lituus

‖n.; pl. Litui [ L. ] 1. (Rom. Antig.) (a) A curved staff used by the augurs in quartering the heavens. (b) An instrument of martial music; a kind of trumpet of a somewhat curved form and shrill note. [ 1913 Webster ]

2. (Math.) A spiral whose polar equation is r2θ = a; that is, a curve the square of whose radius vector varies inversely as the angle which the radius vector makes with a given line. [ 1913 Webster ]

n. [ D. lakmoes; lak lacker + moes a thick preparation of fruit, pap, prob. akin to E. meat: cf. G. lackmus. See Lac a resinous substance. ] (Chem.) 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. [ 1913 Webster ]

Litmus paper (Chem.), unsized paper saturated with blue or red litmus, -- used in testing for acids or alkalies. [ 1913 Webster ]