2. Bark of cinchona officinalis. This tree grows in Quito ; it is confined to the high grounds, and when stripped of its bark soon dies. There are three different kinds of bark to be found in commerce, but whether they be all obtained from the same trees is not known ; the contrary is probable. The following are the most re markable of these varieties. Red Peru vian bark.—This bark is usually in larg' pieces, and is reducible to powder with more ease than the preceding. Its pow der is reddish brown, and has a slightly bitter taste, with a good deal of astrin gency. Yellow Peruvian bark.—This spe cies of bark, first brought into use in this country about the year 1790, has not yet been subjected to a rigorous analysis ; but its constituents do not appear, from the trials which have been made, to differ much from those of the red species. Pale Peruvian bark.—This is the common variety of the bark. It has not yet been subjected to a correct chemical analysis. Its taste is astringent and bitter, and very disagreeable. It is supposed to contain a bitter principle, tannin, extractive, and resin. Besides these, it contains a prin ciple first pointed out by Seguin, and up on which Dr. Duncan, junior, published some experiments. It is distinguished by the property of precipitating infusion of galls ; but as this property is common to a considerable number of substances, it is not sufficient alone to characterize it.
3. Bark of cinch ona caribxa.—This bark was first made known by Dr. Wright, who published a botanical description of the tree, with a figure, in the Philosophi cal Transactions, vol. 67, and an account
of the medicinal properties of the bark in the London Medical Journal for 1787. A description of a tree to which the same name is given, together with a chemical analysis of the bark, was published in the Journal de Physique for 1790, by M. Ira vasseur ; but it is not quite certain that the plants are the same.
4. Bark of the white willow (salix al ba.)—The bark of this tree, which is common enough in Scotland, is remark able for its astringent taste, and has been often used in intermittents by the common people. It has lately been proposed by Bouillon la Grange as an excellent sub stitute for Peruvian bark ; being com posed, according to him, of the very same constituents to which that bark owes its medical virtues. A very superficial ex amination, however, may satisfy any one, that the properties of the two are very far from similar.
5. Bark of quercus nigra.—This tree, to which the name of quercitron has been given, grows spontaneously in North America. Dr. Bancroft discovered, about the year 1784, that the inner bark of this tree contains a great quantity of colour ing matter ; and since that time it has been very generally used by the dyers. To prepare it for them, the epidermis (which contains a brown colouring mat ter) is shaved off, and then the bark is ground in a mill. It separates partly to stringy filaments, and partly into a fine light powder.