Note:  Do not rely on this information. It is very old.


Litmus, a blue colouring-matter prepared in Holland from the archil-producing lichens Roccellce and Lecanorcc, with carbonate and sulphate of lime, as small blue cakes. It is almost exclusively used as a chemical test - a neutral aqueous solution staining bibulous paper violet, the least trace of free acid changing it to red, and free alkali changing it to blue.