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Litmus

(Science: chemistry) 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 colouring matters related to orcin and orcein.

litmus is used as a dye, and being turned red by acids and restored to its blue colour by alkalies, is a common indicator or test for acidity and alkalinity.

(Science: chemistry) litmus paper, unsized paper saturated with blue or red litmus, used in testing for acids or alkalies.

Origin: D. Lakmoes; lak lacker _ moes a thick preparation of fruit, pap, prob. Akin to E. Meat: cf. G. Lackmus. See lac a resinous substance.