The juices of several plants and fruits are expressed with a view to obtain their essential salts, and for various medicinal purposes: they are used either without further preparation, or converted into syrups and extracts. Such juices are generaly obtained by simply bruising the plant in a marble mortar, and then submitting the pulp to a press : some vegetables, however, require the addition of water in the mortar, to express their juice with greater facility. The sap of most antiscorbutic plants, abounding with saline volatile principles, may be disposed to filtration, merely by pouring it in close bottles, and immersing them in boiling water: thus, the saline and volatile particles, in which their medicinal .virtues chiefly consist, may be easily preserved.-See Filtration.
Most vegetable juices coagulate, When exposed to the air, whether they are extracted from the plant by wounds, or flow from it without any external injury: in the latter case, however, it is generally the effect of a disease in the plant, either from a species of canker, or some other internal cause.
Different parts of the same plant often yield juices of opposite qualities. Thus, the sap in the root of the cow-parsnip is of a yellow colour; while that obtained from its stalk is white. The wild or strong scented lettuce, affords the greatest abundance of milky juice, of any known British plant. On wounding the stalk with a knife, there exudes a white ropy liquor ; but, if an incision be made at the top of the stalks, a purple-tinged sap appears, as if cream had been sprinkled over it with a few drops of red wine. In a short time, it acquires a deeper purple, and thickens so, that finally a separation takes place, when the watery part floats on the surface. See Lettuce.