Filix. Fern Root. The dried rhizome of Aspidium Filix-mas; Lin. Syst., Cryptogamia filices; Male Shield Fern; indigenous; it should be collected in summer. Description. The central part, or caudex, is thick and cylindrical, with numerous leaf-stalks surrounding the axis; the spaces intervening between are covered with silky scales, and numerous radicles descending beneath them. The dried root is of a greenish brown colour externally, yellowish within, with a peculiar though slight odour, and a taste at first sweet, then bitter. The colour of the powder is yellowish-green, with a disagreeable odour and a nauseous, bitter, somewhat astringent taste.

Prop. & Comp. In addition to starch, gum, and salts, the male fern contains a volatile oil, resin, and a fixed oil. The active properties of the rhizome are soluble in ether; the etherial extract referred to below, commonly known as the oil of male fern, is of a dark colour, containing the volatile and fixed oil, resin, and colouring matter in solution.

Off. Prep. Extractum Filicis Liquidum. Liquid Extract of Fern Root. [Not officinal in U. S. P.] (Fern root, in coarse powder, two pounds; ether, eighty fluid ounces, or a sufficient quantity to exhaust the root. Prepared by percolation and subsequent evaporation or distillation of the ether.)

Therapeutics. Fern root is used as an anthelmintic, and acts apparently by killing the worms, and thus aiding their expulsion from the intestinal canal. Its use has been attended with much success in cases of tapeworm; it is said to be more useful against the Bothriocephalus latus than against the Taenia solium. It should be given on an empty stomach, and followed after an interval by some mild purgative.

Dose. Of the powder, 60 gr. to 180 gr.; of the liquid extract, 30 min. to 2 fl. drm.

Sub-Class II. Thallogenae. Lichenes.

Cetraria. Iceland Moss. Cetraria islandica; Lichen islandicus; Island Lichen or Moss; Liverwort; the entire lichen obtained in large quantities in Iceland, hence its name.

Desription. Iceland moss consists of a foliaceous thallus, the lobes irregularly subdivided, crisp, cartilaginous, brownish-white, paler beneath, fringed at the edges; bitter and mucilaginous.

Prop. & Comp. The soluble portion is taken up by boiling water. The decoction, on cooling, thickens, and deposits a gelatinous matter; this, when dried, forms a semi-transparent mass, insoluble in cold water, alcohol, or ether, but soluble in boiling water, and striking blue with iodine; this is named Lichenin. Iceland moss contains also & bitter principle, soluble in alcohol and ether, and readily in alkaline solutions, but sparingly so in water; this is crystallizable, and has acid properties; it is called cetraric acid.

Off. Prep. Decoctum Cetrariae. Decoction of Iceland Moss. (Iceland moss, well washed in cold water, one ounce; distilled water, thirty fluid ounces. Reduce to twenty by boiling.)

Therapeutics. Iceland moss, deprived of its bitter principle, is used by the natives of Iceland and Lapland as an article of diet. The decoction is demulcent and slightly tonic. The cetraric acid is said to have been useful in intermittents as a substitute for quinia.

Dose. Of the decoction, 1 fl. oz. to 2 fl. oz.

Lacmus. Litmus. Appendix B. A peculiar blue colouring matter, obtained from Roccella tinctoria, Lecanora tartarea, and other lichenous plants. Litmus is extensively prepared in Holland. Description. It occurs in small cakes, made up of a granular powder, of a fine blue colour.

Prop. & Comp. It is prepared by macerating the lichen for some time in water, rendered alkaline by lime and potash, and mixed with urine; a species of fermentation occurs: the mass becomes first red and afterwards blue; it is then removed and reduced to the proper consistence by sand, lime, etc.

Off. Prep. Tincture of Litmus. Appendix. (Made by digesting one ounce of powdered litmus in ten fluid ounces of proof spirit.)

Blue Litmus Paper. Appendix. (Made by steeping unsized paper in the tincture of litmus, and drying by exposure to the air.)

Red Litmus Paper. (Made by steeping unsized paper in the tincture previously reddened with a trace of sulphuric acid, and dried in the air.)

Uses. - Blue Litmus Paper is used as a test for acids, as any liquid having an acid reaction turns the colour of the paper from blue to red.

Red Litmus Paper, on the contrary, serves to indicate the presence of any substance having an alkaline reaction, which restores the original blue colour to the paper.

The Tincture of Litmus, when diluted with water, may be used as a test for the presence of acids.