CH. XXXVII.] URIC ACID 561 



has received the name on account of the resemblance of the colour 

 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 forma- 

 tion of purpurate of ammonia. On the 

 addition of potash the colour becomes 

 bluer. 



Another reaction that uric acid un- 

 dergoes (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 4 ) or allantoin 

 (C 4 H 6 N 4 3 ) are intermediate products. 

 It is, however, doubtful whether a 



. ., ' . , . . ' ,1 FIG. 429. Various forms of uric acid 



similar oxidation occurs in the normal crystals. 



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 H 4 N 4 3 = uric acid. 

 C 5 H 3 NaN 4 3 = acid sodium urate. 

 C 5 H 2 Na. 2 N 4 3 = 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. 



If uric acid is represented by H 2 U, the normal urates may be represented by 

 M 2 U, and the acid urates by MHU. Bence Jones, and later Sir W. Roberts, 

 considered that the urates actually occurring in urine are what are termed quadri- 

 urates MHU.H 2 U. There is much doubt whether such compounds really exist ; 

 if they do, they are readily decomposed into acid urate, MHU, and free uric acid, 

 HoTJ. 



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

 10 grains (0'5 to 075 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 ammonium 

 urate. After standing for two hours the precipitate is collected on 



2 N 



