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Some of those named here dye the animal fibres, even without mordants; some are Direct Colours possessing mordant-dyeing 750 properties, others are sulphonic acid derivatives of Alizarin Colours, suitable for wool but not for cotton.

The pure substances are best obtained by fusion of the corresponding toluene sulphonic acids with potash.

An important nucleus-synthetic reaction is the saponification of nitriles, which may be obtained by the interaction of potassium cyanide with a halogen substitution derivative or a sulphonic acid.

The aromatic dihydric alcohols are easily condensed with the different sulphonic acids and yield valuable tanning matters.

Alizarin S is a sulphonic acid derivative of alizarin, and since it is much more soluble, it readily yields level colours.