A slight error is introduced by the presence of a trace of oxidizable matter in the glue, and when very great accuracy is required, it is well to make a blank estimation of "not-tannin " without tannin infusion, and deduct of the permanganate consumed ae a correction from the not tannin ; but this may usually be disregarded. Each titration should be made twice, and successive tests should not differ by more than 0.1 cc. of permanganate.
It is obvious that it is impossible by analysis to compare the relative values of different tannins, such as those of rnyrabolans and gambler, or hemlock and valonia. All analysis can reasonably be expected to do ie to give the relative values of different samples of the same substance, or at the most, of materials of the same class. All other comparisons are misleading ; and would be so, even if the exact percentage of each tannin could be calculated ; since the commercial and practical value of different materials does not depend on the quantity of tannin only, but on the character of the leather it produces, hard or soft, dark- or light-coloured, and heavy- or light-weighing.
H. R. P.
Algarobilla.—The seed-pods of Prosopis pallida and P. Algarrobo are known as algarobilla, the two kinds being distinguished as negro and blanco. The trees are abundant in mountainous parts of S. America, notably Chili and the Argentine Republic. The pods contain up to 50 per cent. of a bright-yellow tannin, somewhat resembling that of myrabolans. The friable tannin is readily soluble in cold water, and is so loosely held in the fibrous network of the pod, that great loss is sustained by careless handling. The commerce in algarobilla does not figure in the official trade returns ; but J. Gordon & Co., Liverpool, obligingly state that they imported 50 tons, at an average value of 18/. 10s. a ton, in 1880. Widow Duranty & Son, also of Liverpool, are good enough to add that they received 160 tons in 1881, the first that had reached them for a long time. Havre imported 50 tone in 1881.
The name algarrobo is also applied to Balsamoccupon brevifolium in Chili, and to Hymencea Courbaril (see p. 1666) in Panama.
Chestnut-extract. —The wood of Castanea vesca (see Nuts—Chestnut, pp. 1352-3 ; Timber) contains 14-20 per cent. of a dull-brown tannin. It is quite different from the bark and bark extract of the American chestnut-oak (Quercus sessitylora). Its extract is used largely to modify the colour produced by hemlock-extract, and for tanning and dyeing. The pulverized wood is also extensively employed in France. The imports are included in barks and extiacts, p._1988.
Cork-bark. See Oak-barke.
Cutch, Catechu, or Terra japonica (Fa., Cachou ; GER., Catechu).—The term kdt, kut, or " cutch," is applied to the dried extract, containing 45-55 per cent. of dark-coloured mimo tannic acid, prepared chiefly from two trees :—(1) Acacia Catechu [Mimosa Catechu, M. sundra], tree of 30-40 ft., common in most parte of India and Burma, growing also in the hotter and drier districts of Ceylon, and abundant in tropical E. Africa--the Soudan, Sennar, Abyssinia, the Noer country and Mozambique, though the utilization of its tannin is restricted to India ; (2) A. [M.] Suma, a large tree inhabiting S. India (Mysore), Bengal, and Gujerat.
The process for preparing outch varies slightly in different districts. The trees are reckoned to be of proper age when their trunke are about 1 ft. diam. They are tiaen cut down, and the whole of the woody part, with the exception of the smaller branches and the bark, is reduced to chips : some accounts state that only the darker heart-wood is thus used. The chips are placed with water in earthen jars, arranged in a Belies over a mud fire-place, usually in the open air. Here the water is made to boil, the liquor as it becomes thick and strong being decauted into another vessel, in which The evaporation is continued until the extract is sufficiently inspissated, when it is poured into moulds made of clay, or of leaves pinned together in the shape of cups, or in some districts on to a mat covered with the ashes of burnt cow-dung, the drying in each case being completed by exposure to the sun and air. The product is a dark-brown extract, which is the usual form in which cutch is known in Europe.
In Kuinaon, N. India, a slight modification of the process affords a, drug of very different appearance. Instead of evaporating the decoction to the condition of an extract, the inspissation is stopped at a certain point, and the liquor is allowed to cool, coagulate, and crystallize over twigs and leaves thrown into the pots for the purpose. By this method is obtained from each pot about 2 lb. of loth or catechu, of an ashy-whitish appearance. In Burma, the manufacture and export of vetch form one of the most important items of forest revenue. The quantity of catch exported from the province in 1869-70 was 10,782 tone, valued at 193,602/., of which, nearly half was the produce of manufactories situated in British territory. The article is imported in mats, bags, and boxes, often enveloped in the large leaf of Dipterocarpus tuberculatus. It is brought down from Berar and Nepal to Calcutta. That of Pegu has a high reputation.