258 PRACTICAL PHYSIOLOGY 



to stand overnight. It will then be found that a dark brown sedi- 

 ment, like cayenne pepper, has settled down, and probably also that a 

 brown scum has formed on the surface. Filter and examine the sedi- 

 ment under the microscope. It consists of large dark-brown clumps of 

 crystals, whetstone or barrel-shaped (Fig. 156). These are crystals 

 of uric acid admixed with pigment. They can be purified by solution 

 in 5 per cent. KOH and reprecipitation by HC1. 



EXPERIMENT II. Pure crystals can be obtained from the solid urine of 

 a snake or bird. This urine, which consists of sodium urate, is dissolved 

 in caustic potash and acidified with HC1. Pure uric acid separates out. 



From these two experiments we learn that uric acid exists in 

 urine as a salt. If this salt be decomposed by a mineral acid the 

 liberated uric acid, being very insoluble, is precipitated. 



Since uric acid contains two replaceable hydrogen atoms, it is a dibasic 

 acid, and may be represented by the formula H 2 U. Consequently it 

 may form an acid salt MHU in which only one JJatom is replaced 

 by a monad, 1 or a natural salt M 2 U in which they are both replaced. 



The following are the most important reactions of uric acid. 



EXPERIMENT III. The Murexide Test. Place some uric acid 

 or bird's urine in a capsule, add a few drops of dilute nitric acid, 

 evaporate slowly to dry ness on a water-bath. A yellow residue is 

 obtained (consisting partly of alloxan, see p. 256). Add a drop of 

 ammonia, a deep purple colour results, which is changed to blue by 

 adding caustic soda. This reaction is due to the alloxan changing 

 into ammonium purpurate. 



Another way of doing the test is to leave the dry yellow residue on 

 the water bath till it turns red, and to dissolve it in distilled water, 

 when a purple solution will be produced without adding ammonia. 



EXPERIMENT IV. The last test should be repeated with some urine. 



EXPERIMENT V. Uric acid has the power of reducing metallic 

 oxides in alkaline solution. This may be demonstrated by the 

 following tests. Some urate is dissolved in weak sodium car- 

 bonate solution, which is then poured on to a piece of filter paper 

 moistened with a solution of AgN0 3 . A black stain of reduced silver 

 results. This is called SchifFs reaction. In the presence of neutral 

 salts, and more especially of magnesium mixture (MgCl 2 , NH 4 C1, 

 NH 2 HO), the uric acid and other purin bodies unite with the 

 silver to form a double salt. This salt separates out as a gelatinous 

 precipitate, and is much employed for quantitatively estimating the 

 purin bodies (Salkowski's method). Uric acid can also exercise its 

 reducing powers on cupric salts in alkaline solution. 



1 In these equations M stands for the monad base. 





