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A small quantity of these agents, however, may sometimes form supercompounds with the stuff, and its color may be altered in that way. The oxides of metals produce in the coloring particles, with which they unite, a degree of combustion proportioned to the quantity of oxygen which these particles can take from them. Therefore the colors, which the compounds of metallic oxides and coloring particles assume, are the product of the color peculiar to the coloring particles, and of that peculiar to the metallic oxide: but the coloring particles and metallic oxides must be considered in that state to which they have been reduced by the diminution of oxygen in the oxide, and the diminution of hydrogen in the particles that produce the color. It follows from this, that the metallic oxides, to which the oxygen is only slightly attached, are not fit to serve as intermedia for the coloring particles, because they produce in them too great a degree of combustion; instances of this kind are the oxides of silver, gold, and mercury. The oxides which undergo considerable alterations of color, by giving off more or less of their oxygen, are also bad intermedia, particularly for light shades, because they produce changeable colors; examples of this kind are the oxides of copper, of lead, and of bismuth. The oxides which strongly retain their oxygen, and undergo very little change of color by the loss of a proportion of it, are the most suitable for this purpose; such is particularly the oxide of tin, which quits its menstruum easily, which has a strong affinity for the coloring particles, and which affords them a basis that is very white, and proper for giving a brightness to their shades, without altering them by the mixture of another color. The oxide of zinc is possessed of some of these properties in a considerable degree.

75. To account for the colors, which proceed from the union of the coloring particles with the basis which a mordant gives them, we must attend to the proportion in which the coloring particles unite to that basis. Thus the solution of tin, which produces a very copious precipitate with a solution of coloring particles, and which thereby proves that the oxide of tin enters in a large proportion into the precipitate, has a much greater influence on the color of the precipitate, by the whiteness of its basis, than the solution of zinc, or that of alum, which generally produce much less copious precipitates. The precipitates produced by these two last substances retain very nearly the natural tint which the coloring particles afforded. It is therefore necessary to distinguish, in the action of mordants, the combinations that may take place by their means, between the coloring particles, the stuff, and the intermedium; the proportions of the coloring substances and intermedium; the modifications of color, which may arise from the mixture of the color of the coloring particles, and of that of the basis to which they are united; and the changes which the coloring particles may suffer, from the combustion that may be produced by the substance that is employed as an intermedium. It is evident also, that astringents do not differ essentially from coloring particles; but the 'atter take this name, especially when employed

to produce black with exide of iron, by restor ing this metal to the state of a black oxide, and by their assuming a dark color from the action of oxygen.

76. The notion of an astringent supposes, moreover, the property of combining in a certain quantity with animal substances, giving them thus solidity and incorruptibility; because these two properties are most commonly united. These again are derived from their large share of carbon, a circumstance in their composition which gives them increased tendency to solidity, and greater stability.

77. On this ingenious theory of Berthollet, Dr. Bancroft, an able writer on dyeing, has made some remarks that deserve attention. In his opinion M. Berthollet, in ascribing the decays of vegetable and animal coloring matters in general, to effects or changes similar to those of combustion, has gone much farther than is warrantable by facts. It cannot, he thinks, be his intention, that we should apply the term of combustion to alterations which result from a simple addition of oxygen to coloring matters, with a destruction or separation of any of their component parts; though many of the decays and extinctions of these colors evidently arise only from such simple additions of oxygen. The nitric, sulphuric, and other acids, containing oxygen, have the power not only of weakening, but of extinguishing, for a time, the colors of many tingent matters; not by any effect which can properly be denominated a combustion, but rather by a change in their several attractions for particular rays of light; but none of their parts being destroyed, or carried away, the addition of an alkali, or of calcareous carbonate, will generally undo such alteration, and restore the original color, by decomposing and neutralising the acid or oxygen which had caused the alteration.

78. Of this numerous instances might be given, it being the case of almost all vegetable or animal coloring matters. It will be sufficient to mention, that ink dropped into a glass of diluted nitric, vitriolic, or other acid, will lose its color, and that it may be again restored by adding a suitable portion of vegetable or fossil alkali; and that this may be done several times with the same ink, and therefore the change, or loss of color, could not have been the effect of combustion. If, however, this ink had not been fixed by dyeing in the substance either of wool, silk, linen, or cotton, and the substance so dyed had been dipped into a glass of diluted acid, a considerable part of the coloring matter would have been dislodged, aud separated from the dyed substance, by its affinity with the oxygen or acid; although no combustion had taken place, the color so separated and lost could not be again restored without a second dyeing. This loss of color would be similar to what frequently happens to colors from exposure to the sun and air, by which they are gradually weakened, many of them without any other change of tint than the simple diminution of their original quantity of coloring matter; and this continuing in the more fugitive colors, particularly that of turmeric, the cloth is soon left as white as before it had beer

dyed, without any thing like combustion having ever taken place in it, or in the matter with which it was dyed. It may also be presumed, that colors are not generally impaired by any thing like combustion, from this fact, that there are but few of them which the common muriatic acid does not injure, as much as either the nitric or the sulphuric; and as there can be no combustion without oxygen, and as the common muriatic acid either contains none, or what it does contain is confessedly combined with it by an affinity too powerful to be overcome by any known substance or means, it follows, that the oxygen (if it contain any) cannot be liberated so as to act in the way of combustion upon any other matter; and therefore, when the common muriatic acid changes or destroys the colors, it changes or destroys the affinities upon which they depend, by producing effects different from those of combustion; and as the changes which it produces on colors are in most cases similar to those produced by the nitric, sulphuric, and other acids known to contain oxygen, it is reasonable to conclude, that these also act upon colors, by producing other effects than those of combustion.

79. M. Sennebier exposed a great variety of woods to the action of the sun and air, and found ail their colors very soon affected. The white woods generally became brown, and the red and violet changed either to yellow or black. Guaiacum was rendered green; the oak and the cedar were whitened, as were the brown woods generally; effects which certainly do not resemble those of combustion, any more than the bleaching of wax or tallow by exposure to the air. It is therefore evident, argues Dr. Bancroft, that the color of each particular substance depends on its constitution, producing in it a particular attraction for certain rays of light; and a disposition to reflect or transmit certain other rays; and in this respect it may doubtless suffer very considerable changes from the action or combination of oxygen, without any effects similar to those of combustion. And, indeed, the changes of color which arise from the access of atmospheric air, seldom resemble those which the mere predominance of blackness (the supposed natural color of carbon) would produce; though this may have been the case with the coloring matter of brown or unbleached linen, upon which the experiments of M. Berthollet seem principally to have been made. But whether the action of vital air, or its basis, in promoting the decays and colors, ought to be denominated a combustion or not, Dr. Bancroft is confident, that at least some of them are liable to be impaired, not so much by an accession of oxygen, as by the loss of it. The difference of color in arterial and venous blood had been long noticed, and numerous experiments have shown that the fine vermilion color of the former is produced solely by vital air, which it is capable of acquiring through bladders, the coats of blood-vessels, &c. And Mr. Hassenfratz seems to have proved, that, as this fine red color is gained by a dissolution of oxygen in the arterial blood, so it is lost, and the dark color of the venous blood restored, by a separation of the oxygen, in consequence of its forming a new combination with the hydrogen and carbon of

the same.

80. Dr. Bancroft is also of opinion, that the blue color of indigo depends upon a certain portion of oxygen, for he has found that a solution of indigo, by losing its oxygen, may become as pellucid, and, excepting a very slight yellowish tinge, as colorless as water, and afterwards speedily return through all the shades of yellow and green to its original deep blue, by exposure to atmospheric or vital air. Similar to this, he remarks, is the fact long since observed by the abbè Nollet, of the tincture of archil employed to color the spirit of wine used in thermometers, and which after some time loses its color, but recovers it again upon being exposed to atmospheric air. This also happens to the infusion of turnsole, and to syrup of violets, which lose their colors when secluded from air, and regain them when placed in contact with it. He has also observed various animal and vegetable colors, produced solely by the contact of atmospheric air; and some others, which, when given by dyeing or callico-printing to wool, silk, cotton, &c., though unable to sustain a single day's exposure to the sun and air without manifest injury, were found to receive none from the action of strong nitric or sulphuric acids, but, on the contrary, were perceived by being wetted with them, and even with oxygenated muriatic and sulphuric acids. But the same colors, if covered with linseed oil, were found to decay more quickly from exposure to the sun and air, than if uncovered. These colors, therefore, he contends, could not owe their decay to the contact or combination of oxygen, because they were not only unhurt, but benefited by its concentrated powers in the nitric, the oxygenated muriatic, and sulphuric acids; and also because they were soonest impaired when defended from the access of oxygen, by being covered with linseed oil. Probably the decays of these colors were occasioned by a loss of at least some part of the oxygen which was necessary to their existence, and which the linseed oil assisted in depriving them of, by the strong affinity it has with oxygen.

81. Dr. Bancroft further observes, that, in forming systems, we are apt to draw general conclusions from only a partial view of facts. This M. Berthollet seems to have done, not only in ascribing the decays of vegetable and animal colors, exclusively to effects similar to those of combustion, but also in representing the oxygenated muriatic acid, as an accurate test for anticipating, in a few minutes, the changes which these colors are liable to suffer by long exposure to the action of sun and air; for, says he, though it is true, that the oxygenated muriatic acid, in weakening or destroying colors, gives up to them more or less of the oxygen which it had received by distillation from manganese; and that, by this new combination of oxygen, those affinities for particular rays of light, upon which their colors depend, are liable to be destroyed; it is nevertheless true, that the changes of color so produced are no certain indication of those, which the combined influence of light and air will occasion upon colors in general; there being several colors which are very speedily destroyed by the latter of these causes, though they resist the strongest action of the oxygenated muriatic acid, without suffering any degree of

injury or hurt. The Dr. adds, that M. Berthollet well knows, since nobody has contributed more to ascertain, how much the properties of oxygen are diversified by each particular basis to which it unites; and that it does not, therefore, seem warranta' le to imagine, that its action will not be modified by a basis so powerful as that of the common muriatic acid, or that the united properties of both should represent or resemble those of atmospheric air upon colors, any more than they do in the lungs by respiration; where, instead of supporting life, they would instantly put an end to it.

82. These observations were made in reference to the manner in which M. Berthollet had expressed himself on the subject in his Elemens de l' Art de la Teinture, published in 1791. A new edition of this work was published about the year 1804, in which the author has fully noticed Dr. Bancroft's arguments; refuted some of them; admitted the force of others in part; and, in some respects, has availed himself of the important improvements of Dr. Bancroft.

OF THE DIFFERENCES BETWEEN ANIMAL AND VEGETABLE SUBSTANCES.

83. Before we proceed to treat of the practice of dyeing, it will be necessary to consider some of the leading differences that exist between several of the substances to be dyed, and to point out the processes through which they must pass before they will receive the colors required. The following is the substance of M. Berthollet's opinion relative to this subject :-It is now known, that the composition of animal substances is distinguished from that of vegetables, by their abounding in a particular principle called azote, which is found only in small quantities in vegetables, as well as by their containing much more hydrogen, or base of inflammable air, than is found in the other. From these two causes, the differences observed in the distillation of animal and vegetable substances proceed: the former yield a large quantity of ammoniac or volatile alkali; the latter afford very little, and sometimes yield an acid: the former yield a great deal of oil, the predominant principle in which is hydrogen, which is very volatile and disposed to fly off by a small increase of temperature; while the latter sometimes do not yield it in the least sensible quantity.

84. Dr. Ure in a note, p. 151, vol. I. of his translation of Berthollet's treatise, has the following remarks on this theory. Modern researches do not justify this position of M. Berthollet. Sugar and starch, by the analyses of M.M. Gay Lussac and Berzelius, contain about as much hydrogen as fibrin does, and very little less than gelatin and albumen; while, by my analyses, wool and silk contain less hydrogen than cotton and flax. See Phil. Trans. for

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The first two, independently of the azote, pos sess a marked difference of composition, from their excess of carbon and deficiency of oxygen.

85. In consequence of this composition, animal substances, when set on fire, produce a bright flame, which breaks out at the beginning, but is soon stifled by the charcoal which is formed, and which has peculiar properties; their combustion is accompanied with a penetrating odor, owing to the ammoniac and oil which escape unconsumed; they are liable to putrefaction, in which process ammoniac is produced, as well as in their distillation, by a more intimate union of the azote and hydrogen; while vegetable substances, on the contrary, undergo the vinous and acetous fermentation. It is evident, that, as animal substances contain a considerable quantity of principles disposed to assume an elastic form, they have less cohesive force among their particles than vegetables, and a greater disposition to combine with other substances; hence they are more liable to be destroyed by different agents, and are more disposed to combine with coloring particles.

86. The consequence of this action on animal substances is, that they cannot bear lies, and that alkalis should be used with great caution in the processes employed for dyeing them; whereas no danger is to be apprehended from the use of alkalis with substances of the vegetable kind. Nitric and sulphuric acids have also a consi derable action on animal substances: the former decomposes them, extricates the azote, separates the fatty matter, and forms carbonic acid or fixed air, and oxalic acid or the acid of sugar with a part of the hydrogen and a part of the charcoal; the latter extricates the inflammable gas, probably azotic gas, and reduces the other principles to the state of carbon. Silk bears some resemblance to vegetable substances, from its being less disposed to combine with coloring particles, and by resisting the action of alkalis and acids more powerfully; which may arise either from the same principles being more intimately combined in it than in wool, or, more probably, from its containing less azote and hydrogen. But, though the action of alkalis and acids upon silk be weaker than upon wool, they should still be employed with great caution, because the brightness of color required in silk appears to depend upon the smoothness of its surface, which should, on that account, be preserved unimpaired, with every possible attention. Cotton withstands the action of acids much better than flax or hemp. Even the nitric acid does not destroy it without great difficulty.

OF WOOL.

87. The value of wool, and its fitness for the different kinds of manufacture, depend upon the length and fineness of its filaments. Wool is naturally covered with a kind of grease, which preserves it from moths; so that it is not scoured until it is about to be dyed, or formed into yarn. To scour wool, it is generally put for about a quarter of an hour into a kettle, containing a sufficient quantity of water, mixed with onefourth of putrid urine, heated to such a degree as the hand can just bear, and it must be stirred from time to time with sticks. It is then taken

out, put to drain, and carried in a large basket to a running water, where it is moved about until the grease is entirely separated, and no longer renders the water turbid; it is afterwards taken out, and left to drain. It sometimes loses in this operation more than a fifth of its weight. This operation should be conducted with much care, since the more correctly it is performed, the better is the wool fitted to receive the dye. In this process the ammonia or volatile alkali which exists in the urine, readily combines with the oil of the wool, and forms a soap, which, being soluble in water, is dissolved and carried

off.

88. Wool is dyed in the fleece before it is spun, when it is intended to form cloths of mixed colors; it is dyed after being spun, when intended principally for tapestry: but it is most generally dyed after having been manufactured into cloth. If wool be dyed in the fleece, its filaments, from being separate, absorb a larger quantity of the coloring particles than when it is spun; for the same reason, woollen yarn takes ap more than cloth: but cloths themselves vary considerably in this respect, according to their degree of fineness, or the closeness of their texture. Besides, the variety in their dimensions, the different qualities of the ingredients employed in dyeing, and a difference of circumstances in the process, prevent us from relying upon the precise quantities recommended for the processes. This ought in all dyes to be attended to. It is a fact well known to dyers and others, that the coarse wool from the thighs and tails of some sheep receives the coloring particles with great difficulty. The finest cloth is never fully penetrated with the scarlet dye, hence the interior of the cloth appears always of a lighter shade when cut, and sometimes almost white. For the generality of colors, wool requires to be prepared by a bath, in which it is boiled with saline substances, principally with alum and tartar; but there are some dyes for which the wool does not require such a preparation; then it must be well washed in warm water, and wrung out, or left to drain.

89. The surface of the filaments of wool or hair is not quite smooth; for, although no rough ness or inequality can be discovered, yet they seem to be formed of fine laminæ placed over each other in a slanting direction, from the root of the filament towards the point, resembling the arrangement of the scales of a fish, which cover each other from the head of the animal to its tail. This peculiarity of structure is proved by a simple experiment. If a hair be held by the root in one hand, and drawn between the fingers of the other hand, from the root towards the point, hardly any friction is perceived, and no noise is heard; but if it be seized by the point, and passed in the same manner between the Singers from the point towards the root, a resistance is felt, and a tremulous motion is perceptible to the touch, while the ear perceives a slight noise. Thus it appears, that the texture is not the same from the root towards the point, as it is from the point towards the root. This is farther confirmed by another experiment. If a hair be held between the thumb and fore-finger,

and they be rubbed against each other in the longitudinal direction of the hair, it acquires a progressive motion towards the root. This effect depends not on the nature of the skin of the finger, or on its texture, for if the hair be turned and the point placed where the root formerly was, the motion is reversed, that is, it will still be towards the root.

90. On this peculiarity of structure, which was observed by M. Monge, depend the processes of felting and fulling of hair and wool for different purposes. In the process of felting, the flocculi of wool are struck with the string of the bow, by which the filaments are detached, and dispersed in the air. These filaments fall back on each other in all directions, and, when a layer of a certain thickness is formed, they are covered with a cloth, on which the workman presses with his hands in all parts. By this pressure the filaments are brought nearer to each other; the points of contact are multiplied; the progressive motion towards the root is produced by the agitation; the filaments entangle each other; and the laminæ of each taking hold of those of the others, which are in an opposite direction, the whole is retained in a state of close contexture.

91. Connected with this operation is that of fulling. The roughness on the surface of the filaments of wool, and their tendency to acquire a progressive motion towards the root, produce great inconvenience in the operations of spinning and weaving. This inconvenience is obviated by covering the filaments with a coat of oil, which fills up the pores, and renders the asperities less sensible. When these operations are finished, the stuff must be freed from the oil, which would prevent it from taking the color with which it is to be dyed. For this purpose it is taken to the fulling-mill, where it is beaten with large beetles, in a trough of water, through which clay has been diffused. The clay unites with the oil, which, being thus rendered soluble in water, is carried off by fresh portions of water, conveyed to it. In this way the stuff is scoured; but this is not the sole object of the operation. By the alternate pressure of the beetles, an effect similar to that of the hands of the workman, in the operation of felting, is produced. The filaments composing a thread of warp or woof, acquire a progressive motion; are entangled with the filaments of the adjoining threads; those of the latter into the next, and so on, till the whole become felted together. The stuff is now contracted in all its dimensions, and, participating both of the nature of cloth and of felt, may be cut without being subjected to ravel; and, when employed to make a garment, requires no hemming. In a common woollen stocking web, after this operation, the stitches are no longer subject to run, and, the threads of the warp and woof being less distinct from each other, the whole stuff is thickened, and forms a warmer covering.

OF SILK.

92. Silk in its natural state is coated over with a substance which has generally been considered as a kind of gum or varnish. To this

substance the silk is supposed to owe its elasticity and stiffness. Besides this varnish, the silk usually met with in Europe is impregnated with a substance of a yellow color, and, for most of the purposes for which silk is required, it is necessary to free it from both the varnish and the coloring matter. To effect this, the silk is subjected to the operation of scouring; but it is very obvious that when the silk is to be dyed, the scouring need not be carried so far as is required where it is to remain white. Different colors, also, will require different degrees of scouring; and this difference is generally regulated by the quantity of soap employed: 100 pounds of silk boiled in a solution of twenty pounds of soap, for three or four hours, supplying a little water occasionally because of the evaporation, will be sufficiently prepared to receive the common colors. For blue colors the proportion of soap must be greater; and scarlet, cherry color, &c., require a still greater proportion, because for those colors the ground must be whiter.

93. When silk is to be employed white, it must undergo three operations. The first consists in keeping the hanks of silk in a solution of thirty pounds of soap to 100 of silk: this solution ought to be very hot, but not boiling; when any part of the hanks immersed is entirely free from its gum, which is known by the whiteness it acquires, the hanks are to be shaken over, as the dyers term it, so that the part which was not before immersed, may undergo the same process. They are then taken out and wrung, as the process is finished.

94. In the second operation the silk is put into bags of coarse cloth, each bag containing from twenty-five to thirty pounds. A solution of soap is prepared as in the former case, but with a smaller proportion of soap. In this the bags are boiled for an hour and a half; and that they may not receive too much heat by resting on the bottom of the vessel, they must be constantly stirred during the operation.

95. The third operation is to communicate to the silk different shades, that the white may be rendered more pleasing. These shades are known by different names, as China-white, silverwhite, azure-white, or thread-white. For this purpose a solution of soap is also prepared, of which the proper degree of strength is ascertained by its manner of frothing by agitation. For the China-white, which is required to have a slight tinge of red, a small quantity of anatto is added, and the silk is shaken over in it till it has acquired the shade required. In other whites, a blue tinge is given by adding a little blue to the solution of soap. The azure-white is produced by means of indigo. To prepare the azure, fine indigo is well washed in moderately warm water, after which boiling water is poured upon it. It is then left to settle, and the liquid part only, which contains the finer and more soluble parts, is employed.

96. Some use no soap in the third operation, but, when the second is completed, they wash the silks, fumigate with sulphur, and azure them with river water, which should be very pure. But all these operations are not sufficient to give

silk that degree of brightness which is necessary, when it is to be employed in the manufacture of white stuffs. For this purpose it must undergo the process of sulphuration, in which the silk is exposed to the vapor of sulphur. But before the silk which has been thus treated is fit for receiving colors, and retaining them in their full lustre, the sulphur which adheres to it must be separated by immersion and agitation for some time in warm water, otherwise the colors are tarnished and greatly injured.

97. It has long been an object of considerable importance, to deprive silk of its coloring matter, without destroying the gum, on which its stiffness and elasticity depend. A process for this purpose was discovered by Beaumé, but, as it was not made public, others have been led to it by conjecture and experiment. The following account, given by Berthollet, is all that has transpired concerning this process. A mixture is made with a small quantity of muriatic acid and alcohol. The muriatic acid should be in a state of purity, and entirely free from uitric acid, which would give the silk a yellow color. In the mixture thus prepared, the silk is to be immersed..

98. One of the most difficult parts of the process, especially when large quantities are operated upon, is to produce a uniform whiteness. In dyeing the whitened silk, there is also some difficulty in preventing its curling; hence, it is recommended to keep it constantly stretched during the drying. The muriatic acid seems to be useful in this process, by softening the gum, and assisting the alcohol to dissolve the coloring particles which are combined with it. The alcohol which has been impregnated with the coloring matter may be again separated from it and purified, and may thus serve in future operations, and render the process more economical. This may be effected by distillation with a moderate heat, in glass or stone-ware vessels.

The preparation with alum is a very important preliminary operation in the dyeing of silk. Without this process, few colors would have either beauty or durability. Forty or fifty pounds of alum, dissolved in warm water, are mixed in a vat, with forty or fifty pails of water; and, to prevent the crystallisation of the salt, the solution must be carefully stirred during the mixture. The silk being previously washed and beetled, to separate any remains of soap, is immersed in this alum liquor, and after eight or nine hours is wrung out, and washed in a stream of water: 150 pounds of silk may be prepared in the above quantity of liquor; but when it begins to grow weak, which may be known by the taste, twenty or twenty-five pounds of alum are to be added, and the addition repeated till the liquor acquires an offensive smell. It may then be employed in the preparation of silk intended for darker colors, till its whole strength is dissipated. This preparation of silk with alum must be made in the cold; for when the liquor is employed hot, the lustre is impaired.

OF COTTON.

99. Cotion is the down or wool obtained from the pods of the gossipium, a shrubby plant which

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