Preface to the second volume.
As frequent use is made in these pages of observations drawn from the auxiliary sciences, as affording some light on the medicinal properties of plants, it may be proper to examine how far testimony of this kind is entitled to receive credit in our inquiries and examinations.
There can be no question, that the actual operation of medicines upon the human system, gathered from positive experience, is, in the present state of our knowledge, the only criterion by which we can pronounce, with universal certainty, on their properties. There are nevertheless many things to be learnt from chemical analysis, sensible qualities, and botanical affinity, which may afford us, in some instances certainty, and in most others presumptive evidence of the medicinal characters of vegetables. The correspondence in these respects is frequently so striking, that we can hardly resist the belief, that an entire harmony of properties exists, which, if we are unable fully to comprehend, it is rather owing to the imperfection of science, than to the irregularity of nature.
A few illustrations of this point, taken from general facts already ascertained, will place the subject in a clearer light.
The chemical substances, known by the names of Gum Mucus and Faecula, are constantly emollient, demulcent, and nutritious. They manifest these qualities even when extracted from acrid and poisonous vegetables, as in Arum, Calla, and Jatropha.
Sugar is nutritious and demulcent. When subjected to a spontaneous chemical change by the vinous fermentation, it is universally a strong diffusible stimulus.
Fixed oils are emollient and laxative. Also nutritious.
Volatile oils on the contrary are acrid, stimulating, heating, and antispasmodic.
Tannin and the Gallic acid are uniformly antiseptic and powerfully astringent.
The Acetous, Citric, Tartaric and similar vegetable acids are refrigerant and antiseptic.
Bitter Extractive substances are usually tonic.
Resins, which are bitter and acrid, are commonly cathartic.
Emetine, as separated by Pelletier and Magendie, is powerfully emetic.
Morphium, obtained by Serturner, is a very strong narcotic.
The foregoing are some of the examples, which the present state of Chemistry allows us to observe of affinity between chemical and medicinal characters. With a few exceptions they will be found to be strictly true. Yet the analysis of vegetables is at present but imperfectly known, and an extended investigation is continually bringing new principles to light. We can hardly expect that the business of generalization should be attempted with complete success, until the constituent facts are better understood. From what we already know, however, it is not chimerical to predict, that if the chemistry of vegetables were as perfectly known in all its parts, as in those which we have detailed; their medicinal properties might be inferred, with at least as great certainty, as that which now attends most inferences in the conjectural science of medicine.
In regard to the botanical affinities of plants, as affording evidence of their medicinal powers, much has been said and written. Petiver, Hoffman, Linnaeus, Hasselquist, and recently the learned Professor Decandolle have bestowed much investigation on this subject. It is regarded as a desideratum by all, and as the consummation of botanical science by many, that plants should be so arranged, as that their assemblages should agree, not only in external forms, but in internal qualities and operative powers. Certain general agreements of this kind evidently prevail throughout nature; yet they are so varied, and subject to so many exceptions, that it is difficult to establish them by general scientific descriptions, and when they are rendered too minute they seem to lose much of their importance. It is perhaps as easy to know the properties of plants from their external habit, as to understand the characters of mankind from their physiognomy. Accurate observers know more than they can communicate the means of knowing to others, yet the most accurate are liable to be mistaken. Many vegetables of the closest affinity and resemblance, even species of the same genus, differ wholly from each other in their effects. Witness the species of Cucumis, Convolvulus, and Solanum, some of which are salutary, and others highly deleterious. Nevertheless there are many general truths, or at least general probabilities, by which every one would be influenced, and which have so much importance, that they will never be forgotten. No botanist, even if in danger of starving in a wilderness, would indulge his hunger on a root or fruit taken from an unknown plant of the natural order Luridae, of the Multisiliquae, or the umbelliferous aquatics. On the contrary, he would not feel a moment's hesitation in regard to any of the Gramina, the fruit of the Pomaceae, and several other natural families of plants, which are known to be uniformly innocent in their effects.
The sensible properties of plants afford another clue to their influence on the human system. It is true, that observations derived from this source will not serve us in forming very minute distinctions. They are, however, almost always adequate in vegetable productions, to enable us to distinguish what is innocent and salubrious, from what is noxious and virulent. The brute creation depend wholly upon the powers of sense in selecting their food, and this reliance does not often betray them. In regard to mankind it almost uniformly happens, that what is sweet, delicious, or aromatic, proves nutritive or salutary; while on the other hand, vegetable poisons are nauseous, acrid, and disgusting. It has been observed, that it would have been a sort of treachery in nature to have made it otherwise. Considering the universal dissemination of poisonous plants, and the number of them, which frequent the vicinity of human habitations, this arrangement of Providence, by making ungrateful what is dangerous, has furnished almost the only safeguard from harm, to the inexperienced and unwary.
These remarks have been offered on account of an impression which many persons entertain, that collateral evidences of the characters of plants are worthless and undeserving of attention. Even if the community were composed exclusively of physicians, such an opinion could not be wholly correct. Every one may be called on to form hasty decisions on subjects where his experience is deficient, and where an acquaintance with auxiliary facts might lead him to a correct issue. It is not only curious and instructive to perceive the harmonies of nature, but to every inquirer among her works it must be practically useful. It can no where be more useful, or more deserving of study, than in a new country, where the face of nature presents an ungathered harvest, and where every clue to useful discovery derives importance from its influence and tendency.