This section is from the book "An Experimental History Of The Materia Medica", by William Lewis.
The medicinal materials, in the infancy of physic necessarily few, were by degrees exuberantly multiplied; and new ones are still, from time to time, discovered and introduced. Their estimation and use have been variable; fub-stances at one time in high esteem being often at another disregarded, and those which in one age had fallen into neglect being often in another revived; a fluctuation apparently owing, in many articles, to the fancy or caprice which influences other human things, and in many to ignorance or error. The design of the present work is, to examine the several substances which are or have been in repute; with a view to ascertain, as far as possible, their real powers, and to establish this important part of medicine on a just foundation. It will, perhaps, appear from this examination, that there are some materials of little significance among those which are retained in practice, and many of more utility among those which are overlooked.
The materia medica is commonly understood to comprehend, not only the materials afforded by nature, but many of those also which are prepared, produced, or compounded by art; as minium, pot-ash and soap. In pharmacopoeias, intended as directories, for the preparation of such medicines only as are made in the shops, all those artificial medicinal sub-stances are referred to the materia medica, which are either brought from abroad, or commonly prepared by particular persons, as articles of commerce. A scientific distinction may, however, be fixed, independently of commercial considerations. Productions essentially different, in their medical and other properties, from the subject from which they were produced, and which had nothing analogous to them prae-existing in the subject; as the fixt alkaline falts of vegetables, and the volatile salts of animals; compounds resulting from the coalition of opposite ingredients; of ingredients which in mixture lose their specific powers, and form together a substance of new qualities; as neutral salts and soap; - the constituent parts of natural compounds of this kind, separated and purified by art; as magnefia and the mineral acid spirits; may be considered as distinct medicinal materials, or as articles of the materia medica, wheresoever they are prepared.
It were to be wished, that the several subjects could be methodically arranged, from some qualities subservient to medicinal intentions. This has been attempted by different writers, on different plans, but in my opinion with little success: nor indeed does it seem to be practicable; the qualities of medicines being too intricate, and compounded, and multisariously diversified, to serve for the basis of any useful distribution of them. The division into minerals, animals, and vegetables, and the subdivi-sions into roots, barks, leaves, flowers, etc. are equally exceptionable; some substances not being clearly reducible to either of the three kingdoms, and different parts of one vegetable being commonly made use of. I know of no practicable method that promises any advantage above the alphabetic one; and what conveniency there is in this regards rather the author than the reader.
Each article may be considered in three points of view; as an object of natural, medicinal, and pharmaceutic history.
The office of natural history, so far as it relates to the materia medica, consists, in dis-tinguishing the several substances from one another by criteria drawn from their external form and structure, and in ascertaining their origin and production.
The criteria of natural history are peculiarly adapted to vegetables and animals in their entire and perfect state; the form, structure, and dis-position of the several parts, collectively con-sidered, affording here generally sufficient means, and indeed the only means, of distinguishing each particular species from all others. In this branch of knowledge, of late years so diligently cultivated and so remarkably improved as a general science, little new matter can be expected in a work of the present kind. If, of the accounts given in detail by those who have written professedly on these subjects, the more interesting and useful particulars relative to the medicinal articles, are perspicuously and con-cisely expressed; if the more obvious and invariable discriminative appearances are justly se-lected from the writings of others or assigned from my own observation, so as to render the descriptions strictly definitive or characteriftic, without regard to the systems of naturalists; I have in this point accomplished my intentions.
Of the distinct parts and productions of vegetables and animals, there are many, which cannot be sufficiently discriminated by any external marks, and which require the assistance of characters drawn from pharmacy or pharmaceutic chemistry, that is, from their intrinsic properties. The criteria of natural history, strictly so called, are still more insufficient, and those of chemistry of consequence more neces-sary, in regard to the products of the mineral kingdom; where, oftentimes, one and the same matter assumes different forms, and different kinds of matter the same form. In assigning the criteria drawn from this source, I have endeavoured to determine by experiment those properties, which, at the same time that they are obvious and easily examined, may be fully characteristic of the subject in all its forms.
The medicinal history, or the knowledge of the powers and effects of medicines in the human body, though apparently a most essential branch of the healing art, has been far more incuriously cultivated, and still, perhaps, continues less cleared from the errors of former ages, than any other science. Even in thefe later times; after the arbitrary qualities of cold, hot, dry, and moist, and the ridiculous similitudes and conceits which some enthusiasts of the last century relied on as a test of medicinal activity, had been exploded: the advancement of true medicinal history has met with many obstructions; partly from the officiousness of compilers in collecting and preserving the fictitious virtues; partly from a fondness, in original writers, of aggrandizing their favourite medicines; partly from a fallacy, in ascribing to a particular ingredient in a composition the effects which more powerful ones had produced, a fallacy which the exuberance of mixture made fome-times unavoidable; partly from the difficulty of distinguishing, in many cases, the real effects of medicines from the operations of nature unaf-fifted; partly from a practice, too common among writers on the materia medica, of barely enumerating the diseases, or even the parts of the body in whose general diseases, a medicine had, or was supposed to have, done good, as if diseases of the same parts, or of the same name, were always of the same nature, or were always to be treated by the same remedies. Medicinal history has perhaps suffered also from the misapplication of other sciences.