DECOCTIONS are formed by subjecting the harder parts of plants, which are not easily penetrated by liquids, or are insoluble in water of a low temperature, to the process of boiling, generally in water, but sometimes in oil. By this means much of the substance is dissolved, and the active principles of the plant are imbi bed by the fluid, which is then used medicinally, either internally or externally. This method of extracting the medicinal properties of plants is inadmissible when their powers de pend upon any aromatic or volatile principle, such as essential oils, which are dissipated by the high temperature. In many other in stances this process is not only unnecessary, but injurious, being employed when infusion is sufficient, even when cold water is used for the purpose of infusion. The heat is apt to destroy the very principles which are desired to be obtained in many cases ; and this pro cess is most suitable to those substances which are both nutritious and medicinal, such as Iceland Moss. Many principles which are
dissolved by the water at a boiling tempera ture are deposited by it on cooling ; ou which account the liquid should always be strained while hot, and the contents of the bottle shaken up before each dose be poured out. Aromatic liquids are frequently added to the fluid after it is strained; or the aromatic sub stance may be placed at the bottom of the vessel into which the hot decoction is strained, and, after being allowing to infuse for a few hours, may then be applied to the use in tended. A better method however is to add some aromatic tincture after the straining, as the spirit retards the tendency to decomposi tion, while it imparts the flavour and qualities of the substances from which it had been pre, pared.
The decoctions employed in medicine are so numerous, that a mere list of them would occupy considerable space. They are pre pared from bark, berries, rind, mot, stems, seeds, pods, kernels, &c.