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It does not react with sodium or with phenylhydrazine, but yields dye-stuffs with isatin and phenanthrenequinone.

These bases were discovered in 1876 by Emil Fischer, and they are known as hydrazines, the particular compound thus obtained from aniline being phenylhydrazine.

The latter is obtained by the action of a sulpho-acid of phenylhydrazine on dioxytartaric acid, and is a yellow dye, which is of special interest on account of its extraordinary fastness towards light.

By this method the manufacture of phenylhydrazine and other hydrazines is effected on a large scale—all kinds of amido-compounds and their sulpho-acids can be diazotised and reduced to their hydrazines.

Antipyrine introduced by L. Knorr, leading to manufacture of phenylhydrazine.