146

DYEING AND CALICO PRINTING.

See. In the case of nitrate of iron, a salt is chosen which holds one part ofits oxide in a comparatively loose state, and, when diluted with water, allowsthe feeble affinity of the fibre to remove it from the acid. A similar processtakes place with alum when a portion of the acid has been removed bysalts having an alkaline reaction.

The vegetable and animal mordants differ in their method of adhering tothe fibre. Oil, as employed in Turkey-red dyeing, undergoes a change byexposure to the air, which renders it insoluble and irremovable by ordinaryagents. Albumen and similar substances possess a mechanical affinity forthe cloth; they are applied in the soluble state, and become insoluble by heat,or some other agent which coagulates them. They embrace the fabric in areticulation of fibres, and may be considered as rather holding to the surfacethan as retained in the interior of the fibre.

There now arises the question, what is the nature of the combinationwhich takes place between the mordant and the fibre ? No diredt nor positiveanswer can be given, for the question is involved in doubt and controversy,while the evidence on the various views is so inconclusive that no satisfa&oryresult can be arrived at. The insoluble particles of the mordant may be re-tained simply in a mechanical manner by the fibre, which is supposed to behollow, and to a<5l the part of a trap; or the fibres may be porous, and the acidsolutions penetrating the pores leave their oxides in contact with the walls ofthe cells, from which they cannot escape, either by reason of the narrowness ofthe cells or some supposed conformation of the metallic oxide. The mordants,again, separated by more powerful chemical agents, or through destructionby time, may form a chemical combination with the matter of the fibre. Or thefibre may possess some active power of adhesion on its exterior walls for themetallic oxides, and they may be held together by virtue of the power of contact.All these theories are held, and the sum of the evidence that can be drawnfrom chemical or microscopical observations does not bear more on one sidethan on the other. In the absence of any satisfactory scientific account, wemay undertake the practical supposition that the particles of mordants areheld by the fibres in a state of chemical freedom, capable of exerting all theiraffinities, and drawing to themselves those substances for which they have anattraction.

The affinity which the mordants have for the colouring matter is undoubtedlyof a chemical nature. A given quantity or strength of mordant can combinewith only a certain amount of colouring matter to produce a certain shade,and, as is the case in most chemical combinations, the colour of the resultingcompounds bears no necessary resemblance to that of the constituents. Theshades of colour which any dye-stuff can give differ for each mordant, and forvarious strengths of the same mordant. The same colouring matter will yieldshades that appear quite opposite in their nature, as, for instance, madder,which with weak alumina mordants gives pink, with strong red; with weakiron mordants, lilac or violet, with strong black colours; while a mixture ofiron and alumina mordants yields various shades of chocolate.

A given mordant has not the same properties upon all kinds of fibre. Tinmordant, so extensively employed in woollen dyeing, and yielding fast colours,is a very feeble mordant on cotton, and never gives colours comparable for