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. B. _The Developement of Active Elements._ The accomplishment of such an effect is in many instances the sole object of a pharmaceutical process. It is thus that we obtain pure _Citric acid_ from the juice of the Lemon; _Tartaric acid_, from Cream of Tartar; _Benzoic acid_, from the resinous substance known by the name of _Gum Benzoin_; upon the same principle, the _Muriatic_ and _Nitric_ acids are elicited from the saline compounds in which they exist. _Ammonia_, in its pungent form, is developed from its inodorous _Muriate_; and the fixed alkalies are obtained in their caustic state, from the comparatively mild _carbonates_ in which they naturally exist. But a more striking and instructive instance of the effect of chemical action, in developing an active, or useful principle, cannot perhaps be selected than that of the well known stimulant Plaister, composed of _Muriate of Ammonia_, _Soap_, and _Lead Plaister_, in which the alkali of soap enters into combination with the muriatic acid, when the _Ammonia_, upon which the virtues of the plaister solely depend, is slowly disengaged in the form of gas, producing a powerfully rubifacient and stimulant effect: the “_Cataplasma Fermenti_,” or “_Yeast Poultice_,” is indebted for its antiseptic properties to a similar agency, for they do not depend upon any virtue in the ingredients themselves, but upon their decomposition, and the consequent developement of an active element, which is _Carbonic Acid_. The practitioner unacquainted with the _modus operandi_ of these combinations, would inevitably fall into an error by which their efficacy must be lost; he would hardly apply them as soon as they were formed, nor would he be aware of the necessity of repeating them at short intervals.