The local action of these stimulants would appear to be placed under the dominion of the same law, and perhaps the origin of the custom, so long observed, of mixing together the varieties of snuff, may thus receive a plausible and philosophical explanation; certain it is that by such combination the harsh pungency of each ingredient will be diminished, whilst the general potency of the application, in exciting the nerves, will be increased, and rendered more grateful; the same principle will direct the formation of safe and efficient plaisters and lotions; the _Emplastrum Cumini_ of the London, and the _Emplastrum Aromaticum_ of the Dublin Pharmacopœia, offer examples of its judicious application. ASTRINGENTS. For illustrations see _Form. 51, 58_. EMETICS are certainly more efficient when composed of _Ipecacuan_ united with _Tartarized Antimony_, or _Sulphate of Zinc_, than when they simply consist of any one of such substances in an equivalent dose. See _Form. 63, 65_. CATHARTICS not only acquire a very great increase of power by combination with each other, but they are at the same time rendered less irritating in their operation; the _Extractum Colocynthidis compositum_ affords an excellent example of a compound purgative mass being much more active and manageable, and less liable to irritate, than any one of its components separately taken. Additional examples of this fact are furnished by _Formulæ 70, 76, 78, 79, 81, 88_. In many cases, however, the fact of purgatives thus accelerating and correcting each other’s operation may be explained by considering them as substances endowed with different powers, as already demonstrated, (p. 88), and which will be more fully considered in the third division of this Essay. DIURETICS. Under this class of medicinal agents it may be observed that, _whenever a medicine is liable to produce effects different from those we desire, its combination with similar remedies is particularly eligible_, by which the action of the basis may be directed and fixed; thus the individuals which compose the class of Diuretics are uncertain in their operation, and disposed when exhibited singly to produce diaphoretic, and other contrary effects; it is, therefore, in such cases, highly judicious to unite several of them in one Formula, by which we increase their powers, and are more likely to ensure their operation. _Formulæ 101, 103, 108, 109, 110, 111, 115_, are constructed upon this principle. DIAPHORETICS. Our maxim, “VIS UNITA FORTIOR,” certainly applies with equal truth to this class of medicinal agents. _Form. 122, 124_. EXPECTORANTS. More is frequently to be gained by the co-operation of these remedies than can be obtained by the exhibition of them separately, as in _Form. 134, 135_. DEMULCENTS do not appear to obtain any other benefit from combination than, occasionally, a convenience and efficacy of application arising from a suitable degree of consistence and solubility. See article “_Trochisci_.”