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. 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.
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. 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.