yielded 10 grammes more of crystallized gallic acid, darker coloured than the first crystals. To free these crystals entirely from colouring matter, they were mixed with eight times their weight of water, and about the fifth of their weight of ivoryblack, and the mixture was kept for about a quarter of an hour at the boiling temperature. It was then filtered while hot; on cooling it concreted into a mass of perfectly white crystals of gallic acid, which were separated from the liquid by pressure in a cloth.* Gallic acid obtained by this last process is nearly pure, and it may be freed from the tannin which still adheres to it by sublimation. Class I. Div. II. When thus obtained it is snow white, and is in small crystals Properties. having a foliated fracture. The shape of the crystals is usually a flat rectangular prism, or a flat rectangular plate like mica. Mr. Brooke describes it as a doubly oblique prism.+ But it is evident from his own descriptions, that the sides M, M' of his figure are inclined to each other at right angles, so that the prism must be rectangular and not oblique. The taste of sublimed gallic acid is bitter, and it leaves in the mouth an impression of sweetness. It does not redden paper stained blue with litmus. The acid obtained by Braconnot's process has a sour taste. It is soluble in 1 parts of boiling water, and in 12 parts of cold water. When this solution is heated, the acid undergoes a very speedy decomposition. Alcohol dissolves one-fourth of its weight of this acid at the temperature of the atmosphere. When boiling-hot, it dissolves a quantity equal to its own weight. It is soluble also in ether. The solutions are colourless. Gallic acid in crystals is not altered by exposure to the air. Neither oxygen gas, the simple combustibles, nor azote, seem to have any particular action on it. When the solution of this acid in water is exposed to the air, it gradually acquires a brown colour, and the acid is destroyed; the surface of the liquid becoming covered with mouldiness. It combines with alkaline bodies, separating the carbonic acid if they were in the state of carbonates. The compounds formed have received the name of gallates; but hitherto have scarcely been examined. Indeed they may be said not to exist. For bases, as has been shown by Mr. Sertürner, have the property of acting on gallic acid and speedily decomposing it.‡ * Ann. de Chim. et de Phys. ix. 181. ↑ Annals of Philosophy, vi. 119. Schweigger's Journal, iv. 410. Chap. I. Composition. When nitric acid is poured on this acid, a brisk effervescence takes place, heat is evolved, and the acid is dissolved very rapidly. The solution has a red colour, and when concentrated, crystals of oxalic acid are deposited, and a white insoluble matter also appears. To a solution of 13-19 grains of bicarbonate of potash (containing 6 grains of potash), I added 8 grains of crystallized and anhydrous gallic acid. There was a strong effervescence which accompanied the solution of the acid. But after the whole acid was dissolved the liquid still retained an alkaline taste, and rendered cudbear paper violet as alkalies do. Even the addition of 8 more grains of the acid did not destroy the alkaline nature of the liquid. The solution was at first colourless; but after two days it became green, and this colour gradually deepened so much, that the liquid became opaque and looked like ink. When evaporated to dryness it left a substance like pitch, which dissolved with effervescence in nitric acid, and the solution was brown. It is obvious from this that the acid was decomposed by the action of the potash on it. Accordingly the solution of gallate of potash in water does not exhibit the characteristic properties of gallic acid, when mixed with a solution of sulphate of iron. The characteristic property of gallic acid is to strike a deep blue with the salts of iron, particularly the sulphate. The tannin, which is another constituent of nutgalls, possesses the same property; but it has an exceedingly astringent taste, and throws down a buff coloured precipitate when dropt into an aqueous solution of glue. Tannin itself seems to possess acid qualities; but as it has never been obtained exempt from gallic acid, it is possible that it may owe these qualities to the presence of that substance. Berzelius subjected the gallate of lead to analysis, and found its constituents to be* This gives us 8 for its atomic weight. The acid itself, subjected to analysis, yielded These atomic proportions give 7.875 for the atomic weight of gallic acid. This is 0.125, or an atom of hydrogen less than what results from the analysis of gallate of lead. I am disposed to prefer this last analysis as likely to be most correct; to consider the atom of gallic acid to weigh 8, and its constituents to be Dobereiner informs us, that when 100 parts of gallic acid dissolved in ammonia are left in contact with oxygen gas, they absorb 38-09 parts of that gas, and are converted into ulmin. 100 parts of gallic acid, according to the above estimate, consist of Now 38-09 of oxygen, in order to form water, must combine with 4-76 of hydrogen. There will remain 1·69 of hydrogen. So that ulmin ought to be a compound of This is very little more than an atom of hydrogen, so that ulmin would appear from Dobereiner's observation to be a compound of 1 atom hydrogen, 6 atoms carbon, 3 atoms oxygen." * Gilbert's Annalen, lxxiv. 410. Chap. I. History. Properties. Ellagic Acid. It may be proper to notice here a substance which Braconnot has distinguished by this very absurd name (the French word galle inverted). It possesses imperfectly acid properties, and was extracted from nutgalls at the same time with gallic acid. It would appear from a note of Chevreul that it had been already detected by him, and several of its properties described in the article tannin, published in the chemical part of the Encyclopaedic Methodique, in 1815. But he had not given it a name, and does not seem to have been aware of its acid properties.' Ellagic acid was obtained by Braconnot by filtering the solution of gallic acid obtained from the crystals which had formed spontaneously in the infusion of nutgalls. A powder remained upon the filter, which was ellagic acid mixed with a certain quantity of gallate of lime. To free it from this salt it was treated with a dilute solution of potash, which dissolved the acid with the evolution of a considerable quantity of heat. The solution had an intense yellow colour, and gradually let fall a pretty considerable quantity of pearl-coloured powder, which was separated by the filter and decomposed by dilute muriatic acid. The ellagic acid thus obtained is a white powder with a slight shade of buff. It is tasteless, and is not sensibly soluble even in boiling water. It does not decompose the alkaline carbonates even when assisted by heat. But it unites with caustic soda and potash, and destroys their alkaline characters. These salts are insoluble in water. But they become soluble if a little potash or soda be previously dissolved in that liquid. The solution has a very deep buff colour. The ellagate of ammonia is likewise insoluble, and does not become soluble even when an excess of ammonia is added. When this acid is agitated in lime water it separates the lime from the liquid. Nitric acid does not seem to act upon it at first, but if we continue the digestion it gradually assumes a blood-red colour. Ellagic acid does not combine with iodine. When heated it does not melt, but burns away with a sort of scintillation, without emitting flame. When distilled it leaves charcoal and produces a yellow vapour, which condenses into transparent crystals of a fine greenish-yellow colour. This sublimate is tasteless and insoluble in water, alcohol, and ether; but it dissolves readily in a solution of potash, and communicates a * Ann. de Chim. et de Phys. ix. 329. The vegetable substance to which I gave the name of ulmin, History. was first noticed by Klaproth in 1804, who found it in a matter which had exuded from the bark of the elm. It was noticed by Berzelius in 1810, in his experiments on the bark of the pine, and on Jesuit's bark.+ In the beginning of the year 1813 I published a set of experiments on this substance. About the same time, a similar set of experiments leading to nearly the same conclusions was made by Mr. Smithson.§ In 1830, M. Polydor Boullay published a set of experiments on it, in which he showed that it possessed the properties of an acid, and therefore gave it the name of ulmic acid.|| Ulmic acid appears to be a vegetable substance of very great importance. It not only exists ready formed in bark, but it makes its appearance in a great variety of vegetable decompositions. M. P. Boullay has found that when the sugar of grapes, dissolved in water, is heated with an alkaline ley, it is converted into ulmic acid. It constitutes the essential ingredient of peat and of umber. It would appear also to be an admirable manure, or rather to constitute what is usually called vegetable manure. Ulmic acid when obtained from the exudation of the elm by Properties. dissolving the exudation in water, and precipitating the ulmic acid by an acid, which saturates the potash with which it was in combination, consists of dark brown flocks almost destitute of taste and smell. It dissolves in small quantity in water and alcohol. The solutions have a brown colour, but no taste. What is called moss-water is merely water in which this acid is dissolved. When heated, it swells and catches fire, but does not melt. It combines with the different bases which it neutralizes, and forms compounds analogous to salts, and which therefore may be called ulmates. The solution of ulmate of potash¶ in water has a dark brown colour. Sulphate of iron precipitates it brown or sometimes green. All the acids throw down the ulmic acid by saturating the potash. Nitrate of silver throws it down of * Ann. de Chim. et de Phys. ix. P. 187. Annals of Philosophy, i. 23. † Afhandlingar, iii. 347. § Phil. Trans. 1813, p. 64. Ann. de Chim. et de Phys. xliii. 273. |