Dictionary.net

Lacmus, a blue coloring matter obtained from Roccella tinctoria and other species of lichens; it is reddened by acids and turned blue again by alkalies.

A blue pigment, which reddens when dipped into acids.

A purple dye obtained from certain lichens; known also as turnsole.

A purple colouring matter prepared from certain lichens.

A blue stain from lichens, turned red by acids.

The coloring matter obtained from the species of Rocella, Variolaria, Leconora, and similar lichens. It is used chiefly as a test for acids and alkalis, being colored red by the former and restored to its original color by the latter.

n. [German] A purple dye obtained from a lichen, and firom a kind of spruge common in the south of Europe— it turns blue with alkalies and red with acids.