These preparations are obtained by evaporating the watery or spirituous
solutions of vegetables, and the native juices obtained from fresh
plants by expression, to masses of a tenacious consistence. The London
college does not arrange the extracts under the titles of _watery_ and
_resinous_, which is the arrangement of the Edinburgh Pharmacopœia, nor
under those of simple and resinous, which is the division observed in
that of Dublin, but rejecting all _specific_ distinctions, includes,
under the _generic_ appellation of extract, both the species, as well as
all the _inspissated juices_. Since however the former of these
arrangements will afford greater facilities for introducing the
observations which it is my intention to offer, it is retained in this
work.
The chemical nature of extracts must obviously be very complicated and
variable, depending in a great degree upon the powers of the _menstruum_
employed for their preparation; although Fourcroy and Vauquelin
considered that _one peculiar_ principle was the basis of them all,
which they called _Extract_, _Extractive_, or the _Extractive
Principle_. It is distinguished by the following characters, _viz_.
It has a strong taste, varying in different plants; it is soluble in
water, and in alcohol when it contains water, but is quite insoluble in
_absolute_ alcohol and æther; its aqueous solution soon runs into a
state of putrefaction; by repeated solutions and evaporations, or by
long ebullition, it acquires a deeper colour, and in consequence of its
combination with oxygen it becomes insoluble and inert, a fact which is
of extreme importance as it regards its pharmaceutical relations; it
unites with alumine, and if boiled with its salts, precipitates it,
hence wool, cotton, or thread, impregnated with alum, may be dyed of a
fawn-colour by _extractive_; its habitudes with alkalies are very
striking, combining most readily and forming with them compounds of a
brownish yellow colour, which are very soluble in water; if to a
colourless and extremely dilute solution of extractive, an alkali be
added, a brown or yellowish tint is immediately produced, so that under
certain circumstances I have found an alkali to be a serviceable test in
detecting the presence of extractive matter. The usual brown hue of the
_liquor ammoniæ acetatis_, is owing to the action of the ammonia upon
traces of vegetable extractive contained in the distilled vinegar.
Much confusion has arisen from the word _extract_ having been employed
in this double meaning,—_chemically_ to express a peculiar vegetable
proximate principle, and _pharmaceutically_ to denote any substance
however complicated in its nature, which has been obtained by the
evaporation of a vegetable solution or a native vegetable juice. It is
in the latter sense that it is to be understood in the present article.
The different proximate principles of vegetable matter undergo various
and indefinite changes with such rapidity, when acted upon by heat, that
the process of _extraction_ must necessarily more or less impair the
medicinal efficacy of a plant, and not unfrequently destroy it
altogether, and hence, says Dr. Murray, “with the exception of some of
the pure bitters, as gentian, or some of the saccharine vegetables, as
liquorice, there is no medicine perhaps but what may be given with more
advantage under some other form;” this however is not exactly true, for
when care is taken in the preparation, we are thus enabled to
concentrate _many_ very powerful qualities in a small space, and the
process lately adopted of evaporating the solutions by the aid of steam,
contributes very materially to obviate the failures which so frequently
occurred from a too exalted temperature. There is, for instance, great
reason to suppose that the black colour which so often characterises the
extracts of commerce, is frequently owing to the decomposition and
carbonization of the vegetable matter; the colour therefore of an
extract becomes in some degree a test of its goodness. I have lately
examined the extracts of commerce with some attention, and I find the
presence of iron by no means an uncommon circumstance; when thus
contaminated they afford a very dirty coloured solution, which rapidly
becomes darker on exposure to air. The extracts mentioned in the preface
as made by Mr. Barry, by _evaporating in vacuo_, deserve the attention
of the profession; the principle is without doubt well calculated to
secure the active matter of the plant from those changes to which it is
constantly liable during the ordinary operation of inspissation. The
extracts, thus prepared, are certainly more powerful in their effects,
and some few of them appear also to possess properties which are not to
be distinguished in the Extracts of Commerce; those of narcotic plants,
as _Hemlock_, _Hyoscyamus_, _&c._ are decidedly more efficacious; where
the practitioner directs their use, he should, to prevent any mistake,
add the words _in vacuo præp._ as in _Formula 4_; for on account of the
difference in the strength of these preparations, and of those prepared
by the ordinary method, they cannot be indiscriminately employed. Dr.
John Davy, at my request, has made trial of these extracts in the
Military Hospital at Fort Pitt, and as his results coincide with those
obtained in my own practice, I shall relate, under the history of each
Extract, the comparative conclusions which have been obtained.