COLOURING matter. It has been sup.
posed that a peculiar proximate princi ple exists in vegetables, in which their colour frequently resides, and which has hence received the name of colouring matter.
The colouring matter of vegetables is scarcely ever found insulated, but is mix ed or combined with other principles. In this state it exists in the leaves and flowers, in the bark, and in the wood of the stem and roots. It is extracted, and obtained more pure, by the action of those agents which are capable of dissolving it. In many cases, water, cold or warm, is sufficient for this purpose. if logwood, brazil wood, madder, weld, or querci tron bark, for example, be macerated in water, the matter on which the colour depends is dissolved ; a transparent solu tion, more or less deeply coloured, is ob tained; and, by repeating the macera. tion with water sufficiently, nothing at length remains but the mere ligneous fi bre. Sometimes, however, the colour ing matter is not soluble in water : it is then frequently soluble in alcohol ; and, in a few substances, is even best dissolved by oils essential or expressed.
When the colouring matter is in solu tion, it may be attracted from the solvent by other substances with which it enters into combination : and this, in some mea sure, gives it a more appropriate charac ter. There are some substances even which appear in general to exert strong affinities to colouring matter, particular ly alumina and some of the metallic ox ides. If alumina be diffused or boiled in
a coloured vegetable infusion, it often happens, that the colouring matter com bines with it, and leaves the water of the infusion perfectly colourless. Or if alum be dissolved in a coloured infusion, and it be decomposed by the addition of an al. kali, the alumina, in the moment of its precipitation, attracts the colouring mat ter, forms a coloured precipitate, and, if the due proportions have been observed, the liquid w ill remain colourless. In like manner, if a coloured infusion be boiled with a metallic oxide, it often happens, that the colouring matter is attracted by the oxide. Thus Berthollet obtained combinations by this process of the co louring matter of logwood, and other dye stuffs, with oxide of copper, and oxide of tin. Or if certain metallic salts be dis solved in the infusion, and be then de composed by an alkali, the oxide, in pre cipitating, equally attracts the colouring matter. It is from similar affinities to the colouring matter that it is often attracted by linen, cotton, silk,'or wool, from its solutions ; and even where the affinities of these are not sufficiently powerful, they may be rendered capable of attract ing it, or the combination may be render ed more permanent by their being im pregnated with another substance, which has towards it a still stronger attraction.
See DYEING.