Litmus. A peculiar colouring matter obtained from Roccella Tinctoria and other colorific Lichens. It is officinal solely as a chemical test. Acids turn blue Litmus paper red, but the former colour is restored by solutions of the alkalies. It is an easy and important test for ascertaining the acidity, or otherwise, of the urine and secretions.
Offic. Prep. 1. Tincture of Litmus (Powdered Litmus oz. j.; Proof Spirit fl. oz. x. Prepared by maceration).
2. Blue Litmus Paper (made by steeping unsized paper in tHe Tincture, and drying by exposure to air).
3. Red Litmus Paper (made as the above, the Tincture having been previously reddened by the addition of a very minute quantity of Sulphuric Acid).