JUICE denotes the sap of vegetables, or the liquors of animals.

The juices of several plants are expressed to obtain their essential salts, and for several medicinal purposes, with intention either to be used without further preparation, or to be made into syrups and extracts. The general method of extracting these juices is, by pounding the plant in a marble mortar, and then by putting it into a press. In this way is obtained a muddy and green liquor, which generally requires to be clarified. The juices of all plants are not extracted with equal case. Some plants, even when fresh, contain so little juice, that water must be added whilst they are pounded, otherwise scarcely any juice could be obtained by expression. Other plants, which contain a considerable quantity of juice, furnish by expression but a small quantity of it, because they contain also much mucilage, which renders the juice so viscid that it cannot flow. Water must also be added to these plants to obtain their juice. The juices thus obtained from vegetables by a mechanical method arc not, properly speaking, one of their principles, but rather a collection of all the proximate principles of plants which are soluble in water; such is the saponaceous extractive matter, the mucilage, the odoriferous principle, and the saline and saccharine substances; all of which are dissolved in the water of the vegetation of the plants. Besides these matters, the juice contains some part of the resinous substance, and the green colouring matter, which in almost all vegetables is of a resinous nature. These two latter substances not being soluble in water, are only interposed between the parts of the other principles which are dissolved in the juice, and consequently disturb its transparency. They nevertheless adhere together in a certain degree, and so strongly in most juices, that they cannot be separated by filtration alone. When therefore these juices are to be clarified, some previous preparations must be used by which the filtration may be facilitated. Juices which are acid, and not very mucilaginous, are spontaneously clarified by rest and gentle heat. The juices of most antiscorbutic plants abounding in saline volatile principles, may be disposed to filtration merely by immersion in boiling water; and as they may be contained in close bottles, whilst they are thus heated in a water bath, their saline volatile part, in which their medicinal qualities chiefly consist, may thus be preserved. Fermentation is also an effectual method of clarifying juices which are susceptible of it; for all liquors which have fermented clarify spontaneously after fermentation. But this method is not used to clarify juices, because many of them are susceptible of only an imperfect fermentation, and because the qualities of most of them are injured by that process. The method of clarification most generally used, and indispensably necessary for those juices which contain much mucilage, is boiling with the white of an egg. This matter, which has the property of coagulating in boiling water, and of uniting with mucilage, does accordingly, when added to the juice of plants, unite with and coagulate their mucilage, and separates it from the juice in form of scum, together with-the greatest part of the resinous and earthy matters which disturb its transparency. And as any of these resinous matters which may remain in the liquor, after this boiling with the whites of eggs, are no longer retained by the mucilage, they may be easily separated by filtration.

The juices, especially before they are clarified, contain almost all the same principles as the plant itself; because, in the operation by which they are extracted, no decomposition happens, but every thing remains, as to its nature, in the same state as in the plant. The principles contained in the juice are only separated from the grosser oily, earthy, and resinous parts, which compose the solid matter that remains under the press. These juices, when well prepared, have therefore the same medicinal qualities as the plants from which they are obtained. They must evidently differ from each other as to the nature and proportions of the principles with which they are impregnated, as much as the plants from which they are extracted differ from each other in those respects.

Most vegetable juices coagulate when they are exposed to the air, whether they are drawn out of the plant by wounds, or naturally run out; though what is called naturally running out is generally the effect of a wound in the plant, from a sort of canker, or some other internal cause. Different parts of the same plant yield different juices. The same veins, in their course through the different parts of the plant, yield juices of a different appearance. Thus the juice in the root of the cow-parsnip is of a brimstone colour, but in the stalk it is white.

Amongst those juices of vegetables which are clammy and readily coagulate, there are some which readily break with a whey. The great wild lettuce, with the smell of opium, yields the greatest plenty of milky juice of any known British plant. When the stalk is wounded with a knife, the juice flows out readily like a thick cream, and is white and ropy; but if these wounds are made at the top of the stalks, the juice that flows out of them is dashed with a purple tinge, as if cream had been sprinkled over with a few drops of red wine. Some little time after letting this out, it becomes much more purple, and thickens; and finally, the thicker part of it separates, and the thin whey swims at the top. The whey or thin part of this separated matter is easily pressed out from the curd by squeezing between the fingers, and the curd will then remain white; and on washing with water it becomes like rags. The purple whey (for in this is contained all the colour) soon dries into a purple cake, and may be crumbled between the fingers into a powder of the same colour. The white curd being dried and kept for some time, becomes bard and brittle. It breaks with a shining surface like resin, and is inflammable, taking fire at a candle, and burning all away with a strong flame. The same thick part being held over a gentle heat, will draw out into tough, long threads, melting like wax. The purple cake made from the whey is quite different from this; and when held to a candle scarce flames at all, but burns to a black coal. The whole virtue of the plant seems also to consist in this thin part of its juice; for the coagulum or curd, though looking like wax or resin, has no taste at all; whereas the purple cake made from the serum is extremely bitter, and of a taste somewhat resembling that of opium.

Of the same kind with the wild lettuce arc the throatwort, spurge, and many other plants. These are all replete with a milky juice which separates into curds and whey like that already described. But this, though a common law of nature, is not universal; for there are many plants which yield the like milky juices without any separation ensuing upon their extravasation. The white juice of the sonchus never separates, but dries into an uniform cake; the common red wild poppy bleeds freely with a milky juice; and the heads or capsules of seed bleed not less freely than the rest of the plant, even after the flower is fallen. The juice, on being received into a shell or other small vessel, soon changes its white to a deep yellow colour, and dries into a cake which seems resinous and oily, but no whey separates from it. The tragopogon, or goat’s beard, when wounded, exudes freely a milky juice; it is at first white, but becomes immediately yellow, and then more and more red, till at length it is wholly of a [7:12:629]dusky red. It never separates, but dries together into one cake; and is oily and resinous, but of an insipid taste. The great bindweed also exudes freely a white juice; the flowers, as well as the stalks and leaves, affording this liquor. It is of a sharp taste; and, as many of the purging plants are of this class, it would be worth trying whether this milk is not purgative.

These juices, as well as the generality of others which bleed from plants, are white like milk; but there are some of other colours. The juice of the great celandine is of a fine yellow colour; it flows from the plant of the thickness of cream, and soon dries into a bard cake, without any whey separating from it. Another yellow juice is yielded by the seed-vessels of the yellow centaury in the month of July, when the seeds are full grown. This is very clammy; it soon hardens altogether into a cake without any whey separating from it. It sticks to the fingers like bird-lime, is of the colour of pale amber, and will never become harder than soft wax if dried in the shade; but if laid in the sun it immediately becomes hard like resin. These cakes burn like wax, and emit a very pleasant smell. The great angelica also yields a yellowish juice on being wounded; and this will not harden at all, but if kept several years will still be soft and clammy, drawing out into threads or half-melted resin.

Other kinds of juices very different from all these are those of a gummy nature. Some of these remain liquid a long time, and are not to be dried without the assistance of heat; and others very quickly harden of themselves, and are not inflammable. The gum of the juice of rhubarb leaves soon hardens, and is afterwards soluble in commonwater, and sparkles when put into the flame of a candle. The clusters of the common honeysuckle are full of a liquid gum. This they frequently throw out, and it falls upon the leaves, where it retains its own form. The red hairs of the ros solis are all terminated by large bladders of a thin, watery fluid. This is also a liquid gum; it sticks to the fingers, draws out into long threads, and stands the force of the sun all day. In the centre of each of these dew-drops there is a small red bladder, which stands immediately on the summit of the red hair, and contains a purple juice which may be squeezed out of it. The pinguicula, or butterwort, has also a gummy matter on its leaves, in much greater quantity than the ros solis.

Some plants yield juices which are manifestly of an oily nature. These, when rubbed, are not at all of a clammy nature, but make the fingers glib and slippery, and do not at all harden on being exposed to the air. If the stalk of elecampane be wounded, there flows out an oily juice swimming upon a wate’ry one. The stalks of the hemlock also afford a similar oily liquor swimming upon the other; and in like manner the white mullein, the berries of ivy, the bay, juniper, dogberry tree, and the fruit of the olive, when wounded, show their oil floating on the watery juice. Some of these oily juices, however, harden into a kind of resin. Our ivy yields such a juice very abundantly; and the juice of the small purple-berried juniper is of the same kind, being hard and fat, and not very gummy. If the bark of the common ivy is wounded in March, there will ooze out a tough and greasy matter of a yellowish colour, which, taken up between the fingers, feels not at all gummy or sticking, but melts in handling into a sort of oil, which in process of time hardens and crusts upon the wounds, and looks like brown sugar. It burns with a lasting flame, and smells very strong. The tops of the wild lettuce, and the leaves growing near the tops, if examined with a magnifying glass, show a great number of small bladders or drops of an oily juice of a brownish colour, hardening into a kind of resin; they are easily wiped off when of any size, and are truly an oily juice a little hardened. It is probable, also, that the fine blue flour or powder called the bloom, upon the surface of our common plums, is no other than such an oily juice exudating from their pores in small particles, and hardening into a sort of resin.