Infusions are solutions of vegetable principles in water. The addition of any alcoholic mixture is only made in cases where the medicinal action of the liquor itself is desired, or to act as a preservative. Clear, soft water, as river, rain, or distilled water should be used in the preparation of infusions; hard, or limestone water, from springs or wells, or water holding saline substances in solution, is unfit for this purpose, as such water is apt to occasion precipitates. Drugs containing volatile active constituents, or which are deteriorated by a temperature somewhat elevated, or which contain a principle not desired, and which is not readily dissolved by water at a low degree of heat, are better made into infusions by cold instead of hot water.
Infusions form a very expeditious and convenient mode of exhibiting many medicines, as the most of them readily yield their active constituents in this way without requiring to be very finely divided. The principal objection to them is the difficulty of keeping them for any length of time, in consequence of which they require to be prepared off-handed, and in limited quantity at a time. Mugs containing a movable diaphragm are now much in use for the preparation of infusions. The diaphragm extends to one-third or one-half of the depth of the mug, and contains the vegetable remedy, while the jar is filled with hot or cold water as may be required. A constant circulation is kept up in the fluid by the increased density of the impregnated water carrying it to the bottom, while its place is occupied by the less impregnated fluid, and this continues until the remedy is exhausted of its active soluble principles.
In making infusions with boiling water, starch and other principles are often taken up, whose presence disposes to acidity or moldiness, or perhaps favors reactions which materially impair the infusions; on this account percolation by cold water is often preferable, as it avoids these inconveniences, beside which these infusions have a less tendency to spoil than those made at a boiling temperature. The process of percolation or displacement by cold water, affords infusions of very great strength, and is preferred to any other mode; it requires, however, that the articles should be more finely powdered, as a general thing, than is customary in preparing infusions in the ordinary way. When of too great strength, the infusion may be reduced by dilution with water. Very excellent infusions may be prepared with many medicinal herbs, roots, or barks, by percolating with a fluid composed of 3 parts of water and 1 part of glycerin.
Infusions are better when prepared in glazed earthenware or porcelain vessels fitted with covers, than when prepared in metallic vessels, on account of a liability to chemical alteration from metallic influence, and which frequently impairs the preparation. Infusions containing acids, or saline substances, should always be prepared and kept in glass or china vessels.
In the preparation of infusions, the reactions of agents should always be kept in view. Thus, infusion of chamomile flowers yields precipitates with nitrate of silver, sulphate of iron, gelatin, yellow Peruvian bark, tincture of chloride of iron, corrosive sublimate, and the acetates of lead. Infusion of horseradish undergoes rapid decomposition, and is precipitated with acetate of lead, infusion of galls, nitrate of silver, corrosive sublimate, and the alkaline carbonates. Infusion of cloves is precipitated by the soluble salts of antimony, zinc, iron, silver, lead, and by lime-water. Infusion of cascarilla is precipitated by infusion of galls, acetates of lead, sulphates of zinc and iron, nitrate of silver, and lime-water. Infusion of yellow Peruvian bark is incompatible with potassium, sodium, and ammonium hydroxides and carbonates, lime, magnesia, tannic and gallic acids, and vegetables containing these acids, tartaric acid, oxalic acid, and the soluble tartrates and oxalates. It also affords precipitates with other agents, which, however, do not always injure its efficiency or active principle, as corrosive sublimate, arsenous acid, tartar emetic, gelatinous solutions, soluble salts of iron, silver, and zinc, and many vegetable solutions, as those of cloves, chamomile, calumba, cascarilla, galls, horseradish, catechu, digitalis, senna, orange-peel, rhubarb, valerian, and simaruba. Infusions of senna, gentian, rhubarb, and calumba, are better made with cold water. When boiling water is added to calumba it takes up the starch, and the infusion spoils rapidly. It should be made with cold water, then boiled, and filtered to separate albuminous matter. Infusion of digitalis is precipitated by acetate of lead, sulphate of iron, and infusion of cinchona (Lond.—Phillips, Pharm. Jour. and Trans., 1855, Vol. XIV, pp. 339, 438, 439, 403, 486).
As nearly all vegetable medicines are occasionally administered in the form of infusion, it would be useless to enter into an especial relation of them, further than already explained in the above general rules. They are more commonly prescribed as secondary or auxiliary measures, and are left for the nurse or family to prepare. However, there are a few compound infusions, some of which are of a spiritous nature, which it may be advisable to describe on account of their extensive employment and superior efficiency in the diseases for which they are recommended.
The U. S. P. general method is as follows: "An ordinary infusion, the strength of which is not directed by the physician, nor specified by the Pharmacopoeia, shall be prepared by the following formula: Take of the substance, coarsely comminuted, fifty grammes (50 Gm.) [1 oz. av., 334 grs.]; boiling water, one thousand cubic centimeters (1000 Cc.) [33 fl℥, 391♏]; water, a sufficient quantity to make one thousand cubic centimeters (1000 Cc.) [33 fl℥, 391♏]. Put the substance into a suitable vessel provided with a cover, pour upon it the boiling water, cover the vessel tightly, and let it stand for half an hour. Then strain, and pass enough water through the strainer to make the infusion measure one thousand cubic centimeters (1000 Cc.) [33 f1℥, 391♏]. Caution.—The strength of infusions of energetic or powerful substances should be specially prescribed by the physician "—(U. S. P.).
We give place to formulas for several infusions according to the British Pharmacopoeia, and a number official in the U. S. P., 1870.
INFUSUM EUPATORII (U. S. P., 1870), Infusion of thoroughwort.—Dried boneset, or thoroughwort, 1 troy ounce; boiling water, 1 pint. Macerate 2 hours. Strain. Dose, as an emetic, take freely in tepid condition; as a tonic, use cold in 1 or 2 fluid-ounce doses.
INFUSUM PICIS LIQUIDAE (U. S. P., 1870), Infusion of tar, Tar water.—Tar, 1 pint; water, 4 pints. Mix, agitate frequently throughout 24 hours, pour off infusion, and filter. Dose, as a diuretic, 1 or 2 pints daily. Locally, as a lotion in skin diseases, and as a bladder-wash in chronic cystitis.
INFUSUM SPIGELIAE (U. S. P., 1870), Infusion of spigelia.—Spigelia, ½ troy ounce; boiling water, 1 pint. Macerate 2 hours. Strain. Dose, for young child, ½ to 1 fluid ounce, night and morning; for adult, 2 to 6 fluid ounces.
INFUSUM SALVIA (U. S. P., 1870), Infusion of sage.—Sage, ½ troy ounce; boiling water, 1 pint. Macerate ½ hour. Strain. Dose, 1 fluid ounce. Valuable in spermatorrhoea and night-sweats. Locally, as a mouth-wash, or vehicle for other topical agents.