Water, the proper menstruum of gums, of gellies, and of salts, extracts the gummy and fa-line parts of vegetables, and the gelatinous matter of animals. By the mediation of these principles, it dissolves others of more activity, oils and resins being made miscible with water by the mediation of gums. All the substances which water extracts from vegetables or animals, it dissolves almost unlimitedly, so as, by repeated infusion on fresh parcels of the subjects, to become more and more impregnated with their active parts, till so far loaded as to have its fur-ther action impeded by the diminution of its fluidity: it generally takes up first the lighter and more grateful matter, and afterwards the groffer and more disagreeable; and hence, by skilful management, it may be richly impregnated with the former, without much admixture of the latter. The subject should be moderately dried (unless it be of such a kind as to lose its virtue in drying) as in this state it communicates a remarkably stronger impregnation than an equivalent quantity when fresh. Most leaves and flowers yield a great share of their more active matter by cold maceration, or more readily by warm infusion: by boiling, the dissolving power of the water is for the most part greatly increased, and the volatile parts, if the subject: contained any, are dissipated with its steam.

The vapour which exhales in the boiling of odorous substances, and many of the pungent vegetables, in water, collected and cooled in proper vessels, forms a liquor impregnated with their smell and pungency. This impregnation depends on a subtile principle, whose matrix is a volatile oil; of which oil a part often sepa-rates in its own form, either floating on the fur-face or finking to the bottom according to its gravity; and which, from the specific flavour of the subject being concentrated in it, is dis-tinguished by the name of essential. As this subtile oily matter is here separated from the more fixt gummy parts that rendered it before almost unlimitedly dissoluble, the water now can retain only a certain proportion of it, and generally but a small one: if the distilled water, once saturated so that a part of the oil appears dis-tinct, be redistilled from repeated fresh quantities of the subject, the aqueous fluid receives no further impregnation, and the quantity of oil that separates proves proportionably larger than if fresh water had been used. These oils differ from the expressed ones formerly mentioned, in being dissoluble in spirit of wine, and volatile in the heat of boiling water; on either of which foundations, when expressed oils are mixed with essential ones, whether artificially, or in their ex-pression from subjects that contain both, the two oils may be completely separated from one another. The sophistications of the dearer essential oils, commonly practised, by the admixture of cheaper ones, can be distinguished only by the smell and taste: the smell and taste which they communicate to liquors in certain known quantities, when dissolved in spirit, or, by means of sugar or mucilages, in water, is an useful criterion also of the degree of goodness of the oils when genuine; for the same kinds of vegetables, produced in different foils and seasons, vary not only in regard to the quantity of water they are capable of impregnating by distillation, and in the quantity of oil they afford, but likewise in the strength of the oil itself.

There are some substances whose virtues re-side wholly in an essential oil, and are wholly dissipated in boiling: there are others, which have one virtue residing in an essential oil, and another which remains behind in the decoction, which last may be brought to a concentrated state by evaporating the watery menstruum with a gentle heat till the matter becomes thick or solid: there are others, which contain no oil, and whose virtue remains entire in the infpif-fated extract, provided it has been skilfully prepared, pared. A difference in the quantity of water will in many case occasion a sensible variation in the qualities of the extract, by requiring more or less heat for its evaporation; for, independently of the dissipation of the volatile parts, even those of the more fixt kind suffer a con-siderable change from continued heat: by long boiling with water, sweet substances become nauseous, and the drastic purgatives lose their virulence, without any remarkable separation of their parts. Some have endeavoured to avail themselves of this observation, for converting the stronger cathartics and emetics, afarum, tobacco, and others, into medicines of safety and utility; and report that extracts made from these plants, by long boiling with a large quantity of water, were found to act as mild aperients or deobstruents: these kinds of preparations, however, must necessarily be too precarious in strength to be received in general practice; the abatement of the virulence of the medicine depending on what no care can adjust to one stan-dard, the degree and continuance of the heat.

Pure fpirit of wine, the appropriated diffbl-vent of refins and essential oils, and which dis-solves also certain saline bodies, as the sweet saccharine salts of vegetables; extracts, for the most part, such virtues of vegetable and animal substances, as reside in those principles, or in principles analogous to them. Of the substances, whose virtues reside apparently in these principles, there are many, which give a strong impregnation to water as well as spirit, but few that impart their virtues in an equal degree to the two menstrua: from a compound of pure gum and resin, water, by infusion, extracts directly the gummy matter, and by the intervention of this, a part of the more active resin, leaving great part of the resin undissolved: whereas, contrariwise, pure spirit extracts directly the refin, and leaves undissolved greatest part of the inert gum; of which it does not appear to take up so much as water does of the resin. Hence, in the analyses of these kinds of subjects, it is generally observed that spirit loads itself with their active parts much more than water is capable of doing: that the extracts made with spirit are much smaller in quantity, and proportionably stronger, than the watery extracts: and that the spirituous tinctures, loaded with the resinous parts, grow turbid on the admixture of water, and deposite their pure resin; the gummy matter, that the spirit had taken up, remaining dissolved in the aqueous fluid, and being insufficient in quantity to keep any considerable portion of the resin suspended. Hence saturated refinous tinctures, those espe-cially of the cathartic kind, require, in being diluted for exhibition, an admixture of gummy or saccharine matter, to keep the resin divided, and to prevent its separation: on this foundation may be prepared, from these kinds of tinctures, elegant gummy-resinous extracts; by mixing with them, when infpiffated to the consistence of a balsam, a thick solution of any simple gum or mucilage, and continuing the evaporation, with a gentle heat, till the matter becomes dry. In like manner, the resinous and gummy parts of one subject, or those parts which pure spirit extracts, and which water extracts after spirit, may in some cases be advantageously united into one mass; by separately infpiffating the tincture and decoction to a certain thickness, and then mixing them together.

Pure spirit, which exhales or distils with a much less heat than water, carries off with it, for the most part, much less of the essential oils of vegetables. There are many substances, whose active parts are almost wholly dissipated in the preparation of the watery extract, and almost wholly retained in the spirituous. There are some, however, whose oils are so volatile, as to rife with pure spirit as perfectly as with water; and in this case, the distilled spirit proves sometimes stronger than the distilled water; spirit keeping dissolved all the oil that rises with it; whereas, when water is used, a part of the oil frequently separates.

Wines, as being compounds of water and inflammable spirit, take up such parts of vegetables and animals as are soluble in those liquors; but their dissolving power is somewhat weaker than that of purer mixtures of water and spirit, on account partly of their viscous unctuous matter, and partly of their acid. Wines are impregnated with the active parts of medicines, chiefly by maceration in the cold, or with a very gentle warmth; the heat, which is often advantageously applied for expediting or promoting the action of water and spirit, occasion-ing in wines a disagreeable alteration. Malt liquors are commonly medicated, by macerating the ingredients in them during the fermentation, or boiling or infusing them in the wort.

Acids, both vegetable and mineral, somewhat weaken the dissolving power both of water and spirit on bodies of the resinous and oily kind; and when added to infusions or tinctures, generally precipitate a part of what the men-struum had before taken up: neverthelefs, when acids are intimately combined with vinous spirits into what is called a dulcified liquor, the compound proves a more efficacious menstruum, for some bodies, than the pure vinous spirit.

- Fixt alkalies generally increase the action of water on resinous and oily bodies, and weaken or restrain its action on gummy ones, rendering water incapable of holding pure gums dissolved: they have been supposed to promote the action of spirit on difficultly soluble resinous bodies, but though the alkali deepens the colour of the tincture, the quantity extracted is found to be the same without as with it. - Volatile alkalies precipitate gums from water, like the fixt, but in other cases their effects are more variable.

Expressed oils extract the odoriferous, resinous and oily parts of vegetables; and with these they are sometimes impregnated, both for the purposes of perfumes, and for external medicinal uses. The more fragrant flowers give out their odour by cold maceration. The more fixt resinous parts of the leaves of the plants are extracted, by boiling them in the oil till their watery moisture has exhaled, that is, till they are almost crisp, and the oil appears tinged of a green colour. Animal fats, liquefied and boiled with recent herbs, become in like manner impregnated with their resinous parts, and with the green colour residing therein. It does not appear, that the oils or fats receive, by this management, any valuable virtues: the heat, requisite for making them boil, impresses an ungrateful scent, and dissipates the more volatile parts of the subject: they may be impregnated more elegantly with the active parts of vegetables, by an admixture of essential oils or of spirituous extracts. The spirituous tinctures of the leaves of most plants are of a deep green colour; and the infpiffated extracts, though often brown or black in their solid or consistent state, give generally a like greennefs to fresh spirit, to essential oils, expressed oils and fats.

Air, or its watery moisture, seems to act as a true dissolvent, in the same manner, though not so expeditiously, as water in its groffer form. The astringent virtue of the walnut tree, and the purgative of the damask rose, have been observed to be diffused through the air; though they are obviously not of the volatile but of the fixt kind, not exhalable by heat but dissoluble by menstrua. Hence the atmo-sphere may become impregnated with all those virtues of vegetables, which at least watery menstrua can extract: and hence many medicinal substances are gradually robbed by it of their virtues; powders the moil speedily, as exposing the largest surface to its action.