me 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.

Some other uses of carbolic acid must next be considered. Of the
colouring-matters derived from coal-tar, none is more widely known than
the oldest artificial yellow dye, picric acid. This is a phenol
derivative, and was first obtained as long ago as 1771 by Woulfe, by
acting upon indigo with nitric acid. Laurent in 1842 was the first to
obtain this dye from carbolic acid, from which compound it is still
manufactured by acting upon the sulpho-acid with nitric acid. Chemically
considered, it is trinitrophenol. It has a very wide application as a dye,
and has been used as an explosive agent. A similar colouring-matter was
made from cresol in 1869, and introduced under the name of "Victoria
yellow," which is dinitro-cresol. Other dyes derived directly or
indirectly from phenol will take us back once again to toluene.

A new diazotisable diamido-compound was obtained from this last
hydrocarbon, and introduced in 1886 by Leonhardt & Co. One of the three
isomeric nitrotoluenes furnishes a sulpho-acid which, on treatment with
alkali, gives a compound derived from a hydrocarbon known as stilbene, and
this, on reduction, is converted into the diamido-compound referred to.
The latter, which is a disulpho-acid as well as a diamido-compound, can be
diazotised and combined with phenols, &c. The stilbene azo-dyes thus
prepared from phenol and salicylic acid, like the chrysamines, are yellow
colouring-matters, containing two azo-groups. It is a valuable
characteristic of these secondary azo-dyes that they all po