DYE MATERIALS.

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for instance, is commercially a colouring material, and acetate of aluminacommercially a mordant, but the immediately effedtive colouring matter isalizarine , and the adtive agent of the mordant alumina. The mode of adtionof mordants is manifold as well as complex, and again differs for differentcases. The mordant sometimes serves to render the colouring matterinsoluble, and to fix it to tissues for which the colouring matter has itselfbut little affinity. Again, the mordant may modify or entirely change theshade of the colouring matter; or the mordant may simply render the dye“ fast,” as it is termed, instanced by the peculiar adtion of fatty matter inturkey-red dyeing. In another portion of this chapter we shall speak morein detail of the nature and adtion of mordants.

Colouring matters may, then, be divided into two classes

1. Colouring matters that become fixed and exhibit their peculiar colourwithout the application of an intermediate agent or mordant.

2. Colouring matters fixed by the agency of a mordant with which theycombine.

Aniline-red dyes wool and silk without the aid of a mordant, while it has notthe least affinity for cotton, hemp, or flax. Animal fibres combine chemicallywith fuchsine and analogous colouring matters; indeed, many animal sub-stances themselves adt as mordants. We have arrived at the conclusion,therefore, that a woven tissue can be said to be dyed only when the dye resiststhe adtion of water and of fridtion. The colouring matter must consequentlymechanically adhere to the fibre or be chemically incorporated with it. Thetheory of the fixing of colouring matter cannot be reduced to a single principle,since it varies as well with the nature of the colouring matter as with thenature of the fibrous substance. An insoluble colouring matter can in nocase be deposited alone on the fibre, for the least fridtion would cause it tobecome detached. When a very finely divided insoluble colouring matter ismixed with a solution of gum or gelatine, or any other viscous material, andis thus applied to a tissue, the thickener, when dry, will form upon the surfaceof the fibre a kind of varnish, which will cause the powder to adhere tem-porarily or until brought under the influence of water. Some kinds of mixedgauze tissues not intended to be washed are coloured in this manner—toooften with poisonous compounds. Many of the finer tissues admit only ofthis method of colouring, instanced in the case of mixed raw silk and cottonfabrics. These fabrics are sometimes coloured with Scheele’s green, and thecolouring matter becomes detached by fridtion. It is evident that if any sub-stance can be found possessing the property of glueing, as it were, thecolouring matter to the woven tissue, then it is possible to fix colours other-wise insoluble in neutral solvents. Such a substance we possess in albumen,and from what has been said it will be readily understood that the propertypossessed by albumen of coagulating at a high temperature greatly enhancesits pradtical value, because by its agency colours may be fixed under the adtionof steam.

If it is found desirable to fix to cloth any insoluble substance otherwisethan by means of an agglutinative, it is then indispensable that it be firstdissolved, in order that it may thoroughly penetrate the pores of the fibre andbe uniformly divided. We have here to distinguish two cases which canoccur —