The scientific development of the history of this phenol dye is full of interest, but we can only give it a passing glance. Its interest lies chiefly in the circumstance that it is related to magenta, as was first pointed out by Caro and Wanklyn in 1866. In fact they obtained rosolic acid from magenta by the action of nitrous acid on the latter. We now know that a diazo-salt is first formed under these circumstances, and that the decomposition of this unstable compound in the presence of water gives rise to the rosolic acid. Later researches have shown that by heating rosolic acid with ammonia it is converted into rosaniline. It is also known that the commercial corallin, like the commercial magenta, is a mixture of closely related colouring-matters. The close analogy between magenta and rosolic acid was further shown by Caro in 1866. In the same way that Hofmann found that magenta could not be produced by the oxidation of _pure_ aniline, Caro found that a mixture of phenol and cresol was necessary for the production of rosolic acid when inorganic oxidizers were used. It is indeed this series of investigations upon the phenol dyes--investigations which have been taken part in not only by the chemists named, but also by Graebe, Dale and Schorlemmer, and the Fischers--which led up to the discovery of the constitution of the colouring-matters of the rosaniline group, and, through this, to the far-reaching industrial developments of the discovery as traced in the last chapter. It is evident, from what has been said, that rosolic acid and its related colouring-matters are members of the triphenylmethane group. They are in fact the hydroxylic or acid analogues of the amido-containing or basic dyes of the rosaniline series. In the fragrant blossom of the meadowsweet (_Spiraea ulmaria_) there is contained an acid which is found also as an ether in the oil of wintergreen (_Gautheria procumbens_). This is salicylic acid, a white crystalline compound which has been known to chemists since 1839. In 1860 Kolbe prepared the sodium salt of this acid by passing carbon dioxide gas into phenol in which metallic sodium had been dissolved. It was found subsequently that the same transformation was brought about by heating the dry sodium salt of carbolic acid in an atmosphere of carbon dioxide. This process of Kolbe's is now worked on a manufacturing scale for the preparation of artificial salicylic acid. The acid and its salts and ethers find numerous applications as antiseptics, for the preservation of food, and in pharmacy.