236 THE URL\K 



thus still in stain, quo, and the same may be said of the formation 

 of uratic deposits in the joints and tendons in gout. It appear.-, 

 however, that an increased production of uric acid, contrary to 

 what was formerly supposed, plays no r6le in the causation of the 

 latter disease. 



Properties of Uric Acid. Pure uric acid crystallizes in trans- 

 parent, colorless rhombic platelets, the angles of which are often 

 rounded off. Such crystals are at times seen in urinary sediments, 

 but more commonly the substance is here found in the form of 

 brownish-yellow whetstone-like crystals, which may occur singly, 

 but are frequently arranged in groups. These are quite character- 

 istic, and cannot be confounded with crystals of any other substance 

 that may occur in the urine. 



A great many other forms may, however, also be encountered, 

 such as dumb-bells, somewhat irregular hexagonal platelets, paddle- 

 shaped crystals, etc., the nature of which is not at once apparent. 



Uric acid is almost insoluble in cold water (1 : 40,000), with 

 difficulty also in boiling water (1 : 1800), and insoluble in alcohol 

 and ether. 



In concentrated sulphuric acid and boiling glycerin it dissolves 

 with comparative ease and without undergoing decomposition. It is 

 a dibasic acid, and accordingly combines with bases to form neutral 

 and acid salts. Of these, the neutral salts of potassium and 

 lithium are the most soluble, while the acid salts, and notably acid 

 ammonium urate, are quite insoluble. Its compounds with the 

 alkaline earths are likewise soluble only with great difficulty. In 

 the urine uric acid is said to be present as a quadriurate, viz., as 

 a hyperacid compound, in which one molecule of sodium (viz., 

 potassium or ammonium) is in combination with two molecules of 

 uric acid. Its solubility in the urine is largely dependent upon the 

 amount of water, the reaction, and the presence of mineral salts and 

 possibly of pigments. On standing, however, in the absence of 

 micro-organisms, the quadriurate is decomposed, with the liberation 

 of free uric acid and acid biurates, which latter are then again trans- 

 formed into quadriurates through the agency of the diacid phos- 

 phates, and through a repetition of this process all the uric acid 

 finally separates out, as is shown in the equations : 



(1) HNa.C 5 H 2 N 4 3 .C 5 H 4 NA + H 2 O = C 5 H 4 N 4 O 3 -f HNa.C 5 TI 2 N 4 O 3 



Sodium quadriurate. Trie acid. Acid sodium 



biurate. 



(2) 2HNa.C\H 2 N 4 3 + NaH 2 PO 4 = HNa.C fc H a N 4 O s .C 6 H 4 N 4 O. + Na 2 HPO 4 



Acid biurate. Quadriurate. 



In the urine of birds and reptiles the uric acid is said to occur 

 exclusively in the form of quadriurates. Neutral urates are not 

 found in the urine. Of the compounds which uric acid forms with 

 the salts of the heavy metals, the silver and copper salts deserve 

 especial mention, as some of the methods which are employed in the 

 quantitative estimation of the substance are dependent upon their 



