Litmus

a blue coloring matter procured from certain lichens. It is prepared chiefly in Holland. The lichens are ground fine and allowed to ferment. Alum, potash, and lime are then added. Paper tinged blue by litmus dye is used in the laboratory as a test for acids and alkalies. If a liquid turns litmus-paper red, it contains an acid. If a strip of paper previously reddened in an acid be turned blue again, the liquid contains an alkali.