Cochineal, with alum, will form a crimson ; but, with iron, a black.

Indigo requires no mordant ; its solvent is sulphuric acid, and its colour is due to oxygen.

It is of a greenish yellow, and is soluble in lime water, if deprived of its oxygen.

It may be deoxidized, by the application of some substance having a greater affinity for oxygen protoxide of iron, for example ; but it will reabsorb oxygen from the atmosphere, and again become blue.

142 THE LAWS OF AFFINITY.