Litmus

1. 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.
   
Litmus paper (Chem.),
unsized paper saturated with blue or red litmus, -- used in testing for acids or alkalies.