The medicinal juices of vegetables commonly reside in distinct vessels; and often exude upon the surface, from a spontaneous rupture, or artificial incisions of them. The juices of herbs and trees issue chiefly during the summer heats; as the gum of the cherry-tree, the refin of the fir, the sweet juice of the manna ash, and the unctuous exudations on the leaves of many plants; though some trees, as the birch and maple, yield early in the spring, on being bored or deeply wounded, a very copious sweetish watery juice, of which, in summer, they yield little or nothing, the watery menstruum being now perhaps consumed. Some roots bleed gummy-refmous juices in the spring, as bryony and angelica among us; others in summer, as the afafetida roots in Persia, and the fcammony in Syria. Some fruits, particularly the several varieties of lemons, citrons, and oranges, have numerous vesicles, in their outer rind, filled with a fragrant oil; great part of which may be extracted, by rolling the fruit on a plane stuck full of sharp points, which lay open the oily vesicles, or by rubbing it on a mass of sugar, which imbibes the oil.

From succulent herbs and fruits, the different fluid juices they contain are forced out, mixed, by bruising and pressing them. Vegetables of the sweet or saline kind, as the summer fruits and the acid herbs, several. of the acrid plants, as arum and scurvygrass; and those of the lactescent kind, as dandelion and the spurges; generally give out by this process great part of their active matter along with the watery fluid: but the juices expressed from aromatic herbs, as mint, have, for the most part, little or nothing of the peculiar smell, taste, or virtue, of the subject; and many of the fragrant flowers, as lilies and violets, have their fragrance destroyed by the pressure. The juices of plants, thick, turbid, and very impure when newly ex-pressed, by fettling and repeated {training become clear; many, in this depuration, lose nothing considerable of their virtue: from others, the medicinal parts, not dissoluble in watery fluids, separate and subside along with the feculent matter. To the depurated juices, designed for keeping, a small proportion of rectified spirit of wine may be added, which, on standing for some time, generally throws down a fresh sedi-ment: the liquor is then to be put in small bottles that have been washed with spirit and dried, a little sweet oil poured on the surface so as nearly to fill the bottles, and the mouths slightly ftopt: by this method most of the juices that bear depuration may be preserved, in a cool cellar, for a year or two; excepting perhaps only the very fermentable sweet ones, which can scarcely be long restrained from fermentation without boiling. Those which are not injured in their virtue by evaporation, may be infpiffated to the consistence of a syrup, or of a thick or solid extract: from those of the saline kind, duly depurated and infpiffated, the saline part commonly separates, on long keeping, in a crystalline form.

The kernels of fruits, the seeds that on being triturated with water form an emulsion or milky liquor, and some other vegetable sub-stances, yield, on being strongly pressed, an oil; which, of itself, is flavourless and insipid; but which, in some cases, is impregnated with the smell and taste of the subject. The aromatic seeds and kernels, as anniseeds and nutmegs, and some of the purgative ones, as the ricini, are the principal substances that give out with their oils their peculiar virtues, the oils of most of the others, having no particular impregnation. There are, however, considerable differences among the unflavoured and insipid oils; in their consistence; in their disposition to congeal by cold; in their disposition to grow rancid by heat; in the degree of heat necessary to make them boil; in their power of dissolving certain bodies; and in their combinability with fixt alkalies into soap. The extraction of the oil from the subject is greatly facilitated by heat; and hence the preparers of these oils for mechanic purposes generally warm to a considerable degree either the subject itself, or the iron plates of the press, or both. Where the product is intended for medicinal use, this practice is generally condemned; heat being apt to impress upon the oil an ungrateful flavour, and increase its disposition to become rancid. Nevertheless, though a great heat has undoubtedly these effects, yet a gentle warmth is in some cases necessary, and not, perhaps, very injurious in any: in winter at least, both the subject and the apparatus may be warmed with safety, to the greatest degree of heat that obtains in the shade in summer. The heat is never to be continued longer than the expression requires; and the oil, as soon as drawn, is to be fet in a cold place.