28 URIC ACID. 



arrangement and mother-of-pearl lustre. Oxalic acid acts 'similarly, producing 

 Urea-oxalate (U, C 2 H 2 O 4 ), but the crystals are not so characteristic. Both 

 bodies are quite insoluble in the acid liquids. An important compound (2U-f- 

 Hg (NO 3 ) 2 +3Hg O) is obtained as a heavy amorphous white precipitate, when 

 a dilute solution of U is acted upon by dilute solution of mercuric nitrate in 

 excess. This body is insoluble in neutral or slightly acid liquids, but soluble 

 in nitric acid. On adding sodic carbonate to the solution it is precipitated. 

 Hence, if a solution of mercuric nitrate of known strength is added, drop by 

 drop, to a solution of U acidulated with nitric acid, and the mixture tested 

 from time to time by mixing a drop of it with a drop of sodic carbonate, 

 such mixture will be attended with the formation of an additional white 

 precipitate, so long as there remains any uncombined urea. The moment 

 that all has been used up, the test will indicate the presence of excess of 

 mercuric nitrate in the mixture, by the formation of a precipitate of basic 

 nitrate. 



Uric acid exists in urine chiefly as an acid sodium salt 

 which is deposited in the cold. When this is decomposed 

 by a stronger acid the free acid crystallizes. Ammonium 

 urate occurs only in ammoniacal urine. Uric acid is 

 absent in the urine of herbivorous mammalia, but in that 

 of birds and reptiles it takes the place of urea as the 

 channel for the discharge of nitrogen. In man it is dis- 

 charged in relatively larger quantities in early infancy than 

 in adult life ; its relative proportion to urea is increased a 

 few hours after a full meal. 



The daily discharge of uric acid is increased by certain 

 kinds of dyspepsia, in fever, and in certain chronic diseases. 

 It undergoes oxidation into urea and oxalic acid in the 

 body. 



Uric Acid (C 5 H 4 N 4 O 3 , also called lithic acid) being soluble in water only 

 in the proportion of one part to 14,000, exists as such in extremely small quan- 

 tities in urine. Uric acid crystallizes readily in urine to -which enough 

 hydrochloric acid has been added to decompose its urates. The most common 

 forms of crystals are the so-called whetstone crystals and the sheaf-like bundles 

 of flattened needles, which (as formed in urine) are always of an amber brown 

 colour. Acid sodic urate (C 5 H 2 N 4 O 3 , HNa) is always present in normal 

 urine. In urine of which the urea has undergone transformation into ammonic 

 carbonate, ammonic urate (C 5 H 3 N 4 O 3 , NH 4 ) is deposited in needle-shaped 

 crystals which are often in stellate groups. In ordinary urine, when concen- 

 trated by evaporation and then cooled, an amorphous deposit falls, which 

 consists chiefly of sodic urate. The same body often occurs as a natural 

 subsidence in disease (lateritious sediment). Uric acid and urates reduce 



