1911 Encyclopædia Britannica/Litmus

LITMUS (apparently a corruption of lacmus, Dutch lacmoes, lac, lac, and moes, pulp, due to association with “lit,” an obsolete word for dye, colour; the Ger. equivalent is Lackmus, Fr. tournesol), a colouring matter which occurs in commerce in the form of small blue tablets, which, however, consist mostly, not of the pigment proper, but of calcium carbonate and sulphate and other matter devoid of tinctorial value. Litmus is extensively employed by chemists as an indicator for the detection of free acids and free alkalis. An aqueous infusion of litmus, when exactly neutralized by an acid, exhibits a violet colour, which by the least trace of free acid is changed to red, while free alkali turns it to blue. The reagent is generally used in the form of test paper—bibulous paper dyed red, purple or blue by the respective kind of infusion. Litmus is manufactured in Holland from the same kinds of lichens (species of Roccella and Lecanora) as are used for the preparation of archil (q.v.).