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place, as is generally recommended. The final with metallic solutions. It is not surprising, effect of aluming, in whatever manner performed, therefore, that metallic oxides should serve as a and whatever chemical changes may have taken bond of union between the coloring particles and place in it, consists in the combination of alumine animal substances; but, besides the attraction of with the stuff: this union has probably been im- the oxides for the coloring particles, and for ani. perfect, and the acids only partially separated, mal substances, their solutions in acids possess but becomes complete when the cloth has been qualities which render them noore or less fit to boiled with madder, as in the case of printed act as mordants : thus, those oxides which easily stuffs. But an acid or an alkali may form a su- part with their acids, such as that of tin, are capercompound with the stuff, the coloring matter, pable of combining with animal substances, withand the alumine; for there are some colors which out the aid of coloring particles; it is sufficient are changed by an acid, and restored hy alkalis, to impregnate the wool or silk with a solution of or by calcareous earths, which take the acid from tin, although they be afterwards carefully washed, them, or vice versa; but this supercomposition which is not the case with other metallic soludoes not take place with respect to those colors tions. Some metallic substances afford, in comwhich are esteemed durable, being unchangeable bination, only a white and colorless basis; and by alkalis or acids, which are not strong enough some by the admixture of their own color, modify to destroy their composition.

that which is proper to the coloring particles; 25. The attraction of alumine for animal sub- but in many metallic oxides, the color varies stances is not, however, merely indicated by un- according to the proportion of oxygen they concertain

appearances, nor supposed for the purpose tain, and the proportion of this is easily liable to of being employed in explanations, but is proved change, Upon these circumstances their proby direct experiment. M. Berthollet united them perties in dyeing chiefly depend. together, by mixing an anima: substance with a 28. The affinity of inetallic oxides for subsolution of alum; a double exchange took place, stances of vegetable origin, seems much weaker the alkali entered into combination with the acid than that which they have for animal substances : of the alum, and the alumine, combining with the metallic solutions are, therefore, not well adapted animal substance, was precipitated. He also to serve as mordants for colors in cotton or linen, proved the affinity of alumine" for animal sub- except iton, the oxide of which unites firmly stances by another experiment: having mixed a with vegetable substances, as is shown by ironsolution of glue with a solution of alum, he pre- moulds, which are owing to a real corubinacipitated the alumine by an alkali, and the glue tion of this oxide. Whenever the coloring parwith which it had combined fell down along with ticles have precipitated a metallic oxide from its it. This compound has the appearance of a menstruum, the supernatant liquor contains the semitransparent jelly, and dries with difficulty. disengaged acid, which is commonly capable of Thus, in the preceding experiments, the alkali dissolving a portion of the compound of the coprecipitated the alumine combined with the animalloring substance and oxide, so that the liquor substance, from the uncrystallisable residue of remains colored; but sometimes the whole of the the alum which had been boiled with the wool. coloring particles are precipitated, when the pro

26. The affinity of alumine for most coloring portions have been accurately adjusted : this substances, may also be shown by direct experi- precipitation is facilitated, and rendered more ment. If a solution of a coloring substance be complete, by the presence of the stuff, which mixed with a solution of alum, a precipitation assists, by the tendency it has to unite with the sometimes takes place; but if to the liquor we compound of oxide and coloring particles. Un. add an alkali, which decomposes the alum, and combined metallic oxides have also a very eviseparates the alumine, the coloring particles are dent action on many coloring substances when then precipitated, combined with the alumine, boiled with them, and modify their color; the and the liquor remains clear: this compound has oxide of tin in particular increases the brightness obtained the name of lake. In this experiment, and fixity of many. too much alkali must not be added, because al 29. The compounds of oxides and coloring kalis are capable of dissolving lakes in general. substances are similar to many other chemical No direct experiment has however yet shown, compounds, which are insoluble, when the printhat alumine attracts any vegetable substance ciples of which they are formed are properly proexcept the coloring particles : its affinity for them portioned; but which are capable of being seems much weaker than that which it has for supersaturated by an excess of one of the princianimal substances; hence the acetite of alumine ples, and thence of becoming soluble. Thus a is a better basis for cotton and linen than aluin metallic oxide, united with a coloring substance is, and upon this depend the different means to excess, produces a liquor, the color of which employed to increase the fixity of the coloring will be modified by the oxides ; whereas, when particles of madder in the dyeing of these sub- the coloring matter is not in excess, the comstances.

pound will be insoluble, or nearly so ; these 27. Metallic oxides have so great an affinity effects are very evident in the combination of iron for many coloring substances, that they quit the with the astringent principle. Neutral salts acids in which they were dissolved, and are such as nitre, and particularly muriate of soda, precipitated in combination with them. On the or common salt, act as mordants, and modify other hand, all metallic oxides have the property colors; but it is difficult to ascertain the manner of uniting with animal substances; and these in which they act. M. Berthollet found that different compounds may be formed by mixing the muriate of soda was contained, in substance, an alkali, saturated with an animal substance, in the precipitates produced by some species o

coloring particles, and that these precipitates may have, either on its color or texture, must retained i considerable degree of solubility; it also be considered; and to be able accurately to would seem that a small part of the salt becomes judge of the extent of this action, we must know fixed with the coloring particles and the stuff. the proportions of the principles of which the Salts with calcareous bases also modify colors; mordant is composed ; which of these principles but, as these modifications are nearly similar to remains in an uncombined state in the liquor, those which would be produced by the addition and the proportion or quantity which is seof a small quantity of lime, it is probable that parated. they are decomposed, and that a litile of the lime 32. The coloring particles have been hitherto enters into combinaation with the coloring parti- considered only as substances capable of forming cles and the stuff. By attention to this, we shall different combinations, by which their properties easily discern what combinations are formed by are modified; but they may be altered in their the agency of the different reactives, employed composition, either by other external agents, or in the analysis of coloring substances; but we by the substances with which they unite. The must not forget, that the mordants and the color- stability of a color consists in its power of ing particles have a mutual action on each other, resisting the action of vegetable acids, alkalis, which may change their properties. It is evie soap, and more especially that of the air and dent that, by varying the mordants, we may light; but this power varies exceedingly, accordgreatly multiply the shades obtained from a co- ing to the nature of the color and the species of loring substance ; even to vary their mode of the stuff; for the same durability is not required application may be sufficient: thus we shall in the colors of silk as in those of wool. There obtain different effects by impregnating the stuff is not much obscurity in the action of water, with the mordant, or by mixing the mordant acids, alkalis, or soap: it is a solution brought with the bath; by applying heat, or using exsic- about by these agents: and it appears that a cations, for we operate upon three elective small quantity of acid, or of alkali, sometimes attractions; that of the coloring particles, that of unites with the compound which gives the color; the stuffs, and that of the principle of the mordant; because the color is not destroyed, but only and many circumstances

may

cause variations in changed, and may be restored by taking away the result of these attractions; circumstances this acid ; for instance, hy chalk and ammoniac, which merit further explanation. Exsiccation, or volatile alkali. But this is not the case with or drying, favors the union of the substances respect to the action of air and light. which have an affinity for the stuff, and the de 33. Scheele observed, that the oxygenated compositions which may result from that union; muriatic acid rendered vegetable colors yellow, because the water which held these substances in and he attributed that effect to the property it solution, by its attraction, opposed the action of had of taking up the phlogiston which entered the stuff; but the exsiccation should be slow, in into their composition. Berthollet has shown, order that the substances may not be separated that the properties of the oxygenated muriatic before their mutual attractions have produced acid were owing to the combination of its oxygen their effect.

with the substances exposed to its action; that it 30. Considerable differences must be observed commonly rendered the coloring particles yellow; in the manner of employing the mordant, as but that, by a continuance of its action, it the force of affinity between the stuff and the destroyed their color; without determining in coloring matter is greater or less.

When what this action consisted. Fourcroy afterwards this affinity is strong, the mordant and the co- made several observations on the action of oxygen loring substance may be mixed together; the on the coloring particles, which throw a great compound thus formed, immediately enters into deal of light on the nature of the changes they combination with the stuff. But, when the affi- undergo, chiefly when watery solutions of them nity between the stuff and the coloring particles are left exposed to the air, or have been subjected is weak, the compound formed of the latter and to a boiling heat. He observed that, in consethe mordant may separate, and a precipitation quence of the action of the air, vegetable decoctake place, before it can be attached to the stuff; tions formed pellicles, which lost their solubility, and hence it is, that the mordant which is to and underwent successive changes of color; he serve as the medium of union between the stuff marked the gradations of color thus produced, and the coloring matter, must be combined with and concluded, from his observations, that oxygen the former, before the application of the latter. entered into the composition of the coloring It is from these differences that different pro- particles; that when it combined with them, cesses must be followed in fixing coloring mat- their shade was changed; that the more they ters on animal and vegetable productions. received, the more fixed did their color become;

31. In judging of the effects of mordants, and and that the best method of obtaining permanent the most advantageous manner of applying them, unchangeable colors, for painting, was to choose it is necessary to attend to the combinations such as had been exposed to the action of the which may be formed, either by the action of the oxygenated muriatic acid. ingredients of which they are composed, or by 3.1. In considering the effects of air on colors, that of the coloring matter and the stuff. It is it is necessary to make a distinction between necessary, also, to take into consideration the cir- those produced by metallic oxides, and those cumstances which may tend to bring about these produced by the coloring particles. Berthollet combinations with more or less rapidity, or that is of opinion that the modifications of the former may render them more or less perfect. The action are entirely owing to different proportions of which the liquor in which the stuff is immersed oxygen, but from observation he has been led to

form a different opinion respecting the modifica- if exposed to a certain degree of heat, if the tions of the latter. He observed, that the oxygenated parts have not been removed by a oxygenated muriatic acid exhibited different phe- sufficiently strong lixivium. In the same mannomena with the coloring particles; that some ner, the green parts of vegetables are rendered times it discharged their colors, and rendered white by the oxygenated muriatic acid, but them white; that most frequently it changed beome yellow when boiled. them to a yellow, fawn, or root-colored, brown, 36. From these facts it appears, that oxygen or black, according to the intensity of its action; is capable of whitening, or rendering paler, the and that, when their color appeared only dis- coloring matters with which it unites, perhaps charged or rendered white, heat, or a length of by having produced the effects of a slight comtime, was capable of rendering them yellow. He bustion upon them; or possibly these effects take compared the effect produced by the oxygenated place only afterwards in a gradual manner, but muriatic acid, when the particles are rendered more rapidly, when the whole is exposed to a yellow, fawn-colored, or brown, with the effect certain degree of heat. It is extremely probable, of a slight degree of combustion, and showed that that in all cases a part of the oxygen unites with they were the same; that they were owing to the the coloring matter, without being combined destruction of the hydrogen, which, combining with the hydrogen in particular, and that it is in with the oxygen, morc easily, and at a lower tem- this way that oxygen acts, in rendering the colorperature than charccal does, leaves it predomi- ing maiter of flax more easily soluble in alkalis. nant, so that the natural color of charcoal is more In many other cases oxygen has evidently an or less blended with that which before existed. influence on the changes which take place in the This effect becomes very evident, when sugar, coloring particles of vegetables ; these particles indigo, or the infusion of the gall-nut, or of are formed chiefly in the leaves, flowers, and sumach, are exposed to the action of oxygenated inner bark of trees; by degrees they undergo a muriatic gas; the sugar and the indigo assume a slight combustion, either from the action of the deep color, and afford indisputable marks of a atmospheric air which surrounds them, or from slight combustion; the infusion of the gall-nut, that of the air which is carried by a particular set and that of sumach, let fall a precipitate, which of vessels into the internal parts of vegetables. is not far from being pure charcoal or carbon. 37. Berthollet, therefore, supposes we may These appearances are analogous to those which are explain how the air acts upon coloring matters, observed in the distillation of organised substan- of an animal, or a vegetable nature ; it first comces; in proportion as the hydrogen is extracted bines with them, renders them weaker and paler, in the form of oil, or of gas, the substance grows and by degrees occasions a slight combustion, yellow and at length there remains only a black by means of which the hydrogen which entered cual. If the hydrogen be expelled from an oil, by into their composition is destroyed ; they change heat, it grows brown, evidently in the same way. to a yellow, red, or fawn-color; their attraction

35. Berthollet also found, by other experiments for the stuff seems to diminish; they separate made on alcohol and ether, that the oxygen from it, and are carried off by water: all these united to the marine acid, had the property of effects vary, and take place more or less readily, combining with the hydrogen, which abounds in and more or less completely, according to the these substances, and of thereby forming water. nature of the coloring particles; or rather, from He therefore supposes, that when the oxygenated the nature of the properties which they possess, marine acid renders a color yellow, fawn-colored, in the state of combination into which they have or brown, the effect proceeds from the coloring gone. The changes which occur in the colors, matter having undergone a slight combustion, produced by the union of the coloring particles by which more or less of its hydrogen has been with metallic oxides, are effects compounded of converted into water; and that the charcoal, thus the change which takes place in the coloring rendered predominant, has communicated its own particles, and of that which is undergone by the color. The art of bleaching linen by means of metallic oxide. the oxygen of the atmosphere, of the dew, and of 38. The light of the sun considerably accelethe oxygenated marine acid, he also supposes to rates the extinction of colors. It ought, theredepend on this change of the coloring matter. fore, if this theory be well founded, to favor the The coloring particles of the fax are rendered combination of oxygen, and the combustion soluble in the alkaline lixivia, the action of which thereby induced. Sennebier, who has given ought to be alternate with that of the oxygen. many interesting observations on the effects of These oring particles may be afterwards preci- light on different substances, and particularly on pitated from the alkali, and by evaporation and their colors, attributes these effects to a direct drying become black, and prove the truth of this combination of light with the substances. And theory, both by the color they have acquired, the effects of light on the color of wood, have and by the quantity of charcoal which they yield on long ago been noticed ; it preserves its natural being analysed. But the alkaline solution of the appearance while kept in the dark, but when coloring matter of linen which is of a dark exposed to the light, it becomes yellow, brown, brown color, loses its color almost entirely, by or of other shades. The same writer also rethe addition of a certain quantity of oxygenated marked the varieties which occur in this particumuriatic acid; and the same effect is observable lar in different kinds of wood, and found, that in many other substances, which have assumed a the changes are proportioned to the brightness color originating from a commencement of com of the light, and that they take place even under bustion A piece of linen, whi appears white, water, but that wetted 'wood underwent these may grow yellow in process of time, particularly changes less quickly than that which was dry;

that several folds of riband were required to then becomes more intimate. But many flowers, defend the wood completely, that a single leaf of when in azotic gas, retain their color in perfecblack paper was sufficient, but that, when paper of tion. The tincture of turnsole was placed in any other color was substituted, the change was contact with vital air over inercury, both in the not prevented; a single covering of white paper dark, aud exposed to the light of the sun; the forwas insufficient, but two intercepted the action mer continued unchanged for a considerable of the rays of light.

length of time, and the vital air had suffered no 39. He extended his experiments to a great diminution; the other lost much of its color; number of vegetable substances, in a manner became red; and the air was, in a great measure, that may serve to illustrate different phenomena absorbed, and a small quantity of carbonic acid of vegetation. If a well-made solution of the was produced, which undoubtedly had occasioned green parts of vegetables in alcohol, which has the alteration of color from blue to red. Froin a fine green color, be exposed to the light of the this we may form an idea of some of the changes sun, it very soon acquires an olive hue, and loses of color, produced by a particular disposition of its color in a few minu:es. If the light be weak, the coinponent principles of vegetable substances, the effect is much more slow; and in perfeci when, by their combination with oxygen, they darkness, the color remains without alteration, undergo the effects of a slight combustion, which or, if any change does take place, it requires a may generate an acid, as in the leaves in autumn, great length of time. An alkali restores the which grow red before they become yellow, and green color; but if the change of color in the in the streaks which are seen in flowers, the vegeliquor has been completed, the alkali has no tation of which is becoming weak. effect. No change of color takes place in azotic 42. On the whole it is evident, that coloring gas, nor in a bottle which is exactly full. A substances resist the action of the air more or bottle half full of this green solution was inverted less, according as they are more or less disposed over mercury, by Berthollet, and exposed to the to unite with oxygen, and thereby to suffer more light of the sun; when the color was discharged, or less quickly a smaller or greater degree of the mercury was found to have risen in the bottle, combustion. Light favors this effect, which in and consequently vital air had been absorbed, many cases is not produced without its assistthe oxygen having united with the coloring mat- ance; but the coloring matter, in its separate ter. The precipitate which M. Sennebier men. state, is much more prone to this combustion, tions was not evident; the liquor had continued than when united to a substance, such as alumine. transparent, and retained a slight yellow tinge. which may either defend it by i:s own power of On evaporating this liquor, its color was immedi- resisting combustion, or, by attracting it strongly, ately rendered darker, and became brown; the weaken its action on other substances, which is residuum was black, and in a carbonaceous state. the chief effect of mordants. This last compound

40. Light, therefore, acts by favoring the ah- acquires still greater durability, when it is capasorption of oxygen, and the combustion of the ble of combining intimately with the stuff upon coloring matter. At first, the marks of combus- which it is deposited. Thus the coloring matter tion are not evident; the liquor retains only a of cochineal is easily dissolved in water, and its a slight yellow tinge; but, by the assistance of color is quickly changed by the air; but when heat, the combustion is completed, the liquor united to the oxide of tin, it becomes much becomes brown, and leaves a black residuum. brighter, and almost insoluble in water, though If the vessel which holds the liquor contains no it is still easily affected by the air, and by oxygen gas, the light has no effect on the color- oxygenated muriatic acid; it resists the action of ing matter; azotic gas in this situation suffers no these better, however, when it has formed a triple diminution. The observation, that ribands, or compound with a wooller stuff. But still it is a single leaf of white paper, do not prevent the not to be inferred, that all yellow colors are action of light, deserves atiention, as it shows that owing to the carbonaceous part of the coloring light can pass through coverings which appear 10 substance; very different compounds are capable be opaque, and exert its energy a considerable of producing the same colors; thus, indigo is depth within. Beccaria and Sennebier have very different from the blue of our flowers, from compared the effects of light on ribands of that of oxide of copper, and from that of Prussian various colors; but the differences they have blue. Berthollet does not even suppose, that observed are rather to be attributed to the nature oxygen may not unite in a small proportion with of the coloring matters, than to the colors; for some coloring substances, without weakening a riband dyed with Brasil-wood will lose its the color, or changing it to yellow. Indigo color much sooner than one dyed with cochineal, becomes green by uniting with an alkali, with though the shade should be exactly the same in lime or a metallic oxide; but resumes its color, each.

and quits these substances, when it recovers a 41. Although light greatly accelerates the com- small portion of the oxygen which it had lost. bustion of the coloring particles, and seems even The liquor of the whelk, employed to dye purple, necessary for their destruction in some cases, in is naturally yellowish ; but when exposed to the others it is not required. It was found, by air, and more especially to the sun, it quickly putting some plants into a dark place, in contact passes through various shades, and at length with vital air, that that air was absorbed by some assumes the exquisite purple color of the ancients; of them; and, also, that the rose suffers a change, and which, according to the testimony of Endoand becomes of a deeper hue, when it is not in cià, derived its lustre and perfection from expocontact with vital air, probably because it con sure to the sun's rays. tains a little oxygen, the combination of which 43. It may then be considered as a general

fact, that colors become brighter by their union case, when a vegetable color, reddened by an with a small portion of oxygen. It is on this acid, has been kept in like manner for some time. account found necessary to air stuffs when they Those instances in which acids have been emcome out of the bath, and sometimes even to ployed, which act by giving off their oxygen, take them out of it from time to time, expressly must be excepted, for in these there is an extracfor this purpose; but the quantity of oxygen tion of the color. which, thus becoming fixed, contributes to the 45. From the above remarks on mordants it brightness of the color, is very considerable in must appear very obvious that the practical dyer some cases and the deterioration of shade soon ought to be exceedingly careful in his selection begins. But the action of the air affects not only of substances, giving the preference to those that the coloring matter and the stuff, but also metallic most readily resist the action of the causes which nxides, when they are employed as intermedia; we have specified. because the oxides, which have at first been 46. It may not be improper to notice the deprived of a part of their oxygen by the color- action of these acids on animal substances, in ing particles, may absorb it again. Those then, consequence of its intimate counexion with the the color of which varies according to their pro- subject of mordants. It was observed by M. portion of oxygen, have thereby an influence in Brunwiser, that wood, on being exposed to the effecting the changes which the stuff undergoes. action of the air, assumed different colors: this It is undoubtedly to this cause that the change led him to endeavour to ascertain whence those ohservable in the blue given to wool, by sulphate colors arose, and to produce them by artifiof copper, or blue vitriol, and logwood, is to be cial means. He remarked that on moistening attributed. This blue soon becomes green by the the surface of wood, particularly young wood, action of the air: now copper, which has a blue with nitric acid, it assumed a yellow color; and color, when combined with a small proportion of that, by applying in the same way the muriatic oxygen, assumes a green one by its union with a and sulphuric acids, the wood assumed a violet larger quantity. The change which the coloring color. Hence he inferred that, as all colors are particles undergo, may indeed contribute to this produced by a mixture of yellow, blue, and red, effect; but the coloring particles of the logwood, all those colors which are seen in the leaves, which have themselves a dark color, should rather fruits, and flowers of trees, are owing to the colorbecome brown by combustion, than grow yellow, ing particles which exist in the wood, and are which would be necessary in order to produce there kept in a state of disguise, by the action of a green with the blue. It has been observed, an alkali; that the mineral acids, by taking up that coloring particles in a state of combination this alkali, set the coloring particles at liberty; and were less disposed to be changed by the action that the fixed air, by penetrating the leaves, fruits, of the air, than in an uncombined state. This is and flowers, produces naturally the same efiect, generally the case, but there are some exceptions; by combining with the alkali which kept them an alkali, for instance, produces a contrary effect. disguised. A matrass half filled with an infusion of cochineal, 47. M. de la Folie informs us that having imwas exposed to the light, over mercury; a similar mersed a skein of white silk in nitrous acid of the matrass contained an infusion of cochineal made strength generally used in commerce, the silk in with a little tartar; and in a third, a small quan- three or four minutes assumed a fine jonquille tity of alkali had been added to the infusion. yellow. He washed it several times in water, The second matrass appeared least altered in that it might not be affected by any adhering the same space of time, and in it the absorption acid; the color sustained several trials to which had been least considerable. In the third, the he submitted it, and the silk preserved its lustre color of the liquor became first brown, and was unimpaired. When dipped into an alkaline soluthen discharged; and the absorption of air, tion, a tine orange color was the result. Dr. though inconsiderable, was greater than in the Gmelin observes, that he has given a fine brimtwo others. On evaporation it assumed a brown stone color to silk, by keeping it for a day in cold color; and left a residuum of a yellowish brown. nitric acid, or some hours only, when the acid 44. Similar experiments having been made on

Boiling with soap and water dimidifferent coloring substances, the alkali was found nished the brightness of this color; and it was to darken their color, which grew more and more changed to a fine lemon color, by being kept for brown, and promoted the absorption of air. twelve hours in an alkaline solution; but, when Madder appeared to be the only exception to the solution was employed hot, a fine gold color this rule : its color, which became darker at first, was produced. The different solutions of metals stood better than that of the infusion made with in nitric acid communicated a more or less deep out alkali. The general effect of alkalis on the yellow to silk, as did also the solution of alumine coloring particles is consonant to that which it in the same acid; but those of the calcareous produces on many other substances, such as sul- earth and magnesia had no effect whatever. phur; it favors the absorption of air, because it 48. M. Berthollet also found, that the oxygehas a strong affinity for the substance which is nated muriatic acid has the property of tinging the result of that absorption. From this effect of animal substances yellow; but that it does not alkalis, a fact which has been observed by Becker give them so deep a color as the nitrous acid, may be explained ; viz. that a vegetable infusion, and it weakens them much more than that acid rendered green by an alkali, becomes gradually when properly diluted ; so that the nitrous acid yellow, if left exposed to the air, and that when is far preferable for the different purposes of art. the yellow is completely acids nno It, therefore, appears that the nitrous acid, direstore the original color: but that this is not the luted with a certain quantity of water, gives silk

was warm.

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