Litmus

Litmus is a well-known colouring matter, which is obtained from several lichens, but chiefly from Lecanora tartarea. The lichens are powdered and digested with ammoniacal fluids (urine, for example) till they undergo decomposition. Alum, potash, and lime are then added, and the mixture is allowed to stand till the maximum degree of colour is observed. Sand and chalk are added to give a due degree of solidity, and the mass is then dried in cubes, and is ready for the market. The exact nature of the changes which ensue is not altogether known; it is, however, certain that the pigment is originally red, and that it only becomes blue on the addition of alkalies or of lime. This blue colour is again changed into a red on the addition of a free acid. The use of litmus-paper and tincture of litmus for the purpose of detecting the acidity of fluids, &c. is known to every student of chemistry. See TEST-PAPERS.