This section is from the book "Dental Medicine. A Manual Of Dental Materia Medica And Therapeutics", by Ferdinand J. S. Gorgas. Also available from Amazon: Dental Medicine.
Litmus is a peculiar coloring matter obtained from various species of rocella, of the natural order Lichenes - the
Lichen order, which grow on Alpine or maritime rocks in various parts of the world. The varieties used for chemical purposes are obtained from the European and African coasts and Northern Europe. They yield coloring substances by the reaction of water, air, and ammonia. Litmus is prepared chiefly in Holland; the lichens are coarsely powdered and macerated for several weeks, with occasional agitation in a mixture of urine, lime and potassa or soda, fermentation ensuing; the mass becomes first red and ultimately blue, and is then mixed with calcareous or siliceous matter, to give it consistence. A tincture is formed from this product (litmus 1 part to distilled water 20 parts), and blue litmus paper is made by steeping unsized paper in the liquid and afterward drying it. The blue tincture is rendered deeper in color by indigo. Red litmus paper is prepared in the same manner, except that the tincture, into which the unsized paper is steeped, is reddened by the addition of sulphuric acid.
The chief use of litmus in medicine and dentistry is as a test of acids and alkalies ; blue litmus paper being turned red by acids, while red litmus paper is turned blue by alkalies. In dental practice litmus paper is employed to determine the nature of the oral fluids - whether acid or alkaline.
A single color, viz., purple, has recently been substituted for the ordinary blue and red litmus paper. This purple litmus paper turns red with acids, blue with alkalies. It is claimed to be much more delicate and convenient.