d 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.

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
chrysoidine, but also upon phenols, forming acid azo-colours. This
important fact was made known in 1870 by the German chemists Kekule 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, so