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a yellow color, which is more or less deep, ac- by accumulation, is capable of disguising the cording to the concentration of the acid, its tem- yellow color occasioned by the combustion, perature, and the time of immersion ; that the which it had originally induced. Berthollet has silk must be carefully washed as soon as taken endeavoured to explain the effects which the out of the acid; that this color possesses consi- sulphurous acid produces on colors, by the faderable brightness; and that it may be made cility with which it gives off its oxygen, and has deep without sensibly weakening the silk, which compared them to those of the oxygenated mumay render the process really useful. The color riatic acid; but, although it be true that oxygen may also be modified by the use of alkalis. adheres much more weakly to the sulphurous The solutions of calcareous earth and magnesia than to the sulphuric acid, he does not believe produce no effect upon silk, because they do not that that explanation is founded in truth. contain an excess of acid; but the solutions of 51. It appears from the observation of De la alumine and of all metallic substances, produce Folie, that roses, whitened by the vapor of burna more or less deep yellow, because they all ing sulphur, become green in an alkaline lixivium, contain more or less excess of acid, which acts and red in acids; and M. Berthollet has himself upon the silk like uncombined acid.

observed, that the sulphurous acid reddened the 49. It appears likewise to have been the acid tincture of turnsole, which has a very fading alone that dyed the animal substances yellow, in color, but that it acted only like other acids, on the experiments of M. Brunwiser, and not the infusions of fustic, Brasil-wood and logwood; matter extracted from the wood, as he supposed. and further, that silk which has been exposed to Nor is the yellow color in these cases owing to the vapor of sulphur, exhaled the smell of suliron, as De la Folie supposed; for the purest phurous acid, when moistened with sulphuric nitrous acid, which contains no iron, produces it, acid, although it could not be perceived before as well as that in which the presence of that that odor existed. He therefore supposes, that metal may be supposed to exist. Silk, when put the sulphurous acid commonly unites with the into concentrated nitrous acid, quickly assumes coloring particles, and with the silk, without a deep yellow color, loses its cohesion, and is giving off its oxygen to them, and consequently dissolved; during this solution, the azote, which without producing any combustion; that the enters into the composition of animal substances, product of that combination sometimes loses its is extricated, with a long continued efferves- color entirely, which is probably owing to the cence; if heat be applied, it expels much nitrous semi-elastic state of the oxygen; but sometimes gas, and the liquor immediately acquires a deep combustion may, and even com

mmonly should color and grows brown. At this time, the oxygen take place by degrees, so that the coloring partiof the nitric acid combines with the hydrogen cles, which have been disguised for some time, which abounds in animal substances, forming ought ultimately to leave a yellow color. the oil which is obtained from them by distilla

OF ASTRINGENTS. tion, and which renders them so inflammable. When the acid begins to act, and to render the 52. Astringents deserve particular attention, silk yellow, the same effect should also begin to not only from their great use in dyeing, but as take place. M. Berthollet therefore supposes, possessing a property common to many vegethat the yellow color arises from a commence- tables. Perhaps, says Berthollet, there is no ment of combustion ; but that this combustion property in vegetables concerning which such being very slight, does not sensibly weaken the vague ideas have been currently received. A silk; if, however, the acid be a little too strong, slight relation in taste has frequently been deemed or the immersion too long continued, or if the enongh to rank them in the class of astringents; whole of it be not carried off by careful washing, and every substance has been commonly regarded the silk immediately becomes weak, and is as astringent, or acerb, which turned a solution of burnt. It is, therefore, evident why the nitrous iron black. This effect has been presumed to acid is preferable in this operation to that which arise from one identical principle residing in all is saturated with nitrous gas; for, in the former, the bodies that produce it. Experience has subthe proportion of oxygen being greater, it is sequently shown, that two species of astringents better fitted to produce the effects of combustion, ought to be admitted, viz. tannin and gallic than it becomes in the state of nitrous acid, acid. The gallic acid is obtained from gallThe same explanation ought to apply to the nuts, in which it is found in great plenty. action of the oxygenated muriatic acid on animal 53. The gall-nut is an excrescence found on substances; it differs, however, in some essential the young branches of the oak, and produced by circumstances, which are not easily explained. the puncture of an insect. Different kinds of

50. Silk has been observed to receive a yellow the gall-nut are met with, some inclining to color when the oxygenated muriatic acid is em- white, yellow, green, brown, or red; others, ashployed, which is much lighter than when the colored or blackish. They also differ greatly in nitrous acid is made use of; the sulphurous magnitude, and are either round or irregular, acid discharges it in a great degree, but has no heavy or light, smooth or covered with protubeeffect on the yellow produced by the diluted rances. Those which are small, blackish, knotted, nitrous acid. The oxygenated muriatic acid and heavy, are the best; and are known by the has, however, a much stronger action on the name of Aleppo galls. These astringent subsilk; it soon weakens, and even dissolves it; and stances are almost totally soluble in water by if it be left for some time in this fluid, the yellow long ebullition. Sixteen drachms afforded Neu which at first appeared grows lighter, agreeably mann fourteen of extract; from the remaining to what has already been remarked, that oxygen, two drachms, only four grains could be extracted

by alcohol. And the same quantity treated first trous gas, either uncombined or weakly attached to with alcohol, and then with water, afforded twelve the nitrous acid, renders solutions of iron black, drachms and two scruples of spirituous extract, and even precipitates the metal, by depriving it of and four scruples of watery extract; the residuum a portion of its oxygen. By acting in the same weighed half a scruple more than in the preced- manner, ammoniac produces a black precipitate ing experiment. In the spirituous extract, the with the solutions of iron; in this case, the hytaste is more strong and disagreeable than in the drogen of the ammoniac forms water, by comwatery extract.

hining with the oxygen that is disengaged from 54. Many other very interesting observations the oxide of the iron. Galls precipitate gold and have been made on astringent substances, by silver from their solutions, by reducing them to Messrs. Scheele, Monnet, and Berthollet. The their metallic state; they, therefore, have the latter seems to have proved, that it is not the property of separating the oxygen from those gallic acid which communicates the astringent metals, to which it adheres but slightly; and, properties to the substances that possess it; that from others, that portion which is retained in the the acid itself possesses that property, in a de- weakest degree. "Any infusion of galls, of itself, gree inferior to other astringents; and that readily assumes a deep brown color, by exposure sumach, treated like the galls, in the manner to the air; though it absorbs but a small quantity described hy Scheele, affords no gallic acid, of vital air. The infusion of sumach, and that though it possesses a high degree of astringency; of woods and barks, also acquire a dark color walnut peels, treated in the same way, do not by exposure to the air; so that when acting afford any. The property which the infusion of upon the oxide of iron, by separating a part of common galls has, of reddening certain vegetable its oxygen, an astringent ought itself to acquire a colors, appears to proceed only from the gallic darker color, by which the black should be assisted. acid. The infusions of sumach, or of sloe-bark, 57. Various substances, which have in other which very readily produce a black precipitate, respects different properties, produce black with that of walnut-tree bark, or of quinquina, did solutions of iron. Among these, some are real not exhibit this property; and thence it is evi- coloring particles, and employed as such in dyedent, that the gallic acid does not exist in white ing. Logwood, and even most kinds of coloring galls; for the infusion of these, though it deposit particles, form brown or blackish precipitates a copious sediment on exposure to the air, is not with iron. Sometimes the astringent effect is the gallic acid.

not instantaneous; the color of the precipitate is 55. If the astringent property were owing to at first light; it grows deeper gradually, being an individual principle distributed in different darkened in proportion as the iron loses its vegetables, the precipitates obtained by their oxygen. The infusion of fustic produces, with means, from a solution of iron, would constantly the solution of iron, a yellow precipitate, that form the same compounds, and exhibit the same grows brown by degrees, and becomes black appearances and properties; but the precipitate after a considerable time. But though the proproduced by galls is of a blackish blue: that by perty of precipitating solutions of iron black, logwood has a different shade of blue; that by does not indicate the presence of the same indioak is of a fawn color, or blackish brown; that vidnal principle in the substances which possess by quinquina, a blackish green. They fall down it, there can be no inconvenience in calling it by with different attendant circumstances, and when the name of astringent, provided by that term is fixed on stuffs, are discharged by alum and meant only a property, which is common to a tartar, some much more easily than others; and, great number of substances, and which they may probably, by multiplying experiments, many have in various proportions. other remarkable differences may be discovered 58. The astringent principle is found to preciin the properties of these different precipitates. pitate iron from all acids. "The acids of phosAstringents form with iron different especies of phorus and arsenic only have a stronger attraction compounds, and consequently do not derive than it has for iron. The phosphoric acid was their properties from one principle; but there known to have the property of separating iron must be a property common to different sub- from the sulphuric acid; but all acids, except the stances, to enable them to act uniformly on so- acetous, and probably some other vegetable acids lutions of iron, and to produce precipitates more which have not been tried, redissolve the precior less black, and thus appearing of the same pitate, and make the color disappear, until they

are saturated with an alkali. It is not surprising, 56. The metallic oxides, which unite with the that the astringent principle can unite with mecoloring particles, modify their colors; but some tallic oxides, without having the qualities of an metallic oxides, and particularly that of iron, acid; for animal substances, oils, even alkalis, have colors which vary according to the quantity and lime, have this property. It is well known, of oxygen they contain. Iron, when united with that it is the precipitate composed of iron and only a small quantity of oxygen, has a black the astringent principle, which, by remaining color. If any substance, by uniting with the suspended in the liquor, forms ink. oxide of iron, had the property of taking from it 59. But although chemists considered the a part of the oxygen, which it has when preci- astringent principle as always the same, expepitated from its solution in an acid, this would rience shows, that all astringent substances are not be sufficient to give it a black color; and if the equally proper for producing a beautiful and peculiar color of this substance were not predo- durable black; it is of importance to determine ninant, or of itself inclining to blach, the com which of them may be employed with the greatest pound formed would have a black color; thus ni- success; it is, however, very difficult to make

nature.

comparative experiments on this subject with croy, and Berthollet, made experiments on galls, perfect accuracy, because some substances re- oak-bark, raspings of heart of oak, the external quire much longer boiling than others to extract part of oak, of logwood, and sumach, for the their astriugency; because a difference in their purpose of forming a comparison of their qualicoarseness or fineness, when subjected to ebulli- ties. To ascertain the portion of astringent tion, is sufficient to produce differences in the principle contained in these different substances, results; and because the coloring particles have they took two ounces of each separately, which a greater or less disposition to combine with the they boiled half an hour in three pounds of stuff, according to the proportion of sulphate of water; after the first water they added a second, iron that has heen made use of. Solutions of which underwent a similar ebullition; and coniron in different acids may produce differences in tinued these operations until the substances the results, according to the state of oxygenation appeared exhausted: they then mixed together of the iron in them, according as the proportion the decoctions that had been successively obof that metal is greater or less, and according to tained. A transparent solution of sulphate of the degree of strength which the different acids, iron, in which the proportions of water and when disengaged, are capable of exerting on the sulphate had been exactly determined, was used. newly-formed compound.

They first estimated the quantity of the astrin60. In the dyeing of stuffs also some differ- gent principle, by the quantity of sulphate which ences will be found to arise from their greater each liquor could decompose, and afterwards or less attraction for the coloring particles. Dr. by the weight of the black precipitate which was Lewis has proved in his excellent observa- formed. In order to stop precisely at the point tions on the process of making ink, that no of saturation, they proceeded very slowly in the known astringent, not even sumach, can be sub- precipitation, and towards the end added the stituted for gall-nuts. If, says M. Berthollet, solution of sulphate only drop by drop, and too large a proportion of sulphate of iron beceased at the moment when the last added quanadded to the galls, the ink becomes speedily tity no longer augmented the intensity of the brown, and then passes to yellow, because the black color. When the liquor is too opaque to astringent is destroyed by the action of the oxy- allow its shade of color to be distinguished, a gen, which the sulphate of the iron affords, or small quantity of it is largely diluted with water, progressively attracts from the atmosphere ; for and, by adding to this a little of the solution of we see that oxygen eventually destroys those sulphate of iron at the end of a glass tube, it is coloring substances with which it is coinbined discovered whether or not the point of saturation in too great quantities. When this accident has been attained : if we then wish to get the happens from age, Dr. Lewis found that an infu- precipitate which is formed, the whole must be sion of galls passed over the faded characters diluted with water very copiously. restored them. According to Dr. Ure, the best 63. This operation is an easy and accurate restorative for faded writing is a solution of ferro- mode for manufacturers to determine the proper prussiate of potash, faintiy acidulated, or sul- proportions of astringents, and solutions of iron. phuretled hydrogen water.

Dr. Lewis ascer To saturate the decoction of two ounces of galls, tained, by repeated experiments, that the best three drachms and sixty-one grains of iron were proportion for ink is three parts of gall-nuts to required; the precipitate weighed seven drachms une of sulphate of iron; that cherry-gum, and and twenty-four grains, when collected and dried. plum-tree gum, are as good as gum-arabic for The color of the decoction of oak bark is a deep giving the necessary consistence, and for keeping yellow; a very small portion of sulphate of iron suspended the black molecules which tend to gives it a dirty reddish color, and a larger one fall; and that decoction of logwood employed changes it to a deep brown. The quantity of instead of water for the infusion of the galls sulphate required to saturate the decoction of two improves the beauty of the ink.

ounces of this bark, was eighteen grains. The 61. Mr. Beunie, made many experiments to precipitate, collected and dried, formed coarser determine the best process for giving cotton a and more compact grains, and weighed twentydurable black. He brst tried what solution of two grains; the inner bark of the oak afforded iron gave the finest black to galled cotton; he nearly the same result. But the decoction of afterwards combined different solutions, and the raspings of the heart of oak required for ii 3 examined the durability of the blacks which he saturation one drachm and twenty-four grains produced; and made ihe same experiments on and the precipitate weighed one drachm and galled cotton, with other metals and semimetals; twenty-four grains; the decoction of the external he employed in like manner a great number of wood of the oak produced very little precipitate. astringents, and tried with them cotton which The decoction of sumach acquired a reddish had received different preparations. He found violet color, when a small quantity of the sulthat out of twenty-one species of astringents, oak phate of iron was added. The quantity required saw-dust, the galls of the country, and yellow for its saturation was two drachms eighteen grains. my robolans, were the only substances which the precipitate exactly resembled that afforded produced a fine black, but which was still neither by the galls. And the decoction of logwood so fine nor so durable as that obtained by the became of a sapphire blue color, by the addition of common galls. He also found that the oak saw- sulphate of iron : if the point of saturation be dust is preferable to the bark, employed by the exceeded, the blue becomes greenish and dirty. Gyers of thread, and, being cheaper, may be sub- The exact quantity required for saturation was stituted with advantage.

found to be one drachm forty-eight grains, and the 62. Messrs. Lavoisier, Vandermonde, Four- weiglit of the precipitate was two drachms tweive

grains. The different precipitations made by racter of tannin is that of affording an insoluble oak take place readily; that by logwood, a little precipitate when added to a solution of isinglass, more difficultly, but still more easily than that or any other animal jelly. On this property the which is effected by galls.

art of tanning depends, for which oak bark is 64. It was next ascertained, by trials made generally employed; but the barks of many with cloth, that the quantity of astringent sub- other trees are frequently employed for the same stances required to give a black color of inten- purpose. Professor Proust recommends the sity, to an equal weight of the same cloth, was precipitation of a decoction of galls by powdered proportional to the quantities of astringent prin- carbonate of potassa, for obtaining tannin, washciple, which had been already estimated in each ing well the greenish-gray flakes that fall down kind from the foregoing experiments; but the with cold water, and drying them in a stove. black obtained by the different parts of the oak This precipitate becomes brown in the air, does not resist proofs of color, nearly so well as brittle and shining like a resin, and yet remains that which is produced by galls. Logwood soluble in hot water. In this state the tannin, alone seems not capable of producing so intense he says, is very pure. According to Berzelius, a black as galls or oak; nor does the color tannin consists of hydrogen 4:186 + carbon which it produces stand the test of proofs so 51.160 + oxygen 44:654. well as that produced by galls.

67. M. Berthollet considers the abundance of 65. We shall now consider the astringent charcoal as the essential characteristic of the principle in regard to its property of combining astringent principle; the hydrogen, which it with vegetable and animal substances, particu contains only in small quantity, is however very larly the latter. Silk acquires by galling, which much disposed partially to combine with oxygen: is an operation that consists in macerating a stuff Hence, when an infusion of galls is left in corin a decoction of some astringent substance, a tact with vital air, a small quantity of the air only weight which cannot be taken from it, or dimin- is absorbed, and yet the color of the infusion ished beyond a certain degree, by repeated becomes much deeper; for, in conformity with washing; after which operation the stuff

' when the theory already laid down, the charcoal put into a solution of iron is dyed black, because readily becomes predominant in consequence of the astringent principle, decomposing the sul- the slight combustion, and the color is rendered phate of iron, forms a triple compound with the deeper, and becomes brown. oxide of iron and the stuff which is dyed. A 68. Substances which contain much charcoal, stuff that is galled is likewise capable of combi- and can undergo only a slight degree of combusning with other coloring particles, the colors of tion, ought to possess considerable durability, which thereby acquire fixity, if they do not because charcoal does not combine with oxygen naturally possess it; so that the astringent com- in the ordinary tempera!ure of the air, unless its municates its durability to the triple compound, union be assisted by other attractions, and or perhaps the more complex one which is because slight variations of temperature produce formed; but by this union the color generally no cliange in the dimensions of charcoal; but, becomes of a deeper shade. The astringent on the contrary, substances which contain much principle, by combining with animal substances, hydrogen, and in which the particles of the renders them incapable of corruption, and tends hydrogen are in a state of division, ought to be to render their texture more compact; and in easily decomposed, by the combination of the this the art of tanning consists.

hydrogen with azote or oxygen. The disunion 66. It may be proper to take some notice here of their parts ought to take place from small of the substance denominated tannin, which, variations of temperature, because hydrogen is while it has some properties in common with the dilatable by heat, which the carbonaceous partgallic acid, differs from it in others. Seguin cles are not. When, therefore, the astringent was the first who showed that astringents con- principle is combined with an animal substance, tained a peculiar substance, which, in combining it communicates to it the properties which it with skin, gave it the properties of tanned derives from the charcoal ; the animal substance leather, and that the tanning effect arose from becomes less liable to change from slight varixthe combination thus formed. Tannin may be tions of temperature ; instead of growing putrid, procured by digesting gall-nuts, grape-seeds, it suffers a slight degree of combustion, by the oak-bark, or catechu, in a small quantity of action of the air; for the process of tanning cold water.

The solution, when evaporated, probably could not go on in a perfectly close affords a substance of a brownish-yellow color, vessel. highly astringent, and soluble in water and in 69. On examining the analyses that have been alcohol. According to Mr. Brand, the purest made of indigo, which may be looked upon as form of tannin appears to be derived from the coloring matter least liable to change of any bruised grape-seeds; but even here, he observes, with which we are acquainted, it will be found it is combined with other substances, from which that this substance leaves, in distillation, a it is, perhaps, scarcely separable. I have dever, greater proportion of charcoal than even galls says lie, been able to obtain it of greater purity themselves. M. Berthollet supposes that it is than by digesting powdered catechu in water at also to this abundance of charcoal, that the 33° or 34°, filtering and boiling the solution, durability of the color of indigo is to be attribuwhich, on cooling, becomes slightly turbid, and ted, and that the proportion of this principle 15 is to be filtered again, and evaporated to dry- the chief cause of the difference observed in the ness; cold water, applied as before, extracts durability of colors; but the force of adhesion nearly prre tannin. The most distinctive cha- may also have great influence, for a principle

which combines intimately with another sub- particles have for wool, silk, cotton, and linen, stance, ought to form with it a more permanent they unite more or less readily and intimately compound, than one which has only a slight with them: and thence arises ihe first cause of disposition to unite with it; now the astringent variation in the processes employed, according principle possesses a very strong disposition to to the nature of the stuff, and of the coloring form intimate combinations, especially with substance employed. And by the affinity which animal substances.

the coloring particles have for alumine and 70. Upon the same principles may be ex- metallic oxides, they form compounds with plained the fixity communicated to coloring these substances, in which their color is more or particles hy alumine, and by those metallic less modified, and becomes more fixed, and less oxides which are not liable to contain different affected by external agents than before. This proportions of oxygen, such as the oxide of tin, compound being formed of principles which and some others. The different coloring sub- have separately the power of uniting with vegestances, capable of uniting with metallic oxides, table substances, and more especially with have an action upon them, analogous to that of animal substances, preserves this property, and astringents. The oxides are deprived of more forms a triple compound with the stuff; and the or less of their oxygen, according to the force color, which has been again modified by the with which they retain it, the strength of attrac- formation of this triple union, acquires a greater tion with which the coloring particles tend to degree of fixity, and of indestructibility, when combine with them, the proportions in which exposed to the action of external agents. they meet with each other, and the greater or less 73. The coloring particles have often so great disposition of the coloring particles towards an affinity for alumine and metallic oxides, that combustion.

they separate them from acids wbich held them 71. The coloring particles also suffer a change in solution, and fall down with them; but the in their constitution from these circumstances: affinity of the stuff is sometimes necessary, in thus the solutions of iron render brown all the order that this separation may take place. The colors into which oxide of iron can enter, oxides of metals, which combine with the coloralthough it has only a green or yellow color in ing particles, modify their colors, not only by the state in which it is held in solution by acids, their own, but also by acting upon their compoand this effect goes on increasing to a certain sition by their oxygen. The change which the degree; but the alteration of the coloring parti- coloring particles thereby suffer, is similar to that cles may afterwards be carried so far as to spoil occasioned by the air, which injures every color their color, and to diminish their tendency to in a greater or less degree. In the two different combination; the oxide of iron is then brought principles which constitute the air or the atmosback to the yellow color by the oxygen which it phere, it is only the oxygenous gas that acts upon attracts, and is capable of retaining. The action the coloring particles." "It combines with them, of metallic oxides and the coloring particles on weakening their color, and rendering it paler; each other, explains the changes observed in but presently its action is principally exerted on solutions of the coloring particles, when mixed the hydrogen, which enters into their composiwith metallic solutions. The effect is gradual, tiou, and it then forms water. This effect, conas has been shown with respect to fustic. It tinues M Berthollet, ought to be considered as sometimes happens that the mixture does not a true combustion, whereby the charcoal which even gros turbid immediately, but loses its enters into the composition of the coloring partransparency by degrees; the precipitation be- ticles becomes predominant, and the color comgins; the sediment is formed; and its color monly changes to yellow, fawn color, or brown; becomes gradually deeper. In producing these or the injured part, by uniting with what remains effects, light has sometimes a corisiderable share. of the original color, causes other appearances of

72. Upon the whole, we may conclude, that a different kind. The combustion of the colormetallic colors should be distinguished from ing particles is increased by light, and frequently those which are peculiar to substances of the cannot take place without its aid; it is indeed in vegetable and animal kind: that the colors of this way that it contributes to the destruction of metals are modified and changed by oxidation, colors. Heat promotes it also, but less powerand by the proportion of oxygen with which fully than light, provided its intensity be not they are combined; and that vegetable and very great. The effects of the nitric acid, the animal substances may themselves possess a oxygenated muriatic acid, and even the sulphupeculiar color, which varies in the different states ric acid, when they make the color of the subthrough which they pass, or they may owe their stances upon which they act pass to a yellow colors to colored particles, either combined, or and even to black, are to be attributed to a comsimply mixed with them. These are the parti- bustion of a similar nature. cles which are extracted from different substan 74. The effects of combustion may, however, ces, and which undergo different preparations, be concealed, by the oxygen combining with the in order to render them proper for the various coloring particles, without the hydrogen being purposes of dying. And the coloring particles particularly acted upon by it. But colors are possess chemical properties which distinguish more or less fixed, in proportion to the greater or them from all other substances: the attinities less disposition of the coloring particles to suffer which they have for acids, alkalis, earths, this combustion. There are some substances metallic oxides, oxygen, wool, silk, cotton, and also capable of acting on the color of stuffs, by a linen, from the principal of these properties. stronger affinity, or by a solvent power; and in In proportion to the attivity which the coloring this consists the action of acids, alkalis, and soap

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