk of all other animals, as far as it has hitherto been examined, consists nearly of the same ingredients; but there is a very great difference in their proportion.

Woman's milk has a much sweeter taste than cow's milk. When allowed to remain at rest for a sufficient time, a cream gathers on its surface. This cream is more abundant than in cow's milk, and its colour is usually much whiter. After it is separated, the milk is exceedingly thin, and has the appearance rather of whey, with a bluish white colour, than of creamed milk. None of the methods by which cows milk is coagulated succeed in producing the coagulation of woman's milk. It is certain, however, that it contains curd; for if it be boiled, pellicles form on its surface, which have all the properties of curd. Its not coagulating, therefore, must be attributed to the great quantity of water with which the curd is diluted.

Though the cream be churned ever so long, no butter can be obtained from it; but if, after being agitated for some hours, it be allowed to remain at rest for a day or two, it separates into two parts; a fluid which occupies the inferior part of the vessel, pellucid, and colourless, like water and a thick white unctuous fluid, which swims on the surface. The lowermost fluid contains sugar of milk and some curd; the uppermost does not differ from cream except in consistence. The oily part of the cream, then, cannot be separated by agitation from the curd. This cream contains a greater portion of curd than the cream of cow's milk.

When this milk, after the curd is separated from it, is slowly evaporated, it yields crystals of sugar of milk, and of muriat of soda. The quantity of sugar is rather greater than in cow's milk. According to Haller, the sugar obtained from cow's milk is to that obtained from an equal quantity of woman's milk as 35 : 38; and sometimes as 37 : 67, and in all the intermediate ratios.

Thus it appears, that woman's milk differs from that of cows in three particulars:

1. It contains a much smaller quantity of curd. 2. Its oil is so intimately combined with its curd, that it does not yield butter. 3. It contains rather more sugar of milk.

Parmeater and Deyoux ascertained, that the quantity of curd in woman's milk increases in proportion to the time after delivery. Nearly the same thing has been observed with respect to cow's milk.

Asses milk has a very strong resemblance to human milk; it has nearly the same colour, smell, and consistence. When left at rest for a sufficient time, a cream forms upon its surface, but by no means in such abundance as in woman's milk. This cream, by very long agitation, yields a butter, which is always soft, white, and tasteless; and, what is singular, very readily mixes again with the butter-milk; but it may be again separated. rated by agitation, while the vessel, which contains it, is plunged in cold water. Creamed after milk is thin, and has an agreeable sweetish taste. Alcohol and acids separate from it a little curd, which has but a small degree of consistence. The serum yields sugar of milk and muriat of lime.

After milk therefore differs from cows milk in three particulars:

1. Its cream is less abundant and more insipid. 2. It contains less curd. 3. It contains more sugar of milk: the proportion is 35:80.

Goats milk, if we except its consistence, which is greater, does not differ much from cows milk. Like that milk, it throws up abundance of cream, from which butter is easily obtained. The creamed milk coagulates just as cows milk, and yields a greater quantity of curd. Its whey contains sugar of milk, muriat of lime, and muriat of soda.

Ewes milk resembles almost precisely that of the cow. Its cream is rather more abundant, and yields a butter which never acquires the consistence of butter from cows milk. Its curd has a fat and viscid appearance, and is not without difficulty made to assume the consistence of the curd of cows milk. It makes excellent cheese.

Mares milk is thinner than that of the cow, but scarcely so thin as human milk. Its cream cannot be converted into butter by agitation. The creamed milk coagulates precisely as cows milk, but the curd is not so abundant. The serum contains sugar of milk, sulphat of lime, and muriat of lime.

Sect IX. Of Saliva.

The fluid secreted in the mouth, which flows in considerable quantity during a repast, is known by the name of saliva. No accurate analysis has hitherto been made of it, though it possesses some very singular properties.

It is a limpid fluid like water, but much more viscid; it has neither smell nor taste.

Its specific gravity, according to Hamburger, is 1.067. When agitated, it froths like all other adhesive liquids; indeed it is usually mixed with air, and has the appearance of froth.

It neither mixes readily with water nor oil; but by trituration in a mortar, it may be mixed to with water as to pass through a filter. It has a great affinity for oxygen, absorbs it readily from the air, and gives it out again to other bodies. Hence the reason why gold or silver, triturated with saliva in a mortar, is oxidized; as Duttenhofer has observed; and why the killing of mercury by oils is much facilitated by spitting into the mixture. Hence also, in all probability, the reason that saliva is a useful application to sores of the skin. Dogs, and several other animals, have constantly recourse to this remedy, and with much advantage.

Saliva is coagulated by oxy muriat of mercury, by alcohol, and by nitre. Therefore, in all probability, it contains albumen and gelatine, or some analogous substances.

When 100 parts of saliva are distilled, there come over 80 parts of water nearly pure, then a little carbonat of ammonia, some oil, and an acid, which perhaps is the profile. The residuum amounts to about 1.56 parts, and is composed of muriat of soda and phosphat of lime.

The tartar of the teeth, which is a crust deposited from saliva, consists, as Fourcroy has ascertained, of the phosphat of lime.

The pancreatic juice has never been examined with much attention; but it does not appear, from the experiments that have been made, to differ much from saliva.

Sect X. Of Bile.

Bile is a liquid of a yellowish green colour, an unctuous feel, and bitter taste, is secreted by the liver; and in most animals considerable quantities of it are usually found collected in the gall bladder.

Great attention has been paid to this liquid by physicians; because the ancients were accustomed to ascribe a very great number of diseases, and even affections of the mind, to its agency. The most accurate chemical analysis of it which has hitherto appeared is that of Mr Cadet, which was published in the Memoirs of the French Academy of Sciences for the year 1767. Several important observations had been previously made on it by Boyle, Boerhaave, Verheyen, Ramsay, and Baglivi; and some facts have since been added to our chemical knowledge of bile by Maclure and Fourcroy. The experiments have chiefly been confined to the bile of oxen, known in this country by the name of gall; because it is most easily procured in large quantities.

The specific gravity of bile seems to vary, like that of all other animal fluids. According to Hartmann, it is 1.027. When strongly agitated, it lathers like soap; and for this reason, as well as from a medical theory concerning its use, it has been often called an animal soap.

It mixes readily with water in any proportion, and assumes a yellow colour; but it refuses to unite with oil when the two fluids are agitated together; the instant that they are left at rest, the oil separates and swims on the surface.

When muriatic acid is poured upon bile, let it be ever so fresh, an odour of sulphurated hydrogen gas is constantly exhaled. When on 100 parts of ox-bile four parts of strong muriatic acid are poured, the whole instantly coagulates; but in some hours the greater part becomes again fluid; and when passed through the filter it leaves 0.26 of a white matter, which has all the properties of albumen. This matter was detected by Ramsay; who found that it could be precipitated from bile by alcohol, acetic acid, sulphat of potash, and muriat of soda. Cadet ascertained, that 100 parts of ox bile contain about 0.52 of albumen. It is precipitated in a state of purity by oxy muriatic acid, provided that acid be not employed in excess.

The muriatic acid solution, after the separation of the albumen, has a fine grey green colour. When concentrated by some hours evaporation in a glass crucible on hot coals, it deposits a very copious precipitate, and loses almost the whole of its green colour. By longer evaporation, a new precipitate, similar to the first, appears, and the remaining liquid assumes the colour of beer. This precipitate possesses all the properties of the resin of bile. In its most dilute it amounts to 10.8 parts. The same substances may be obtained from bile by nitric acid; but the resin in that Bile has a yellow colour, and its properties are somewhat altered.

If 100 parts of bile be gently evaporated to dryness by a very moderate heat, the dry mass only weighs 10 parts, and has a brownish black colour. When exposed to a strong heat in a crucible, this matter swells up, takes fire, and emits very thick fumes. The residuum amounts to 1.09. By lixiviation with water, 1.87 of crystallized soda may be obtained; consequently 100 parts of bile contain, according to Mr. Kirwan's table, c.403546 of pure soda. But it is evident that, by this method, part of the soda must have been evaporated; therefore 100 parts of bile contain more than c.403546 of soda. Besides the soda, there is found also a small portion of muriate of soda.

Cadet found the residuum, after the separation of the salts, of a black colour; it gave some traces of iron. He also obtained a calcareous salt from bile, which he considered as a sulphate; but it is more than probable that it was phosphat of lime.

Cadet also obtained from bile, by evaporating the muriatic acid solution after the separation of the resin, a salt which crystallized in trapeziums; it had a sweetish taste, and was considered by him as analogous to sugar of milk.

Thus we see that bile contains the following ingredients:

1. Water, 2. Resin, 3. Albumen, 4. Soda, 5. A sweetish salt, 6. Muriate of soda, 7. Phosphat of lime, 8. Iron.

The proportion of these ingredients has by no means been ascertained. The presence of iron has been denied in bile, because it gives no blue precipitate with prussic alkali, and because tincture of nut-galls does not give it a black colour. But these reasons are insufficient to overturn the experiment of Cadet, who actually found it in bile.

When four parts of vinegar and five of bile are mixed together, the mixture has a sweet taste, and does not coagulate milk. The lactic acid has precisely the same effect as vinegar.

When bile is distilled in a water bath, it affords a transparent watery liquor, which contracts a pretty strong odour, not unlike that of musk or amber, especially if the bile has been kept for some days before it is submitted to distillation. The residuum is of a deep brownish green; it attracts moisture from the air, and dissolves readily in water. When distilled in a retort, it affords a watery liquor of a yellowish colour, and impregnated with alkali, oil, carbonat of ammonia, carbolic acid, and hydrogen gas. The coaly residuum is easily incinerated. Bile, exposed to a temperature between 65° and 85°, soon loses its colour and viscosity, acquires a nauseous smell, and deposits whitish mucilaginous flakes. After the putrefaction has made considerable progress, its smell becomes sweet, and resembles amber. If bile be heated, and slightly concentrated by evaporation, it may be kept for many months without alteration.

Sect. XI. Of Biliary Calculi.

Hard bodies sometimes form in the gall bladder, or in the duct through which the bile passes into the intestinal canal, and stop up the passage altogether. These biliary calculi have got the name of biliary calculi or gallstones. As they are formed in the midst of bile, and as the substances of which they are composed must be derived from the bile, it is proper to give an account of them here, because their properties cannot fail to throw some additional light on the nature of bile itself.

Biliary calculi, all of them at least which have been hitherto examined with attention, may be divided into three classes.

1. The first kind comprehends those which have a white colour, and a crystallized, shining, lamellated structure.

2. The second is dark coloured, and has precisely the appearance of inspissated bile. Both these kinds are combustible.

3. The third kind comprehends those gall stones which do not flame, but gradually waste away at a red heat.

We shall take a view of each of these kinds of biliary calculi in their order. For the greater part of the chemical knowledge which has been hitherto acquired of them, the world is chiefly indebted to Mr. Fourcroy.

1. The first species of biliary calculi was pointed out for the first time by Haller, in a dissertation published in 1749. Walther afterwards added several new facts; and at last it was accurately described by Vieq d'Azy. It is almost always of an oval shape, sometimes as large as a pigeon's egg, but commonly about the size of a sparrow's; and for the most part only one calculus (when of this species) is found in the gall bladder at a time. It has a white colour; and when broken, presents crystalline plates or flake, brilliant and white like mica, and having a soft greasy feel. Sometimes its colour is yellow or greenish; and it has constantly a nucleus of inspissated bile.

Its specific gravity is lower than that of water: Gren found the specific gravity of one o.8033.

When exposed to a heat considerably greater than that of boiling water, this crystallized calculus softens and melts, and crystallizes again when the temperature is lowered. It is altogether insoluble in water; but alcohol dissolves it with facility. Alcohol, of the temperature of 167°, dissolves 1/5 of its weight of this substance; but alcohol, at the temperature of 60°, scarcely dissolves any of it. As the alcohol cools, the matter is deposited in brilliant plates resembling talc or boracic acid. It is soluble in oil of turpentine. When melted, it has the appearance of oil; when suddenly heated, it evaporates altogether in a thick smoke. It is soluble in pure alkalies, and the solution has all the properties of a soap. Nitric acid also dissolves it; but it is precipitated unaltered by water.

This matter, which is evidently the same with the crystals which Cadet obtained from bile, and which he considered as analogous to sugar of milk, has a strong resemblance to spermaceti. Like that substance, it is of an oily nature, and inflammable; but it differs from it in a variety of particulars.

Since it is contained in bile, it is not difficult to see how it may crystallize in the gall bladder if it happens to be more abundant than usual; and the consequence must 2. The second species of biliary calculus is of a round or polygonal shape, of a grey colour exteriorly, and brown within. It is formed of concentric layers of a matter which seems to be inspissated bile; and there is usually a nucleus of the white crystalline matter at the centre. For the most part there are many of this species of calculi in the gall-bladder together; indeed it is frequently filled with them. Their size is usually much smaller than that of the last species.

This is the most common kind of gall-stone. It may be considered as a mixture of inspissated bile, and of the crystalline matter which forms the first species; and the appearance of calculi of this kind must vary considerably, according to the proportion of these ingredients.

3. Concerning the third species of gall-stone, very little is known with accuracy. Dr Saunders tells us, that he has met with some gall-stones insoluble both in alcohol and oil of turpentine; some which do not flame, but become red, and consume an ash like a charcoal. Haller quotes several examples of similar calculi.

Gall-stones often occur in the inferior animals, particularly in cows and hogs; but the biliary concretions of these animals have not hitherto been examined with attention.

**Sect. XII. Of Tears.**

That peculiar fluid which is employed in lubricating the eye, and which is emitted in considerable quantities when we express grief by weeping, is known by the name of tears. For an accurate analysis of this fluid, chemistry is indebted to Messrs Fourcroy and Vauquelin. Before their dissertation, which was published in 1791, appeared, scarcely anything was known about the nature of tears.

The liquid called tears is transparent and colourless like water; it has scarcely any smell, but its taste is always perceptibly salt. Its specific gravity is somewhat greater than that of distilled water. It gives to paper stained with the juice of the petals of mallows or violet, a permanently green colour, and therefore contains a fixed alkali. It unites with water, whether cold or hot, in all proportions. Alkalies unite with it readily, and render it more fluid. The mineral acids produce no apparent change upon it. Exposed to the air, this liquid gradually evaporates, and becomes thicker. When nearly reduced to a state of dryness, a number of cubic crystals form in the midst of a kind of mucilage. These crystals possess the properties of muriate of soda; only they tinge vegetable blues green, and therefore contain an excess of soda. The mucilaginous matter acquires a yellowish colour as it dries.

This liquid boils like water, excepting that a considerable froth collects on its surface. If it be kept a sufficient time at the boiling temperature, parts of it evaporate in water; and there remain about 0.4 parts of a yellowish matter, which by distillation in a strong heat yield water and a little oil; the residuum consists of different saline matters.

When alcohol is poured into this liquid, a mucilaginous matter is precipitated in the form of large white flakes. The alcohol leaves behind it, when evaporated, traces of muriate of soda and soda. The residuum which remains behind, when inspissated tears are burnt in the open air, exhibits some traces of phosphat of lime and phosphat of soda.

Thus it appears that tears are composed of the following ingredients:

1. Water, 2. Mucilage, 3. Muriate of soda, 4. Soda, 5. Phosphat of lime, 6. Phosphat of soda.

The saline parts amount only to about 0.01% of the whole; or probably not so much.

The mucilage contained in the tears has the property of absorbing oxygen gradually from the atmosphere, and of becoming thick and viscid, and of a yellow colour. It is then insoluble in water, and remains long suspended in it without alteration. When a sufficient quantity of oxy muriatic acid is poured into tears, a yellow flaky precipitate appears absolutely similar to this inspissated mucilage. The oxy muriatic acid loses its peculiar odour; hence it is evident that it has given out oxygen to the mucilage. The property which this mucilage has of absorbing oxygen, and of acquiring new qualities, explains the changes which take place in tears which are exposed for a long time to the action of the atmosphere, as is the case in those persons who labour under a fitful lacrimation.

The mucus of the nose has also been examined by Fourcroy and Vauquelin. They found it composed of mucous precisely the same ingredients with the tears. As this fluid is more exposed to the action of the air than the tears, in most cases its mucilage has undergone less or more of that change which is the consequence of the absorption of oxygen. Hence the reason of the greater viscidity and consistence of the mucus of the nose; hence also the great consistence which it acquires during colds, where the action of the atmosphere is assisted by the increased action of the parts.

**Sect. XIII. Of Sinovia.**

Within the capsular ligament of the different joints of the body, there is contained a peculiar liquid, intended evidently to lubricate the parts, and to facilitate their motion. This liquid is known among anatomists by the name of sinovia.

Whether it be the same in different animals, or even in all the different joints of the same animal, has not been determined; as no accurate analysis of the sinovia of different animals has been attempted. The only analysis of sinovia which has hitherto appeared is that by Mr Margueron, which was published in the 14th volume of the Annales de Chimie. He made use of sinovia obtained from the joints of the lower extremities of oxen.

The sinovia of the ox, when it has just flowed from the joint, is a viscid semi-transparent fluid, of a greenish or white colour, and a smell not unlike frog spawn. It very soon acquires the consistence of jelly; and this happens equally whether it be kept in a cold or a hot temperature, whether it be exposed to the air or excluded from it. This consistence does not continue long; the sinovia soon recovers again its fluidity, and at the same time deposits a thread-like matter.

Sinovia mixes readily with water, and imparts to it a great deal of viscidity. The mixture froths when agitated; becomes milky when boiled, and... and deposits some pellicles on the sides of the dish; but its viscosity is not diminished.

When alcohol is poured into finovia, a white substance precipitates, which has all the properties of albumen. One hundred parts of finovia contain 4.52 of albumen. The liquid still continues as viscous as ever; but if acetic acid be poured into it, the viscosity disappears altogether, the liquid becomes transparent, and deposits a quantity of matter in white threads, which possesses the following properties:

1. It has the colour, smell, taste, and elasticity of vegetable gluten. 2. It is soluble in concentrated acids and pure alkalis. 3. It is soluble in cold water, the solution froths; acids and alcohol precipitate the fibrous matter in flakes. One hundred parts of finovia contain 11.86 of this matter.

When the liquid, after these substances have been separated from it, is concentrated by evaporation, it deposits crystals of acetate of soda. Finovia, therefore, contains soda. Margueron found that 100 parts of finovia contained about 0.71 of soda.

When strong sulphuric, muriatic, nitric, acetic, or sulphurous acid is poured into finovia, a number of white flakes precipitate at first, but they are soon redissolved, and the viscosity of the liquid continues. When these acids are diluted with five times their weight of water, they diminish the transparency of finovia, but not its viscosity; but when they are so much diluted that their acid taste is just perceptible, they precipitate the peculiar thready matter, and the viscosity of the finovia disappears.

When finovia is exposed to a dry atmosphere it gradually evaporates, and a feely residuum remains, in which cubic crystals, and a white saline efflorescence, are apparent. The cubic crystals are muriat of soda. One hundred parts of finovia contain about 1.75 of this salt. The saline efflorescence is carbonat of soda.

Finovia soon putrefies in a moist atmosphere, and during the putrefaction ammonia is exhaled. When finovia is distilled in a retort there comes over, first water, which soon putrefies; then water containing ammonia; then empyreumatic oil and carbonat of ammonia. From the residuum muriat and carbonat of soda may be extracted by lixiviation. The coal contains some phosphat of lime.

From the analysis of Mr Margueron it appears that finovia is composed of the following ingredients:

- 11.86 fibrous matter, - 4.52 albumen, - 1.75 muriat of soda, - .71 soda, - .70 phosphat of lime (n), - 80.57 water, - 100.00

Sect. XIV. Of Semen.

The peculiar liquid secreted in the testes of males, and destined for the impregnation of females, is known by the name of semen. The human semen alone has hitherto been subjected to chemical analysis. Nothing is known concerning the seminal fluid of other animals. Vauquelin published an analysis of the human semen in 1791.

Semen, when newly ejected, is evidently a mixture of two different substances: the one, fluid and milky, of foam, which is supposed to be secreted by the prostate gland; the other, which is considered as the true secretion of the testes, is a thick mucilaginous substance, in which numerous white shining filaments may be discovered. It has a slight disagreeable odour, an acid irritating taste, and its specific gravity is greater than that of water. When rubbed in a mortar it becomes frothy, and of the consistence of pomatum, in consequence of its containing a great number of air bubbles. It converts paper stained with the blossoms of mallows or violets to a green colour, and consequently contains an alkali.

As the liquid cools, the mucilaginous part becomes transparent, and acquires greater consistence; but in about twenty minutes after its emission, the whole becomes perfectly liquid. This liquefaction is not owing to the abstraction of moisture from the air, for it loses instead of acquiring weight during its exposure to the atmosphere; nor is it owing to the action of the air, for it takes place equally in close vessels.

Semen is insoluble in water before this spontaneous liquefaction, but afterwards it dissolves readily in it. When alcohol or oxy muriatic acid is poured into this solution, a number of white flakes are precipitated. Concentrated alkalis facilitate its combination with water. Acids readily dissolve the semen, and the solution is not decomposed by alkalis; neither indeed is the alkaline solution decomposed by acids.

Lime destroys no ammonia from fresh semen; but after that fluid has remained for some time in a moist and warm atmosphere, lime separates a great quantity from it. Consequently ammonia is formed during the exposure of semen to air.

When oxy-muriatic acid is poured into semen, a number of white flakes precipitate, and the acid loses its peculiar odour. These flakes are insoluble in water, and even in acids. If the quantity of acid be sufficient, the semen acquires a yellow colour. Thus it appears that semen contains a mucilaginous substance, analogous to that of the tears, which coagulates by absorbing oxygen. Mr Vauquelin obtained from 100 parts of semen six parts of this mucilage.

When semen is exposed to the air about the temperature of 60°, it becomes gradually covered with a transparent pellicle, and in three or four days deposits small transparent crystals, often crossing each other in such a manner as to represent the spokes of a wheel. These crystals, when viewed through a microscope, appear to be four-sided prisms, terminated by very long four-sided pyramids. They may be separated by diluting the liquid with water, and decanting it off. They have all the properties of phosphat of lime. If, after the appearance of these crystals, the semen be still allowed to remain exposed to the atmosphere, the pellicle on

(n) Mr Hatchett found only 0.268 of phosphat of lime in the finovia which he examined. He found, however, traces of some other phosphat; probably phosphat of soda. Phil. Trans. 1799, p. 246. on its surface gradually thickens, and a number of white round bodies appear on different parts of it. These bodies also are phosphat of lime, prevented from crystallizing regularly by the too rapid abstraction of moisture. Mr Vauquelin found that 100 parts of semen contain three parts of phosphat of lime. If at this period of the evaporation the air becomes moist, other crystals appear in the semen, which have the properties of carbonat of soda. The evaporation does not go on to complete exsiccation, unless at the temperature of 77°, and when the air is very dry. When all the moisture is evaporated, the semen has lost 0.9 of its weight, the residuum is semi-transparent like horn, and brittle.

When semen is kept in very moist air, at the temperature of about 77°, it acquires a yellow colour, like that of the yolk of an egg; its taste becomes acid; it exhales the odour of putrid fish, and its surface is covered with abundance of the hyphas septica.

When dried semen is exposed to heat in a crucible, it melts, acquires a brown colour, and exhales a yellow fume, having the odour of burnt horn. When the heat is raised, the matter swells, becomes black, and gives out a strong odour of ammonia. When the odour of ammonia disappears, if the matter be lixiviated with water, an alkaline solution may be obtained, which, by evaporation, yields crystals of carbonat of soda. Mr Vauquelin found that 100 parts of semen contain one part of soda. If the residuum be incinerated, there will remain only a quantity of white ashes, consisting of phosphat of lime.

Thus it appears that semen is composed of the following ingredients:

- 90 water, - 6 mucilage, - 3 phosphat of lime, - 1 soda,

100

Sect. XV. Liquor of the Amnios.

The fetus in the uterus is enveloped in a peculiar membranous covering, to which anatomists have given the name of amnios. Within this amnios there is a liquid, distinguished by the name of the liquor of the amnios, which surrounds the fetus on every part. This liquid, as might have been expected, is very different in different animals, at least the liquor amnii in women and in cows, which alone have hitherto been analysed, have not the smallest resemblance to each other. These two liquids have been lately analysed by Vauquelin and Buniva, and the result of their analysis has been published in the 33rd volume of the Annales de Chimie.

1. The liquor of the amnios of women is a fluid of a slightly milky colour, a weak but pleasant odour, and a faint taste. The white colour is owing to a curdy matter suspended in it, for it may be obtained quite transparent by filtration. Its specific gravity is 1.005. It gives a green colour to the tincture of violets, and yet it reddens very decidedly the tincture of turpentine. These two properties would indicate at once the presence of an acid and of an alkali. It froths considerably when agitated. On the application of heat it becomes opaque, and has then a great resemblance to milk diluted with a large quantity of water. At the same time it exhales the I. quire of the Amnios.

Acids render it more transparent. Alkalies precipitate an animal matter in small flakes. Alcohol likewise produces a flaky precipitate, which, when collected and dried, becomes transparent, and very like glue. The infusion of nut galls produces a very copious brown coloured precipitate. Nitrat of silver occasions a white precipitate, which is insoluble in nitric acid, and consequently is muriat of silver.

When slowly evaporated it becomes slightly milky, a transparent pellicle forms on its surface, and it leaves a residuum which does not exceed 0.012 of the whole. By lixiviating this residuum, and evaporating the ley, crystals of muriat and carbonat of soda, may be obtained. The remainder, when incinerated, exhales a fecid and ammonical odour, resembling that of burning horn; the ashes consist of a small quantity of carbonat of soda, and of phosphat and carbonat of lime.

Thus we see that the liquor of the human amnios is composed of about

\[ \begin{align*} 98.8 \text{ water}, \\ 1.2 \left\{ \begin{array}{l} \text{albumen,} \\ \text{muriat of soda, soda,} \\ \text{phosphat of lime, lime,} \end{array} \right. \end{align*} \]

While the fetus is in the uterus, a curdy-like matter is deposited on the surface of its skin, and in particular parts of its body. This matter is often found collected in considerable quantities. It is evidently deposited from the liquor of the amnios; and consequently the knowledge of its peculiar nature must throw considerable light upon the properties and use of that liquor. For an analysis of this substance we are also indebted to Vauquelin and Buniva.

Its colour is white and brilliant; it has a soft feel, and very much resembles newly prepared soap. It is insoluble in water, alcohol, and oils. Pure alkalies dissolve part of it, and form with it a kind of soap. On burning coals it decrepitates like a fat, becomes dry and black, exhales vapours which have the odour of empyreumatic oil, and leaves a residuum which is very difficultly reduced to ashes. When heated in a platinum crucible it decrepitates, lets an oil exude, curls up like horn, and leaves a residuum, consisting chiefly of carbonat of lime.

These properties show that this matter is different from every one of the component parts of the liquor of the amnios, and that it has a great resemblance to the fat. It is probable, as Vauquelin and Buniva have conjectured, that it is formed from the albumen of that liquid, which has undergone some unknown changes. It has been long known, that the parts of a fetus which has lain for some time after it has been deprived of life in the uterus, are sometimes converted into a kind of fatty matter. It is evident that this substance, after it is deposited upon the skin of the fetus, must preserve it in a great measure from being acted upon by the liquor of the amnios.

2. The liquor of the amnios of the cow has a viscidity similar to mucilage of gum arabic, a brownish red colour, an acid and bitter taste, and a peculiar odour, not unlike that of some vegetable extracts. Its specific gravity is 1.028. It resembles the tincture of turpentine, and Liquefied and therefore contains an acid. Muriat of barytes causes a very abundant precipitate, which renders it probable that it contains sulphuric acid. Alcohol separates from it a great quantity of a reddish coloured matter. When this liquid is evaporated, a thick frothy scum gathers on the surface, which is easily separated, and in which some white acid-tasted crystals may be discovered. By continuing the evaporation, the matter becomes thick, and viscid, and has very much the look of honey. Alcohol boiled upon this thick matter, and filtered off, deposits upon cooling brilliant needle-formed crystals nearly an inch in length. These crystals may be obtained in abundance by evaporating the liquor of the amnios to a fourth part of its bulk, and then allowing it to cool. The crystals soon make their appearance. They may be separated and purified by washing them in a small quantity of cold water. These crystals have the properties of an acid.

If after the separation of this acid the liquor of the amnios be evaporated to the consistence of a syrup, large transparent crystals appear in it, which have all the properties of sulphate of soda. The liquor of the amnios of cows contains a considerable quantity of this salt.

Thus it appears that the liquor of the amnios of cows contains the following ingredients:

1. Water, 2. A peculiar animal matter, 3. A peculiar acid, 4. Sulphate of soda.

The animal matter possesses the following properties: It has a reddish brown colour, and a peculiar taste; it is very soluble in water, but insoluble in alcohol, which has the property of separating it from water. When exposed to a strong heat it swells, exhales first the odour of burning gum, then of empyreumatic oil and ammonia, and at last the peculiar odour of prussic acid becomes very conspicuous. It differs from gelatin in the viscosity which it communicates to water, in not forming a jelly when concentrated, and in not being precipitated by tan. It must be therefore ranked among the very undefined and inaccurate class of animal mucilages.

When burnt, it leaves a very large coal, which is readily incinerated, and leaves a little white ash, composed of phosphate of magnesia, and a very small proportion of phosphate of lime.

The acid substance is of a white and brilliant colour; its taste has a very slight degree of sourness; it reddens the tincture of turmeric; it is scarcely soluble in cold water, but very readily in hot water, from which it separates in long needles as the solution cools. It is soluble also in alcohol, especially when assisted by heat. It combines readily with pure alkalies, and forms a substance which is very soluble in water. The other acids decompose this compound; and the acid of the liquor of the amnios is precipitated in a white crystalline powder. This acid does not decompose the alkaline carbonates at the temperature of the atmosphere, but it does so when assisted by heat. It does not alter solutions of silver, lead, or mercury, in nitric acid. When exposed to a strong heat, it froths and exhales an odour of ammonia and of prussic acid. The properties are sufficient to show that it is different from every other acid. Vauquelin and Boniva have given it the name of amniotic acid. It approaches nearest to the fæcalactic and the uric acids; but the fæcalactic acid does not furnish ammonia by distillation like the amniotic. The uric acid is not so soluble in hot water as the amniotic, it does not crystallize in white brilliant needles, and it is insoluble in boiling alcohol; in both which respects it differs completely from amniotic acid.

Sect. XVI. Of Urine.

No animal substance has attracted more attention than urine, both on account of its supposed connection with various diseases, and on account of the very singular products which have been obtained from it. Mr. Boyle, and the other chemists who were his contemporaries, were induced to attend particularly to this liquid, by the discovery of a method of obtaining phosphorus from it. Boerhaave, Haller, Haupt, Margraf, Pott, Rouelle, Proust, and Klaproth, successively improved the method of obtaining the phosphoric salts from urine, or added something to our knowledge of the component parts of these salts. Scheele added greatly to our knowledge of urine by detecting several new substances in it which had not been suspected. Cruickshank has given us a very valuable paper on urine in the second edition of Rolle's Diabetes; and Fourcroy and Vauquelin have lately published the most complete analysis of it which has hitherto appeared.

Fresh urine is a liquid of a peculiar aromatic odour, an orange colour, of greater or less intensity, and an acrid saline taste.

Its specific gravity varies from 1.005 to 1.033.

1. It reddens paper stained with turmeric and with the juice of radishes, and therefore contains an acid.

2. If a solution of ammonia be poured into fresh urine, a white powder precipitates, which has the properties of phosphate of lime. The presence of this substance in urine was first discovered by Scheele. If lime water be poured into urine, phosphate of lime precipitates in greater abundance than when ammonia is used; consequently the acid which urine contains is the phosphoric. Thus we see that the phosphate of lime is kept dissolved in urine by an excess of acid. This also was first discovered by Scheele. This substance is most abundant in the urine of the sick. Berthollet has observed, that the urine of gouty people is less acid than that of people in perfect health. The average quantity of phosphate of lime in healthy urine is, as Cruickshank has ascertained, about \( \frac{1}{3} \) of the weight of the urine.

3. If the phosphate of lime precipitated from urine be examined, a little magnesia will be found mixed with it. Fourcroy and Vauquelin have ascertained that this is owing to a little phosphate of magnesia which urine contains, and which is decomposed by the alkali or lime employed to precipitate the phosphate of lime.

4. When fresh urine cools, it often lets fall a brick-coloured precipitate, which Scheele first ascertained to be crystals of uric acid. All urine contains this acid, even when no sensible precipitate appears when it cools. For if a sufficient quantity of clear and fresh urine be evaporated to \( \frac{1}{4} \) of its weight, a flaky powder precipitates to the bottom, and attaches itself in part very firmly to the vessel. This part may be dissolved in pure alkali, and precipitated again by acetic acid. It exhibits all the properties of uric acid. The quantity of uric acid in urine is very various. During intermittent terrestrial fevers it is deposited very copiously, and has been long known to physicians under the name of tertian sediment. This sediment always makes its appearance at the crisis of fevers. In gouty people, the same sediment appears in equal abundance towards the end of a paroxysm of the disease (r). And if this sediment suddenly disappears after it has begun to be depolished, a fresh attack may be expected.

5. If fresh urine be evaporated to the consistence of a syrup, and muriatic acid be then poured into it, a precipitate appears which possesses the properties of benzoic acid. Scheele first discovered the presence of benzoic acid in urine. He evaporated it to dryness, separated the saline part, and applied heat to the residue. The benzoic acid was sublimed, and found crystallized in the receiver. The method which we have given is much easier; it was first proposed by Fourcroy and Vanquelin. By it very considerable quantities of benzoic acid may be obtained from the urine of horses and cows, where it is much more abundant than in human urine. In human urine it varies from about to 1/32 of the whole.

6. When an infusion of tan is dropped into urine, a white precipitate appears, having the properties of the combination of tan and albumen, or gelatin. Urine, therefore, contains albumen and gelatin. These substances had been suspected to be in urine, but their presence was first demonstrated by Seguin, who discovered the above method of detecting them. Their quantity in healthy urine is very small. Cruikshank found that the precipitate afforded by tan in healthy urine amounted to 1/10th part of the weight of the urine. It is to these substances that the appearance of the cloud, as it is called, or the mucilaginous matter, which is sometimes deposited as the urine cools, is owing. It is probable that healthy urine contains only gelatin and not albumen, though the quantity is too small to admit of accurate examination; but in many diseases the quantity of these matters is very much increased. The urine of tropical people often contains so much albumen, that it coagulates not only on the addition of acids, but even on the application of heat. In all cases of impaired digestion, the albuminous and gelatinous part of urine is much increased. This forms one of the most conspicuous and important distinctions between the urine of those who enjoy good and bad health.

7. If urine be evaporated by a slow fire to the consistence of a thick syrup, it assumes a deep brown colour, and exhales a fetid ammoniacal odour. When allowed to cool, it concretes into a mass of crystals, composed of all the component parts of urine. If four times its weight of alcohol be poured upon this mass, at intervals, and a slight heat be applied, the greatest part of it is dissolved. The alcohol, which has acquired a brown colour, is to be decanted off, and distilled in a crucible in a sand bath, till the mixture has boiled for some time, and acquired the consistence of a syrup.

Suppl. Vol. II. Part II.

By this time the whole of the alcohol has passed off, and the matter, on cooling, crystallizes in quadrangular plates which intersect each other. This substance is urea, which composes 1/4 of the urine, provided the watery part be excluded. To this substance the taste, smell, and colour of urine are owing. It is a substance which characterizes urine, and constitutes it what it is, and to which the greater part of the very singular phenomena of urine are to be ascribed.

The colour of urine depends upon the urea; the greater the quantity, the deeper is the colour. It may be detected by evaporating urine to the consistence of a syrup, and pouring into it concentrated nitric acid. Immediately, a great number of white shining crystals appear in the form of plates, very much resembling crystallized boracic acid. These crystals are urea combined with nitric acid.

The quantity of urea varies exceedingly in different urines. In the urine voided soon after a meal, very little of it is to be found, and scarcely any at all in that which hysterical patients void during a paroxysm.

8. If urine be slowly evaporated to the consistence of a syrup, a number of crystals make their appearance in it. Two of these are remarkable by their form; one of them consists of small regular octahedrons; which, when examined, are found to possess the properties of muriat of soda. Urine, therefore, contains muriat of soda. It is well known that muriat of soda crystallizes in cubes; the singular modification of its form in urine is owing to the action of urea. It has been long known that urine saturated with muriat of soda deposits that salt in regular octahedrons.

9. Another of the salts which appear during the evaporation of urine has the form of regular cubes. This ammonia, salt has the properties of muriat of ammonia. Now the usual form of the crystals of muriat of ammonia is the octahedron. The change of its form in urine is produced also by urea.

10. The saline residuum which remains after the separation of urea from crystallized urine by means of alcohol, has been long known under the names of fusible salt of urine and microcosmic salt. Various methods of obtaining it have been given by chemists from Boerhaave, who first published a process, to Rouelle and Chauvins, who gave the method just mentioned. If this saline mass be dissolved in a sufficient quantity of hot water, and allowed to crystallize spontaneously in a cloche vessel, two sets of crystals are gradually deposited. The lowermost set has the figure of flat rhomboidal prisms; the uppermost, on the contrary, has the form of rectangular tables. These two may be easily separated by exposing them for some time to a dry atmosphere. The rectangular tables effloresce and fall to powder, but the rhomboidal prisms remain unaltered.

When these salts are examined, they are found to have the properties of phosphates. The rhomboidal prisms consist of phosphate of ammonia united to a little phosphate of soda; the rectangular tables, on the contrary,

(r) The concretions which sometimes make their appearance in gouty joints have been found to consist chiefly of uric acid. This singular coincidence deserves the attention of physiologists: it cannot fail, sooner or later, to throw light, not only upon gout, but upon some of the animal functions. ANIMAL SUBSTANCES.

Urine.

Trary, are phosphat of soda united to a small quantity of phosphat of ammonia. Urine, then, contains phosphat of soda and phosphat of ammonia.

Thus we have found that urine contains the twelve following substances:

1. Water, 2. Phosphoric acid, 3. Phosphat of lime, 4. Phosphat of magnesia, 5. Uric acid, 6. Benzoic acid, 7. Gelatine and albumen, 8. Urca, 9. Muriat of soda, 10. Muriat of ammonia, 11. Phosphat of soda, 12. Phosphat of ammonia.

These are the only substances which are constantly found in healthy urine; but it contains also occasionally other substances. Very often muriat of potash may be distinguished among the crystals which form during its evaporation. The presence of this salt may always be detected by dropping cautiously some tartarous acid into urine. If it contains muriat of potash, there will precipitate a little tartar, which may easily be recognised by its properties.

Urine sometimes also contains sulphat of soda, and even sulphat of lime. The presence of these salts may be ascertained by pouring into urine a solution of muriat of barytes, a copious white precipitate appears, consisting of the barytes combined with phosphoric acid, and with sulphuric acid, if any be present. This precipitate must be treated with a sufficient quantity of muriatic acid. The phosphat of barytes is dissolved, but the sulphat of barytes remains unaltered.

No substance putrefies sooner, or exhales a more detestable odour during its spontaneous decomposition, than urine; but there is a very great difference in this respect in different urines. In some, putrefaction takes place almost instantaneously as soon as it is voided; in others, scarcely any change appears for a number of days. Foureroy and Vaupelin have ascertained that this difference depends on the quantity of gelatine and albumen which urine contains. When there is very little of these substances present, urine remains long unchanged; on the contrary, the greater the quantity of gelatine or albumen, the sooner does putrefaction commence. The putrefaction of urine, therefore, is, in some degree, the test of the health of the person who has voided it; for a superabundance of gelatine in urine always indicates some defect in the power of digestion.

The rapid putrefaction of urine, then, is owing to the action of gelatine on urea. We have seen already the facility with which that singular substance is decomposed, and that the new products into which it is changed are, ammonia, carbonic acid, and acetic acid. Accordingly, the putrefaction of urine is announced by an ammoniacal smell. Mucilaginous flakes are deposited, consisting of part of the gelatinous matter. The phosphoric acid is saturated with ammonia, and the phosphat of lime, in consequence, is precipitated. Ammonia combines with the phosphat of magnesia, forms with it a triple salt, which crystallizes upon the sides of the vessel in the form of white crystals, composed of six-sided prisms, terminated by six-sided pyramids. The uric and benzoic acids are saturated with ammonia; the acetic acid, and the carbonic acid, which are the products of the decomposition of the urea, are also saturated with ammonia, and notwithstanding the quantity which exhales, the production of this substance is so abundant, that there is a quantity of unsaturated alkali in the liquid. Putrefied urine, therefore, contains chiefly the following substances, most of which are the products of putrefaction:

Ammonia, Carbonat of ammonia, Phosphat of ammonia, Phosphat of magnesia and ammonia, Urat of ammonia, Acetite of ammonia, Benzoat of ammonia, Muriat of soda, Muriat of ammonia;

Besides the precipitated gelatine and phosphat of lime.

The distillation of urine produces almost the same changes; for the heat of boiling water is sufficient to decompose urea, and to convert it into ammonia, carbonic and acetic acids. Accordingly, when urine is distilled, there comes over water, containing ammonia dissolved in it, and carbonat of ammonia in crystals. The acids contained in urine are saturated with ammonia, and the gelatine and phosphat of lime precipitate.

Such are the properties of the human urine. The urine of other animals has not hitherto been examined with equal care; but it is certain that it differs very considerably from that of men. The urine of cows and horses, and of all ruminating animals, for instance, contains carbonat of lime, without any mixture of phosphat of lime. It contains also a much greater proportion of benzoic acid than that of man.

Sect. XVII. Of the Urinary Calculus.

It is well known that concretions not unfrequently form in the bladder, or the other urinary organs, and occasion one of the most dismal diseases to which the human species is liable.

These concretions were distinguished by the name of urinary calculi, from a supposition that they are of a stony nature. They have long attracted the attention of physicians. Chemistry had no sooner made its way into medicine than it began to exercise its ingenuity upon the urinary calculi; and various theories were given of their nature and origin. According to Paracelsus, who gave them the ridiculous name of dudleeb, urinary calculi were intermediate between tartar and stone, and composed of an animal resin. Van Helmont pronounced them anomalous coagulations, the offspring of the faults of urine, and of a volatile earthly spirit, produced at once, and destitute of any viscid matter. Boyle's De L'Est extracted from them, by distillation, oil, and a great quantity of volatile salt. Boerhaave supposed them compounds of oil and volatile fats. Hales extracted from them a prodigious quantity of air. He gave them the name of animal tartar, pointed out several circumstances in which they resemble common tartar, and made many experiments to find a solvent of them. Drs Whytt and Alton pointed out alkalies as solvents of calculi. It was an attempt to discover a more perfect solvent that induced Dr Black to make those experiments which terminated in the discovery of the nature of the alkaline carbonat.

Such was the fate of the chemical analysis of calculus, when, in 1776, Scheele published a dissertation on the subject in the Stockholm Transactions; which was succeeded by some remarks of Mr Bergmann. These illustrious Animal Substances.

The greater number of calculi consist of uric acid. All those analysed by Scheele were composed of it entirely. Of 300 calculi analysed by Dr Pearson, scarcely one was found which did not contain a considerable quantity of it, and the greater number manifestly were formed chiefly of it. Fourcroy and Vauquelin found it also in the greater number of the 300 calculi which they analysed.

The presence of this acid may easily be ascertained by the following properties: A solution of potash or soda dissolves it readily, and it is precipitated by the weakest acids. The precipitate is soluble in nitric acid, the solution is of a pink colour, and tinged the skin red.

2. Uric acid is easily detected by its rapid solubility in fixed alkaline leys, and the odour of ammonia which is perceived during the solution. It is not so often present in urinary calculi as the last mentioned substance. No calculus has hitherto been found composed of it alone, except the very small polygonal calculi, several of which sometimes exist in the bladder together.

It is most usually in thin layers, alternating with some other substance, very easily reduced to powder, and of the colour of ground coffee.

3. Phosphat of lime is white, without lustre, fiery, friable, stains the hands, paper, and cloth. It has very much the appearance of chalk, breaks under the forceps, is insipid, and insoluble in water. It is soluble in nitric, muriatic, and acetic acids, and is again precipitated by ammonia, fixed alkalies, and oxalic acid.

It is never alone in calculi. It is intimately mixed with a gelatinous matter, which remains under the form of a membrane when the earthy part is dissolved by very diluted acids.

4. Phosphat of magnesia and ammonia occurs in white, semitransparent, lamellar layers; sometimes it is crystallized on the surface of the calculi in prisms, or what are called dog's tooth crystals. It has a weak sweetish taste, it is somewhat soluble in water, and very soluble in acids, though greatly diluted. Fixed alkalies decompose it.

It never forms entire calculi. Sometimes it is mixed with phosphat of lime, and sometimes layers of it cover uric acid or oxalat of lime. It is mixed with the same gelatinous matter as phosphat of lime.

5. Oxalat of lime is found in certain calculi, which, from the inequality of their surface, have got the name of mariform or malberry-shaped calculi. It is never alone, but combined with a peculiar animal matter, and forming with it a very hard calculus, of a grey colour, difficult to saw asunder, admitting a polish like ivory, exhaling, when sawed, an odour like that of semen. Insoluble and indecomposable by alkalies; soluble in very diluted nitric acid, but slowly, and with difficulty. It may be decomposed by the carbonates of potash and soda. When burnt, it leaves behind a quantity of pure lime, which may be easily recognized by its properties.

6. Silica has only been found in two instances by Fourcroy and Vauquelin, though they analysed 300 calculi. No other chemist has observed it. It must therefore be considered as a very uncommon ingredient of these concretions. In the two instances in which it occurred, it was mixed with uric acid and the two phosphats above mentioned.

7. Animal matter appears to compose the cement which binds the different particles of the calculus together, and in all probability it is the cause which influences its formation. It is different in different calculi. Sometimes it has the appearance of gelatine or albumen, at other times it resembles urea. It deserves a more accurate investigation.

No general description of the different calculi has hitherto appeared; but Fourcroy and Vauquelin are at present occupied with that subject. They propose to classify them according to their composition; to point out their different species and varieties; to give a method of detecting them by their appearance; to analyse the animal matter by which they are cemented; and to apply all the present chemical knowledge of the subject in the investigation of the cause, the symptoms, and the cure, of that dreadful disease which the urinary calculi produce. As their labour is already very far advanced, it would be unnecessary for us to attempt any classification of calculi. Indeed every attempt of that kind, by any person who has not had an opportunity of analysing a very great number of calculi, must be to exceedingly imperfect as scarcely to be of any use.

We shall satisfy ourselves with the following remarks, deduced almost entirely from the observations which these celebrated chemists have already published.

Many calculi consist entirely, or almost entirely, of uric acid. The animal matter, which serves as a cement to these calculi, appears to be urea. Calculi of this kind may be dissolved by injecting into the bladder solutions of pure potash or soda, so much diluted as not to act upon the bladder itself. The gritty substance, which many persons threatened with the stone discharge along with their urine, which has been called gravel, consists almost constantly of uric acid. It may therefore serve as an indication that the subsequent stone, if any such form, is probably composed of uric acid.

The two phosphats, mixed together, sometimes compose calculi. These calculi are very brittle, and generally

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(a) Brugnatelli found also phosphat of lime, with excess of acid, in calculi. See Ann. de Chim. xxxii. 183. ANIMAL SUBSTANCES.

rally break in pieces during the extraction. Such calculi may be dissolved by injecting into the bladder muriatic acid, so much diluted as scarcely to have any taste of acid.

The phosphates never form the nucleus of a calculus. They have never been found covered with a layer of uric acid, but they often cover that acid. Hence it would seem that the existence of any extraneous matter in the bladder disposes these phosphates to crystallize. When extraneous bodies are accidentally introduced into the bladder, and allowed to lodge there, they are constantly covered with a coat of phosphate of ammonia and magnesia, or of the two phosphates mixed.

As the phosphate of ammonia and magnesia is not an ingredient of fresh urine, but formed during its putrefaction, when it exists in calculi, it would seem to indicate a commencement of putrefaction during the time that the urine lodges in the bladder. But putrefaction does not take place speedily in urine, unless where there is an excess of albumen and gelatin; consequently we have reason to suppose, that these substances are morbidly abundant in the urine of those patients who are afflicted with calculi consisting of the phosphates; hence also we may conclude, that their digestion is imperfect. It will no doubt be objected, that droppings of people are not peculiarly subject to calculi; but their urine is only morbidly albuminous when the disease is beginning to disappear, and then there seems to be a deficiency of urea; at least their urine has not been observed to putrefy with uncommon rapidity. Besides, there seems to be some animal matter present, which serves as a cement to the phosphate in all cases where calculi form.

Urate of ammonia is only found alone in the very small polygonous calculi which exist, several together, in the bladder. In other cases it is mixed with uric acid. It sometimes alternates with uric acid or with the phosphates. It is dissolved by the same substance that acts as a solvent of uric acid.

Oxalate of lime often forms the nucleus of calculi composed of layers of uric acid or of the phosphates. It forms those irregular calculi which are called moriform. These calculi are the hardest and the most difficult of solution. A very much diluted nitric acid dissolves them but very slowly. As oxalic acid does not exist in urine, some morbid change must take place in the urine when such calculi are deposited. Brugnatelli's discovery of the instantaneous conversion of uric acid into oxalic acid by oxy-muriatic acid, which has been confirmed by the experiments of Fourcroy and Vauquelin, throws considerable light upon the formation of oxalic acid in urine, by showing us that uric acid is probably the basis of it; but in what manner the change is actually produced, it is not so easy to say.

The calculi found in the bladder of other animals have not been examined with the same care. Some of them, however, have been subjected to an accurate analysis. No uric acid has ever been found in any of them.

Fourcroy found a calculus extracted from the kidney of a horse composed of three parts of carbonat of lime, inferior and one part phosphate of lime. Dr Pearson examined a urinary calculus of a horse; it was composed of phosphate of lime and phosphate of ammonia. Brugnatelli found a calculus extracted from the bladder of a fowl, which was exceedingly hard, composed of pure carbonat of lime, including a soft nucleus of a fetid and urinous odour. Bartholdi examined another calculus of a pig, the specific gravity of which was 1.9200. It consisted of phosphate of lime. Dr Pearson found a calculus taken from the bladder of a dog composed of phosphate of lime, phosphate of ammonia, and an animal matter. He found the urinary calculus of a rabbit, of the specific gravity 2, composed of carbonat of lime and some animal matter.

The composition of the different animal concretions hitherto examined may be seen in the following table:

| Horse. | Carbonat of lime and phosphate of lime | |--------|--------------------------------------| | Horse. | Phosphate of lime and phosphate of ammonia | | Sow. | Carbonat of lime and animal matter | | Dog. | Phosphate of lime and of ammonia, and animal matter | | Rabbit.| Carbonat of lime and animal matter |

We have now given an account of all those secretions which have been attentively examined by chemists. The remainder have been hitherto neglected; partly owing to the difficulty of procuring them, and partly on account of the multiplicity of other objects which occupied the attention of chemical philosophers (x). It remains for us now to examine by what processes these different secretions are formed, how the constant waste of living bodies is repaired, and how the organs themselves are nourished and preserved. This shall form the subject of the following chapter.

CHAP. III. OF THE FUNCTIONS OF ANIMALS.

The intention of the two last chapters was to exhibit a view of the different substances which enter into the composition of animals, as far as the present limited state of our knowledge puts it in our power. But were our enquiries concerning animals confined to the mere ingredients of which their bodies are composed, even supposing the analysis as complete as possible, our knowledge of the nature and properties of animals would be imperfect indeed.

How are these substances arranged? How are they produced?

(x) The chief of these secretions are the following: 1. Cerumen, or ear-wax, is at first nearly liquid, and of a whitish colour. It gradually acquires consistence. Its taste is very bitter. Said to be insoluble in alcohol; but soluble in hot water. Does not become rancid by keeping. 2. The humours of the eye. 3. The milky liquor, secreted by the thyroid gland. 4. Mucus of the lungs, intestinal canal, &c. 5. Smegma of the areola of the breasts, glans penis, vagina, subcutaneous glands, &c. 6. Marrow. produced? What purposes do they serve? What are the distinguishing properties of animals, and the laws by which they are regulated?

Animals resemble vegetables in the complexities of their structure. Like them, they are machines nicely adapted for particular purposes, constituting one whole, and continually performing an infinite number of the most delicate processes. But neither an account of the structure of animals, nor of the properties which distinguish them from other beings, will be expected here.

There have been already treated of sufficiently in the articles ANATOMY and PHYSIOLOGY (Encyc.), to which we beg leave to refer the reader. We mean only, in the present chapter, to take a view of those processes which are concerned in the production of animal substances, which alone properly belong to chemistry. The other functions are regulated by laws of a very different nature, which have no resemblance or analogy to the laws of chemistry or mechanics.

1. Every body knows that animals require food, and that they die sooner or later if food be withheld from them. There is indeed a very great difference in different animals, with regard to the quantity of food which they require, and the time which they can pass without it. In general, this difference depends upon the activity of the animal. Those which are most active require most, and those which move least require least food.

The cause of this is also well known; the bodies of animals do not remain stationary, they are constantly wasting; and the waste is generally proportional to the activity of the animal. It is evident, then, that the body must receive, from time to time, new supplies, in place of what has been carried off. Hence the use of food, which answers this purpose.

2. We are much better acquainted with the food of animals than of vegetables. It consists of almost all the animal and vegetable substances which have been treated of in the former part of this article; for there are but very few of them which some animal or other does not use as food. Man uses as food chiefly the muscles of animals, the seeds of certain grapes, and a variety of vegetable fruits. Almost all the inferior animals have particular substances on which they feed exclusively. Some of them feed on animals, others on vegetables. Man has a greater range; he can feed on a very great number of substances. To enumerate these substances would be useless; as we are not able to point out with accuracy what it is which renders one substance more nourishing than another.

Many substances do not serve as nourishment at all; and not a few, instead of nourishing, destroy life. These last are called poisons. Some poisons act chemically, by decomposing the animal body. The action of others is not so well understood.

3. The food is introduced into the body by the mouth, and almost all animals reduce it to a kind of pulpy consistence. In man and many other animals this is done in the mouth by means of teeth, and the saliva with which it is there mixed; but many other animals grind their food in a different manner. See PHYSIOLOGY (Encyc.) After the food has been thus ground, it is introduced into the stomach, where it is subjected to new changes. The stomach is a strong soft bag, of different forms in different animals; in man it has some resemblance to the bag of a bagpipe. In this organ the food is converted into a soft pulp, which has no resemblance to the food when first introduced. This pulp has been called chyme.

4. Since chyme possesses new properties, it is evident that the food has undergone some changes in the stomach, and that the ingredients of which it was composed have entered into new combinations. Now, in what manner have these changes been produced?

At first they were ascribed to the mechanical action of the stomach. The food, it was said, was still farther triturated in that organ; and being long agitated backwards and forwards in it, was at last reduced to a pulp. But this opinion, upon examination, was found not to be true. The experiments of Stevens, Résumur, and Spallanzani demonstrated, that the formation of chyme is not owing to trituration; for on including different kinds of food in metallic tubes and balls full of holes, in such a manner as to screen them from the mechanical action of the stomach, they found, that these substances, after having remained a sufficient time in the stomach, were converted into chyme, just as if they had not been included in such tubes. Indeed, the opinion was untenable, even independent of these decisive experiments; the moment it was perceived that chyme differed entirely from the food which had been taken; that is to say, that if the same food were triturated mechanically out of the body, and reduced to pulp of precisely the same consistence with chyme, it would not possess the same properties with chyme; for whenever this fact was known, it could not but be evident that the food had undergone changes in its composition.

The change of food into chyme, therefore, was ascribed by many to fermentation. This opinion is indeed very ancient, and it has had many zealous supporters among the moderns. When the word fermentation was applied to the change produced on the food in the stomach, the nature of the process called fermentation was altogether unknown. The appearances, indeed, which take place during that process, had been described, and the progress and the result of it were known. Chemists had even divided fermentations into different classes; but no attempt had been made to explain the cause of fermentation, or to trace the changes which take place during its continuance. All that could be meant, then, by saying that the conversion of food into chyme in the stomach was owing to fermentation, was merely, that the unknown cause which acted during the conversion of vegetable substances into wine or acid, or during their putrefaction, acted also during the conversion of the food into chyme, and that the result in both cases was precisely the same. Accordingly, the advocates for this opinion attempted to prove, that air was constantly generated in the stomach, and that an acid was constantly produced; for it was the vinous and acetic fermentations which were assigned by the greater number of physiologists as the cause of the formation of chyme. Some indeed attempted to prove, that it was produced by the putrefactive fermentation; but their number was inconsiderable, compared with those who adopted the other opinion.

Our ideas respecting fermentation are now somewhat more precise. It signifies a slow decomposition, which takes place when certain animal or vegetable substances are mixed together at a given temperature; and the consequence... consequent production of particular compounds. If therefore the conversion of the food into chyme be owing to fermentation, it is evident that it is totally independent of the stomach any farther than as it supplies temperature; and that the food would be converted into chyme exactly in the same manner, if it were reduced to the same consistence, and placed in the same temperature out of the body. But this is by no means the case; substances are reduced to the state of chyme in a short time in the stomach, which would remain unaltered for weeks in the same temperature out of the body. This is the case with bones; which the experiments of Stevens and Spallanzani have shewn to be soon digested in the stomach of the dog. Further, if the conversion of food into chyme were owing to fermentation, it ought to go on equally well in the stomach and oesophagus. Now, it was observed long ago by Ray and Boyle, that when voracious fish had swallowed animals too large to be contained in the stomach, that part only which was in the stomach was converted into chyme, while what was in the oesophagus remained entire; and this has been fully confirmed by subsequent observations.

Still farther, if the conversion were owing to fermentation, it ought always to take place equally well, provided the temperature be the same, whether the stomach be in a healthy state or not. But it is well known, that this is not the case. The formation of chyme depends very much on the state of the stomach. When that organ is diseased, digestion is constantly ill performed. In these cases, indeed, fermentation sometimes appears, and produces flatulence, acid eructations, &c., which are the well-known symptoms of indigestion. These facts have been long known; they are totally incompatible with the supposition, that the formation of chyme is owing to fermentation. Accordingly that opinion has been for some time abandoned, by all those at least who have taken the trouble to examine the subject.

The formation of chyme, then, is owing to the stomach; and it has been concluded, from the experiments of Stevens, Reaumur, Spallanzani, Scopoli, Brugnatelli, Carini, &c., that its formation is brought about by the action of a particular liquid secreted by the stomach, and for that reason called gastric juice.

That it is owing to the action of a liquid, is evident; because if pieces of food be inclosed in clothe tubes, they pass through the stomach without any further alteration than would have taken place at the same temperature out of the body; but if the tubes be perforated with small holes, the food is converted into chyme.

This liquid does not act indiscriminately upon all substances: For if grains of corn be put into a perforated tube, and a granivorous bird be made to swallow it, the corn will remain the usual time in the stomach without alteration; whereas if the husk of the grain be previously taken off, the whole of it will be converted into chyme. It is well known, too, that many substances pass unaltered through the intestines of animals, and consequently are not acted upon by the gastric juice. This is the case frequently with grains of oats when they have been swallowed by horses entire with their hulls on. This is the case also with the seeds of apples, &c., when swallowed entire by men; yet these very substances, if they have been previously ground sufficiently by the teeth, are digested. It appears, therefore, that it is chiefly the bulk or outside of these substances which resists the action of the gastric juice. We see also, that trituration greatly facilitates the conversion of food into chyme.

The gastric juice is not the same in all animals; for many animals cannot digest the food on which others live. The conium maculatum (hemlock), for instance, is a poison to man instead of food, yet the goat often feeds upon it. Many animals, as sheep, live wholly upon vegetables; and if they are made to feed on animals, their stomachs will not digest them; others, again, as the eagle, feed wholly on animal substances, and cannot digest vegetables.

The gastric juice does not continue always of the same nature, even in the same animal: it changes gradually, according to circumstances. Graminivorous animals may be brought to live on animal food; and after they have been accustomed to this for some time, their stomachs become incapable of digesting vegetables. On the other hand, those animals which naturally digest nothing but animal food may be brought to digest vegetables.

What is the nature of the gastric juice, which possesses these singular properties? It is evidently different in different animals; but it is a very difficult task, if not an impossible one, to obtain it in a state of purity. Various attempts have indeed been made by very ingenious philosophers to procure it; but their analysis of it is sufficient to shew us, that they have never obtained it in a state of purity.

The methods which have been used to procure gastric juice are, first, to kill the animal whose gastric juice is to be examined after it has fasted for some time. By this method, Spallanzani collected 37 spoonfuls from the two first stomachs of a sheep. It was of a green colour, undoubtedly owing to the grass which the animal had eaten. He found also half a spoonful in the stomach of some young crows which he killed before they had left their nest.

Small tubes of metal, pierced with holes, and containing a dry sponge, have been swallowed by animals; and when vomited up, the liquid imbibed by the sponge is squeezed out. By this method, Spallanzani collected 481 grains of gastric juice from the stomachs of five crows.

A third method consists in exciting vomiting in the morning, when the stomach is without food. Spallanzani tried this method twice upon himself, and collected one of the times 1 oz. 32 gr. of liquid; but the pain was so great, that he did not think proper to try the experiment a third time. Mr Goffe, however, who could excite vomiting whenever he thought proper by swallowing air, has employed that method to collect gastric juice.

Spallanzani has observed, that eagles throw up every morning a quantity of liquid, which he considers as gastric juice; and he has availed himself of this to collect it in considerable quantities.

It is almost unnecessary to remark how imperfect these different methods are, and how far every conclusion drawn from the examination of such juices must deviate from the truth. It is impossible that the gastric juice, obtained by any one of these processes, can be pure; because in the stomach it must be constantly mixed mixed with large quantities of saliva, mucus, bile, food, &c. It may be questioned, indeed, whether any gastric juice at all can be obtained by these methods; for as the intention of the gastric juice is to convert the food into chyme, in all probability it is only secreted, or at least thrown into the stomach when food is present.

We need not be surprised, then, at the contradictory accounts concerning its nature, given us by those philosophers who have attempted to examine it; as they relate not so much to the gastric juice, as to the different substances found in the stomach. The idea that the gastric juice can be obtained by vomiting, or that it is thrown up spontaneously by some animals, is, to say the least of it, very far from being probable.

According to Brugnatelli, the gastric juice of carnivorous animals, as hawks, kites, &c., has an acid and resinous odour, is very bitter, and not at all watery; and is composed of an uncombined acid, a resin, an animal substance, and a small quantity of muriatic acid. The gastric juice of herbivorous animals, on the contrary, as goats, sheep, &c., is very watery, a little muddy, has a bitterish taste, and contains ammonia, an animal extract, and a pretty large quantity of muriatic acid. Mr Carminati found the same ingredients; but he supposes that the ammonia had been formed by the putrefaction of a part of their food, and that in reality the gastric juice of these animals is of an acid nature.

The accounts which have been given of the gastric juice of man are so various, that it is not worth while to transcribe them. Sometimes it has been found of an acid nature, at other times not. The experiments of Spallanzani are sufficient to show, that this acidity is not owing to the gastric juice, but to the food. He never found any acidity in the gastric juice of birds of prey, nor of serpents, frogs, and fishes. Crows gave an acidulous gastric juice only when fed on grain; and he found that the same observation holds with respect to dogs, herbivorous animals, and domestic fowls. Carnivorous birds threw up pieces of shells and coral without alteration; but these substances were sensibly diminished in the stomachs of hens, even when inclosed in perforated tubes. Spallanzani himself swallowed calcareous substances inclosed in tubes; and when he fed on vegetables and fruits, they were sometimes altered and a little diminished in weight, just as if they had been put into weak vinegar; but when he used only animal food, they came out untouched. According to this philosopher, whose experiments have been by far the most numerous, the gastric juice is naturally neither acid nor alkaline. When poured on the carbonat of potash, it causes no effervescence.

Such are the results of the experiments on the juices taken from the stomach of animals. No conclusion can be drawn from them respecting the nature of the gastric juice. But from the experiments which have been made on the digestion of the stomach, especially by Spallanzani, the following facts are established:

1. The gastric juice attacks the surfaces of bodies, unites to the particles of them which it carries off, and cannot be separated from them by filtration. It operates with more energy and rapidity the more the food is divided, and its action is increased by a warm temperature. The food is not merely reduced to very minute parts; its taste and smell are quite changed; its sensible properties are destroyed, and it acquires new and very different ones. This juice does not act as a ferment; so far from it, that it is a powerful antiseptic, and even restores flesh already putrefied. There is not the smallest appearance of such a process; indeed, when the juice is renewed frequently, as in the stomach, substances dissolve in it with a rapidity which excludes all idea of fermentation. Only a few air bubbles make their escape, which adhere to the alimentary matter, and buoy it up to the top, and which are probably extricated by the heat of the solution.

With respect to the substances contained in the stomach, only two facts have been perfectly ascertained: The first is, that the juice contained in the stomach of oxen, calves, sheep, invariably contains uncombined phosphoric acid, as Macquart and Vauquelin have demonstrated. The second, that the juice contained in the stomach, and even the inner coat of the stomach itself, has the property of coagulating milk and the serum of blood. Dr Young found, that seven grains of the inner coat of a calf's stomach, infused in water, gave a liquid which coagulated more than 100 ounces of milk; that is to say, more than 6857 times its own weight; and yet, in all probability, its weight was not much diminished.

What the substance is which possesses this coagulating property, has not yet been ascertained; but it is evidently not very soluble in water; for the inside of a calf's stomach, after being steeped in water for six hours, and then well washed with water, still furnishes a liquor on infusion which coagulates milk. And Dr Young found, that a piece of the inner coat of the stomach, after being previously washed with water, and then with a diluted solution of carbonat of potash, still afforded a liquid which coagulated milk and serum.

It is evident, from these facts, that this coagulating substance, whatever it is, acts very powerfully; and that it is scarcely possible to separate it completely from the stomach. But we know at present too little of the nature of coagulation to be able to draw any inference from these facts. An almost imperceptible quantity of some substances seems to be sufficient to coagulate milk. For Mr Vaillant mentions in his Travels in Africa, that a porcelain dish which he procured, and which had lain for some years at the bottom of the sea, possessed, in consequence, the property of coagulating milk when put into it; yet it communicated no taste to the milk, and did not differ in appearance from other cups.

It is probable that the saliva is of service in the conversion of food into chyme as well as the gastric juice. It evidently serves to dilute the food; and probably it may be serviceable also, by communicating oxygen.

3. The chyme, thus formed, passes from the stomach into the intestines, where it is subjected to new changes; converted and at last converted into two very different substances, chyle and excrementitious matter.

4. The chyle is a white coloured liquid, very much resembling milk. It is exceedingly difficult to collect it in any considerable quantity, and for that reason it has never been accurately analysed. We know only in general that it resembles milk; containing, like it, an albuminous part capable of being coagulated, a serum, and globules which have a resemblance to cream. It contains also different salts; and, according to some, a substance scarcely differing from the sugar of milk. It is probable also that it contains iron; but if so, it must be... 6. Concerning the process by which chyle is formed from chyme, scarcely anything is known. It does not appear that the chyme is precisely the same in all animals; for those which are herbivorous have a greater length of intestine than those which are carnivorous.

It is certain that the formation of the chyle is brought about by a chemical change, although we cannot say precisely what that change is, or what the agents are by which it is produced. But that the change is chemical, is evident, because the chyle is entirely different, both in its properties and appearance, from the chyme. The chyme, by the action of the intestines, is separated into two parts, chyle and excrement: the first of which is absorbed by a number of small vessels called lacteals; the second is pushed along the intestinal canal, and at last thrown out of the body altogether.

After the chyme has been converted into chyle and excrement, although these two substances remain mixed together, it does not appear that they are able to decompose each other; for persons have been known seldom or never to emit any excrementitious matter per anum for years. In these, not only the chyle, but the excrementitious matter also, was absorbed by the lacteals; and the excrement was afterwards thrown out of the body by other outlets, particularly by the skin: in consequence of which, those persons have constantly that particular odour about them which distinguishes excrement. Now in these persons, it is evident that the chyle and excrement, though mixed together, and even absorbed together, did not act on each other; because these persons have been known to enjoy good health for years, which could not have been the case had the chyle been destroyed.

7. It has been supposed by some that the decomposition of the chyme, and the formation of chyle, is produced by the agency of the bile, which is poured out abundantly, and mixed with the chyme, soon after its entrance into the intestines. If this theory were true, no chyle could be formed whenever any accident prevented the bile from passing into the intestinal canal: but this is obviously not true; for frequent instances have occurred of persons labouring under jaundice from the bile ducts being stopped, either by gallstones or some other cause, so completely, that no bile could pass into the intestines; yet these persons have lived for a considerable time in that state. Consequently digestion, and therefore the formation of chyle, must be possible, independent of bile.

The principal use of the bile seems to be to separate the excrement from the chyle, after both have been formed, and to produce the evacuation of the excrement out of the body. It is probable that these substances would remain mixed together, and that they would perhaps even be partly absorbed together, were it not for the bile, which seems to combine with the excrement, and by this combination to facilitate its separation from the chyle, and thus to prevent its absorption. It also stimulates the intestinal canal, and causes it to evacuate its contents sooner than it otherwise would do; for when there is a deficiency of bile, the body is constantly constipated.

8. The excrement, then, which is evacuated per anum, consists of all that part of the food and chyme which was not converted into chyle, entirely altered however from its original state, partly by the decomposition which it underwent in the stomach and intestines, and partly by its combination with bile. Accordingly we find in it many substances which did not exist at all in the food. Thus in the dung of cows and horses there is found a very considerable quantity of benzoic acid. The excrements of animals have not yet been subjected to an accurate analysis, though such an analysis would throw much light upon the nature of digestion. For if we knew accurately the substances which were taken into the body as food, and all the new substances which were formed by digestion; that is to say, the component parts of chyle and of excrement, and the variation which different kinds of food produce in the excrement, it would be a very considerable step towards ascertaining precisely the changes produced on food by digestion, or, which is the same thing, towards ascertaining exactly the phenomena of digestion. The only analysis which has hitherto been made on human excrement is that of Homberg; and as it consisted merely in subjecting it to distillation, it is needless to give an account of it. Of late, as Mr. Fourcroy informs us, the subject has been resumed in France, and we may soon expect some very curious and important additions to our knowledge.

Mr. Vaquelin has already published an analysis of the fixed parts of the excrements of fowls, and a comparison of them with the fixed parts of the food; from which some very curious consequences may be deduced.

He found that a hen devoured in ten days 11111.843 grains troy of oats. These contained

- 136.509 gr. of phosphat of lime, - 219.548 silica,

356.057.

During these ten days she laid four eggs; the shells of which contained 94.776 gr. phosphat of lime, and 453.417 gr. carbonat of lime. The excrements emitted during these ten days contained 175.529 gr. phosphat of lime, 184.494 gr. of carbonate of lime, and 185.266 gr. of silica. Consequently the fixed parts thrown out of the system during these ten days amounted to

- Grams: - 274.305 phosphat of lime, - 511.911 carbonat of lime, - 185.266 silica,

Given out 971.482 Taken in 356.057

615.425

Consequently the quantity of fixed matter given out of the system in ten days exceeded the quantity taken in by 615.425 grams.

The silica taken in amounted to 219.548 gr. That given out was only 185.266 gr.

Remains 34.282

Consequently there disappeared 34.282 grams of silica.

The phosphat of lime taken in was 136.509 gr. That given out was 274.305 gr.

137.796

Consequently Consequently there must have been formed, by digestion in this fowl, no less than 137,796 grains of phosphat of lime, besides 511,911 grains of carbonat. Consequently lime (and perhaps also phosphorus) is not a simple substance, but a compound, and formed of ingredients which exist in oat-feed, water, or air, the only substance to which the fowl had access. Silica may enter into its composition, as a part of the silica had disappeared; but if so, it must be combined with a great quantity of some other substance.

These consequences are too important to be admitted without a very rigorous examination. The experiment must be repeated frequently, and we must be absolutely certain that the hen has no access to any calcareous earth, and that she has not diminished in weight; because in that case some of the calcareous earth, of which part of her body is composed, may have been employed. This rigour is the more necessary, as it seems pretty evident, from experiments made long ago, that some birds at least cannot produce eggs unless they have access to calcareous earth. Dr. Fordyce found, that if the canary-bird was not supplied with lime at the time of her laying, she frequently died, from her eggs not coming forward properly. He divided a number of these birds at the time of their laying eggs into two parties: to one he gave a piece of old mortar, which the little animals swallowed greedily; they laid their eggs as usual, and all of them lived; whereas many of the other party, which were supplied with no lime, died.

The intestines seldom or never are destitute of gases, which seem to be evolved during the process of digestion; and may therefore, in part, be considered as excrementitious matter. The only person who has examined these gases with care, is Mr. Jurine of Geneva. The result of his analysis is as follows. He found in the stomach and intestines of a man who had been frozen to death, carbonic acid gas, oxygen gas, hydrogen gas, and azotic gas. The quantity of carbonic acid was greatest in the stomach, and it diminished gradually as the canal receded from the stomach; the proportion of oxygen gas was considerable in the stomach, smaller in the small intestines, and still smaller in the great intestines; the hydrogen and azotic gases, on the contrary, were least abundant in the stomach, more abundant in the small intestines, and most abundant in the larger intestines; the hydrogen gas was most abundant in the small intestines. It is well known that the flatus discharged per anum is commonly carbonated hydrogen gas; sometimes also it seems to hold sulphur, or even phosphorus in solution.

The chyle, after it has been absorbed by the lacteals, is carried by them into a pretty large vessel, known by the name of thoracic duct. Into the same vessel likewise is discharged a transparent fluid, conveyed by a set of vessels which arise from all the cavities of the body. These vessels are called lymphatics, and the fluid which they convey is called lymph. In the thoracic duct, then, the chyle and the lymph are mixed together.

Very little is known concerning the nature of the lymph, as it is scarcely possible to collect it in any quantity. It is colourless, has some viscosity, and is said to be specifically heavier than water. It is said to be coagulable by heat; if so, it contains albumen; and, from its appearance, it probably contains gelatine. Its quantity is certainly considerable, for the lymphatics are very numerous.

The chyle and lymph being thus mixed together, are conveyed directly into the blood vessels. The effect produced by their union in the thoracic duct is not known, but neither the colour nor external properties of the chyle is altered. In man, and many other animals, the thoracic duct enters at the junction of the left subclavian and carotid veins, and the chyle is conveyed directly to the heart, mixed with the blood, which already exists in the blood vessels. From the heart, the blood and chyle thus mixed together are propelled into the lungs, where they undergo farther changes.

The absolute necessity of respiration, or of some respiration analogous, is known to every one; and few are ignorant that in man, and hot blooded animals, the organ by which respiration is performed is the lungs. For a description of the respiratory organs, we refer to the article Anatomy, Encycl., and the reader will find an account of the manner in which that function is performed in the article Physiology, Encycl. But what are the changes produced upon the blood and the chyle by respiration? What purposes does it serve to the animal? How comes it to be so indispensably necessary for its existence? These are questions which can only be answered by a careful examination of the phenomena of respiration.

It has been long known that an animal can only breathe a certain quantity of air for a limited time, after which it becomes the most deadly poison, and produces suffocation as effectually as the most noxious gas, or a total absence of air. It was suspected long ago that this change is owing to the abstraction of a part of the air; and Mayow made a number of very ingenious experiments in order to prove the fact. Dr. Priestley and Mr. Scheele demonstrated, that the quantity of oxygen gas in atmospheric air is diminished; and Lavoisier demonstrated, in 1776, that a quantity of carbonic acid gas, which did not previously exist in it, was found in air after it had been for some time respired. It was afterwards proved by Lavoisier, and many other philosophers, who confirmed and extended his facts, that no animal can live in air totally destitute of oxygen. Even fish, which do not generally respire, die very soon, if the water in which they live be deprived of oxygen gas. Frogs which can suspend their respiration at pleasure, die in about forty minutes, if the water in which they have been confined be covered over with oil. Insects and worms, as Vaquelin has proved, exhibit precisely the same phenomena. They require oxygen gas as well as other animals, and die like them if deprived of it. They diminish the quantity of oxygen gas in which they live, and give out, by respiration, the very same products as other animals. Worms, which are more retentive of life than most other animals, or at least not so much affected by poisonous gases, absorb every particle of the oxygen gas contained in the air in which they are confined before they die. Mr. Vaquelin's experiments were made on the grillus viridiflumus, the limax flavus, and helix pomatia.

The changes which take place during respiration are:

1. Part of the oxygen gas respired disappears. 2. Carbonic acid is produced. 2. Carbonic acid gas is emitted. 3. Water is emitted in the state of vapour.

The first point is to ascertain exactly the amount of these changes. Though a great many experiments have been made on this subject by different philosophers, the greatest confidence ought to be put in those of Lavoisier, both on account of his uncommon accuracy, and on account of the very complete apparatus which he always employed.

He put a guinea-pig into 708,980 grains troy of oxygen, and after the animal had breathed the gas for an hour, he took it out. He found that the oxygen gas now amounted only to 592,253 gr. Consequently there had disappeared 116,736. The carbonic acid gas formed was 130,472. This was composed of about 94,234 oxygen, and 36,238 of carbon. Consequently supposing, as Mr. Lavoisier did, that the oxygen absorbed had been employed in the formation of the carbonic acid gas, there still remained to be accounted for 22,502 grains of oxygen which had disappeared. He supposed that this had been employed in the formation of water, a quantity of which had appeared. If so, the water formed must have amounted to 26,429 grains; which was composed of 3,917 hydrogen, the rest oxygen*.

Since the water emitted was not actually ascertained, this experiment can only be considered as an approximation to the truth. Accordingly that very ingenious philosopher contrived an apparatus to ascertain the quantity of oxygen gas absorbed by man, and the quantity of carbonic acid gas and water emitted by him during respiration. This apparatus he had constructed at an expense at least equal to L.500 sterling.

The experiments were completed, and he was preparing them for publication, when, on the 8th of May 1794, he was beheaded by order of Robespierre, after having vainly requested a fortnight's delay to put his papers in order for the press. Thus perished, in the 51st year of his age, the man who, if he had lived a few years longer, promised fair to become the rival of Newton himself. Chemistry, as a science, is deeply indebted to him. He saved it from that confusion into which the thoughtless ardour of many of his contemporaries were plunging it headlong: he arranged and connected and simplified and explained the multitude of insulated facts, which had been accumulating with unexampled celerity; and which, had it not been for his happy arranging genius, might have retarded, instead of advanced, the progress of the science. He reduced all the facts under a few simple heads, and thus made them easily remembered and easily classified. In a few years more, perhaps, he would have traced their general principles to their sources, established the science on the complete induction, and paved for his successors a road as unerring as that which Sir Isaac Newton formed in mechanical philosophy.

Mr. Lavoisier's experiments have never been published, but fortunately Mr. de la Place has given us the result of them†. He informs us that it was as follows: A man, at an average, consumes, in twenty-four hours, by respiration, 32,48437 ounces troy of oxygen gas; that is to say, that a quantity of oxygen gas, equal to that weight, disappears from the air which he respires in twenty-four hours; that he gives out by respiration, in the same time, 15.73 oz. troy of carbonic acid gas, and 28.55 oz. of water in the state of vapour.

Total 44.28 Oxygen.

The carbonic acid gas is composed of 10.486 and 5.243 carbon. The water of 24.2675 and 4.2825 hydrogen.

Total of the oxygen emitted: 34.75416

Total absorbed: 32.48437

So that there is 2,369,7916 ounces of oxygen emitted more than is absorbed by respiration. Thus it appears that, by respiration, the absolute quantity of oxygen in the blood is diminished.

Dr. Menzies found that a man, at a medium, draws in at every respiration 43.77 cubic inches of air, and that 1/3rd of that quantity disappears. Consequently, according to him, at every respiration 2.1885 cubic inches of oxygen gas are consumed. Now 2.1885 cubic inches of that gas amount to 0.68669 gr. troy.

Supposing, with Hales, that a man makes 1200 respirations in an hour, the quantity of oxygen gas consumed in an hour, will amount to 824,028 grains, and in 24 hours to 19775,672 grains, or 41,214 ounces troy. This quantity exceeds that found by Lavoisier considerably; but the allowance of oxygen for every respiration is rather too great. Indeed, from the nature of Dr. Menzies's apparatus, it was scarce possible to measure it accurately.

The quantity of water given out by respiration, as determined by Hales, amounts in a day to 20.4 oz.* but his method was not susceptible of great accuracy. We may therefore, on the whole, consider Lavoisier's determination as by far the nearest to the truth of any that has been given.

There is, however, a very singular anomaly, which becomes apparent when we compare his experiments on the respiration of the guinea-pig with those on the respiration of man.

The guinea-pig consumed in 24 hours 5.8368 oz. troy of oxygen gas, and emitted 6.5246 oz. of carbonic acid gas. Man, on the other hand, consumes in the same time 32.48437 oz. of oxygen gas, and emits only 15.73 oz. of carbonic acid gas. The oxygen gas consumed by the pig is to the carbonic gas emitted as 1:00 = 1:12; whereas in man it is as 1:000 = 0:484. If we could depend upon the accuracy of each of these experiments, they would prove, beyond a doubt, that the changes produced by the respiration of the pig are different, at least in degree, from those produced in man; but it is more than probable that some mistake has crept into one or other of the experiments. We have more reason to suspect the first, as it was made before 1778, at a time when a great many circumstances, necessary to insure accuracy, were unknown to Lavoisier.

Such are the substances imbibed and emitted during respiration. It still remains for us to determine what are the changes which it produces on the blood.

It has been long known that the blood which flows in the veins is of a dark reddish purple colour, whereas the arterial blood is of a florid roselet colour. Lower observed that the colour of the venous blood was converted into that of arterial during its passage through the No chyle can be distinguished by its white colour in the blood after it has passed through the lungs. The changes, then, which take place upon the appearance of the blood are two: 1st, It acquires a florid red colour; 2nd, The chyle totally disappears. Now to what are these changes owing?

Lower himself knew that the change was produced by the air, and Mayow attempted to prove that it was by absorbing a part of the air. But it was not till Dr Priestley discovered that venous blood acquires a scarlet colour when put in contact with oxygen gas, and arterial blood a dark red colour when put in contact with hydrogen gas, or, which is the same thing, that oxygen gas instantly gives venous blood the colour of arterial; and hydrogen, on the contrary, gives arterial blood the colour of venous blood; it was not till then that philosophers began to attempt any thing like an explanation of the phenomena of respiration. Two explanations have been given; one or other of which must be true.

The first is, that the oxygen of the air, which disappears, combines with a quantity of carbon and hydrogen given out by the blood in the lungs, and forms with it carbonic acid gas and water in vapour, which are thrown out along with the air expired.

The second is, that the oxygen gas, which disappears, combines with the blood as it passes thro' the lungs; and that, at the instant of this combination, there is let free from the blood a quantity of carbonic acid gas and of water, which are thrown out along with the air expired.

The first of these theories was originally formed by Lavoisier, and it was embraced by La-Place, Crawford, Gren, and Girtanner, with a small variation. Indeed it does not differ, except in detail, from the original hypothesis of Dr Priestley, that the use of respiration is to rid the blood of phlogiston; for if we substitute carbon and hydrogen for phlogiston, the two theories precisely agree. Mr Lavoisier attempted not to prove its truth; he only tried to shew that the oxygen absorbed corresponds exactly with the quantity of oxygen contained in the carbonic acid and the water emitted. This coincidence his own experiments have shown not to hold; consequently the theory is entirely destitute of proof, as far as the proof depends upon this coincidence.

The other hypothesis was proposed by Mr de la Grange, and afterwards supported and illustrated by Mr Hassenfratz.

In order to discover what the real effects of respiration are, let us endeavour to state accurately the phenomena as far as possible.

In the first place, we are certain, from the experiments of Priestley, Girtanner, and Hassenfratz, that when venous blood is exposed to oxygen gas confined over it, the blood instantly assumes a scarlet colour, and the gas is diminished in bulk; therefore part of the gas has been absorbed. We may consider it as certain, then, that when the colour of venous blood is changed into arterial, some oxygen gas is absorbed.

In the second place, no chyle can be discovered in the blood after it has passed through the lungs. Therefore the white colour of the chyle at least, is destroyed by respiration, and it assumes a red colour. Now if the red colour of the blood be owing to iron, as many have supposed, this change of colour is a demonstration that oxygen has combined with the iron; for we have seen already, that iron, if it exists in chyle, as it probably does, is in the state of a white oxyd. Consequently, when converted into a red oxyd, it must absorb oxygen. Even though iron be not the colouring matter of the blood, it would still be probable that the change of colour of the chyle depends on the fixation of oxygen; for Berthollet and Fourcroy have shown that in several instances substances acquire a red colour by that process.

We may consider it as proved, then, that oxygen enters the blood as it passes through the lungs.

In the third place, when arterial blood is put in contact with azotic gas, or carbonic acid gas, it gradually assumes the dark colour of venous blood, as Dr Priestley found. The same philosopher also observed that arterial blood acquired the colour of venous blood when placed in vacuo. Consequently this alteration of colour is owing to some change which takes place in the blood itself, independent of any external agent.

The arterial blood becomes much more rapidly and deeply dark coloured when it is left in contact with hydrogen gas placed above it. We must suppose therefore that the presence of this gas accelerates and increases the change, which would have taken place upon the blood without any external agent.

If arterial blood be left in contact with oxygen gas, it gradually assumes the same dark colour which it would have acquired in vacuo, or in contact with hydrogen; and after this change oxygen can no longer restore its scarlet colour. Therefore it is only upon a part of the blood that the oxygen acts; and after this part has undergone the change which occasions the dark colour, the blood loses the power of being affected by oxygen.

Mr Hassenfratz poured into venous blood a quantity of oxy-muriatic acid; the blood was instantly decomposed, and assumed a deep and almost black colour. When he poured common muriatic acid into blood, the colour was not altered. Now oxy-muriatic acid has the property of giving out its oxygen readily; consequently the black colour was owing to the instant combination of a part of the blood with oxygen.

The facts therefore lead us to conclude, with La Grange and Hassenfratz, that during respiration the oxygen, which disappears, enters the blood; that during the circulation this oxygen combines with a certain part of the blood; and that the venous colour is owing to this new combination. We must conclude, too, that the substance which causes this dark colour leaves the blood during its circulation thro' the lungs, otherwise it could not be capable of affuming the florid colour. Now we know what the substances are which are emitted during respiration; they are water and carbonic acid gas. It must be to the gradual combination of oxygen, then, during the circulation, with hydrogen and carbon, that the colour of venous blood is owing. And since the same combination takes place every time that the blood passes through the lungs, we must conclude, that it is only a part of the hydrogen and carbon which is acted upon each time. Let us now attempt, with these data, to form some notion of the decomposition which goes on during the circulation of the blood.

It is probable that, during a considerable part of the day, there is a constant influx of chyle into the blood, and we are certain that lymph is constantly flowing in blood. Now it appears, from the most accurate observations hitherto made, that neither chyle nor lymph contain fibrina, which forms a very conspicuous part of the blood. This fibrina is employed to supply the waste of the muscles, the most active parts of the body, and therefore, in all probability, requiring the most frequent supply. Nor can it be doubted that it is employed for other useful purposes. The quantity of fibrina in the blood, then, must be constantly diminishing, and therefore new fibrina must be constantly formed. But the only substances out of which it can be formed are the chyle and lymph, neither of which contain it. There must therefore be a continual decomposition of the chyle and lymph going on in the blood-vessels, and a continual new formation of fibrina. Other substances also may be formed; but we are certain that this must be formed there, because it does not exist previously. Now, one great end of respiration must undoubtedly be to assist this decomposition of chyle and complete formation of blood.

It follows, from the experiments of Fourcroy formerly enumerated, that fibrina contains more azot, and less hydrogen and carbon, than any of the other ingredients of the blood, and consequently also than any of the ingredients of the chyle. In what manner the chyle, or a part of it, is converted into fibrina, it is impossible to say: we are not sufficiently acquainted with the subject to be able to explain the process. But we can see at least, that carbon and hydrogen must be abstracted from that part of the chyle which is to be converted into fibrina: And we know, that these substances are actually thrown out by respiration. We may conclude, then, that one use of the oxygen absorbed is, to abstract a quantity of carbon and hydrogen from a part of the chyle by compound affinity, in such proportions, that the remainder becomes fibrina: therefore one end of respiration is to form fibrina. Doubtless the other ingredients of the blood are also new modified, though we know too little of the subject to throw any light upon it.

But the complete formation of blood is not the only advantage gained by respiration: the temperature of all animals depends upon it. It has been long known, that those animals which do not breathe have a temperature but very little superior to the medium in which they live. This is the case with fishes and many insects. Man, on the contrary, and quadrupeds which breathe, have a temperature considerably higher than the atmosphere: that of man is 98°. Birds, who breathe in proportion a still greater quantity of air than man, have a temperature equal to 103° or 104°. It has been proved, that the temperature of all animals is proportional to the quantity of air which they breathe in a given time.

These facts are sufficient to demonstrate, that the heat of animals depends upon respiration. But it was not till Dr Black's doctrine of latent heat became known to the world, that any explanation of the cause of the temperature of breathing animals was attempted. That illustrious philosopher, whose discoveries form the basis upon which all the scientific part of chemistry has been reared, saw at once the light which his doctrine of latent heat threw upon this part of physiology, and he applied it very early to explain the temperature of animals.

According to him, part of the latent heat of the air inspired becomes sensible; and of course, the temperature of the lungs, and the blood that passes through them, must be raised; and the blood, thus heated, communicates its heat to the whole body. This opinion was ingenious, but it was liable to an unanswerable objection: for if it were true, the temperature of the body ought to be greatest in the lungs, and to diminish gradually as the distance from the lungs increases; which is not true. The theory, in consequence, was abandoned even by Dr Black himself; at least he made no attempt to support it.

Lavoisier and Crawford, who considered all the changes operated by respiration as taking place in the lungs, accounted for the origin of the animal heat almost precisely in the same manner with Dr Black. According to them, the oxygen gas of the air combines in the lungs with the hydrogen and carbon emitted by the blood. During this combination, the oxygen gives out a great quantity of caloric, with which it had been combined; and this caloric is not only sufficient to support the temperature of the body, but also to carry off the new formed water in the state of vapour, and to raise considerably the temperature of the air inspired. According to these philosophers, then, the whole of the caloric which supports the temperature of the body is evolved in the lungs. Their theory accordingly was liable to the same objection with Dr Black's; but they obviated it in the following manner: Dr Crawford found, that the specific caloric of arterial blood was 1.0352, while that of venous blood was only 0.8928. Hence he concluded, that the instant venous blood is changed into arterial blood, its specific caloric increases; consequently it requires an additional quantity of caloric to keep its temperature as high as it had been while venous blood. This addition is so great, that the whole new caloric evolved is employed: therefore the temperature of the lungs must necessarily remain the same as that of the rest of the body. During the circulation, arterial blood is gradually converted into venous; consequently its specific caloric diminishes, and it must give out heat. This is the reason that the temperature of the extreme parts of the body does not diminish.

This explanation is certainly ingenious; but it is not quite satisfactory; for the difference in the specific calorics, granting it to be accurate, is too small to account for the great quantity of heat which must be evolved. It is evident that it must fail to the ground altogether, provided, as we have seen reason to suppose, the carbonic acid gas and water be not formed in the lungs, but during the circulation.

Since the oxygen enters the blood, and combines with it in the state of gas, it is evident that it will only part at first with some of its caloric; and this portion is chiefly employed in carrying off the carbonic acid gas and the water. For the reason that the carbonic acid leaves the blood at the instant that the oxygen gas enters it, seems to be this: The oxygen gas combines with the blood, and part of its caloric unites at the same instant to the carbonic acid, and converts it into gas; another portion converts the water into vapour. The rest of the caloric is evolved during the circulation when the oxygen combines with hydrogen and carbon, and forms water and carbonic acid gas. The quantity of caloric evolved in the lungs seems not only sufficient to carry off the carbonic acid and water, which the diminution Animal Substances

The motion of the specific calorie (if it really take place) must facilitate; but it seems also to raise the temperature of the blood a little higher than it was before.

For Mr John Hunter constantly found, that the heat of the heart in animals was a degree higher than any other part of the body which he examined. Now this could scarcely happen, unless the temperature of the blood were somewhat raised during respiration.

Thus we have seen two uses which respiration seems to serve. The first is the completion of blood by the formation of fibrin; the second is the maintaining of the temperature of the body at a particular standard, notwithstanding the heat which it is continually giving out to the colder surrounding bodies. But there is a third purpose, which explains why the animal is killed so suddenly when respiration is stopped. The circulation of the blood is absolutely necessary for the continuance of life. Now the blood is circulated in a great measure by the alternate contractions of the heart. It is necessary that the heart should contract regularly, otherwise the circulation could not go on. But the heart is stimulated to contract by the blood; and unless blood be made to undergo the change produced by respiration, it ceases almost instantaneously to stimulate.

As the blood receives oxygen in the lungs, we may conclude that the presence of oxygen is necessary to its stimulating power.

Thus we have reason to suppose, that chyle and lymph are converted into blood during the circulation; and that the oxygen gas supplied by respiration is one of the principal agents in this change. But besides the lungs and arteries, there is another organ, the sole use of which is also to produce some change or other in the blood, which renders it more complete, and more proper for the various purposes to which it is applied. This organ is the kidney.

For the structure of the kidneys, which in man and quadrupeds are two in number, we refer to Anatomy. A very great proportion of blood passes through them; indeed, we have every reason to conclude, that the whole of the blood passes through them very frequently.

These organs separate the urine from the blood, to be afterwards evacuated without being applied to any purpose useful to the animal.

The kidneys are absolutely necessary for the continuance of the life of the animal; for it dies very speedily when they become by disease unfit to perform their functions; therefore, the change which they produce in the blood is a change necessary for qualifying it to answer the purposes for which it is intended.

As the urine is immediately excreted, it is evident that the change which the kidneys perform is intended solely for the sake of the blood. It is not merely the abstraction of a quantity of water and of salts, accumulated in the blood, which the kidney performs. A chemical change is certainly produced, either upon the whole blood, or at least on some important part of it; for there are two substances found in the urine which do not exist in the blood. These two substances are urea and uric acid. They are formed, therefore, in the kidneys; and as they are thrown out, after being formed, without being applied to any useful purpose, they are certainly not formed in the kidneys for their own sake. Some part of the blood, then, must be decomposed in the kidney, and a new substance, or new functions substances, must be formed; and the urea and uric acid must be formed at the same time, in consequence of the combined action of the affinities which produce the change on the blood; and being useless, they are thrown out, together with a quantity of water and salts, which, in all probability, were useful in bringing about the changes which take place in the arteries and in the kidneys, but which are no longer of any service after these changes are brought about.

The changes operated upon the blood in the kidneys are hitherto altogether unknown; but they must be important.

Provided the method of analyzing animal substances were so far perfected as to admit of accurate conclusions, considerable light might be thrown upon this subject, by analyzing with care a portion of blood from the emulsified vein and artery separately, and ascertaining precisely in what particulars they differ from each other.

Thus we have seen that the principal changes which the blood undergoes, as far at least as we are at present acquainted with them, take place in the lungs, in the kidneys, and in the arteries. In the lungs, a quantity of water and carbonic acid gas is emitted from the blood, and in the kidney the urine is formed and separated from it. There seems also to be something thrown out from the blood during its circulation in the arteries, at least through those vessels which are near the surface of the body: For it is a fact, that certain substances are constantly emitted from the skins of animals. These substances are known in general by the name of perspirable matter, or perspiration. They have a great resemblance to what is emitted in the lungs; which renders it probable, that they are both owing to the same cause; namely, to the decomposition produced in the blood by the effects of respiration. They consist chiefly of water in a state of vapour, carbon, and oil.

The quantity of aqueous vapour differs very considerably, according to circumstances. It has been shown our vapours to be greatest in hot weather, and in hot climates, and after great exercise; and its relation to the quantity of urine has been long known. When the aqueous vapour perspired is great, the quantity of urine is small, and vice versa.

The most accurate experiments on this matter that we have seen are those of Mr Cruikshank. He put his hand into a glass vessel, and noted its mouth at his wrist by means of a bladder. The interior surface of the vessel became gradually dim, and drops of water trickled down. By keeping his hand in this manner for an hour, he collected 30 grains of a liquid, which possessed all the properties of pure water. On repeating the same experiment at nine in the evening (thermometer 62°), he collected only 12 grains. The mean of these results is 21 grains. But as the hand is more exposed than the trunk of the body, it is reasonable to suppose that the perspiration from it is greater than that from the hand. Let us therefore take 30 grains per hour as the mean; and let us suppose, with Mr Cruikshank, that the hand is 1/4th of the surface of the body. The perspiration in an hour would amount to 1800 grains, and in 24 hours to 43200 grains, or 7 pounds 6 ounces troy. He repeated the experiment again after hard exercise, and collected in an hour 48 grains of water*. He found also, that this aqueous vapour pervaded his flocking without difficulty; and that it made its way thro' a flannel leather glove, and even through a leather boot, though in much smaller quantity than when the leg wanted that covering†.

It is not difficult to see why the quantity of watery vapour diminishes with cold. When the surface of the body is exposed to a cold temperature, the capacity of the cutaneous vessels diminishes, and consequently the quantity which flows through them must decrease.

When the temperature, on the other hand, is much increased, either by being exposed to a hot atmosphere, or by violent exercise, the perspired vapour not only increases in quantity, but even appears in a liquid form. This is known by the name of sweat. In what manner sweat is produced, is not at present known; but we can see a very important service which it performs to the animal.

No sooner is it thrown upon the surface of the skin than it begins to evaporate. But the change into vapour requires heat; accordingly a quantity of heat is absorbed, and the temperature of the animal is lowered. This is the reason that animals can endure to remain for some time in a much higher temperature without injury than could have been supposed.

The experiments of Fillet, and the still more decisive experiments of Fordyce and his associates, are well known. These gentlemen remained a considerable time in a temperature exceeding the boiling point of water.

Besides water, it cannot be doubted that carbon is also emitted from the skin; but in what state, the experiments hitherto made do not enable us to decide. Mr Cruikshank found, that the air of the glass vessel in which his hand and foot had been confined for an hour, contained carbonic acid gas; for a candle burned dimly in it, and it rendered lime-water turbid*. And Mr Jurine found, that air which had remained for some time in contact with the skin, consisted almost entirely of carbonic acid gas†. The same conclusion may be drawn from the experiments of Ingenhousz and Millay‡.

Now it is evident, that the carbonic acid gas which appeared during Mr Cruikshank's experiment, did not previously exist in the glass vessel; consequently it must have either been transmitted ready formed through the skin, or formed during the experiment by the absorption of oxygen gas, and the consequent emission of carbonic acid gas. The experiments of Mr Jurine do not allow us to suppose the first of these to be true; for he found, that the quantity of air allowed to remain in contact with the skin did not increase. Consequently the appearance of the carbonic acid gas must be owing either to the emission of carbon, which forms carbonic acid gas by combining with the oxygen gas of the air, or to the absorption of oxygen gas, and the subsequent emission of carbonic acid gas; precisely in the same manner, and for the same reason, that these substances are emitted by the lungs. The last is the more probable opinion; but the experiments hitherto made do not enable us to decide.

Besides water and carbon, or carbonic acid gas, the skin emits also a particular odoriferous substance. That every animal has a peculiar smell, is well known: the dog can discover his master, and even trace him to a distance by the scent. A dog, chained some hours after his master had set out on a journey of some hundred miles, followed his footsteps by the smell, and found him on the third day in the midst of a crowd*. But it is needful to multiply instances of this fact; they are too well known to every one. Now this smell must be owing to some peculiar matter which is constantly emitted; and this matter must differ somewhat either in quantity or some other property, as we see that the dog easily distinguishes the individual by means of it. Mr Cruikshank has made it probable that this matter is an oily substance; or at least that there is an oily substance emitted by the skin. He wore repeatedly, night and day for a month, the same sort of fleecy hosiery during the hottest part of the summer. At the end of this time he always found an oily substance accumulated in considerable masses on the nap of the inner surface of the vest, in the form of black tears. When rubbed on paper, it makes it transparent, and hardens on it like grease. It burns with a white flame, and leaves behind it a charry residuum†.

It has been supposed that the skin has the property of absorbing moisture from the air; but this opinion has not been confirmed by experiments, but rather the contrary.

The chief arguments in favour of the absorption of whatever the skin, have been drawn from the quantity of moisture discharged by urine being, in some cases, not only greater than the whole drink of the patient, but even than the whole of his drink and food. But it ought to be remembered that, in diabetes, the disease here alluded to, the weight of the body is continually diminishing, and therefore part of it must be constantly thrown off. Besides, it is scarcely possible in that disease to get an accurate account of the food swallowed by the patients; and in those cases where very accurate accounts have been kept, and where deception was not to much practised, the urine was found not to exceed the quantity of drink*. In a case of diabetes, related with much accuracy by Dr Gerard, the patient was bathed regularly during the early part of the disease in warm water, and afterwards in cold water; he was weighed before and after bathing, and no sensible difference was ever found in his weight†. Consequently, in that case, the quantity absorbed, if any, must have been very small.

It is well known, that thirst is much alleviated by cold bathing. By this plan, Captain Bligh kept his men cool and in good health during their very extraordinary voyage across the South Sea. This has been considered as owing to the absorption of water by the skin. But Dr Currie had a patient who was suffering for want of nourishment, a tumor in the esophagus preventing the possibility of taking food, and whose thirst was always alleviated by bathing; yet no sensible increase of weight, but rather the contrary, was perceived after bathing. It does not appear, then, that in either of these cases water was absorbed.

Further, Seguin has shown that the skin does not absorb water during bathing, by a still more complete experiment: He dissolved some mercurial salt in water, and found that the mercury produced no effect upon a person that bathed in the water, provided no part of the the cuticle was injured; but upon rubbing off a portion of the cuticle, the mercurial solution was absorbed, and the effects of the mercury became evident upon the body. Hence it follows irresistibly, that water, at least in the state of water, is not absorbed by the skin when the body is plunged into it, unless the cuticle be first removed.

This may perhaps be considered as a complete proof that no such thing as absorption is performed by the skin; and that therefore the appearance of carbonic acid gas, which takes place when air is confined around the skin, must be owing to the emission of carbon. But it ought to be considered, that although the skin cannot absorb water, this is no proof that it cannot absorb other substances; particularly, that it cannot absorb oxygen gas, which is very different from water. It is well known, that water will not pass through bladders, at least for some time; yet Dr Priestley found that venous blood acquired the colour of arterial blood from oxygen gas, as readily when these substances were separated by a bladder as when they were in actual contact. He found, too, that when gases were confined in bladders, they gradually lost their properties. It is clear from these facts, that oxygen gas can pervade bladders; and if it can pervade them, why may it not also pervade the cuticle? Nay, farther, we know from the experiments of Cruikshank, that the vapour perspired passes through leather, even when prepared so as to keep out moisture, at least for a certain time. It is possible, then, that water, when in the state of vapour, or when dissolved in air, may be absorbed, although water, while in the state of water, may be incapable of pervading the cuticle. The experiments, then, which have hitherto been made upon the absorption of the skin, are altogether insufficient to prove that air and vapour cannot pervade the cuticle; provided at least there be any facts to render the contrary supposition probable.

Now that there are such facts cannot be denied. We shall not indeed produce the experiment of Van Mons as a fact of that kind, because it is liable to objections, and at best is very indefinite. Having a patient under his care who, from a wound in the throat, was incapable for several days of taking any nourishment, he kept him alive during that time, by applying to the skin in different parts of the body, several times a day, a sponge dipped in wine or strong soup*. A fact mentioned by Dr Watson is much more important, and much more decisive. A lad at Newmarket, who had been almost starved in order to bring him down to such a weight as would qualify him for running a horse race, was weighed in the morning of the race day; he was weighed again just before the race began, and was found to have gained 30 ounces of weight since the morning; yet in the interval he had only taken a single glass of wine. Here absorption must have taken place, either by the skin, or lungs, or both. The difficulties in either case are the same; and whatever renders absorption by one probable, will equally strengthen the probability that absorption takes place by the other (n).

16. We have now seen the process of digestion, and the formation of blood, as far at least as we are acquainted with it. But to what purpose is this blood of animals employed, which is formed with so much care, and for the formation of which so great an apparatus has been provided? It answers two purposes. The parts of the body which the body is composed of, bones, muscles, ligaments, walls of membranes, &c., are continually changing. In youth, they are increasing in size and strength, and in mature age they are continually acting, and consequently continually liable to waste and decay. They are often exposed to accidents, which render them unfit for performing their various functions; and even when no such accident happens, it seems necessary for the health of the system that they should be every now and then renewed. Materials therefore must be provided for repairing, increasing, or renewing all the various organs of the body. Phosphat of lime and gelatin for the bones, fibrin for the muscles, albumen for the cattages and membranes, &c. Accordingly all these substances are laid up in the blood; and they are drawn from that fluid as from a storehouse whenever they are required. The process by which the different parts of the blood are made part of the various organs of the body is called assimilation.

Over the nature of assimilation the thickest darkness still hangs; there is no key to explain it, nothing to lead us to the knowledge of the instruments employed. Facts, however, have been accumulated in sufficient numbers to put the existence of the process beyond the reach of doubt. The healing, indeed, of every fractured bone, and every wound of the body, is a proof of its existence, and an instance of its action.

Every organ employed in assimilation has a peculiar office; and it always performs this office whenever it has materials to act upon, even when the performance of it is contrary to the interest of the animal. Thus, the stomach always converts food into chyme, even when the food is of such a nature that the process of digestion will be retarded rather than promoted by the change. If warm milk, for instance, or warm blood, be thrown into the stomach, they are always decomposed by that organ, and converted into chyme; yet these substances are much more nearly assimilated to the animal before the action of the stomach than after it. The same thing happens when we eat animal food.

On the other hand, a substance introduced into an organ employed in assimilation, if it has undergone precisely the change which that organ is fitted to produce, is not changed, acted upon by that organ, but passed on unaltered to the next assimilating organ. Thus it is the office of the intestines to convert chyme into chyle. Accordingly, whenever chyme is introduced into the intestines, they perform their office, and produce the usual change; but if chyle itself be introduced into the intestines, it is absorbed by the lacteals without alteration. The experiment, indeed, has not been tried with true chyle, because it is scarce possible to procure it in sufficient quantity; but when milk, which resembles chyle pretty accurately, is thrown into the jejunum, it is absorbed unchanged by the lacteals*.

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(n) The Abbé Fontana also found, that after walking in moist air for an hour or two, he returned home some ounces heavier than he went out; notwithstanding he had suffered considerable evacuation from a brisk purge purposely taken for the experiment. This increase, indeed, might be partly accounted for by the absorption of moisture by his clothes. Again, the office of the blood vessels, as assimilating organs, is to convert chyle into blood. Chyle, accordingly, cannot be introduced into the arteries without undergoing that change; but blood may be introduced from another animal without any injury, and consequently without undergoing any change. This experiment was first made by Lower, and it has since been very often repeated.

Also, if a piece of fresh muscular flesh be applied to the muscle of an animal, they adhere and incorporate without any change, as has been sufficiently established by the experiments of Mr J. Hunter. And Buvina has ascertained, that fresh bone may, in the same manner, be grafted on the bones of animals of the same or of different species.

In short, it seems to hold, at least as far as experiments have hitherto been made, that foreign substances may be incorporated with those of the body, provided they be precisely of the same kind with those to which they are added, whether fluid or solid. Thus chyle may be mixed with chyle, blood with blood, muscle with muscle, and bone with bone. The experiment has not been extended to the other animal substances, the nerves, for instance; but it is extremely probable that it would hold with respect to them also.

On the other hand, when substances are introduced into any part of the body which are not the same with that part, nor the same with the substance upon which that part acts; provided they cannot be thrown out readily, they destroy the part, and perhaps even the animal. Thus foreign substances introduced into the blood very soon prove fatal; and introduced into wounds of the flesh or bones, they prevent these parts from healing.

Although the different assimilating organs have the power of changing certain substances into others, and of throwing out the useless ingredients, yet this power is not absolute, even when the substances on which they act are proper for undergoing the change which the organs produce. Thus the stomach converts food into chyme, the intestines chyme into chyle, and the substances which have not been converted into chyle are thrown out of the body. If there happen to be present in the stomach and intestines any substance which, though incapable of undergoing the changes, at least, by the action of the stomach and intestines, yet has a strong affinity, either for the whole chyme and chyle, or for some particular part of it, and no affinity for the substances which are thrown out, that substance passes along with the chyle, and in many cases continues to remain chemically combined with the substance to which it is united in the stomach, even after that substance has been completely assimilated, and made a part of the body of the animal. Thus there is a strong affinity between the colouring matter of madder and phosphat of lime. Accordingly, when madder is taken into the stomach, it combines with the phosphat of lime of the food, passes with it through the lacteals and blood vessels, and is deposited with it in the bones, as was proved by the experiments of Duhamel. In the same manner milk, indigo, &c., when taken into the stomach, make their way into many of the secretions.

These facts shew us, that assimilation is a chemical process from beginning to end; that all the changes are produced according to the laws of chemistry; and that we can even derange the regularity of the process by introducing substances whose mutual affinities are too strong for the organs to overcome.

It cannot be denied, then, that the assimilation of food consists merely in a certain number of chemical decompositions which that food undergoes, and the consequent formation of certain new compound. But what are the agents employed in assimilation merely chemical agents? We cannot produce anything like these changes on the food out of the body, and therefore we must allow that they are the consequence of the action of the animal organs. But this action, it may be said, is merely the secretion of particular juices, which have the property of inducing the wished for change upon the food; and this very change would be produced out of the body, provided we could procure these substances, and apply them in proper quantity to the food. If this supposition be true, the specific action of the vessels consists in the secretion of certain substances; consequently the cause of this secretion is the real agent in assimilation. Now, can the cause of this secretion be shown to be merely a chemical agent? Certainly not. For in the stomach, where only this secretion can be thrown to exist, it is not always the same, but varies according to circumstances. Thus eagles at first cannot digest grain, but they may be brought to do it by perishing in making them use it as food. On the contrary, a lamb cannot at first digest animal food, but habit will also give it this power. In this case, it is evident that the gastric juice changes according to circumstances. Now this is so far from being a case of a chemical law, that it is absolutely incompatible with every such law. The agent in assimilation, then, is not a chemical agent, but one which acts upon different principles. It is true, indeed, that every step in the process is chemical; but the agent which regulates these chemical processes, which prevents them from acting, except in particular circumstances and on particular substances, and modifies this action according to circumstances, is not a mere chemical agent, but endowed with very different properties.

The presence and power of this agent will be still more evident, if we consider the immunity of the stomach of the living animal during the process of digestion. The stomach of animals is as fit for food as any other substance. The gastric juice, therefore, must have the same power of acting on it, and of decomposing it, that it has of acting on other substances; yet it is well known that the stomach is not affected by digestion while the animal retains life; though, as Mr Hunter ascertained, the very gastric juice which the living stomach secretes often dissolves the stomach itself after death. Now what is the power which prevents the gastric juice from acting on the stomach during life? Certainly neither a chemical nor mechanical agent, for these agents must still retain the same power after death. We must, then, of necessity conclude, that there exists in the animal an agent very different from chemical and mechanical powers, since it controls these powers according to its pleasure. These powers therefore in the living body are merely the servants of this superior agent, which directs them so as to accomplish always one particular end. This agent seems to regulate the chemical powers, chiefly by bringing only certain substances together which are to be decomposed, and by keeping at a distance those substances which would interfere with, or diminish, or spoil the product, or injure the organ. And we see that this separation is always attended to even when the substances are apparently mixed together. For the very same products are not obtained which would be obtained by mixing the same substances together out of the body that are produced by mixing them in the body; consequently all the substances are not left at full liberty to obey the laws of their mutual affinities. The superior agent, however, is not able to exercise an unlimited authority over the chemical powers; sometimes they are too strong for it; some substances accordingly, as madder, make their way into the system; while others, as arsenic, decompose and destroy the organs of the body themselves.

But it is not in digestion alone that this superior agent makes the most wonderful display of its power; it is in the last part of assimilation that our admiration is most powerfully excited. How comes it that the precise substances wanted are always carried to every organ of the body? How comes it that fibrina is always regularly deposited in the muscles, and phosphat of lime in the bones? And what is still more unaccountable, how comes it that prodigious quantities of some one particular substance are formed and carried to a particular place in order to supply new wants which did not before exist? A bone, for example, becomes dilated and unfit for the use of the animal; a new bone therefore is formed in its place, and the old one is carried off by the absorbents. In order to form this new bone, large quantities of phosphat of lime are deposited in a place where the same quantity was not before necessary. Now, who informs this agent that an unusual quantity of phosphat of lime is necessary, and that it must be carried to that particular place? Or granting, as is most probable, that the phosphat of lime of the old bone is partly employed for this purpose, who taught this agent that the old bone must be carried off, new modelled, and deposited anew? The same wonders take place during the healing of every wound, and the renewing of every diseased part.

These operations are incompatible with the supposition that the body of animals is a mere chemical and mechanical machine; and demonstrate the presence of some agent besides, which acts according to very different laws.

But neither in this case is the power of this agent over the chemical agents, which are employed, absolute. We may prevent a fractured bone from healing by giving the patient large quantities of acids. And unless the materials for the new wanted substances be supplied by the food, they cannot, in many cases, be formed at all. Thus the canary bird cannot complete her eggs unless she be furnished with lime.

It is evident that the supreme agent of the animal body, whatever that agent may be, acts according to fixed laws; and that when these laws are opposed by those which are more powerful, it cannot overcome them. These laws clearly indicate design; and the agent has the power of modifying them somewhat according to circumstances. Thus more phosphat of lime is sent to a limb which requires a new bone, and more lime than usual is taken into the system when the hen is laying eggs. Design and contingency are considered by us as infallible marks of consciousness and intelligence. That they are infallible marks of the agency of mind is certain; but that they are in all cases the proofs of immediate consciousness and intelligence, as the Stahlians supposed, cannot be affirmed without running into inconsistencies. For we ourselves are not conscious of those operations which take place during assimilation.

To say that a being can act with design without intelligence, we allow to be a flat contradiction, because design always implies intelligence. There must therefore be intelligence somewhere. But may not this intelligence exist, not in the agent, but in the being who formed the agent? And may not the whole of the design belong in reality to that being?

May not this agent, then, be material, and may not the whole of assimilation be performed by mere matter, acting according to laws given it by its maker? We answer, that what is called matter, or the substances enumerated in the first part of Chemistry (Suppl.), act always according to certain attractions and repulsions, which are known by the name of mechanical and chemical laws.

The phenomena of assimilation are so far from being cases of these laws, that they are absolutely inconsistent with them, and contrary to them; consequently the agent which presides over assimilation is not matter. Concerning the nature of this substance it is not the business of this article to inquire; but as it possesses properties different from matter, and acts according to very different laws, it would be an abuse of terms to call it matter.

We would give it the name of mind, were it not that Animal metaphysicians have chosen to consider intelligence as principle the essence of mind; whereas this substance may be conceived to act, and really does act, without intelligence. There is no reason, however, to suppose, with some, that there are two substances in animals: one possessed of consciousness as its essence, and therefore called mind or soul in man; another, destitute of consciousness, called the living principle, &c., employed in performing the different functions of assimilation, aborption, &c. It is much more reasonable to suppose, that in every animal and vegetable there is a peculiar substance, different from matter, to which their peculiar properties are owing; that this substance is different in every species of animal and vegetable; that it is capable of acting according to certain fixed laws which have been imposed upon it by its Creator, and that these laws are of such a nature that it acts in subservience to a particular end; that this substance in plants is probably destitute of intelligence; that in man and other animals it possesses intelligence to a certain extent, but that this intelligence is not essential to its existence nor to its activity; that it may be deprived of intelligence altogether, and afterwards recover it without altering its nature. Physiologists have given it the name of living principle, because its presence constitutes life. Perhaps it would be proper to distinguish that of animals by the name of animal principle. Upon what the intelligence of the animal principle depends, it is impossible to say; but it is evidently connected with the state of the brain. During a trance, or an apoplectic fit, it has often been lost for a time, and afterwards recovered.

17. Besides assimilation, the blood is also employed in forming all the different secretions which are necessary for the purposes of the animal economy. These have been enumerated in the last chapter. The process is similar to that of assimilation, and undoubtedly the agents in both cases are the same; but we are equally equal ignorant of the precise manner in which secretion is performed as we are of assimilation.

18. After these functions have gone on for a certain time, which is longer or shorter according to the nature of the animal, the body gradually decays, at last all its functions cease completely, and the animal dies. The cause of this must appear very extraordinary, when we consider the power which the animal has of renewing decayed parts; for it cannot be doubted that death proceeds, in most cases at least, from the body becoming incapable of performing its function. But if we consider that this power is limited, and that it must cease altogether, when those parts of the system begin to decay which are employed in preparing materials for future assimilation, our surprise will, in some measure, cease. It is in these parts, in the organs of digestion and assimilation accordingly, that this decay usually proves fatal. The decay in other parts destroys life only when the waste is so rapid that it does not admit of repair.

What the reason is that the decay of the organs causes death, or, which is the same thing, causes the living principle either to cease to act, or to leave the body altogether, it is perfectly impossible to say, because we know too little of the nature of the living principle, and of the manner in which it is connected with the body. The last is evidently above the human understanding, but many of the properties of the living principle have been discovered; and were the facts already known properly arranged, and such general conclusions drawn from them as their connection with each other fully warrant, a degree of light would be thrown upon the animal economy which those, who have not attended to the subject, are not aware of.

No sooner is the animal dead, than the chemical and mechanical agents, which were formerly servants, usurp the supreme power, and soon decompose and destroy that very body which had been in a great measure reared by their means. But the changes which take place upon animal bodies after death, are too important, and too intimately connected with the subject of this article to be passed over lightly. They shall therefore form the subject of the next chapter.

**CHAP. IV. OF THE DECOMPOSITION OF ANIMAL SUBSTANCES.**

All the soft and the liquid parts of animals, when exposed to a moderate temperature of sixty-five degrees or more, pass with more or less rapidity through the following changes. Their colour becomes paler, and their consistence diminishes; if it be a solid part, such as flesh, it softens, and a serous matter sweats out, whose colour quickly changes; the texture of the part becomes relaxed, and its organization destroyed; it acquires a faint disagreeable smell; the substance gradually sinks down, and is diminished in bulk; its smell becomes stronger and ammoniacal. If the subject be contained in a close vessel, the progress of putrefaction, at this stage, seems to slacken; no other smell but that of a pungent alkali is perceived; the matter effervesces with acids, and converts syrup of violets to a green. But if the communication with the air be admitted, the urinous exhalation is dissipated, and a peculiar putrid smell is spread around with a kind of impetuosity; a smell of the most insupportable kind, which lasts a long time, and pervades every place, affecting the bodies of living animals after the manner of a ferment, capable of altering the fluids; this smell is corrected, and as it were confirmed by ammonia. When the latter is volatilized, the putrefactive process becomes active a second time, and the substance suddenly swells up, becomes filled with bubbles of air, and soon after subsides again. Its colour changes, the fibrous texture of the flesh being then scarcely distinguishable; and the whole is changed into a soft, brown, or greenish matter, of the consistence of a poultice, whose smell is faint, nauseous, and very active on the bodies of animals. The odorant principle gradually loses its force; the fluid portion of the flesh assumes a kind of consistence, its colour becomes deeper, and it is finally reduced into a friable matter, rather deliquescent, which being rubbed between the fingers, breaks into a coarse powder like earth. This is the last state observed in the putrefaction of animal substances; they do not arrive at this term but at the end of a considerable time.

In carcases buried in the earth, putrefaction takes place much more slowly; but it is scarcely possible to observe its progress with accuracy. The abdomen is the part gradually dilated with elastic fluids which make their appearance in it, and at last it bursts and discharges a horribly fetid and noxious gas; at the same time a dark coloured liquid flows out. If the earth be very dry, and the heat considerable, the moisture is often absorbed so rapidly, that the carcase, instead of putrefying, dries, and is transformed into what is called a mummy.

Such are the phenomena when dead bodies are left to putrefy separately. But when great numbers of carcases are crowded together in one place, and are so abundant as to exclude the action of external air, and other foreign agents, their decomposition is entirely the consequence of the reciprocal action of their ingredients themselves upon each other, and the result is very different. The body is not entirely disintegrated or converted into mould, but all the soft parts are found diminished remarkably in size, and converted into a peculiar saponaceous matter. This singular change was first accurately observed in the year 1786.

The burial ground of the Innocents in Paris having become noxious to those who lived in its neighbourhood, on account of the disagreeable and hurtful odour which it exhaled, it was found necessary to remove the carcases to another place. It had been usual to dig very large pits in that burial ground, and to fill them with the carcases of the poorer sort of people, each in its proper bier; and when they were quite full, to cover them with about a foot depth of earth, and to dig another similar pit, and fill it in the same manner. Each pit held between 1000 and 1500 dead bodies. It was in removing the bodies from these pits that this saponaceous substance was found. The grave-diggers had acquired, by long experience, that about thirty years were required before all the bodies had undergone this change in its full extent. Every part of the body assumed the properties of this substance. The intestines and viscera of the thorax had completely disappeared; but what is singular enough, the brain had lost but little of its size or appearance, though it was also converted into the same substance.

This saponaceous matter was of a white colour, soft to the touch, and melted, when heated, like... PART III. OF DYEING.

Mankind have in all periods of society manifested a fondness for beautiful and gaudy colours. Naked savages at first applied them to their skin. This was the case with the Britons, and with the Gauls, too, in the time of Caesar; it is even still the practice in the South Sea islands, and many parts of America. When mankind had advanced so far towards civilization as to wear garments, they naturally transferred to them the colours which they admired. Hence the origin of dyeing, which is of such antiquity, that it precedes the earliest records left us by profane authors. We see from the book of Genesis the great progress which it had made in the time of the patriarchs.

Dyeing seems to have originated in India, and to have spread gradually from that country to the west. The Indians were the inventors of the method of dyeing cotton and linen, which was not understood in Europe before the conquests of Alexander the Great. The Phoenicians excelled in the art at a very early period. It was from them that the Jews purchased all the dyed stuffs described in Exodus. The Phoenician dyers seem to have confined their art to wool; silk was unknown to them, and linen was usually worn white. From them the art of dyeing passed to the Greeks and Romans.

During the fifth century, the Western Empire was overturned by the northern nations, and with it the arts and sciences, which had flourished under the protection of the Romans, disappeared. A few of the arts, indeed, were preserved in Italy, but they were obscured and degraded. By degrees, however, a spirit of industry began to revive in that country. Florence, Genoa, and Venice, becoming rich commercial cities, carried on a considerable intercourse with the Grecian empire, where many of the arts had been preserved. This intercourse was much increased by the crusades. The Italian cities became rich and powerful; the arts which distinguish civilized nations were cultivated with emulation, and dyeing, among others, was rapidly improved.

In the year 1429, the first treatise on dyeing made its appearance at Venice, under the name of Morigola in modern del'arte de tentori. Giovanni Ventura Rotetta collected, with great industry, all the processes employed by the dyers of his time, and published them in 1538, under the title of Picto. For many years dyeing was almost exclusively confined to Italy; but it gradually made its way to France, the Low Countries, and to Britain. The minister Colbert, who employed his talents in extending the commerce and manufactures of France, paid particular attention to the art of dyeing. In the year 1672, he published a table of instructions, by which those who practised the art were laid under several very improper restrictions. But the bad effects of these were in a good measure obviated by the judicious appointment of men of science to superintend the art. This plan, begun by Colbert, was continued by the French government. Accordingly, Dufay, Hellot, Macquer, and Berthollet, successively filled the office. It is to this establishment, and to exertions of the celebrated chemists who have filled it, that France is indebted for the improvements she has made in the art of dyeing during the course of the 18th century. Under the direction of Dufay, a new table of regulations was published in 1737, which superseded that of Colbert. In Britain, though dyeing has been carried on for many years with great success, very little progress was made in investigating the theory of the art. The Royal Society, indeed, soon after its institution, recommended it to some of its members; but as no treatise made its appearance in consequence of this, it seems very soon to have lost their attention. Lewis, many years after, published some very important remarks on dyeing; but they were confined to a few processes. The British dyers satisfied themselves with a translation of Hellot. Such was the state of the art when the article Dyeing in the Encyclopaedia was drawn up. It consists chiefly of an abstract of Hellot's treatise. But within the last 30 years, the attention of men of science has been very much turned to this complicated art. In Sweden has appeared the treatise of Scheffer, and Bergman's notes on it; in Germany, the experiments of Beckmann, Poerner, and Vogler, and the dissertation of Francheville; in France, the treatises of D'Ambouray, D'Aphigay, Hauffmann, Chaptal, and above all, of Berthollet; in this country, the ingenious remarks of Delaval, of Henry, and the valuable treatise of Dr Bancroft; besides many other important essays. These, together with the progress of the science of chemistry, on which the theory of dyeing depends, have thrown so much new light upon the art, that we find ourselves under the necessity of tracing the whole over again. We shall pass over, however, very slightly those parts of the art which have been sufficiently explained in the article Dyeing, Encycl.

To understand the art of dyeing, we must be acquainted with the substances on which it is practised, with the nature of colours, and with the method of permanently changing the colour of bodies. These three things we shall consider in the three following chapters. In the first, we shall give an account of the substances of which garments are usually made, with which alone the art of dyeing is concerned; in the second, we shall inquire into the nature of colour; and in the third, explain the theory of dyeing, as far as it is at present understood. In some subsequent chapters, we shall give a general view of the processes by which the different colours are given to stuffs.

**Chap. I. Of the Substances used for Clothing.**

The substances commonly employed for clothing may be reduced to four; namely wool, silk, cotton, linen. As there is no name in the English language which includes all these substances, we shall take the liberty, in the remainder of this article, to use the word cloth for that purpose. They are all made into cloths, of some kind or other, before they can be useful as articles of clothing.

1. Wool, as is well known, is the hair which covers the bodies of sheep; it differs from common hair merely in fineness and softness. Its filaments possess a considerable degree of elasticity; they may be drawn out beyond their usual length, and afterwards recover their form when the external force is removed. The surface of wool and hair is by no means smooth; no inequality, indeed, can be perceived by a microscope; nor is any resistance felt when a hair is laid hold of in one hand, and drawn between the fingers of the other, from the root towards the point; but if it be drawn from the point towards the root, a resistance is felt which did not take place before, a tremulous motion is perceived, and a noise may be distinguished by the ear. If, after laying hold of a hair between the thumb and fore finger, we rub them against each other in the longitudinal direction of the hair, it acquires a progressive motion towards the root; the point gradually approaches the fingers, while the root recedes from them; so that the whole hair very soon passes through between the fingers.

These observations, first made by Mr Monge, demonstrate that the surface of hair and wool is composed, either of small laminae, placed over each other in a slanting direction from the root towards the point, like the scales of a fish—or of zones, placed one above another, as takes place in the horns of animals.

On this structure of the filaments of hair and wool depend the effects of felting and fulling. In both of these operations, the filaments are made, by an external force, to rub against each other; the position of their apertures prevents them from moving, except in one direction: they are mutually entangled, and obliged to approach nearer each other. Hence the thickness which cloth acquires in the fulling mill. The filaments have undergone a certain degree of felting, and are interwoven like the fibres of a hat. The cloth is contracted both in length and breadth; it may be cut without being subject to ravel; nor is there any necessity for hemming the different pieces employed to make a garment. See Felting and Fulling, in this Suppl.

Wool is naturally covered with a kind of grease, which preserves it from moths. This is always removed before the wool is dyed; because its presence is very prejudicial to the success of that operation. The apertures of the surface of woolly fibres would impede the converting of it into thread by spinning; but they are in a great measure covered, previous to that operation, by soaking the wool with oil. The oil must also be removed before the wool be dyed: this process is called Scouring, which see in this Suppl.

We have already, in the second part of this article, given an account of what is at present known concerning the composition of wool and hair. It would be foreign to the subject of this chapter, to describe the method of spinning and weaving wool.

Wool is of different colours; but that which is white is preferred for making cloth; because it answers better for the purposes of dyeing than any other kind.

2. Silk is a substance spun in fine threads by the silk worm. Its fibres are not scaly like those of wool; neither have they the same elasticity; but silk, in its natural state, before it has undergone any preparation, has a considerable degree of stiffness and elasticity. In this state it is known by the name of raw silk. It is covered with a kind of gummy varnish, which may be removed by scouring with soap. The scouring deprives it of its stiffness and elasticity. Raw silk is of a yellow colour, owing to yellow resinous matter with which it is naturally combined. We have given the method of separating this matter, and also the gum, in the article Bleaching, Supplement.

Silk, before it is dyed, is always freed from its gum, and generally also from its resin. It may be dyed without Cotton is a fine downy substance, contained in the pods of different species of gossypium. The species from which the greater part of the cotton brought to this country is taken is the *herbacum*. The quantity imported annually into Britain is very great; in 1786 it amounted to 20 millions of pounds. Cotton varies greatly, according to the plant on which it grows, and the climate where it is cultivated. The chief differences are in colour, and in the length, fineness, and strength of the filaments.

No aperitives can be discovered on the surface of these filaments; but Levenhoek observed, by means of a microscope, that they are triangular, and have three sharp edges. This is probably the reason of a well-known fact, that cotton cloth, when applied by way of dressing, always irritates a sore.

Some cottons are naturally white; others a fine light yellow, as those of which hankin is made; but most commonly cotton is of a dirty brownish yellow colour, which must be removed before the fluff can be dyed. This is done by the process of bleaching. The fibres of cotton, even after being bleached, retain almost always some lime and oxyd of iron, which must be removed before we attempt to dye the cotton; because their presence would spoil the colour. This is done by steeping the cotton for some time in water acidulated with sulphuric acid.

Cotton, like silk, may be dyed without the assistance of heat. It is not nearly so easy to dye cotton any particular colour as it is to dye wool or silk. If wool and cotton be put into the same dyeing vessel, the wool frequently acquires the wildest for colour before the cotton has lost any of its original whiteness.

4. Lint, from which linen is made, is the inner bark of the *linum usitatissimum*, or flax; a plant too well known in this country to require any description.

The flax, when ripe, is pulled and steeped for some days in water, in order to separate the green coloured glutinous matter which adheres to the inner bark. This matter undergoes a degree of putrefaction; carbolic acid gas and hydrogen gas, are disengaged; it is decomposed, and carried off by the water. If the water, in which the flax is steeped, be completely stagnant, the putrefaction is apt to go too far, and to injure the fibres of the lint; but in a running stream, it does not go far enough, so that the green matter still continues to adhere to the lint. Flax, therefore, should be steeped in water neither completely stagnant, nor flowing too freely, like a running stream.

The flax is afterwards spread upon the grass, and exposed for some time to the air and sun; this improves the colour of the lint, and renders the woody part so brittle, that it is easily separated by the action of the lint mill. The subsequent operations, of dressing, spinning, weaving, and bleaching, do not belong to this article.

The fibres of lint have very little elasticity. They appear to be quite smooth; for no aperitives can be perceived by the microscope, nor detected by the feel; nor does linen irritate sores, as is the case with cotton.

Linen may be dyed without the assistance of heat; but it is more difficult to give it permanent colours than even cotton.

Thus we have given a short description of wool, silk, cotton, and linen. The first two are animal substances; the two last vegetable. The animal contain much azot and hydrogen; the vegetable much carbon. The animal are readily destroyed by acids and alkalies; the vegetable withstand the action of these substances better; even nitric acid does not readily destroy the texture of cotton. The animal substances are more easily dyed than the vegetable; and the colours which they receive are more permanent than those given to cotton and linen by the same processes.

Such are the properties of the cloths on which the art of dyeing is exercised. But what is the nature of these colours which it is the object of that art to communicate? We shall examine this subject in the following chapter.

CHAP. II. Of Colours.

All visible objects, as has been long ago sufficiently established, are seen by means of rays of light falling off from them in all directions, and partly entering the eye of the spectator.

1. For the theory of light and vision we are indebted to Sir Isaac Newton. He first demonstrated, that light is composed of seven rays, differing from each other in refrangibility, and other properties. Each of these rays is distinguished by its particular colour. Hence their names, red, orange, yellow, green, blue, indigo, violet. By mixing together these different rays, in various proportions, all the colours known may be obtained. Thus red and yellow constitute orange; yellow and blue constitute green; blue and red constitute purple, violet, aurora, &c., according to their proportions. When all the rays are mixed together, they form a white.

2. Bodies differ very much from each other in their power of reflecting light. Some reflect it in vast quantities; others reflect but little, as charcoal. In general, the smoother the surface of a body is, the greater is the quantity of light which it reflects. Hence the effect of polishing in increasing the brightness of bodies. But it is not in the quantity of the light reflected alone that bodies differ from each other; they differ also in the quality of the light which they reflect. Some bodies reflect one or more particular species of ray to the exclusion of the rest. This is the reason that they appear to us of different colours. Those bodies which reflect only red rays are red; those that reflect yellow rays are yellow; those that reflect all the rays equally are white; those that reflect too little to affect the eye are black. It is to the different combinations of rays reflected from the surface of bodies that all the different shades of colour are owing.

Colour, then, in opaque bodies, is owing to their disposition to reflect certain rays of light, and to absorb the different rest; in transparent bodies, to their disposition to transmit certain rays, and to absorb the others. But this subject has been discussed at sufficient length, in the article Optics, Encyc.; to which, therefore, we beg leave to refer the reader. Here we mean only to inquire into the cause of this disposition of the particles of bodies.

3. Sir Isaac Newton, to whom we are indebted for the existence of optics as a science, made a set of experiments to ascertain the changes of colour which thin plates of matter assume in consequence of an increase or difference of dimi- diminution of their thickness. These experiments were of a very delicate nature; but Newton conducted them with so much address, and varied and repeated them with so much industry, that he was enabled to render them surprisingly accurate.

Upon a large double convex lens of a 50 feet focus, he placed the plane surface of a plano-convex lens, and pressed the lenses slowly together. A circle, of a particular colour, appeared in the centre, where the two glasses touched each other. This circle gradually increased in diameter as the pressure was augmented; and at last a new circle, of another colour, occupied the centre, while the first colour assumed the form of a circular ring. By increasing the pressure, a new coloured circle appeared in the centre, and the diameter of the other two increased. In this manner he proceeded, till he produced no less than 25 different-coloured circular rings. These he divided into seven orders, on account of the repetition of the same colour. They were as follows, reckoning from the central colour, which was always black:

1. Black, blue, white, yellow, red. 2. Violet, blue, green, yellow, red. 3. Purple, blue, green, yellow, red. 4. Green, red. 5. Greenish blue, red. 6. Greenish blue, pale red. 7. Greenish blue, reddish white.

These different colours were occasioned by the thin film of air between the two glasses. Now this film varies in thickness from the centre of the lens towards the circumference; that part of it which causes the black colour is thinnest, and the other coloured circles are occasioned by air gradually increasing in thickness. Newton measured the relative thickness of the air which produced each of these coloured circles; and he found it as follows:

| Colour | Thickness | |--------------|-----------| | Black | 1 | | Blue | 2 | | White | 5 | | Yellow | 7 | | Red | 8 | | Violet | 11 | | Blue | 14 | | Green | 15 | | Yellow | 16 | | Red | 18 | | Purple | 21 | | Blue | 23 | | Gr. blue | 46 | | Red | 52 | | Gr. blue | 58 | | Red | 65 | | Gr. blue | 71 | | Reddish white| 77 |

The absolute thickness of these films cannot be ascertained, unless the distance between the two glasses, at that part where the black spot appears, were known. Now there is no method of measuring this distance; but it certainly is not greater than the thousandth part of an inch.

He repeated these experiments with films of water, and even of glass, instead of air; and he found, that in these cases the thickness of the films, reflecting any particular colour, was diminished, and that this diminution was proportional to the density of the reflecting film.

From these experiments Sir Isaac Newton concluded, that the disposition of the particles of bodies to reflect or transmit particular rays depended upon their size and their density; and he even attempted to ascertain the size, or at least the thickness, of the particles of bodies from their colours. Thus a particle of matter, whose density is the same with that of glass, which reflects a green of the third order, is of the thickness of $\frac{1}{16}$ of an inch.

In the year 1765, Mr Delaval published, in the Philosophical Transactions, a very ingenious paper on the same subject. In this paper, he endeavours to prove, by experiment, that the colours of metallic bodies depend upon their density. He takes it for granted, at the same time, that the size of the particles of bodies is inversely as the density of bodies. The densest bodies, according to him, are red; the next in density, orange; the next, yellow; and so on, in the order of the refrangibility of the different rays. Some time after, the same ingenious gentleman, in his Experimental Inquiry into the Cause of the Permanent Colours of Opaque Bodies, extended his views to animal and vegetable substances, and endeavoured to prove the truth of Newton's theory by a very great number of experiments.

Such is a view of the opinion of Newton and Delaval respecting the cause of bodies reflecting or transmitting particular rays of light, as far, at least, as that theory relates to colours. They ascribed this cause solely to the size and the density of the particles of bodies.

By particles, it is evident that nothing else can be meant than the integrant particles of bodies. Newton, indeed, does not express himself precisely in this language; but it is plain that nothing else could be his meaning. Mr Delaval undoubtedly is of that opinion.

According to the Newtonian theory of colour, then, it depends solely upon the size of the integrant particles of bodies whose density is the same; and upon the size and the density jointly of all bodies.

It is evident that the truth of the Newtonian theory must depend upon its coincidence with what actually takes place in nature, and that therefore it can only be determined by experiment. Newton himself produced but very few experiments in support of it; and though this deficiency was amply supplied by Mr Delaval, it is needless for us to adduce any of these here; because, from the prodigious accumulation of chemical facts since these experiments were made, the very basis upon which they stood has been destroyed, and consequently all the evidence resulting from them has been annihilated. They proceeded on the supposition, that acids render the particles of bodies smaller, and alkalies larger than they were before, without producing any other change whatever in the bodies on which they act. To attempt a refutation of this opinion at present would be unnecessary, as it is well known not to be true.

Let us therefore compare the Newtonian theory of colour with those chemical changes which we know for certain to alter the size of the particles of bodies, in order to see whether they coincide with it. If the theory be true, the two following consequences must hold

(1) Newton, however, pointed out an exception to this law, concerning which Mr Delaval has been more explicit. Combustible bodies do not follow that law, but some other. Mr Delaval has supposed, that this deviation is owing to the presence of phlogiston. hold in all cases: 1. Every alteration in the size of the integrant particles of bodies must cause these particles to assume a different colour. 2. Every such alteration must correspond precisely with the theory; that is to say, the new colour must be the very colour, and no other, which the theory makes to result from an increase or diminution of size.

Now neither of these consequences holds in fact. We have no method indeed of ascertaining the sizes of the integrant particles of bodies, nor of measuring the precise degree of augmentation or diminution which they suffer; but we can in many cases ascertain whether any new matter has been added to a particle, or any matter abstracted from it; and consequently whether it has been augmented or diminished; which is sufficient for our present purpose.

For instance, whatever be the size of an integrant particle of gold, it cannot be denied that an integrant particle of oxyd of gold is greater; because it contains an integrant particle of gold combined with at least one integrant particle of oxygen. Now the colour both of gold and of its oxyd is yellow, which ought not to be the case, according to the Newtonian theory. In like manner, the amalgam of silver is white, precisely the colour of silver and of mercury; yet an integrant particle of the amalgam must be larger than an integrant particle either of silver or of mercury. Many other instances besides these will occur to every one, of changes in the size of the particles taking place without any change of colour. All these are incompatible with the Newtonian theory.

It may be said, perhaps, in answer to this objection, that there are different orders of colours; that the same colour is reflected by particles of different sizes; and that the increased particles, in the instances above alluded to, retain their former colour, because the increment has been precisely such as to enable them to reflect the same colour in the next higher order.

This very answer is a complete proof that the Newtonian theory is not sufficient to account for the colours of bodies; for if particles of different sizes reflect the same colour, size certainly is not the only cause of this reflection. There must be some other cause, very different from size. Nor is this all; the most common colour which remains after an increase of the size of the integrant particles of bodies is white; yet white does not appear in any of the orders except the first, and therefore its permanence cannot be accounted for by any supposition compatible with the Newtonian theory.

Even when alterations in the colour of bodies accompany the increase or diminution of the size of their particles, these alterations seldom or never follow an order which corresponds with the theory. As for metals, it is self-evident that their colour does not depend upon their density. Platinum is the densest body known, and yet it is not red, as it ought to be, but white like tin; a metal which has little more than one third of the density of platinum.

The green oxyd of iron, when combined with prussic acid, becomes white; yet the size of its particles must be increased. Now this change of colour is incompatible with the theory; for, according to it, every change from green to white ought to be accompanied by a diminution instead of an increase of size. A particle of indigo, which is naturally green, becomes blue by the addition of oxygen, which must increase its size. This change is also incompatible with the theory. But it is unnecessary to accumulate instances, as they will naturally occur in sufficient number to every one.

It follows incredibly from these facts, that the Newtonian theory is not sufficient to explain the cause of colour; or what causes bodies to reflect or transmit certain rays, and to absorb the rest.

4. We have endeavoured, in the article Chemistry, Suppl., to show, that bodies have a particular affinity for the rays of light; and that the phenomena of light depend entirely upon these affinities. Indeed this consequence follows from the properties of light established by Newton himself. We shall not repeat here the proofs upon which the existence of these affinities is founded: the reader may easily satisfy himself by consulting the article above referred to.

Every coloured body, then, has a certain affinity for some of the rays of light. Those rays for which it has a strong affinity are absorbed by it and retained, and the other rays for which it has no affinity are either reflected or transmitted, according to the nature of the body and the direction of the incident ray. Thus a red body has an affinity for all the rays except the red; it absorbs therefore the other five, and reflects only the red; a green body absorbs all but the green rays, or perhaps the red and yellow; a black body has a strong affinity for all the rays, and therefore absorbs them all; while a white body, having no strong affinity for any of the rays, reflects or transmits them all.

If affinity, as we have endeavoured to show in the article Chemistry, Suppl., be an attraction of the same nature with gravitation, and increasing as the distance diminishes, it must depend upon the nature of the attracting particles. Now the only differences which we can conceive to exist between the particles of bodies, are differences in size, in density, and in figure. Changes in these three things will account for all the varieties of affinity. Now if affinity depends upon these three things, and if colour depends upon the affinity between the particles of bodies and the different rays of light, as cannot be denied, it is clear that the cause of the colour of bodies may be ultimately resolved into the size, density, and figure, of their particles. Newton's theory, then, was defective, because he omitted the figure of the particles, and ascribed the whole to variations in size and density.

When we say, then, that colour is owing to affinity, we do not contradict the opinion of Newton, as some philosophers have supposed, but merely extend it: Newton was not mistaken in saying, that colour depends upon the size and the density of the particles of bodies; his mistake lay in supposing that it depends upon these alone.

5. Since the colour of bodies depends upon their affinity for light, and since every body has a certain colour, because it absorbs and retains particular rays while it transmits or reflects the rest, it is evident that every body must continue of its first colour till one of two things happen; either till it be saturated with the rays which it absorbs, and of course cease to absorb any more, or till its particles change their nature, by being either decomposed or combined with some new substance. We have no positive proof that the first cause Dyeing Substances.

Cause of change ever occurs, as many substances have been exposed to the action of light for a very long time without any change of colour. The absorbed light seems to make its escape, either in its own form, or in some unknown or unsupposed one. The second cause of change is very common; indeed its action may be detected in almost every case of alteration in the colour of bodies. The green oxyd of iron, by combining with oxygen, becomes red; and this red oxyd, when combined with prussic acid, affumes a blue colour, and with gallic acid a black colour. The cause of this change of colour, when the composition of a body changes, is obvious; every change of composition must alter the affinity, because it must of necessity produce changes in the size, density, or figure of the particles, or perhaps in all of these. Now if the affinity of a body for other bodies be altered, it is natural to suppose that it will be altered also for light. Accordingly this happens in most instances. It does not, however, take place constantly, for very obvious reasons. It may happen that the new density, size, or figure of the altered body is such, as to render it still proper for attracting the very same rays of light which it formerly attracted. Just as iron, after being combined with a certain dose of oxygen, is converted into green oxyd, which still retains an affinity for oxygen.

It is evident from all this, that in most cases the permanence of colour in bodies will depend upon the permanence of their composition, or on the degree of facility with which they are acted upon by those bodies, to the agency of which they are exposed.

In dyeing, the permanence of colour is of very great importance. Of what value is the beauty of a colour, provided that colour be fugitive or liable to change in some other. In all cases, therefore, it is of consequence to attend to the substances to which dyed cloth is exposed, and to ascertain their action upon every particular dyeing ingredient. Now the bodies to which dyed cloth is almost constantly exposed are air and light; the combined action of which has so much influence, that very few dyes can resist it.

It is evident that those substances which have a strong affinity for oxygen cannot retain their colour, provided they be able to take it from atmospheric air. Thus the green colour of green oxyd of iron and of indigo is not permanent, because these substances readily absorb oxygen from air. In order, then, that a colour can have any permanence, the coloured body must not have so great an affinity for oxygen as to be able to take it from air. Those bodies have in general the most permanent colours which are already saturated with oxygen, and therefore not liable to absorb more. Such is the case with red oxyd of iron.

All coloured bodies are compounds; some of those only excepted which still retain an affinity for oxygen. Coloured bodies, therefore, are composed of several ingredients; and in every coloured body, at least some of the ingredients have a strong affinity for oxygen. Now, before the colour of a body can be permanent, its ingredients must be combined together by so strong affinities, that oxygen gas is unable to decompose it by combining with one or more of its ingredients and carrying it off. If this decomposition take place at once, it is impossible for the colour of a body to have any permanence. If it takes place slowly, the colour of the body gradually decays. The action of oxygen gas upon bodies is much increased in particular circumstances. Almost all coloured bodies are decomposed by oxygen gas by the assistance of heat. Thus if wheat flour be exposed to the heat of 448°, it loses its white colour, and becomes first brown and then black. At this temperature it is decomposed, and a part, or even the whole of its hydrogen, combining with oxygen, flies off. Cloth is scarcely ever exposed to so high a temperature; but there are other circumstances in which it may be placed which may have a similar effect. Thus the action of light seems in some substances to be similar to that of heat, and to facilitate the decomposition of the coloured matter by the combination of some of its ingredients with oxygen.

Coloured bodies, in order to have permanent colours, must not be liable to be decomposed by other substances more than by oxygen. For instance, if they contain oxygen and hydrogen, these two bodies must not be liable to combine together and form water, nor must oxygen and carbon be liable to combine and form carbonic acid gas. Light seems to have a tendency to decompose many bodies in this manner, and even to carry off oxygen from them in the form of oxygen gas. Thus it renders the nitrate of silver black by carrying off part of its oxygen, and it reduces oxy-muriatic acid to common muriatic acid by the same means.

These are the causes which induce a change in the colour of coloured bodies, as far as they have been traced; namely, the addition of oxygen, the abstraction of oxygen, partial decomposition by some one of their ingredients combining with oxygen, complete or partial decomposition by the ingredients entering into new combinations with each other. The coloured matters used in dyeing are very liable to these changes, because they are in general animal or vegetable substances of a very compound nature. Of course their ingredients have often no very strong affinity for each other, and therefore are very liable to decomposition; and every one of the ingredients has in general a very strong affinity for oxygen. This renders the choice of proper colouring matters for dyeing a very important point. In order to have permanency, they must not be liable to the above changes, not to mention their being able also to withstand the action of soap, acids, alkalies, and every other substance to which dyed cloth may be exposed.

It becomes therefore a point of some consequence to be able to ascertain whether cloth dyed of any particular colour be permanently dyed or not. The proper method of ascertaining this is by actually exposing such cloth to the sun and air; because as these are the agents to which it is to be exposed, and which have the most powerful action, it is clear, that if it withstand them, the colour must be considered as permanent. But this is a tedious process. Berthollet proposed exposing such cloth to the action of oxy-muriatic acid; those colours that withstand it being considered as permanent. This method answers in many cases; but it is not always to be depended on; for it destroys some permanent colours very speedily, and does not alter others which are very fading*. But we shall have occasion to return to this subject afterwards.

Dyers divide colours into two classes; namely, simple and compound. The simple colours are those which cannot Dyeing in General.

From the theory of colour laid down in the last chapter, it follows, that permanent alterations in the colour of cloth can only be induced two ways; either by producing a chemical change in the cloth, or by covering its fibres with some substance which possesses the wished-for colour. Recourse can seldom or never be had to the first method, because it is hardly possible to produce a chemical change in the fibres of cloth without spoiling its texture and rendering it useless. The dyer, therefore, when he wishes to give a new colour to cloth, has always recourse to the second method.

1. The substances employed for this purpose are called colouring matters, or dye stuffs. They are for the most part extracted from animal and vegetable substances, and have usually the colour which they are intended to give to the cloth. Thus a blue colour is given to cloth by covering its fibres with indigo, a blue powder extracted from a shrub; a red colour, by the colouring matter extracted by water from an insect called cochineal, or from the root of a plant called madder.

2. Mr Delaval has published a very interesting set of experiments on colouring matters in the second volume of the Manchester Memoirs. He has proved, by a very numerous set of experiments, that they are all transparent, and that they do not reflect any light, but only transmit it: For every colouring matter which he tried, even when dissolved in a liquid, and forming a transparent coloured solution, when seen merely by reflected light, was black, whatever was the colour of the matter; but when seen by transmitted light, it appeared of its natural colour. This discovery, which Mr Delaval has established very completely, and to which, as far at least as dye stuffs are concerned, there are but few exceptions, is of very great importance to the art of dyeing, and explains several particulars which would otherwise be unintelligible.

Since the particles of the colouring matter with which cloth, when dyed, is covered, is transparent, it follows, that all the light reflected from dyed cloth must be reflected, not by the dye stuff itself, but by the fibres of the cloth below the dye stuff. The colour therefore does not depend upon the dye alone, but also upon the previous colour of the cloth. If the cloth be black, it is clear that we cannot dye it any colour whatever; because no light in that case is reflected; none can be transmitted, whatever dye stuff we employ. If the cloth were red, or blue, or yellow, we could not dye it any colour except black; because as only red, or blue, or yellow rays were reflected, no other could be transmitted (x). Hence the importance of a fine white colour when cloth is to receive bright dyes: It then reflects all the rays in abundance; and therefore any colour may be given, by covering it with a dye stuff which transmits only some particular rays.

3. If the colouring matters were merely spread over the surface of the fibre of cloth by the dyer, the colours produced might be very bright, but they could not be permanent; because the colouring matter would very soon rubbed off, and would totally disappear whenever the cloth was washed, or even barely exposed to the weather. The colouring matter, then, however perfect a colour it possesses, is of no value, unless it also adheres so firmly to the cloth, that none of the substances usually applied to cloth in order to clean it, &c., can displace it. Now this can only happen when there is a strong affinity between the colouring matter and the cloth, and when they are actually combined together in consequence of that affinity.

4. Dyeing, then, is merely a chemical process, and can only consist in combining a certain colouring matter with the fibres of cloth. This process can in no instance be performed, unless the dye stuff be first reduced to its integrant particles; for the attraction of aggregation between the particles of dye stuffs is too great to be overcome by the affinity between them and cloth, unless they could be brought within much smaller distances than is possible, while they both remain in a solid form. It is necessary, therefore, previously to dissolve the colouring matter in some liquid or other, which has a weaker affinity for it than the cloth has. When the cloth is dipped into this solution, the colouring matter, reduced by this contrivance to a liquid state, is brought within the attracting distance; the cloth therefore acts upon it, and by its stronger affinity takes it from the solvent, and fixes it upon itself. By this contrivance, too, the equality of the colour is in some measure secured, as every part of the cloth has an opportunity of attracting to itself the proper proportion of colouring particles.

The facility with which cloth imbibes a dye, depends upon two things, namely, the affinity between the cloth and the dye stuff, and the affinity between the dye stuff and its solvent. It is directly as the former, and inversely as the latter. It is of importance to preserve a due proportion between these two affinities, as upon that proportion much of the accuracy of dyeing depends. If the affinity between the colouring matter and the cloth be too great, compared with the affinity between the colouring matter and the solvent, the cloth will take the dye too rapidly, and it will be scarce possible to prevent its colour from being unequal. On the other hand, if the affinity between the colouring matter and the solvent be too great, compared with that

(x) These remarks hold only on the supposition, that the whole of the surface is of the given colour, which in many instances is not the case. Dyeing substances.

Dyeing in that between the colouring matter and the cloth, the cloth will either not take the colour at all, or it will take it very slowly and very faintly.

Wool has the strongest affinity for almost all colouring matters, silk the next strongest, cotton a considerably weaker affinity, and linen the weakest affinity of all. Therefore, in order to dye cotton or linen, the dye stuff should in many cases be dissolved in a solution for which it has a weaker affinity than for the solvent employed in the dyeing of wool or silk. Thus we may use oxyd of iron dissolved in sulphuric acid, in order to dye wool; but for cotton and linen, it is better to dissolve it in acetic acid.

5. Were it possible to procure a sufficient number of colouring matters having a strong affinity for cloth, to answer all the purposes of dyeing, that art would be exceedingly simple and easy. But this is by no means the case: if we except indigo, the dyer is scarcely possessed of a dye stuff which yields of itself a good colour sufficiently permanent to deserve the name of a dye.

This difficulty, which at first sight appears insurmountable, has been obviated by a very ingenious contrivance. Some substance is pitched upon which has a strong affinity both for the cloth and the colouring matter. This substance is previously combined with the cloth, which is then dipped into the solution containing the dye stuff. The dye stuff combines with the intermediate substance; which, being firmly combined with the cloth, secures the permanence of the dye. Substances employed for this purpose are denominated mordants (v).

The most important part of dyeing is undoubtedly the proper choice and the proper application of mordants, as upon them the permanency of almost every dye depends. Every thing which has been said respecting the application of colouring matters, applies equally to the application of mordants. They must be previously dissolved in some liquid, which has a weaker affinity for them than the cloth has to which they are to be applied; and the cloth must be dipped, or even steeped, in this solution, in order to saturate itself with the mordant.

Almost the only substances used as mordants are, earths, metallic oxyds, tan, and oil.

6. Of earthy mordants, by far the most important and most generally used is alumina. It was used as a mordant in very early ages, and seems indeed to have been the very first substance employed for that purpose. Alumina has a very strong affinity for wool and for silk; but its affinity for cotton and linen is a good deal weaker.

It is used as a mordant in two states; either in the state of alum, in which it is combined with sulphuric acid and a little potash; or in the state of acetite of alumina, in which it is combined with acetic acid.

Alum was employed as a mordant very early. The ancients, indeed, do not seem to have been generally acquainted with pure alum; they used it in that state of impurity in which it is found native; of course it was used in dyeing long before the nature of its ingredients was understood, and therefore long before the part which it acts was suspected. Indeed, it is but a very short time since the office which mordants perform was suspected: the first person that hit upon it was Mr Keir; he gave an account of the real use of mordants in his translation of Macquer's Dictionary, published in 1771.

Alum, when used as a mordant, is dissolved in water, and very frequently a quantity of tartar is dissolved along with it. Into this solution the cloth is put and kept in till it has absorbed as much alumina as is necessary. It is then taken out, and for the most part washed and dried. It is now a good deal heavier than it was before, owing to the alumina which has combined with it. The tartar serves two purposes: the potash which it contains combines with the sulphuric acid of the alum, and thus prevents that very corrosive substance from injuring the texture of the cloth, which otherwise might happen; the tartarous acid, on the other hand, combines with part of the alumina, and forms a tartarite of alumina, which is more easily decomposed by the cloth than alum.

Acetite of alumina has been introduced into dyeing since the commencement of the 18th century; and, like many other very important improvements, we are indebted for it to the ignorance of the calico printers, who first introduced it. As they did not understand the nature nor use of the mordants which they employed, they were accustomed to mix with their alum an immense farrago of substances, a great proportion of which were injurious instead of being of service. Some one or other had mixed with alum acetite of lead: the good effects of this mixture would be soon perceived; the quantity of acetite was gradually increased, and the other ingredients omitted. This mordant is now prepared, by pouring acetite of lead into a solution of alum: a double decomposition takes place, the sulphuric acid combines with the lead, and the compound precipitates in the form of an insoluble powder; while the alumina combines with the acetic acid, and remains dissolved in the liquid. This mordant is employed for cotton and linen, which have a weaker affinity than wool for alumina. It answers much better than alum, the cloth is more easily saturated with alumina, and takes, in consequence, both a richer and a more permanent colour.

Besides alumina, lime is sometimes used as a mordant. Cloth has a strong enough affinity for it; but in general it does not answer well, as it does not give so good a colour. When used, it is either in the state of lime-water or of sulphate of lime dissolved in water.

7. Almost all the metallic oxyds have an affinity for metal cloth; but only two of them are extensively used as mordants, namely, the oxyds of tin and of iron.

The oxyd of tin was first introduced into dyeing by Kutter (z), a German chemist, who brought the secret to London in 1543. This period forms an era in the history of dyeing. The oxyd of tin has enabled the moderns

(v) This term, imposed by the French dyers before the action of mordants was understood, signifies biters or corroders. These bodies were supposed to act merely by corroding the cloth. Mr Henry of Manchester has proposed to substitute the word baths for mordants; but that word is too general to answer the purpose well.

(z) Mr Delaval has supposed, that the Tyrians were acquainted with the use of tin in dyeing, and Mr Hen- Dyeing Substances.

Dr Bancroft has proposed to substitute a solution of tin in a mixture of sulphuric and muriatic acid, instead of nitro-muriat of tin, as a mordant for wool. This mordant, he informs us, is much cheaper, and equally efficacious. It may be prepared by dissolving somewhat less than one part of tin in two parts of sulphuric and three of muriatic acid; at the degree of concentration at which they are commonly sold in this country.

This mordant, like the others, must be dissolved in a sufficient quantity of water, in order to be used.

Iron, like tin, is capable of two degrees of oxidation; but the green oxid absorbs oxygen so readily from the atmosphere, that it is very soon converted into the red oxid. It is only this last oxid which is really used as a mordant in dyeing. The green oxid is indeed sometimes applied to cloth; but it very soon absorbs oxygen, and is converted into the red oxid. This oxid has a very strong affinity for all kinds of cloth. The permanency of the iron spots on linen and cotton is a sufficient proof of this. As a mordant, it is used in two states; in that of sulphate of iron, and acetate of iron. The first is commonly used for wool. The salt is dissolved in water, and the cloth dipped in it. It may be used also for cotton; but in most cases acetate of iron is preferred. It is prepared by dissolving iron, or its oxid, in vinegar, four beer, &c., and the longer it is kept, the more it is preferred. The reason is, that this mordant succeeds best when the iron is in the state of red oxid. It would be better then to oxidate the iron, or convert it into rust, before using it; which might easily be done, by keeping it for some time in a moist place, and sprinkling it occasionally with water. Of late, pyrolignous acid has been introduced instead of acetous. It is obtained by distilling wood or tar.

8. Tan, which has been already described in the first part of this article, has a very strong affinity for cloth, and for several colouring matters. It is therefore very frequently employed as a mordant. An infusion of sumach, or of sumach (a), or any other substance containing tan, is made in water, and the cloth is dipped in this infusion, and allowed to remain till it has absorbed a sufficient quantity of tan. Silk is capable of absorbing a very great proportion of tan, and by that means acquires a very great increase of weight. Manufacturers sometimes employ this method of increasing the weight of silk.

Tan is often employed also, along with other mordants, in order to produce a compound mordant. Oil is also used for the same purpose in the dyeing of cotton and linen. The mordants, with which tan most frequently is combined, are alumina and oxid of iron.

Besides these mordants, there are several other substances frequently used as auxiliaries, either to facilitate the combination of the mordant with the cloth, or to alter... Dyeing substances.

Mordants not only render the dye permanent, but have also considerable influence on the colour produced. The same colouring matter produces very different dyes, according as the mordant is changed. Suppose, for instance, that the colouring matter be cochineal; if we use the alum mordant, the cloth will acquire a crimson colour; but the oxyd of iron produces with it a black. These changes, indeed, might naturally have been expected; for since the colour of a dye stuff depends upon its affinity for light, every new combination into which it enters, having a tendency to alter these affinities, will naturally give it a new colour. Now, in all cases, the colouring matter and mordant combine together; the colour of the cloth, then, must be that which the particles of the dye and of the mordant, when thus combined together, exhibit. Indeed some mordants may be considered in the light of colouring matters also, as they always communicate a particular colour to cloth. Thus, iron communicates a brown colour, and iron and tan together constitute a black dye.

In dyeing, then, it is not only necessary to procure a mordant, which has a sufficiently strong affinity for the colouring matter and the cloth, and a colouring matter which possesses the wished-for colour in perfection, we must procure a mordant and a colouring matter of such a nature, that when combined together they shall possess the wished-for colour in perfection. It is evident, too, that a great variety of colours may be produced with a single dye stuff, provided we can change the mordant sufficiently.

Every thing which tends to weaken the affinity between the mordant and the cloth, or between the mordant and the colouring matter, and every thing which tends in any way to alter the nature of the mordant, must injure the permanency of the dye; because, whenever the mordant is destroyed, there is no longer anything to cause the dye stuff to adhere; and when its nature is altered, the colour of the dye must alter at the same time. All the observations, then, which were made in the last chapter, concerning the nature of colouring matters, and the changes to which they are subject, apply equally to mordants. These substances, indeed, are scarcely liable themselves to any alteration. They are of a much more simple nature, in general, than dye stuffs; and therefore not nearly so liable to decomposition. But when the colouring matter itself is altered, it comes to the same thing. Its affinity for the mordant being now destroyed, there is nothing to retain it.

As the permanency of a dye depends upon the degree of affinity between the mordant and the colouring matter, it is clear, that a dye may want permanency, even though it resist the oxy muriatic acid, and all the other fumes tests propounded by chemists. These substances may happen to have very little action on the dye stuff, and therefore may not affect it; yet it may soon disappear, in consequence of its want of affinity for the mordant.

The colouring matter with which cloth is dyed, does not cover every portion of its surface; its particles attach themselves to the cloth at certain distances from each other; for cloth may be dyed different shades of the same colour, lighter or darker, merely by varying the quantity of colouring matter. With a small quantity, the shade is light; and it becomes deeper as the quantity increases. Now this would be impossible, if the dye stuff covered the whole of the cloth. Newton has demonstrated, that colours are rendered faint when the rays of light, which occasion them, are mixed with white rays. Consequently, from cloth dyed of a light shade, a considerable quantity of white rays passes off unchanged; but this could not be the case if the stuff were covered with coloured matter; because all the white rays would be decomposed as they pass through the coloured matter. Therefore, in light shades, the colouring matter does not cover the cloth; its particles adhere to it, at a certain distance from each other, and from every part of the cloth which is uncovered, the white rays pass off unchanged. Even when the shade of colour is as deep as possible, the colouring particles do not cover the whole of the cloth, but are at a certain distance from each other. This distance, undoubtedly, is diminished in proportion to the depthness of the shade; for the deeper the shade, the smaller is the number of white rays which escape undecomposed; the more, therefore, of the surface is covered, and consequently, the smaller is the distance at which each of them is placed. A shade may be even conceived to vary, that not a particle of white light escapes the action of the colouring matter; in which case, the distance between the particles of colouring matter could not exceed double that distance at which a particle of matter is able to act upon light.

That the particles of colouring matter, even when the shade is deep, are at some distance, is evident from this well-known fact, that cloth may be dyed two colours at the same time. All those colours, to which the dyers give the name of compound, are in fact two different colours applied to the cloth at once. This cloth gets a green colour, by being first dyed blue and then yellow. The rays of light that pass from green cloth thus dyed are blue and yellow; by the mixture of which it is well known that green is produced. In this case, it is clear, that each of the colouring matters performs the very same office as if it were alone; and that the new colour is not produced by the combination of the two colouring matters. That part of the white light, reflected from the cloth, which passes through the blue colouring matter, is decomposed; and the blue rays only transmitted; and that part of the white light which passes through the yellow colouring matter is also decomposed, and only the yellow rays transmitted. It is clear, therefore, that both of the colouring matters equally cover the naked fibres of the cloth; consequently the one must be placed in the intervals of the other; wherefore the particles of each of the colouring matters are at some distance. Now the same effect happens how deep ever the shade be; and it makes no difference which of the two dyes be first given. Nay, if one of the dyes have a strong affinity for the cloth, and the other only a weak affinity, the latter will soon disappear, and leave the cloth of the colour which the first dye gives it.

The difference, then, in the shade of colour, and also the compound colour which cloth may receive, depend entirely upon the distance between the particles of the colouring matters attached to the cloth, and the possibility Dr Roxburgh, who first drew the attention of manufacturers to the *verum tinctorium*, a tree very common in India, from the leaves of which indigo may be extracted with much advantage, has given a much shorter method of obtaining that pigment. The leaves are kept in a copper full of water, supported at the temperature of 160°; till they assume a yellowish hue, and the liquid acquire a deep green colour. The liquid is then to be drawn off, agitated in the usual manner, till the blue floccule appear; and then the indigo is precipitated with lime water.

This process, which succeeds equally well with the *indigofera*, shows us that the plants, from which indigo may be extracted, contain a peculiar green pollen, soluble in water. The intention, both of the fermentation of the common method, and of the scalding, according to Dr Roxburgh's method, is merely to extract this pollen. Mr Hauffman first showed, that this green basis of indigo has a strong affinity for oxygen; and the subsequent experiments of Drs Roxburgh and Bancroft have confirmed his observations, and put them beyond the reach of doubt. It gradually attracts oxygen from the air; in consequence of which, it acquires a blue colour, and becomes insoluble in water. The agitation is intended to facilitate this absorption, by exposing a greater surface to the action of the air. The lime water, by absorbing a quantity of carbonic acid, with which the green pollen seems to be combined, greatly facilitates the separation of the indigo.

The method of preparing indigo, and of applying it to the purposes of dyeing, seems to have been very early known in India. But in Europe, though it had been occasionally used as a paint*, its importance as a *plum dye stuff* was not understood before the middle of the 15th century. It is not even mentioned in the *Pitthe*, which was published in 1548. At that period, then, the use of indigo must have been unknown to the Italian dyers. The Dutch were the people who first imported it from India, and made its importance known in Europe. It was afterwards cultivated in Mexico and the West Indies with such success, that the indigo from these countries was preferred to every other. In consequence of this preference, they supplied almost the whole of the European market. But within these few years, the East Indian indigo, owing entirely to the enlightened exertions of some of our own countrymen, has recovered its character, and is now imported, in very considerable quantities, into Britain.

The indigo of commerce has different shades of colour, according to the manner in which it has been prepared, and the proportion of foreign substances with which it is mixed. The principal shades are copper colour, violet, and blue. That indigo, which has the smallest specific gravity, is always most esteemed because it is most free from impurities. Bergman†, *fig. v.*, found the purest indigo of commerce which he could procure, composed of

- 47 pure indigo, - 12 gum, - 6 resin, - 22 earth, - 13 oxyd of iron.

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(*n*) Proust informs us, that he found magnesia, even abundantly, in indigo.—Nicholson's *Jour.* III. 323. Pure indigo is insoluble in water, alcohol, ether, and oils; neither alkalies nor earths have any action on it; none of the acids hitherto tried have any effect on it, except the nitric and sulphuric. Nitric acid very soon converts it into a dirty white colour, and at last decomposes it completely*. When the acid is concentrated, it even sets fire to the indigo (c); when it is diluted, the indigo becomes brown, crystals make their appearance, resembling those of oxalic and tartarous acids; and there remains behind, after the acid and the crystals are washed off, a viscid substance, of a very bitter taste, and possessing many of the properties of a resin†.

Concentrated sulphuric acid dissolves indigo readily, and much heat is evolved. The saturated solution is opaque, and consequently black; but it assumes a deep blue colour when diluted with water. This solution is well known in commerce under the name of liquid blue. Bancroft has given it the name of fulphat of indigo. During the solution of the indigo, some sulphurous acid, and some hydrogen gas, are evolved‡, and the blue colour of the indigo is much heightened. These facts have led Bancroft to suppose, that the indigo, during its solution, combines with an additional quantity of oxygen*. This may possibly be the case, but the phenomena are not sufficient to establish it: for the hydrogen gas and sulphurous acid evolved may owe their formation, not to the action of the sulphuric acid on indigo, but upon the impurities with which it is always mixed; and the improvement of the colour may be owing to the absence of these impurities. The carbonates of fixed alkalies precipitate slowly from fulphat of indigo a blue coloured powder, which possesses the properties of indigo; but it is soluble in most acids and in alkalies. Pure alkalies destroy the colour and properties of fulphat of indigo; they destroy also precipitated indigo §. These facts give some probability to Bancroft's opinion; but they do not establish it: because the differences between common and precipitated indigo may depend merely on the state of greater minuteness to which it is reduced, which prevents the attraction of aggregation from obstructing the action of other bodies. Even fibre, when newly precipitated, is soluble in many menstrua.

Indigo has a very strong affinity for wool, silk, cotton, and linen. Every kind of cloth, therefore, may be dyed with it, without the assistance of any mordant whatever. The colour thus induced is very permanent; because the indigo is already saturated with oxygen, and because it is not liable to be decomposed by those substances, to the action of which the cloth is exposed. But it can only be applied to cloth in a state of solution; and the only solvent known being sulphuric acid, it would seem at first sight that the sulphuric acid solution is the only state in which indigo can be employed as a dye.

The fulphat of indigo is indeed often used to dye wool and silk blue; but it can scarcely be applied to cotton and linen, because the affinity of these substances for indigo is not great enough to enable them readily to decompose the fulphat. The colour given by fulphat of indigo is exceedingly beautiful: it is known by the name of Saxon blue; because the process, which was discovered by councillor Barth in 1740, was first carried on at Großenhain in Saxony. The method of the original inventor was very complicated, from the great number of useless ingredients which were mixed with the fulphat. But these ingredients were gradually laid aside, and the composition simplified by others, after the nature of it, which was for some time kept secret, became known to the public. The best process is that of Mr Poerntz*.

One part of indigo is to be dissolved in four parts of concentrated sulphuric acid; to the solution one part of dry carbonat of potash is to be added, and then it is to be diluted with eight times its weight of water. The cloth must be boiled for an hour in a solution containing five parts of alum and three of tartar for every 32 parts of cloth. It is then to be thrown into a water bath, containing a greater or smaller proportion of the diluted fulphat of indigo, according to the shade which the cloth is intended to receive. In this bath it must be boiled till it has acquired the wished for colour. The alum and tartar are not intended to act as mordants, but to facilitate the decomposition of the fulphat of indigo. Bergman ascertained that alum possesses this property. The alkali added to the fulphat answers the same purpose. These substances, also, by saturating part of the sulphuric acid, serve, in some measure, to prevent the texture of the cloth from being injured by the action of the acid, which is very apt to happen in this process.

But fulphat of indigo is by no means the only form of that pigment employed in dyeing. By far the most common method, and indeed the only method known before 1740, is to deprive indigo of the oxygen to which it owes its blue colour, and thus to reduce it to the state of green pollen; and then to dissolve it in water by means of alkalies, or alkaline earths, which in that state act upon it very readily. Indigo is precisely in the state of green pollen when it is first extracted from the plant in the scalding process described by Dr Roxburgh. If, therefore, there were any method of stopping short here, and of separating the pigment while it retains its green colour, it would be precisely in the state best adapted for dyeing. Nothing more would be necessary but to dissolve it in water by means of an alkali, and to dip the cloth into the solution†.

But as indigo is not brought home to us in that state, the dyer is under the necessity of undoing the last part of the indigo-maker's process, by separating again the oxygen, and restoring it to its original green colour. Two different methods are employed for this purpose. The first of these methods is to mix with indigo a solution of some substance which has a stronger affinity for oxygen than the green basis of indigo. Green oxyd of iron, for instance, and different metallic sulphurets. If, therefore, indigo, lime, and green fulphat of iron, be mixed together in water, the indigo gradually

(c) The combustion of indigo by nitric acid, of the density 1.52°, was first published by Mr Sage; but Woulfe appears to have observed the fact before him, and to have pointed it out to Rouelle, who showed it in his lectures. Preysl, Niebelson's Jour. III. 325. gradually loses its blue colour, becomes green, and is diffused, while the green oxyd of iron is converted into the red oxyd. The manner in which these changes take place is obvious. Part of the lime decomposes the sulphate of iron; the green oxyd, the instant that it is set at liberty, attracts oxygen from the indigo, decomposes it, and reduces it to the state of green pollen. This green pollen is immediately dissolved by the action of the rest of the lime. In like manner, indigo is dissolved, when mixed in water, with pure antimony and potash, or with sulphuret of arsenic and potash. For these interesting facts we are indebted to Mr Hauffman.

The second method is to mix the indigo in water with certain vegetable substances which readily undergo fermentation. During this fermentation, the indigo is deprived of its oxygen, and dissolved by means of quicklime or alkali, which is added to the solution. The first of these methods is usually followed in dyeing cotton and linen; the second, in dyeing wool and silk.

In the dyeing of wool, woad and bran are commonly employed as vegetable ferments, and lime as the solvent of the green base of the indigo. Woad contains itself a colouring matter precisely similar to indigo; by following the common process, indigo may be extracted from it. In the usual state of woad, when purchased by the dyer, the indigo which it contains is probably not far from the state of green pollen. Its quantity in woad is but small, and it is mixed with a great proportion of other vegetable matter. Before the introduction of indigo into Europe, woad alone was employed as a blue dye; and even as late as the 17th century, the use of indigo was restricted in different countries, and dyers obliged to employ a certain quantity of woad (a). But these absurd restrictions were at last removed, and woad is now scarcely used in dyeing, except as a ferment to indigo. The blue colouring matter, however, which it contains, nullifies all cases, contribute considerably to the dye.

A sufficient quantity of woad, mixed with bran, is put into a wooden vessel filled with warm water, whose temperature is kept up sufficiently to ensure fermentation. Afterwards quicklime and indigo are added. The indigo is deprived of its oxygen, and dissolved by the lime. When the solution is complete, the liquid has a green colour, except at the surface, where it is copper coloured, or blue, because the indigo at the surface absorbs oxygen from the air, and assumes its natural colour. The woollen cloth is dipped in, and passed through the liquid as equably as possible, piece after piece; those pieces being first dyed which are to assume the deepest shade. No part of the cloth should come in contact with the sediment, which would spoil the colour. When the cloth is first taken out of the vat, it is of a green colour; but it soon becomes blue, by attracting oxygen from the air. It ought to be carefully washed, to carry off the uncombined particles. This solution of indigo is liable to two inconveniences: 1. It is apt sometimes to run too fast into the putrid fermentation; this may be known by the putrid vapours which it exhales, and by the disappearing of the green colour. In this state it would soon destroy the indigo altogether. The inconvenience is remedied by adding more lime, which has the property of moderating the putrefactive tendency. 2. Sometimes the fermentation goes on too languidly. This defect is remedied by adding more bran or wood, in order to diminish the proportion of quicklime.

Silk is usually dyed blue by the following process:

Six parts of bran, and five of indigo, with nearly one part of madder, are stirred into a sufficient quantity of water, in which five parts of common potash of commerce is dissolved. The liquid is kept at a temperature proper for fermentation. When the indigo, deprived of its oxygen by the fermentation, is dissolved by the potash, the liquid assumes a green colour. The silk, previously well scoured, is put into the solution in small quantities at a time; then wrung out of the dye, and hung up in the open air, till the green colour which it has at first is changed into blue. By this method, silk can only be made to receive a light blue colour. In order to give silk a dark blue, it must previously receive what is called a ground colour; that is, be previously dyed some other colour. A particular kind of red dye-stuff, called archil (e), is commonly employed for this purpose.

The madder employed in the above process may, at first sight, appear superfluous; it seems, however, to contribute something to the colour.

Cotton and linen are dyed blue by the following process:

One part of indigo, one part of green sulphate of iron, and two parts of quicklime, are stirred into a sufficient quantity of water. The solution is at first green, but it gradually assumes a yellow colour, and its surface is covered with a shining, copper coloured pellicle. The cloth is to be allowed to remain in the solution for five or six minutes. When taken out, it has a yellow colour; but on exposure to the atmosphere, it soon becomes green, and then blue, in consequence of the absorption of oxygen. The indigo, in this process, seems to be deprived of a greater quantity of oxygen than is necessary to reduce it to the state of green pollen. Mr Hauffman has observed, that the cloth acquires a much deeper colour, provided it be plunged, the instant it is taken out of the dyeing vat, into water acidulated with fulphuric acid. It is usual to dip the cloth into a succession of vats, variously charged with colouring matter; beginning with the vat which contains least colouring matter, and passing gradually to those which contain most. By this contrivance the cloth is dyed more equally, than it probably would be, if it were plunged all at once into a saturated solution of colouring matter.

Sect. II. Of Yellow.

The principal colouring matters employed to dye yellow are weld, saffron, and quercitron bark.

1. Reseda luteola, known in this country by the name of

(b) The employment of indigo was strictly prohibited in England in the reign of Queen Elizabeth; nor was the prohibition taken off till the reign of Charles II. It was prohibited also in Saxony. In the edict it is spoken of as a corrosive substance, and called food for the devil. Colbert restricted the French dyers to a certain quantity of it.

(e) This will be described in a subsequent section. Weld is a plant which grows wild very commonly in Scotland, and in most European countries. Cultivated weld has a more slender stem than the wild kind, but it is more valuable, because it is much more rich in colouring matter. It is an annual plant, of a yellowish green colour, furnished with a great number of small leaves. When ripe it is pulled, dried, tied up in parcels, and in that state sold to the dyer.

Weld readily yields its colouring matter to water. The saturated decoction of it is brown; but when sufficiently diluted with water it becomes yellow. Acids render its colour somewhat paler, but alkalies give it a deeper shade. When alum is added to it, a yellow coloured precipitate falls down, consisting of alumina combined with the colouring matter of weld. The affinity therefore of this colouring matter for alumina is so great, that it is able to abstract it from sulphuric acid. Its affinity for oxyd of tin is at least equally great; for muriat of tin causes a copious bright yellow precipitate, composed of the colouring matter and the oxyd combined. Most of the metallic salts occasion similar precipitates, but varying in colour according to the metal employed. With iron, for instance, the precipitate is dark grey, and with copper brownish green.

2. The morus tinctoria is a large tree which grows in the West India islands. The wood of this tree is of a yellow colour, with orange veins. The French call it yellow wood (bois jaune); but the English dyers have given it the absurd name of old fustic (v). This wood has been introduced into dyeing since the discovery of America. The precise time is not known; but that it was used in England soon after the middle of the 17th century, is evident from Sir William Petty's paper on Dyeing, read to the Royal Society soon after its institution. In that paper particular mention is made of old fustic.

Fustic gives out its colouring matter with great facility to water. The saturated decoction of it is of a deep reddish yellow colour; when sufficiently diluted it becomes orange yellow. Acids render it turbid, give it a pale yellow colour, and occasion a slight greenish precipitate, which alkalies redissolve. Alkalies give the decoction a very deep colour, inclining to red; some time after they have been added, a yellow matter separates from the liquid, and either twines on the surface, or adheres to the sides of the vessel. Alum, sulphate of iron, of copper, and of zinc, produce precipitates composed of the colouring matter combined respectively with the bases of these different salts; and the colour varies according to the substance with which this colouring matter is combined. With alumina it is yellow; with iron, yellowish brown; with copper, brownish yellow; and with zinc, greenish brown.

3. The quercus nigra, to which Dr Bancroft has given the name of quercitron, is a large tree which grows naturally in North America. Dr Bancroft discovered, about the year 1784, that the bark of this tree contains a great quantity of yellow colouring matter, and since that time it has been introduced into dyeing with much advantage. To prepare it for the dyer, the epidermis is shaved off, and then it is ground in a mill. It separates partly into stringy filaments, and partly into a fine light powder. Both of these contain colouring matter, and therefore are to be employed; but as they contain unequal quantities, they should be used in their natural proportions.

Quercitron bark readily gives out its colouring matter to water at the temperature of 100°. The infusion has a yellowish brown colour, which is rendered lighter by acids, and darker by alkalies. Alum occasions a scanty precipitate of a deep yellow colour; muriat of tin, a copious bright yellow precipitate; sulphate of tin, a dark olive precipitate; and sulphate of copper, a precipitate of a yellow colour inclining to olive.

Besides these dye stuffs there are others occasionally used by dyers. The following are the most remarkable:

Genista tinctoria, or dyers broom. This plant yields a very inferior yellow; it is only used for coarse woollen stuffs.

Serratula tinctoria, or saw-wort. This plant yields a yellow nearly of the same nature with weld; for which, therefore, it is a good substitute.

Juglans alba, or American hickory. The bark of this tree yields a colouring matter exactly similar to that of quercitron bark, but much smaller in quantity.

Anota is a name given to a red palke formed of the berries of the bixa orellana, a tree which is a native of America. This palke yields its colouring matter to a solution of alkali in water. The solution affords an exceedingly beautiful yellow dye, but very fading, and incapable of being fixed by any known mordant.

Turmeric is the root of the curcuma longa, a plant which grows both in the East and West Indies. It is richer in colouring matter than any other yellow dye stuff. It yields very beautiful yellows, but too fading to be of much use, and no mordant has any influence in contributing to their permanence.

5. Yellow colouring matters have too weak an affinity for cloth to produce permanent colours without some use of mordants. Cloth, therefore, before it be dyed yellow, is always prepared by combining some mordant or other with it. The mordant most commonly employed for this purpose is alumina. Oxide of tin is sometimes used when very fine yellows are wanted. Tin is often employed as a subsidiary to alumina, in order to fix it more copiously on cotton and linen. Tartar is also used as an auxiliary to brighten the colour; and muriat of soda, sulphate of lime, and even sulphate of iron, in order to render the shade deeper.

6. The yellow dyed by means of fustic is more permanent, but not so beautiful as that given by weld or quercitron. As it is permanent, and not much injured by acids, it is often used in dyeing compound colours where

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(r) The rhiz cotinus, or Venice sumach, is a small shrub, formerly employed as a yellow dye, but now almost out of use. The French call it fustic, from which word it is probable, as Dr Bancroft supposes, that our dyers formed the term fustic. When the morus tinctoria was introduced as a dye-stuff, they gave it the same name; but in order to distinguish the two, they called the sumach, which was a small shrub, young fustic; and the morus, which was a large tree, old fustic. See Bancroft, i. 412. where a yellow is required. The mordant is alumina. When the mordant is oxyd of iron, fustic dyes a good permanent drab colour.

Weld and quercitron bark yield nearly the same kind of colour; but as the bark yields colouring matter in much greater abundance, it is much more convenient, and, upon the whole, cheaper than weld. It is probable, therefore, that it will gradually supersede the use of that plant. The method of using each of these dye stuffs is nearly the same.

7. Wool may be dyed yellow by the following process: Let it be boiled for an hour, or more, with about 1/4th of its weight of alum, dissolved in a sufficient quantity of water. It is then to be plunged, without being rinsed, into a bath of warm water, containing in it as much quercitron bark as equals the weight of the alum employed as a mordant. The cloth is to be turned through the boiling liquid till it has acquired the intended colour. Then a quantity of clean powdered chalk, equal to the hundredth part of the weight of the cloth, is to be stirred in, and the operation of dyeing continued for eight or ten minutes longer. By this method a pretty deep and lively yellow may be given fully as permanent as weld yellow.

For very bright orange, or golden yellow, it is necessary to have recourse to the oxyd of tin as a mordant. A fine orange yellow may be given to woollen cloth, by putting, for every ten parts of cloth, one part of bark into a sufficient quantity of hot water; after a few minutes, an equal weight of muri-fulphat of tin is to be added, and the mixture well stirred. The cloth acquires the wished for colour in a few minutes when brilliantly turned in this bath.

The same process will serve for producing bright golden yellow, only some alum must be added along with the tin. For the brightest golden yellow, the proportions sufficient for dyeing 100 parts of cloth are, 10 parts of bark, 7 parts of muri-fulphat of tin, and 5 parts of alum. All the possible shades of golden yellow may be given to cloth merely by varying the proportion of the ingredients according to the shade.

In order to give the yellow that delicate green shade so much admired for certain purposes, the same process may be followed, only tartar must be added in different proportions according to the shade. Thus to dye 100 parts of cloth a full bright yellow, delicately inclining to green, 8 parts of bark, 6 of muri-fulphat, 6 of alum, and 4 of tartar, are to be employed. The tartar is to be added at the same time with the other mordants. If the proportion of alum and tartar be increased, the green shade is more lively; to render it as lively as possible, all the four ingredients ought to be employed in equal proportions. As these fine lemon-yellows are generally required only pale, 10 parts of each of the ingredients will be sufficient to dye about 300 parts of cloth.

By adding a small proportion of cochineal, the colour may be raised to a fine orange, or even an aurora.

8. Silk may be dyed different shades of yellow, either by weld or quercitron bark, but the last is the cheapest of the two. The proportion should be from 1 to 2 parts of bark to 12 parts of silk, according to the shade. The bark, tied up in a bag, should be put into the dyeing vessel while the water which it contains is cold, and when it has acquired the heat of about 100°, the silk previously alumined, should be dipped in, and continued till it attains the wished for colour. When the shade required is deep, a little chalk or pearlash should be added towards the end of the operation. When a very lively yellow is wanted, a little muri-fulphat of tin should be added, but not too much, because tin always injures the glossiness of silk. The proportions may be 4 parts of bark, 3 of alum, and 2 of muri-fulphat of tin.

Silk is dyed fine orange and aurora colours by annotta. The process is merely dipping the silk into an alkaline solution of annotta. To produce the orange shade the alkali is saturated with lemon juice. The colours thus produced are exceedingly beautiful, but they want permanency.

9. The common method of dyeing cotton and linen yellow, has been described in the article Dyeing in the Encyclopedia. The cloth is first soaked in a solution of alum, and then dyed in a decoction of weld. After this it is soaked for an hour in a solution of sulphat of copper, and, lastly, it is boiled for an hour in a solution of hard soap. This process, besides the expense of it, is defective; because the yellow is neither so beautiful nor so permanent as it might be if the mordant were used in a different form.

The method recommended by Dr Bancroft is much more advantageous, yielding more permanent and beautiful colours at a smaller expense. The mordant should be acetite of alumina, prepared by dissolving 1 part of acetite of lead, and 3 parts of alum, in a sufficient quantity of water. This solution should be heated to the temperature of 100°, the cloth should be soaked in it for two hours, then wrung out and dried. The soaking may be repeated, and the cloth again dried as before. It is then to be barely wetted with lime water, and afterwards dried. The soaking in the acetite of alumina may be again repeated; and if the shade of yellow is required to be very bright and durable, the alternate wetting with lime water, and soaking in the mordant, may be repeated three or four times. By this contrivance a sufficient quantity of alumina is combined with the cloth, and the combination is rendered more permanent by the addition of some lime. The dyeing bath is prepared by putting 12 or 18 parts of quercitron bark (according to the depth of the shade required), tied up in a bag, into a sufficient quantity of cold water. Into this bath the cloth is to be put, and turned round in it for an hour, while its temperature is gradually raised to about 120°. It is then to be brought to a boiling heat, and the cloth allowed to remain in it after that only a few minutes. If it be kept long at a boiling heat the yellow acquires a shade of brown.

Another way of dyeing cotton and linen very permanent yellows, would be to imitate the method adopted for dyeing cotton in the East. That method is indeed exceedingly tedious, but it might be very much shortened by carefully attending to the uses of the ingredients. The essential part of the process is to cause the alumina to combine in sufficient quantity with the cloth, and to adhere with sufficient firmness to ensure a permanent colour. This is accomplished by using three mordants; first oil, then tan, and lastly alum. The combination of these three substances produces a mordant which ensures a very permanent colour.

The cotton is first soaked in a bath composed of a sufficient quantity of oil, and mixed with a weak solution... Dyeing Substances.

The colour which it communicates to cloth is exceedingly permanent, but being far inferior in beauty to those which may be obtained from cochineal, it has been but little employed by dyers since that splendid pigment came into common use.

2. Cochineal is likewise an insect, a species of coccus cochinealis. Linnaeus distinguishes it by the name coccus calli. It inhabits different species of cacti, but the most perfect variety is confined to the cactus coccinellifer. The cochineal insect was first discovered in Mexico; the natives had employed it in their red dyes before the arrival of the Spaniards. It became known in Europe soon after the conquest of Mexico; and the beauty of the colour which it communicates to cloth very soon attracted general attention. For many years it was mistaken for a vegetable production, as had been the case also with the kermes. Different accounts of its real nature had indeed appeared very early in the Philosophical Transactions; but the opinion of Pomet, who insisted that it was the seed of a particular plant, gained so much credit, that it was not entirely destroyed till the publication of Mr Ellis's paper in the 52nd volume of the Philosophical Transactions, which established the contrary beyond the possibility of doubt.

The female cochineal insect remains like the kermes, during her whole life adhering to a particular spot of the tree on which it feeds. After fecundation, her body serves merely as a nidus for her numerous eggs, and gradually swells as these advance towards maturity. In this state the insects are gathered, put into a linen bag, which is dipped into hot water to destroy the life of the young animals contained in the eggs, and then dried. In this state they are sent to Europe and sold to the dyer.

The quantity of cochineal disposed of in Europe is very great. Bancroft informs us, that the Spaniards annually bring to market about 600,000 lbs. of it. Hitherto the rearing of the insects has belonged almost exclusively to that nation. Other nations have indeed attempted to share it with them, but without any remarkable success; as the Spaniards use every precaution to confine the true cochineal, and even the species of cactus on which it feeds, to Mexico. Mr Thiry de Menonville was fortunate enough to procure some specimens of both, and to transfer them in safety to St Domingo; but after his death, the insects were allowed to perish. The wild cochineal insect, which differs from the cultivated kind merely in being smaller, and containing less colouring matter, was produced in St Domingo, in considerable quantities, before the commencement of the present war. Several spirited British gentlemen have lately contrived to procure the insect; and vigorous efforts are making to rear it in the East Indies. We have not yet learned the success of these attempts; but we have reason to hope everything from the zeal and abilities of those gentlemen who have taken an active part in the enterprise.

Cochineal readily gives out its colouring matter to water. The decoction is of a crimson colour, inclining to violet: It may be kept for a long time without putrefying or losing its transparency. Sulphuric acid gives

(g) We ought to mention, that this process, or at least one very similar, has been long well known to the calico printers of this country. Most of their brown yellows, or drabs, are dyed with iron. Dyeing Substances

gives it a red colour, inclining to yellow, and occasions a small fine red precipitate. Tartar gives it a yellowish red colour, which becomes yellow after a small quantity of red powder has subsided. Alum brightens the colour of the decoction, and occasions a crimson precipitate. Muriat of tin gives a copious fine red precipitate; sulphate of iron, a brownish violet precipitate; sulphate of zinc, a deep violet precipitate; acetite of lead, madder, and sulphate of copper, violet precipitates.

Water is not capable of extracting the whole of the colouring matter of cochineal; but the addition of a little alkali or tartar enables the water to extract the whole of it.

3. Archil (it) is a paste formed of the lichen rocella, pounded and kept moist for some time with tile urine. It gives out its colouring matter to water, to alcohol, and to a solution of ammonia in water.

The lichen rocella grows abundantly in the Canary islands, from which it is imported and sold to the dyers. Other lichens are likewise used to dye red, especially the parellus, from which the pigment called luteum, and by chemists turfor, is prepared; the emphalodes and turtureum, which are often employed in this country to dye coarse cloths. To these many others might be added; but the reader may consult the treatises of Hoffmann and Wefring on the subject.

4. The rubia tinctorum is a small well-known plant, cultivated in different parts of Europe for the sake of its roots, which are known by the name of madder. They are about the thickness of a goose quill, somewhat transparent, of a reddish colour, and a strong smell. They are dried, cleaned, ground in a mill, and in that state used by dyers.

Madder gives out its colouring matter to water. The infusion is of a brownish orange colour; alum produces in it a deep brownish red precipitate; alkaline carbonates, a blood red precipitate, which is redissolved on adding more alkali. The precipitate occasioned by acetite of lead is brownish red; by nitrate of mercury, purplish brown; by sulphate of iron, a fine bright brown. After the red colouring matter has been extracted from madder by water, it is still capable of yielding a brown ortho, colour.

5. Carthamus tinctorius is an annual plant, cultivated in Spain, Egypt, and the Levant, for the sake of its flowers, which alone are used in dyeing. After the juice has been squeezed out of these flowers, they are washed repeatedly with salt water, pressed between the hands, and spread on mats to dry. Care is taken to cover them from the sun during the day, and to expose them to the evening dews, in order to prevent them from drying too fast. Such is the method followed in Egypt.

The flowers of carthamus contain two colouring matters; a yellow, which is soluble in water, and a red, insoluble in water, but soluble in alkaline carbonates. The method of preparing them above described, is intended to carry off the yellow colouring matter, which is of no use, and to leave only the red. After the flowers are thus prepared, they are of a red colour, and have lost nearly one-half of their weight. An alkaline ley readily extracts their colouring matter, which may be precipitated by saturating the alkali with an acid. Lemon juice is commonly used for this purpose, because it does not injure the colour of the dye. Next to citric, sulphuric acid is to be preferred, provided too great a quantity be not used. The red colouring matter of carthamus, extracted by carbonat of soda, and precipitated by lemon juice, constitutes the rouge employed by the ladies as a paint. It is afterwards ground with a certain quantity of talc. The fineness of the talc, and the proportion of it mixed with the carthamus, occasion the difference between the cheaper and dearer kinds of rouge.

6. Brazil wood, or fernamboue, as it is called by the Brazilians, is the wood of the ceajapina crista, a tree wood, which grows naturally in America and the West Indian islands. It is very hard; its specific gravity is greater than that of water; its taste is sweetish; its colour, when fresh cut, is pale; but after exposure to the atmosphere, it becomes reddish.

Brazil wood yields its colouring matter to alcohol, and likewise to boiling water. The decoction is of a fine red colour. The mineral acids make it yellow, and occasion a reddish brown precipitate. Oxalic acid causes an orange red precipitate. Fixed alkali gives the decoction a crimson colour, inclining to brown; ammonia, bright purple. Alum occasions a copious crimson precipitate, especially if alkali is added at the same time. Sulphate of iron renders the decoction black. The precipitate produced by muriat of tin is rose coloured; that by acetite of lead of a fine deep red.

The decoction of Brazil wood is fitter for dyeing after it has stood some time, and undergone a kind of fermentation.

7. None of the red colouring matters has so strong an affinity for cloth as to produce a permanent red, without the assistance of mordants. The mordants employed are alumina and oxyd of tin; oil and tan, in certain processes, are also used; and tartar and muriat of soda are frequently called in as auxiliaries.

8. Coarse woollen stuffs are dyed red with madder.

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(h) If we believe Tournefort, this dye stuff was known to the ancients. They employed it to dye the colour known by the name of purple of Amorgos, one of the Cyclades islands. If this account be accurate, the knowledge of it had been lost during the dark ages. It was accidentally discovered by a Florentine merchant about the year 1300, who observed, that urine gave a very fine colour to the lichen rocella. Mr Dufay discovered, that archil possesses the property of tingling indelibly white marble, of forming veins, and giving it the appearance of jasper. See Mem. Par. 1732.

(i) The tincture of archil is used for making spirit of wine thermometer. It is a singular fact, that this tincture becomes gradually colourless when excluded from the contact of air, and that it again recovers its colour when exposed to the atmosphere. The phenomenon was first observed by the Abbé Nollet, and described by him in an essay, published among the memoirs of the Academy of Sciences for 1742. or archil; but fine cloth is almost exclusively dyed with cochineal; though the colour which it receives from kermes is much more durable. Brazil wood is scarcely used, except as an auxiliary; because the colour which it imparts to wool is not permanent.

Wool is dyed crimson, by first impregnating it with alumina by means of an alum bath, and then boiling it in a decoction of cochineal till it has acquired the wished-for colour. The crimson will be finer if the tin mordant be substituted for alum; indeed it is usual with dyers to add a little nitro-muriat of tin when they want fine crimsons. The addition of archil and potash to the cochineal, both renders the crimson darker and gives it more bloom; but the bloom very soon vanishes. For paler crimsons, one half of the cochineal is withdrawn, and madder substituted in its place.

Wool may be dyed scarlet, the most splendid of all colours, by first boiling it in a solution of muri-fulphat of tin; then dyeing it pale yellow with quercitron bark, and afterwards crimson with cochineal. For scarlet is a compound colour, consisting of crimson mixed with a little yellow. This method was suggested by Dr Bancroft, who first explained the nature of the common method. The proportions which he gives are eight parts of muri-fulphat of tin for 100 parts of cloth. After the cloth has been boiled in this solution for a quarter of an hour, it is to be taken out, and about four parts of cochineal, and two and a half parts of quercitron bark, are to be thrown into the bath. After these are well mixed, the cloth is to be returned again to the bath, and boiled in it, till it has acquired the proper colour.

The common process for dyeing scarlet is as follows: Twelve parts of tartar are dissolved in warm water; then one part of cochineal is added, and soon after ten parts of nitro-muriat of tin. When the bath boils, 100 parts of cloth are put in, turned briskly through the bath, boiled in it for two hours; then taken out, aired, washed, and dried. Into another bath eleven parts of cochineal are put; and after its colouring matter is sufficiently extracted, 28 parts of nitro-muriat of tin are added. In this bath the cloth is boiled for an hour, and then washed and dried.

Every preceding writer on dyeing took it for granted, that the yellow tinge necessary for scarlet was produced by the nitro-muriat of tin, or rather by the nitric acid of that compound, and that the tartar was only useful in enlivening the colour. But Dr Bancroft ascertained, by actual experiment, that nitro-muriat of tin has no such effect; that cloth, impregnated with this or any other tin mordant, and afterwards dyed with cochineal, acquires only a crimson colour, unless tartar be added; that the tartar has the property of converting part of the cochineal to yellow; and therefore is the real agent in producing the scarlet colour. Good scarlet, indeed, cannot be made without tin; because every other mordant fulfils the colour, and renders it dull.

Silk may be dyed crimson by steeping it in a solution of alum, and then dyeing it in the usual way in a cochineal bath. But the common process is to plunge the silk, after it has been alummed, into a bath formed of the following ingredients: Two parts of white galls, three parts of cochineal, three-sixteenths of tartar, and three-sixteenths of nitro-muriat of tin, for every fifteen parts of silk. The ingredients are to be put into boiling water in the order they have been enumerated; the bath is then to be filled up with cold water; the silk put into it, and boiled for two hours. After the bath has cooled, the silk is usually allowed to remain in it for three hours longer.

The colours known by the names of poppy, cherry, rose, and flesh colour, are given to silk by means of carthamus. The process consists merely in keeping the silk, as long as it extracts any colour, in an alkaline solution of carthamus, into which as much lemon juice as gives it a fine cherry colour has been poured. To produce a deep poppy red, the silk must be put successively into a number of similar baths, and allowed to drain them. When the silk is dyed, the colour is brightened by plunging it into hot water acidulated with lemon juice. The silk ought to be previously dyed yellow with annatto.

Cherry red is produced the same way, only the annatto ground is omitted, and less colouring matter is necessary. When a flesh colour is required, a little soap should be put into the bath, which softens the colour, and prevents it from taking too quickly.

To lessen the expense, some archil is often mixed with carthamus for dark shades.

The same shades may be dyed by means of brazil wood, but they do not stand.

Silk cannot be dyed a full scarlet; but a colour approaching to scarlet may be given it, by first impregnating the stuff with muri-fulphat of tin, and afterwards dyeing it in a bath composed of four parts of cochineal and four parts of quercitron bark. To give the colour more body, both the mordant and the dye may be repeated. A colour approaching scarlet may also be given to silk, by first dyeing it crimson, then dyeing it with carthamus, and lastly yellow without heat.

Cotton and linen are dyed red with madder. The process was borrowed from the East; hence the colour is often called Adrianople or Turkey red. The cloth is first impregnated with oil, then with galls, and lastly with alum, in the manner described in the last linen red section. It is then boiled for an hour in a decoction of madder, which is commonly mixed with a quantity of blood. After the cloth is dyed, it is plunged into a soda ley, in order to brighten the colour. The red given by this process is very permanent, and when properly conducted it is exceedingly beautiful. The whole difficulty consists in the application of the mordant, which is by far the most complicated employed in the whole art of dyeing.

Cotton may be dyed scarlet by means of muri-fulphat of tin, cochineal, and quercitron bark, used as for silk; but the colour is too fading to be of any value.

Sect. IV. Of Black.

1. The substances employed to give a black colour to cloth are red oxyd of iron and tan. These two substances stances have a strong affinity for each other; and when combined, assume a deep black colour, not liable to be destroyed by the action of air and light. The affinity which each of them has for the different kinds of cloth has been already mentioned.

2. Logwood is usually employed as an auxiliary, because it communicates lustre, and adds considerably to the fulness of the black. It is the wood of the tree called by Linnaeus *haematoxylum campechianum*, which is a native of several of the West India islands, and of that part of Mexico which surrounds the Bay of Honduras. It yields its colouring matter to water. The decoction is at first a fine red bordering on violet, but if left to itself it gradually assumes a black colour. Acids give it a deep red colour; alkalies a deep violet, inclining to brown. Sulphate of iron renders it as black as ink, and occasions a precipitate of the same colour. The precipitate produced by alum is dark red; the fustic, permanent liquid becomes yellow with red.

3. Cloth, before it receive a black colour, is usually dyed blue. This renders the colour much fuller and finer than it otherwise would be. If the cloth be coarse, the blue dye may be too expensive; in that case a brown colour is given by means of walnut peels.

4. Wool is dyed black by the following process. It is boiled for two hours in a decoction of nut galls, and afterwards kept for two hours more in a bath composed of logwood and sulphate of iron, kept during the whole time at a scalding heat, but not boiled. During the operation it must be frequently exposed to the air; because the green oxyd of iron, of which the sulphate is composed, must be converted into red oxyd by absorbing oxygen, before the cloth can acquire a proper colour. The common proportions are five parts of galls, five of sulphate of iron, and 30 of logwood for every 100 of cloth. A little acetate of copper is commonly added to the sulphate of iron, because it is thought to improve the colour.

5. Silk is dyed nearly in the same manner. It is capable of combining with a very great deal of tan; the quantity given is varied at the pleasure of the artist, by allowing the silk to remain a longer or shorter time in the decoction. After the galling, the silk is put into a solution of sulphate of iron, which is usually mixed with a certain quantity of iron filings and gum. It is occasionally wrung out of the bath, exposed for some time to the air, and again immersed. When it has acquired a sufficiently full colour, it is washed in cold water, and afterwards steeped in a decoction of soap to take off the harshness, which silk always has after being dyed black.

6. It is by no means so easy to give a full black to linen and cotton. The cloth, previously dyed blue, is steeped for 24 hours in a decoction of nut galls. A bath is prepared, containing acetate of iron, formed by saturating acetic acid with brown oxyd of iron. Into this bath the cloth is put in small quantities at a time, wrought with the hand for a quarter of an hour, then wrung out and aired, again wrought in a fresh quantity of the bath, and afterwards aired. These alternate processes are repeated till the colour wanted is given. A decoction of alder bark is usually mixed with the liquor containing the nut galls.

It would probably contribute to the goodness and permanence of the colour, if the cloth, before being galled, were impregnated with oil, by being steeped in a mixture of alkaline ley and oil combined, as is practised for dyeing cotton red.

Sect. V. Of Brown.

That particular brown colour, with a cast of yellow, which the French call fauve, and to which the English writers on dyeing have appropriated the word fawn, though in fact a compound, is commonly ranked among simple colours; because it is applied to cloth by a single process. The substances employed to produce this colour are numerous; but we shall satisfy ourselves with enumerating the following:

Walnut-peels are the green covering of the walnut. When first separated, they are white internally; dyes, but soon assume a brown, or even a black colour, on exposure to the air. They readily yield their colouring matter to water. They are usually kept in large casks, covered with water, for above a year, before they are used. To dye wool brown with them, nothing more is necessary than to steep the cloth in a decoction of them till it has acquired the wished-for colour. The depth of the shade is proportional to the strength of the decoction. The root, as well as the peel of the walnut tree, contains the same colouring matter, but in smaller quantity. The bark of the birch, also, and many other trees, may be used for the same purpose.

It is very probable, that the brown colouring matter is in these vegetable substances combined with tan. This is certainly the case in fumach, which is often employed to produce a brown. This combination explains the reason why no mordant is necessary; the tan has a strong affinity for the cloth, and the colouring matter for the tan. The dye stuff and the mordant are already, in fact, combined together.

Chap. V. Of Compound Colours.

Compound colours are produced by mixing together two simple ones; or, which is the same thing, by dyeing cloth first one simple colour, and then another. The result is a compound colour, varying in shade according to the proportions of each of the simple colours employed.

Compound colours are exceedingly numerous, varying almost to infinity, according to the proportions of compound ingredients employed. They may be all arranged under the four following classes:

1. Mixtures of blue and yellow, 2. blue and red, 3. yellow and red, 4. black and other colours.

To describe all the different shades which belong to each of these classes, would be impossible; and even if it were possible, it would be unnecessary; because all the processes depend upon the principles laid down in the preceding chapters, and may easily be conceived and varied by those who understand these principles. In the following sections, therefore, it will be sufficient to mention the principal compound colours produced by the mixture of simple colours, and to exhibit a specimen or two of the mode of producing them.

Sect. I. Of Mixtures of Blue and Yellow.

The colour produced by mixtures of blue and yellow is Dyeing Substances.

Wool is usually dyed green by giving it first a blue colour, and afterwards dyeing it yellow; because, when the yellow is first given, several inconveniences follow: the yellow partly separates again in the blue vat, and communicates a green colour to it; and thus renders it useless for every other purpose, except dyeing green. Any of the processes for dyeing blue, described in the last chapter, may be followed; care being taken always to proportion the depth of the blue to the shade of green which is required. The cloth thus dyed blue may receive a yellow colour, by following the processes described in the last chapter for that purpose. When the sulphate of indigo is employed, it is usual to mix all the ingredients together, and to dye the cloth at once; the colour produced is known by the name of Saxon, or English green. One of the most convenient methods of conducting this process is the following:

Six or eight parts of quercitron bark, tied up in a bag, are to be put into the dyeing vessel, which should contain only a small quantity of warm water. When the water boils, six parts of muriatic sulphate of tin, and four parts of alum, are to be added. In a few minutes, the dyeing vessel should be filled up with cold water, till the temperature is reduced to about 130°. After this, as much sulphate of indigo is to be poured in as is sufficient to produce the intended shade of green. When the whole has been sufficiently stirred, a hundred parts of cloth are to be put in, and turned briskly for about fifteen minutes, till it has acquired the wished-for shade. By this method, a much more beautiful colour is obtained than is given by the usual process, in which tin is employed to give the yellow shade.

Silk, intended to receive a green colour, is usually dyed yellow first, by means of weld, according to the process described in the last chapter; afterwards, it is dipped into the blue vat, and dyed in the usual manner. To deepen the shade, or to vary the tint, decoctions of logwood, anota, fulvic, &c., are added to the yellow bath. Or silk may be dyed at once green, by adding suitable proportions of sulphate of indigo to the common quercitron bark bath, composed of four parts of bark, three parts of alum, and two parts of muriatic sulphate of tin.

Cotton and linen must be first dyed blue, and then yellow, according to the methods described in the last chapter. It is needless to add, that the depth of each of these colours must be proportioned to the shade of green colour which it is the intention of the dyer to give.

Sect. II. Of Mixtures of Blue and Red.

The mixture of blue and red produces violet, purple, and lilac, of various shades, and known by various names, according to the proportion of the ingredients employed. When the colour is deep, and inclines most to blue, it is called violet; but when the red is prevalent, it gets the name of purple. When the shade is light, the colour is usually called lilac. For violet, therefore, the cloth must receive a deeper blue; for purple, a deeper red; and for lilac, both of these colours must be light.

Wool is usually dyed first blue; the shade, even for violet, ought not to be deeper than that called sky blue; afterwards it is dyed scarlet, in the usual manner. The violets and purples are dyed first; and when the vat is somewhat exhausted, the cloth is dipped in which is to receive the lilac, and the other lighter shades. By means of sulphate of indigo, the whole process may be performed at once. The cloth is first alummed, and then dyed in a vessel containing cochineal, tartar, and sulphate of indigo, in proportions suited to the depth of the colour required. A violet colour may also be given to wool, by impregnating it with a mordant composed of tin dissolved in a mixture of sulphuric and muriatic acids, formed by dissolving muriate of soda in sulphuric acid; to which solution a quantity of tartar and sulphate of copper is added. The wool is then boiled in a decoction of logwood till it has acquired the wished-for colour.

Silk is first dyed crimson, by means of cochineal, in the usual way, excepting only that no tartar, nor solution of tin, is employed: It is then dipped into the indigo vat till it has acquired the wished-for shade. The cloth is often afterwards passed through an archil bath, which greatly improves the beauty of the colour. Archil is often employed as a substitute for cochineal: The silk first receives a red colour, in the usual way, by being dyed in an archil bath; afterwards it receives the proper shade of blue. The violet, or purple, given by this process is very beautiful, but not very lasting.

Silk may be dyed violet or purple at once, by first treating it with a mordant, composed of equal parts of nitro-muriate of tin and alum, and then dipping it into a cochineal bath, into which a proper quantity of sulphate of indigo has been poured. But this dye is fading; the blue colour soon decays, and the silk becomes red.

Cotton and linen are first dyed blue, then galled, then soaked in a decoction of logwood; some alum and acetite of copper are added to the decoction, and the cloth is soaked again. This process is repeated till the proper colour is obtained. The colour produced by this method is not nearly equal in permanency to that described in this Supplement under the word iron; to which we beg leave to refer the reader. The process there described has been long-known; but Mr Chapital has simplified it somewhat.

Sect. III. Of Mixtures of Yellow and Red.

The colour produced by the mixture of red and yellow is orange; but almost an infinity of shades results from the different proportions of the ingredients, and from the peculiar nature of the yellow employed. Sometimes blue is combined with red and yellow on cloth; the resulting colour is called olive.

Wool may be dyed orange by precisely the same process which is used for scarlet, only the proportion of red must be diminished, and that of yellow increased. When wool is first dyed red with madder, and then yellow with weld, the resulting colour is called cinnamon colour. The mordant, in this case, is a mixture of alum and tartar. The shade may be varied exceedingly, by using other yellow dye stuffs instead of weld, and by varying the proportions, according to circumstances. Thus a reddish yellow may be given to cloth, by first dyeing it yellow, and then passing it through a madder bath.

Silk is dyed orange by means of carthamus: the method method has been described in the last chapter. Cinnamon colour is given to it by dyeing it, previously alummed, in a bath composed of the decoction of logwood, Brazil wood, and tumeric mixed together.

Cotton and linen receive a cinnamon colour by means of weld and madder. The process is complicated. The cloth is first dyed with weld and acetite of copper, then dipped in a solution of sulphate of iron, then galled, then alummed, and then dyed in the usual way with madder.

For olive, the cloth is first dyed blue, then yellow, and finally passed through a madder bath. The shade depends upon the proportion of each of these colours. For very deep shades the cloth is also dipped into a solution of sulphate of iron. Cotton and linen may be dyed olive by dipping them into a bath, composed of the decoction of four parts of weld and one of potash, mixed with the decoction of Brazil wood and a little acetite of copper.

**Secr. IV. Of Mixtures of Black with other Colours.**

Strictly speaking, the mixtures belonging to this section are not mixtures of black colours with other colours, but combinations of the black dye with other colours; the ingredients of which, galls and brown oxyd of iron, being both mordants, variously modify other colouring matters by combining with them. Thus if cloth be previously combined with brown oxyd of iron, and afterwards dyed yellow with quercitron bark, the result will be a drab of different shades, according to the proportion of mordant employed. When the proportion is small, the colour inclines to olive or yellow; on the contrary, the drab may be deepened or saddened, as the dyers speak, by mixing a little sumach with the bark. The precautions formerly mentioned in applying the oxyd must be observed.

It is very common to dip cloth already dyed some particular colour into a solution of sulphate of iron, and galls or some other substance containing tan, called the black bath, in order to alter the shade, and to give the colour greater permanency. We shall give a few instances: greater minuteness would be inconsistent with the nature of this article.

Cloth dyed blue, by being dipped into the black bath, becomes bluish grey. Cloth dyed yellow, by the same process, becomes blackish grey, drab, or yellowish brown. Cloth previously alummed, and dyed in a decoction of cochineal and acetite of iron, acquires a permanent violet colour inclining to brown, or a like, if the dyeing vessel be somewhat exhausted. Cloth steeped in a mordant, composed of alum and acetite of iron dissolved in water, and afterwards dyed in a bath composed of the decoction of galls and madder mixed together, acquires a fine deep brown. The method of varying the shades of linen and cotton will be readily conceived, after we have given an account of calico printing, which forms the subject of the next chapter.

**Chap. VI. Of Calico Printing.**

Calico printing is the art of communicating different colours to particular spots or figures on the surface of cotton or linen cloth, while the rest of the stuff retains its original whiteness.

This ingenious art seems to have originated in India, where we know it has been practised for more than 2000 years. Pliny indeed informs us, that the Egyptians were acquainted with calico printing; but a variety of circumstances combine to render it more than probable that they borrowed it from India. The art has but lately been cultivated in Europe; but the enlightened industry of our manufacturers has already improved prodigiously upon the tedious processes of their Indian masters. No art has risen to perfection with greater celerity: a hundred years ago it was scarcely known in Europe; at present, the elegance of the patterns, the beauty and permanency of the colours, and the expedition with which the different operations are carried on, are really admirable.

A minute detail of the processes of calico printing would not only be foreign to the plan of this article, but of very little utility. To the artist the processes are already known; an account of them therefore could give him no new information; while it would fatigue and disappoint those readers who wish to understand the principles of the art. We shall content ourselves, therefore, with a short view of these principles.

Calico printing consists in impregnating those parts of the cloth which are to receive a colour with a mordant, and then dyeing it as usual with some dye stuff or other. The dye stuff attaches itself firmly only to that part of the cloth which has received the mordant, cotton. The whole surface of the cotton is indeed more or less tinged; but by washing it, and bleaching it for some days on the grass with the wrong side uppermost, all the unmordanted parts resume their original colour, while those which have received the mordant retain it. Let us suppose, that a piece of white cotton cloth is to receive red stripes; all the parts where the stripes are to appear are penciled over with a solution of acetite of alumina. After this, the cloth is dyed in the usual manner with madder. When taken out of the dyeing vessel, it is all of a red colour; but by washing and bleaching, the madder leaves every part of the cloth white except the stripes impregnated with the acetite of alumina, which remain red. In the same manner, may yellow stripes, or any other wished-for figure, be given to cloth, by substituting quercitron bark, weld, &c. for madder.

When different colours are to be given to different parts of the cloth at the same time, it is done by impregnating it with various mordants. Thus if stripes be drawn upon a cotton cloth with acetite of alumina, and other stripes with acetite of iron, and the cloth be afterwards dyed in the usual way with madder and then washed and bleached, it will be striped red and brown. The same mordants with quercitron bark give yellow, and olive or drab.

The mordants employed in calico printing are acetite of alumina and acetite of iron, prepared in the manner described in the third chapter of this part. These mordants are applied to the cloth, either with a pencil or by means of blocks, on which the pattern, according to which the cotton is to be printed, is cut. As they are applied only to particular parts of the cloth, care must be taken that none of them spread to the part of the cloth which is to be left white, and that they do not interfere with one another when more than one are applied. Dyeing Substances.

If these precautions be not attended to, all the elegance and beauty of the print must be destroyed. It is necessary, therefore, that the mordants should be of such a degree of confidence that they will not spread beyond those parts of the cloth on which they are applied. This is done by thickening them with flour or starch when they are to be applied by the block, and with gum arabic when they are to be put on with a pencil. The thickening should never be greater than is sufficient to prevent the spreading of the mordants; when carried too far, the cotton is apt not to be sufficiently saturated with the mordant; of course the dye takes but imperfectly.

In order that the parts of the cloth impregnated with mordants may be distinguished by their colour, it is usual to tinge the mordants with some colouring matter or other. The printers commonly use the decoction of Brazil wood for this purpose; but Bancroft has objected to this method, because he thinks that the Brazil wood colouring matter impedes the subsequent process of dyeing. It is certain, that the colouring matter of the Brazil wood is displaced during that operation by the superior affinity of the dye stuff for the mordant. Were it not for this superior affinity, the colour would not take at all. Dr Bancroft advises to colour the mordant with some of the dye stuff afterwards to be applied; and he cautions the using of more for that purpose than is sufficient to make the mordant distinguishable when applied to the cloth. The reason of this precaution is obvious. If too much dye be mixed with the mordant, a great proportion of the mordant will be combined with colouring matter; which must weaken its affinity for the cloth, and of course prevent it from combining with it in sufficient quantity to ensure a permanent dye.

Sometimes these two mordants are mixed together in different proportions; and sometimes one or both is mixed with an infusion of sumach or of nut galls. By these contrivances, a great variety of colours are produced by the same dye stuff.

After the mordants have been applied, the cloth must be completely dried. It is proper for this purpose to employ artificial heat; which will contribute something towards the separation of the acetous acid from its base, and towards its evaporation; by which the mordant will combine in a greater proportion, and more intimately with the cloth.

When the cloth is sufficiently dried, it is to be washed with warm water and cow dung, till all the flour or gum employed to thicken the mordants, and all those parts of the mordants which are uncombined with the cloth, are removed. The cow dung serves to entangle these loose particles of mordants, and to prevent them from combining with those parts of the cloth which are to remain white. After this the cloth is thoroughly rinsed in clean water.

Almost the only dye stuffs employed by calico printers are, indigo, madder, and quercitron bark or weld. This last substance, however, is now but little used by the printers of this country, except for delicate greenish yellows. The quercitron bark has almost superseeded it; because it gives colours equally good, and is much cheaper, and more convenient, not requiring so great a heat to fix it. Indigo, not requiring any mordant, is commonly applied at once either with the block or a pencil. It is prepared by boiling together indigo, potash made caustic by quicklime, and orpiment; the solution is afterwards thickened with gum (k). It must be carefully secluded from the air, otherwise the indigo would soon be regenerated, which would render the solution useless. Dr Bancroft has proposed to substitute coarse brown sugar for orpiment. It is equally efficacious in decomposing the indigo and rendering it soluble; while it likewise serves all the purposes of gum.

When the cloth, after being impregnated with the mordant, is sufficiently cleansed, it is dyed in the usual manner. The whole of it is more or less tinged with the dye stuff. It is well washed, and then spread out for some days on the grass, and bleached with the wrong side uppermost. This carries the colour off completely from all the parts of the cotton which has not imbibed the mordant, and leaves them of their original whiteness, while the mordanted spots retain the dye as strongly as ever.

Let us now give an example or two of the manner in which the printers give particular colours to calicoes. Some calicoes are only printed of one colour, others have two, others three, or more, even to the number of eight, ten, or twelve. The smaller the number of colours, the fewer in general are the processes.

1. One of the most common colours on cotton prints is a kind of nankeen yellow, of various shades, down to a deep yellowish brown or drab. It is usually in stripes or spots. To produce it, the printers befear a block, cut out into the figure of the print, with acetite of iron thickened with gum or flour; apply it to the cotton; which, after being dried and cleaned in the usual manner, is plunged

(k) Different proportions are used by different persons. Mr Hauffman mixes 15 gallons of water with 16 pounds of indigo well ground (or a greater or smaller quantity, according to the quality of the indigo and the depth of colour wanted); to which he adds 30 pounds of good carbonat of potash, placing the whole over a fire; and as soon as the mixture begins to boil, he adds, by a little at a time, 12 pounds of quick lime, to render the alkali caustic, by absorbing its carbonic acid. This being done, 12 pounds of red orpiment are also added to the mixture; which is then stirred, and left to boil for some little time, that the indigo may be perfectly dissolved; which may be known by its giving a yellow colour immediately upon being applied to a piece of white transparent glass. M. Oberkampf, proprietor of the celebrated manufactory at Jouy near Versailles, uses a third more of indigo; and others use different proportions, not only of indigo, but of lime, potash, and orpiment; which all seem to answer with nearly equal success; but with the best copper-coloured Guatemalan indigo, it is certain that a good blue may be obtained from only half the quantity prescribed by Mr Hauffman, by using as much stone, or oyster shell lime, as of indigo, nearly twice as much potash, and a fourth part less of orpiment than of indigo. See Bancroft, I. 113. plunged into a potash ley. The quantity of acetite of iron is always proportioned to the depth of the intended shade.

2. For yellow, the block is befeameared with acetite of alumina. The cloth, after receiving this mordant, is dyed with quercitron bark, and then bleached.

3. Red is communicated by the same process, only madder is substituted for the bark.

4. The fine light blues, which appear so often on printed cottons, are produced by applying to the cloth a block befeameared with a composition consisting partly of wax, which covers all those parts of the cloth which are to remain white. The cloth is then dyed in a cold indigo vat; and after it is dry, the wax composition is removed by means of hot water.

5. Lilac, flea brown, and blackish brown, are given by means of acetite of iron; the quantity of which is always proportioned to the depth of the shade. For very deep colours, a little sumach is added. The cotton is afterwards dyed in the usual manner with madder, and then bleached.

6. Dove colour and drab, by acetite of iron and quercitron bark.

When different colours are to appear in the same print, a greater number of operations are necessary. Two or more blocks are employed, upon each of which that part of the print only is cut which is to be of some particular colour. These are befeameared with different mordants, and applied to the cloth, which is afterwards dyed as usual. Let us suppose, for instance, that three blocks are applied to cotton; one with acetite of alumina, another with acetite of iron, a third with a mixture of these two mordants, and that the cotton is then dyed with quercitron bark, and bleached. The parts impregnated with the mordants would have the following colours:

- Acetite of alumina, - - - Yellow, - Iron, - - - Olive, drab, dove (1), - The mixture, - - - Olive green, olive.

If part of the yellow be covered over with the indigo liquor, applied with a pencil, it will be converted into green. By the same liquid, blue may be given to such parts of the print as require it.

If the cotton be dyed with madder instead of quercitron bark, the print will exhibit the following colours:

- Acetite of alumina, - - - Red, - Iron, - - - Brown, black, - The mixture, - - - Purple.

When a greater number of colours are to appear; for instance, when those communicated by bark and those by madder are wanted at the same time, mordants for part of the pattern are to be applied; the cotton is then to be dyed in the madder bath and bleached; then the rest of the mordants, to fill up the pattern, are added, and the cloth is again dyed with quercitron bark and bleached. This second dyeing does not much affect the madder colours; because the mordants, which render them permanent, are already saturated.

Suppl. Vol. II. Part II.

(1) According to the proportion of acetite of iron employed.

Sometimes a new mordant is also applied to some of the madder colours; in consequence of which they receive a new permanent colour from the bark. After the last bleaching, new colours may be added by means of the indigo liquor. The following table will give an idea of the colours which may be given to cotton by these complicated processes.

I. Madder dye. - Acetite of alumina, - - - Red, - Iron, - - - Brown, black, - Ditto diluted, - - - Lilac, - Both mixed, - - - Purple.

II. Bark dye. - Acetite of alumina, - - - Yellow, - Iron, - - - Dove, drab, - Lilac and acetite of alumina, Olive, - Red and acetite of alumina, Orange.

III. Indigo dye. - Indigo, - - - Blue, - Indigo and yellow - - Green.

Thus no less than 12 colours may be made to appear together in the same print by these different processes.

These instances will serve to give the reader an idea of the nature of calico printing, and at the same time afford an excellent illustration of the importance of mordants in dyeing.

If it were possible to procure colours sufficiently permanent, by applying them at once to the cloth by the pencil, block or the pencil, as is the case with the mordants, the art of calico printing would be brought to the greatest possible simplicity; but at present this can only be done in one case, that of indigo; every other colour requires dyeing. Compositions indeed may be made by previously combining the dye stuff and the mordants. Thus yellow may be applied at once by employing a mixture of the infusion of quercitron bark and acetite of alumina; red, by mixing the same mordant with the decoction of alumina, and so on. Unfortunately the colours applied in this way are far inferior in permanency to those produced when the mordant is previously combined with the cloth, and the dye stuff afterwards applied separately. In this way are applied almost all the fugitive colours of calicoes which wailing or even exposure to the air destroys.

As the application of colours in this way cannot always be avoided by calico printers, every method of rendering them more permanent is an object of importance. We shall therefore conclude this chapter with a description of several colours of this kind proposed by Dr Bancroft, which have a considerable degree of permanence.

A yellow printing colour may be formed by the following method: Let three pounds of alum, and three ounces of clean chalk, be first dissolved in a gallon of hot water, and then add two pounds of sugar of lead; stir this mixture occasionally during the space of 24 or 36 hours, then let it remain 12 hours at rest, and afterwards decant and preserve the clear liquor; this being ing done, pour so much more warm water upon the remaining sediment, as after stirring and leaving the mixture to settle will afford clear liquor enough to make, when mixed with the former, three quarts of this aluminoous mordant or acetite of alumine. Then take not less than six, nor more than eight, pounds of quercitron bark properly ground; put this into a tinned copper vessel, with four or five gallons of clean soft water, and make it boil for the space of one hour at least, adding a little more water, if at any time the quantity of liquor should not be sufficient to cover the surface of the bark: the liquor having boiled sufficiently, should be taken from the fire, and left undisturbed for half an hour, and then the clear decoction should be poured off through a fine sieve or canvas strainer. This being done, let fix quarts more of clear water be poured upon the same bark, and made to boil ten or fifteen minutes, both having been first well stirred; and being afterwards left a sufficient time to settle, the clear decoction may then be strained off, and put with the former into a shallow wide vessel to be evaporated by boiling, until what remains, being joined to the three quarts of aluminoous mordant before mentioned, and to a sufficient quantity of gum or patte for thickening, will barely suffice to make three gallons of liquor in the whole. It will be proper, however, not to add the aluminoous mordant, until the decoction is so far cooled as to be but little more than blood warm; and these being thoroughly mixed by stirring, may afterwards be thickened by the gum of Senegal or by gum arabic, if the mixture is intended for pencilling; or by a paste made with starch or flour, if it be intended for printing.

By substituting a pound of muriato-sulphat of tin for the aluminoous mordant in the above composition, a mixture may be formed which affords a very bright and full yellow, of considerable durability.

Sulphat of tin, mixed with a decoction of quercitron bark, communicates to cotton a cinnamon colour, which is sufficiently permanent.

When the decoctions of quercitron bark and logwood are boiled together, and suitable proportions of fulphat of copper and of verdigris are added to them, with a little carbonat of potash, a compound is formed, which gives a green colour to cotton. Bancroft has made trial of this; and though it has not fully answered his expectation, his attempts were attended with sufficient success to determine him to persevere in his experiments.

If acetite of iron be mixed with a decoction of quercitron bark, and the mixture be properly thickened, the compound will communicate to cotton a drab colour of some durability. This compound, mixed with the olive colouring liquor above described, will produce an olive. If a solution of iron, by a diluted muriatic acid, or by a diluted nitric acid, be employed for this purpose instead of iron liquor, it will produce colours a little more lasting; but these solutions should be employed sparingly, that they may not hurt the texture of the linen or cotton to which they are intended to be applied.

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