Note: Do not rely on this information. It is very old.
Gallocyanines, a number of blue and violet dyes, which are obtained from gallic acid and some of the tannins by the action of certain aniline derivatives - nitrosodimethylaniline. That from gallic acid is the best known, and receives the name solid violet. They give blue, violet, or red solutions, according to the solvent employed, frequently exhibiting a fine fluorescence. For dyeing cottons a tin or chromium mordant is generally employed, but for the dyeing of silk or wool no mordant is necessary.