Those only should be decocted whose principles consist of mucilage, gum, or resin, and require boiling to extract them.
The compact resinous woods, roots and barks yield their virtues most freely while fresh. Dry, they yield little to cold or moderately warm water, and require it to be boiling. By this process the grosser, more fixed saline and mucilaginous parts are dissolved, the resinous melted out, and the volatile dissipated.
Infusions.—These are watery solutions of vegetable matter obtained by maceration, either in hot or cold water, with the assistance of ebullition. In selecting and conducting the operation, the following general rules should be observed:
"1st. Infusion should always be preferred before decoction, where the virtues of the vegetable substance reside in volatile oil, or in principles which are easily soluble, whereas, if they depend upon resino-mucilaginous particles, decoction is an indispensable operation.
"2d. The temperature employed must be varied according to the circumstances of each case, and infusion made with cold is in general more grateful but less active than one made with heat.
"3d. The duration of the process must likewise be regulated by the nature of the substances; for the infusion will differ according to the time in which the water has been digested on the materials; thus the aroma of the plant is first taken up, then in succession the coloring, astringent, and gummy parts.
Decoctions.—"These are solutions of the active principles of vegetables, obtained by boiling them in water.
"1st. Those principles only should be decocted whose virtues reside in principles which are soluble in water.
"2d. If the active principle be volatile, decoction must be an injurious process; and if it consists of extractive matter, long boiling, by favoring its oxidizement, will render it insipid, insoluble, and inert.
"3d. The substances to be decocted should be previously bruised or sliced, so as to expose an extended surface to the action of the water.