a yellow color, which is more or less deep, according to the concentration of the acid, its temperature, and the time of immersion; that the silk must be carefully washed as soon as taken out of the acid; that this color possesses considerable brightness; and that it may be made deep without sensibly weakening the silk, which may render the process really useful. The color may also be modified by the use of alkalis. The solutions of calcareous earth and magnesia produce no effect upon silk, because they do not contain an excess of acid; but the solutions of alumine and of all metallic substances, produce a more or less deep yellow, because they all contain more or less excess of acid, which acts upon the silk like uncombined acid."

49. It appears likewise to have been the acid alone that dyed the animal substances yellow, in the experiments of M. Brunwiser, and not the matter extracted from the wood, as he supposed. Nor is the yellow color in these cases owing to iron, as De la Folie supposed; for the purest nitrous acid, which contains no iron, produces it, as well as that in which the presence of that metal may be supposed to exist. Silk, when put into concentrated nitrous acid, quickly assumes a deep yellow color, loses its cohesion, and is dissolved; during this solution, the azote, which enters into the composition of animal substances, is extricated, with a long continued effervescence; if heat be applied, it expels much nitrous gas, and the liquor immediately acquires a deep color and grows brown. At this time, the oxygen of the nitric acid combines with the hydrogen which abounds in animal substances, forming the oil which is obtained from them by distillation, and which renders them so inflammable. When the acid begins to act, and to render the silk yellow, the same effect should also begin to take place. M. Berthollet therefore supposes, that the yellow color arises from a commencement of combustion; but that this combustion being very slight, does not sensibly weaken the silk; if, however, the acid be a little too strong, or the immersion too long continued, or if the whole of it be not carried off by careful washing, the silk immediately becomes weak, and is burnt. It is, therefore, evident why the nitrous acid is preferable in this operation to that which is saturated with nitrous gas; for, in the former, the proportion of oxygen being greater, it is better fitted to produce the effects of combustion, than it becomes in the state of nitrous acid. The same explanation ought to apply to the action of the oxygenated muriatic acid on animal substances; it differs, however, in some essential circumstances, which are not easily explained.

by accumulation, is capable of disguising the yellow color occasioned by the combustion, which it had originally induced. Berthollet has endeavoured to explain the effects which the sulphurous acid produces on colors, by the facility with which it gives off its oxygen, and has compared them to those of the oxygenated muriatic acid; but, although it be true that oxygen adheres much more weakly to the sulphurous than to the sulphuric acid, he does not believe that that explanation is founded in truth.

51. It appears from the observation of De la Folie, that roses, whitened by the vapor of burning sulphur, become green in an alkaline lixivium, and red in acids; and M. Berthollet has himself observed, that the sulphurous acid reddened the tincture of turnsole, which has a very fading color, but that it acted only like other acids, on infusions of fustic, Brasil-wood and logwood; and further, that silk which has been exposed to the vapor of sulphur, exhaled the smell of sulphurous acid, when moistened with sulphuric acid, although it could not be perceived before that odor existed. He therefore supposes, that the sulphurous acid commonly unites with the coloring particles, and with the silk, without giving off its oxygen to them, and consequently without producing any combustion; that the product of that combination sometimes loses its color entirely, which is probably owing to the semi-elastic state of the oxygen; but sometimes combustion may, and even commonly should take place by degrees, so that the coloring particles, which have been disguised for some time, ought ultimately to leave a yellow color.

OF ASTRINGENTS.

52. Astringents deserve particular attention, not only from their great use in dyeing, but as possessing a property common to many vegetables. Perhaps, says Berthollet, there is no property in vegetables concerning which such vague ideas have been currently received. A slight relation in taste has frequently been deemed enough to rank them in the class of astringents; and every substance has been commonly regarded as astringent, or acerb, which turned a solution of iron black. This effect has been presumed to arise from one identical principle residing in all the bodies that produce it. Experience has subsequently shown, that two species of astringents ought to be admitted, viz. tannin and gallic acid. The gallic acid is obtained from gallnuts, in which it is found in great plenty.

by alcohol. And the same quantity treated first with alcohol, and then with water, afforded twelve drachms and two scruples of spirituous extract, and four scruples of watery extract; the residuum weighed half a scruple more than in the preceding experiment. In the spirituous extract, the taste is more strong and disagreeable than in the watery extract.

54. Many other very interesting observations have been made on astringent substances, by Messrs. Scheele, Monnet, and Berthollet. The latter seems to have proved, that it is not the gallic acid which communicates the astringent properties to the substances that possess it; that the acid itself possesses that property, in a degree inferior to other astringents; and that sumach, treated like the galls, in the manner described by Scheele, affords no gallic acid, though it possesses a high degree of astringency; walnut peels, treated in the same way, do not afford any. The property which the infusion of common galls has, of reddening certain vegetable colors, appears to proceed only from the gallic acid. The infusions of sumach, or of sloe-bark, which very readily produce a black precipitate, that of walnut-tree bark, or of quinquina, did not exhibit this property; and thence it is evident, that the gallic acid does not exist in white galls; for the infusion of these, though it deposit a copious sediment on exposure to the air, is not the gallic acid.

55. If the astringent property were owing to an individual principle distributed in different vegetables, the precipitates obtained by their means, from a solution of iron, would constantly form the same compounds, and exhibit the same appearances and properties; but the precipitate produced by galls is of a blackish blue: that by logwood has a different shade of blue; that by oak is of a fawn color, or blackish brown; that by quinquina, a blackish green. They fall down with different attendant circumstances, and when fixed on stuffs, are discharged by alum and tartar, some much more easily than others; and, probably, by multiplying experiments, many other remarkable differences may be discovered in the properties of these different precipitates. Astringents form with iron different species of compounds, and consequently do not derive their properties from one principle; but there must be a property common to different substances, to enable them to act uniformly on solutions of iron, and to produce precipitates more or less black, and thus appearing of the same

nature.

56. The metallic oxides, which unite with the coloring particles, modify their colors; but some metallic oxides, and particularly that of iron, have colors which vary according to the quantity of oxygen they contain. Iron, when united with only a small quantity of oxygen, has a black color. If any substance, by uniting with the oxide of iron, had the property of taking from it a part of the oxygen, which it has when precipitated from its solution in an acid, this would be sufficient to give it a black color; and if the peculiar color of this substance were not predominant, or of itself inclining to black, the compound formed would have a black color; thus ni

trous gas, either uncombined or weakly attached to the nitrous acid, renders solutions of iron black, and even precipitates the metal, by depriving it of a portion of its oxygen. By acting in the same manner, ammoniac produces a black precipitate with the solutions of iron; in this case, the hydrogen of the ammoniac forms water, by combining with the oxygen that is disengaged from the oxide of the iron. Galls precipitate gold and silver from their solutions, by reducing them to their metallic state; they, therefore, have the property of separating the oxygen from those metals, to which it adheres but slightly; and, from others, that portion which is retained in the weakest degree. Any infusion of galls, of itself, readily assumes a deep brown color, by exposure to the air; though it absorbs but a small quantity of vital air. The infusion of sumach, and that of woods and barks, also acquire a dark color by exposure to the air; so that when acting upon the oxide of iron, by separating a part of its oxygen, an astringent ought itself to acquire a darker color, by which the black should be assisted.

57. Various substances, which have in other respects different properties, produce black with solutions of iron. Among these, some are real coloring particles, and employed as such in dyeing. Logwood, and even most kinds of coloring particles, form brown or blackish precipitates with iron. Sometimes the astringent effect is not instantaneous; the color of the precipitate is at first light; it grows deeper gradually, being darkened in proportion as the iron loses its oxygen. The infusion of fustic produces, with the solution of iron, a yellow precipitate, that grows brown by degrees, and becomes black after a considerable time. But though the property of precipitating solutions of iron black, does not indicate the presence of the same individual principle in the substances which possess it, there can be no inconvenience in calling it by the name of astringent, provided by that term is meant only a property, which is common to a great number of substances, and which they may have in various proportions.

58. The astringent principle is found to precipitate iron from all acids. The acids of phosphorus and arsenic only have a stronger attraction than it has for irou. The phosphoric acid was known to have the property of separating iron from the sulphuric acid; but all acids, except the acetous, and probably some other vegetable acids which have not been tried, redissolve the precipitate, and make the color disappear, until they are saturated with an alkali. It is not surprising, that the astringent principle can unite with metallic oxides, without having the qualities of an acid; for animal substances, oils, even alkalis, and lime, have this property. It is well known, that it is the precipitate composed of iron and the astringent principle, which, by remaining suspended in the liquor, forms ink.

59. But although chemists considered the astringent principle as always the same, experience shows, that all astringent substances are not equally proper for producing a beautiful and durable black; it is of importance to determine which of them may be employed with the greatest success; it is, however, very difficult to make

comparative experiments on this subject with perfect accuracy, because some substances require much longer boiling than others to extract their astringency; because a difference in their coarseness or fineness, when subjected to ebullition, is sufficient to produce differences in the results; and because the coloring particles have a greater or less disposition to combine with the stuff, according to the proportion of sulphate of iron that has been made use of. Solutions of iron in different acids may produce differences in the results, according to the state of oxygenation of the iron in them, according as the proportion of that metal is greater or less, and according to the degree of strength which the different acids, when disengaged, are capable of exerting on the newly-formed compound.

60. In the dyeing of stuffs also some differences will be found to arise from their greater or less attraction for the coloring particles. Dr. Lewis has proved in his excellent observations on the process of making ink, that no known astringent, not even sumach, can be substituted for gall-nuts. If, says M. Berthollet, too large a proportion of sulphate of iron be added to the galls, the ink becomes speedily brown, and then passes to yellow, because the astringent is destroyed by the action of the oxygen, which the sulphate of the iron affords, or progressively attracts from the atmosphere; for we see that oxygen eventually destroys those coloring substances with which it is combined in too great quantities. When this accident happens from age, Dr. Lewis found that an infusion of galls passed over the faded characters restored them. According to Dr. Ure, the best restorative for faded writing is a solution of ferroprussiate of potash, faintly acidulated, or sulphuretted hydrogen water. Dr. Lewis ascertained, by repeated experiments, that the best proportion for ink is three parts of gall-nuts to one of sulphate of iron; that cherry-gum, and plum-tree gum, are as good as gum-arabic for giving the necessary consistence, and for keeping suspended the black molecules which tend to fall; and that decoction of logwood employed instead of water for the infusion of the galls improves the beauty of the ink.

61. Mr. Beunie, made many experiments to determine the best process for giving cotton a durable black. He first tried what solution of iron gave the finest black to galled cotton; he afterwards combined different solutions, and examined the durability of the blacks which he produced; and made the same experiments on galled cotton, with other metals and semimetals; he employed in like manner a great number of astringents, and tried with them cotton which had received different preparations. He found that out of twenty-one species of astringents, oak saw-dust, the galls of the country, and yellow myrobolans, were the only substances which produced a fine black, but which was still neither so fine nor so durable as that obtained by the common galls. He also found that the oak sawdust is preferable to the bark, employed by the dyers of thread, and, being cheaper, may be substituted with advantage.

62. Messrs. Lavoisier, Vandermonde, Four

roy, and Berthollet, made experiments on galls, oak-bark, raspings of heart of oak, the external part of oak, of logwood, and sumach, for the purpose of forming a comparison of their qualities. To ascertain the portion of astringent principle contained in these different substances, they took two ounces of each separately, which they boiled half an hour in three pounds of water; after the first water they added a second, which underwent a similar ebullition; and continued these operations until the substances appeared exhausted: they then mixed together the decoctions that had been successively obtained. A transparent solution of sulphate of iron, in which the proportions of water and sulphate had been exactly determined, was used. They first estimated the quantity of the astringent principle, by the quantity of sulphate which each liquor could decompose, and afterwards by the weight of the black precipitate which was formed. In order to stop precisely at the point of saturation, they proceeded very slowly in the precipitation, and towards the end added the solution of sulphate only drop by drop, and ceased at the moment when the last added quantity no longer augmented the intensity of the black color. When the liquor is too opaque to allow its shade of color to be distinguished, a small quantity of it is largely diluted with water, and, by adding to this a little of the solution of sulphate of iron at the end of a glass tube, it is discovered whether or not the point of saturation has been attained: if we then wish to get the precipitate which is formed, the whole must be diluted with water very copiously.

63. This operation is an easy and accurate mode for manufacturers to determine the proper proportions of astringents, and solutions of iron. To saturate the decoction of two ounces of galls, three drachms and sixty-one grains of iron were required; the precipitate weighed seven drachms and twenty-four grains, when collected and dried. The color of the decoction of oak bark is a deep yellow; a very small portion of sulphate of iron gives it a dirty reddish color, and a larger one changes it to a deep brown. The quantity of sulphate required to saturate the decoction of two ounces of this bark, was eighteen grains. The precipitate, collected and dried, formed coarser and more compact grains, and weighed twentytwo grains; the inner bark of the oak afforded nearly the same result. But the decoction of the raspings of the heart of oak required for it 3 saturation one drachm and twenty-four grains and the precipitate weighed one drachm and twenty-four grains; the decoction of the external wood of the oak produced very little precipitate. The decoction of sumach acquired a reddish violet color, when a small quantity of the sulphate of iron was added. The quantity required for its saturation was two drachms eighteen grains. The precipitate exactly resembled that afforded by the galls. And the decoction of logwood became of a sapphire blue color, by the addition of sulphate of iron: if the point of saturation be exceeded, the blue becomes greenish and dirty. The exact quantity required for saturation was found to be one drachm forty-eight grains, and the weight of the precipitate was two drachms twelve

grains. The different precipitations made by oak take place readily; that by logwood, a little more difficultly, but still more easily than that which is effected by galls.

64. It was next ascertained, by trials made with cloth, that the quantity of astringent substances required to give a black color of intensity, to an equal weight of the same cloth, was proportional to the quantities of astringent principle, which had been already estimated in each kind from the foregoing experiments; but the black obtained by the different parts of the oak does not resist proofs of color, nearly so well as that which is produced by galls. Logwood alone seems not capable of producing so intense a black as galls or oak; nor does the color which it produces stand the test of proofs so well as that produced by galls.

65. We shall now consider the astringent principle in regard to its property of combining with vegetable and animal substances, particularly the latter. Silk acquires by galling, which is an operation that consists in macerating a stuff in a decoction of some astringent substance, a weight which cannot be taken from it, or dimin ished beyond a certain degree, by repeated washing; after which operation the stuff when put into a solution of iron is dyed black, because the astringent principle, decomposing the sulphate of iron, forms a triple compound with the oxide of iron and the stuff which is dyed. A stuff that is galled is likewise capable of combining with other coloring particles, the colors of which thereby acquire fixity, if they do not naturally possess it; so that the astringent communicates its durability to the triple compound, or perhaps the more complex one which is formed; but by this union the color generally becomes of a deeper shade. The astringent principle, by combining with animal substances, renders them incapable of corruption, and tends to render their texture more compact; and in this the art of tanning consists.

66. It may be proper to take some notice here of the substance denominated tannin, which, while it has some properties in common with the gallic acid, differs from it in others. Seguin was the first who showed that astringents contained a peculiar substance, which, in combining with skin, gave it the properties of tanned leather, and that the tanning effect arose from the combination thus formed. Tannin may be procured by digesting gall-nuts, grape-seeds, oak-bark, or catechu, in a small quantity of cold water. The solution, when evaporated, affords a substance of a brownish-yellow color, highly astringent, and soluble in water and in alcohol. According to Mr. Brand, the purest form of tannin appears to be derived from bruised grape-seeds; but even here, he observes, it is combined with other substances, from which it is, perhaps, scarcely separable. I have never, says he, been able to obtain it of greater purity than by digesting powdered catechu in water at 33° or 34°, filtering and boiling the solution, which, on cooling, becomes slightly turbid, and is to be filtered again, and evaporated to dryness; cold water, applied as before, extracts nearly pure tannin. The most distinctive cha

racter of tannin is that of affording an insoluble precipitate when added to a solution of isinglass, or any other animal jelly. On this property the art of tanning depends, for which oak bark is generally employed; but the barks of many other trees are frequently employed for the same purpose. Professor Proust recommends the precipitation of a decoction of galls by powdered carbonate of potassa, for obtaining tannin, washing well the greenish-gray flakes that fall down with cold water, and drying them in a stove. This precipitate becomes brown in the air, brittle and shining like a resin, and yet remains soluble in hot water. In this state the tannin, he says, is very pure. According to Berzelius, tannin consists of hydrogen 4186 + carbon 51.160+ oxygen 44.654.

67. M. Berthollet considers the abundance of charcoal as the essential characteristic of the astringent principle; the hydrogen, which it contains only in small quantity, is however very much disposed partially to combine with oxygen: Hence, when an infusion of galls is left in contact with vital air, a small quantity of the air only is absorbed, and yet the color of the infusion becomes much deeper; for, in conformity with the theory already laid down, the charcoal readily becomes predominant in consequence of the slight combustion, and the color is rendered deeper, and becomes brown.

68. Substances which contain much charcoal, and can undergo only a slight degree of combustion, ought to possess considerable durability, because charcoal does not combine with oxygen in the ordinary temperature of the air, unless its union be assisted by other attractions, and because slight variations of temperature produce no change in the dimensions of charcoal; but, on the contrary, substances which contain much hydrogen, and in which the particles of the hydrogen are in a state of division, ought to be easily decomposed, by the combination of the hydrogen with azote or oxygen. The disunion of their parts ought to take place from small variations of temperature, because hydrogen is dilatable by heat, which the carbonaceous particles are not. When, therefore, the astringent principle is combined with an animal substance, it communicates to it the properties which it derives from the charcoal; the animal substance becomes less liable to change from slight variations of temperature; instead of growing putrid, it suffers a slight degree of combustion, by the action of the air; for the process of tanning probably could not go on in a perfectly close vessel.

69. On examining the analyses that have been made of indigo, which may be looked upon as the coloring matter least liable to change of any with which we are acquainted, it will be found that this substance leaves, in distillation, a greater proportion of charcoal than even galls themselves. M. Berthollet supposes that it is also to this abundance of charcoal, that the durability of the color of indigo is to be attributed, and that the proportion of this principle is the chief cause of the difference observed in the durability of colors; but the force of adhesion may also have great influence, for a principle

which combines intimately with another substance, ought to form with it a more permanent compound, than one which has only a slight disposition to unite with it; now the astringent principle possesses a very strong disposition to form intimate combinations, especially with animal substances.

70. Upon the same principles may be explained the fixity communicated to coloring particles by alumine, and by those metallic oxides which are not liable to contain different proportions of oxygen, such as the oxide of tin, and some others. The different coloring substances, capable of uniting with metallic oxides, have an action upon them, analogous to that of astringents. The oxides are deprived of more or less of their oxygen, according to the force with which they retain it, the strength of attraction with which the coloring particles tend to combine with them, the proportions in which they meet with each other, and the greater or less disposition of the coloring particles towards combustion.

71. The coloring particles also suffer a change in their constitution from these circumstances: thus the solutions of iron render brown all the colors into which oxide of iron can enter, although it has only a green or yellow color in the state in which it is held in solution by acids, and this effect goes on increasing to a certain degree; but the alteration of the coloring particles may afterwards be carried so far as to spoil their color, and to diminish their tendency to combination; the oxide of iron is then brought back to the yellow color by the oxygen which it attracts, and is capable of retaining. The action of metallic oxides and the coloring particles on each other, explains the changes observed in solutions of the coloring particles, when mixed with metallic solutions. The effect is gradual, as has been shown with respect to fustic. It sometimes happens that the mixture does not even grow turbid immediately, but loses its transparency by degrees; the precipitation begins; the sediment is formed; and its color becomes gradually deeper. In producing these effects, light has sometimes a considerable share. 72. Upon the whole, we may conclude, that metallic colors should be distinguished from those which are peculiar to substances of the vegetable and animal kind: that the colors of metals are modified and changed by oxidation, and by the proportion of oxygen with which they are combined; and that vegetable and animal substances may themselves possess a peculiar color, which varies in the different states through which they pass, or they may owe their colors to colored particles, either combined, or simply mixed with them. These are the particles which are extracted from different substances, and which undergo different preparations, in order to render them proper for the various purposes of dying. And the coloring particles possess chemical properties which distinguish them from all other substances: the affinities which they have for acids, alkalis, earths, metallic oxide. oxygen, wool, silk, cotton, and linen, from the principal of these properties. In proportion to the affiuity which the coloring

particles have for wool, silk, cotton, and linen, they unite more or less readily and intimately with them: and thence arises the first cause of variation in the processes employed, according to the nature of the stuff, and of the coloring substance employed. And by the affinity which the coloring particles have for alumine and metallic oxides, they form compounds with these substances, in which their color is more or less modified, and becomes more fixed, and less affected by external agents than before. This compound being formed of principles which have separately the power of uniting with vegetable substances, and more especially with animal substances, preserves this property, and forms a triple compound with the stuff; and the color, which has been again modified by the formation of this triple union, acquires a greater degree of fixity, and of indestructibility, when exposed to the action of external agents.

73. The coloring particles have often so great an affinity for alumine and metallic oxides, that they separate them from acids which held them in solution, and fall down with them; but the affinity of the stuff is sometimes necessary, in order that this separation may take place. The oxides of metals, which combine with the coloring particles, modify their colors, not only by their own, but also by acting upon their composition by their oxygen. The change which the coloring particles thereby suffer, is similar to that occasioned by the air, which injures every color in a greater or less degree. In the two different principles which constitute the air or the atmosphere, it is only the oxygenous gas that acts upon the coloring particles. It combines with them, weakening their color, and rendering it paler; but presently its action is principally exerted on the hydrogen, which enters into their compositiou, and it then forms water. This effect, continues M Berthollet, ought to be considered as a true combustion, whereby the charcoal which enters into the composition of the coloring particles becomes predominant, and the color commonly changes to yellow, fawn color, or brown; or the injured part, by uniting with what remains of the original color, causes other appearances of a different kind. The combustion of the coloring particles is increased by light, and frequently cannot take place without its aid; it is indeed in this way that it contributes to the destruction of colors. Heat promotes it also, but less powerfully than light, provided its intensity be not very great. The effects of the nitric acid, the oxygenated muriatic acid, and even the sulphuric acid, when they make the color of the substances upon which they act pass to a yellow and even to black, are to be attributed to a combustion of a similar nature.

74. The effects of combustion may, however, be concealed, by the oxygen combining with the coloring particles, without the hydrogen being particularly acted upon by it. But colors are more or less fixed, in proportion to the greater or less disposition of the coloring particles to suffer this combustion. There are some substances also capable of acting on the color of stuffs, by a stronger affinity, or by a solvent power; and in this consists the action of acids, alkalis. and soap.