188 HIPPURIC ACID. 



Hippuric acid is monobasic, and forms salts which (except the 

 iron salts) are readily soluble in water ; from these solutions, if 

 sufficiently concentrated, excess of hydrochloric acid precipitates 

 the acid in fine needles. When heated with concentrated mineral 

 acids it is resolved into benzoic acid and glycin. The same de- 

 composition occurs readily in presence of putrefactive organisms. 



Apart from the characteristics already stated the acid may be 

 recognised by the following reactions. When gently heated in a 

 small tube the acid does not at once sublime as does benzoic acid, 

 but melts and solidifies again on cooling. If more strongly heated 

 it melts as before, but is now decomposed, yielding a sublimate of 

 benzoic acid accompanied by an odour like that of new hay, while 

 oily red drops are observed in the tube. When treated with boil- 

 ing nitric acid (see above sub benzoic acid) and evaporated to 

 dryness the residue on being heated yields the marked and 

 characteristic odour of nitrobenzol (Llicke's reaction J ). As al- 

 ready stated hippuric acid owes its formation in the body to a 

 union of benzoic acid with glycin, so that its source must be 

 sought for in the modes by which benzoic acid (aromatic sub- 

 stance) is introduced into or arises in the body. The source is 

 probably of more than one kind. Hay and grass were long since 

 stated 2 to contain some substance which yields hippuric acid in 

 the body : this may be extracted by means of dilute sulphuric 

 acid, less readily by caustic potash. 3 More recent researches have 

 shown the presence in grass, hay, and many fruits and berries not 

 only of some benzoic acid but also of substances such as quinic 

 acid (OH) 4 C 6 H 7 . COOH, which readily yield benzoic acid and are 

 hence a source of hippuric acid. 4 A further source is found in 

 the aromatic (benzoic) products of the putrefaction of proteids, 

 such as in especial phenyl-propionic acid (C 6 H 5 . CH 2 . CH 2 . COOH) 5 

 which in its amidated form is more particularly a product of the 

 decomposition of vegetable proteids, 6 and yields benzoic acid by 

 oxidation. This substance has been found in the rumen of cows 

 fed with hay. 7 These facts coupled with the marked occurrence 

 of putrefactive changes in the alimentary canal of herbivora 

 probably account for the preponderance of hippuric acid in their 

 urine. In carnivora it appears that some traces of hippuric acid 

 may be observed during starvation, originating here from the 

 aromatic residues of the tissue proteids; also during an exclu- 

 sively meat-diet. 8 When fed on a mixed diet some of the 



1 Arch.f. path. Anat. Bd. xix. (1860), S. 196. 

 - ,Nleissner u. Shepard, Die Hippursditre. 1866. 



3 Weiske, Zt.f. Biol. Bd. xn. (1876), S. 241. 



4 For refs. see Salkowski u. Leube. Die Lehre vom Harn, 1882, S. 131. 



5 E. u. H. Salkowski, Zt. f. phi/siol. Chem. Bd. vn. (1885), S. 161. 



9 Schulze u. Barbieri, Ber. d'. d. chem. Ges. 1883, S. 1711. Jn. f. prakt. Chem. 

 (N.F.) Bd. xxvu. (1883), S. 337. 



7 Tappeiner, Zt. f. Biol. Bd. XXH. (1886), S. 236. 



8 Salkowski, E., Ber. d. d. chem. Gesell. 1878, S. 500. Arch. f. path. Anat. Bd. 

 73 (1878), S 421. 



