UREDO. 



[ 663 ] 



URIC ACID. 



des Sc. nat. 3 ser. vii. p. 12; 4 ser. p. 77 ; 

 Leveille, ibid. 3 ser. vhi. p. 369 ; De Bary, 

 Brandpilze, Berlin, 1853; Fries, Summa 

 Veg. p. 509; Unger, Exanthem. Plant.; and 

 the works cited under the Genera. 



UREDO, Pers. See UREDINEI. 



URIC ACID and URATES, or lithic acid 

 and lithates. Uric acid may easily be pro- 

 cured in small quantity from human urine, by 

 adding a few drops of dilute muriatic acid, 

 and setting the liquid aside for some hours, 

 when it subsides in crystals. In larger 

 quantity it may be obtained by heating the 

 excrement of serpents with excess of dilute 

 solution of potash, until the odour of ammo- 

 nia has disappeared, and filtering the solution 

 whilst hot into dilute muriatic acid, when it 

 falls in a colourless state. Or the excrement 

 may be digested, without heat, with excess 

 of strong sulphuric acid, the mixture set 

 aside that the impurities may subside, and 

 subsequently poured gradually into a large 

 quantity of distilled water. 



It exists also in the excrement of birds, 

 in the urine of Mollusca and insects, and of 

 all the Mammalia, excepting those which 

 are herbivorous ; it has also been found in 

 the human blood, of which it is probably a 

 normal constituent in minute quantity, al- 

 though mostly secreted with the urine as 

 soon as formed. 



In the natural state of solution in the 

 urine, uric acid exists combined with soda 

 and ammonia, but it is frequently found as 

 an abnormal deposit in the human urine, 

 and is often precipitated after the secretion 

 has been evacuated, from the occurrence of 

 an acid fermentation. The crystals of the 

 free acid are sometimes also met with in the 

 urine or excrement of the lower animals, as 

 Insects, &c, 



Uric acid is but little affected by water, 

 alcohol, acetic or muriatic acid ; slowly so- 

 luble in solution of ammonia, but readily in 

 solution of potash, from which it is re-pre- 

 cipitated by a dilute acid. 



The crystals belong to the right rhombic 

 prismatic system. 



Their various forms are represented in PI. 

 8. figs. 1-10, and fig. 15. Those in fig. 1 

 are frequently met with as natural deposits 

 from human urine ; although most of the 

 same forms, with those in fig. 15, are 

 also found in the artificially precipitated 

 acid. The most common and characteristic 

 form is the rhomb (a), the side view being 

 linear or rectangular. When the urine is 

 strongly acid, the crystals often appear stri- 



ated from the presence of linear fissures (c, 

 d). Sometimes they are narrower and more 

 elongate, with a prismatic form (e). They 

 are frequently aggregated, and either fused 

 into twin crystals (/, g\ or form aigrettes or 

 tufts (k, I, m, n, o). The other forms are 

 noticed in the description of the plate. 



The crystals forming a natural deposit are 

 almost invariably coloured, from combining 

 with the colouring matter of the urine; 

 sometimes their colour is very brilliant (fig. 

 4); they may also be coloured artificially by 

 precipitation from a solution of purpurate of 

 ammonia (fig. 3), madder, &c. 



The test for uric acid is the production of 

 the colour of purpurate of ammonia or mu- 

 rexide, which may be effected by dissolving 

 the crystals or suspected substance in a small 

 quantity of dilute nitric acid, gently evapo- 

 rating the solution to dryness, and adding a 

 little ammonia to the residue, or exposing it 

 to the vapour of ammonia, when the red 

 colour becomes visible. But the rhombic 

 form, when present, with the action of potash 

 and dilute acid, would be sufficient to di- 

 stinguish uric acid from most substances. 



The formation of the crystals of uric acid 

 presents an interesting object for examina- 

 tion. A drop or two of solution of uric acid 

 in potash is first placed upon a slide and 

 covered with thin glass; a little dilute 

 muriatic acid is then applied to the edge of 

 the liquid, or a drop of strong acetic acid 

 placed near its edge, so that the vapour may 

 be absorbed by the liquid. The latter soon 

 becomes turbid from the formation of a pre- 

 cipitate of numerous molecules and granules. 

 If the turbid liquid be watched under the 

 microscope, a minute crystal will presently 

 be seen to form suddenly in some part of 

 the field. The molecules and granules then 

 slowly dissolve immediately around the cry- 

 stal, leaving this in the middle of a clear 

 space. The crystal now enlarges, and the 

 surrounding molecules gradually disappear, 

 until they at last entirely vanish from the 

 field. By careful inspection it may easily 

 be seen that the crystal is not formed by the 

 conflux of the precipitated molecules, but is 

 deposited from a state of solution. 



Some crystals of uric acid polarize light 

 splendidly, and some of the feathery crystals 

 (PI. 8. fig. 8 e) possess considerable analytic 

 power. 



The forms of the crystals and crystalline 

 groups of the urates are represented in PI. 8. 

 figs. 11-14 ; they are not very characteristic, 

 and the aid of chemistry is required for de- 



