The dye introduced by Caro in 1874 is the brominated phthalein of resorcinol. The phthalein itself is a yellow dye, and the solutions of its salts show a splendid and most intense greenish yellow fluorescence, for which reason it is called "fluorescein." When brominated, the latter furnishes a beautiful red colouring-matter known as "eosin" (Gr. [Greek: eos], dawn), and the introduction of this gave an industrial impetus to the phthaleins which led to the discovery of many other related colouring-matters now largely used under various trade designations. About a dozen distinct compounds producing different shades of pink, crimson and red, and all derived from fluorescein, are at present in the market, and a few other phthaleins formed by heating phthalic anhydride with other phenolic compounds instead of resorcinol or pyrogallol (_e.g._ diethylamidophenol), are also of industrial importance. By converting nitrobenzene into a sulpho-acid, reducing to an amido-sulpho-acid, and then fusing with alkali, an amido-phenol is produced, the ethers of which, when heated with phthalic anhydride, give rise to red phthaleins of most intense colouring power introduced by the Baden Aniline Company as "rhodamines." It remains to point out that the scientific spirit which prompted the investigation of the phthaleins in the first instance has followed these compounds throughout their technological career. The researches started by v. Baeyer were taken up by various chemists, whose work together with that of the original discoverer has led to the elucidation of the constitution of these colouring-matters. The phthaleins are members of the triphenylmethane group, and are therefore related to magenta, corallin, malachite green, methyl violet, and the phosgene dyes.