These are _watery_ solutions of vegetable matter, obtained by
maceration, either in cold or hot[536] water without the assistance of
ebullition. In selecting and conducting the operation, the following
general rules should be observed.
I. _Infusion should always be preferred to decoction, where the
medicinal 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._
II. _The temperature employed must be varied according to the
circumstances of each case;[537] an infusion made in the cold, is
in general more grateful, but less active, than one made with
heat._
III. _The duration of the process must likewise be regulated by the
nature of the substances, or the intention of the prescriber; 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 colouring, astringent,
and gummy parts._
Infusions are liable to undergo decompositions by being kept, and
therefore, like decoctions, they must be regarded as _extemporaneous_
preparations. Unless the dose of them be otherwise stated, it is
generally from f℥j to f℥ij.