46 HUMAN PHYSIOLOGY 



seems to be to change the injurious ammonia formed by 

 proteid metabolism into a harmless compound. 



It is a question, however, whether all the urea formed in 

 the body is derived from ammonia or ammonia derivatives. 

 It is probable that some of the urea can also be directly split 

 off from the proteid. 



3. Uric acid, C 5 H 4 N 4 O 3 , has the structural formula: 



/NH-CO 



\ || pCO. [Diureid of trioxyacrylic acid.] 



\NH-C-NH X 



Pure uric acid crystallizes in colorless, rhombic prisms, but 

 directly from the urine it yields bundles of colored dumb- 

 bell and whetstone crystals. Uric acid is but slightly solu- 

 ble in cold water (0.05 g in one liter); it is a little more 

 soluble in hot water (0.5 g in one liter) or in the presence 

 of urea ; it is insoluble in alcohol and ether. As a dibasic 

 acid it forms neutral and acid salts. The neutral alkali 

 salts are quite soluble in water; the acid salts are also more 

 soluble than the free acid. But these are precipitated even 

 by the cooling of the urine and, as they take with them the 

 pigments of the urine, they form reddish precipitates (sedi- 

 mentum lateritium). 



The salts of uric acid with the alkali-earths, most of the 

 metals, and also ammonia are not very soluble in water. 



Because of the comparative insolubility of uric acid, it is 

 easily deposited in the kidneys, ureters and the tissues of the 

 body (gravel, gout). 



If uric acid to which nitric acid has been added is evap- 

 orated to dryness and to the residue ammonia is added, a 

 reddish-violet color results which gives place to a bluish 

 violet on addition of sodium hydrate (murexide test for uric 

 acid). 



By careful oxidation of uric acid, allantoin and carbon 

 dioxide are formed: C 5 H 4 N 4 O 3 + O + H 2 O = C 4 H 6 N 4 O 3 

 (allantoin) + CO 2 . 



