as a direct yellow colouring-matter, but
as a starting-point in the manufacture of other azo-dyes. The use of acid
yellow for this last purpose will be dealt with again in the next chapter.

There is one other use for aniline yellow which dates from the year of its
discovery, when Dale and Caro found that by adding sodium nitrite to
aniline hydrochloride and heating the mixture, a blue colouring-matter is
produced. The latter was introduced in 1864 under the name of "induline."
It was shown subsequently by the scientific researches of several chemists
that the blue produced by Dale and Caro's method results from the action
of the aniline salt on the aniline yellow, which is formed by the action
of the nitrous acid on the aniline and aniline salt. This explanation was
proved to be correct in 1872 by Hofmann and Geyger, who prepared the
colouring-matter by heating aniline yellow and aniline salt with alcohol
as a solvent. These chemists established the composition and gave it the
name of "azodiphenyl blue." Later, viz. in 1883, the manufacture was
improved by Otto Witt and E. Thomas, and the dye, under the old name of
"induline," is now largely manufactured by first preparing aniline yellow
and then heating this with aniline and aniline salt. The colouring-matter
as formed by this method is basic and insoluble in water; it is made acid
and soluble by treatment with sulphuric acid, which converts it into a
sulpho-acid. Induline belongs to the sober-tinted colours, and produces a
shade somewhat resembling indigo. Closely related thereto is a bluish-grey
called "nigrosine," obtained by heating nitrobenzene with aniline, as well
as a certain bluish by-product obtained during the formation of magenta,
and known as "violaniline."

It will be convenient here to pause and reflect upon the great industrial
importance of the two coal-tar hydrocarbons upon which we have thus far
concentrated our attention. Their uses are by no means exhausted as yet,
but they have already been made to account for such a number of valuable
products that the reader may find it useful to have the results presented
in a collected form. This is given below as a chronological summary--

1856. Mauve discovered by Perkin; leading to manufacture of aniline.

1860. Arsenic acid process for magenta discovered; leading to
manufacture of arsenic acid.

1860. Aniline blue discovered; leading in 1862 to soluble and Nicholson
blues.

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