It is not necessary to extend our researches beyond the range of the
Pharmacopœiæ,[265] to collect a variety of interesting and important
examples, in illustration of this division of our subject; if we require
a striking example of the agency of chemical combination in destroying
the identity of the original constituents, and of giving origin to a
compound of new powers, it may be exemplified by the well known instance
of _Sulphate of Potass_, a substance possessing but a weak affinity for
water, and exerting but little energy upon the animal œconomy; whereas
the two ingredients of which it consists are distinguished for the
extreme eagerness with which they unite with water, and for the caustic
activity which they display in their action upon animal matter.
Under this head the class of metals will also present itself to our
consideration, all the individuals of which, with the exception perhaps
of iron, are perfectly inert and harmless; even arsenic, lead, copper,
and mercury, which in certain states of combination constitute some of
the most virulent of known substances, exert no action upon the living
system, unless they be in union with some other body; but when so
united, how valuable do they become, and what various medicinal effects
may they not be made to produce.
The _Acetic Acid_ and _Ammonia_ become neutralized by combination with
each other, affording a compound of new virtues. _Sulphate of Zinc_, and
_Acetate of Lead_, when mixed together in solution, decompose each
other, and the _Acetate of Zinc_ which is formed, affords a more
valuable remedy than either of the former salts, as an application in
ophthalmia. The “_Mistura Ferri Composita_” of our Pharmacopœia offers
another example of the same chemical resource. I also beg the reader to
refer to the construction of _Formula 82_, which presents an instance of
a purgative draught being produced by combination, in which the original
properties of every element are entirely changed. See also _Formula 87_,
the chemical actions of which are more complicated, but no less
instructive than the preceding one; the ingredients of the formula are
the _Carbonates of Soda_ and _Magnesia_,—_Sulphate of Iron_,—_Diluted
Sulphuric Acid_, and _Water_—and when mixed together, the following
decompositions would appear to take place; the free _Sulphuric Acid_,
together with that which exists in the _Sulphate of Iron_, being just
sufficient to decompose the _Carbonates of Soda_ and _Magnesia_, forms
two neutral _Sulphates_ (viz. _Sulphates of Soda_ and _Magnesia_,) and
thereby disengages a volume of _Carbonic Acid gas_, which not only
increases the purgative operation of the new saline compounds, but, by
its excess, holds in solution the _Carbonate of Iron_, which is formed
by the decomposition of the _Sulphate_, and which in that state displays
an effect powerfully tonic.
Before we quit the consideration of medicinal compounds as the results
of chemical action, it is expedient to remind the practitioner of the
essential difference between _Mixture_ and _Combination_, a difference
which affects the medicinal virtues no less than the chemical characters
of bodies; it is determined by ample experience, that substances will
produce effects upon the living system when presented in a state of
simple mechanical mixture, very different from those which the same
medicines will occasion when they are combined by the agency of chemical
affinity, as is well exemplified in the comparative effects of alcohol
as existing in ardent spirits, and in wine (see _Vinum_); or in the
relative powers of _Mercury_ in the _Unguentum Hydrargyri_ of the London
College, and the _Unguentum Oxidi Hydrargyri cinerei_ of the
Pharmacopœia of Edinburgh, (see _Unguent. Hydrargyri_); the former of
which is a true chemical compound, whereas the latter is a simple
mixture of its ingredients.