Having, in the commencement of this inquiry, stated that all the principles of combination, capable of practical application in the construction of extemporaneous formulæ, are exemplified in the composition of the various productions of Nature, I shall conclude the present section by shewing, that _many of our most valuable vegetables owe their useful properties to the joint operation of the several distinct and different ingredients which enter into their composition_. How many substances does Nature produce in the vegetable kingdom, in which the permanent tonic quality of bitterness exerts its influence in union with the transient stimulating powers of an aromatic principle? indeed there is a series of vegetable remedies of this kind: commencing with those that are simply bitter, we gradually proceed through the different species, each blending as we advance an increasing proportion of aroma, until we arrive at those in which the aromatic quality greatly preponderates. _Peruvian Bark_ may be said to combine within itself the properties of _bitterness_, _astringency_, and _aroma_; a fact which suggested the probability of our being able to produce an artificial compound that might emulate the effects of Cinchona, and to a certain extent the idea appears to have been realised; for we are told by Dr. Cullen, that he frequently succeeded in the cure of an intermittent by a combination of _Oak Bark_ and _Gentian_, when neither bitters nor astringents, separately, produced the least impression; and I am informed by Dr. Harrison, that in the Horncastle Dispensary, of which he was for many years physician, he never employed any other remedy for curing the ague of Lincolnshire than equal parts of _Bistorta_ (astringent) and _Calamus Aromaticus_ (bitter and aromatic), neither of which plants, _individually_, ever produced the least benefit in such diseases. Berzelius attempted to produce a compound of this description by adding to the bark of the _Ash_ some _Tormentil root_ and _Ginger_; and he observes that it acted as an excellent tonic, and that according to the experiments of his friends it seemed to cure quartan agues.[261] In the aromatic barks and woods, such as those of the _Canella_, _Orange-peel_, _Sassafras_, &c. the aromatic principle[262] is combined with a bitter ingredient; a union which proves of singular service in the formidable bowel complaints so common in tropical climates.