and in combination principally with sodium, forms the white friable concretions often improperly called chalk stones. Uric acid is best prepared from the dried urine of the boa by dissolving I part of the powdered mass in from 40 to 50 parts of boiling water, to which an excess of caustic potash is added, sufficient to bring the whole of the acid into solution: during this operation ammonia escapes abundantly. The heat must be continued until the ammoniacal odour has disappeared. The brownish liquid thus procured contains an impure potassic urate, which must be filtered while hot, and decomposed by adding a slight excess of hydrochloric acid. Uric acid is immediately separated in minute white crystals; these must be washed with cold water, dissolved a second time in solution of potash, and once more precipitated by hydrochloric acid; the product, after being thoroughly washed, and then dried, is quite pure. Uric acid crystallizes in rhombic tables, the outlines of which are frequently rounded; but when it is deposited from animal fluids the form of the crystals is often much modified. If a cold saturated solution of potassic urate be decomposed by hydrochloric acid, large crystals of the acid are obtained with 2 H2O. This water is expelled when the acid is heated to 212°.* Pure uric acid is a white crystalline powder requiring 10,000 parts of cold water for solution, to which, however, it imparts a feebly acid reaction. Uric acid is insoluble in alcohol and in ether, but it is dissolved by concentrated sulphuric acid, and is deposited in a hydrated condition on diluting the solution. The urates of potassium and of ammonium are more soluble than the free acid. The acid urates of ammonium, of sodium, and of calcium are frequent ingredients of urinary calculi; the proportion of the calcium salt, however, is always very small. reaction of uric acid with nitric acid and ammonia is highly characteristic, and it enables the acid to be detected in very minute quantities; the method of proceeding will be described under the head of murexid (1638). By the action of reducing agents, such as a very dilute amalgam of sodium, Strecker found that first one atom of oxygen, and then a second atom could be removed from uric acid, xanthine being first obtained, and then hypoxanthine in considerable quantity (1613). The (1620) Urates.-Uric acid is dibasic. Its salts have been * When uric acid is long boiled with a solution of potash, the elements of water are assimilated, and potassic uroxanate (K2,H,N, 3 H,e) is formed in small quantity. On the addition of a strong acid, uroxanic acid is separated in sparingly soluble, microscopic, tetrahedral crystals (H ̧¤ ̧H ̧Ñ‚→). URATES. 2 4 803 carefully examined by Bensch (Liebig's Annal. liv. 189, and lxv. 181). All the urates are but sparingly soluble in water; they are much more soluble in alkaline solutions, especially when heated. They are also freely soluble in solutions of borax. Two urates of potassium may be formed: the acid salt (KHЄ,H,N12) is the one best known. It is precipitated in a granular form when carbonic anhydride is transmitted through a solution of the normal salt. It is soluble in about 80 parts of boiling water, from which, on cooling, it is deposited in amorphous flocculi; it requires about 8co parts of cold water for solution. Normal potassic urate (K‚¤¡Í ̧Ñ‚Ð ̧) is obtained by saturating a cold dilute solution of potash, free from carbonate, with uric acid suspended in water, and boiling down the solution in a retort. At a certain degree of concentration the salt is deposited in fine anhydrous needles: the clear liquid must be decanted from them, and they must be washed with dilute alcohol. The salt requires about 44 parts of cold water for solution, and 35 of boiling water. Its solution is slowly decomposed by ebullition. This salt has a caustic taste, and absorbs carbonic acid rapidly from the air. 2 5 2 4 5 2 4 4 There are two urates of sodium; the normal urate being (Na,ЄH2N,,,H,O), the acid urate being (NaH,Є.H ̧Ñ‚Ð ̧); they are each somewhat less soluble than the corresponding potassium salts. The acid urate of ammonium (H ̧NH,¤ ̧H,N,,) is the only urate of ammonium which is known. It usually forms a white amorphous mass, which is soluble in 1800 parts of cold, and 240 of boiling water; it is deposited in fine needles from a boiling aqueous solution of it, which contains an excess of ammonia. Lithium urate is more soluble than any other of these salts. Hence lithia water is occasionally prescribed to gouty patients, and to others who suffer from a superabundance of uric acid. Only the acid urate of magnesium (MgH,, 2 ¤ ̧Н ̧Ñ‚Ð ̧‚ 6 H ̧¤) is known; but both a normal (Єa¤ ̧H,N ̧ ̧) and an acid urate of calcium (ЄaH2, 2 ЄH2NO3, 2 H,→) may be obtained. 5 21 4 4 5 4 32 (1621) Products of the Decomposition of Uric Acid.--When uric acid is submitted to distillation it does not fuse, but undergoes decomposition, producing ammonium carbonate, hydrocyanic acid, cyanuric acid, urea, and some empyreumatic products. Uric acid is decomposed when heated with fused caustic potash, ammonia is evolved, and the residue contains potassic cyanide mixed with potassic carbonate and oxalate: we have already referred to the action of reducing agents upon it. But the most remarkable chemical peculiarity of uric acid is the facility with which it is altered by oxidizing agents, and the number of definite 804 PRODUCTS OF THE DECOMPOSITION OF URIC ACID. and crystallizable compounds to which it gives rise. These products were first investigated by Liebig and Wöhler (Liebig's Annal. xxvi. 241); and the importance of the acid as an excrementitious product has led to further minute investigations by others subscquently, particularly by Schlieper, by Strecker, and still more fully by Baeyer (Liebig's Annal. cxxvii., exxx., and exxxi.). The oxidation of uric acid may be effected in various ways, such as by means of nitric acid, by peroxide of lead, by potassic ferricyanide mixed with caustic potash; the products of oxidation differing with the energy of the action according to the nature of the agent employed. But notwithstanding the numerous researches of which uric acid has been the subject, the exact mode of its construction still remains unknown; for it has never been broken up into the actual molecules of the bodies from which it was formed, nor have the attempts to produce it synthetically been hitherto attended with success. Gerhardt proposed to regard uric acid as the diureide of tartronic acid, in accordance with the equation:— but it is doubtful whether this supposition is correct, inasmuch as the ureic acids are very unstable, whilst the uric acid is not so; besides which, it has not as yet been possible to obtain uric acid from tartronic acid and urea. It is, however, certain that when uric acid is subjected to oxidation in the presence of water, the nitrogen is wholly removed in the form of urea, which often breaks up, owing to the action. of the acids or alkalies employed in the process; whilst the remaining non-azotised constituents of the uric acid furnish either mesoxalic acid, or a product of its decomposition by further oxidation : The abstraction of urea from uric acid is, however, generally effected in two stages, the first portion being removed more easily than the second. If one atom of urea only is got rid of, alloxan is formed; if both atoms are removed, mesoxalic acid is the product. When, for instance, dilute nitric acid acts upon uric acid, alloxan is obtained : PRODUCTS OF THE DECOMPOSITION OF URIC ACID. 805 whilst the second atom of urea is removed when the alloxan is boiled with baryta water: In both cases the urea, in the act of liberation, is in great measure decomposed by the action of the acid and of the alkali, which are not represented in the equations. The last equation shows that alloxan may itself be regarded as the monureide of mesoxalic acid, since the alloxan would be derived from the action. of one molecule of the acid upon one of urea, with elimination of an atom of water. The decomposition of uric acid, according to Hardy, may be effected at common temperatures by means of bromine, which is to be gradually added in excess, to finely-divided uric acid suspended in water. Under these circumstances the uric acid is wholly resolved into alloxan and urea, while hydrobromic acid is formed by the removal of two atoms of hydrogen from one of uric acid :— Uric acid. Alloxan. Urea. + 2 HBr. The following table contains a list of the most important derivatives of uric acid : 806 RELATIONS OF THE OXIDIZED Mesoxalic acid may be made to furnish the key to the greater number of compounds enumerated in the foregoing table. If submitted to the action of reducing agents, such as sodium-amalgam, the mesoxalic combines with 2 additional atoms of hydrogen, and yields tartronic acid, and tartronic acid, by a further reducing action, is converted into malonic acid : If each of these three acids be submitted to oxidizing agents, it loses one molecule of carbonic oxide, and furnishes a new acid containing two atoms only of carbon :— Now each of the six acids just mentioned is susceptible of being combined with one or with two atoms of urea whilst the elements of water are separated. Compounds so obtained constitute what Baeyer terms monureides, when a single molecule of urea enters into their formation; when two atoms of urea unite with the elements of each atom of the acid diureides are formed. In the table on the following page the numerous ureic compounds of this kind are arranged so as to indicate their mutual relations. In the first column are the generating acids; in the second the corresponding monureides formed by the union of the acid with one atom of urea, and elimination of a single atom of water, as in the formation of oxaluric acid from oxalic acid:— In the third column are the more numerous monureides formed by the action of the acid in the same horizontal line upon one molecule of urea, with elimination of two atoms of water; as in the formation of parabanic from oxalic acid :— The fourth column indicates diureides formed with the acid and 2 molecules of urea, with elimination of 1 atom of water; whilst |