But this would make the atomic weight only 23, which does not agree with that deduced from the analysis of the carbazotates. To make them agree we must suppose 12 atoms carbon 24 atoms azote 16 atoms oxygen 29-75 approaches nearest to 30-41, the atomic weight deduced from the analysis of carbazotate of barytes. But Dr. Buff, who has published several interesting papers on indigo, assures us that Liebig ascertained that carbazotic acid is a compound of 16 atoms carbon 4 atoms azote 10 atoms oxygen 7.5 7.0 10.0 24.5* But where this determination appeared I do not know. It is equally inconsistent with Liebig's analysis of the carbazotates, and of carbazotic acid.+ SECTION XLIII.-OF INDIGOTIC ACID. Class I. Div. II. The existence of this acid was first pointed out by Chevreul History. in his paper on the action of nitric acid on indigo; but for the first account of its nature and properties we are indebted to Dr. Buff.‡ Fourcroy and Vauquelin had indeed observed it, but they mistook it for benzoic acid. The method of preparing the indigotic acid is as follows: Very dilute nitric acid is raised to the boiling point, and indigo in fine powder added by little and little. A considerable frothing takes place, the colour of the indigo is immediately destroyed. Indigo is to be added as long as the effervescence continues, taking care to prevent the concentration of the acid, by adding every now and then a little boiling water. The gas evolved consisted of equal volumes of carbonic acid and deutoxide of azote. The yellow coloured liquid thus obtained was separated while hot, from the resinous matter which had been deposited. Ann. de Chim. et de Phys. xxxix. 296. + Welter's bitter principle obtained by digesting silk in nitric acid, appears from Liebig's experiments to be also carbazotic acid. But silk yields a much smaller quantity of this acid than indigo. ‡ Ann. de Chim. et de Phys. xxxvii. 160; xxxix. 290; xli. 174. Chap. I. Properties. On cooling it let fall indigotic acid in very ill formed crystals. The indigotic acid thus obtained being still impure is to be pressed between folds of blotting paper, to free it from nitric acid as completely as possible, and then dried on the water bath. Hot water now dissolves indigotic acid, and leaves a quantity of insoluble matter behind. The aqueous solution (which is very dilute) is to be mixed with recently precipitated carbonate of lead suspended in water, taking care to stop when the effervescence (at first strong) begins to diminish, and the liquid ceases to be transparent. Unless this precaution be attended to, much of the acid precipitates in the state of disindigotate of lead. The liquid is now left till the impurities fall to the bottom. The transparent liquid being drawn off consists of a solution of pure indigotate of lead. The lead being separated by sulphuric acid, and the solution concentrated, yields crystals of pure indigotic acid. The crystals are snow white, very bulky while moist, but they diminish very much in volume, and lose their crystalline appearance when dried. The lustre is that of silk, and the colour exceedingly white. The indigotic acid has a weak, acidulous, and bitter taste. It slightly reddens infusion of litmus. 1000 parts of cold water dissolve only one part of this acid; but boiling water dissolves it in any proportion whatever. The solution is colourless. It is volatile. When heated in a glass tube it melts and sublimes without decomposition, giving out a sharp acid smell. On cooling it crystallizes in six-sided prisms. When heated over an open flame it takes fire and burns with a strong flame and the evolution of much smoke. When digested in concentrated nitric acid it is converted into carbazotic acid. Dilute muriatic and sulphuric acids have no action on it; but when heated in concentrated sulphuric acid a brown coloured solution is obtained, from which water throws down brown flocks. It is not decomposed by chlorine, nor altered. Nascent hydrogen is absorbed by it, and it assumes a copper-red colour, and bluish-red flocks are gradually thrown down. Indigotic acid gives a blood-red colour to the solution of peroxide of iron. In order to determine the atomic weight of this acid, M. Buff analyzed five different combinations of it with oxide of lead; but the results varied so much that there could be no doubt that every one of the salts examined contained a mixture of some of the others. The two that seemed to be the purest were obtained the first, by dissolving a little carbonate of lead in a great deal of indigotic acid, and setting the solution aside. Semitransparent yellow needles were deposited, which were freed from all adhering indigotic acid, by digesting them in alcohol. These crystals are soluble in water, but are decomposed at the same time. They are anhydrous, and composed of Indigotic acid Oxide of lead 26.71 14 The other salt was obtained by pouring a solution of nitrate of lead into indigotate of potash, raised to a boiling temperature. The liquid remained for some time transparent, but fine deep yellow needles gradually fell, which were insoluble in water. They were composed of Indigotic acid Oxide of lead 27.08 If we consider this last salt as a bi-indigotate of lead, and take the mean of the results, we obtain 26.89 for the atomic weight of indigotic acid. But the analysis of indigotate of potash, a salt in which no foreign substance existed, gave for the atomic weight of this acid 35-22. For it was composed of Indigotic acid 35.22 6 While the analysis of indigotate of mercury gave 35.5 for the atomic weight of this acid. Class I. Div. 11. Atomic weight. M. Buff analyzed the acid by means of oxide of copper, and composition, obtained the following constituents: Azote 7.55 48.21 The atomic proportions which suit these numbers are the fol This is less than the number derived from the indigotate of potash. If we were to admit the presence of 2 atoms of azote in order to get rid of the half atom, the atomic weight would come out 35, which I think likely to be the true number. Chap. I. History. SECTION XLIV.—OF URIC ACID. Various attempts were made by chemists to ascertain the nature of the calculous concretions which occasionally form in the kidneys and bladder, and produce one of the most painful diseases to which we are liable. These attempts were attended with very little success, till Scheele published a set of experiments on the subject in 1776. He examined several of these urinary calculi, and found them composed chiefly of a peculiar acid, the properties of which he described. His results were soon after confirmed by Bergman, who had engaged in asimilar set of experiments about the same time.* To the acid thus discovered, Morveau gave at first the name of bezoardic, which was afterwards changed into lithic by the French chemists, when they contrived the new chemical nomenclature in 1787. This last term, in consequence chiefly of the observations and objections of Dr. Pearson, was afterwards laid aside, and the name uric acid substituted in its place. Scheele ascertained that uric acid exists always in human urine. Experiments on the urinary calculi were published by Mr. Higgins in 1789,† and by Dr. Austin in 1791; but little was added to our knowledge of uric acid till Dr. Wollaston published his admirable paper on the calculous concretions in 1797. Dr. Pearson published a copious set of experiments on the same subject in 1798, in which he enumerates his trials on uric acid, and endeavours to prove that it is not entitled to the name of acid, but ought to be classed among animal oxides. This drew the attention of Foureroy and Vauquelin to the subject: they published an excellent treatise on urinary calculi, in which they demonstrate that it possesses the properties of an acid, and confirm the observations of Bergman and Scheele. But for the most complete account of uric acid we are indebted to Dr. Henry, who made it the subject of his thesis published in 1807, and afterwards inserted a paper on it in the second volume of the new series of Manchester Memoirs. To obtain pure uric acid, Dr. Henry dissolved pulverized calculi (previously known to be composed chiefly of that acid) in a ley of potash, and precipitated the uric acid by means of muriatic or acetic acids. The powder thus obtained was first washed with a little ammonia, to remove any adhering * Scheele, i. 199 and 210. French Trans. + Comparative view of the phlogistic and antiphlogistic theories, p. 283. foreign acid, and then edulcorated with a sufficient quantity of warm water. I obtained it in considerable quantity from the fæces of the boa constrictor. This serpent voids its excrements about once a month. They are white and solid, and have a certain resemblance to the album græcum voided by dogs. It was digested in caustic potash by which almost the whole of it was dissolved. The solution was drawn off clear from the undissolved sediment, and mixed with a sufficient quantity of pure acetic acid to saturate the whole of the potash. A copious white precipitate fell, which being thoroughly washed with water, and dried in a gentle heat, was pure uric acid. Class I. Div. II. It was sometimes in the state of a white impalpable powder, Properties. sometimes in small four-sided prisms, having considerable lustre. It is tasteless, very white, very light, and insoluble both in water and alcohol. In concentrated sulphuric acid it speedily assumes the form of a jelly, and upon applying a gentle heat a complete solution is obtained, without altering the colour or transparency of the acid. In nitric acid, even though dilute, it dissolves with effervescence, and when the solution is evaporated to dryness, it assumes a fine pink colour, which becomes much deeper when water is added, so as to have a near resemblance to carmine. In this state it stains wood, the skin, &c. of a beautiful red colour. The watery solution of this matter loses its red colour in a few hours, and it cannot afterwards be restored. When the solution of uric acid in nitric acid is boiled, a quantity of azotic gas, carbonic acid gas, and of prussic acid, is disengaged.+ Dr. Pearson, by repeated distillations, converted the residue into nitrate of ammonia. When chlorine gas is made to pass into water containing this acid suspended in it, the acid assumes a gelatinous appearance, then dissolves; carbonic acid gas is emitted, and the solution yields by evaporation muriate of ammonia, binoxalate of ammonia, muriatic acid, and malic acid.‡ It combines with the different bases, and forms a genus of salts called urates, for the examination of which we are chiefly indebted to Dr. Henry. When digested with water holding carbonate of soda in solution, it gradually dissolves, while the carbonic acid is disengaged. The solution, how much soever of the acid is added, still con ⚫ Scheele, Bergman, Pearson, and Henry. Brugnatelli; Ann. de Chim. xxvii. 267. Fourcroy, x. 222. |