A substance which turns red in acid and blue in alkali.
Litmus, which is prepared from several lichens, is a vegetable dye-substance, which on contact with alkaline fluids becomes blue, and on contact with acid fluids, red. Slips of paper, impregnated with litmus, form a valuable test for the acidity of the secretions and discharges.
A blue dyestuff made by treating coarsely powdered lichens, such as those of the genus Roccella, with ammonia.