215

DYEING.

alum ; so that it more easily quits its menstruum, to combine withthe stuft' ami colouring particles. Another advantage is, that theacid liquor, from which the alumine is separated, has much lessaction on the colour when it consists of the acetous, than when itconsists of a stronger acid, such as the sulphuric. In short, theacetite of alumine not having the property ot crystallizing, themordant, which is thickened with starch or gum, to prepare it forbeing applied to the block on which the design is engraved, doesnot curdle, as it would it it contained alum capable oi crystalliz-ing. 15y attending to the operation performed upon a piece oflinen cloth, we find, that when it has been impregnated by themordant, in the manner determined by the design, it is put into ahath of madder; the whole of the cloth becomes coloured,'hutthe tinge is deeper in those parts which have received the mor-tlant; there the colouring particles have combined with the alu-inine and the cotton, so that a triple compound has been formed,and the acetous acid separated from its basis is carried oil' in thebath. Thus the colouring particles, combined with tin; alumineand the stuff', are much more difficultly a fleeted by externalagents, than when they are in a separate state, or combined onlywith the stulf, without any intermediate bond of union: and onthis property the operations, to w hich the cloth is ailerwards sub-jected, are founded : after it has been maddered, it is boiled withbran, and spread upon the grass; and these operations are alter-nately repeated until the ground becomes white. The colouringparticles, which have not united with the alumine, are altered intheir composition, dissolved, and separated, while those that havecombined with it remain, and are preserved, without alteration;and thus, the design alone remains coloured. It seems that thisdestruction of the colouring particles, by exposure on the grass,and boiling with bran, is accomplished in the same manner as thatof the colouring particles of flax, and admits of the same explana-tion. The only difference consists in substituting bran tor alka-lies, because they would dissolve a part of the colouring matter,which is fixed by the alumine, and would change its colour; in-stead of which, the bran, having a much weaker action on thissubstance, affects only the colouring particles, which, by the ac-tion of the air, have been disposed more easily to solution. If,however, instead of the mordant, a solution of iron be employed,the same phenomena takes place; the. colouring particles decom-pose the solution of iron, and form a triple compound with thestuff; but, instead ot red, we get from the madder brown coloursof-different shades, down even to black: and, by uniting thesetwo mordants, alum and iron, we have mixed colours, incliningto red on the one hand, and to black on the other, such as mor-dore and puce colour. Other colours are also procured by sub-stituting dyers weed for madder; and by means of these two co-louring substances, indigo, and the two mordants above mention-ed, we obtain most of ttie different shades that are observable instuffs which are printed. The substances which compose a mor-dant are sometimes incapable of decomposing each other solely bvtheir own attractions; but the attraction of the stuff for one oftheir constituent parts brings about a decomposition and new com-binations; and sometimes this effect is not produced or completedwithout the aid of the attraction of the colouring particles. Thisappears to he the case in the mixture of alum and tartar, one oftlie most common mordants employed in the dyeing of wool.The attraction of alumine for animal substances, is not merely in-dicated by uncertain appearances, nor supposed for the purpose ofbeing employed in explanations, but is proved by direct experi-ment. M. Herthollet united them together, by mixing an animalsubstance with a solution of alum; a double exchange took place,the alkali entered into combination with the acid of the alum, andthe alumine, combining with the animal substance, was precipitat-ed. lie alo proved the attraction of alumine-for animal sub-stances by another experiment; having mixed a solution of glue.with a solution of alum, he precipitated the alumine by an alkali,and the glue with which it had combined fell down along with it.This compound has the appearance of a semi-transparent jelly, andgrows dry with difficulty. Thus, in the preceding experiments,tlie alkali precipitated the alumine combined with the animal sub-stance, irom the uncry.stallizable residue of tlie alum which hadbeen boiled with the wool. Tlie attraction of alumine for mostcolouring substances, may also be shewn by direct expeiiment.If a solution of a colouring sub,tance be mixed with a solution ofaium, a precipitation sometimes takes place; but if to the liquor

we. add an alkali, which decomposes the alum, and separates thealumine, the colouring particles are then precipitated, combinedwith the alumine, and tlie liquor remains clear: this compoundhas got the. name of lake. In this experiment, too much alkalimust not be added, because alkalies are capable of dissolving lakesin general. No experiment has, however, yet shewn, that alu-mir.e attracts any vegetable substance except the colouring parti-cles: its attraction for them seems much weaker than that whichit has for animai substances; hence the acetite of alumine is a bet-ter basis for cotton and linen than alum is, and upon this dependthe different means employed to increase tlie fixity of the colour-ing particles of madder in the dyeing of these substances. Theoxides or calces of metals have so great an altraction for many co-louring substances, that they quit the acids in which they weredissolved, and are precipitated in combination with (hem. Onthe other hand, ail metallic oxides have the property of unitingwith animal substances; and these different compounds may heformed bv mixing an alkali saturated with an animal substance,with metallic solutions. It is not surprising, therefore, that me-tallic oxides should serve as a bond of union between tlie colour-ing partic les and animal substances; but, besides the attraction ofthe oxides for the colouring particles, and for animal substances,their solutions in acids possess qualities which render them moreor less fit to act as mordants: thus, those oxides which easily partwith their acids, such as that of tin, are capable of combining withanimal substances, without the aid of colouring particles; it is suf-ficient to impregnate tlie wool or the silk with a solution of tin,although they be afterwards carefully washed, which is not thecase with other metallic solutions. Some metallic substancesafford, in combination, only a white and colourless basis; andsome by the admixture of their own colour, modify that which isproper to the colouring particles; hut in many metallic oxides,the colour varies according to the proportion of oxygen they con-tain, and the proportion of this is easily liable to change. Uponthese circumstances their properties in dyeing chiefly depend.The greater or less disposition of the stuff to unite with the colour -ing particles, must frequently occasion considerable differences intlie mode of employing tlie mordant: thus, when this dispositionis strong, the mordant may be mixed with tlie colouring sub-stance; the compound then formed unites with the stuff immedi-ately: but if it be weak, the compound formed by tlie colouringparticles, and (he substance employed as an intermedium, may se-parate, and be precipitated, before it can be attached to the stuff'.To prevent this inconvenience, we must begin by attaching to thestulf the substance which is to serve as the medium of union, be-tween it and tlie colouring particles. Hut to judge of the effectsof mordants, and of the most advantageous manner of employingthem, we must pay attention, first, to the combinations w hich maybe produced, either by the action of tlie substances which com-pose them, or by that ot the colouring particles and the stuff;3dly, to the circumstances which may concur in bringing aboutdiese combinations more or less quickly, or in rendering themmore or less complete; 3d!v, to the action that the liquor in whichthe stuff is immersed may have, either on its colour or texture;and in order to foresee wiiat that action may be, it is necessary toknow the proportions of tlie principles, which enter into the com-position ot the mordant, and what will be left in an uncombinedstate in the fluids.

Of the Action of different Substances on Colours.

The colouring particles have been hitherto considered only assubstances capable of forming different combinations, by whichtheir properties are modified; but they may be altered in theircomposition, either by other external agents, or by the substancewith which they unite. We shall begin with an examination ofthe changes produced by external agents, because they are moreeasily ascertained. The stability of a colour consists in its powerof resisting the action of vegetable acids, alkalies, soap, and moreespecially that of the air or light; but this power varies exceed-ingly, according to the nature of the colour and tlie species of tliestuff; for the same durability is not required in the colours of silkas in those ot wool. '1 here is not much obscurity in the action ofwater, acids, alkalies, or soap: it is a solution brought about bythese agents; and it appears that a small quantity of acid, or of al-kali, sometimes unites with the compound which gives the co-lour ; because the colour is not destroyed, but only changed, and

may