LITMUS (corruption of 1.emus.. from Inc, from Pers. b(1,-. lac, front Skt. lav-in4‘et, from laksa, hundred tlotusand: so ealled from the great of the insect in a single nest). A coloring matter of vegetable origin, indispensable in chemical laboratory work. It permits the chem ist readily to ascertain whether a substance sub mitted to him for examination is acid or alkaline, and is therefore employed in every step of ana lytic determinations. Acids turn blue litmus red; alkaline solutions, on the contrary, turn red litmus blue. In laboratories litmus is used either in the form of a solution, or in the form of test paper impregnated with it. Litmus is manufac tured principally in Holland, from Lecanora tar tarca and other lichens. The process employed is similar to that used in the manufacture of archil (q.v.) ; the lichens are ground into a pulp with water, and potassium carbonate and ammonia are added. Under the action of these substances and
of atmospheric air, the mass gradually assumes a blue color, owing to the formation of the color ing principles of litmus. Chalk or gypsum is then added, to render the mass thick enough to be formed into rectangular cakes, which are dried and brought into the market. The chemistry of is but little known; the following, how ever, have been shown to be its chief coloring principles, viz. azolitmin. whose brownish-red color is turned blue by ammonia; purplish-red erythrolein, which, by the action of alkalies, is turned purple; and erythrolitmin, whose red color is changed by alkalies to blue. These coloring substances exist • in commercial litmus in the form of lakes. i.e. combined with metallic hydrox ides.