Edward Bancroft: Experimental Researches concerning the Philosophy of permanent Colours...

The Quaterly Review
April & July, 1814
London printed.
New-York: Reprinted by Eastburn, Kirk & co.
Printed by Abraham Paul.

Art. XIV. Experimental Researches concerning the Philosophy of permanent Colours, and the best Means of producing them by Dyeing, Calico Printing, &c. By Edward Bancroft, M.D. F.R.S. 2 vols. 8vo. pp.1124. London. 1813.

'The art of dyeing is one of the most useful and the most wonderful of all known arts; and,' according to Mr. Chaptal, whom Dr. Bancroft quotes, 'if any art can inspire mankind with a noble pride, it is this art, which not only imitates, but even exceeds nature, in the richness, splendour, and permanency, of the colours that it affords.' We are bound to suppose that there is some foundation for the opinion thus enounced by a philosopher so respectable as Mr. Berthollet: but we should find ourselves a little at a loss to demonstrate, with mathematical precision, what other utility the art of dyeing possesses, than that of affording employment and emolument to a great number of individuals, whose business it is to render it subservient to the innocent gratification of the taste and luxury of others, without any direct advantage of such a kind as can properly be denominated 'utility; at least in the same sense that the production and preparation of food are understood to possess utility. We recollect, however, but one instance, in the history of all ages and all nations, of an individual who carried his ideas of the propriety of pursuing utility only, to the exclusion of the mere gratification of the eye, so far as to refuse to use or wear any article of furniture or clothing which had ever been submitted to the dyer's art: his coat was white, his hat "was drab, and his shoes were brown, as nature had left the raw materials; but he found no followers, even among the sectaries whose principles he had in this manner practically caricatured; and the consent of m'ankind, so universally and unequivocally expressed, must be allowed to be imperative, in rendering a compliance with custom and taste, in these respects, almost as indespensable as a submission to the irresistible dictates of hunger, thirst, and cold.

The first volume of Dr. Bancroft's elaborate and valuable work was published in 1794; but- it now appears with so many alterations and additions, as to possess a considerable share of novelty and interest. In the introduction, the author defines the sense in which he applies the terms characteristic of the different kinds of colouring substances; substantive colours being such as afford a permanent tint by simple application without mixture, and adjective such as require a mixture with some other substance to fix them; this substance, serving as a bond of union, is called a mordant, though it is sometimes difficult to ascertain which of two substances that are mixed is the more properly considered as the colour, and which as the mordant. If the substances thus employed are mixed before their application to the materials to be dyed, the compound is called by Dr. Bancroft a prosubstantive colour; but it happens more commonly, that the colours are more effectually fixed, by applying the substances concerned in succession, as if the particles wedged each other in, after their penetration into the pores of the materials. There is, hewever, a complete fallacy in Dr. Bancroft's reasoning on the effect of heat in opening the pores, so that the colouring particles may be compressed when they cool again (p. 90); since these particles are at least as much contracted by the effect of cold as the substances which they they penetrate, and often much more. That the colouring particles are only partially distributed in or upon the surface of the substance coloured, is evident from the mixture of two tints, when the materials producing them are applied in succession; and a fugitive colour, constituting one of those tints, is not found to be in any degree fixed by applying a more permanent colour upon it; so that a compound green thus constituted will generally fade into a blue. In many cases it might be supposed that the absorption of oxygen contributed to wedge the particles more firmly into their situations; but it sometimes happens, on the other hand, that oxygen appears rather to be extricated than absorbed, while the colour is acquiring its lustre by exposure to the light, as Dr. Bancroft has found with respect to the Tyrian purple.

Of the mordants employed for fixing colours, one of the most extensively useful, and the most unequivocally entitled to the denomination, is alum, which is attracted by the fibres of many animal and vegetable substances, and, remaining attached to them, serves to unite them to the colouring matter, by leaving its earth as a common bond of union. The name alumen is found in Pliny, and Beckman suspects that it may be of Egyptian origin; it is true that, according to Kircher's vocabulary, the Egyptian word, synonymous with alum, is oben, but we find alom signifying cheese, which may possibly have had some connexion with the coagulating power of this highly astringent substance. Dr. Bancroft observes, that alum, and its use in dyeing, must have been known to the ancients long before the time of Pliny, since they never employed tin, and either alum or tin is absolutely necessary for obtaining a scarlet from kermes.

Dr. Bancroft proceeds to trace the art of dyeing from the ancients, whom he proves to have been acquainted with many of the most important processes, through the middle ages, when it w«s but imperfectly preserved in Italy, to the latest improvements, as described in the works of Macquer, Keir, Henry, Berthollet, Chaptal, Vitalis, Scheffer, Poerner, and Dambourney, as well as to those which he has himself introduced, both in theory and in practice. He divides the body of his work into four parts; the first, after some general discussions respecting colour, and substances to be coloured, is devoted to the subject of substantive colours, whether animal, as the Tyrian purple, vegetable, as indigo, or mineral, as iron; the second to the adjective colours of animal origin, as kermes and cochineal, lac, and Prussian blue; the third to vegetable adjective colours, as weld, quercitron, madder, Brazil wood and logwood: and the fourth to compound colours of various kinds, and particularly the mixtures whieh produce black dyes, and inks of all descriptions.

It has been a common opinion, that the general cause of the destruction of colour, by exposure to the air, is the combination of the colouring substance with oxygen; but Dr. Bancroft observes, that the nitric acid imparts oxygen much more readily than the oxymuriatic, though it has incomparably less effect on colours, and that the action of either of these acids is no direct test of the effect of exposure to the air. Thus a red dyed with madder, on an aluminous basis, was much more rapidly destroyed by the oxymuriatic acid, than a purple obtained from logwood and tin, though the sun and air would have acted on these colours in a manner totally different. A black dye also, which was little affected by the oxymuriatic acid, was totally destroyed by the nitrous, although this acid had incomparably less effect than the former on a Turkey red. The colouring matter of the buccinum was exposed in its recent state to the sun's rays (p.. 151), immersed in oxymuriatic acid; and became purple more rapidly than when simply moistened with water; although afterwards, when placed in the dark, the acid had completely bleached the coloured substance in a few hours. Dr. Bancroft is of opinion that this singular substance, which is found in several testaceous animals, and has at first the appearance of a limpid mucilage, emits some phosphoric acid together with oxygen, while it acquires its colours, and that this is the cause of the offensive smell, which has always been observed to accompany the change.

The colour of indigo is also dependent on the different degrees of its oxygenization: the recent juice is nearly colourless; the absorption of a small portion of oxygen renders it green, and that of a larger blue; in the latter state it is insoluble in water, and with the assistance of agitation, collects into flocculi, the separation being commonly promoted by the addition of lime water, or some other alkali, as a precipitant. It is found not only in the indigoferae, but also in the isatis, or woad, and in some other vegetables: it is capable of distillation or sublimation by a moderate heat, and affords, when burnt, a fine purple smoke. This smoke is noticed by Pliny as observable in an Indian colour, which is thus identified with indigo. In its blue state it is incapable of attaching itself to animal or vegetable fibres; and the process of fermentation in the dyer's vat serves to restore its green colour and its solubility, apparently, as Dr. Bancroft has shown, by the abstraction of oxygen. The materials employed for this purpose are generally a decoction of woad, madder, and bran, to which lime and indigo are added. When the fermentation has been continued a sufficient time, the internal parts of the liquor are green and the surface only blue. Sometimes bran and madder only are used with indigo, a little potash being added to promote mote the solution. Another mode of deoxygenizing the indigo is to mix it with the green sulfate of iron, or the nitrate or muriate of tin: one part of indigo is rendered slowly soluble in warm water by the addition of two of the sulfate of iron and two of lime; part of the lime setting the protoxyd of iron at liberty to deoxygenize the indigo, which is then dissolved, with the assistance of the remainder of the lime, to which a little potash is sometimes added. In calico printing, the indigo is ground with the sulfate of iron and applied to the calico, together with a proper thickening of starch or gum, by means of blocks formed according to the pattern required; the calico is then immersed alternately in lime water and in a solution of the sulfate of iron, until the indigo is become sufficiently dissolved to afford a fixed colour. In this topical application of indigo, red orpiment is sometimes substituted for the sulfate of iron; and Dr. Bancroft has found that tugar is capable of producing a similar effect.

It is well known that indigo may be dissolved in the sulfuric acid without changing its colour; the blues dyed with this solution are called Saxon blues; they are less permanent than those which are derived from the green liquor; and the indigo is deprived of the property of burning with a purple smoke.

The gardenia genipa is mentioned as a plant affording a very permanent bluish black, which might be advantageously employed either for dyeing or as an ink, in countries which allow it to be obtained in a recent state.

'The fine rose colours of safflower, extracted by crystallized soda, and precipitated by citric acid, and then slowly dried in the shade, being afterwards finely ground with the purest talc, produces the beautiful paint by which ladies give to their cheeks the bloom of youth and health, and which the French distinguish from carmine by the name of rouge vegetale.'

The colouring matter thus derived from the carthamus possesses but little durability; it affords a striking illustration of the Newtonian doctrine of transmitted and reflected colours in the form in which it is sold as a pink dye spread out on saucers. The 'talc' employed for making rouge is generally that which is now called French chalk, but which will not, like the true French chalk, extract spots of grease.

The finest substantive purples are obtained from lichens. Many species of this genus are employed, after maceration with animal substances containing ammonia, for violet, purple, and crimson dyes: the lichen roccella, or orchil, is the principal; the lichen tartareus is also used in this country, under the name cudbear, given it by Dr. Cuthbert Gordon, who introduced it. After fermentation the substance is kept moist in casks for a year or two two before it comes to perfection: the colours which it affords are however unfortunately fugitive.

The purple obtained from gold, by means of tin, is found to depend on the deoxygenization of the gold by the solution of tin; a similar effect may be obtained by impregnating silk or cotton with glue, eggs, or other animal substances, together with sugar or orpiment, and applying to them the solution of gold. The nitromuriate of platina affords, in combination with a muriatic solution of tin, a fixed colour like that of arterial blood.

'Respecting the antiquity of calico printing,' Dr. Bancroft observes, (p. 346) 'that Pliny describes the Egyptians as practising a species of topical dying, or calico printing, which, as far as can be discovered from his general terms, appears to have been similar to that which, many ages after, was found to exist in Hindostan, and other parts of India, and was from thence introduced into this and other countries of Europe. He says the Egyptians began by painting or drawing on white cloths, (doubtless linen or cotton,) with certain drugs, which in themselves possessed no colour, but had the property of attracting or absorbing colouring matters. After which these cloths were immersed in a heated dying liquor, and though they were colourless before, and though this dying liquor was of one uniform colour, yet when taken out of it soon after, they were found to be wonderfully tinged of different colours, according to the different natures of the several drugs which had been applied to their different parts;' and ' that these colours, so wonderfully produced from a tincture of only one colour, could not afterwards be discharged by washing.'

The art of calico printing has been much improved and simplified in modern times, especially by the mixture of the acetate of 'lead with the aluminous mordant, forming an acetate of alumine. A still more economical method, lately invented, is to employ the acetic acid in the form of the pyrolignic, obtained by the destructive distillation of wood, and to substitute lime for lead. The acetate of iron is also now generally prepared from the pyrolignic acid. MM. Thenard and Roard found that the acetate of alumine, when exposed to a warm atmosphere, lost some of its acid, leaving an excess of alumine; but alum, tartar, and the salts of tin, are attached to the fibres of the cloths impregnated with them in their entire state, and may be recovered by repeated washings, until they are decomposed by the operation of the colouring substances. The method of employing some of these substances may be understood from the following description of the processes.

'When pieces of calico have been printed with iron liquor, whether it be applied to those which either have received, or are intended to receive, the aluminous mordant also, they are to be thoroughly dried by a stove heat, and afterwards passed through the mixture of cow dung and warm water/ which is supposed not only to cleanse them more effectually, but possibly to communicate to them some animal impregnation subservient to the durability of the colours; they are afterwards, in the language of the calico printers, to be streamed, or extended in running water, and beat, to remove all the loose or uncombined particles of the mordant, and thus fit them to be dyed with either madder, sumach, weld, or quercitron bark; these being the principal and almost the only adjective colouring matters so employed by calico printers, and sufficient (excepting the blue from indigo) to produce, with the aluminous and ferruginous mordants, all the various colours seen and admired on pointed calico.'

'Ex. gr. If pieces of calico, to which these mordants have been applied, both separately and mixed, be put into a dying vessel, with water scarcely blood warm, and in which three, four, or five pounds of madder in powder for each piece have been previously mixed, and they be turned, as usual, through the liquor by the winch, gradually, but slowly, raising the heat, so that it may only reach the boiling point at the time when the calicoes will have been sufficiently dyed, the several pieces will be found to have imbibed colour in every part. The figures or places to which the unmixed iron liquor was applied will have been dyed black, and those on which the aluminous mordant was printed will be red, of different shades, if the mordants had been used at different degrees of concentration; and, if both mordants were mixed and applied in different proportions, such applications will have produced various shades of purple, violet, chocolate, and lilac colours, whilst the parts, or grounds, intended to be ultimately left white, will manifest a considerable brownish red discolouration; but as the madder colour producing it is not [there] united to the calico, by the affinity or attraction of any intermediate basis, it will not be able, as in other parts, to resist the action of exterior agents, and may therefore (as is usually done) be removed, and the grounds made white by boiling the pieces in water soured by fermented bran, and by afterwards spreading them for some days (according to the season) upon the grass, where, with the well-known treatment, the colours dyed upon a basis will become brighter, whilst that without one will completely disappear.

'Calico printed with the same mordants, and dyed with the quercitron bark, (quercus nigra, Linn.) will acquire fixed and bright yellows of different shades, upon the aluminous basis, and various drab colours upon that of iron. A mixture of these bases will produce olive colours. Along with these it is usual to produce black impressions at the same time by previously applying to the calico a mordant composed of iron liquor and gall, by which figures which, without the galls, would only .have manifested a dark drab colour, are made black by dying with the quercitron bark, and if the dying be conducted as 1 shall hereafter direct, the grounds will be so little discoloured, that no exposure upon the grass wiil be required as is necessary with madder and weld, an advantage which has nearly put an end to the use of weld in calico printing.'—p. 377.

The colouring matter of kermes, derived from the coccus ilicis, our author considers as identical with that of cochineal, although combined combined with some of the astringent substances derived from tbe tree. The scarlet afforded by cochineal was unknown in its highest perfection till the year 1630, when the singular power of the oxyd of tin, in exalting its colours, was discovered in Holland: it was soon after communicated to one of the celebrated MM. Gobelins at Paris, and contributed to the perfection of the colours of their tapestries. The nitrate or nitromuriate of tin, commonly used by the dyers, affords a crimson colour, which is converted to scarlet by the tartar employed in the process. Dr. Bancroft has invented an ingenious method of saving this valuable colour, by substituting a yellow dye, in particular the quercitron bark, for the acid substance which changes the colour of the cochineal, and thus forming a compound instead of a simple scarlet. The colour thus obtained is more durable than the common scarlet, but, as it is said, not quite so brilliant by day-light, although somewhat brighter by candle-light. The solution of tin, called spirit by the dyers, is usually made with one pound of aqua fortis, two ounces of sea salt or sal ammoniac, half a pound of water, and two ounces of grained tin, added by degrees. Dr. Bancroft attempted to substitute for it a muriate of tin, but found the corrosive quality of this combination extremely injurious to the cloth: he however succeeded much better with a muriosulfate.

The process employed in dyeing scarlet on wool requires a mixture of all the materials concerned, before their application to the cloth: to apply them in succession, as is either indispensable or highly advantageous in many other cases, would here be impracticable: a fact which renders it necessary to be very cautious in all theoretical reasonings respecting the use of mordants. With silk and cottons, the reverse is true, at least with respect to all dyes except the quercitron.

'Since the preparation or manufacture of Morocco leather has been established in this country, cochineal is employed to communicate the beautiful colour of that, which is called red Morocco; though in Persia, Armenia, Barbary, and the Greek islands, a similar colour was originally produced by the use of either kermes or lac. As a basis for the colouring matter of cochineal, goat skins, deprived of their hair by lime water, and properly cleansed, are impregnated, on that which was the hairy side, with a saturated solution of alum, applied repeatedly and equally by a sponge, and, after an interval of three or four days, a decoction of cochineal, which has been strained, is applied also by a sponge, to the same side or surface, a little, but not much, more than blood warm, lest it should crisp the leather. This application is repeated from time to time, until a colour sufficiently full and equal has been produced. Afterwards the skins are soaked in bran liquor, and then tanned by a decoction of either galls or sumach, or of both mixed together. I have found that by substituting a diluted muriosulfate of tin, for the solution of alum, or by employing a mixture of both upon goat skins in a suitable state of preparation, the colour subsequently produced was considerably improved, at least in vivacity.'—ii. 167.

Dr. Bancroft appears to have bestowed considerable labour and ingenuity on an attempt to obtain the colouring matter of stick lac in a state of purity, and separate from the resin: but it seems to be extremely difficult to exhibit it in an extractive form, without impairing the beauty of the colour. The separation may be partly effected by employing water not hotter than 190°, which dissolves the colouring matter, and leaves the resin, with some other impurities, undissolved; and in this state the colour is as fine as that of cochineal, and somewhat more durable: but it loses its brilliancy in the process of evaporation. The finest parts are also more easily powdered than the rest, and will pass through a sieve, while a great portion of the impurities remains behind.

The peculiar colouring matter of Prussian blue, which Dr. Bancroft cannot readily allow to be an acid, as it has usually been denominated by chemists, affords, with the quercitron bark, a fine green, and as our author first discovered, a good brown with copper. In order to investigate the nature of the green thus obtained, he took a piece of cotton which had been printed in stripes with iron liquor and galls, with iron liquor only, with iron liquor and acetate of alumine, and with this acetate only: he dyed it first with quercitron bark, which made the first stripe black, the second dark drab, the third olive, and the fourth yellow: he then took prussiated potass, acidulated with sulfuric acid, and immersing the cotton in it for a minute, he found the colouring matter of the galls and quercitron bark discharged where the basis of iron had been employed: so that the first stripe was become a dark blue, and the rest paler, as they stood in order: when there was less excess of acid, the colouring matter of the galls remained, and that of the bark only was discharged: when the liquor was perfectly neutral, the colour of the quercitron bark was discharged from the ferruginous basis, but not from the aluminous; so that the second stripe, with iron liquor, had become blue, and the third a fine green, while the yellow remained perfect on the fourth, and the part not printed became white, being freed from the discolouration of the bark. In these cases of change of colour, the displacement of one of the substances by the other is sufficiently proved by the state of the liquor, in which the substance displaced is found dissolved.

Among the vegetable adjective colours, weld, the reseda luteola, holds the first place, as having been the longest in use: besides this, young fustic, the rhus cotinus, sumach, the rhus coriaria, old rustic, the morus tinctoria, and French berries, the rhamnus tinctoria, are the most generally known of the yellow dyes. But all of these these appear to have been in some measure superseded by Dr. Bancroft's discovery of the utility of the quercitron bark: a discovery of which the advantages were secured to him by act of parliament for a term of years, although he failed in his application for an extension of that term in 1798, notwithstanding the advan/tage which the public had derived from the singularly liberal manner in which he had conducted the monopoly. In less than twelve months after that time, the bark rose to three times the price at which he had constantly supplied it, and at which he would have been bound to supply it for another term of seven'years, if the bill had been passed. He has presented us with an immense variety of experiments and of practical directions relating to the use of this substance, and his-communications must be of great value to the intelligent manufacturer.

We cannot altogether agree with the author in the decided preference which he appears to entertain for the process of bleaching by the oxymuriatic acid, (II. 176.) which seems at present to have become almost universal, not a little to the advantage of manufacturers and tailors, and to the prejudice of the public in general. Sir Humphry Davy has found that, even when neutralised by an excess of lime, the muriatic acid, formed during the process of bleaching, is injurious to the fibres of the cloth; (Elem. p. 242.) and whatever precautions it may be possible to employ for avoiding this evil, we are persuaded, from continued experience, that they are not commonly adopted by manufacturers, either in bleaching cotton or paper.

Madder, the rubia tinctorum, is a very well known and important vegetable, much employed for dying red with an aluminous basis, on common woollen cloths. Its effect, in giving a red colour to the bones of animals that feed on it, was first observed by Lemnius in the 16th century, and is now well known to physiologists. Madder does not appear to be capable of affording a prosubstantive colour; and it is absolutely necessary that the basis should be separately applied to the linen or cotton which is to b« dyed with it. Galls are commonly employed by practical dyers as a preparation for the aluminous impregnation, in order to promote the attachment of the alum to the cloth; but they add nothing to the durability of the colour.

The rubia peregrina, or Smyrna madder, is principally used in the complicated process for dyeing the Turkey red on cotton, with the assistance of oil, alum, galls, and some blood, which seems to brighten the colour, besides that of the substances which have passed through the alimentary canal of the sheep, carrying with them some of the gastric fluids, and which, in Dr. Bancroft's opinion, mon, are highly conducive to the stability of the colour, although some former chemists have much underrated their efficacy.

Brasil wood is the heart of the caesalpina echinata; its name is not derived from the country which affords it; the old name of kermes having been gratia di brasife, implying the colour of fire or live coals; and the country of Brazil was afterwards so denominated, from its producing this substance, which affords a dye of the same hue. The rose-colour, which it communicates to water, is destroyed by confinement for a few days with sulfureted hydrogen: but the same effect was not produced by a protoxyd of tin, which would have destroyed the colour of indigo. Acids make the infusion yellow, but alum reconverts it to red, and affords a precipitate, which is employed as an inferior sort of carmine; and the addition of an alkali throws it down in great abundance.

Logwood, the haematoxyton campechianum, is sometimes used for dyeing purple, with a mordant of muriosulfate of tin, tartar, and sulfate of copper; the latter Dr. Bancroft thinks superfluous, since it affords only a fugitive colour; but the most extensive employment of logwood is for dyeing, black.

For browns, the mangrove bark, rhizophora mangle, the mahogany, and several species of walnuts are recommended, principally with bases of iron. Galls give, with an aluminous basis, a tawn, or light cinnamon colour.

In the fourth part of the work, the author gives a few practical directions only respecting the mixture of colours, and proceeds to the subject of black dyes and writing ink. He objects altogether to the chemical distinctions and definitions of the gallic and tannic principles, as contained in astringent vegetables, and is rather disposed to consider the colouring matter as distinct, and as not agreeing in general with the characteristic marks of any particular chemical combination. Thus, the quercitron, mangrove, and mahogany bark are astringent to the taste, precipitate glue, and tan leather, but produce no darker tint than an olive. Catechu tans admirably well, but affords only a snuff-colour with iron: on the other hand, the walnut bark and logwood afford a black ink, but are not astringent to the taste, and do not precipitate glue. Dr. Bancroft imagines that logwood has no claim to the title of an astringent in a medical sense; but we have reason to think that in this respect he is greatly mistaken. We may add the artichoke to the number of vegetables which have no astringent taste, and yet blacken iron: the Jerusalem artichoke, which has a taste nearly similar to that of the common artichoke, does not possess the same property. The gallic acid, so called, blackens the peroxyds of jrap, but Dr. Bancroft is rather disposed to attribute this effect to some accidental mixture of the colouring matter, than to the essential constitution of the substance itself.

Ink may be deprived of its blackness by a stream of sulfureted hydrogen, which can only act by combining with the oxygen: by exposure to the air it recovers its colour, with a fresh supply of oxygen. It is well known that the ink of the ancients was carbonaceous: Indian ink, according to Cuvier, is obtained from some species of sepia. Dr. Bancroft finds the best proportion of galls three times the weight of the sulfate of iron; and if a portion of logwood be employed, half as much of the galls may be omitted: the sulfate of copper, recommended by Chaptal, he thinks useless: it is true that it tends to prevent mouldiness: but we have found inconvenience from its corroding and blunting the penknife, when it happens to touch it. Dr. Bancroft directs twelve ounces of galls to be boiled with six of logwood in five quarts of soft water for two hours, the decoction to be strained, and made up one gallon, to which five ounces of sulfate of iron, five of gum arabic, and two of muscovado sugar are to be added. A simpler mode, lately recommended by a celebrated chemist, is to infuse three ounces of galls, one of logwood, one of sulfate of iron, and one of gum arabic in a quart of cold water for a week, and to add four grains of corrosive sublimate, in order to prevent mouldiness. We may add, that when economy is an object, the soluble parts of the galls may be much more effectually extracted by the repeated affusion of fresh portions of the water, than by steeping them in the whole at once.

The best black woollen cloths are first dyed red with madder, and blue with indigo or woad, since without- this preparation it would be necessary to use so much of the common black dye as would materially injure the texture of their fibres. Sometimes, in coarser cloths, logwood only is employed for the first dye, with a salt of copper: in this manner a black is produced, which inevitably turns in a short time to a brown.

In dyeing silk black, the galls are applied first, being more strongly attracted by the silk than the iron, and they may therefore be considered as the true mordant; after this, alternate immersions in a solution of sulfate of iron, and in a decoction of logwood are frequently repeated, in order to obtain a deep black; but to cotton, the iron is usually applied first. Some of the black vats, with iron and various vegetable substances, are suffered to remain unemptied for centuries, being supposed to have their qualities improved by age. For a prosubstantive topical black, the addition of vinegar and nitric acid to galls and sulfate of iron is found to increase the durability of the dye, without any corrosion of the substance of the calico.

Dr. Bancroft has investigated the whole subject of black dyes and inks with great attention, and has made many elaborate experiments respecting them. Some of these experiments have indeed been productive of no immediate practical improvement; but in these, and in other similar instances, he describes his failures with a degree of candour which' does him no less honour than his success on more fortunate occasions. We sincerely wish that he may be enabled long to continue his favourite pursuits, and that the public may hereafter profit by the 'additions' contingent on 'the prolongation of a life, of which the sixty-ninth year is now passing away.'

In vol. ii. p 325, L 16, by the 'oxide,' we suppose is meant the oxide of tin; and p. 361, l. 10, by 'a little,' probably a little lime.

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