Various, Encyclopaedia Britannica, 11th Edition…
“ LITMUS (apparently a corruption of lacmus, Dutch lacmoes, lac, lac, and moes, pulp, due to association with “lit,” an obsolete word for dye, colour; the Ger. equivalent is Lackmus, Fr. tournesol) , a colouring matter which occurs in commerce in the form of small blue tablets, which, however, consist mostly, not of the pigment proper, but of calcium carbonate and sulphate and other matter devoid of tinctorial value. Litmus is extensively employed by chemists as an indicator for the detection of free acids and free alkalis. ”
