These are solutions of the active principles of vegetables obtained by
boiling them in water. To decide upon the expediency of this form of
preparation, in each particular case, requires a knowledge of the
chemical composition of the substance in question. In conducting the
operation, the following rules must be observed.
1. _Those substances only should be decocted, whose medicinal powers
reside in principles which are soluble in water._
2. _If the active principle be volatile, decoction must be an
injurious process; and, if it consist of extractive matter, long
boiling, by favouring its oxidizement, will render it insipid,
insoluble, and inert._
3. _The substances to be decocted should be previously bruised, or
sliced, so as to expose an extended surface to the action of the
water._
4. _The substances should be completely covered with water, and the
vessel slightly closed, in order to prevent, as much as possible,
the access of air: the boiling should be continued without
interruption, and gently._
5. _In compound decoctions, it is sometimes convenient not to put in
all the ingredients from the beginning, but in succession, according
to their hardness, and the difficulty with which their virtues are
extracted; and if any aromatic, or other substances containing
volatile principles, or oxidizable matter, enter into the
composition, the boiling decoction should be simply poured upon
them, and covered up until cold._
6. _The relative proportions of different vegetable substances to
the water, must be regulated by their nature; the following general
rule may be admitted; of roots, barks, or dried woods, from ʒij to
ʒvj to every pint of water; of herbs, leaves, or flowers, half that
quantity will suffice._
7. _The decoction ought to be filtered through linen, while hot, as
important portions of the dissolved matter are frequently deposited
on cooling; care must be also taken that the filtre is not too fine,
for it frequently happens, that the virtues of a decoction depend
upon the presence of particles which are suspended in a minutely
divided state._
8. _A decoction should be prepared in small quantities only, and
never employed, especially in summer, forty-eight hours after it has
been made. It should be considered as an extemporaneous preparation,
but introduced into the pharmacopœia for the purpose of convenience,
and for the sake of abridging the labour of the physician._
It is very important that the water employed for making decoctions,
should be free from that quality which is denominated _hardness_.
The officinal decoctions may be classed into simple and compound
preparations.