Infusions are aqueous solutions of vegetable substances obtained without the aid of boiling. In this respect only do they differ from decoctions, in the manufacture of which boiling is resorted to. Infusions are prepared by digesting the vegetable substance (root, bark, &c.) in hot or cold water in a covered earthenware vessel. Cold water is preferable when the active principle is very volatile, or when it is desired to avoid the solution of some ingredient in the vegetable which is soluble in hot, but not in cold water. For example, in preparing the infusion of calumba cold water is preferable, because it takes up the bitter principle (which is the essential ingredient), and leaves the starch-matter undissolved. In most cases, however, boiling water is employed. Infusions are preferred to decoctions when the active principle volatilises at a boiling heat, as in the case of essential oils; or when ebullition readily induces some chemical change, as in the case of senna.
Infusions may also be prepared by Percolation (q.v.), a process which is extensively employed in the preparation of tinctures. When thus prepared they are less liable to decay than when prepared on the old system.
The fresh infusion, while possessing a finer flavour, is in danger of being superseded in pharmaceutical practice by the concentrated infusion. On account of the trouble and expense involved in making small quantities of the fresh preparations, recourse is frequently had to the concentrated ones, which, when diluted with seven times their bulk of distilled water, more or less represent the fresh article. Where the active principle is a volatile one it is very difficult to retain the full aroma in the concentrated state, and to this question much pharmaceutical attention has been turned. The concentrated infusions contain from 20 to 25 per cent. of alcohol, which is essential for their preservation. The simple infusions may be preserved for a short time by the addition of a trace of chloroform.