The distinction between _acid_ and _basic_ dye-stuffs is largely due to certain characteristics possessed by many of them. Thus magenta, which is the salt of the base known as Rosaniline, belonging to the basic colouring matters, a group of dyes which do not possess the fastness of colour peculiar to acid dyes, has a limited application. But by treatment with sulphuric acid magenta is converted into an acid magenta, and this dye has wider application than the basic salt. Similarly the basic dye called aniline blue is insoluble in water, and therefore has only a limited use, but by treatment with sulphuric acid it is converted into alkali blue, soluble blue and so on, which dissolve readily in water and are good fast colours. The acid dyes generally have a weaker colouring power than the basic dyes, but they produce very even shades. -------+-------------------+-----------------+----------------- Colour.| Acid. | Basic. | Substantive. -------+-------------------+-----------------+----------------- Yellow | Metanil yellow. | Auramine. | Cotton yellow. and | Paper yellow. | Chrysoidine. | Chrysophenine. Orange.| Orange II. | | | Naphthol yellow S.| | | Quinoline yellow. | | | | | Red. | Fast red A. | Rhodamine. | Congo red. | Cotton scarlet. | Paper scarlet. | Benzopurpurin. | Erythrine. | Safranine. | Oxamine red. | Ponceau. | Magenta. | | | | Blue | Water blue 1 N. | Methylene blue. | Azo blue. and | Fast blue. | Victoria blue. | Violet.| Acid violet. | New blue. | | | Indoine blue. | | | Methyl violet. | | | Crystal violet. | | | | Brown | Naphthylamine | Bismarck brown. | | brown. | Vesuvine. | | | | Black | Nigrosine. | Coal Black B. | | Brilliant black B.| | | | | Green | | Diamond green. | | | Malachite green.| -------+-------------------+-----------------+----------------- The difference in the composition of the basic and acid dyes is taken advantage of in the dyeing of paper pulp to secure a complete distribution of the colouring matter upon the pulp, with the result that the intensity of colour is increased, its fastness strengthened, and the process of dyeing generally rendered more economical. This is effected by the judicious addition of a suitable acid dye to the pulp already coloured with the basic dye.