The oldest method of separating tannins from other constituents is that applied by Pelouze to the preparation of commercial gallotannic acid from gall-nuts. The finely pulverised material is placed in a percolator and exhausted with commercial ether containing water and alcohol. The liquid separates, on standing, into 2 layers of which the lower contains most of the tannin in a tolerably pure form, dissolved in water and alcohol with a little ether, while the upper mainly ethereous layer contains the gallic acid. Gall-nuts thus treated yield 35-40 per cent. of tannin. If equal parts of ether and 90 per cent. alcohol are used, a larger yield is obtained, but the liquid does not separate into 2 layers, and it is questionable if the product is so pure. For Chinese galls, washed ether acts better than ether alcohol. The tannin may be still further purified by dissolving in a mixture of 1 part water with 2 of ether, when 3 layers are formed, of which the lowest contains nearly pure tannin. These methods are applicable to the dried or highly concentrated extracts of many tanning materials. Many tannins may be separated from their strong aqueous solution in a state of considerable purity by first agitating with ether to remove gallic acid, and then saturating with common salt, and shaking well with acetic ether, which takes up the tannin. Another method is to extract with alcohol, evaporate to a small bulk at as low a temperature as possible, and treat at once with a considerable quantity of cold water. The infusion is then precipitated with successive small quantities of lead acetate; the first and last portions of the precipitate are filtered off and rejected as contaminated with colouring matters and other impurities, while the remainder, after rapid washing, is suspended in water and decomposed with sulphuretted hydrogen. The filtrate is shaken with ether to remove gallic acid, and the aqueous portion is evaporated at a low temperature in a partial vacuum to a thin syrup, and the drying completed over sulphuric acid in vacuo. General Chemistry. The natural tannins are all compounds of carbon, hydrogen and oxygen only. They all contain the benzene group of carbon atoms, but their ultimate structure is, except in the case of gallotannic acid, very imperfectly understood, and probably differs considerably in type in different members of the family.