HIPPURIC ACID. 60 1 
in many fruits and in the cortical parts of most plants. Vegetable food 
greatly increases, therefore, the excretion of hippuric acid. 
But the vegetable aromatic compounds are not the sole source of the 
urinary hippuric acid. In the decomposition of proteids, which occurs 
in the bowel, aromatic residues split off. Precursors of benzoic acid 
(mainly, perhaps, phenylpropionic acid) are thus formed, and after 
oxidation they appear in the urine as hippuric acid. The metabolism 
of the tissue proteids themselves, moreover, may yield precursors of the 
same kind, so that even in starvation hippuric acid does not wholly dis- 
appear from the urine. 
This dual origin (from aromatic precursors in the diet chiefly, hut 
likewise from proteid metaholisni) is found also in the case of the other 
aromatic constituents of the urine (p. 605). 
Upon a mixed diet the excretion of hippuric acid in human urine 
amounts to about 07 grms. per diem; upon a diet rich in fruits it may 
he raised to three or four times this. In herhivora the quantity is much 
larger; the urine of cattle, for instance, often contains as much as 
2 per cent., though, as might he expected, that of sucking calves only 
contains small amounts. 
Properties. — It forms four-sided prismatic crystals ending in two or 
four facets, and often grouped in clumps (Fig. 53), of which the melting 
point is about 187°. It is but slightly soluble in cold water or alcohol ; 
but both these solvents dissolve it easily when hot, It is soluble in 
acetic ether, but not so in most other organic liquids. If heated to 240° 
it decomposes, benzoic acid subliming out and a reddish residue being left 
behind: When first heated at this temperature a hay-like odour is given 
oh', which is succeeded by that of prussic acid. When 1 toiled with strong 
hydrochloric acid it splits up into its components, benzoic and amido- 
acetic acids. The growth of the Micrococci'* urece can bring about tins 
decomposition, so that stale specimens of urine often contain benzoic in 
place of hippuric acid. Taken to dryness with nitric acid, it yields an 
odour of nitrobenzene. 
Solutions of hippuric acid react acid to litmus, and even when very 
dilute they impart a violet colour to congo-red. By the use of the latter 
indicator Briicke proved the absence of the free acid from the urine. It 
is present always as salts. It forms salts with bases, but does not 
combine with acids. Its iron compound is insoluble in hot water, and 
may he employed in separating the acid from its solutions. 
Isolation and estimation. — The method of Bunge and Schmiedeherg 1 
consists in making an alcoholic extract of the urinary solids, evaporating off 
the spirit, dissolving the residue in water, and, after acidifying with hydro- 
chloric acid, shaking up repeatedly with successive quantities of acetic-ether. 
On evaporating the latter, impure crystals of the acid are obtained, the 
impurities heing removed by treatment with petroleum-ether, in which 
hippuric acid is insoluble. 
Tests. — The substance is recognised by its crystalline form, by its 
melting point, by its behaviour on heating, and by the formation of its 
insoluble iron compound when neutral ferric chloride is added to its 
solutions. 
In addition to hippuric acid, minute quantities of its homologue 
pTbenaceturic acid (phenylacetylglycin), C 6 H 5 .CH 2 CO — NH.CH 2 .COOH, 
1 Arch. f. exper. Path. u. Pharmakol., Leipzig, Bd. vi. S. 235. 
