e of
this acid is the nitroso-group, and compounds containing this group are
nitroso-derivatives. In 1879, Otto Witt found that the nitroso-group in
nitrosodimethylaniline acted as an oxidizing group, and enabled this
compound to act upon certain diamido-derivatives of benzene and toluene,
with the formation of unstable blue compounds, which on heating the
solution changed into red colouring-matters of the azine group. This
process soon bore fruit industrially, and azines of a red, violet, and
blue shade were introduced under the names of neutral red, violet, and
blue, Basle blue, &c., some of these surviving at the present time.

We have now to turn to another chapter in the history of dimethylaniline.
In 1876, Lauth discovered a new colour test for one of the
diamidobenzenes. By heating this base with sulphur, and oxidizing the
product, a violet colouring-matter was formed, and the same compound was
produced by oxidizing the base in an aqueous solution in the presence of
sulphuretted hydrogen. Lauth's violet was never manufactured in quantity
because the yield is small; but in the hands of Dr. Caro the work of Lauth
bore fruit in another direction. Instead of using the diamidobenzene, Caro
used its dimethyl-derivative, and by this means obtained a splendid blue
dye, which was introduced under the name of "methylene blue." Here again
we find scientific research reacting on technology. A few words of
chemical explanation will make this manufacture intelligible. By the
action of reducing agents on nitro and nitroso-compounds, the nitro and
nitroso-group become converted into the amido-group. Thus when
nitrobenzene is reduced by iron and an acid we get aniline; similarly when
nitrosodimethylaniline is reduced by zinc and an acid we get
amidodimethylaniline, and this is the base used in the preparation of
methylene blue. By oxidizing this base in the presence of sulphuretted
hydrogen, the colouring-matter is formed. Other methods of arriving at the
same result were discovered and patented in due course, but the various
processes cannot be discussed here.

Lauth's violet and methylene blue became the subjects of scientific
investigation in 1879 by Koch, and in 1883 a series of brilliant
researches were commenced by Bernthsen which extended over several years,
and which established the constitution of these compounds. It was shown
that they are derivatives of diphenylamine containing sulphur as an
essential constituent. The