URIC ACID. 



[ 794 ] URINARY DEPOSITS. 



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 

 lias 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 ani- 

 mals, as Insects, &c. 



Uric acid is l3ut 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 

 PL 12. figs. 1-10, and fig. 1/5. 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 (), the side view being linear 

 or rectangular. When the urine is strongly 

 acid, the crystals often appear striated from 

 the presence of linear fissures (c, d). Some- 

 times they are narrower and more elongate, 

 with a prismatic form (e). They are fre- 

 quently aggregated, and either fused into 

 twin crystals (/, #), or form aigrettes or 

 tufts (&, /, m, , 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 artifi- 

 cially 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 

 evaporating 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 distinguish this acid from most sub- 

 stances. 



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 mu- 

 riatic 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 crys- 

 tal, 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 coalescence 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. 12. fig. 8 e) possess considerable ana- 

 lytic power. 



The forms of the crystals and crystalline 

 groups of the urates are represented in PI. 

 12. figs. 11-14 ; they are not very charac- 

 teristic, and the aid of chemistry is required 

 for determining with certainty the com- 

 position of the respective crystals. 



The urate of ammonia may be prepared 

 artificially by adding ammonia to a boiling 

 mixture of uric acid and water j the urate of 

 lime by mixing urate of potash with chloride 

 of calcium ; the urate of soda by dissolving 

 uric acid in solution of soda ; and the urate 

 of magnesia by mixing solutions of sulphate 

 of magnesia and urate of potash. 



The presence of an excess of uric acid in 

 the blood is the chemical expression of 

 gout ; it leads to an abnormal precipitation 

 of urates in various parts of the body. In 

 cartilage the cells are the chief depositories 

 of the urates of soda and lime, and they form 

 the centres of the stellate bundles of crys- 

 tals by which the tissue is permeated. 

 The appearance of cartilage thus affected is 

 very characteristic ; and each cartilage cell 

 is surrounded by radiating tufts of crystals 

 which nearly or quite touch the extremities 

 of other groups radiating from neighbouring 

 cells. See URINARY DEPOSITS. 



BIBI,. Rindfleisch, Path. Hist. ii. 270; 

 and that of CHEMISTRY, Animal. 



URINARY DEPOSITS. We shall give 

 here a list of the deposits most commonly 

 occurring in the human urine, with the re- 

 ferences to the plates in which they are 



