want, cumidine had to be made on a large
scale, but here practical difficulties interposed themselves. The quantity
of cumene in the light oil is but small, and it is associated with other
hydrocarbons which are impurities from the present point of view, and from
which it is separated only with difficulty. A new source of the base had
therefore to be sought, and here again we find chemical science
ministering to the wants of the technologist.

It was explained in the last chapter that aniline and similar bases can be
methylated by heating their dry salts with methyl alcohol under pressure.
In this way dimethylaniline is made, and dimethyltoluidine or
dimethylxylidine can similarly be prepared. Now it was shown by Hofmann in
1871, that if this operation is conducted at a very high temperature, and
under very great pressure, the methyl-alcohol residue, _i.e._ the
methyl-group, does not replace the amidic hydrogen or the hydrogen of the
ammonia residue, but the methylation takes place in another way, resulting
in the formation of a higher homologue of the base started with. For
example, by heating aniline salt and pure wood-spirit to a temperature
considerably above that necessary for producing dimethylaniline, toluidine
is formed. In a similar way, by heating xylidine hydrochloride and methyl
alcohol for some time in a closed vessel at about 300 deg. C. cumidine is
produced. Hofmann's discovery was thus utilized in 1882, and by its means
the base was manufactured, and cumidine scarlet, very similar in shade to
cochineal, became an article of commerce.

While the development of this branch of the colour industry was taking
place by means of the new naphthol disulpho-acids, the cultivation of the
fertile field of the azo-dyes was being carried on in other directions. It
came to be realized that the fundamental discovery of Griess was capable
of being extended to all kinds of amido-compounds. The azo-dyes hitherto
introduced had all been derived from amido-compounds containing only one
amido-group, and they accordingly contained only one azo-group; they were
_primary_ azo-compounds. It was soon found that aniline yellow, which
already contains one azo-group as well as an amido-group, could be again
diazotised and combined with phenols so as to produce compounds containing
two azo-groups, _i.e._ _secondary_ azo-compounds. The sulpho-acid of
aniline yellow--Graessler's "acid yellow"--was the first source of azo-dyes
of this cl