The chemist whose name will always be associated with the compounds of
this group is the late Dr. Peter Griess of Burton-on-Trent. He commenced
his study of the action of nitrous acid on organic bases in 1858, and
from that time till the period of his death in 1888, he was constantly
contributing to our knowledge of the resulting compounds. In 1866, he and
Dr. Martius established the composition of aniline yellow, and the
following year Caro and Griess did the same thing for the Manchester
brown. In 1877 Hofmann and Witt established the constitution of
chrysoidine, the final outcome of all this work being to show that the
three colouring-matters belonged to the same group. The further
development of these discoveries has been one of the most prolific sources
of new colouring-matters. A brief summary of our present position with
respect to this group must now be attempted.

When nitrous acid acts upon an amido-derivative of a benzenoid hydrocarbon
in the presence of a mineral acid, there is formed a compound in which the
amido-group is replaced by a pair of nitrogen atoms joined together in a
certain way, which is different to the mode of combination in the azines.
This pair of nitrogen atoms is combined on the one hand with the
hydrocarbon residue, and on the other with the residue of the mineral
acid. The resulting compound is very unstable; its solution decomposes
very readily, and generally has to be kept cool by ice. Freezing machines
turning out large quantities of ice are kept constantly at work in
factories where these produces are made. The latter are known as
"diazo-compounds"--Griess's compounds _par excellence_--and they are
prepared on a large scale by dissolving a salt of the amido-base,
generally the hydrochloride, in water with ice, and adding sodium nitrite.
The result is a diazo-salt; aniline, for example, giving diazobenzene
chloride, and toluidine diazotoluene chloride. Similarly all
amido-derivatives of a benzenoid character can be "diazotised." The
importance of this discovery will be seen more fully in the next chapter.
At present we are more especially concerned with aniline.

The extreme instability of the diazo-salts enables them to combine with
the greatest ease with amido-derivatives and with other compounds. The
very property which in the early days rendered their investigation so
difficult, and which taxed the ingenuity of chemists to the utmost, has
now placed these compounds in the fron