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Some Direct Colours, indeed, are of little value, owing, for example, to their sensibility to acids, until they have been diazotized and developed, the usual developers being β-naphthol, resorcinol, phenol and phenylene-diamine.

The other compound, resorcinol, was known to chemistry ten years before it was utilized as a source of colouring-matters.

After mixing the ointment heat it until the resorcinol crystals melt to prevent any irritation of the skin from them.

Further, an excellent group of coloring matters, giving fast browns and greens with copper and iron mordants respectively, is formed by naphthol green, resorcinol green, gambin, and dioxin.

The condensation product—prepared as described for resorcinol, but requiring higher temperature—is a brick-red powder, insoluble in water.