Uric acid is insoluble in alcohol and ether. Uric acid is with facility dissolved in a solution of the common or tribasic phosphate of soda, as also in solutions of many other salts of the alkalies, particularly in higher temperatures. In these cases uric acid combines with part of the base, and thereby gives rise to the formation of acid salts, which impart to the solution an acid reaction. Of these processes we shall have to speak when treating of the sediments. Crystallography. The most perfect and pure crystals of uric acid that can be obtained are oblong square plates; and if it were not for the combinations, the plates themselves would pass as rectangular and quadratic, but in fact they are rhombic plates with extremely obtuse angles. A secondary prism may take off the angles of the primary plate (plate or scale being only another expression for flat prism, or prism in which only two axes are developed). When the secondary prism begins to predominate, we have an hexagonal plate. In this form uric acid is frequently obtained in precipitating it with acetic acid from its ammoniacal solution. In this form it also crystallizes from its solution in hydrochloric acid. Plate I, fig. 5 and 6. In many cases the one pair of opposite angles of a plate become more acute, the other more obtuse; and plates may be observed varying from the rectangular form to that of needles. To these elementary forms all the crystals of uric acid occurring either in the form of spontaneous deposits in the urine or as products of a chemical process may be referred. Occasionally twin crystals, composed of two rhombic plates crossing at right angles, or at an angle of 45°, are observed. There is another multiplication of spinous crystals imitating the square plate, or placed upon the square plate. This modification is usually large, and frequently obtained from urine by the addition of nitric acid. The reader being thus in possession of the regular forms is enabled to classify all the irregularities usually occurring in spontaneous deposits. He will at a glance be able to put each of the forms in figure 1 on Plate II in its proper place. The common rhombic plates of uric acid deposits, when formed rather quickly round another substance, such as granules of urates, present holes in their substance, and irregularities, such as usually attend a hasty formation. obtuse angles frequently become rounded off, less frequently the pointed angles. In both cases the rhombic plates become The oval. This rounding off of the obtuse angles is characteristic of the so-called lozenges. It would be useless to describe all the varieties of crystallization in which uric acid deposits may occur. If the reader is once acquainted with the types, he will have no difficulty in classing any deposit that may occur to him. As yet no rule has been found according to which one or the other of the varieties of crystals is formed. Concentration and temperature of the urine, the relative quantities of its ingredients, particularly the absence or presence of colouring matter, and its amount, the nature and quantity of the acid by which uric acid is precipitated, determine the nature of the crystallization. It is the same in producing crystals artificially. If the urine be kept in a warm place for that purpose, the crystals will be the largest that can be obtained by any means. I have obtained crystals one quarter of an inch in length. Uric acid has the property of polarizing light. The large plates obtained artificially polarize only faintly, as they are invariably covered with a sort of dew. But the more transparent the crystals become, the more beautiful will be the colours they exhibit under polarized light. Crystals obtained from urine by acetic acid and from a solution in hydrochloric acid are particularly suited for the experiment on polarization. Metamorphoses of Uric Acid by different agents. Under the influence of heat, uric acid is decomposed without being fused. By dry distillation, a sublimate is obtained, consisting of cyanuric acid, urea, carbonate of ammonia, and cyanide of ammonium. At the same time a great deal of hydrocyanic acid escapes, and some carbon remains behind, which is porous and contains nitrogen. Concentrated nitric acid (4 parts of 1:42 specific gravity) dissolves uric acid (1 part) under effervescence, carbonic acid and nitrogen being disengaged. The reaction consists in the formation of alloxan, urea, and nitrous acid; the latter decomposing urea, in the moment of formation, into carbonic acid and nitrogen (vide Urea, p. 46). Alloxan remains in large colourless rhombic octahedra, which are disintegrated under the influence of air, and are readily soluble in water. The solution reddens litmus paper, and imparts a purple colour to the skin. On dissolving uric acid in dilute nitric acid, alloxantine is obtained, which is distinguished by producing a purple reaction, when ammonia is added to its warm solution. This colour is due to the formation of murexide, otherwise known under the name of purpurate of ammonia. This reaction is diagnostic of uric acid. On making uric acid and water into a pap and gradually adding peroxyde of lead, and keeping the mixture near the boiling point, the lead is transformed into the oxalate of lead, carbonic acid is evolved with effervescence, and the filtered fluid deposits crystals of allantoine on cooling. The motherliquor yields urea. This reaction consists in the first instance in the transformation of uric acid into allantoine, carbonate of lead being formed. Part of the allantoine is further decomposed into urea and oxalic acid. C1HNO + 2H + 2Pb O2 = CH ̧NO + 2(PыO, CO1) 10 4 Uric acid. 6 6 6 Water. Peroxyde of Allantoine. Carbonate of lead. lead. CH ̧NO + 2HO + 2PbO, = 2(C,H ̧Ñ2O) + 2(PbO, CO) 61 4 6 Allantoine. Water. Peroxyde of Urea. Oxalate of lead. All substances derived from uric acid may be transmuted either into urea and oxalic acid, or into urea and mesoxalic acid. This has led to the assumption of three radicals in uric acid, of which the one, Cy (cyanogen), is the radical of urea; the second one, CO, (oxalyl), the radical of oxalic acid; and the third, CO (mesoxalyl), of mesoxalic acid. Upon this basis the derivates of uric acid have been grouped by Gerhardt' as follows: It will be found useful to have mentioned these relations, as we shall have to explain several phenomena on their basis. Determination of Uric Acid in Urine.' On adding to the solution of any urate, either hydrochloric, nitric, or acetic acid, uric acid is precipitated. If the solution of the urate is concentrated, the precipitate will fall immediately, and consists of amorphous hydrated uric acid, becoming crystalline by heat or standing. But if the solution. is dilute, no immediate precipitate is formed, but crystals of uric acid form after some (twenty-four to forty-eight) hours of standing; and they will be the larger in proportion to the time required for their formation. To obtain, therefore, uric acid from urine, it is precipitated by the addition of an acid, no matter whether the urine is normal, or contains sugar, albumen, or any of the soluble constituents of blood.2 If the urine be normal, and contain no albumen, nitric acid is best used, and the urine is allowed to stand in a warm place, a sand-bath, or any warm corner. A temperature of 98° F. (36-67° C.) has, moreover, the advantage of not admitting the precipitation of any urates, which are not easily acted upon by the dilute acid. The crystals formed in higher temperatures are much larger, and therefore much easier to be collected, than those produced at the ordinary temperature. A quantitative analysis, therefore, has better chances of accuracy in this way. I do not recommend hydrochloric acid for precipitating, because, uric acid being more soluble in it than in water, the analysis becomes incorrect. In some instances (for example a case of chronic rheumatism) I have obtained no definable precipitate by hydrochloric acid in a sample of the same urine, in which nitric acid produced a number of large plates on the surface of the fluid and the sides of the glass. Moreover, hydrochloric acid favours the acid fermentation and development of certain descriptions of confervoid vegetations, which act as yeast-cells on urates, decomposing them very rapidly. This process is entirely prevented by nitric acid. If the urine contain Heintz, Poggend. Ann.,' lxx, p. 122. Lehmann, ‘Journ. für Pract. Chem.,' xxv, p. 17; Lehrb. der Physiol. Chem.,' 1853, i, p. 197. Demonstration of Uric Acid, vide E. v. Gorup- Besanez, Anleit. zur Zoochem. Analyse,' 1854, 2d edit., p. 186. 2 Gerhardt, loc cit., albumen, acetic acid, or the common phosphoric acid, are best used. The crystallization of uric acid may be said to be completed when all the crystals are of nearly uniform size. But as we possess no other indication on this point, the acidulated fluid must be allowed to stand rather longer, to make sure of a perfect precipitation. The crystals are collected into a deposit, washed by decanting, and collected on a filter. It has been estimated by Heintz that the loss incurred by the imperfect insolubility in acidulated urine of uric acid amounts to 9 parts in 100,000 of the urine used for the analysis, and that this loss is not increased by the presence of sugar, albumen, or the soluble constituents of blood. In all cases this loss is compensated by a certain amount of red or brownish colouring matter, which is mechanically but intimately adherent to the particles composing the crystals. If the urine contain bile, the uric acid crystals may contain so much colouring matter, that it may be necessary to purify them of it. This result is best obtained by the following method of Lehmann: The urine is evaporated to the consistence of a syrup, which is extracted with alcohol. The insoluble part is treated with a dilute solution of caustic potash; the solution so obtained is made boiling hot, and precipitated by acetic acid, when all the uric acid falls down in a pure state. It is freed from the adherent impure fluid by washing with water containing some acetic acid. To the weight of the uric acid so obtained, the above estimated loss must be added. In speaking of quantities it may be as well to mention that, for the quantitative analysis by precipitation, a measured quantity of urine of not less than 100 c.c., or better, 150 c.c., is necessary. To this in a beaker are added 5 per cent. of the ordinary acids. For this method it is moreover, from obvious reasons, advisable not to use urine of a very low specific gravity; but if the latter be less than 1012, to concentrate the fluid by evaporation on the water-bath to about the normal average specific gravity. For the method of Lehmann, about 50 c.c. of urine will be sufficient to give tolerable results; though here, as everywhere, the relative loss diminishes in proportion to the increase in the quantities operated on. The precipitate of uric acid obtained by either method, after being collected on a filter of known weight, and there once more washed with water acidulated with acetic acid, is now dried in the hot-water bath. When dry, it is placed between two watch-glasses, fitting closely to each other by ground edges, and held together by brass clips made for that |