424 DYEING.

have been already considered. In the sequel of this article an automatic dyeing vat willbe described.

The extracts of solutions of native dye-stuffs may be divided into two classes, in refer-ence to their habitudes with the oxygen of the atmosphere; such as continue essentiallyunaltered in the air, and such as suffer oxydation, and thereby precipitate a determinatecoloring matter. The dyes contained in the watery infusions of the different vegetableand animal substances which do not belong to the second class, are feebly attached totheir solvents, and quit them readily for any other bodies that possess an attraction forthem. On this principle, a decoction of cochineal, logwood, brazil wood, or a solutionof sulphate of indigo, by digestion with powdered bone black, lose their color, in conse-quence of the coloring particles combining by a kind of capillary attraction with theporous carbon, without undergoing any change. The same thing happens when well-scoured wool is steeped in such colored liquids; and the color which the wool assumesby its attraction for the dye, is, with regard to most of the above colored solutions, butfeeble and fugitive, since the dye may be again abstracted by copious washing with simplewater, whose attractive force therefore overcomes that of the wool. The aid of a hightemperature, indeed, is requisite for the abstraction of the color from the wool and thebone-black, probably by enlarging the size of the pores, and increasing the solvent powerof the water.

Those dye-baths, on the contrary, whose coloring matter is of the nature of extractiveor apotheme, form a faster combination with stuffs. Thus the yellow, fawn, and browndyes, which contain tannin and extractive, become oxygenated by contact of air, and in-soluble in water; by which means they can impart a durable dye. When wool is impreg-nated with decoctions of that hind, its pores get charged by capillarity, and when the liquidbecomes oxygenated, they remain filled with a color now become insoluble in water. Asimilar change to insolubility ensues when the yellow liquor of the indigo vat gets oxydizedin the pores of cotton and wool, into which it had been introduced in a fluid state. Thesame change occurs when protosulphate of iron is converted into persulphate, with thedeposition of an insoluble peroxyde in the substance of the stuff. The change hereeffected by oxydation can, in other circumstances, be produced by acids which have thepower of precipitating the dye-stuff in an insoluble state, as happens with decoction offustic.

Hence we perceive that the dyeing of fast colors rests upon the principle, that thecolors dissolved in the vat, during their union with the stuff, should suffer such a changeas to become insoluble in their former menstruum. The more this dye, as altered in itsunion with the stuff, can resist other menstrua or agents, the faster it will be. This isthe essential difference between dyeing and painting; or applying a coat of pigment de-void of any true affinity for the surface.

If we mix a clear infusion of a dye with a small quantity of a solution of an earthyor metallic salt, both in water, the limpid liquids soon become turbid, and there grad-ually subsides sooner or later, according to the nature of the mixture, a coloredprecipitate, consisting of the altered dye united with a basic or subsalt. In this com-pound the coloring matter seems to act the part of an acid, which is saturated by asmall quantity of the basis, or in its acid relationship is feeble, so that it can alsocombine with acids, being in reference to them a base. The decomposition of a salt, asalum, by dyes, is effected principally through the formation of an insoluble subsalt?with which the color combines, while a supersalt remains in the bath, and modifies, byits solvent reaction, the shade of the dyed stuff. Dyed stuffs may be considered acomposed of the fibrous body intimately associated with the coloring matter, the oxy >and acid, all three constituting a compound salt. Many persons have erroneous yimagined, that dyed goods contained none of the acid employed in the dye bath;they forget that even potash added to alum does not throw down the pure earthy bas ?but a subsalt; and they should not ascribe to coloring matter a power of decomposi iat all approaching to that of an alkali. Salts, containing strong acids, saturate a v jlarge quantity of coloring matter, in proportion to their place in the scale of C ' |CDQ * requivalents. Mere bases, such as pure alumina, and pure oxyde of tin, have no P°of precipitating coloring matter; when they seem to do so, they always contain sacid.

Such salts, therefore, as have a tendency to pass readily into the basic state, are Pp^j gliarly adapted to act as mordants in dyeing, and to form colored lakes. Magnesia a itsas fine a white powder as alumina, and answers equally well to dilute Jakes,soluble salts cannot be employed to form lakes, because they do not pass into tne_ \

state. This illustration is calculated to throw much light upon dyeing process

general. j the

The color of the lake depends very much upon the nature of the acid, anbasis of the precipitating salt. If it be white, like alumina and oxyde of tin, t itewill have, more or less, the color of the dye, but brightened by the reflection