Mordants, or Mordicants, in dyeing, signify those substances, which are employed for the purpose of macerating the stuffs, and rendering them capable of imbibing the tinging matter. Of this nature are, the sulphat of alumine ; the acidulous tartrite of pot-ash; the solution of tin in nitro-muriatic, and oxygenated muriatic acids ; the vegetable astringent principle, of gallic acid; acetite of alumine; sulphat of copper, or blue vitriol; arsenic ; acetite of copper, or ver-digrease ; and the sulphat of zinc, or white vitriol.

Mordicants act on stuffs in two different ways : 1. By parting with a portion of their oxygen, in consequence of which the substance of such stuff's is changed, and their attraction for the pigment or colouring matter is increased; and, 2. By altering, in a similar manner, the nature of the pigment, and rendering it capable of coagulation. Thus, the colouring matter undergoes several changes, and receives various degrees of a lighter or darker shade.

It would exceed our limits, to point out the different mordicants, which are adapted to certain colours, or to particular stuffs; for these can be only ascertained by experience. But, as the fine Turkey red communicated to cotton by means of madder, depends principally on the mordants employed in that process : and, as the knowledge of these is involved in considerable obscurity, by the jealousy or avarice of dyers, we shall subjoin a few hints on their effects in dyeing cotton red, selected from the memoir lately published by M. Chaptal, in the " Annales de Chi-mie."

The three principal mordants used in this operation are, oil, galls,and alum.

The greatest caution is necessary in choosing the oil, which ought to be similar to that employed in' painting, and to contain a large portion of the extractive principle. Hence, this oil .should not be com-pletely saturated with the alkali' but, previously to giving the red dye to the stuff, it ought to be combined with a weak solution of soda (or of pot-ash, if the former, alkali cannot be easily procured) ; and the cotton duly impregnated with this preparation; by which every part of it will thoroughly imbibe the oil'. The next process is that of galling, for which purpose, galls only should be employed, as no other vegetable astringent is equally efficacious.

The last mordicant is alum, which not only possesses the pro-, perty of brightening the red tint produced by madder, but at the same time contributes, by its decomposition, and the fixity of its earth or aluminous base, to give solidity to the colour, in order to judge of its effects in dyeing cotton, it will be sufficient to mix a decoction of galls with a solution of alum. The mixture will immediately become turbid, and a greyish precipitate be formed; which, On being dried, is insoluble both in water and in alkaline ley. Great care, however, is requisite, that the aluminous solution be not too hot, lest part of the astringent principle, obtained from the galls, escape the cotton, a nd the alum be composed in the immersion; a circumstance by which the power of the mordant is necessarily diminished, and the colour is impaired.

It is, therefore, to be attributed wholly to the united effects of the three principles (oil, the astringent, principle, and the earth, or base of alum), which serve as a mordant in dyeing red with madder. If these be employed separately, they. will neither produce the same fixity, nor afford a similar brilliancy of colour.