These preparations are generally _extemporaneous_, in which different
ingredients are mingled together in the liquid form, or, in which solid
substances are diffused through liquid, by the medium of mucilage or
syrup: for prescribing mixtures the following general rules may be laid
down.
I. Substances which are capable of entering into chemical combination,
or of decomposing each other, ought not to be mixed together,
unless it be with a view of obtaining the new products as a
remedy.
II. Transparency is not a necessary condition,[302] and hence
insoluble powders may be advantageously introduced into mixtures,
if the following precautions be observed.