There are substances, in which all the foregoing means of investigation fail; and which operate by some latent power, of which they give little or no intimation to the senses. Of this kind are most of the purgative, emetic, and narcotic plants; those, which from their deleterious effects when taken in certain quantities, are called poisonous; and most of the metallic bodies and their preparations. Experiments on brutes are here of use, but of limited use: for if prudence requires us to refrain from substances which are noxious to brutes, it does not always authorize us to venture on such as may to them be innocent: experience shews, that the crocus of antimony, of which a grain or two operate on the human body as a virulent cathartic or emetic, may be given to horses in the quantity of an ounce, without producing any very remarkable effect: that a moderate dose of jalap throws a dog into convulsions, who could well bear a much greater quantity of opium than could be given with safety to a man. The virtues of these kinds of substances can be known only from their effects in the human body itself: and as, of all medicines, they have the most obvious and apparent effects; they are, happily, of all medicines, thole which admit of the least deception, and in which, of consequence, we can most avail our-selves of the observations of former writers. Indeed many of them being now received in general practice, their powers have been determined by general experience.

With such assistances as I could draw from these sources or from my own experience, 1 have endeavoured to point out chiefly the primary effects of the several subjects, or the immediate sensible operation which constitutes their true medical character. I judged it useless to enter into an enumeration of diseases in which a medicine is or is not proper, when the falu-tary or pernicious effects, which it produces in those diseases, are no other than obvious consequences of its general power: it nevertheless appeared frequently necessary to specify some particular cases, as being either illustrative of the general power, or subservient to its disco-very, or where it could not be precisely ascer-tained.

The pharmaceutic history of simples, closely allied to the medicinal, regards, chiefly, the variations of their qualities in different states and forms naturally or artificially induced; the separability or non-feparability of their active principles by different menstrua or different operations; and their miscibility or non-mifci-bility one with another. In these properties, remarkable diversities and contrarieties are ob-served among the different medicinal simples, even among those in which no material disagree-ment has been generally suspected. Thus, the virtue of some vegetables accompanies the fluid which they yield on being pressed, while that of others remains behind locked up and concentrated in the subject, and that of others is destroyed in the operation: some plants, in being dried, lose all their virtue, some have their virtue improved, and some have it changed to another kind: some, by infusion, give out their virtue both to water and to spirit of wine, some to water only, some to spirit only, and some neither to one nor the other. Nor can these diversities be reduced to any general rules, or any otherwise determined than by a separate examination of each particular article.

This province belongs peculiarly to chemis-try; but notwithstanding its obvious importance to the practice of medicine, even the medical chemists have been very remiss in the cultivation of it. I know only of two persons, whose labours have been considerable, and whose success may be applauded. Neumann, one of the first who, rejecting the useless analy-ses of vegetables made by vehemence of fire, endeavoured to separate their component parts, unaltered, by means of menstrua; examined by this method a considerable number of the officinal drugs, not indeed directly in a medicinal view, but in the way of a general chemical inquiry into the products of nature. Cartheufer, confining himself more closely to medicinal considerations, followed nearly Neumann's plan so far as it included these, and made fundry valuable additions.

In the present work, the inquiry is extended to a far greater number of simples, and con-ducted likewise on somewhat different principles. The quantity of matter, which water or spirit extract from a plant, or which either menstruum extracts after the action of the other, is, medicinally, of little importance to be known; unless it be known also, what are the precise qualities of the several preparations, whether the virtue of the plant resides in the part extracted by one or the other menstruum, or whether in this separation of the parts of the subject, any active matter is discovered which was not perceptible before. And on the other hand, though the qualities of the infufions, extracts, etc. be very carefully and minutely examined; yet if they are described independently of one another, and if no account is taken of the remaining substance of the plant, or of the vapour that exhales in the infpiffation of the spirituous tincture; it will be impossible, in many cases, to judge between the dissolving powers of water and spirit, or whether either is a complete menstruum for the active parts, or whether the spirituous extract retains the full virtue of the subject, or whether a part of the virtue exhales or distils with the spirit. Without embarrassing the reader with a minute history of experiments, I have given only their result, or the general pharmaceutic habitude of the subject deduced from them: it is in trying to make these general deductions from the experiments hitherto published, that their insufficiency, in regard to the greater number of the articles, is most conspicuous. I have never-theless been obliged, in some cases, by the multiplicity of the labour, and the difficulty of procuring specimens of some few articles, to be sa-tisfied with such information as those experiments afford.

To prevent the necessity of frequent repetitions under the particular subjects, some obser-vations of a general nature are here premifed.