In the fragrant blossom of the meadowsweet (_Spiræa 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.
Salicylic acid is employed also for the manufacture of certain azo-dyes in a way that it will be very instructive to consider, because the process used may be taken as typical of the general method of preparing such compounds. Solutions of diazo-salts act not only upon amido- and diamido-compounds, as we have seen in the case of aniline yellow and chrysoïdine, but also upon phenols, forming acid azo-colours. This important fact was made known in 1870 by the German chemists Kekulé and Hidegh, but more than six years elapsed before this discovery was taken advantage of by the technologist. Large numbers of these acid azo-dyes are now made from various diazotised amido-compounds combined with different phenols and phenolic acids. The mode of procedure is to diazotise the amido-compound by sodium nitrite and hydrochloric acid in the manner already described, and then add the diazo-salt solution to the phenolic compound dissolved in alkali. The colouring-matter is at once formed. Salicylic acid possesses the characters both of an acid and a phenol. It combines readily with diazo-salts under the circumstances described, and gives rise to azo-dyes, some of which are of technical value.
The manufacture of azo-dyes from salicylic acid brings us into contact with certain amidic compounds which figure so largely in the tinctorial industry that they may be conveniently dealt with here. These bases are not azo-compounds themselves, but they are prepared from azo-compounds, viz. from the azobenzene and azotoluene which were spoken about in the last chapter. When these are reduced by acid reducing-agents, they become converted into diamido-bases which are known as benzidine and tolidine respectively. These bases can be diazotised, and as they contain two amido-groups, they form double diazo-salts, _i.e._ tetrazo-salts, which are capable of combining with amido-compounds, or phenols, in the usual way. Thus diazotised benzidine and tolidine combine with salicylic acid to form valuable yellow azo-dyes known as "chrysamines." The dyes of this class obviously contain two azo-groups.