458 PHYSIOLOGICAL CHEMISTRY. 



made by placing a fragment of uric acid in a porcelain dish, adding 

 a drop of nitric acid, and carefully evaporating over a flame. To the 

 dry residue a drop of ammonia water is added, which produces a 

 beautiful purplish red color. (Plate VIII., 4.) This reaction occurs, 

 however, also with a number of substances which are similar to, but 

 more complex in composition than uric acid. 



The quantitative estimation of uric acid in urine is best accomplished 

 by adding 10 c.c. of hydrochloric acid to 250 c.c. of urine, setting 

 aside for 24 hours in a cool place, and collecting the crystals of uric 

 acid on a small filter which has been previously weighed. The 

 crystals are washed with a little water, and dried at 100 C. (212 F.). 

 As uric acid is not entirely insoluble, 0.0038 gramme has to be added 

 for every 100 c.c. of urine employed for the analysis. 



If the urine (to be tested for uric acid) be very dilute, it should be 

 evaporated to about one-half its bulk before adding hydrochloric 

 acid ; if it contain albumin, this should be removed by adding a drop 

 of acetic acid, boiling and filtering. 



Hippuric acid, C 9 H 9 NO 3 (Benzyl-glycocol, Benzyl-amido-acetic acid), 

 is a normal constituent of human urine, but is found in much larger 

 quantities in the urine of herbivora. Its constitution may be con- 

 sidered as ammonia in which two hydrogen atoms are replaced by the 



/C 7 H 5 

 radicals of benzoic and acetic acid respectively, thus, N_ C 2 H 3 O 2 . 



\H 



Hay, and especially aromatic herbs, contain benzoic acid, or com- 

 pounds having a similar composition, and a portion of these com- 

 pounds is eliminated in hippuric acid. Administration of benzoic 

 acid increases the amount of hippuric acid in urine. 



When pure, hippuric acid crystallizes in transparent, colorless, 

 odorless prisms, which have a bitter taste, and are sparingly soluble 

 in water. 



Analytically, hippuric acid is characterized 



1. By giving a sublimate of benzoic acid, and an odor of hydro- 

 cyanic acid, when heated in a dry test-tube. 



2. By giving a brown precipitate with ferric chloride. 



3. By giving off benzene and ammonia when heated with calcium 

 hydroxide. 



4. By evolving an intense odor of nitro-benzene when evaporated 

 to dryness with a few drops of nitric acid. 



Other organic substances, such as kreatin, kreatinin, xanthin, lactic 



