A dyestuff extracted from certain lichens (Roccella tinctoria
, Lecanora tartarea
, etc.), as a blue amorphous mass which consists of a compound of the alkaline carbonates with certain coloring matters related to orcin and orcein. [1913 Webster
" Litmus is used as a dye, and being turned red by acids and restored to its blue color by alkalies, is a common indicator or test for acidity and alkalinity."