Extracts, in a technical sense, are medicinal preparations of vegetable principles, got either by extracting these from the plants by means of a solvent or menstruum, and then evaporating the liquid down to about the consistency of honey, or by expressing the juice of the plants and evaporating; this last is properly inspissated juice. Extracts, therefore, contain only those vegetable principles that are either held in solution in the juices of the plants themselves, or are soluble in the liquid employed in extracting them, and at the same time are not so volatile as to be lost during evaporation. Now, as many extractive matters are more or less volatile, it makes a great difference whether the operation is conducted at a low or at a high temperature. Besides the loss of volatile constituents by prolonged or excessive heating, extracts become more or less changed and inert owing to the readiness with which vegetable principles are destroyed when exposed to heat and air. On this account it is usual to avoid evaporation as much as possible, and, where this is impracticable, evaporation in vacuo is resorted to. Extracts are called watery or alcoholic according as the menstruum employed is water or alcohol. Ether is also used in extracting. Different plants, of course, afford different extracts, some being of the nature of bitters, others being used as pigments, tannin, &c.
Liquid extracts are those which are not evaporated so far as to form a paste, and it is usual to make them of such a strength that one fluid ounce contains the active ingredients of one ounce by weight of the drug.