We may discover the operation of such a principle in some of the more active compounds presented to us by nature: many herbs owe their efficacy to a cause of this kind. _Elaterium_, as I have ascertained by experiment, contains a purgative element, _sui generis_, (_Elatin_) and a bitter principle, which in itself is quite inert, and yet its presence in the compound renders the alimentary canal more susceptible of the impression of the active ingredient, and therefore increases its force. See _Extract_. _Elaterii._ The history of _Senna_ will afford some interesting facts in farther elucidation of this subject; the leaves of this plant, like _Elaterium_, appear to contain an active principle, in combination with a bitter, which latter ingredient, although destitute of purgative properties, considerably increases those of the former; for if this be removed, as happens when Senna is transplanted into the south of France, the purgative principle is weakened, but may be again restored by the artificial addition of some bitter extractive. The fruit or pods of Senna[249] contain only the purgative principle, and are therefore comparatively feeble, unless the defect be compensated by art. Dr. Cullen has observed that a much smaller quantity of the leaves is required for a dose if they be infused in company with some bitter plant; and it has been found that the watery infusion of _Rhubarb_ is rendered more purgative by the addition of _Calumba_. The experiments of Seguin have established beyond all doubt that the active principle of vegetable astringents is a peculiar element, to which the name of _Tannin_ has been given; but the efficacy of this ingredient is undoubtedly enhanced by the presence of the gallic acid with which it is usually associated, although this acid, when separated from the native combination, is incapable of producing the least astringent effect; that peculiar flavour which we so commonly experience in unripe fruits, and which we designate by the term _acerbness_, is the result of a combination of the astringent principle with some vegetable acid. The relative sweetness of sugar, when in different degrees of purity, depends upon the operation of the same law of combination; _pure sugar_, as Dr. Mac Culloch has very justly observed, however paradoxical it may appear, _is not so sweet as that which is impure_; the sweetness of _melasses_, compared with that of refined sugar, is too well known to require more than a bare mention; the vegetable extractive matter in this case, increases the effect of the saccharine principle with which it is combined; for the same reason grapes, differing very materially in their proportion of saccharine matter, may seem _to the taste_ equally sweet, and such in fact is the case on comparing the luscious grapes of Spain, with the _Chasselas_ of Paris; and yet the vinous produce is entirely different, the result of the one being a sweet and luscious