So called from the acidity of its leaves. The plants of this acid nature employed in medicine belong to the genus oxalis L. and are the O. Acetosella Sp. Pi. 620, O. Cornuculata623; O. Cernua Wildenow, vol. ii. p. 717. This genus of plants, by the labours of Jacquin and Thunberg, is considerably augmented; and the last edition of Linnaeus, by Wildcnow, contains 83 species, the greater number of which are subacid, containing, in modern chemical language, super-oxalate of potash, viz. the alkali more than saturated with the oxalic acid. Some of these species are more acid than others: the common wood sorrel is the least so. Some species of the rumex have had the same appellation, (Rumex Acetosa L. Sp. Pi. 481,) as the leaf stalks are sour; and the same acidity occurs in the leaf stalks of the rheum compactum, a plant nearly allied to the docks. The juice of sorrel is sometimes used as an agreeable refrigerating drink in fevers, and sometimes the leaves are boiled in milk, to form a pleasant whey. Externally, they arc thought to promote suppuration, particularly in indolent scrofulous humours. The seeds are slightly astringent; and indeed we seldom find astringency in any part of a plant, but we discover acidity in some other. The expressed juice is now never used, and the conserve is rejected from the dispensatories: it is pleasing as a conserve, but nearly inert as a medicine. The salt of lemons, as it is called, is only the salt of wood sorrel, and sometimes supposed to be cream of tartar, with a little sulphuric acid. It is chiefly used for taking out the stains of ink from linen; and. were the muriatic acid added, the salt would be. scarcely inferior in this power.

A great part of the acid of sorrel may be obtained in the form of a concrete salt, which is more acid than that of tartar, more easily soluble in water, and less, if at all, purgative; the wood sorrel yields near one-hundredth part of the weight of the fresh leaves.

Different methods have been proposed to separate the mucilage of the expressed juices, which is the great impediment to their crystallization. The method of Stahl, and the elder chemists, consisted in repeated affusions of alcohol. The process of Scheele is now generally adopted: it consists in combining the acid with calcareous earth, which forms a neutral nearly insoluble in water: this neutral may then be repeatedly washed, and the vegetable acid recovered by the addition of the sulphuric. This was the process recommended for the salt of lemons, and is the method by which Mr. Coxwells concrete salt is prepared. But he wisely, we have been told, directs the chalk to be combined with the acid in the country; and it is imported in the form of the earthy salt; so that the fruit is not liable to injury by packing, and the inconveniencies of a sea voyage.