After botany had been regularly cultivated, and methodized into systems; it was observed that several of the plants, which had been ranged together from their agreement or affinity in botanic characters, agreed or were allied also in medicinal virtue. As nature appeared to have in some instances established a connection of this kind, it was fondly presumed that she had done so in all; that, the virtues of some particular plants being known, thole of all the others, ranked in the same botanic class, might also be inferred; and accordingly rules have been drawn up, for judging of the virtues of plants upon this principle, by botanists of the greatest name. But so far is a similarity of virtue from obtaining through the several genera which con-stitute one class of vegetables, that frequently it does not obtain through the several species of one genus: there are solanums, lettuces, herb-mercuries, cucumbers, mushrooms, etc. efcu-lent and deleterious; and even the same individual often varies, from culture or other circumstances, as much as two plants which have no botanic affinity.

The chemists, in like manner, extending the discoveries of their useful art beyond the proper limits, endeavoured to investigate the virtues of plants from the substances into which they are resoluble by fire; and in this view, the French academicians analysed almost all those made use of in medicine. From their experiments it appears, that the substances thus, obtained have no resemblance in quality to the original vegetable, and can afford no foundation for judging of its virtues; that plants the most remote in virtue, purgative and astringent, poisonous and esculent, are changed by force of fire into similar principles. It is matter of concern, that these analyses should have been preserved in the posthumous works of a writer so judicious as Mr. Geoffroy, while the editor was senfible that the author himself, in his later years, disap-proved of them.

There are, nevertheless, in most vegetables, certain sensible qualities, either obvious, or easily discoverable; which afford, under due reftrictions, an excellent test of their virtues; and in which indeed, oftentimes, their virtues wholly consist. Aromatics, acrids, fetids, astringents, bitters, sweets, acids, unctuous and mucilaginous substances, which comprehend the greater number of the articles of the vegetable kingdom, operate generally by such qualities as are the immediate objects of smell and taste; and from the degree of force with which they affect those senses, their degree of medicinal efficacy may be generally inferred. The smells and tastes of the several materials, on which some have already laid considerable stress, but which for the most part have been either wholly neglected, or regarded only as they affect the medicine in point of elegance, I have examined with no little care: and though it is not to be supposed, that the particular degree of each can be precisely determined; or its particular species, especially in regard to smell, fully expressed in words, any otherwise than by com-parison with substances more known; or that any exact limits can be always fixed, as between fetid and aromatic, grateful and ungrateful; I nevertheless flatter myself, that the observations of this kind will furnish, in many inftances, sufficient data to the physician for judging what may be expected from materials he has not experienced. In this part, as in the descriptive history, great assistance has been drawn from pharmaceutic chemistry: for, in many vegetables, the active matter is so far divided and diluted by the herbaceous inert substance; and in others, different kinds of active matter are so blended together; that they cannot be disco-vered, or distinguished, till they are extracted, or separated from one another, by the operations of chemistry.

Some other experiments are, in particular cases, very useful auxiliaries in this inquiry. Thus, a solution of vitriol of iron, made in water, is by many vegetables, turned to a black colour; by others, a solution of sulphur, made in alkaline liquors, is rendered milky or turbid, and of a strong fetid smell. It is not known, that any vegetable substance produces the first of these effects, but those which have an astrin-gent power; or that any produces the second, but those partaking of an acid, which unites with and neutralizes the alkaline matter in the liquor, and disengages the sulphur which was thereby kept dissolved. By these criteria, lower degrees of astringency and acidity are often discovered, than the taste gives any notice of.

The effects of medicines on the fluid and solid parts of dead animals; as their producing or resolving coagulations, relaxing or contracting the fibres, promoting or retarding putrefaction, or varying the degree and the species as well as the facility of the resolution; afford likewise, in some cases, considerable light into their medicinal operation. Of these cases, however, the number appears to be much smaller than seems to have been imagined by the generality of those who have prosecuted these inquiries; who have not, perhaps, sufficiently considered, how different is the operation of medicines on animal substances in a vital and in an inanimate state; and how much the fluids of an animal are influenced by the action of medicines on the solid parts: it is probable, that the operation of mod medicines is immediately or principally upon the solids, and that the fluids are in most cases only consequentially affected. The fluids most likely to answer any useful pur-pose, in thefe forts of trials, are those which are secreted into the alimentary canal: experiments on blood seem to be of no medical utility: green vitriol, mixed with the blood drawn from a vein, instead of rendering it more florid, the common medical effect of this chalybeate pre-paration, changes it grey: mercurials examined in the same manner, discover nothing of that remarkable colliquation, which they produce in the blood of living animals.