CII. XXXIX.] URIC ACID 591 



excess of creatinine is then excreted by the kidneys. The small 

 amount of creatinine excreted in diseases of the liver also supports 

 the view that that organ is responsible for creatinine formation. 



These views will no doubt be subjected to the usual tests of 

 criticism and renewed research; they certainly appear to explain 

 some of our previous difficulties, though the ultimate fate of the 

 muscular creatine is still unsolved. 



Uric Acid. 



Uric Acid (C 5 N 4 H 4 3 ) is, in mammals, the medium by which a 

 very small quantity of nitrogen is excreted from the body. It is, 

 however, in birds and some reptiles the principal nitrogenous con- 

 stituent of their urine. It is not present in the free state, but is 

 combined with bases to form urates. 



It may be obtained from human urine by adding 5 c.c. of hydro- 

 chloric acid to 100 c.c. of the urine, and allowing the mixture to 

 stand for twelve to twenty-four hours. The crystals which form are 

 deeply tinged with urinary pigment, and though by repeated solution 

 in caustic soda or potash, and precipitation by hydrochloric acid, 

 they may be obtained fairly free from pigment, pure uric acid is more 

 readily obtained from the solid urine of a serpent or bird, which con- 

 sists principally of the acid ammonium 

 urate. This is dissolved in soda, and 

 then the addition of hydrochloric acid 

 produces as before the crystallisation of 

 uric acid from the solution. 



The pure acid crystallises in colour- 

 less rectangular plates or prisms. In 

 striking contrast to urea it is a most in- 

 soluble substance, requiring for its solu- 

 tion 1900 parts of hot and 15,000 parts 

 of cold water. The forms which uric 

 acid assumes when precipitated from 

 human urine, either by the addition of 



,, , , . . , f . . ., PIG. 382. Various forms of unc acid 



hydrochloric acid or in certain patho- crystals, 



logical processes, are very various, the 



most frequent being the whetstone shape ; there are also bundles of 

 crystals resembling sheaves, barrels, and dumb-bells (see fig. 382). 



The murexide test is the principal test for uric acid. The test 

 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 



