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Lacmus.—Litmus.

A peculiar blue coloring matter obtained from Roccella tinctoria, Acharius, and other lichens.
Nat. Ord.—Lichenes.
COMMON NAMES AND SYNONYMS: Litmus, Turnesole, Tournesol, Lacca caerulea, Lacca musica.

Botanical Source.Roccella tinctoria, or Orchilla weed, is a small, dry lichen, with a rounded, glaucous, nearly erect thallus, forked and subdivided into numerous branchy, roundish, gray, yellowish, or brownish threads; the apothecia are scattered, red and elevated; the disks are flat, caesius, pruinose, and as broad as the border.

History, Preparation, and Description.—Roccella tinctoria is found on the maritime rocks of the eastern Atlantic Islands, as the Azores, Canaries, etc.; the western coast of South America, south of England, Portland Islands, Scilly Islands, and various other countries. Litmus was formerly obtained from this plant alone, but other lichens have now in a great measure supplanted it, as the Roccella fuciformis, or Angola weed, from Angola and Madagascar; the Lecanora tartarea, or Tartarean moss, from Norway and Sweden; the Variolaria dealbata, from Auvergne and the Pyrenees, and some others.

LACMUS, or Litmus, was formerly prepared only in Holland, but at present is manufactured from various lichens in Italy, France and Britain. It is made "by macerating powdered lichen for several weeks, with occasional agitation, in a mixture of urine, lime, and potashes, in a wooden trough under shelter. A kind of fermentation takes place, and the lichen becomes first reddish, and subsequently blue. When the pulp has acquired a proper blue color, it is placed in brass or steel molds, and the cakes thus obtained are subsequently dried. An addition of aqueous ammonia answers the same purpose as that of urine in the above mixture." Litmus is imported in the form of small, rectangular, light and friable cakes of an indigo-blue color. Examined by the microscope, we find sporules and portions of the epidermis, and mesothallus of some species of lichen, moss, leaves, sand, etc. Its odor is that of indigo and violets" (P.). Litmus is usually mixed with chalk or gypsum in order to form it into cakes.

Chemical Composition.—The chromogenic bodies in the lichens mentioned are crystallizable phenols and phenol acids. To the latter class belong lecanoric acid, discovered in 1842 by Schunck, with which beta-orsellinic acid of Stenhouse (1848) is identical; erythrinic and roccellic acids (Heeren, 1830), usnic acid, evernic acid, etc. (For details regarding these acids, see Husemann and Hilger, Pflanzenstoffe, Vol. I, p. 303.) They are in themselves colorless, but become converted into coloring matters by the joint action of water, air, and ammonia.

Lecanoric acid (C16H14O7, Gerhardt and Hesse), crystallizes in white stellate needles soluble in 2500 parts of boiling water with acid reaction, more soluble in hot acetic acid, also soluble in alcohol and ether. Its melting point is 153° C. (307.7° F.), and it forms crystallizable salts with acids. Heated with water, alcohol or aqueous alkalies, lecanoric acid adds one molecule of water and is converted into crystallizable orsellinic acid (C8H8O4). This when continuously boiled with water, loses carbonic acid and forms orcin or dihydroxy-toluene (C7H8O2, or C6H3.CH3.[OH]2), which is also obtained by dry distillation of lecanoric acid. Orcin is the chromogene body proper of this group. It forms colorless needles of sweetish, nauseating taste, is easily soluble in water, alcohol, and ether; ferric chloride produces with it a violet coloration. Exposed to light and air it turns reddish. In alkaline solution it changes to red or brown upon exposure to the air. In contact with moist air containing ammonia, it is converted into orcein (C7H7NO3), a brown substance soluble in aqueous alkalies with purple-red color, being precipitated from this solution by acids. Orceïn is the coloring principle of orseille or archil (see below).

Orcin, when exposed to moist and ammoniated air in the presence of alkali carbonates, is converted into azolitmin, the blue coloring matter of litmus. The coloring bodies in litmus, according to Dr. Kane (Chem. Centralblatt, 1841, p. 567; also see Pereira, Mat. Med., edition by J. Carson, 1846), are: (1) A purplish-red semifluid material, erythrolein. It is soluble in ether and alcohol, and yields with ammonia a rich purple solution; (2) a crystalline body of a light red color, erythrolitmin, nitrogen free, soluble in alcohol, but sparingly so in ether and water, and striking blue with ammonia; (3) a brownish-red, noncrystalline body, the chief coloring principle of litmus, named azolitmin (C7H7NO4); it turns blue with alkalies, is insoluble in alcohol and ether, and sparingly soluble in cold water; (4) a small amount of a bright-red body, spaniolitmin, which is colored blue by alkalies; water dissolves it sparingly; insoluble in alcohol and ether.

Action and Uses.—Orcin resembles resorcin in its effects upon skin diseases. It is said to be a decided antiseptic, and to cause death in toxic doses by paralyzing the heart-muscle.

Litmus is employed in urinary, chemical, and pharmaceutical analysis, and is a familiar test for free acids and alkalies. The acids impart a red color to blue litmus; the alkalies restore the original blue color to the reddened litmus. Carbonate of calcium dissolved in water by a considerable excess of carbonic acid, will also restore the blue color of reddened litmus. It is used either in infusion, or in the form of litmus paper. The infusion, sometimes erroneously called tincture of litmus, is made by adding 1 part of litmus to 25 parts of distilled water, to which, for the purpose of preserving it about 1/10 part of spirit or alcohol may be added.

Litmus Paper.—BLUE LITMUS PAPER (Charta Exploratoria Caerulea) is prepared by dipping strips of paper in a clear and strong infusion of litmus, or by rushing the infusion over the paper. White unsized paper is the best for this purpose; and the infusion may be made by adding 1 part of litmus to 6 parts of boiling water. Good litmus paper should be of uniform color, neither too light nor too dark, and when carefully dried, should be kept in well-stopped vessels in a dark place; when it has a purplish tint, it is a more delicate test for acids than when pure blue. An extremely delicate test-paper may be made by almost neutralizing the alkali contained in the litmus; thus: Divide the filtered infusion of litmus into two parts; stir one portion with a glass rod which has been previously dipped into very dilute sulphuric acid, and repeat this until the liquid begins to look reddish; then add the other portion of liquid, and immerse the paper in it (P.).

RED LITMUS PAPER (Charta Exploratoria Rubefacta) is best prepared by dipping the blue paper in a very dilute acetic or hydrochloric acid, merely acid enough to redden it.

Related Products.—The following pigments are produced from the same plant which yields lacmus. They are similarly prepared, excepting that alkalies, caustic soda, or potash, are not added to the ammoniacal mixture (see Chemical Composition above).

ORCHIL, or ARCHIL, is used for dyeing, coloring, and staining. There are two kinds, called blue orchil and red orchil, which differ merely in the degree of their red tint. They are deep-reddish purple liquids, or pasty masses, with an ammoniacal odor. Orchil is prepared by steeping the lichens in an ammoniacal liquor, in a covered wooden vessel.

Preparation: Tincture of cudbear - Compound tincture of cudbear

CUDBEAR, or Persio, is obtained by the same process as orchil, and when the proper purplish-red color has been developed, the mixture is dried in the air and reduced to fine powder. It is used as a dye, and sometimes as a test for acids and alkalies. (An interesting article on the manufacture and chemistry of orchil, cudbear, and litmus, by Dr. Crace-Calvert, is to be found in Pharm. Jour. Trans., Vol. II, 1871, pp. 514 and 535.)


King's American Dispensatory, 1898, was written by Harvey Wickes Felter, M.D., and John Uri Lloyd, Phr. M., Ph. D.



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