LITMUS (German, Lackmus·, French, Tournesoι) t 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 carbonate and sulphate of lime and other matter devoid of tinctorial value. Litmus is never used as a dye, but is extensively employed by chemists as a reagent for the detection of free acids and free alkalies. 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 RocceUa and Lecanora) as are used for the preparation of Archil <2∙ v ∙)∙

What orcein is to archil, a substance “azolitmin,” which Kane extracted from it, seems to be to litmus. At any rate, Kane’s analysis supports the idea of its being formed from orcin, thus:—

C 7 H 8 O 2 + NH 3 + O 3 =H 2 O + C 7 H 9 NO 4 . Orcin. Azolitrnrn.

A solution of this substance when treated with nascent hydrogen loses its colour. So does litmus solution when left to itself in closely stoppered bottles. When pi eserved in contact with air it retains its colour.