Litmus (Ger. Lackmus), a blue coloring matter prepared from rocella tinctoria and related lichens. The various species of rocella are found upon the rocks of the coast of the Mediterranean and other warm countries; they are known in commerce as archil or orchella weed, and are designated by the names of the countries which produce them. (See Archil.) They are used for dyeing, and when prepared by fermentation with potash or soda, they produce litmus. The lichen is macerated for several weeks in water, to which urine, lime, and potash have been added. Exposed to the air, the mixture undergoes a fermentation, becoming at first reddish, and ultimately blue. When the pulpy mass has assumed the proper blue color, it is pressed into a mould to form small rectangular cakes, plaster or clay being sometimes added to increase the bulk. As found in commerce, litmus is in small squares, light, friable, of the color of poor indigo, and of an odor that has been compared to that of violets. It consists of several peculiar coloring matters, together with the remains of lichens and such earthy substances as may have been added.

The sole use of litmus is as a test for acids and alkalies, it being reddened upon contact with an acid, and the blue color being at once restored by an alkali. - Litmus paper is the form in which litmus is used as a test. To prepare this, a strong infusion of litmus is made with boiling water; this is divided into two parts; dilute sulphuric acid is gradually added to one of these portions until it assumes a red color, after which the two portions of liquid are mixed. As from the manner of preparation litmus is likely to contain an excess of alkali, this method is adopted to render it as nearly neutral as may be, and thus increase its sensitiveness. Unsized paper is dipped into this infusion, and after it is dry cut in strips of convenient size and preserved in a stoppered bottle to prevent access of acid fumes. The paper thus prepared is a very sensitive test for acids. Another portion is prepared by adding acid to the blue infusion until it is red, and dipping paper in this; this is red litmus paper, and serves as a test for the presence of alkalies, which restores the normal blue color.