t rank as colour generators. When a
diazo-salt acts on an amido-derivative there is formed a compound which is
more or less unstable, but which readily undergoes transformation under
suitable conditions into a stable substance in which two hydrocarbon
residues are joined together by the pair of nitrogen atoms. These products
are dye-stuffs, known as "azo-colours," and aniline yellow, Bismarck
brown, and chrysoidine are the oldest known technical compounds belonging
to the group. The parent substance is "azobenzene," and these three
colouring-matters are mono-, di- and triamido-azobenzene respectively.

A new phase in the technology of tar-products was entered upon when Witt
caused a diazo-salt to act upon diamidobenzene. This was the first
industrial application of Griess's discovery. Azobenzene, which was
discovered by Mitscherlich in 1834, and azotoluene are now manufactured by
reducing nitrobenzene and nitrotoluene with mild reducing agents. These
parent compounds are not in themselves colouring-matters, but they are
transformed into bases which give rise to a splendid series of azo-dyes,
as will be described subsequently. Let it be recorded here that these two
compounds are to be added to the list of valuable products obtained from
benzene and toluene. And it must also be remembered that the introduction
of these azo-colours has necessitated the manufacture on a large scale of
sodium nitrite as a source of nitrous acid. Without entering into
unnecessary detail it may be stated broadly that this salt is made by
fusing Chili saltpetre, which is the nitrate of sodium, with metallic
lead, litharge or oxide of lead being obtained as a secondary product.
Then again, the manufacture of Bismarck brown requires dinitrobenzene,
this being made by the nitration of benzene beyond the stage of
nitrobenzene. The brown is made by reducing the dinitrobenzene to
diamidobenzene, and then treating a solution of the latter with sodium
nitrite and an acid. The azo-colour is formed at once, and no special
refrigeration is required in this particular case.

It has already been stated that the old aniline yellow of 1863 is no
longer used on account of its fugitive character. In 1878 Graessler found
that by the action of very strong sulphuric acid this azo-compound could
be converted into a sulpho-acid in just the same way that magenta can be
converted into acid magenta. Under the name of "acid yellow" this
sulpho-acid is now used, not only