These are solutions of vegetable matter, obtained by maceration either in cold or in boiling water. As in the case of decoction, the substance must be sliced or bruised if in a recent state, or pulverized if dry, in order to expose a large surface to the action of the menstruum. The term infusion, in pharmaceutical language, is confined to watery solutions.

The substances which water, without the aid of boiling, can extract from vegetable matter submitted to its action, are gum, mucus, extractive, tannin, the bitter and narcotic principles, gum-resin, volatile oil, acids, and alkalies; a range, which includes most of the principles on which the medicinal properties of plants depend. These principles, also, are less liable to be altered by infusion than by decoction; and, consequently, this form of preparation is to be preferred in every instance to which it is applicable. The strength and quality of the infusions are varied by the degree of temperature of the water: those made with hot water being necessarily stronger; but, particularly in the case of bitters, cold infusions are more grateful.

In making infusions, when heat is required, the vessel is to be placed near the fire, so that the temperature of the water may be kept up to the necessary point for a sufficient length of time to produce the effect intended. Perhaps it might be an advantage, were the external surface of infusion-pots covered with a metallic coating, and polished; by which, as the heat would be much more slowly radiated than from the vessels usually employed, the effect of it would be more uniform and certain in promoting the solvent powers of the water.

Infusions, like decoctions, are liable to undergo spontaneous decomposition, if kept even for a few days; and therefore the London College has properly directed a pint only to be made at one time: thus regarding them as extemporaneous preparations.