URIC ACID. 761 XIRIM AND THUMMIM. Tile best and most read.v mode of obtaininc; uric acid on a large scale is from guano, or from the excrements of snakes or fowls, which consist largely of urates. For this purpose the mass is boiled with potash, which expels any ammonia that is present, and a stream of carbonic acid is then passed through the strained potash solution, which throws down acid urate of potash. Tiiis precipitate is dissolved in water, and decom- posed by hydrochloric acid, which throws down the uric acid in minute crystals. The form in which this acid crystallizes is liable to great variations. Sometimes we have flat tablets re- sembling whetstones, or sections with a double knife thi-ough biconvex lenses; at other times, more or less perfect rhombic tablets, and some- times hexagonal plates, barrel-shaped prisms, and toothed crystals. It is, however, difficult to identify uric acid crystals by direct inspection; so that, to prove the presence of uric acid, tests of a chemical nature must be applied. The so- called murexid reaction constitutes an excellent test: the substance submitted for examination is treated with a little strong nitric acid, and the mixture is evaporated to dryness at a gentle heat ; the residue is moistened with a trace of ammonia, which, if uric acid was present in the original siibstance. produces a brilliant purple coloration ; a drop of caustic soda changes the color to a reddish bhie that disappears on warm- ing the solution. The test is based on the fact that warming with nitric acid converts urea into purpuric acid, the acid ammonium salt of which — called murexid — has a brilliant reddish- purple color. By the oxidation of uric acid two series of in- teresting products are obtained. If nitric acid is used as the oxidizing agent, uric acid splits up into urea and alloxan (a ureide). If, instead of nitric acid, potassiiim permanganate is era- ployed, uric acid breaks up into carbonic acid and allaiifoin (another ureide). Alloxan can be readily transformed into parabanic acid (the ureide of oxalic acid), and from the latter urea and oxalic acid can be obtained with eqiial readi- ness. On the other hand, allantoin can be con- verted, by a series of simple reactions, into urea, allanturic acid (glyox-l urea), parabanic acid (oxalyl urea), and hydantoic acid, the latter being closely allied to hydantoin (glycolyl urea). The substituted ureas (ureides) obtained from uric acid may also be prepared directly from urea. (See Ureas. Compound.) It is, there- fore, clear that uric acid and urea are chemi- cally closely related to each other. In mammals nitrogenous substances are disintegrated mainly into urea; in birds the main product of disin- tegration is uric acid. The formation of both urea and uric acid takes place most probably in the liver. The urine of mammals sometimes con- tains allantoin, hydrated parabanic acid, oxalic acid, and other products, which are obtained in the laboratory by oxidizing uric acid. The products of decomposition of uric acid were first thoroughly investigated, in a series of celebrated researches, by Liebig and Wiihler. Then the chemical relationship of urea and uric acid was clearly established. It has since been shown that uric acid is a di-ureide, i.e. a sub- stitution product containing two molecules of urea. After much painstaking research, carried on by some of the best clicmists of the century, it was possible to demonstrate the true chemical constitution of uric acid, and finally llorliac- zewsky succeeded in preparing this important organic substance by a synthetic nietlui(l ; Bch- rend and Roo.sen subsequently discovered an- other synthesis of uric acid, which is relatively simple and ])erfectly well understood in the light of our knowledge of the constitution of uric acid. The chemical constitution of uric acid is rep- resented by the following structural formula, tirst proposed by Medicus: .NH— O— NH / II );co CO C— NH^
I
^NH— CO Uric acid. URICO'NIUM, or VIR'ICO'NITJM. An an- cient Roman city of Britain, the site of which is about four miles to the east of Shrew.sbury, and partly occupied by the village of Wroxeter. It is mentioned by Ptolemy as existing in the beginning of the second century of our era, and it is prob- able that there was a Roman station here before A.D. 70. The traces of the ancient wall sliow that it had a circumference of about three miles, and through it ran the great Roman road now known as Watling Street. Exploration of the re- mains began in 1859. On the north side of the old wall was probably the Basilica, and on the south the public baths. A few inscriptions and numerous small objects, such as coins, brooches, hairpins, pottery, sculptures, etc., found in the ruins are in the museinn at Shrewsbury, and some of the buildings excavated have since been covered. The human remains found in the ex- cavations show that the city was sacked and burned by enemies, probably the West Saxons, in the sixth century. Of this, however, there is no historic record. The ruins seem to have re- mained with little change, except the gradual process of decay, till about the twelfth century, when they were used as material for other build- ings. Consult J. C. Anderson, The Roman City of Vriconium at Wroxeter (London, 1867). URIEL (Heb. tJricl, Light of God). One of the seven archangels, who personified light and had his dwelling in the sun. He appears in Jlilton's Paradise Lost and Longfellow's Golden, Legend. U'RIM AND THUM'ldM (Heb. urlni wetummun) . The name given to two objects, perhaps pebbles or stones in the shape of dice, kept in the oracle-pouch that hung upon the breast of the Jewish high priest (Ex. xxviii. .'^O; see Epiiod) . They were used to obtain an oracular decision revealing the Divine will. While we do not know the precise manner in which this de- cision was obtained, analogy from similar cus- toms among ancient nations makes it probable that the stones were drawn by lot out of the pouch, and, according to the one drawn, an af- firmative or negative answer to a question was given. An entirely satisfactory explanation of the two mysterious names has not yet been fur- nished, but all the evidence points in favor of a connection with two Babylonian stems («'»r« and tamu), from which are derived ttrtu, 'de-
