DYE MATERIALS.

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b. By passing the tissue through any bath or solution containing an

oxidising material, such as a chromate.

c. By the introduction into the colouring mixture which is to be

printed on the cloth of some oxidising substance, which on ex-posure to an elevated temperature gives off oxygen.

IV. The colour is dissolved in a suitable solvent, which is capable ofbeing volatilised by exposure to air and heat. This method isemployed in the fixing of the acetates of iron and alumina, forexample, and is sometimes accompanied by oxidation. Steam-heatis generally used to effeCt the volatilisation.

V. The colour is formed upon the fibre by double decomposition or

displacement effected by means of two salts, or a salt and an alkalinehydrate, or an alkaline carbonate. One of these compounds isapplied to the fibre in solution, while the other is dissolved in asuitable fluid, and the fibrous material is passed through this solution.Examples : fixing the oxides of iron, chromium, copper, manganese,&c. This process also serves for the fixing of certain mordants, andis employed in many cases with acetate of alumina mordants inconjunction with the dung-bath. Oxidation is sometimes necessaryat a subsequent stage for the development of the colour.

VI. The constituent elements of a colour are presented to the fibre insolution by chemical means, and in such a manner that—either underthe subsequent influence of moisture and heat or by lapse of time—they unite to form an insoluble and adhesive colour. Under thishead may be included the dissociation of the ferro- and ferri-cyanhydric acids by the influence of heat; the result of suchdissociation is subsequently oxidised to obtain a deposit of Prussianblue.

Notwithstanding the many subdivisions of our classification, there are somecases to which it is not applicable. To these belong those instances of mul-tiple reactions, or of special agency, which will need presently a moredetailed treatment.

The colouring materials which we shall pass in review may be divided intotwo groups, viz.—

Mineral Products, andOrganic Substances.

Among the members of the latter group we shall meet with a large numberof substances of vegetable and animal origin which have not been reproducedartificially, and the constitution of which is imperfectly known, exhibiting noother relation than that of similarity in colour. Others, again, have been re-produced artificially, and are chemically well defined.