CH. xxxvn.] URIC ACID. 557 



to the purple of the ancients, which was obtained from certain 

 snails of the genus Murex. It is performed as follows : place a little 

 uric acid or a urate in a capsule ; add a little dilute nitric acid 

 and evaporate to dryness. A yellowish-red residue is left. Add 

 a little ammonia carefully, and the residue turns violet ; this is 

 due to the formation of purpurate of ammonia. On the addition 

 of potash the colour becomes bluer. 



Another reaction that uric acid undergoes (though it is not 

 applicable as a test) is, that on treatment with certain oxidising 

 reagents urea and oxalic acid can be obtained from it. Alloxan 

 (C 4 H 2 N 2 04) or allantoin (C4H 6 N 4 3 ) are intermediate products. 

 It is, however, doubtful whether a similar oxidation occurs in the 

 normal metabolic processes of the body. 



Uric acid is dibasic, and thus there are two classes of urates 

 the normal urates and the acid urates. A normal urate is one in 

 which two atoms of the hydrogen are replaced by two of a monad 

 metal like sodium ; an acid urate is one in which only one atom 

 of hydrogen is thus replaced. The formulae would be 



C 5 H4N 4 03=uric acid. 



3 =acid sodium urate. 

 = normal sodium urate. 



The acid sodium urate is the chief constituent of the pinkish 

 deposit of urates, which often occurs in urine, and is called the 

 lateritious deposit. 



The quantity of uric acid excreted by an adult varies from 7 to 

 10 grains (0*5 to 0-75 gramme) daily. 



The best method 'for determining the quantity of uric acid in 

 the urine is that of Hopkins. Ammonium chloride in crystals is 

 added to the urine until no more will dissolve. This saturation 

 completely precipitates all the uric acid in the form of ammo- 

 nium urate. After standing for two hours the precipitate is 

 collected on a filter, washed with saturated solution of ammonium 

 chloride, and then dissolved in weak alkali. From this solution 

 the uric acid is precipitated by neutralising with hydrochloric 

 acid. The precipitate of uric acid is collected on a weighed filter, 

 dried, and weighed ; or the crystals may be dissolved in sodium 

 carbonate solution, and titrated with standard solution of potas- 

 sium permanganate, until a diffused pink flush appears throughout 

 the solution. 



Origin of Uric Acid. Uric acid is not made by the kidneys. 

 When the kidneys are removed uric acid continues to be formed 

 and accumulates in the organs, especially in the liver and spleen. 

 The liver has been removed from birds, and uric acid is then 



