INFUSION, in chemistry, taken in its most general sense, consists in placing compound bodies in a liquor intended to be impregnated with some of their principles, either without heat, or with a heat less than that of the boiling liquor.
From this definition we may see that infusion is one of the principal operations of the analysis by menstruum, in the same manner in which decoction is, of which infusion may be considered as the first degree.
Infusion may be made in watery, spirituous, oily, acid, or alkaline liquors, according to the nature of the matters to be infused, and of the principles intended to be extracted. It is nevertheless practised upon vegetable matters only; and almost always for
the preparation of remedies, which are also called infusions.
Aromatic plants, and other odoriferous vegetable matters, are generally infused, when their odoriferous principle is to be preserved, in which their virtue consists, and which is at the same time so volatile as to be dissipated and lost by the heat of ebullition. To preserve still better these volatile principles, the smallest heat requisite for the extraction ought to be employed; and these infusions ought to be made in matrasses, or other vessels, which may be exactly closed.
The principles extracted by infusion are different according to the menstruum employed. Pure water may be impregnated with the odoriferous principle, or spiritus rector, with saline, saponaceous, mucilaginous, extractive principles. Spirit of wine dissolves the spiritus rector, the essential oils, those resinous matters the base of which is an oil of the nature of essential oils, and the greatest part of the saponaceous extractive matter. Those infusions made by spirit of wine are particularly called tinctures, especially in pharmacy. Oils dissolve the spiritus rector, and any oily matter. Acids and alkalis dissolve the earthy substances of vegetables, and almost all their other principles: but they disguise them, alter them, and change considerably their virtues by the combination. They are therefore not much used for these kinds of extractions.