THE J. PAUL GETTY MUSEUM LIBRARY Digitized by the Internet Archive in 2015 https://archive.org/details/elementsofartofd00bert_1 ELEMENTS OF THE ART of DTING, CONTAINING THE THEORY OF DYING IN GENERAL, AS FAR AS IT RESPECTS THE PROPERTIES OF COLOUFaX 5UBSTANCES. Tranflatcd from ihe fsltnch of M. B ERTHO LLET, MEMBER 0^ TliE ACADEMY OF SCIENCES OF PAR '5, E D I N B U R G Hi PRINTZD LAWRIE AND SYMINC-TO' I 7 9. TP B5^ ADVERTISEMENT. iHE Publifliers were preparing to lay before thePablic, a complete tranllation of M. BerthoUet's whole works, by an able hand : the tranflation was confiderably ad- vanced, and fome progrefs had been made in the print- ing, when the unexpeded appearance of another tranf- lation of the fame book in London, rendered it eligible for them to defift from the undertaking, as the Lon- don market was thus pre- occupied. Senfible, however, that the tranflation which they were about to publifti may well bear a comparison with that which has appeared, and believing that the firfl part, which they printed, may, even feparately, prove of con- fiderable utility to Students of Chemiftry, and Practi- tioners in the Art of Dying ; they, upon thefe grounds, prefume to offer it to the Public. The general properties of colouring fubflances, and of thofe other bodies which are employed to fix colours upon the matters to which they are applied j the adion of ii Advertisement. of air and light upon colours ; the influence of tfle nitric and oxigenated muriatic acids in colouring ani- mal fubftances ; the nature of galls and of aftringents in general ; with the characters of wool, filk, and flax as fubjecls for the Art of Dying, are the topics treated of in the part of M. BerthoUet's book now publifheci in an Enjlifli drefs. ELEMENTS OF THE ART OF DYING. PART FIRST. OF DYING IN GENERAL. SECTION FIRST. • F THE GENERAL PROPERTIES OF COLOURIXG SUBSTANCES. CHAP. I. Of colouring Particles^ and their Affiniiics. THE phyfical theory of colours is foreign to the objed of this work. Before confidering the characlers of colouring lubilances, Idiallonly indulge myfelf in a few reflections on the manner m which the particles of bodies abforb certain rays of light, tranfmit or reflect others, and in that way produce colour. Newton has demonflrated, in his Optics, that tran- fparent bodies, according to the difference of their thicknefs and denfity, reflect rays of one colour, and tranfmit thofe of another. A |5 OF COLOURING PARTICLES, M. Laval has endeavoured to extend the proofs of the Newtonian theory to all the colours of bodies permanently coloured, and he has collected a great number of experiments, in order to prove that the difference of colour in thefe bodies is exactly in pro- portion to the denfity of their conflituent parts. The application of the beautiful experiments upon colours refleded by thin and tranfparent plates to bodies permanently coloured, is founded upon analo- gies which did not efcape the genius of Newton ; but "we cannot as yet fay how far it is poflible to obtain direct proofs of this theory, or to deduce from it any thing certain with regard to particular colours. For, I/?, The fame colours are fuccelTively produced by plates, the thicknels of which increafe in a progreffive order ; fo that we cannot, from the colour produced by a tranfparent plate, whatever its kind may be, de- termine its precife thicknefs. 2d, The tenuity of the particles of bodies and their denfity are two elements, which, according to Newton, combine in the tranfparent plates to produce colour ; and a plate of water, air, or glafs^ ought to be of different degrees of thicknefs, in order to refleQ: the, fame colour. But we have no method of determining either the tenuity or the denfity of the particles of bodies. The fpecific gravity is not fuffi- cient for this purpofe, for fmall particles, with very numerous pores, may give the fame fpecific gravity as more extenfive pores, and a fmalier number of large particles. Thus, * Recherches experimentales de changements de coulcurs, dans ?es corps opaaues et aaturelkment colgres.. AND THEIR AFFINITIES. 3 Thus, when M. Laval aflerts that the order of co- lours which metals give to glafs is in proportion to their fpecific gravity, he refutes himfelf by the fa6ls he adduces ; for he obferves, that we can make glafs yellow by means of iron, lead, or filver, the fpecific gravities of which are very different. Mercury com- bined with a fmall quantity of oxigen is black, but by combining it with a greater quantity it becomes red. Iron pafles through almofl every variety of colour, according to the quantity of oxigen with which it is combined ; and the changes of fpecific gravity that refult from the different proportions of oxigen are very fmall, and far lefs confiderable than the differen- ces which exifl: among th^ fpecific gravities of thefe metals. The fame author alledges that alkalies give a green tinge to many blue vegetable colours, becaufe they increafe the denfity of their particles ; and that acids change them to a red, becaufe in diffolving they attenuate them : but alkalies alfo diffolve die greater part of colouring particles, and render thent more mifcible with water than they are in their natural ftate ; they mufl therefore aft in a manner analogous to acids : befides, acids diffolve blue co- louring particles without changing their colour : thus the folution of indigo, in the fulphuric acid, retains its blue colour although it be diluted with a great quantity of water ; but according to M. Laval's mode of rea- foning, the denfity of the blue particles ought then to undergo a confiderable change. In fliort, many vege- table fubllances are changed by means of a very weak acid, or a fmall quantity of an alkali, from a red colour A 2 to 4 4 PF COLOURING PARTICLES, to a green, without our being able to perceive any of the (hades which ought to intervene, The tenuity and denfity of the particles are not the only ciicumftances which ought to be confidered ; it is evident, even from the experiments of Newton, that their chemical nature has much influence up- pn their colours ; for we cannot doubt, that a force which exerts a ftrong adion upon the rays in their refradion, will alfo afFc£t their refledion. In comparing the refradive powers of different fub- ftances, Newton found that inflammable bodies pof- feflfed this power in a much higher degree than fub- ftances not inflammable : he deduced from his obfer- Yations the furpri2ing conclufion that the diamond muft contain much inflammable matter, that water is an intermediate fubfl:ance, between inflammable and uninflammable bodies, and that it is water which affords to vegetables their inflammable principle: truths that, of late only, have been underftood, and demonfl:rated by experiment. There fl:ill remain many interefting obfervations for thofe who wifli to follow the path of the great Newton, and to compare the refradive power of the different gafes and other fubftances, the conftituent principles of which are at prefent known. Is it hy- drogen or carbon which contributes mofl: to the re- fradive power of inflamm.able fubllances ? By attend- ing to the experiments which have been made upon the diamond, we would be inclined to conflder it as. carbon pure and cryfl:alized ; but in oil of turpentine, which has a confiderable refradive power, hydrogen forms the prevailing ingredient. Many AND THEIR AFFINITIES. 5 Many chemical experiments prove that oxigen nlfo has a confiderable affinity with light ; it combines with it without feparating any of its rays ; and re- fumes an elaflic (late by this combination, which it al- ways forms when not prevented by a ftronger affini- ty. From this it happens, that when we expofe to the light oxigenated muriatic acid, the oxigen, which is but feebly retained, quits the acid, and affumes the elaftic Hate by becoming faturated with the principle of light : the fame difengagement is produced when v/e expose nitric acid to the rays of the fun, but with greater dif- ficulty, becaufe in this acid the oxigen is more ftrong- ly combined. Oxigen is alfo difengaged in the fame manner from fome oxids, or metallic calces. When a principle enters in a large proportion into any fubflance, it generally renders itfelf evident by the properties which it communicates, notwithftand-^ ing the changes they may undergo. Thus oxigen, in combining with metals, communicates to them the property of adling more forcibly upon the rays of light ; but the action which they then exert is un- equal with regard to rays of different kinds, until the affinity of the oxigen is fufficiently weakened by heat, fo that all the rays may combine with it, and ren- der it elaftic. It is on this account that metallic oxids change their colours by ffight variations in the propor- tions of oxigen. In this view the oxid of manganefe prefents a very remarkable phoenomenon. When it is faturated with oxigen, it appears black if its particles are colleded together ; but if they are extended or dif- perfed as in glafs, into which they are made to enter only in a certain proportion, it gives a red colour, A 3 which 6 Or COLOURING P.^RTICLES, which proves that the apparent black is owing to the red particles being brought into clofer contad. If we deprive it of a great part of its oxigen, either by carbon or by any other fubftance which can feparate this principle, the glafs becomes perfectly tranfpa- rent, and lofes its colour ; but we can re-produce the colour by means of nitre, or any other fubftance that can reflore to manganefe the oxigen which it had loil. . It is in this manner v*^e explain its ufe in the art of making glafs ; it deftroys the carbonaceous- fubftances which might alter the purity of the glafs, and by this means it lofes the property of giving it a colour, provided ii be added only in the proper proportion. In metallic oxids, and in many mineral fubftan- ces, all the parts are equally coloured ; but it is not fo with vegetable or animal fubftances ; in them, colour is generally owing to particles which are mixed or combined with the parts that form thefe fubftances ; and vegetables fometimes contain dilier- ent colouring particles at different periods of their ex- iftence, and in different parts. The colouring molecules of vegetable and animal fub- ftances are almoft the only ones ufed in dying; but they are often modified by admixtures. In this treatife they will be diftinguifhed by the name of Colouring par- ticles. Thefe colouring particles do not exhibit, in their compofition, the fmiplicity of mineral fubftances, and their colours are not affefted in the fame manner by oxigen. We ftiall examine, in one of the following chapters, the manner in which this principle ads up- on and decompofes them. Some AND THEIR AFFINITIES. 7 Some chemifts have confidered iron as the caufe of all vegetable and animal colours ; and M. Becker has employed, to fupport this opinion, arguments taken from the property that a metal, fo generally difFufed, has, of afliiming a great variety of colours, in the (late of an oxid, in folutions, and in vitrifications Iron indeed appears to be contained in all vegetable and animal fubftances, but in an extremely fmall quantity. The oak, which is one of thofe vegetable fubftances that ought to give the greatefl: refiduum, affords by combuftion only of its weight in afhes, and thefe aQies do not contain -rh^ of iron. Can we explain, by fo fmall a quantity, the rich and fhining colours with which vegetables aire enamelled ? Is there any true relation between the readinefs with which fome of thefe colours are changed, by acids, by alkalies, and by air, and that conftant fuccefTion of colours which iron affumes, according to its ftate of oxidation ? I know that the opinion 1 combat may be fupport- ed by the authority of Bergman, who has endeavour- ed to prove that indigo owes its colour to the iron, which it contains | ; but I fliall only remark, that it is eafy to prove that this great chemift has been de- ceived with regard to this point. By means of the pruffiate of alkali, he obtained from the afhes of an ounce of indigo from 30 to 32 grains of PrufTian blue, and he calculates the iron it contained at 1 8 or A 4 20 * Specimen fiftens experimenta circa mutationem colorum quo- rumdam vegctabllium a corporibus falibu?, cum corollariis ; auC- tore J. F. Adolph9 Becker, 1779. ■\ Analyfe chyfhique dc Tindigo. Mem. des Say. etr. torn., IX, g OF COLDUKING PARTICLES, 20 grams ; but, in fome other places, he proves that the iron contained in any fubflance does not form more than a fifth part of the Pruffian blue that we obtain from its folution, and he has every where elfe employed this calculation, which is very exad:. He ought then to have calculated the iron that he ob- tained from an ounce of indigo at no more than fix grains ; but in the experiments which follow, he proves that the greateft part of this' iron may be dif- folved by the muriatic acid, without any alteration of the colouring particles, fo that the greateft part of the metal does not enter into their compofition. It follows clearly from this, that the colouring parti- cles of this fubftance contain a quantity of iron fo. fmall, as to a-fied its colour only in a very inconfider- able degree. It appears to me alfo, that it is upon very weak grounds that phyfiologifls have concluded, from the experiments of Menghini, that iron is the immediate caufe of the colour of the blood. The methods of chemical: analyfis which we pof- fefs, do not enable us to determine with fufficient precifion the compofition of the colouring particles, in order to knov/ from what principles they derive their properties, and it can be ihown that a very differ-. ent compofition may give rife to a colour of the fame kind. The particles of indigo differ much from thofe which give a blue colour to many flowers. We have a great number of yellow fubftances, differing; widely in their properties, and yet affording colours nearly fimilar in appearance. There are fome colours which are fimple ; there are others which are owing to a mixture of thefe colours, and AND THEIR AFFINITIES. 5 and which are confeqiiently compound. Newton has given a general rule for producing any colour whatever, by means of the other prifmatic colours. Le Blon has fhown, that we can procure every colour which the art of painting requires, by a mixture of red, of blue, and of yellow *. Dufay found, that we can obtain the fame elFedl in dying, and on this account he concludes that there are only three primitive colours in nature, from which all the reft are derived. This opinion has been adopted by many philofophers, but it is ealy to prove that it is without foundation. There is this difference between fimple colours and colours formed by the union of different rays, that the former feen through the prifm retain their fimpli- city, while the latter are decompofed. But the green produced by the rays of this colour fuiFers no decom- pofition by the prifm ; it poifeffes then the character of a fimple and primitive colour, but that which is compound is feparated into yellow and blue. Al- though the green that we obtain in dying is owing to a mixture of yellow and blue, yet we ought not to con- fider all greens as a combination of thefe two kinds of colouring particles; thus the green oxid of copper can- not owe its colour to particles of a different nature : and the green colour of plants is undoubtedly produ- ced by a homogeneous fubftance, as well as the greater part of the iliades which exift in nature. I fay, that the greater part of vegetable colours ap- pear to me to be owing to a homogeneous fubftance^ but this fubftance may produce green, for example, by * L'Harmonle du colons dans la psinture reduite en pratic^ue. io OF COLOURING PARTICLES, by the refledlion of a green ray, or by that of yelfow and blue rays ; and there are feveral colours, which are neceffarily produced by the refledion of different idnds of rays. If the green colour of plants be owing to two fubilaRces, one of which is yel- low, and the other blue; it appears furprifmg that we cannot feparate them, or at lead change their proportions by fome folvent ; however, there are fub- llances which contain colouring particles of different kinds, fuch as madder, as we fliall afterwards fee in the fecond part.' The green, which I take as an example of a real compound colour produced by dying, makes it evi- dent, that the colouring particles cover only a part of the furface of the fluff, although it appears of a full and uniform colour ; for, after the fluff has been dyed blue, the yellow parts are fixed on it, and produce the green; and what proves that it is upon the parts of the fluff, and not upon thofe of the indigo that they are chiefly fixed, is, that although we begin the dying with a yellow, if we employ a yellow that is not very fixed, it may be deflroyed by the air, and ef- pecially by chemical agents, while the blue remains almofl without any alteration. From this, it appears that it is only neceffary that rays of different colours fhould be brought together^ in order to give us an idea of a fimple colour. In- deed we can produce a uniform colour by the care- ful mixture of wool of different colours, and in painting we obtain fimple fliades by the mixture of diiierent ingredients which do not combine to- gether. AND THEIR AFFINITIES. li The colouring particles form different combina^ tions, and they are applied to wool, to filk, to threap, and to cotton, either alone, or previoufly combined with other fubftances. The art of dying confifts in employing the affinities of the colouring particles in or- der to extract, diffolve, and afterwards apply, and fix them on the fubftances to be dyed. Some have attempted to diftinguifh colouring parti- cles into extra6live and refmous ; but this dirifion can only give imperfe£l and falfe ideas of their pro- perties ; for there are colouring particles, which, not being foluble in water, might be confidered as refi- nous, were they not equally infoluble in alcohol. Such are the red colouring part of the carthamus, (baflard faffron), which is dilfolved only by means of an alkali, and indigo ; which is neither foluble in wa- ter nor in alcohol, but which is readily diffolved in the fulphuric acid, and by means of certain procelTes is rendered foluble in alkalies. The colouring particles which are diffolved in wa- ter cannot be compared to the mucilaginous and ex- traclive parts of vegetables ; fmce the property which thefe fubftances poffefs, of diffolving in water, affords no idea of the properties which are effential to them ; fuch as their relations with the chemical agents which are ufed in dying, with air, with light, and with vegetable and animal fubftances. There is another error, hurtful to the progrefs of true theory ; that of attempting to explain the pro- perties of colouring fubftances by the mucilaginous, refmous, earthy, faline, and oily particles, which are fuppofed to enter into their compofition in the man- ner ii OF COLOUP.ING PART'ICLES", ner that Mr Poerner has chiefly attempted. An error fimilar to the abfard explanations that have often been given of the action of medicines. (Vid. Mater* Med. Cartheuf.) Thus we fee, that in explaining the caufe of co- lours, and the nature of colouring particles, authors have fallen into two mifrakes ; one, that of endeavouring to explain the action which the particles of colouring fubftances exert upon the rays of light, by their denfity and thicknefs, without having any method to determine, or without any regard to the affinity depending on their •chemical compofition ; the other, that of comparing on fome feeble analogies, the colouring particles with mu- cilages and refms, and endeavouring to explain their properties from the parts which are fuppofed to enter into their compofition ; while their colouring proper- ties ought rather to be determined by diretl experi- ment, than explained by any imaginary compofition. It was little confonant to true theoryj which is only the refult of obfervation, to attribute to laws purely mechanical the adhefion of colouring particles to the fubftances which we dye ; the action of mordojits^ and the difference of the colours of the true and falfe dye. Hellot, to whom we are indebted for the beft work extant on dying, has deceived himfelf completely on this fubjedt ; his theoretical ideas have often in- fluenced his obfervations and the principles which he has eftablifhed, as any one may be convinced of by reading his work. We may judge of his theory by the following paffage ; " I believe we may lay it down as a general principle in the art of which I treat, that " the whole invifible mechanifm of dying confifts in " dilating - i^ND THEIR AFFINITIES. IJ dilating the pores of the body to be dyed, in depo- *• fiting in them the particles of a foreign fubftance, and retaining them there by a kind of cement, which nei- *' ther water nor rain, nor the rays of the fun can alter. *' In choofmg colouring particles of fuch tenuity, that they maybe retained, properly wedged in the pores of ** the fubftance opened by the heat of boiling water, " then contracted by cold, and afterwards coated with a kind of varnifh, that the fairs employed in pre- paring the llutfs had left in the pores ; from this ** it follows, that the pores of the tibres of wool, which we have formed, or mean to form into ftufl's, " ought to be cleanfed, enlarged, varnifhed, and then • * contradled ; fo that the colouring particle may be retained like a diamond in the beafel of a ring*." We have reafon to befurprized,that Macquer, who was one of the lirfl: who entertained juft notions with regard to chemical affinities, Ihould have been ie- duced by the ideas of Hellot f. " It would be eafy, *' in this place," fays he, " to explain the manner in *' which mordanU act in dying, and to develope the caufe of the true and falfe dye^ but thefe objects have been treated fo well by M.Tlellot, that 1 need ** only refer the reader to his work.'* However, 'M. Dufay had already perceived that the colouring particles were difpofed according to their difierent natures to form a fironger or weaker ad- hefion to the filaments which receive them : he ob- ferves, very properly, that, without this cifpofition, the * L'Art de la teinture dcs laincs. p. 42. ■J- L'Art de la teinture en foie, av«nt-prcpcs, p. S. M OF COLOURING PARTICLES, the fluffs ought only to afllime the colour of the dy- ing vat, and to divide equally the colouring particles with it } whereas the liquor of the vat becomes foroe- times as clear as water, and yields all the colouring particles to the fluff ; which feems (fays he) to prove that the ingredients adhere lefs flrongly to the water than to the particles of the wool. He adds, that we can account in the fame manner for feveral other ap- pearances ; but that fuch an explanation is flill unfatisfaclory, and that he would abandon it with- out reluctance, if a more probable one were propo- fed. Bergman feems to have been the firft who referred the phenomena of dying completely to chemical principles * : having dyed wool and filk in a folution of indigo in the fulphuric acid much diluted with water, he explains, in the following manner, the ef- fedls that he obferved in this operation ; he attributes them to the precipitation occafioned by the great- er affinity fubfifling between the wool and filk, and the blue particles, than betv/een the fame par- ticles and the acidulated water : he remarks, that this affmity of the wool is fufficiently flrong to deprive the liquor entirely of its colouring particles, but that the weaker affinity of the filk can only diminifh the pro.^ portion of thefe particles in the vat; and he has made it appear, that upon thefe different affinities depend both the durability of the colour and the intenfity which it affumes. This is the proper method of confidering the pheno- mena * Analyfe de I'indigo, Mem. des Sav. etre. T. IX. et dans les notes fur le traite de SchefFer. AND ITHEIR AFFINITIES. IJ mena of dying ; they are true chemical phenomena, which ought to be analyfed Uke all thofe that depend upon the adlion which bodies exert in confequence of their peculiar nature. Colouring particles have chemical properties, which diitinguilh them from all other fubflances ; they have affinities which are peculiar to themfelves ; in confequence of thefe affinities, they combine with acids, alkalies, metallic oxids, with fome earths, and principally with alumine: they frequently precipitate the oxids and alumine from the acids which hold them in folution ; in other circumftances, they combine with falts, and form with them fupra-compounds which unite to wool, to filk, to cotton, and to thread. In general, they form with thefe laft fubftances, by means of alumine, or of a metallic oxid, a much more inti- mate combination than without this admixture. The difference of the affinity of the colouring par- ticles with wool, filk, and cotton, is fometimes fuch, that they refufe to combine with one of thefe fubfhan- ces, while they combine very readily wifh another ; thus, cotton is not affccled in the vat which dyes wool of a fcarlet colour. Dufay caufed a piece of fluff to be made, of which the warp was wool and the woof cotton : in order to be certain that the wool and cot- ton received exa£lly the fame preparation, he made the piece pafs through the hands of the fuller ; but the wool took on the colour of fcarlet, and the cotton remained white. It is this difference of affinity that obliges us to vary our preparations and proceffes, ac- cording to the nature of the fubflance that we wiffi to dye of any particular colour. Thcfc l6 OF COLOURING PARTICLES, Thefe confiderations ought to fix the plan we are to follow in explaining the art of dying. It is undoubtedly proper to attempt to afcertain what the conftituent parts of colouring particles are ; for although this analyfis alone cannot as yet lead us to extenfive conclufions, we fhall fee, however, that we are already able to determine in what certain changes produced in thefe particles by differ ent agents eonfift. It is of the utmoft confequence to determine the re- lations of the colouring fubflance, ly?, With thefub- ftances that may be employed as folvents ; idly^ With thofe which can by their combination .nodify their colour, increafe their brightnefs, and affift in fixing their union with the fubflances dyed ; 3^//^, With the different agents which can alter their colour, and principally with air and light. It muft not be forgotten, that the qualities of free colouring particles are changed when they enter into combination with any fubftance; but if this combina- tion unites with the fluff, it undergoes itfelf new chan- ges. Thus the properties of the colouring parts of co- chineal are modified by its combination with the oxid of tin, and thofe of the fubftance which refult from this combination are alfo modified in combining with wool or with filk ; 10 that the knowledge we acquire by exa- mining fubffances by themfelves, can only inform us as to the preparations they may be made to under- go ; that which we acquire with regard to the com- binations with fubflances employed to fix or to in- creafe the beauty of colours, may direct as to thofe proceffes in dying that ought to be adopted or that may AND THEIR AFFINITIES. may be attempted ; but it is only direcc experiment made with the dilTerent fubllances to be dyed, that can verify our conjectures and bring procelFes into ufe. In this feries of cbiervations, we are frequently able to explain the circumdances of an operation which we owe entirely to a blind practice, improved by the trials of many ages we fcparate from it every thing fuperiiuous ; we limpiify what is complicated ; and we employ analogy in transferring to one procefs what has been found ufeful in anotlier. But there is fllll a great number of facts which we cannot ex- plain, and which e!ude all theory : we m.uil then con- tent ourfelves with det.uling the procefTes of the art 5 not attempting idle explanation?, but waiting till expe- dience throw greater light upon the fiibjedl. It follows from the preceding cbfcrvations, that the changes produced by acids and aikaiies in many ve- eetable colours, and of which chemiits make crreat uie in afcertainlng the namre of different fubllances, are owing to the combination which is formed be- tween the colouring particles and the acids or alkalies. We may compare the compounds that are formed to ^leutral irks, which poiTefs properties diilerent from thofe of ih^ir component parts, but in which one of the componerit parts may become fuperabundant, fo that the properties fhall prevail. Vv''e obferve thi^ flare of combinatif'i >je: A-een the colouring particles of cochi- neal and the aciJulaied tartar ot pota^, jr cream of Tar- tar ; by evaporating flovvly a folution of (his fait in a decoction of cochineal crylbls are formed wh'ch preferve a beautiful riioy cciour, much deeper and B more OF COLOURING PARTICLE'S. more lively than that of the liquor in which they 2it€ formed. There are acids, particularly the nitric acid, whichy after being united with colouring particles, alter the colour they at firfl produced, render it yellow, and de- flroy it. They acl then by means of one of their com- ponent parts, the oxigen, in a manner that will be ex- plained when we come to treat of the action of air, and of the oxyginated muriatic acid upon colouring par- ticles. The blue are not the only colours that become red by the aQion of acids, and green by that of alkalies ; the greater part of red colours, that of the rofe for example, are heightened by acids, and become green by alkalies ; and fome green colours, fuch as the green decodion of Burdock, according to the teflimony of M. Nofe, and the green juice of the berries of Buck- thorn, according to the obfervation of M. Becker, be- come alfo red by acids. This property, common to the mofl ordinary ve- getable colours, feems to prove, that there is a ftrong analogy between the greater part of their colouring particles : and it was not without foundation that Lin- n83us imagined that the red colour in vegetables was owing to an acid, and was a proof of its exiftence in them * j but there are many vegetables which con- tain * The colour of feme flowers appears to be naturally modified By a very weak acid without being decidedly red ; fuch is the flower' of the violet, the juice of which becomes blue when it is allowed to remain for a time in a veffel of tin j probably becaufe the acid it contained combines with the oxidated part which is found on the farface OF iM0RDAXT5. ig tain art iciJ fully elaborated, without having any red tblour. Colouring; particles have their affinities with acids, nlkalies, earths, and metallic oxids, which partly con- fHtute their chemical properties, and according to which their colours undergo greater or lefs changes. Thefe particles form then with the fluff * upon which they are fixed, a combination which retains on- ly a part of their primitive properties, in uniting with allum, metallic oxids, and fome other fubftances. They are alio modified ; but the properties of thefe lafl: combinations are ftill the fame when united with the fluff. All thefe changes are fi milar to what we obferve in other chemical combinations. They will afterwards be examined more in detail. CHAP. IT. Of Mordants, WE give the name of Mordant to fubflance^ employed in facilitating or modifying the com- bination' of the colouring particles with the flufia which are to be dyed. B 2 Mordant! furfacc of the reiTtl ; fuch alfo Is th^ flower of the mallow, uhich becomes blue by fimple drying. Thefe recover their primitive hue from the addition of a very fmall quantity of acid. Obferv. Phyfico. chym. fur les couleurs. Journ. de Phyf. torn. VIII. * Throughout this work, the word JIuJ' means any thing that is the fubjecl of dying, whaterer may be its nature or form. 20 OF MOPvDANTS. Mordants deferve the greareft attention ; it Is by their means we vary colours, that we increafe their bri 'htnefs, that v/e fix and render them more durable. The analyfis of the action of mordants is what in a pecuhar manner converts dying into a chemical art, and a more extenfive knowledge of their effects muft contribute much to the perfedion of this art. 1 (hall defcribe, in the third feftian, the effential pro- perties of the chemical agents which are chiefly em- ployed as mordants ; 1 fliall afcertain, in treating of each colouring fubflance, the effects they produce upon it. In this chapter I ihall examine in what the adion of the principal mordants confifts, and endeavour to determine hov/ their affinities intervene between the colouring particles and the (tuff, and how they influence the quahties of the colours. A mordant is not always a fimple agent ; for among the ful)ilances of which it is compofed, there are fom.e- times formed new combinations, fo that the fubftances employed do not a6t directly, but the combinations which refult from their mixture. Sometimes we. mix the mordant with the colouring^ particles, at other times we impregnate the fluff with it; in other circumftances we unite thefe two methods, or wc mry dye fucceilively with liquors containing dif- ferent fubftances, the lalt of which can a£t only upon the particles with v*'hich ihe fluff is already impreg- rared. As the art of printing callicoes exhibits niany pro- ceffes, in which it is eafy to profecute the effects cf mordants^ 1 fliall take a few examples from it, which "wili render more intelligible the theory afterwards to be OF MORDAXTS. 21 he explained. The mcrJani applied to cloths that are to receive different degrees of red, is prepared by dif- folvlnt; in 81bs. of hot water 3lbs. of alum, and i lb. of acctite of lead, or fu'^ar of lead ; and we add to it 2 nz. of potafn, and afterwards 2 cz. of chalk in powder The alum is decompofe 1 by the acetire of lead, be- caufe the oxid of lead combines with the fulphuric acid, and forms an infoluble fait which is precij:>itated ; the bafe of the alum or alumine combines at the fame time with the acetous acid, by which an acetire of alumine is formed ; the chalk and the potafli fcrve to faturate the excefs of the acid. One great advantage that refults from the formation of the aceriteof the alumine is, that the alumine is re- tained in it by an affinity much weaker than in alum, fo that it parts more readily with its folvent, to combine with the fluff and the colouring particles. A fecond advantage i?, that the acid liquor from which the alumine is feparated a£ls lefs powerfully up'^n the colour when it is the acetous acid, than when it is a flronger acid, fuch as the fulphuric. Laflly, the acetite cf alumine not pofTcfling the pro- perty of cryflaliizing, the mordant, that is thickened by means of (larch or gum, before applying it to the mould on which the pattern is engraved, does not be- come rough and unequa', as it might if it contained alum that would cryOallize. Let us follow the operation which is performed up- on a piece of cloth when it is impregnated by the mcr- B 3 dant * Wc Aiill be mete particular on this head in treating of T:adderp OF MORDANTS. 4ant in the manner determined by the pattern ; it is put into a madder vat ; the whole piece is qoloured, but the colour is deeper upon the parts that have received the mordant. There the colouring particles are combined \yith the alumine, and with the cotton ; fo that it is this triple combination that is formed, and the acetous acid being difengaged from its bafe, is left fufpended in the vat. Thecolouringparticles combined with the alumincand the flufF are far lefs eafiiy afTeded by external agents than when they are either ifolated or united to the fluff with- out any intermediate fubftance ; and it is upon this property that the operations are founded to which the cloth is afterwards fubjeded ; after having pafTed through the madder vat, it is boiled v/ith bran, and expcfed upon the bleaching field ; and thefe opera- tions are alternately repeated, till the bafe becomes white. The colouring particles, which are not uni- ted with the alamine,are altered in their compcfition, diffolved, and feparated ; while thofc which are combi- ned, refifl, and are preferved without alteration ; fo that the (lamp alone remains coloured. This deftrudion of the colouring particles, by expo- fure on the held, and by boiling with bran, feems to be produced, and ought to be explained in the fame manner with that of the colouring particles of thread y the only difference in the procefs is, that we employ bran inftead of alkalies, becaufe the latter would dif- folve a part of the colouring fubftance which is fixed by the alumine, and would alter its colour \ whereas the bran, having a much lefs afHnity for this fubftance, f6ts only upon the colouring particles that have been difpofed CT MORDANTS. 23 difpofed by the influence of the air, to diffolve more readily. If, inftead of the mordant we fpeak of, we employ a folution of iron, the fame phenomena take place ; the colouring particles decompofe the folution of iron, and form a triple combination with the fluff. But, inftead of a red colour, we obtain from the madder different (hades of brown, from the lightefl even to a black, and by uniting the mordant of the alum to that of the iron, we obtain mixt colours, which are partly red, and partly black ; fuch are the mordoreox golden, and the puce or flea colour. In fhort, we procure other colours, by ufmg woad inflead of madder ; and by means of thefe two colour- ing fubilances, indigo blue, and the two mordants of which we have been fpeaking, we obtain the greater part of the varied fhades that we obferve on printed cloths. The fubftances which enter into the compofition of a mordant cannot fometimes be decompofed by the force of their own affinities ; but the aflinity of che lluffforoneof the conftituent parts occafions a decom- pofition, and new combinations, and fometimes this effe£l cannot be produced, or at leall complcated, without the aid of the affinity of the colouring parti- cles. This appears to be the cafe with the mixture of alum and tartar, which is the mordant chiefly employ- ed in the dying of wool. I diffolved equal weights of alura and tartar \ this lafl: fait acquired by the mixture a greater degree of folubility than natural ; but by evaporation, and by a fecond cryftallization, the alum and the tartar fepara- B 4 ted. 24 PJ' MORDANTS. ted, fo that they had not be^.n decompofed. I boiled half an oance of alum v/idi an ounce of wool for an hour; a precipitate was formed, which I wafiied careful- ly : this precipitate was chiefly compofed of fmall fila- ments of wool encruued with earth. I expofed it to the action of the fulphuric acid, dried it by evapora- tion, and diflblved the refiduum, from w^hich I obtain^ ed cry dais of alum ; it depofited carbonaceous parti- cles. I evaporated the liquor which had been boiled with the wool; I obtained from it a few grains of alum; the remainder did not crydaliife. I rediliolved it, and precipitated the alumine by an alkali ; the precipitate was of a Hate colour, it became black upon burning charcoal, and gave out alkali ae vapours. "We fee from this experiment, that the wool decom- pofed the alum, that a part of the alumine combined with the filaments which were the mod ifolated and the mod weakly kept together by the force of aggre- gation ; that an animal matter had been diffolved and precipitated by the alkali from the triple combination which was formed. I made the fauic trial with half an ounce of alum and two drachms of tartar ; in this there w^as no pre- cipitate formed. I obtained, by evaporation, afinallpart of the tartar and forne crydals cf alum of a very irre- gular figure ; the remainder did not crydallize. I di- luted it v/ith water, and precipitated it with potafh, and I obtained, by evaporation, a fait that burned like tartar. The wool which had been boiled with the alum, be- came rough to the touch, but the other prefervediis foft- nefs J the former aHamed with the madder a duller co- lour« OF MORDANTS. «5 iour, though lefs deep ; and the latter a colour rnorc full and livtly. I cannot give iniietail the explanation of what took place iu thel'e experiments^ but we lee, ly?, That ihe wool had begun to decoinpofe the alum, that it combined with a part of thealumine, and that '-ho part of the alum which retained its siumine had diiTolved a fmall c^uantity of an anhnal fubftance. Thar the tartar and the alum, which are not mutually decompoiecl when they acl only by meir own aiTmiiies, may aci upon one another when their affini- ties are akered by thofe of the wool. 3^/, The tartar appears to be particularly ufeful in moderating the too great adion of the alum upon the wool, which is foniewhat injured by it ; thus no ufe is made of tartar for the aluming of filk and thread, which have lefs adlion upon the alum than wool. As the decompofition of the alum by the tartar and the wool is only produced by affinities, which lofe their equilibrium with difficulty ; and as, cf confe- quence, it can only be performed llowly, we may per- ceive why it is advantage-. ^us to keep for fome days in a UKnll place, the ftuif impregnated with alum and tartar, as is commonly advifed. The Lilt refult of the a'uniing, in whatever manner it m^y have been performed, and whatever the chemi- cal changes may be which have been produced, con- fifts in the combination of the aluin with the RufT. This combination is perhaps at fird im.perfe^V, having parted with only a porlion of the acids ; but it is completed, OF I«ORDANTS. completed by boiling the wool with madder, juft as we have feen it performed on printed cloths. An acid and an alkali may, at the fame time, be u- nited with the fluff, the colouring fubftance, and the alumine ; for there are colours which are altered by an acid, and reftored by an alkali, or by a calcareous earth, which deprives them of their acid, or produces an ef- fect contrary to it. But this fupracompofition takes place only with colours which may be confidered as fixed, and w^hich are unalterable by acids or alkalies that are not fufficiently flrong to deftroy their compofi- tion. The afEnlty of alumine for animal fubftances is not Indicated by doubtful phenomena only, nor invented to ferve merely for explanation ; it is demonftrated by dired experiments. 1 have formed this combination by mixing an alkali, faturated with an animal fubftance, with a folution of alum ; a double exchange takes place ; the alkali unites with the acid of the alum, while the alumina is precipitated in combination with the animal fubftance ^. I have proved, by another experiment, the affinity of the alumine for animal fubilances. After having mixed a folution of ftrong glue and a folution of alum, I precipitated the alumine with an alkali; it carried along with it the ftrong glue with which it had been com- bined : this combination has the appearance of a femitranfparent jelly, and is dried with difficulty f . We have feen alfo in the preceding experiments, chat the alkali had precipitated the alumine combined with * Mem. de Tacad. 1784. f Ann. de cb^ra. tome IV. p. 155. OF MORDANTS. vith the animal fubftance, from the uncryilallifable refiduum of the alum which had been boiled with the wool. The affinity of alumine for the greater part of co- louring fubilances may alfo be demonftrated by dirccl; experiment. If we mix a folution of a colouring fub-^ ftance with a folution of alum, a precipitate is fometimes formed ; but if we add to the liquor an al- kali which dccompofes the alum, and feparates the alumine from it, then the colouring particles are preci- pitated in combination with the alumine, and the liquor remains clear. It is to this combination that we give the name of Lac: we muft not, in making this experiment, ufe too great a quantity of alkali, becaufe the alkali has the property of re-diffolving the greater part of lacs. No d'lxtdi experiment has hitherto proved the affi- nity of alumine for any vegetable fubftances, except the colouring particles ; it appears to be much weaker than that which it has for animal fubftances ; it is for this reafon that the acetite of alumine, as has formerly been obferved, is a better mordant for cotton and thread than alum, and on this depend the difierent methods employed to increafe the folidity of the colouring par- ticles of the madder in the dying of thefe fubftances. The metallic oxids have fuch an affinity with fome colouring particles, tha: in order to preci.^itate themfelves in combination with them, they quit the acids by which they are held in folution. On the other hand, all the metalHc oxids have the property of combining with animal fubftances ; and I l^ave formed thefe different combinations by mi.c.ng #Ln alkali, faturated with animal matter, with metallic foiutions OF MORDANTS. folutions It need not then appear furprifing, that me- tallic oxids may ferve to promote the union between the colouring particles and animal fubftances ; but, be- fides the affinity of oxids for the colouring particles and animal fubilances, there are in their acid folutions pro- perties which render them m.ore or lefs fit lo ferve as mordants : thus oxids that part readily with their acids, fuch as the oxld of tin, may combine with animal fubflances without the aid of colouring particles ; it is enough to impregnate the wool or the filk with the folarion of tin, although we afterwards wafh them carefully, which does not happen with fome other metallic folutions. Some metallic fubftances bring Into their combina- tions only a white and colourlefs bafe, there are others that modify, by the union of their colours, that which is peculiar to the colouring particles ; but in many me- tallic oxids the colour varies according to the propor- tion of the oxigen that is fixed in them, and this pro- portion may readily undergo fome changes. On thefe circumftances, which will be examined in the fequel of this feCtion, depend principally their properties in dying. The afHnicy of metallic oxids for fubftances of a vegetable nature, appears to be much weaker than that which they have for animal fubftances, and it is even unknown whether they can form true combina- tions with them. Thus, metallic folutions are not very fit to ferve as mordants ^or the colours of cotton and thread. It is necelTary, however, to except iron, the * Mem. de I'acud. 1784, OF MORDANTS. 2$ the oxiJ of which combines ftrongly with vegeta- ble fubftances, as the fpots of mildew, which are pro- duced by a true combination of this oxid, demon- (Irate. When the colouring particles have precipitated a metallic oxid from its folvent, the fupernatant liquor retains the acid which has been fet at liberty ; it has generally the power of diffolving a portion of the com- bination of the colouring fubftance with the metallic oxid, fo that the liquor remains coloured ; but fome- times all the colouring particles are precipitated when the proportions are very exa6l ; this precipitation is facilitated and rendered more complete by the pre- fence of the fluff, which ads alfo by the tendency which it has to unite with the combination of metallic oxids and colouring particles. The free metallic oxids, when boiled with feverrf colouring fubflances, have alfo a very confiderabie action upon them by which their colour is changed. The oxid of tin in a particular manner increafes the brightnefs and folidity of fome colours. We may compare the combinations of oxids w^ith colouring fubflances to feveral other chemical combi- nations, that are infoluble when the principles whith form them exift in the proper proportions ; but w hich may be combined w^ith an excefs of one of thefe prin- ciples, and in that way be rendered foluble. Thus,- * a metallic oxid combined with an excefs of colouring matter forms a liquor the colour of which is modified by the oxid ; v;hereas, when the colouring particles are not in excefs, the combination is infoluble or near- ly ^0 MORDANl*$. ly fo*'* Thefe efFecls are very evident in the combina- tion of iron with the aftringent principle. Neutral falts, fuch as nitre, and efpecially muriate of fcda, ferve as mordants^ and to modify colours ; it is difficult to determine in what manner they ad. I have found that the muriate of foda exifts in fubftance in the precipitates that it forms with fome colouring fub- ftances j and that thefe precipitates retain a tolerable degree of folubiUty. It would appear that a fmall part of the fait is fixed with the colouring particles and the fluff. Salts with a calcareous bafe alfo modify colours 5 but as thefe changes are almoft fimilar to thofe that a fmall quantity of quick lime would occafion, it is pro- bable that they are deccmpofed, and that a fmall por-- tion of the lime enters into combination with the co- louring particles of the (luff. If we attend to what has already been faid, the com- binations which are formed by the adion of the differ- ent re-agents that are employed in the analyfis of co- louring fubftances will beeafily underftood; but itmuft not be forgotten, that mordants and colouring parti- cles exert a mutual action, by which their properties may be altered. We have already perceived, that bjr varying the mordants we can multiply prodigioufly the fhades that are obtained from a colouring fubftance; ititevcnfuffici- entto vary the method of applying thctti. Th^s different refuks are obtained by impregnating the fluff with a mordant^ * ObferTgtions fur la combinaifon des oxicks metalHques avec les pinies ailringentes & Ics parties colorantes des vegetaux. (Ann. chyni. tern, 1, j OF IVTOHDAWTS. mordant, or by mixing the mordant in the dying vat, by making ufe of heat, or by employing deficcation ; for we have three reciprocal affinities, that of the co- louring particles, that of the fluff, and that of the principles of the mordant', and many circumftances can produce variations in the refult of each of thefe affinities, which merit fome explanations. Deficcation favours the combination of the fubflances which have an affinity with the fluff, and the decom- pofirions that may refult from this combination ; be- caufe the water which holds thefe fubflances in folu- tion oppofes, by its affinity, the aclion of the fluff ; but the deficcation ought to be flow, that the fubflances may not feparate before their reciprocal affinities have produced their effecl. From this it may be un- derftood in what manner the repeated deficcations aft which are employed in fome procefTes. The greater or lefs difpofition that the fluff has to combine with the colouring particles produces fome- times confiderable changes in the manner in which the rmrdant ought to be employed ; thus, when thisr difpofition is great, we can mix the mordant with the colouring fubllance : the combination which is form- ed unites inflantly with the fluff ; but if it a£ls only by a weak affinity, the combination which Is formed between the colouring particles and the mordant may be fcparated and precipitated before it is fixed upon the fluff. In order to prevent this inconvenience, it rs necelfary to begin by fixing upon the fluff the fub- flance which is to ferve as the intermedium between it and the colouring particles. From thefe obferva- tions, I fhall deduce the explanation of the differences which of AIR At^D LIG^T which exift between the proceffes erhployed for dyiiig black, and for dying wool or filLwith cochineal. Independent of the confiderations which follow, it is neceffary then, in order to judge of theeffeds of mor- dants, andof themoft advantageous manner of applying them, to attend, iji, To the combinations which ought to be formed, either by the adion of the fubftances which compofe them, or by that of the colouring particles, and of the ftuif : 2 J, To the circumftances which may concur in forming thefe combinations, more or lefs readily, and more or lefs completely^: 3^/, To the adion the liquor may have in wliich the ftuff 13 imrncrfed, either upon its colour or texture ^ and in order to fcr.;!'ee what this aclion may be, it is: neceifary to know the proportions of the ingredients l?/hich enter into the cornpofition of the mordant ^ and which ought to be fet at liberty in the liquor. Mordants ciTC not taken from the clafs of falts alone: vegetable ?.'"d animal fubllances ferve mutually as morda nts certain circumftances ; thus, in the pro- cefs for dying the red of Andrinople, (which will be defcribed in the fecond part), the cotton ought to be impregnated, or rather combined with an animal fubdance : thus, the aftringent principle is frequently employed as an intermedium betv/een the colouring- particles and the -luiT. Nobody has hitherto varied Mordants, and the manner of applying them fo much, as' M. Poerner; and his works, in this refpeci, may be of great ufe, particularly the one which has been lately tranflaiedc' CHAP. OP AIR AND LIGHT UPON COLOURS. 33 CHAP. IIL Of the adicn of different Subjiantes^ and particularly of Air and Light, upon Colours. WE have hitherto confidered the colouring par- ticles only as fubftances that can form dif^ ferent combinations, in confeqaence of which their ; properties are changed ; but they may be altered in their compofition, either by external agents, or by the fubftances themfelves with which they are com- bined. It is proper to begin, by examining the chan- ges produced by external agents, becaufe they are the moft eafily afcertained. The property of refifting vegetable acids, alkalies, foap, and particularly the adion of air and of light, conftitutes the durability of a colour ; but this proper- ty is differently eilimated, according to the nature of the colour, and according to the kind of the ftutf ; for the fame durability is not required in feme colours of filk, as in thofe of wool. There is nothiiig obfcure in the adion of water, acids, alkalies, or foap ; it is a folution produced by means of thefe agents ; and fometimes it appears that a fmall portion of the acid or alkali unites with the com- bination which forms the colour, fmce the colour is not deftroyed, but changed only ; and we can reftore it by faturating the acid, by means of chalk or am- moniac. It is otherwife with the a£lion of air and of light : till of late, it was not known in what their action cor.fifted ; C but 34 AIR- AND LIGHT but at prefent much greater progrefs has been made in this fubjecl ; and we perceive that experiments on* ly are wanting to enable us to arrive at a very accu- rate knowledge of it. Scheele had obferved, that the oxigenated muriatic acid rendered vegetable colours yellow ; and he attri- buted this effecl to its property of abftradi ng the phlogillon which entered into their compofition, I have ihown * that the properties of the oxigenated muriatic acid are owing to the oxigen tiiat is taken from it by the fubftances on which it acts: that it generally changes the colouring particles to a yel- iow ; but that, by a cpntinued adion, it deftroys their colour, without afcejrtaining in what that adion confifts. M. Fourcroy afterwards made many obfervations which throw much light on the changes that oxi- gen produces on colouring particles j chiefly when their aqueous folution is expofed to the air, or when boiled : he has obferved, that by the aftion of air vegetable decodions form pellicles which iofe their fokibility, and gradually change their co- lour : he has attended to the gradations of thofe that are obtained in that way ; and he has con- cluded from his obfervations, that oxigen enters into the compofition of the colouring particles ; that when it combines with them their ftiades are changed ; that ihe more they receive of it, the more fixed their co- lour becomes ; and that the method of obtaining un- alterable colours for painting, is to chufe thofe that have * Mem. de TAcad. lySj, UPON COLOURS. 35 iiave been fubmltted to the aclion of the oxigenated muriatic acid I have been particularly engaged in endeavouring to afcertain the aclion of air upon colours of different natures, by analyfing that of the oxigenated muriatic acid f in a memoir, of which I ilull give a fliort ab- flracl. It is necefTary to diftinguilh, with regard to the ef- feds of air, the colours of the metallic oxids from thofe of the colouring particles. I have proved, that the modifications of the firil are entirely owing to their different proportions of oxigen, as I have faidiii the firft chapter of this feclion : but 1 have been led by my obfervations to a very different opinion, with refpecl to the modifications of th-e other colouring par- ticles. I have obferved that the oxigenated muriatic acid exhibits different appearances with the colouring par- ticles ; that fometimes it effaces the colour, and ren- ders it white ; that more frequently it changes it to a yellow, to a fawn, to a brown, and to a black, accord- ing to the infenfiiy of its aclion ; and that when the colour appears to be effaced' or bleached, hear, or the lapfe of time, are fufficient to render them yellow. I have compared the effeft produced by the oxige- nated muriatic acid, when the colouring particles are rendered yellow, fawn, or brown, with the effecls of C 2 flight * Ann. de chym. tome V. f V/id. tome VI. Mem. fur l'a(5lion que I'acide murlatiquc orj, ^ne exerce far ies parties coloantes. go pr A.!R AND LIGHT flight combuflion. and I have found that they are per- fedlly the fame ; that they are owing to the deftruc-? tion of the hydrogen, which, by conibining with the oxigtn more readily, and at a lower temperature than carbon, leaves it in a fuperabundant quantity, fo that the colour peculiar to the carbon is more or lefs mix- ed with that which exifted before. This effeO: is very evident, when we fubjeft fugar, indigo, an infufien of gall nuts, or of fumach, to the action of the oxigenated muriatic acid ; the fugar and indigo affume a deep colour, and prefent unequivocal marks of flight combuftion ; the infufion of gall nuts, or of fumach, aftbrds a black depofition, which is carbon almoft pure. Thefe phenomena are fimilar to thofe obferved in the diilillation of an organized fubftance ; in pro- portion as the hydrogen is difengaged under the form of oil, or in the ftate of a gas, the fubftance be- comes yellow, and at laft there remains only a black charcoal. If we expell hydrogen from an oil by heat, it becomes equally brown. By other experiments which I have made on alco- hol and sether, I have proved that the oxigen united \vith the niuriatic acid poiTeffes the property of com- bining, and of forming water with the hydrogen, which abounds in thefe fubftances. When then the oxigenated rnuriatic acid produces a yellow, fawn, or brown colour, this is occafioned by the colouring fubftance undergoing a flight combuf- tion, in which a greater or lefs portion of its hydro- gen forms water, and then the carbon becoming the prevaiiing ingred'ent communicates its own colour. ' I have UPON COLOUIlS. 37 I have made It appear, that it is upon this change of the colouring particles by the oxigen of the atmof- phere, by dew, and by the oxigenated muriatic acid, that tlie art of bh^achnig cloth is founded. The colour- ing particles of thread are rendered foluble by the al- kaline ley, which ought to be alternate with the adion of oxigen. We can afterwards feparate from the al- kali thefe colouring particles, which being dried and brought together, are black, and demonftrate the truth of this theory by the colour which they have ailumed, and by the quantity of carbon which their anaiylis affords. The alkaline folution of the colouring particles of a cloth that is of a dark brown, loK ^ aimoft the whole of its colour, when a certain quantity of the oxigena- ted muriatic acid is poured upon it } and the fame ef- fed may be obferved in many other fubftauces that have afTumed a colour which depends on the com- mencement of cotnbullion, A cloth may appear white, and yet become yellow in courfe of time ; efpecially if it is fubjeded to a Certain degree of heat, when it has not been deprived of its oxigenated particles by a ley fufficiently (trong : in the fame manner, the green parts of vegetables are bleached by the oxigenated muriatic acid, but they be- come yellow by boiHng. Thefe- acls prove that oxigen can whiten, or render the colouring particles paler with which it combines, whether it may have begun to produce the effeOis of combuftion, or that it produces thofe effeds after- l^^ards in a gradual manner ; and efpecially when their C 3 eombination t 3^ dF AIR AND LIGHT combination is fubjeded to a confiderable degree of heat. It is very probable, that in all thefe cafes, a part of the oxigen unites with the colouring particles without combining in a particular manner with the hydrogen ; and that this is the reafon why the colouring parti- cles of thread beconre, by the action of oxigen, more foluble than formerly with alkalies. The action of oxigen 5 in many other circumRances, has certainly much influence upon the changes which are produced in the colouring particles of vegetables ; thefe particles are formed chiefly in the leaves, in the flowers, and in the fecond bark of trees : they gradu- ally undergo a flight combuftion, either by the a^lioa of the furrounding atmofpheric air, or by that of the air which is carried into the interior parts of vegetables by the ^vafa propria: hence the greater part of trees contain colouring particles erf a fawn colour, approach- ing more or lefs to a yellow, red, or brown, w^hich by^ means of this combuflion are thickened and at length' thrown out of the vafcular fibres of the bark, of which I find they form the greatefl: part. Thefe obfervations inform us in what manner the air ads upon colouring fubftances of a vegetable or animal nature : it at firfl: combines w ith them, weak- ens them, renders them pale, and by degrees excites a flight combuftion, by which the hydrogen that en- ters into their compofition is deflroyed ; they become yellow, red, or fawn ; their affinity with the fluff ap- pears to be weakened, they are detached from it, they are carried along by the water, and all thefe effects "/ary and are produced more or lefs readily, and more or' lef^ \ tjPdN COLOURS. 39 lefs completely, according to the nature of the colour- ing particles, or rather according to the properties they poirifs in that ftate of combination in which theyexift. The changes which happen in the colours that de- pend on the combination of the colouting particles witn metallic oxids, are a compound effed of the change which is produced in the colouring parti- cles, and of that which the metallic oxid under- goes. I hc light of the fun accelerates much the deftruc- tion of colours ; it ought then (if the prefent theory be juft,) to favour the combination of the oxigen^ and the combuftion by that means produced. M. Sennebier has made a great number of int.-reft- ing obfervations with regard to the efFeQts that light produces upon different fubftances, and particularly upon colours ; he attributes thefe elfeds to a diredb combination of the light with thefe fubftances The influence of light upon the colour of wood has been obferved for a great length of time : in the dark, it prefervES its natural flate^ but expofed to light ic becomes of a yellow, a brown, and a variety of other fhadefi. M. Sennebier obferved the differences which occur in this particular between different kinds of wood : he found that the changes were proportioi'al to the intenfity of the light ; that they took place even under water, yet that moiftened wood under\A'ent thefe changes lefs readily than dry ; that feveral folds of ribbon were neceffary to pr^ferve th<. wood com- pletely ; that black paper was fufHcL:at 5 but that C 4 the ^ Mom. Pl-iyfico cbyrn. fur Tlnfluence dc la lumiere folaire. T. c, 2^ 4® OF AIR A'm LIGHT the change was not prevented by other colours ; tffaf a fmgle fold of white paper was infufficient, but that two intercepted the action of light. This learned philofopher has extended his experi- ments to a great number of vegetable fubftances, which may ferve to explain many phenomena of vege- tation that are foreign to our prefent fubje^l. But in order to fhow the a6^ion of the fun upon colours, it will be fufficient to examine the phenomena exhibited by the folution of the green parts of vegetables in al- cohol. If we expofe to the light of the fun this folution,, which is of a beautiful green, it ailumes an olive colour, which is afterwards in a few minutes de- ftroyed : if the light has little intenfity, the effed takes place much more flowly ; but in perfed dark- Befs, the colour k preferved without change, or at leaflithe change is very gradual. M. S^nnebier fays, that an alkali reftores the green colour; but if the liquor has once been completely deflroyed, the alkali produces no change in it. He obferved, that in azotic gas, or phlogifticated air, the colour was not changed ; and that it fufFered no change if the jar was full. I inverted in mercury a jar half filled with a green folution, and expof^d it to the light of the fun : when the colour was deflroyed, the mercury was found rai- fed in the jar, and confequently vital air had been ab- forbed ; the oxigen had combined with the colouring particles. I did not obferve the depofition which Mo Sennebier mentions; the liquor remained tranfparent, and was tinged of a ciea.r yellow colour. UPON COLOURS. 41 I evaporated this liquor, its colour inftantly became deep and brown, the refiduum was black, and in the (late of carbon. The light then a£led by promoting the abforption of oxigen, and the combination of the colouring par- ticles : at firft, the marks of cambuflion were not per- ceptible, the liquor had only a feeble yellow colour, but the combulHon was foon completed by the ac- tion of heat ; the liquor became brown, and left a black refiduum. If the veflel in which the liquor is kept does not contain oxigenous gas the light has no adion upon the colouring particles : the azotic gas does not fuffer any diminution. It is neceflary to attend to an obfervation I have al- ready made; ribbons, and a fingle fold of white paper, do not prevent the adion of light. It can penetrate through coverings which appear to us opake, and ex- ert its influence to a greater or lefs depth. Beccari, and after him M. Sennebier, have compa- red the elFeds that light produces upon ribbons of dif- ferent colours ; but the differences which they have obferved ought rather to be attributed to the nature of the colouring fubftances with which the ribbons were dyed, than to the colours themfelves ; for a rib- bon dyed with brafil wood will lofe its colour much fooner than one dyed with cochineal, although the fhade be the fame. Although light accelerates greatly the combufiion of the colouring particles, and its prefence even ap- pears to be neceflary to the deftruclion of fome of them, yet in others this effed takes place without the alfiltance of light. Having placed in the dark, differ- ent 4^ OF AIR AND LiGH'f ent plants in contad with vital air, I obferved that fome of them abforbed it. I alfo obferved, that a rofe was changed and aiTumed a darker tinge without the contad of air, evidently becaufe it contained a finall quantity ofoxigen, the combination of which af- terwards became clofer ; but many kinds of flowers preferved their colour perfectly in azotic gas. T placed a quantity of the tinclure of turnfole in €onta6l with vital air over mercury in the dark, and another quantity in the light of the fun ; the former remained a long time without alteration, and without the vital air fuffering any diminution ; the latter loft much of its colour and became red, the vital air was for the moft part abforbed and formed a fmall quanti- ty of carbonic acid, which evidently had changed thef blue colour into a - ed. This obfervation may give us an idea of fome of the? changes of colour that are produced by a particular difpofition of the principles exifting in a vegetable fubftance, when by its combination with oxigen it expc-- riences the effeds of a flight combuftion that may give rife to fome acid ; in the leaves for example, which be- com.e red in Autumn before becoming yellow, and in the ftreaks that are obferved in flowers that begin to lanpuifli. It is then proved that light favours the abforptloii of oxigen by the colouring particles, and by this means Combuftion is produced, the comtiion efiecl of which is a fuper-abun dance of carbonaceous particles. We muil attribute to the fame caufe the deflruc^ tion obferved in fttifls themfelves by the a<5lion of light. Among many examples that I might quote/' ITFON COLOURS. iff fays Dufay, I (ball only mention a filk curtain of a " falfe crimfon colour, which had remained for a ** long time flretched out behind a wine4ow ; all the parts oppofite to the panes of glafs were completely faded, while thofe that correfponded to the frame " were much lefs injured : bcfides this, the filk itfelf •* was almoft deflroyed in the faded parts, and the ** curtain was torn with the lead force in thcfe ** places, while in others it preferred nearly its ufual ** degree of (Irength An apparent contradiction here prefents itfelf: it is to the adion of the light of the fun that the produc- tion of vegetable colours is owing ; it difengages oxi- gen from the nitric acid, from the cxigenatcd muriatic acid, from fome metallic oxids, and from plants in ve- getation ; it prod\ices then, to fpeak fo, the oppoHte of combuftion, and when it contributes to the de- ftruclion of colours, it ferves to fix the oxigen, and to produce a kind of combuftion. In the fame n^ianner phofphorus is not changed by the oxigenated muria- tic acid in the dark, even with the aSlftance of heat ^ but it undergoes combuftion, ?nd is converted into phofphcric acid by the prefence of light f. I am ignorant what the circumftances and what the affinities are that produce at one time the one effect,, and at another time the other, but both are equally certain. h appears to m.e that thefe phenomer.a may be compared to thofe of double aftinities. h is cer- tain that calcareous earth yields the fulphuric acid to potalh;, • Mera. dc I'acid. 1737. f hUaa, fur Tacid'^ Kwrin dcpV.lo^illlqt-;^ I'lSc. 44 OP AtR. AND l^IOHT potafli, and yet by means of an exchange, calcrtreoirs earth can expell potafh from its combination with ful- phuric acid. I fee, befides, in nature a great number of pheno- mena of the fame kind *, thus, in fpirituous fermen- tation the hydrogen is accumulated in the liquor, and by the progrefs of the fame effect alone the proportion of the hydrogen is diminiflied 5 it enters into combination with the oxigen, which becomes fu- perabundant when the fermentation palfes to the acid ftate. The putrefadion which takes place in animal fubftances may be confidered as an operation oppo- fed to the phenomena of growth and nutrition. Colouring fubftances refift then the a6i:ion of air, in proportion as they are more or lefs difpofed to com- bine with oxigen, and to undergo by its a^ion com- buftion more or lefs readily, and in a greater or lefs degree. The light promotes this combuftion, and in many cafes it does not take place without its afTiftancc; but ifolated colouring particles are much more difpo- fed to undergo this combuftion than when they are combined with any fubftance, fuch as alumine^ whether it be that this defends them by its incombuftibility, or to fpeak fo, that it reftrains the force of their affi- nity, and weakens their aftion upon other fubftances ;^ it is in this that the utility of mdrdanis ch'ie^y confifts. In ftiort, this laft combination acquires a ftill greater degree of unalterability when it is intimately combined' with the ftuff. Thus the colouring part of cochineal is readily dif- fblved in water, and is fpeedily changed by expcfure to the air ; when it is combined with the oxid of tin^- it UPON COLOURS. 45 t s of a much more lively colour, and becomes almofl infoluble in water ; but it is ftill eafily changed by the air, and by the oxigenated muriatic acid, and it refifts thefe agents better when it forms a triple combinatioa with a piece of woollen ftufF. We muil not conclude from what has been faid, that all yellow colours are owing to the carbonaceous part of colouring fubftances : very different compofi- tions may produce the fame colour ; thus, indigo is very different from the blue of flowers, from the blue of the oxid of copper, and fiom Pruffian blue. I do not affert that oxigen cannot unite with certain colouring particles In a fmall quantity, without weak- ening their colour, or changing them to a yellow. We (hiill fee in the chapter on indigo, that it has a green colour when it is combined with an alkali, with lime, or a metallic oxid ; but that it refumes its colour, >nd is feparated from thefe folvents in recovering a fmall portion of the oxigen which it had loft. The liquor of the buccinrjm^ which is employed to dye pur- ple, is naturally of a yellowifh colour, upon being ex- pofed to the air, and efpecially to the fun, it paffes readily through different fhades and affumes at hfh the colour that was fo precious among the antients *. We may confider it as a general facl, that colours acquire a greater degree of brightnefs, by combu.ing with a fmall quantity of oxigen ; it is on this account that ♦ Dccouverte d'une nouvelie reloture de pourpre, S:c. par Reau- mur. Mem. de Pacad. 1711. According to the teilimoay o^" Eu- docius the purple acquired its brightnefs, and arrived at jerfcaion by cxpofing^ it to the rays of the lun. 46 or AIR AND LIGHT that It IS neceffary to agitate in the air (luffs whick come out of the dying vat, and fometimes to take them out at proper intervals to expofe them to the air ; but the quantity of oxigen whii h is fixed, and which contributes in this manner to the brightnefs of colours, is very inconfiderable in feme, and the fading very fpeedily commences. The adion of the air does not alter the colouring par- ticles, and the ftuff alone : it extends to metallic oxids when they are ufed as auxiliaries, becaufe the oxids, which are at firft deprived of a part of their oxigen, may recover it again, as we fliall fee when we come to explain the aftion of aftringents. Thofe which have a colour varying according to the proportion of oxigen, have a confiderable influ- ence upon the changes that the colour undergoes. The change obferved in the blue produced on wool, by means of fulphate of copper and logwood, is evidently owing-to this caufe^ This blue is changed very quickly to a green by the a6cion of air j for we know that copper, which has a blue colour when it is combined with a fmall por- tion of oxigen, acquires a green by combining it with a greater quantity of this principle. Indeed the change that the colcurinsr particles undergo may con- tribute to produce this effecl ; but the colouring par- ticles of logwood, which are naturally of a deep colour, ought duriiig combuflion to become brown rather than yellow, which would be neceiTary to pro- duce green with blue. I have faid that colouring particles which are in a ^ ftate of combination;, are lefs difpofed to be altered by tIPON COLOURS. 47 by the a£iion of the air than when they are uncombined. This in general holds true. There are, however, fome exceptions : alkali procuces a contrary eiled. I put an infufion of cochineal in a jar, which I tilled half full, and cxpofed to the lij^ht over mercury. A finii- br jar contained an infufion of cochineal made with a fmall quantity of tartar ; and in a third jar I added to the infufion a fmall quantity of alkali : the infufion in the fecond jar fulfered the leafl alteration, and in it alfo the lead abforptlon took place. In the third, the liquor at firft aRumed a brown co- lour, which was aficrwarvis deflroyed; and the abforp- tionofair, although iuGcnfiderable, was greater than in the other two. I had it evaporated j it aifumed a brov/n colour, and the refiduum was of a yellov/ brown. I have made fimilar experiments upon many colour- ing fubftances : an alkali deepens their colour which becomes darker, and it promotes the abforpt'on of air. Madder alone appears to be an exception to this : its colour, which at firfl: becomes very deep, is better prefcrved than that of the infufion made with- out the alkali. The common effect of the alkali upon the colour- ing particles is fimilar to that which it produces up- on many other fubftances, fuch as fulphur ; it promotes the abforption of the air, becaufe it has a ilrong aliini- ty with the refult of this abforption. i explain by this action of the alkali an obfervation of M. Becker's, which is, that when an alkali has ren- dered a vegetable infufion green, we fee it gradually become yellow if we allow it to remain expofed to the ^ir, and when once the yellow is formedj acids cannot Inng 4t OF THE YELLOW COLOUR, bring back the primitive colour again ; but the fame thing does not take place when we expofe, during the fame time, a vegetable colour that has been changed to a red by means of an acid. It is neceffary, how- ever, to except acids which may aft by parting with their oxigen, as we fhall immediately fee, for then the colour is equally deftroyed. CHAP. IV. Of the yellow colour produced in ani?nal fuhjlances by the nitric and oxigenaied muriatic acids. THE adion which the nitric and oxigenated mu- riatic acids exert upon animal fubftances, hat fuch a relation to the fubject of the preceding chap- ter, that I have thought it proper not to feparate them, although the yellow colour which is produced by thefe acids upon wool, and particularly upon filk, might deferve a place among the procelTes of the art. M. Brunwifer * having obfjsrved that wood affu- med different colours by expofure to the air, endea- voured to afcertain from what fource thefe colour* originated, and to produce them artificially : he ob- ferved, that by moiftening ^he furface of wood, efpe- cial y young wood, which is not thoroughly dried, with * Verfucbe mit mineraltfchen fauern geiflern aufden holzern far* htu zuziehcn. 1770, in abhandlungen des Baierifchen AcademiCf PRODUCED IN ANIMAL SUBSTANCES. 49 with the nitric acid, and it affuined a yellow colour; and that by repeating the fame operation with the mu- riatic or fulphuric acids it aflumed a violet colour, and he confidered this violet as compofed of a blue and a red colour. From thefe obfervations he con- cludes, that fmce all colours are produced by the mix- ture of yellow, of blue, and of red, therefore thofe ^hich are obferved in leaves, fruits, and fiowers, muft be owing to colouring particles which exifl in the wood, and are concealed there by an alkali : that the mineral acids, by feizing upon this alkali, fet the co- louring particles at libert y; and that fixed air, by pene- tratingthe leaves, fruits, and flowers, produces naturally the fame effect by combining with the alkali that kept thele particles concealed. This author has endeavoured to apply his experiments and fuppofed difcovery to the arts*. After having m.oiilened chips of wood with the nitric acid, he poured w^ater upon them, 'filtrated the liquor, and employed it for dying ftufTs made of . w^ool, of filk, and goats hair, of a durable yellow ; which is produced, according to him, by the yellow co- louring particles contained in the wood being ex- tracted or fet at liberty by the nitric acid. De la Folie relates |, that having plunged a fkain of white filk into the nitric acid, or aquafortis, of the degree of concentration at which it is fold in the (hops, in three or four minutes it took on a beautiful jonquil yellow colour : he wafned it repeatedly with water, D left * Entdeckung verfchisdencr vegcrabilifchen farbematerialen fei- den und wollcn zeu^e fchon und dauerbaft geib zu farbea 177 1« t Journal de Phys. T. IV. p. 349. ^0 OF THE YELLOW COLOUR, I'-^ it fliould be altered by the acid that might remain Ji-j -h-.g to it. This colour fuftained feveral trials to vYni he fubjecled it, and'the filk preferved its luf- tre entire. Yvhen dipt into an alkaline folution, it af- fumed a beautiful orange colour. We find many experiments upon'this procefs, in a difiertation pubiiHied by M. Gmelin * : he fays, he gave a beautiful fulphur colour to filk by keeping it for a day in cold nitric acid, or for a few hours when the acid was warm. Boiling with foap water dimi- nifhed the brightnefs of this colour. It was changed to a fine citron colour by keeping the filk for twelve hours in an alkaline folution, and it afTumed a beauti- ful golden colour when this folution tvas warm. All metallic folutions by the nitric acid give to filk a yellow colour more or lefs deep, as well as the folu- tion of alamine by the fame acid ; but the folution of calcareous earth, or of magnefia, produces no effedt. This folution, mixed with the folution of gold, af- fords, according to la Folic f, a purple colour, that h fixable upon filk ; but M. G myelin obtained by this procefs only a common yellow. He proves alfo, that the different yellows which IM. Struve had faid were obtained from metallic folutions J are owing entirely to the acid of thefe folutions ; the only exception that ought perhaps to be made is, with refpedt to the folution of mercury, which gives a copper colour to Bergman * J. Frieder. Gmelin, prof. Gotting, de tingendo per nitri acid- um, five nudum, five terra aut metallo faiuratum ferico. Erfurti 1785. t Journ. de Phyf. T. VIIL t Berneriches ma^azin der nsturkunft und wilTenfchaften, vol. i. PRODUCED IN ANIMAL SUBSTANCES. 5 I Bergman was acquainted with this procels when he pubhilied his notes to the treatife of Schctler : he i'ays, that common aquafortis gives to wool and to filk, in three or four minutes, a clear, beautiful, and dura- ble yellow ; that it is necelfary to wafh them both im- mediately, and that the more the acid is dephlogiflica- teJ, the greater is the effect that is produced. From the firft experiments that I made upon the oxigenated muriatic acid, I found it had alfo the property of giving a yellow colour to animal fubflan- ccs ; but it gave them a colour much lefs deep than the nitric acid, and it injured them much more than this acid when properly diluted with water, fo that the nitric acid is greatly preferable for the purpofes of art. To refume thcfe obfervations ; the nitric acid, dilu- ted with a certain q\!antity of water, gives to filk a yellow colour, more or lefs deep, according to the concentration of the acid, the temperature, and the longer or fliorter time of immerfion ; it is necelTary to wafii the filk carefully on taking it out of the riad: this colour has afufTicient degree of brightncfs : It may be rendered deep without any vifible injury to the fi;k, fo tha^ this procefs may be of real ufe. The colour ay be modified by alkalies. The folutions of calcareous earth, or of magnefia, produce no effect upon filk, becaufe they do not con- tain an excefs of acid ; the folution of alumine, and thofe of all metallic fubftances, produce, on the con- trary, a darker or lighter yellow; becaufe they all ccn- D 2 tain * EiTui far I'Art de la Tcintui'e. 53 OF THE YELLOW COLOUR^ tain a greater or lefs excefs of acid, v/hich a£ls upon the filk in the fame manner as a free acid ; it was the acid alone, therefore, which gave the yellow colour to animal fub fiances in the experiments of M. Brunwifer, and not the particles extracted from the w^ood. The yellow colour is not owing to iron, as de la Fo- lie has alledged ; for the purefi: nitric acid, which of confequence contains no iron, produces it as well as that in which a fmall quantity of this metal is fuppo- fed to exift. If we put filk Into concentrated nitric acid, it readi- ly alTumes a deep yellow colour, lofes all its force of adhefion, and is diflblved : during this folution, the azote, which enters into the compofition of animal fubftances, is difenga: ed, and occafions a tedious efFer- vefcence : if we apply beat, a great quantity of ni- trous gas is difeijgaged, and at the beginning the li- quor affumes a deep colour, and becomes brown. At this time the oxigen of *he nitric acid combines un- queflionably with the hydrogen that abounds in ani- mal fubftances, vvhlch affifls in forming the oil we ob- tain from them in diflillation, and which renders them fo inflammable. The fame efFeO: ought to take place when the acid begins to a<5l upon the filk and to render it yellow. I believe, therefore, that the yel- low colour arifes from an incipient combuftion, as I have explained in the preceding chapter. This flight degree of combuftion does not injure the filk in any fenfible manner ; but if the acid is too much concen- trated, if the immerfion is continued too long, or if W9 * Mem. de TAcad. 17 85. PRODUCED IN ANIMAL SUBSTANCES, 53 t^e do not remove, by a careful wafaing, whatever might adhere to it, the filk is foon very much inju- red, and, according to common language, which af- fords a very juft idea, // is burned. We can now perceive why, according to the obferva- tion of Bergman, thenitrid acid, which he called the de- phlogilHcated, is for this operation to be preferred to that which is faturated with nitrous gas : for in the former the proportion of oxigen being greater, it is on that account more fit to produce the effecls of combuf- tion until it is brought back to the (late of nitrous acid. The fame circumftance ought undoubtedly to ex- plain the action of the oxigenated muriatic acid upon animal fubltances ; yet it differs in fome effential points, all of which I cannot explain, as they require a great number of obfervations. Silk allumcs, with the oxigenated muriatic acid, a colour much lighter than with the nitric : the fulphu- rous acid caufes it in a great meafure to difappear, but it has no adlion upon the yellow produced by diluted nitric acid. The oxigenated muriatic acid has, how- ever, a much more lively adion upon filk : it quick- ly injures it, and even diflblves it : if we allow the filk to remain fome time in this liquor, the yellow, which at firfl: appears, is weakened, agreeably to the obfer- vation I formerly made, that the yellow colour, which is owing to the combuflion firft produced, may be difguifed by the accumulation of oxigen. By the readinefs with which the fulphurous acid parts with its oxigen, I once endeavoured to explain* D 3 the * Suite des Exper. far i'icidc fulfureux. Ann. de Chym. torn, II. 54 OF THE YFIlLOW COLOtTR. the efeds it produces upon colours, and I comparef^ them to thofe of the oxigenated muriatic acid ; and ahhoagh it be true that oxigen adheres much more weakly to the fulphurous than to the fulphuric acid^ jet I do not believe that the explanation I gave is well founded. According to the obfervation of de la Folie rofes whitened by the vapour of burning fulphur, become green in an alkaline ley, and red in acids. I have my- felf obferved, that the fulphurous acid reddens the tin£lure of tournfole, which has a very fleeting colour^ and that it acts only like other acids upon infufions of yellov/ v.'ood, on fernambouc, and logwood. I have befides obferved, that filk which has been expofed to the vapour of fulphur, exhales the odour of fulphu- rous acid, when we moiflen it with the fulphuric, al- though this odouf was not formerly perceptible. I think then, that the fulphurous acid combines ge- nerally with the colouring particles and wiih filk, without parting with its oxigen to them, and confe- quently without combuflion taking place ; that the re- lult of this combination fometimes lofes all colour, a property depending probably on the femi-elaflic (late of the oxigen ; but fometimes the combudion may take place, and it ought in general to do fo by de- grees ; fo that the colouring particles, after having, been long concealed, will at lail alTume a yellow co- loiir. This fubje^c I think might eafily be elucidated by fome experiments. CHAP. * Journ. de Phyf. 1774. Astringents in general, kc. 55 CHAP. V. Of Aftrhigents in gaieraly and particularly of Galls. ASTRINGENTS dcfcrve particular attention, not only becaufe they are of great ufe in dying, but becaufe they exhibit a property common to a great number of vegetables. There is, perhaps, no property in vegetables of which we entertain fuch vague ideas as of this ; v/e of- ten judge of it from a weak refemblance in the taftc, and both in medicine and in the arts under the name of aflringenr, we rank alum and many vege- tables, the properties of which are, however, exceed- ingly different : in general, we confider as an aflrin- gent, every fubftance which gives a black colour to a- folution of iron ; we fuppofe, that this effedl is owing to one and the fame principle exifting in all the fub- ftances which produce it : we apply the term aftrin- gent to this principle, and at prefent it is confidered as a peculiar acid to which w^e give the name of gallic acid, from the gall nut in which it exills. However, feveral learned chemifts have of late made a great number of interefling obfervations upon aftrin- gents. I (hall now prefent an abflrad of them ; but as the gall nut has been the ufual fubjecl of thefe ob- fervations, it is neceffary to begin by defcribing it. The gall nut is an excrefcence which is found up- on the young branches of the oak, particularly on the fpecies that we call the hard oak, which grows in the Levant, Iltria, Provence, and Gafcony. This excref- cence ^(5 OF ASTRINGENTS IN GE'^ERAl^ cence is produced by the bite of an infect, which de- pofits its eggs in the faiall inclfion it makes in the Spring; the juice which tranfudes from this wound thickens, accunii>lates, and ferves as a fheker to the young infe6l, until it be able to make its efcape : when the infed does not obtain an outlet, we find it dead in the gall nut : fometimes after it has quitted its ha- bitation, other infeds enter and occupy it. There iire difiercnt kinds of gall nuts ; fome of them are white, yellow, green, brown and red ; others are afh-coloured and blackifn ; they vary greatly in fize ; they are round or irregular, light or heavy ; fome of them are fmooth, and others are covered with knobs ; thofe which are fmatl, knobby, blackifh, and heavy, are the beft : it is thefe lali: that are known by the name of the palls of Aleppo,- and they are brought from Aleppo, Tripoli, and Smyrna. Galls are aimoft entirely difTolved by long boiling : fixteen drachms afforded to Newman fourteen drachms of extra6t ; alcohol diiTolved only four grains of the refiduum, which weighed two drachms. The fame quantity, infufed firft in alcohol, and af- tcrv/ards in water, aiTorded twelve drachms two fcru- pies of a fpLrituous, and four fcruples of an aqueous ex-- tracl ; the refjduum weighed half a fcrupk more than :n the preceding experiment. '1 he fpirituous extract- had a ftronger and more difagreeabie tafte than the other M. Monnet, to whom chemiftry is indebted for a- g?eat number o. important cbfervations f , confiders Macquei; * The chemical works Cufp.ar Newman, by William Lesvis. 4" Traite des Eau:i mincralei, p. 3040 AND PARTICULARLY OF CALLS. 57 Macquer as the firfl who attributed the formation of ink to a precipitation of iron by the aflringent principle, the theory of which he has hiinfelf eftabliflied ; he ima- gines that the iron his a greater affinity to this prin- ciple than to the acid with which it was united ; but the acid diifolves a part of the precipitate, provided it be not too much dilated with water. Iron is not the only metallic fubilance which may be precipitated ^rom its folvent by the aftringent prin- ciple : M. Monnet has obferved, that gold, filver, copper, zinc, cobalt, platina, and mercury, are alfo precipitated by it. The metallic fubflance the moft eafily precipitated is mercury : its folution mixed v»^ith an infufion of galls affumes a brick colour, but the precipitate which is formed foon becomes of a reddifli grey. Copper is very readily precipitated : the precipitate IS at firfl green, but it becomes of an afh-coloured grey, and in drying affumes a reddilh coppery glofs. Zinc forms a green afli-coloured precipitate. The precipitate of cobalt is of a clear blue ; but it does not preferve this colour long, for it foon becomes of an afh-coloured grey. Silver is precipitated very flowly : the precipitate, which is at firfl of the colour of burnt coffee, affumes, when dried on pr.per, a fliining filvery luflre. Gold is precipitated almofl infenfibly : the precipi- tate, when collected and dried, is in every reipecl fimi- lar to that of CafTius. M. Monnet collecled the precipitate of iron by the aflringent principle upon a filter : it was of a beauti- ful deep blue : he thinks it might be employed with advantage cS OF ASTRINGENTS IN GENERAL, advantage in painting. He tried to diflblve it In the nitric and faiphuric acids, but they afted upon it very ilowly J having accelerated their action by heat, he obtained by lihratlon beautiful yellovi^ tinctures, efpe- cially that of the nitric acid, which refembled a foiu- tion of gold : no precipitation took place, and the li- quors remained always clear and limpid : alkali oc- cafioned no precipitation ; it produced only a deeper colour. M. Monnet vi^as led by this obfervation, to difiblve this precipitate diredly in a folution of fixed alkali by means of boiling j he obtained a folution as red as blood, in vv^hich water produced no precipita- tion. The acadeixiicians of Dijon have added to thefe fads feveral intereftinc: obfervations. Galls afford by diilillation, a limpid phlegm, which gradually becomes brown, then a yellow oil, after that an oil deeper in colour, and empyreumatic. All thefe liquois afford a black precipitate with the ful- phate or vitriol of iron; the carbonaceous refiduum has no longer any peculiar property ; it reddens in the fire without bein^r reduced to aflies. o Even cold water diffolves the aftringent principle : an ounce of galls afforded by this means tTiiee drachms of a nondeliquefccnt and very flyptic ex- trad. Fixed and volatile oils and aether dilTolve the aftrin- gent princi])le. , The infufion of galls gives a red colour to blue pa- peY, and the tincture of tournfole ; bui it has no ac- tion upon fvrup of violets. The AND PARTICULARLY O? CALLS. 59 Thefulphuric acid itc>;ives fiom ga!Is-a red colour; the nitric an amber, and the muriatic a biown : thefe three acids precipitate the fulpliate of iron of a black colour, when they are faturated with alkalies. The acetous acid loaded with the aftringent princi- ple, does not require this addition of alkali to give a black precipitate. The phofphoric acid produces no change upon the Infufion of galls; but this mixture renders the fclurion of fulphate of iron muddy, and a white precipitate is formed. The fulphure of alkali is decompofed by the infufion of galls : in filtrating the liquor afterwards, the refi- duum docs not precipitate the iron of a black colour, but the liquor that pafics through the filter produces this elfecl: ; it appears to be a combination of alkali and of the aftringent principle. The carbonate of fixed alkali gives a reddifli colour to the infufion of galls : this mixture forms a brown precipitate with the fulphate of iron. The cauRic alkali gives a red brown colour to the infufionof galls; and precipitates, at the fame time, the fulphate of iron of a black colour : this precipitate when properly wafhed is not attraded by the magnet: it dilTolves without eHervercence in the mineral acids, but is not acled on by the acetous. The mixture of the prufliate of alkali, and of the tincture rr alcohol of galls, affords a black precipitate with the foluilon of iron : the fulphuric acid at firfl augments the intcnfity of the colour, and at laft de- ftro)s it. The 6o OF ASTRINGENTS IN GENERAL, The fame work contains a great rlumber of obferva- tions upon the precipitation of metallic fubllances by the aftringent principle : galls do not alter the folu- tions of arfenic in any degree ^ and the oxid of this metal, mixed with an infufion of galls, does not pre- vent its action upon iron. The aftringent principle forms ink with all the acid fohitions of iron, except thofe of th6 phofphoric and arfenical acids. To conclude ^ we fmd in that work fome e:^peri- ments upon ink, and upon the depofition which it af- fords when largely diluted with water. Ink is not de- compofed by the acetous acid, but the mineral acids quickly deprive it of its colour ; the fulphuric acid produces heat in it without efFervelcence ; the liquor becomes yellow, and forms a greyifh depofuion. Al- kalies reftore the colour that acids have made to dif- appear, but by repeating the alternate action of the acids and alkalies, the colouring principle is at length deitroyed. The fame academicians have obferved another fact, w^hich may become important, and upon which we ftiall afterwards fee M. Laval has bedowed more at- tention ; it is the adion of galls upon iron. A cold infufion of them gives a violet thige to iron filings, but by boiling them together, apart of the iron is dif- folved, and the liquor alTumes a violet colour, which approaches much to a black. M. Laval fays, he long ago obferved, that galls in- fufed in difiilied water alone, difToIve iron with the greatefl readinefs ; that with a fmiple folution of this metal in an infufion of galls, he not only produced the deepeft black colour, and the moil indelible ink, but AND PARTICULARLY OF GALLS. 6i but alfo, by -plunging filks and woolen fluffs into this black tindure, he dyed them, without the addition of any acid, of the deepefl and moft indeftrudible black. Dr Prieflley obtained inflammable, or hydrogenous gas, from a mixture of iron filings, powdeted galls and water : this experiment proves, that galls act upon iron like acids, by promoting the decompofition of water, and the confequent difengagement of its hy- drogenous gas. Even before M, Monnct afcertaincd the manner in which the aflringent principle combines wuh the iron, and forms the black colouring particles which may be precipitated, Dr Lewis had made fome in- terefting obfervations upon this combination, which he endeavoured to render peculiarly ufeful in the arts. He obferves, that if we dilute common ink with a fufficient quantity of water, the black is precipitated, and the liquor remains clear ; he remarks, that it is this black part which is fixed upon fluifs, and w^hich ferves to colour them : he endeavoured from this to d etermine the moft convenient proportions of the af- trlngent, and of the folution of iron, both as to the quantity of the black precipitate, and as to its quality. I'he experiments of Scheele have fuch an influence on the opinion that is formed concerning the nature cf the aftringent principle, that I fliall relate them in his own words : *' I pafled through a large fieve a pound of galls, " and infufed the powder wuh 2 and 3-4ths pints " of water in a glafs globe. I allowed it to remain in that ftate four days, during which time it was fre- quently ftirred with a glafs reed, I filtrated the li- quor 6l OF ASTRINGENTS IN GENERAL, " quor, which was clear, and of the colour of French *^ wine : I expofed it to the open air in the fame glafs globe, covered only with a bit of grey paper. This preparation was made in the month of June. A month afterwards I again looked at this infufion, and I found it covered with a thick mouldy pelli- cle; there was no precipitate formed, and it had a more acid, but not more allringent tafte than for- " merly. I replaced this infufion in the fame globe, covered as formerly with grey paper. Five weeks " after this I examined it again ; almoft one half of it " was evaporated. I found a precipitate two inches in thicknefs, and on the furface a mucous pel- licle ; it had loft wholly its ftyplic tafte, although ^' it fiill gave a black colour to the vitriol of iron. I ^' filtrated the infufion, and expofed it once more to " the open air ; by the following Autumn the great- " eft part of it v/as evaporated ; what remained was ^' mixed with a great quantity of precipitate. I mix- cd all thefe precipitates together, and poured cold ^' water upon them ; after they had been depofited, I " decanted off the liquor, and poured on as much warm water as was neceflary for their fclution. ^' I filtrated the whole ; the liquor was of a brown " yellow ; I evaporated it v. ith a gentle heat : during the evaporation a part like fine fand was precipita- " ted ; another part formed cryftals at the bottom difpofed towards the light ; this fait was of a green colour, and notwithftandiiig repeated folutions and cryftallizatlons, it was impolhble to procure it whiter. ^' This tilt of galls has the following properties : ^' I- AND PARTICULARLY OF GALLS. 63 I. It has an acid tafle ; efFervefces with chalk, and gives a red colour to the infufion of tournfok. "2. An ounce and a half of water are required " to dilToive half an ounce of this fait completely ; but " as foon as the folution becomes cold, the whole *' forms a concrete mafs, c^mpofed of fmall cryflals. " Half an ounce requires twelve ounces of cold water " to difiblve it. 3. It diifolves very readily in fpirit of wine ; for half an ounce of this fait, only the half of boiling " fpirit of wine is necelTiry ; but if wc ufe cold fpirit ^' of wine, two ounces are required for half an ounce of the fait. " 4. It burns readily in an open crucible on the fire, and in melting exhales an agreeable odour ; but it afterwards affords a charcoal that is with diiTi- *' culty reduced to afhes. " 5. DiiUHed in a retort it becomes at firft fluid, and affords an acid phlegm. No oil comes over, but *' towards tl^e end a white fublimate rifes which at- " taches itfelf to the neck of the retort, and remains " fluid as long as it is warm ; but it afterwards cryftal- ** lifes ; we find in the retort a great quantity of char- " coal. This fublim.ate has almofl the fmell and taile " of the benzoinic acid, or fait of benzoin : it dilToives as readily in water a- ^d in fpirit of wine ; it reddens *^ the infufion of tournfole ; and, what is very remark- *' able, it precipitates metallic folutions of their differ- ent colours and the vitriol of iron of a black. " 6. The folution of the fait of galls poured into a " folution of gold, gives it a dark green colour, and " at v64 0¥ ASTRINGENTS IN GENERALj at lad a powder is precipitated, which is the gold revived. " 7. The folution of filver becomes brown, and " depofits by heat a greyifh powder, which is the fil- ver revived. " 8. The folution of mercury is precipitated of an orange yellow. " 9. ihe folution of copper affords a brown pre- cipitate. " 10. The vitriolic folution of iron becomes black ; " the more water that is added, the colour becomes deeper. ''11. Lead dilTolved in vinegar is precipitated of a " white colour. 12. Bifmuth gives a yellow citron precipitate. 13. The molybdenic acid becomes of an ob- fcure yellow colour, without affording any precipi- tate. " 14. Platina, zinc, the arfenical acid, tin, cobalt, " and manganefe, do not fuffer any change. 15. The folutions of lime, magnefia, alumine, " and barytes, are not decompofed, but lime-water affords a copious gray precipitate. " 16. The fait of galls is converted into the fac- ^' charine acid by diflilling nitrous acid upon it in the ufual manner. " The white precipitate that we obtain when the acetite of lead is precipitated by galls, may be again decompofed by the vitriolic acid, and we obtain the " fait of galls in its greatefl purity ; however, as the infufion of galls precipitates the acetite of lead, I ^" thought I fliculd have been able to have procured AND PARTICULARLY OF QALLS. 65 ■^^ this fait in a manner llill more expididous ; but this did not fuccetd ; for when I had deconipofed this precipitate by' means of the vitriolic acid, I again found my infufion of galls pufTelT^^d of its ordinary aitringent talte. If we difllll with a violent fire, we obrdn an a.ciduiated phi '^nn, the odour of which is n n diiagreeabie : no oil palles " over^ but towards the end a volatile Hilt rl'es, funi- lar to that obtained from the diflillcJ fait of galls, and polledl-d of the fame properties. Frcm this it appears, that this fait cxids completely farmed in ihe i.'ifufion of galls, although it cannot be obtain- " ed by means of ordinary cryltallization ; for it is fo *• intimately combined with fome mucilaginous or other matter, that it cannot be fepiirated without an internal commotion, or without fermentation.'* Thefe experiments of Scheele have induced the greater part of chemiRs to confider the gallic acid as the aifringent principle, and to conclude that it exiifs in all vegetable fubrcances which precipitate folutions of iron of a black colour ; that this precipitate is a com- bination of the gallic acid, and of ^he iron which quits the other. acids to unite with it. I have repeated and varied the experiments of Scheele upon galls, and I have obferved, i. That by following the procefs pointed out by this great chemifl there were green pellicles luccelTively formed, owing to a byjfus which couid not be produced in a velTel well (lopped. - 2. That in a fnnilar vefiel where evaporation -could not take place, pure, tranfparent, and yellow cryftals were depofited during the Winter, without E the €5 OF ASTRINGENTS IN GENERAL, the influence of the external air, and that confequcn|:f ly the gallic acid exifls in the galls, and is not produ- ced by ' he abforption of oxigen^ as I had formerly conjedured. 3. That if we evaporate one half of the liquor before fliuriing it up in the veffel, it aifumes a deep colour ci^rinnhe evaporation, and depofites afterwards a great quantity of a cryftalline fubitance, but lefs pure, and of a deeper colour, 4. That the acid obtained in this manner, or by the procefs of Scheele, and afterwards diffolved in wa- ter, is alif^vays decompofed by evaporation ; that black pellicles are feparated from it ; that it becomes brown by fimple expqfure to the air ; that, after repeated ope- rations, nothing more is obtained than an ill-formed pryftallization of a grey colour, more or lefs yellow or brown ; and that if the evaporation is repeated a number of times, this fait is completely deflroyed. Heat, and efpecially boiling, accelerate this deftruc- tion : this is the true reafon, 1 believe, why this fait is not obtained by evaporating the decoclion of galls ; an extract then remains which refembles perfedly that of galls, and which fuddenly precipitates folutions of iron of a black colour. 5. The folution of the gallic acid alTumes a fine; green colour with the fixed and volatile alkalies, ei- ther free or combined with the carbonic acid ; and this colour becomes fo deep, that it appears black if the gallic acid and the alkaline folution are brought together : with lime water the gallic acid affumes a reddifh brown colour, the red tinge is difTipated, and a CO- AND PARTICULARLY OF GALLS. ^ :t copious depofition takes place of a, grey colour, ap- jDr caching to brown. In the mixture of the gallic acid v/ith the carbo- jiate of alkali, I have not obfcrved an effcrvelcence, any more than with the carboTiate of lime or of magne- fia : but this fubftance reddens the tindure of tourn- fole, c^' radifhes, &c. like acids. 6. The infufion of common galls produces the liime -cfle^l upon vegetable colours as the gallic acid, but the infafion of white galls has -no aclion upon them. Paper tinged with tournfole is not altered by the infufion of furaach, of the bark of the plumb-tree, af the bark of the walnut, or of Peruvian bark- Having made four fucceflive decodlons of galTs, which had previoufly undergone Scheele's procefs, the two laft did not redden paper tinged with tournfole, although they precipitart^d ccploufly the foJutions of iron. By fpontaneous evaporation I could perceive no mark of the galHc acid in the two lad decodions. 7. A folution of the ulphate of iron, mixed witfi a foluiion of the gallic acid of the highefl poffible de- gree of purity, is not di'durbcd, and does not aff'.mie any colour at firfl ; it is only by dei;rfes that it be- comes black, witliout lofmg its tranfparency, or at ieaft it does not begin to lofe it till after fome hours : a fmall quantity of the infufion of galls produces in- flantaneoufly a colour much more intenfe and opake. Thefe obfervations prove, in a fatisfadory manner, that it is not the gallic acid that communicates the ailringent property to the fubjlances which polTefs it : that this acid itfelf polfefTes this property in a lower ^legree than other aftringents, E 2 Indeed 68 OF ASTRINGENTS IN GENERAL, ^ Indeed the fumach, which is managed like galls by Scheele's procefs, does not afford any gallic acid, al- though it poffelTes the aftringent property in a high degree ; the green fhell of nuts treated in the fame manner alfo gives none. It appears that the proper- ty, which the infufion of cornnion ?alls has of, redden- ing certain vegetable colours, depends entirely on the gallic acid ; fmce the infufion of fumach, that of the bark of the plumb-tree, which readily afford a black precipitate, the infufion of the bark of the walnut, and of the Peruvian bark, do not pofiefs this property ; and from this it appears, that the gallic acid does not exifl in white galls. The infufion of white galls expofed to the air may readily deceive us ; for it forms a co* pious precipitate, but this is not the gallic acid. If the aftringent property were owing to one and the fame principle, diftributed among different vegetables, the precipitates that are obtained by their means from a folution of iron fnould be the fame combination, and ihould prefent the fanje appearances and the fame properties • but it is quite otherwife ; the precipitate produced by galls is of a blackifh blue ; that from iogvv'ood has another ihade of blue ; that f;*om the oak is of a fawn, or blacklfli brown ; that from the Peru* vian bark of a blackifh green: the circumftances which attend their depofition are different, and when fixed upon fluffs, fome of them are deftroyed much more readily than others by alum and tartar. ~ It can- not be doubted, that by multiplying the experiments, we fhall flill find many remarkable differences among the properties of thefe various precipitates. Aflringents then form different kinds of combina- tions AND PARTICULARLY OF GALLS. tions With iron, and confequently their aflringent pro- perty does not depend upon one and the fame princi- ple cxifling in different vegetables ; fiill, however, there mufl be fome property common to thefe differ- ent fubftances, which enables them to acl in the fame uniform manner upon foluiions of iron, and to pro- duce precipitates, which being more or lefs black, ap- pear to be of the fame nature, at lead when not exa- mined with attention. I have obferved, in analyfmg the effeds of mordants^ that metallic oxids, by combining with the colouring particles, modify their colours ; but the colour of fome metallic oxids, and particularlyof the oxid of iron, varies according to the quantity of oxigen which they contain. Iron combined with only a fmall portion of oxigen i$ of a black colour, and forms what we call martial asthiops. It is fufficient that a fubftance, in combin- ing with an oxid of iron, be able to feparate from it a part of the oxigen which it retains, when precipi- tated from an acid folution, in order to give it a black colour ; and if the particular colour of this fubllance does not predominate, or if it approaches to black, the combination'which is formed will be black alfo. Thus nitrous gas, free, or weakly combined in the nitrous acid, .blackens the folutions of iron, as I have already ffiown and it precipitates this metal by de- priving it of a part of its oxigen. E 3 It * Mem. de I'acad. 1785, p. 338. The caufe of the produc- tion of black by the gallic acid, has not efcaped the fagacity of M. Fourjroy, '* it colours iron," fays he, " bccaufe it ciakcs it approach to the qietallic ftate." 0-F ASTRINGENTS IN GENERAI^- It is alfo by ailing in this manner that ammoniac forms a black precipitate with the folutions of iron ; for the hydrogen of the ammoniac forms water with fheoxigen that is taken from the oxid of iron. BiTt galls, according to the obfervations of M. Monnet, and the academicians of Dijon already quo- ted, precipitate filver and gold from their folutions^ by reftoring them to the metallic ftate. They have then the property of abftraffing oxigen from thofe metals to which it adheres fiightly, and of taking from others the portion that is lead intimately com- bined. The infufion of galls itfelf affames readily a deep brown colour, by remaining expofed to the air ; how- ever, I have found that it does not abforb much oxi- gen. The infufion of fumach, and thofe of mod other vegetable fubilances, particularly of woods and barks,, aifume in the fame manner a brown colour by expo- fure to the air : fa that by ading upon the oxid of iron, and by abftractinga part of its oxigen, an aftrin- gent ought itfelf to affume a brown colour which mufl concur in producing the black. We fee then, why many fubdances, which in other refpe£i:s have different properties^ may produce black with the folutions of iron. Among thefe fubflances^ there are fome which have true colouring particles^ and which are employed as fuch in dying ; for exam- ple, logwood, and even the greater part of colouring particles, form brown or blackifh precipitates with iron. Sometimes the aftringent effect is not inilantane- pus, but only produced by degrees 5 the colour of the precipitate U at firft dear, it deepens, and becomes brawn '\ AND Particularly of galls. y% brown gradually, in proportion as it lofes its oi igen : we obferve this in the infufion of yellow wood ; witf> a folution of iron, it forms a yellow precipitate, which gradually becomes brown, and afterwards black, Although the property of precipitating folutions of iron of a black colour, does not announce the exift- ence of one and the fame principle in the fubllan- ces which polTefs it, yet it may not be improper to exprefs it by the name of the aftringent principle, provided we underftand by this word only a pro- perty common to a great number of fubftances, and a property which may exift in them in very different de- grees. The aftringent principle precipitates iron from al- mofl: all acids ; it appears from the experiments gf the academicians of Dijon, that the phofphoric and arfeni- cal acids alone, have a greater affinity with it than the oxid of iron, and we know that the phofphoric dcid has the property of feparating iron from the ful- phuric acid ; but all acids re-diffolve this precipitate, and make its colour difappear until they be faturated with an alkali, except the acefous acid ; and proba* bly the other vegetable acids that have not yet been tried. We need not be furprifed, that the aftringent prin* ciple can combine with metallic oxids without poffef- fmg the qualities of an acid ; for animal fubflances, 6ils, and even alkalies, and lime, have the fame pro- perty. It is this precipitate compofed, of iron and the aftrin- gent principle, which, by remaining fufpended in the liquor, forms ink 5 an objed thatis fufficiently intereft- E 4 ing 7^ 1 OF ASTRINGENTS IN GENERAL,' ing l|;th on its own account and its connedlion- with drying black, to arreft our attention for a mo- ment. When we pour a fmall quantity of the folution of iron into an infufion of galls, at firO: we perceive the Mquor become black at the point of eon tad ; but the precipitate which is formed, is re-dilToIved in the aftrin- gent liquor: by continuing to pour a fufficient quanti- ty of the folution of iron, all the liquor becomes black, and lofes its tranfpareney ; if in this ftate we dilute it whh a very great quantity of water, the pre- cipitate is gradually depofited, but the depontion is very flow in forming, when we ufe only a fmall pro- portion of the folution of iron ; on the contrary, it forms much more readily, if we employ a quantity of the folution of iron fo great, that all the aftringent principle may combine with it. A fuperabundance of the fulphate of iron alfa accelerates this effedt* The ffiftie phenomena take place here which are ob- ferved in other precipitations. The precipitate which is formed is foluble to a certain degree in the aftrin- gent folution ; when the folution is farther advanced, the precipitation begins ; but the precipitate is ftili kept fufpended until the a^lion of the aftringent prin- ciple is fufficiently v/eakened by the admixture of wa- ter ; if the iron is in a quantity fufficient to exhauft the adion of the aftringent principle, the preci- pitate is not retained, but depofited much more quickly. The gum we add to ink oppofes the depofition of the colouring part ; it ferves to fix more of this co- louring part to a fmgle ftroke of the pen, to prevent it kom AND PARTICULARLY OF GALL^/ from running, and to preierve it from the adion of the air. The gum of the cherry and plumb trees, ac- cording to Lewis, anfwers as well as gum-arabic. The principles which 1 have eflablilhed ferve to ex- plain feveral obfervations that Lewis has made upon this fubjed, and of which I lhall now relate the prin- cipal. Ink that is made by fimple infufion is of a pale co- lour, and becomes black only by keeping : but if we wifh to have it at once a black ink, it is neceifary to imitate the dyers, who boil the aftringents in water for a long time, and afterwards add the fulphate of iron : for, i. By boiling, we diffolve a greater quan- tity of the aflringent principle ; 2. By adding the ful- phate of iron to the liquor, while Hill very warm, we more readily obtain an eiFed analogous to the flight eombuftion that takes place, when the oxigen com- bines with the ailringent principle. Recent ink, as well as fluffs immediately taken out of the dying vat, afliimes always a deeper black by the contad of air ; it appears, that this effed is owing to the oxigen which the galls feparate from the iron, not being fufficient for the degree of combuftion neceffary to produce the black required ; and that this combuf- tion is completed by the oxigen which is attraded froni the atmofphere. Ink, in the formation of which a large proportion of the fulphate of iron has been employed, readily be- <;omes brown by the adion of the fun, and of the air, and at lafl: yellow ; becaufe the iron which is not fatu- rated with the aftringent principle continues to attrad powerfully the cxigen, and by this it lofes its black colour, f4 ASTRINGENTS IN'o^NERAt, colour, and affumes one that approaches more ancJ more to a yellow ; and, at the fame time, the combuf- tion of the aftringent principle proceeds rapidly, efpe- cially by the influence of light ; but the ink is much more durable if the aftringent principle exifls in a fuf- ficient proportion^ The bell, according fo Lewis, is compofed of three parts of galls and one of the fulphate of iron. He found that characters which had become brown or yellow, recovered their colour in confequence of be- ing moiflened by an infufion of galls. Dr Blagden has ufed the prufTiate of alkali with fuccefs to recover writings worn out by age, and he found it more ad- vantageous than galls.- Although chemifts have confidered the aftringent •is one and the fame principle, experience has fhown that all aftringent fubftances are not equally fit for producing a beautiful and folid black: it is o/ confequence to afcertain thofe which may be employed with fuccefs ; but it muft be obferved, that it is very difficult to give precifion to comparative experiments made upon this fubjedl, becaufe fome fubftances re- quire much longer boiling than others for the extrac- tion of the aftringent principle ; and the finer or coarfer form under which we fubjedt thefe fubftances to boiling, may aftbrd different refults ; and becaufe' the colouring particles have a ftronger or weaker dif- pofition to combine with the ftuff, according to the proportions of the fulphate of iron that have been em- ployed. It is neceffary alfo to obferve, that the folutions of iron by different acids may produce variations in the refult^y AND 1^^RTICULARLY OF GALLS. 7^, reCulu, according to the more or lefs oxigenated ftatc in which the iron exids, according to the greater or lefs proportion of this metal, and according to the ftronger or weaker adion that the dilFerent acids, now fet at liberty, may exert upon the combination which is formed. Lafliy, in tire dying of a piece of fluff, the ftronger or weaker affmity of the fluff for the colouring particles, may alfo produce changes in the refults. I^wis has obferved, that the decodion of logwood ufed infteadof fmiple water for the infufion of galls, increafes- the beauty of the ink without rendering it more pale. Sumach, the bark of the plumb, the pomegranate, the flowers of the pomegranate, the roots of biftort, tor- mentil, the bark of the oak, do not poffefs the effi- cacy of galls. In his experiments on dying, he found that fumach, the bark of the oak, and the faw-duff of the fame wood, could only fupply the place of galls,, by employing them in larger proportions. M. Beunie has publifned a great number of inter* efling experiments * ; his objecl was, to determine the befl; method lo dye cotton of a fixed black. He tried at firll what folution of iron gives the moft beautiful black to galled cotton ; afterwards he combined dif- ferem folutions ; he tried the folidity of the blacks he produced ; he made the fame trials with other metals, andfemi-metals upon galled cotton; he employed a great number of altringents in the fame manner, and he fubjecled to trial cottons which had been prepared * Memoirc fur la Teinture en noir, qui a remporte le prix ^It U foi- «ietc iiteraire dc Bruxelici cq J771. Rotterdam, I777> ^6 Ot ASTRINGENTS IN GENERAL^ in different ways. We fliall find, in the fecllon of the fecond part which treats of dying black, the pro- ceffes to which thefe repeated experiments have given rife. Of twenty-one kinds of aftringents compared with galls, the faw-dull: of oak, the galls of this country, and the myrobalan citrons, are the only fubilances that produce a beiiutiful black, but which is dill neither fo beautiful nor fo folid as that produced by common galls. He found that the faw-dull: of oak is preferable to the bark which is employed by the dyers of thread, and he remarks that it is cheaper. M. M. Lavoifier, VandermoDde, Fourcroy, and myfelf, have been engaged in making experiments upon different aflringents, in order to draw up a re* port which the academy entrutled to us. The fub- ftances upon which we judged it proper to make com- parative trials were galls, oak bark, 'the rafpings of oak, taken from the interior part of the wood, the rafpings of the inner bark of the oak, logwood and fu- inach. To determ.ine the proportion of the aPtrin- gent principle contained in thefe fubftances, we took fuccefTively two ounces of each, and boiled them for half an hour, in three pounds of water ; after the firfi:, we ufed a fecond water, which underwent a fimilar boiling, and we continued thefe operations until the fubftances appeared to be exhaufted ; we mixed the decoctions together that we had fuccefTively obtain- ed ; we employed a very clear folution of fulphate of iron, in v/hich we knew exactly the proportion of the water and the fulphate ; we eflimated, at firft, the quantity c/ the aftringent principle by the quantity of the AND PARTICULARLY OF GALLS. 77 the fulphate that cdch liquor decompofed, and after- wards by the wclv ht of the black precipitate that was formed. In order to Hop at the precife point of fatu- ration, ic is necelfary to carry on the precipiia'don very flowly, by pouring in the fotution of fulphate towards the end only drop' by drop, and by topping direftly, when a new addliion of this fubitance does not in- creafe the inienfiiy of the black colour. When the liquor is too opake, fo that we cannot aiftinguifh the flia ie of the coL-^ur, we dilate a fmail pordon of it with a great quanticy of water, and by adding a h>tle of the folution of the fulphate of iron in the etid of a glafs tube, we (hall be able to know if it has reached the point of iaturation ; if we wifh to preferve the pre- cipitate that is formed, it is nectlTary to dilute the liquor largely with water. '1 his operation deferves attention, becaufe It affords an eafy and accurate method of determining, in the workfiiops, the proper proportions of aRringents and of folutions of iron. Three drachms 6i grains of galls were neceflary to faturate the decoction of 2 ounces of galls ; the precipitate collecled and dried weighed 7 drachms 24 grains. The deco6lion of oak bark is of a deep yellow co- lour ; a very fmall quantity of fulphate of iron gives it a dirty red cojour : a farther addition changes it to a black brown. The quantity of fulphate necef^ fary to faturate the decociion of 2 ounces of this bark was 1 8 grains ; the precipitate, confiding of larger particles, which were more difficult to divide, when cglleded and dried weighed 22 grains j the liber, or in- ner OF ASTRINGENTS IN GENERAL, ner bark of the oak, prefents nearly the fame refults^ The deco6lion of the faw-duft ot the heart of the oak requires, for its faturatiori, i drachm and 24 grains ; the precipitate was precifeiy of the fame weight ; the decoction of the inner bark of the oak affords very little precimtat^. The decoction of fumach affumes a red violet co- lour, when we add to it a fmall quantity of the folu- ti(5n of the fulphate of iron. The quantity neceffary for its faturaticn was 2 drachms and 18 grains. An accident prevented us from afcertaining the weight of ihe precipitate ; it refembled perfcclly that afforded by galls. The decodion of logwood receives a fapphire blue colour from the addition of the fulphate of iron. If we exceed the point of faturation, the blue becomes of a dirty greenifli colour. The quantity neceffary for complete faturation was found to be 1 drachm 48 grains, and the weight of the precipitate v;as 2 drachms 1 2 grains. All the precipitates of the oak are form- ed readily ; that of logwood with a little more dif? ficulty, but ftill with more facility than that of galls. We afterwards found by trials made upon woollen fluff, that the quantity of aftringents neceffary to pro- duce a black colour equally deep, on equal v/eights of the fame ftuft^, are proportional to the quantity of the aflringent principle in each fubftance, which we had afcertained by the foregoing experiments : but the black obtained from the different parts of the oak did not ftand boiling nearly fo well as that produced by galls. u AND PARTICULARLY OF GALLS. 79 It does not appear that loc^wood alone can produce a black fo deep as galls, or oak; and, beliJcs, the co- lour that it produces does not {land boiling fo well a$ the one obtained from galls. We read in ihe memoirs of Stockholm, for the year 1763, that the uvaurfi gadiered in 'Utumn, and dried with care, in order rhat iis leaves may remain green, may be ufed as a fubflitute for galls. The theory which I have given with regard to af- tringents is far from being complete in all its differ- ent parts ; but it eftabliflies principles, the application of which may afterwards be made by experiments, that ought to be much more varied and more accurately performed than any', that have hitherto been attempt- ted. It will be neceffary to confine ourfelves to the prin- cipal aftringents only, becaufe the greater number of vegetables polTefs more or lefs of an aftringent proper- ty, as may be feen from the trials of Bergius * and M. Durande f . After having confidered the aflringent principle in its relations with the folurions of iron, it remains to confider it with regard to the property which it has of combining with vegetable and animal fubflances, prirticularly the latter. Silk acquires by galling |, which confifts in macerating a ftuff in a decoction of galls, a weight that cannot be taken from it by re- peated * Materia med:ca e regno vegetabili. f Mem. fur Ics parties aftringentes indi.sjenes. Acad, de Dijon 1783. X I (hall apply the word galling to denote the combination of an aflringent with afluA^; of whatever nature either of them may be* So OF ASTRINGENTS IN GENERAL, peated wafhings ; or, at kaft, that can only be di- minifhed to a certain degree : after this operation, the ftuff, immerfed into a folution of iron, is dyed black ; becaufe the aflringent principle, by decompofing the fulphate of iron, forms a triple combination with the oxid of the iron and the fluff. A fluff that has been galled may combine with ether colouring particles, in cpnfequence of which their colours will acquire foliiiity if they have it not of themfelves ; fo that the. aftringent comfuunicates its own fixednefs to the triple coaibination, and fome- times to a more compound one that is formed, and commonly the colour becomes deeper by this combi- jiation. By combining with animal fubflances, the aflrin- gent principle fecures them from corruption, and tends to contradl the texture of their parts; it is in this that tanning, the bafis of the preparation of leather con- fifls. The prize which the Academy of Sciences has pro- pofed, with regard to this important art, will certain- ly procure more extenfive information, and ferve to promote the progrefs of the art : but I hope the con- jedures I have formed will not be altogether ufelefs to thofe who fliail turn their attention to this fub- An ounce of galls, diflilled with a ftrong heat, af- fords 3 drachms of carbon; an ounce of fugar, 2 -drachms 12 grains ; an ounce of the colouring parti- cles of thread, 2 drachms 24 grains ; an ounce of plumb-tree well dried, 1 drachm 48 grains ; an ounce of dry walnut-tree, alfo i drachm 48 grains : fo that galls AND PARTICULARLY OF GALL3. tt galls afford almofl: double the quantity of carbon that hard and dry wood does. I have elfewhere remark- ed that when the infufion of galls, and that of fu- niach, are decompofed by means of the oxigenated mu^ riatic acid, a copious precipitate of carbon is formed, particularly in the former. From thefe obfervations, I confider the fuperabun- dance of carbon as the eflential charadleriftic of the aflringent principle : the hydrogen, which exifts only in a fmall quantity, has a ftrong tendency to combine in part with oxigen : from this it happens, that when we allow the infufion of galls to remain in con- tad with vital air, only a fmall abforption of vital air takes place, although the colour of the infufion be- comes much deeper ; for in conformity with the theo-^ ry I have delivered in the third chapter, and particularly in the Amiaks de Chymie^ the carbon, by means of this flight combuftion, becomes the prevailing ingredient, and the colour deepens and becomes brown. A fubflance which contains a great quantity of car- bon, and which can undergo only a flight degree of combuilion, fhould be of a very fixed nature, becaufe carbon does not combine with oxigen in the ordinary temperature of the atmofphere, unlefs this combina- tion be affifted by other affinities, and becaufe flight changes in the temperature do not produce any in the dimcnfions of the carbon : on the contrary, fub- flances which contain much hydrogen, and in which the particles of the hydrogen are very much divided, ought to be eafily decompofed by the combination of F the * Ann. fie Chym. torn, VL 82 OF ASTRINGENTS IN GENERAL, the hydrogen with azote, or with oxigen. The fepa- ration of their particles ought to be produced by fmall changes in the temperature, becaufe the hydrogen is dilatable by heat, while the carbonaceous particles are not. It is in this manner, according to the ingenious remark of M. Monge, that a very fufible metal, mix- ed with a metal that is m.uch lefs fo, renders the lat- ter brittle in a degree of heat, which, by fufmg the former, deftroys the continuity of the particles of the mixture. When, therefore, the adringent principle is com- bined with an animal fubdance, it communicates to it the properties which it derives from carbon ; the animal fubftance becomes lefs eafily altered by flight changes in the temperature ; inftead of becoming pu- trid, it undergoes a flight combuflion by the a£lion of air : for, in all likelihood, tanning could not be per- formed in a veflTel clofely fnut : the effe'3: of this com- builion is the contraction of the panicles of the fub- jecl, and this tends afterwards to preferve it from pu- trefaclion. The preliminary operations of tanning confifi: chiefly in extracting all the fatty pa^ts w'hich are dinuf^d in the cellular texture of fis^ins, fo as to allow the afl:ringent principle, and even the air, to pe- netrate through them. When I examine the analyfis that has been made of indigo, which may be confidered as the leaft change- able of all known colouring bodies, I obferve, that this fubflance affords by diilillation a greater propor- tion of carbon than galls themfelves. I omit in thefe calculations the*carbon which enters into the Gompofiticn of the thick oil that we principally ob- tain AND PARTICULARLY OF GALLS. 83 lain from indigo, and that which is di^engai^ed un- der the form of carbonic acid, and of carbonated hy- dro^^-enous gas. It appears to me, that we muft alfo attribute the fixity of the colour of indigo to this fuperabundance of carbon, and that the ditTerent proportion of this principle, is the chief caufe of the diiTerence obferved in the fixity of colours ; ,but the force of adhefion may alfo have much influence ; for a principle that unites intimately with any fubllance, ougiit to form a more lafting combination than another principle which is more feebly difpofed to unite with it; but the aftrin- gent principle pofl'elTes this tendency to combine inti- mately in a very high degree, particularly with ani-- mal fubilances. I fliali explain, by the fame principles, the fixity that is communicated to the colouring par- ticles by alumina, and thofe metallic oxids which are not fubjecl to variations in the quantities of oxigen* they contain, fuch as the oxid of tin. All colouring fubflances which can combine with metallic oxids exert upon them an aclion analogous to that of aflringents. Oxids are by this means more or lefs deprived of their oxigen, according to the force with which they retain it ; the ftrength of the affinity by which the colouring particles tend to com- bine with them ; the proportion in which they meet, and the greate'r or lefs difpofition the colouring parti- cles have to undergo conibuftion. The colouring particles, on their part, undergo a change in their nature proportional to thefe cir- cumftances ; thus the folutions of iron change all co- lours to a brown, into which the oxid of iron can en- F 2 tcr, $4: RECAPITULATION OF THE ter, although it has only a green or yellow colour ia the ftate in which it is kept in folution by acids ; and this efFe6t goes on increafing to a certain point ; but the alteration of the colouring particles may afterwards be carried to a degree at which their colour is deftroyed, and their tendency to combination diminilhed ; the oxid of iron is then brought back to a yellow, by the oxigen which it attrads and retains. The mutual adibn of colouring particles, and of metallic oxids, explains the changes that are obferved in the folutions of the colouring particles when mix- ed with metallic folutiong. The efFe6l produced is gradual, as I have already obferved with regard to yel- low wood. Sometimes the mixture is not difturbed at firft ; it lofes its tranfparency by degrees ; precipitation be- gins ; a depofition is formed ; and its colour becomes deeper and deeper. Light has confiderable influence in producing thefe effeds. CHAP. VI. Recapitulation of the theory delivej^ed in this fedion* IT is neceffary to dlftingulfli metallic colours from thofe which belong to vegetable and animal fub- ftances. The colours of metals are modified and changed by oxidation, according to the proportion of oxigen that is combined with them. Vegetable )»ORECOING THEORY. Vegetable and animal fubflances may polTcfs of themfelves a particular colour, varying in the different ftates through which they pafs, or they may owe their colours to colouring particles, which are either combined or limply mixed with them. There arc particles of this kind, extraded from a variety of fub- itances, which undergo different preparations, in order to ferve the purpofes of dying. Colouring particles have chemical properties which didinguifh them from all other fubflances ; the affinities they have with acids, alkalies, earths, metal- lic oxids, oxigen, wool, fdk, cotton, and thread, con- flit ate the moft important part of thefe properties. In proportion to the affinity which the colouring particles have with wool, filk, cotton, and thread, they combine more or lefs readily, and more or lefs intimate- ly, with each of thefe fubflances ; and from this arifes a primary difference in the procefTes we employ occa- fioned by the nature of the fluff, and that of the co- louring fubflancc. By the afnnity that the colouring particles have for alumine, and the metallic oxids, they form with thefe fubflances a combination, in which their colour is more or lefs modified, and becomes more fixed, and lefs eafily affe£led by external agents than formerly. This combination, being formed of principles which feparately poffefs a difpofition to unite with vegetable, and principally with animal fubflances, retains this property ; it forms a triple combination with the flufi, and the colour which has been again modified in con^ fequencs of this triple union, acquires a greater de- F 3 grec 86 RICAPITULATION OF THE gree of fixity, and a greater power of refifting ezzteV' The colouring particles frequently have fuch an af- finliy for the alumine and metallic oxids, as to feparate ihem from acids which kept them in folution, and form with them a precipitate ; but fometimes the affinity of the fluff is necelTary to effeduate this feparation. The metallic oxids which combine with the co- louring particles not only modify their colours by their own peculiar colour, but alfo a6l upon their compofition by the oxigen they contain. The change that the colouring particles undergo by this means is fimilar to that which they fufFer from the air, by which all colours are more or lefs injured. Of the two principles which form atmofpheric air, it is only the vital air, or oxigenous gas, that ads upon the colouring particles ; it combines .with them, and then^weakens their colour and renders it pale : but its action h foon exerted, chiefly upon the hydrogen, which enters into their compofition, and with it forms 7vvater. This eiTed: muft be confidered as a true com- builion. By it the carbon, which enters into the com- pofition of the colouring palticles, becomes prevalent ; and the colour paiTes generally to a yellow, to a fawn, to a brown ; or this degradation, by mixing with what remains of the firfl colour, produces other appear- ances. Light promotes the combudion of the colouring particles, which frequently cannot be performed with- out its aiTiftance ; and it is in this way that it contri- butes to defiroy colours, HeU alfo favours it, but " ' . ' lefs FOREGOING THEORY. 87 lefs effedually than light, till raifed to a certain de- gree. It is on a fimilar combudion that the efFeds of the nitric, the oxigenated muriatic, and fulphuric acids de- pend, when they change the colour of the fubitances upon which they a£l: from a yellow to a black The effecls of combuflion may be concealed by the oxigen combining with the colouring particles, with- out exerting its action in a particular manner upon the hydrogen. Colours are more or lefs folid, and more or lefs fix- ed, according to the greater or lefs tendency that the colouring particles have to undergo this combuflion, and that in a greater or lefs degree. There are fome fubftances which may a6l as fol vents on the colours of fluft's, in confequence of their fupe- F 4 rior * There are a great number of other natural phenomena, bc- fides thofe I have already mentioned (Ann. de Chcm. T. VI.) in Vr'hich the formation of water appears to me to be undeniable j thus the bitter tafte of unripe fruits is probably owing to oxigen feebly combined ; but during maturation, the fruit v^hich was hard be- comes foft; water is formed ; the juice becomes faccharine and co- pious : but fugar contains a large portion of carbon. When the fruits are too ripe, and fulfer the degeneration that is natural to them, their colour indicates the fuperabundance of carbon. It ap- pears from the experiments of the celebrated 'Ingenhoufz (Exper. fur les vcge, T. II.) that combuHion takes place on the furf^ce of fruits expofed to the fun, to fuch a degree as to produce caibo- nic acid. Dung, either alone or mixed with ftraw or chaff, is converted into mould by a combuft'on that Is even accompanied with qonfi- derable heat, fo that the carbon becomes fuperabundant in the lat- ter. On this depend perhaps many of the phenomena of vegetation. S8 RECAPITULATION OF THE rior affinity : of this kind are acids, alkalies, and foap; however, a fmall part of thefe agents may fometimes be fuperadded to the fluff, and in that way alter its colour. Metallic oxids produce in the colouring particles with which they unite a combuftion proportional to the quantity of oxigen which can b^ taken from them by thefe particles. The colours that the combinations of the metallic oxids with the colouring particles aifume, are then the conjoined efFe£t of the colour belonging to the co- louring particles, and of that of the metallic oxid j but it is neceffary to confider the colouring particles and the metallic oxids in the ftate to which the dimi- nution of oxigen reduces the latter, and the diminu- tion of hydrogen the former. It follows from this, i. That metallic oxids, to which the oxigen adheres very weakly, are not fit to be employed as intermediate fubflances to the colour- ing particles, becaufe they would produce too great a degree of combuflion ; fuch are the oxids of filverj gold, and mercury. 2. That oxids which undergo confiderable changes in their colour, by parting with a greater or lefs quan- tity of oxigen, are alfo unfit for intermediate fubilances, efpecially for clear fhades, becaufe they produce changeable colours ; fuch are the oxids of copper, l«ad, and bifmuth. 3. That the oxids which have a ftrong affinity for their oxigen, and which fuffer little change of colour by the lofs of a portiim of it, are the fitteft for this ^ purpofe 5 fuch in particular is the oxid of tin, that readily FOREGOING THEORY. 89 readily quits its folvent, that has a flrong affinity with tlie colouring particles, and affords them a very white bafe, and one proper to give brightnefs to their fliades, without altering them by the mixture of any other colour. The oxid of zinc polTefles in part thefe pro^ perties. To account for the colours rcfulting from the com- bination of the colouring particles with the bafe that a 7nordant affords them, it is like wife ncceffary, to at- tend to the proportion in which the colouring particles unite with this bafe ; thus the folution of tin, which forms a very copious precipitate with a folution of co- louring particles, and by this iliows that the oxid of tin enters in a large proportion into the precipitate, has much greater influence by the wbitenefs of the bafe upon the colour of the precipitate than the folution of zinc, or that of alum, which generally form a much Icfs ccnfiderable quantity of precipitate. The preci- pitatc-G produced by thefe two lafl fubflances retain nearly the natural colour of the colouring particles. It is neceffary, therefore, in the adion of mordants, to diftinguifli the combinations which may be formed by their means between the colouring particles, the fluff, and the intermediate fubftance, the proportions of the colouring and intermediate fubitances, the mo- difications of colour that may take place from the mix- ture of the colour of the colouring particles, and of the bafe to which they are united, and, laftly, the changes that the colouring particles may undergo from the combuflion which may be produced by the inter- mediate fubflance. Afhingents go RECAPITULATION, &C. Aftrlngents do not owe their diftinguifliing quality to an acid, or to any other identical principle ; but to the property which they poflefs, of combining with the oxid of iron, of bringing it back to the flate of ablackoxid, and of afluming themfelves a browncolour from the combuflion which they by this means under- go. Galls, which we ought to confider as the reprefen- tatives of the clafs of aflringents, readily undergo a flight combuflion, that gives them a deep brown colour ; but this combuflion, which requires only a fm.all quantity of oxigen, is fpeedily flopt without af- fedling their properties. Thefe fubflances owe their durability to the great quantity of carbon they contain; and as they pofTefs the property of combining with fome vegetable fub- flances, with many colouring particles, and efpecially with animal matters, they ferve as an intermediate fubftance between them, and communicate to them their own durability. SECTION [ 91 ] SECTION SECOND. ON THE OPERATIONS OF DYING IN GENERAL, CHAP I. Of the dijlinguijlolng charaSlers of ivooL filk^ cotton^ and fax, and of the operations by which they are prcpa* red for dying. ARTICLE I. Ohftfvations on the d'ljjerence between animal and vegeU '.'. fib' Jlatices. 1DARE not flatf:er myfelf whh the hope of ex- • plaining many of t je properties which depend ca the compofition of organised matter ; but the knc-w- ledge we have already acquired, wiih regard to the compofition of vegetable and animal fublrauccs, will enable us, in fome degree, to difcover the caufe of the different tendencies that vi^ool, fiik, coiton, and fi.ix have to unite with the colouring particles, or with ilie bafes which mordnpts riTord. Wool and filk belong to the clafs of animal ; cotton, flax, and hemp, to that of ve?;etable fubuances. The principal diftingulfliing chatacler in the com- pofition of vegetable and animal fubilances is, i. ^"" ni- mal fubftances contain a great quantity of a pecui r principle, azote ^ which in the e^aflic (late forms what .3 CTiWcd phlogi/Iicated gas, or phlogiflicdfed air. Tl.''^ is found only in very fmali quantity in vjj;etablt:^ : 2. 92 DIFFERENCE BETWEEN ANIMAL 2. Animal fabftances contain a greater quantity of hydrogen or the bafe of inflammable gas. The differences that are obferved in the diftilla- tion of animal and vegetable fubflances, arife from thefe two caufes : the former afford a great quantity of ammoniac, which is a compound of azote and hy- drogen ; the latter afford only a fmall quantity, and in general give rather an acid. The former afford a great quantity of oil, the prevailing ingredient of which is hydrogen : this oil, at a temperature fome- what high is (Irongly difpofed to feparate, and to be evaporated ; the latter fometimes do not afford the fmallefl quantity. In confequence of this compofi- tion, animal fubflances produce at the commencement of their combuftion, a very bright flame, but this is quickly fuffocated by the carbon that is formed, and which poffeffcs peculiar chara6i:ers. Their com- buftion is accompanied by a penetrating odour, owing to the ammoniac and oil that are formed, and efcape the inflammation ; they are fubjecl to putrefaction, during which, ammoniac is evolved, as in diftillation, by the more intimate union of the azote and hydrogen ; while, on the contrary, vegetables undergo only a fpi- rituous or an acid fermentation. In this place, I merely allude to opinions that I have explained at full length in fome other memoirs. The particles of animal fubftances, containing a number of principles difpofed to affume the elaf- tic ftate, adhere lefs flirongly among themfelves, than thofe of vegetables, and are more difpofed to combine with other fubftances. On AND VEGETABLE SUBSTANCES. 93 On this account, they are more liable to be deftroy- ed by different agents, and more difpofed to combine with the colom-ing particles. Thus the fixed alkalies, either pure or caufiic, de- ftroy animal lubftances, bccaufe they combine with them ; in confequence of which, they become faturated and lofe their cauflicity From this a6lion of alkalies on animal fubftances, it happens, that they cannot bear the adion of leys, and that alkalies ought to be employed with great caution in the procefles of dying, while, on the con- trary, no dread can be entertained refpeding the ufe of alkalies for vegetable fubftances. The nitric and fulphuric acids have alfo a ftrong adion upon animal fubflances. The former decom- pofes them, difengages the azote, feparates the fat, and forms the carbonic and oxalic acids, with a part of the hydrogen, and a part of the carbon ; the latter difengages inflammable gas, probably alfo azotic gas, and reduces the other principles to a carbonaceous ftate. Silk appears to approach in fome degree to vegeta- ble fubftances, by a weaker tendency to unite with the colouring particles, and by a greater refiftance to the action of acids and alkalies, which may arife ei- ther from the fame principle being more intimately combined in it than in wool, or more probably from th;^ fmaller quantity of azote and hydrogen it con- tains y but although the aQion that alkalies and acids exert * Mem. dc TAcad. 1783. 94 OF WOOL. exert upon filk, be not fo ftrong as upon wool, It Is neverthelefs neceflary to ufe them with great caution, becaufe the brightnefs of the colour we wifh for in- filk, appears to depend upon the fmoothnefs of its furface, which mud not be injured. Cotton refifts the a6tlon of acids better than flax or hemp, and it is only with difHculty that we are able to defiroy it by the nitric acid. ARTICLE II. Of Wool. The principal differences of wool confifl: in the length and finenefs of its filaments. Wool which has very fine filaments is referved for the befl fuperfine cloth. The finefl is brought from Spain ; M. d'Au- benton has proved that we can obtain wool in France, not inferior to that from Spain, by making a careful feledlion of rams, and by confining the fheep in parks through the whole year. As the fight alone is apt to deceive us with regard to the fmenefs of wool, and as it is of confequence to manufacturers to know it with accuracy, he has given a method of determining it with the neceflary precifion, by ufmg a micrometer, in order to compare with the microfcope, the finenefs of the wools we vv^ifh to examine, with that which we choofe as a term of comparifon ^, Although long wool be not fo fine as Spanifh wool, and therefore cannot be employed for fuperfine cloths, it * Mem. de I'acad. 1779. Infliuflions pourles bergers & pourlcs proprietaires de iroupeaux. OF WOOL. . 95 it Is, however, very ufeful for the woolen fluffs known by the name of Englifh cloths ; and as the (heep which afford it have by far the flrongefl fleeces, the advan- tages obtained are not inferior to thofe from the fine wool ; befides, the cloths that are formed of their wool are cheaper, and have a more extenfive fale ; the Englifh owe the profperity of their commerce in part to the abundance of this wool. But the breed of fheep that afford the one or the other kind of wool, depends in fome meafure on the nature of the paflu- rage, a circumftance that ought to make us careful in the choice of it. It is of fo great confequence to our manufactures to multiply our flocks, and to improve our wool, that no opportunity of calling the attention of the public to this object fhould be loft. Wool is naturally covered with a kind of greafe that is called tjie fweat. This coating preferves it from moths, fo that wool is only fcoured when it is to be dyed or fpun. To fcour wool, it is put for a quarter of an hour into a copper, containing a fufficient quantity of v/a- ter mixed with a fourth part of putrid urine heated till the hand can jufl bear it, and flirred from time to time with a flick ; it is then taken out and drained ; after this it is put into a large bafket, placed in running water, and flirred till the greafe is entirely wafhed out, and renders the water no longer milky ; it is then ta- ken out again and drained. It fometimes lofes du- ring this operation more than a fifth of its weight. It is of importance that the fcouring be performed with care. $6 OF WOOL. care, becaufe by means of it the wool is more difpofed to take on the dye. The ammoniac, or the volatile alkali, which is form- ed in putrid urine, combines with the greafe, and forms a foap with it, by which it is rendered foluble in water. Wool is dyed in the fleece, or without being fpun, cfpecially when it is intended to be formed into cloths of mixt colours ; or it may be dyed after it is fpun, and it is then chiefly intended for tapeftry j but it is moll: commonly dyed in the form of cloth. "When wool is dyed in the fleece,its feparated filaments abforb a greater quantity of colouring particles than when it is fpun ; for the fame reafon, fpun wool takes up more than cloth ; but cloths vary much in this refpeft, according to their degree of finenefs, and their loofer or clofer texture ; befides, the difference of their dimenfions, the changeable qualities of the ingredients ufed in dying, and the different circumflances of ope- rations, prevent us from trufling to the precife quan- tities recommended in the procefTes that are defcribed. This confideration ought to be extended to all dyes. For the greater part of coiours wool requires to be boiled in a vat with faline fubflances, principally with alum and tartar; this is what we call bouillon, and v/hich we will have an opportunity to defcribe in various pro- cefTes; but there are dyesforwhichwool does notrequire thefe preparations ; it is then neceffary to wet it with lukewarm water, and afterwards to exprefs it or al- low it to drain. This precaution is generally neceifa- ry with ail thofe fubilances we mean to dye, in order that CF WOOL. 97 that the colour may be more eafily Introduced, and more equally liiftributed. M. Monge has explained the procefs of felt-making, and the eflecls of fulling, from the external conforma- tion of the wool and hair of animals. He has made a number of curious obfervations on this fubjed:, the principal of which I fiiall relate in his own words : " We can difcover nothing peculiar in the furface of fibres of tlie wool and hair of animals ; how- " ever, the furfaces of thefe objecls are not fmooth, they mufl be formed, either of plates laid upon one another from the root to the point, almofL in the fam.e manner as the fcales of fifiies rell upon one another " from the head to the tail ; or, perhaps, rather of zones placed above one another^ as is obfcrved in " horns. " If we take a hair by the root in one hand, and make it Aide between two fingers of the other, from " the root to'v^^ards the point, we feel almofl no fric- tion or refinance, and v.-e hear no noife; but if, In laying hold of it by the point, we make it pafs in the fame manner between the finnrers cf the other hand, from the point towards the root, we feela refifl- ance that did not occur in the former crJe ; a trem.u- " lous motion is produced perceptible to the touch, and alfo manifefled by a fenfible noife. " We have already feen, tha-t the texture of the *' furface of a hair is not the fame from the root to the *' point as from the poin: towards the root ; and that a hair, when it is preiVed between the finger?, •* finds greater refiflance in fliding, and taking a pro- grcilive motion towards the point, than in taking the G «• fame 9^' 0? WOCL. fame motion towards the root ; but £5 this ttx- " ture is the principal object of the prefent memoir, li inay be necexTary to prove it ftili farther by fome other obfeTvationSc " If, after having laid hold of a hair between the " thumb and foref:nger, we make thefe fide alternate- ly upon orxe another in the direction of the length " of the hair, the hair takes a progreiTive motion in that dire£tion, aild its' motion is always direded to- wards the root. This effedt h not connected, ei- ther with the nature or texture of the fkin of the fin- gers ; for if we turn the hair fo that the point may be in the place of the root, tke motion takes place in a ^- contrary direction 5 that is to fay, the motion is al- " ways dired:ed towards the rooto " Thefe remarks, to which M. Monge has added '*'mahy others, are all made upon a human hair taken " as an exam.ple ; bat they are equally true when ap- " plied to horfes hair, to the fibres of wool, and, in ge- ^' neral, to the hairs of all animals. The furface of all thefe objects is formed of rigid plates, placed a- bove one another in the form of tiles, from the root to the point, which allow progreffive motion towards- the root, and oppofe a fimilar motion towards the point. " It is this conformation v/nich is the principal caufe of the aptnefs for felt-making, that the hairs of all: *^ animals pofiefs. " In ffiort, the hatter, by flriklng the flocks of wool " with the cord of his bow, detaches and feparates each fibre in particular ; thefe fibres fall back upon one another in every direSion on the table^ where they form OF WOOL. g<) form a layer of a certain thicknefs ; the workman then covers them with a piece of cloth, which he prefles \i"ith open hands, moving the hands in dif- ferent diredlions. The prefTure briiigs the fibres of the wool together, and iacreafes the points of con- *' tad. The agitation gives to each of them a pro- greiTive motion towards the root ; by means of this motion the fibres are interwoven with one another, and the plates of each fibre, by catching ihofe of " other fibres that lie in a contrary direclion, keep the whole in the ccmpad textm'e which the preflnre has made it affuine. In proportion as the texture be- comes compa6l, the prefTure of the hands ought to be incrcafdd, both to render it more compa^l, as well as to fapport the progreflive motion of the fi- bres and their interlacement, which find then a greater refiflance ; but in the whole of this opera- ticn the fibres of the wool lay hold only of one another, and not of the cloth, the fibres of which, " as has been already obferved, are fmooth, and do ^' not afford the fame opportunity, *^ The ftrudure of the furface of the fibres of wool, *' and the hairs of animals, does not alone conllitute *' their aptitude for felt- making; it is not fiifKcient that each fibre can affume a progreffive motion tcf- ^' wards the root, nor that the inclined plates, by lay- " ing hold of one another, preferve the texture in the ftate produced by preffure : it is alio neceffary that ^' the fibres be not firaight like needles ; by means of ^' agitation each of them would continue its progref- five motion without changing its direclion, and the effed: of the operation would be the feparation of 2 " the ICO O? V/OCL. the v^hole froin the centre, without producing any interlacement. It is neceffary then, that each fibre ' be curled, that the extremity which is next the ' root be difpofed to change its direOiicn perpetually, ' to warp itfelf around new fibres, and to return up- ' on itfelf, if determined by any change in the pofition ' of the other parts of its body. It is becaufe the ' wool is naturally formed in this manner that it is ' fo proper for felt-making, and that it can be em- ' ployed without any* necefhty of making it under- ' go any previous preparation. But the hairs of the rabbit, the hare, and the ' beaver, are naturally ftraight ; they cannot be em- ' ployed for felt-making, without having under- ' gone a prelixuinary operation, which confills in rub- ' bine: them, before takinn; them of the fkin, with a ' bru(h impregnated with a folution of mercury in ' the nitric acid* This fojution^ by ading only upon one fide of the hairs, changes their direciion from a flraight line, and gives to them the aptnefs to felt-making which wool naturally poffefTes. " The operation of fulling woolen fluffs has fo great a relation with felt-making, that I believe it will be neceffary here to enter into fome details on the fubjed. The afperities on the furfaceof the fibres of wool, and their tendency to take a progrefTive motion to- wards the root, is an obftacle to the fpinnlng of wool and the making of flufls. In order to fpin wool, it is neceffary to teaze it, and after w^ards to coverall the fibres with a layer of oil, which by filling OF WOOL. 10 r filling the cavities renders the afpcrlties lefs perccp- " tible, in the fame manner as we put a laver of oil on a fmooth file to render it ftill fmoother. When ^' the piece of (tuff is made, it is neceffary to cleanfe it " from this oil, which gives it a difagreeablefmell, and " might prevent it from taking on the dye we wifli to *' give it : for this piirpofe, they are carried to thefiil- " ling machine, where they are beat with mallets in a trough full of water mixed with a quantity " of clay. The clay combines with the oil, and ren- ders it foluble in water ; the whole is made to pafs ^' through frefli water brought to it by the machine " itfelf, and after a certain time the fluff is fcoured. *' But fcouring is not the only objccl of fulling; ' the alternate preffure that the mallets exert upon ' the piece of fluff, efpecially when the fcouring is far ' advanced, produces an effedl analogous to thatocca- ' floned by the hands of the hatter ; the fibres of v/oo!, ' which compofe one of the threads of the vrarp or ' woof, take a progreinve motion, and infmuate them- ' felves into the contiguous threads, then into thofc ' v/hich follow ; and, in a fliort time, ail the threads of the warp, as well as of the woof, are felted together. The fluff, after having undergone a contraction in all its dimenfions, partakes both of the nature oi a web, and that of a felt. We can cut it without its fcparating into threads, and it is not neceflary to hem the dif- ferent pieces of which a gj^rment is formed. If it is an ordinary knitting of wool, the flitch is no longer difpofed to run when it happens to break: in fliort, the threads of the warp and v>'oof being neither fodif- G 3 ^ tina I02 OF SILK. tin6l nor feparated in fo eafy a manner, the (lufF \>%'^ " comes thicker, and forms warmer cloatbing." ARTICLE m, OfSilL Silk is naturally varnifned by a kind of fabflance which we confider as a gum, and to which it owes its ftiftnefs and elafticity. That which is moPc common in our country, contains, befides this, a yel- jow colouring matter. The greater number of ufes for which filk is intend- ed, require not only that it be deprived of its co- louring matter, but alfo of its gura. Vv'e obtain both purpofes by means of foap, and w^e give the name of drefiing (decreitfa^e^J to the operation by v/hich we give to filk its whitenefs and its flexibility. The dreHmg fhculd not be fo complete for filks that are to be dyed as for thofe that are to be made •white ; and it even ought to differ according to the co- lour which the fliks are to receive. This difference con- fifts chiefly in the proportion of foap.that is employ- ed; thus for ordinary colours we boil the filk for three or four hours, in a folution of tv/enty pounds of foap to the hundred weight of f:lk, taking care to hll the copper frora tim.e to time with water, fo as to fiave it always in fufficieiU quantity. We increafe the quantity of foap for H Iks that are to be dyed blue, s.nd efpecially for thofe that are to be dyed of a pop- py or cherry colour, he. becaufe a whiter bafe is re- quired for thefe than for lefs delicate colours. .e? SILK. 103 in treating of each particular colour, I fhall mention the quantity of foap proper for the filk that is to re- ceive it. When filk is to be ufed white, it is made to under- go three operations ; the firft is called Ungumming : it confifls in keeping hanks of filk in a folution of thirty pounds of foap to the hundred weight of filk ; this folution fliould be made very warm, but not fo as to boil. When that part of the hank which is in tl e liquor is completely deprived of its gum, which is known from its whitenefs and flexibility, we turn the hanks upon the rods, in order to make the part that had not been immerfed undergo the fame o:era- tion ; and we take the hanks cut of the vat, wringing them on pegs in proportion as the ungumming is performed. The fecond operation is called Boiling ; the fiiks are fluit up in bags of coarfe cloih, from twenty- five to thirty pounds of filk in each bag, which is termed a pocket: : a foap vat is prepared, fimilar to thefirft, on- ly diminiiliing the quanlity of foap, and it is made to boil for an hour and an half, taking care to move the bags one over another, to prevent thofe at the bot- tom of the copper from undergoing too great a degree of heat. The third operation is called Whitening : tins oper- ation is chiefly intended to give a flight fhade to filk, which renders the white more agreeable, and by means of which it obtains diflergnt names. Thus we make a diilin(£tion between china white, filver white, azure white, and thread or milk white. A folution c^i fo?p is made, fo that by beating *t v;it'i a fliick, ^ G 4 froth 104 OF SILK. froth is produced, that enables us to judge whether U be of the proper ilrength ; and for china white, which ought to have a reddifh hue, we add a httle arnotto^ and keep the fiik there till it acquires the (hade de- iired. We give to other whites a tinge rriore or leis blue, by means of azure added to the folution of foap ; but we had previouily put in foine of it in the boil- ing. To prepare azure, we take fine indigo, and afrer having wailied it tv/o or three times in water mode- rately warm, v. e pound it in a mortar, and pour boil- ing water upon it : we allow it to fettle, and we ufe the -liquor which retains only the moft fubtile parts: it is this we call Azure. We can fubftitute for it a fmall quantity of the liquor of a new indigo vat. At Lyons, where they produce a more fhining white than at Paris, they ufe no foap for the third ope- ration j but after the fecond, they waih the fdks, fui- phur them^, and add the azure in river water. In this method, it is of confequence to ufe Vv^ater that is very clear. When filks are fmooth, and have taken on the ftiade %ve defire, they are wrung and dried. The white obtained by the means we have defcri- bed is not fuiliciently bright for hlks intended for white flufts j but it is alfo neceffary to expofe them to the vapour of fulphur ; thi^ operation is defcribed un- der the article SulphuRo As foap appears to injure the luftre of filk, the aca- demy of Lyons in the year 1761 propofed, as the fub- je£t for a prize, to difcover a method of drefiing filks without foap, The prise vvas adjudged to M. Rigaut de OF sir.K. J05 deStQuentin, who, inftead of foap, propofed to em- ploy a folution of fait of fjda, or carbonate of foda, diluted in a quantity of water fuJicicnt to prevent ic from injuring the filk : but this method mud have been found inconvenient, fince it is not at all employ- ed, although it be well known, and eafily pracliftd. M. TAbbe Collomb * has publifhed a number of obfervations on the dreffing of filk, by m.eans of a- ter alone, which merit particular attention. Havint^ perceived that a Ikain of yellow filk, which he boiled for about three hours in common water, had lofi: al- mo(l an ei^t of its weight, he repeated the boiling twice, and m this .w^ay reduced the weight almofl a fourth. Silk w^hich has fufiered this lofo of weight preferves however a yellow colour, or rather the colour of cha- moy leather, which prevents it from being ufcd for fluffs that are to remain white, or to take on colours, the beauty of which depends on the whitenefs of the bafe to which they are applied : but it affumes colours very well which its own tinge cannot injure ; thus the black that it takes on is preferable to that of filk dref- fcd with foap. After this operation the filk remains very flrong and tough ; threads of it, compared with fmiilar threads drefled with foap, are able to fuftaia weights that break the latter. It required eight hours boiling to dilToIve all the var- nifli of the filk, and by this it lofl a little more than a fourth * Ohfvrvations for U diilv] J^jion du T^inis d. L fci?, Jcurn. de Phyf. Acur 1785. O? SILK, fourth part of its weight ; but the boiling fliould be continued longer when the mercury in the barometer is low, becaufe the greater the weight of the atmof- phere, the higher is the degree of heat that the water affumes in boiling. This confideration led M. Collomb to try the boil- ing of filk in Papin's digefler, and indeed it required only an hour and a quarter to produce a complete fo- Jution of the varnifh, although the degree of heat was inferior to that which ought to produce the eife<3:s ob- ferved by philofophers in this kind of concentrated boiling. I have feen a pattern piece of filk dreffed by M, Collomb ; it appeared to me to pofTefs the qualities mentioned by that author, ' but it was leis flexible and foft than filk drefied with foap. M. le Camus, a learned naturalift of the Academy at Lyons, fent me a fmall quantity of the fubftance that is feparated from filk by means of water in the operation, of M. Collumb ; this fubftance was black, brittle, and {hining in its fraclures ; it afforded by diflillation the products of animal fubflances ; it diffolved very rea- dily in hot water, and left very little refiduum upoa the filter; the foluticn, which was clear and of a greeniili yellow colour^ was not fenfibly altered by a- cids ox by alkalieso The folution of alum produced a dirty white pre- cipitate ; that of the fulphate of copper a black brown precipitate ; that of the fulphate of iron a brown pre- cipitate ; the nitro-muriatic folution of tin a white pre;- clpitate ; the acetite of lead a brown precipitate. All thefe precipitates were fcanty, and more or lefs •vifcid^ o? SILK. ?e7 yifcid. The infufion of galls and that of famach produced a white precipitate. Alcohol did not dilfolve this fubflance, even by boiling, but only a yellow colouring matter. This fo- lution afforded by evaporation a refiduuni in fcales of an amber colour. 1 mixed feme drops of the muriatic acid with neaily two ounces of alcohol that I had boiled, upon twenty grains of the gum of fi k, ^ complete folution was produced ; but this fubftancc \n cooling affumed the form of a jelly. The fubllance that we feparate from the filk m drefling is therefore of an animal nature ; it is for this reafon that the foapy waters employed in the drefling, putrify quickly ; when it is no longer retained by its affinity with the filk it diflulves readily in water, but; not in alcohol. Although it be not of a vegetable na- ture, we fee the term Gum may be applied to it with great propriety. The yellow colouring part is foi ubis in alcohol. When this part is feparated, the gum be- comes brown : it would appear that this colour is pro- duced by the heat it undergoes in boiling, fmce, When, we feparate only the yellow colouring part, according to the procefs of M. Baume, of which we fhall after- wards fpeak, the filk remains white. ■ In the procefs of M. Coilomb the gum is alfo fepa- rated, and carries along with it only a part of the yellow colouring particles : but in the drefiing with foap, both the gum and yellow colouring panicles are feparated from the fiik. I boiled yellow filk in a retort, where the vapours being more clofely confmed than in the open ai^, cuglit to produce a degree of heat fuperior to that of boilirg in ioS OF SILK. in the open air: after four hours boiling, the filk had loft nearly a fourth part of its weight, but it prefer- red its colour almofl: entire. 1 boiled, in the fame man- ner, a pattern in water impregnated with marine fait ; it became whiter, but loft lefs of its weight, although the degree of heat was certainly increafed by the re- fiftance which the fait made to the evaporation : a part of the marine fait perhaps combined with the filk. Were we to try other falts, we might probably find fome which, without injuring the filk, would produce a more ready folution of the gum and colouring part. When filk is intended to be formed into blond lace and gauzes, it ought to preferve the ftifFnefs and elafti- city v/hich are natural to it : the greater part of that produced in our climate has a yellow colour, and it is the white filk from China that is chiefly ufed for thefe purpofes ; but as it is obtained at a price which does not permit our manufadures to fupport a rival- lliip with the EngliOi, through whofe hands it comes to us, efpecially as they retain the fineft for their own manufactures, we have endeavoured to difcover a me- thod of depriving yellow filk of its colouring part without afFe£ling its gum, and confequently without depriving it of its elafticlty. M. Baume has refolved this interefting problem ; but he has kept his procefs fecret : fome artifts, to wb^om he had communicated it, or directed by fome hints, have fucceeded in performing this procefs : however, it appears to be fubject lo accidents, which increafe the expence by the lofs they occafion ; fo that it has not hitherto been generally employed, notwith- fiandinstheadvantagesthat it holds out to our manufac- tures, OF SILK. jC9 tares. We fliall point out what has tranfplred to the public refpecling this procefs. A fmall quantity of the muriatic acid is mixed with alcohol, and the lilk plunged into it. It is necefTary that the muriatic acid be pure, and do not contala any nitric acid, becaufe this laft would give the filk a yellow tinge. It appears that the moft difficult pni t of the procefs is to produce a white that is equal, ef- pecially when we operate upon large quantities. It appears alfo, that we fmd great difticulty in dryinp; white filk without its becoming wrinkled ; for this reafon it is proper to keep it extended while drying : the procefs would be too expenfive, if we did not ob- tain the alcohol that is loaded with the colouring parr, and make it ferve for fubfequent operations. It is ne- cefTary, therefore, to diflill it with a gentle heat, in an earthen or glafs vefTel. It appears from the above-mentioned experiments, that the muriatic acid fcrves in this procefs to foften the gum, fo as to enable the alcohol to diiTolve the co- louring part that is combined with it. Aluming ought to be confidered as one of the ge- neral operations of dying upon filk, becaufe without alum, the greater part of the colours we apply to filk would neither polTefs beauty nor folidity. To perform this procefs, we put into a ca(k or tub about forty or fifty buckets of water, with forty or fif- ty pounds of Roman alam, which had previoully been dilToIved in a copper full of water, fufHciently warm ; flirring carefully the mixture, in order to prevent the crvftallization of the alum. After i:0 OF COTTON* After having wafhed the filk by beetling it, and ^'ven by wringing it upon the pegs to extract the foap it may have retained, we plunge it into the alum vat, where it is allcv/ed to remain nine hours ; after which we wring it with the hand over the tub, and Carry it to the river to wafii it. V/e can make a hiindred and fifty pounds of filk pafs through a vat like the preceding, without any ne* ceiTity for adding any m.ore alum ; but when we per* Geive that the vat begins to weaken, which a little ex- perience enables us to do by the tafte, we difTolve twenty to twenty-five pounds of alum, which is put in« to the vat as formerly, and we continue to renew the vat in this manner, till it begins to emit a bad fmelL We then endeavour to exhauft it by palling the filks through it which are intended for dark co- lours, fuch as brov/ns, marones, &c. and we throw it a- way in order to m.ake a new folution. Silks are alv/ays alumed in the cold, for when they are alumed in a w^arm vat they are apt to lofe a pare of their luftre. ARTICLE lY. Of Cotton. Cottoia is the dow^n, or foft hair, that is con- tained in the feed-pod of a tree or fhrub v/hictt grows in warm countries. This down is feparz- ted from the feeds by means of a kind of mill which it covers, Clim.ate hasagreat influence apon the qualities of cot- ton, and the different kinds of the cott®'n tree feem to' agree 61? COTTOI^. Ill agree wdl with heac*. But agreat variety of cotton tree^ are to be found in the iflands of America; and it appears from what M. Bennet f has flated, that the colonies have hitherto neglecled to make a icle6cion of thofe which would be the mofl: profitable, and in ccnfequence of this, they have loft a great part of the advantages they might have obtained from this valuable produc- tion. The principal difTerences of cotton confift in the length of its filaments, their finenefs, their folidity, and colour. The colour of cotton varies from a deep yellow to a white ; that of Siam and Bengal has the deepeft co- lour, and it is often formed into fluffs which prefervc its natural colours. The mofl: beautiful kinds are not the whiteft, but it is neceffary to whiten them by pro- ceffes funilar to thofe by which we whiten thread, but fewer and lefs tedious operations are required for cot- ton than for thread. Inftead of thefe operations, vv'e may employ the oxigenated muriatic acid. Befides fa- ying time, we give it a more beautiful white than by the ordinary bleaching, and from the obfervaiions of M. Decroinlle, the ftuff appears more difpofed to take on the beautiful colours of dying. I have bleach- ed with fuccefs the yellow cottons of St Domingo, the ugly and tenacious colour of which renders it ufelefs m commerce. In * Effki fur Ics caraSleres qui diftinguent les cotons des divaffes parties du monde, <5cc ; pa"- M. Quatremere Disjonval. f TranfadioHs of the fociety inftituted at London for the enccu- ragemeat of arts, manufa but it is on this account that hemp tliat has been ftecp- cJ in a rapid ftream h deacient in flexibility and foftnefs. If the deeping is performed inflngnant and corrup- ted water, the henip acquires a brown colour j but be- H 2 fides I 16 OF FLAX. fides this, it lofes its folidity, and exhales vapours v/hich produce fatal diforders. ^ It appears then, that the fleeping is mofi: advantage- ouily performed in pools planted at the fides of rivers, fo that the water may be eafily renewed to prevent the putrefadion from being hurtful to the hemp or injurious to health, and yet not fafncient to prevent the degree necelTary to render the gelatinous fabRance foluble in water. M. Rofier found that the fleeping would be per- formed when the hemp is covered Avlth a layer of earth, and he advifes that method ; M. Prozet ^ has propofed to dilTolve a fmall quantity of cauflic alkali in the v/ater in which it is performed, to increafe its iolvent power and prevent putrcfadlion ; but it ap- pears from the experiments of Dr Home that the al- i;ali retards the operation of fleeping and renders the fiax brittle f . While flax is fieeping and during the drying which ought to precede and follow it, the green colouring particles undergo a change fnnilar to that obferved in the green fubflance of plants which are expofed to the action of the air and of light ; the colour changes to 'A yellow, to a fawn, and even to a brown, in confe- quence of the combudion which I explained in the firll fection. A great part is then foluble in alkalies without requiring to be any further oxigenated ; fo that by trcullng it with an alkaline folution we can fe- j-,arate from it a confiderablc portion of its colouring paiticles, * Mem. fur le rouiiuge du chanvre. f Effai fur is blar.chiment uts p. 270. OF fLAX. 117 particles, which remain perhaps combined with a part of the gluten. It is upon the Iblution of thefe colour- ing panicles that the procefs publiflied by the prince of St Sever for obtaining fine lint from hemp is found- ed. He orders the hemp to be lixiviated with a folu- tion of two parts of foda to one of lime, afterwards to impregnate it with foap to keep it in digeflion to wafil it well, and afterwards to comb it. 1 have attempted to bleach lint thoroughly by the method I employ for thread ; but although its fibres fhould lofd very little of their folidity by this procefs, they acquire l:o'.vever a greater difpofiiion to feparate, and they become more difficult to fpin, and form a thread much lefs folid. I iiave made a number of comparative experiments up-~>n lint, prepared by a method iimilar to that of the Prince de S. Sever, and Unt of the fame kind treated in the common manner. The firft method, afforded a greater proportion of tow, and the thread obtained by it was not finer than by the fecond, after the latter had been lixiviated , but the former v/as lefs foHd : however, there is an advantage which deferves atten- tion, that in the drefrmg, the lint treated in the for- mer method was free of that pov/der v/hich is fo dan- gerous to the workmen. The beauty that we produce in lint, by the pre- liminary lixiviations, does not ftem to pofTefs the ad- vantages that its beautiful appearance gives us reafon to expeft, fmce wc only diiiolve that portion of the colouring fubflance which would be carried av;ay by the firft lixiviations with which wc beq^in the bleach- ing llS m FLAX, ing. The great degree of finenefs we give it, Is pro^ bably obtained at the expence of the length and foli* dity of its filaments, A clergyman of the department of the Somme has rendered his mini (try refpedable by employing him^ felf with an object in which the real profperity of the people isfo much interefted. M. Brale haseltablifh- cd near Amiens a kind of public fchool, where he examines and endeavours to difcover the bed me- thod for the culture of hemp, for fleeping it and fot preparing the lint. His procefs is not attended with the inconvenience I met with in the lixiviation of the lint ; he lleeps the hemp as foon as it is taken from the earth : After fleeping he feparates the bark by a particular manipulation and the dipping it in a foiution of black foap ; he wafhes it with great care before drying ; the colouring part, which is foluble only in alkalies, may flill be dilfolved and carried a- way by the water afTiiled by a little foap ; the lint 13 then much whiter ; it divides more readily without doing fo too much ; and we have the lixiviaiions which ought to precede the bleaching. Ordinary thread and cloth that is woven contain a colouring fubflance vv^hich may be feparated by fnnple ^ lixivations ; but there is a portion of this fubftance that is truely comxbined with the vegetable fibres and which can only be feparated by decompofii^g it by the f combuition it undergoes in combining with oxigen, as I have explained in the third chapter of the firft feftion. Thread lofes by the operation of bleaching from a fourth to a third part of its weight. In 1 OF FLAX 119 In order to Jifpofe thrrad to take on the dye it is made to underojo the 'operations of drefTinc;, aluming, and galling, deicribed under the article ccttcn. t^~i tjn 'jTi 'J^ T n r. r n d. ^ V c p 1 4 THfc i. PAUL GerTY aNTER UBftARY