434 THE URINE. 



reptiles, and is the end-product of their proteid metabolism, as 

 urea is that of mammals. When pure it crystallizes in the form 

 of small rhombic crystals, which vary much in shape when de- 

 posited from the urine, and this is said to depend upon the nature 

 of the pigment with which they are associated. 



Uric acid is soluble in cold water to the extent of but 1 part 

 in 15,000 ; 1 liter of boiling water will dissolve ^ gram. It 

 is soluble in sulphuric acid, but insoluble in ether and alcohol. 



The presence of uric acid is recognized by means of the 

 murex id-test, or, as it is sometimes called, WeidePs reaction. It 

 is applied as follows, the method being that recommended by 

 Hopkins, which he regards as the most delicate method of apply- 

 ing it: If a small quantity of uric acid is placed upon a watch- 

 glass, a little strong nitric acid or a few drops of bromin-water 

 added, and the whole dried upon the water-bath, an orange-red 

 residue is obtained, which, if touched with a drop of ammonia, 

 yields a fine purple color. If a minute quantity of sodium hydrate 

 solution is subsequently added, the purple color changes to blue, 

 while on warming the alkaline solution all color is discharged. 

 The water-bath should always be used for evaporation in applying 

 this test, and if the watch-glass is allowed to remain in the bath 

 for a considerable time after evaporation is complete, a red color 

 will develop without further treatment, and the residue will dis- 

 solve to a purple solution in distilled water. It is on account 

 of this purple color that the test receives its name. From the 

 genus Murex were obtained various colors ; two of these mollusks, 

 Murex brandaris and Murex truneuhis, yielded a secretion which, 

 when exposed to the air, became purple. The celebrated Tyrian 

 purple was obtained from these gastropods. 



Uric acid is dibasic, and there are formed by it three forms of 

 salts : Neutral tirafeg, which do not occur in the urine ; acid urates 

 or b i urates ; and quadriurates. 



As already stated, uric acid does not exist free in urine ?. f. T 

 in recently passed urine ; though after standing and being cooled 

 uric acid is deposited as such. The deposit of uric acid which 

 ordinarily occurs is that known as brick-dust or fatcritious deposit, 

 and consists of urates in an amorphous condition. Although these 

 are usually regarded as biurates, Roberts considers them to be 

 quad ri urates. 



Sources of Uric Acid. In a paper concerning the sources of uric 

 acid, published in the Brooklyn Medical Journal under the title 

 44 The Genesis of Uric Acid," Chittenden prefaces his consideration 

 of the subject by emphasizing two vital points : 



First, the close chemical relationship of the purin bases, viz., 

 adeniu, hypoxanthin, guanin, xanthin, and the methyl xanthins, 

 theobromin, caffein, etc., together with uric acid, which is likewise 



