Some practitioners, whose opinions I always receive with respect, have considered these views respecting the influence of solubility as savouring too much of the refinement of theory, and instances have been suggested which would appear to invalidate their pretensions; upon examination, however, it will be found that such exceptions are but apparent, and depend upon the solvent action of the gastric fluid. Thus the _Protoxide of Iron_ would appear to be soluble in the fluids of the stomach, and is consequently an active medicine, whereas the _Peroxide_ of the same metal, being insoluble under such circumstances, requires to be combined with an acid, as in the _Tinctura Ferri Muriatis_, to render it efficacious. The same remark will probably apply to the oxides of antimony. I shall conclude this section upon the influence of Solubility, by the relation of an anecdote which may tend to confirm the justness of the views I have offered, more satisfactorily perhaps than any additional arguments derived from a scientific examination of chemical and medicinal facts; in as much as it presents us with a practice, the utility of which has been discovered by unassisted experience, and must consequently be independent of theory. The American Indians, whenever they undertake a long journey, and are likely to be destitute of provisions, employ Tobacco for the purpose of counteracting the uneasy sensations of hunger, and in its preparation for such a purpose they adopt an expedient for modifying its powers, and protracting its effects, which affords an instructive illustration of the influence of solubility; it consists in combining the juice of Tobacco, with the pulverized shells of snails, cockles, and oysters;[271] the mass is dried, and formed into pills, of a convenient size to be held between the gum and lip, which being gradually dissolved and swallowed, fulfil the intention required.[272] V. TO AFFORD AN ELIGIBLE FORM. A. _By which the efficacy of the remedy is enhanced._ After the views which have been submitted in the progress of the present inquiry, it is evident, that the _form_ in which a remedy is administered may exert some influence upon its medicinal effects; for additional proofs of this fact, and for more particular directions, see _Decocta_, _Infusa_, _Tincturæ_, _Misturæ_, _Pilulæ_, _Pulveres_, &c. When a substance, or a combination of substances, requires the addition of some other one, for the purpose of imparting a convenient, agreeable, or efficacious form, _a vehicle should always be selected, whose effects will be likely to correspond with the intention of the other ingredients_. This precept may be exemplified by a reference to _Form. 80, 134_, and others, the _key-letters_ of which announce the _modus operandi_ of their respective _vehicles_. B. _By which its aspect or flavour is rendered more agreeable._