62 ORGANIC CONSTITUENTS. 



puric acidj a well-marked yellow colour is produced ; no such 

 change is effected on the addition of this test to a solution of 

 uric acid. On its addition to a solution of hippurate of potash, 

 a copious orange- coloured deposit is thrown down^ which, on 

 the application of heat, forms a red resinous mass, soluble in 

 alcohol, but insoluble in water; when added to a solution of 

 urate of potash, a precipitate is likewise thrown down, which 

 is at first of a brownish red colour, but rapidly becomes yellow. 

 The composition of this acid is represented by the formula^ 

 C,g Hg NO^ + HO. In its physical characters it strongly re- 

 sembles benzoic acid, and there can be no doubt that these two 

 acids have been often confounded : there is, moreover, a close 

 analogy between them. They both belong to the benzoyl series, 

 although the exact place of hippiu'ic acid cannot be at present 

 assigned to it with certainty. Oxidising agents (as nitric acid, 

 or sulphuric acid and binoxide of manganese) convert hippuric 

 into benzoic acid ; and a similar change occurs in the urine if 

 it be kept for any time. Conversely, benzoic and cinnamic 

 acids are converted in the organism into hippuric acid." 



Hippuric acid forms soluble crystallizable salts with the 

 alkalies and alkaline cai'ths. 



Diagnosis. Hippuric acid may be distinguished by its crys- 

 talline form, its solubility in alcohol, its behaviour when heated, 

 and its reaction with perchloride of ii"on. Nitric acid will suffice 

 to distinguish it from ui'ic acid. 



15. Uric Oxide. 



Uric oxide, xanthic oxide, urous acid. This substance is a 

 very rare ingredient in vesical calculi. It was discovered by 

 Marcet, who gave it the name xanthic oxide ; it has since been 

 met with by Laugier, Stromeyer, and Dulk, and it is said to 

 have been recently detected in guano, by Unger. 



Urinary calculi which contain this ingredient are dissolved 

 in caustic potash ; the mic oxide is precipitated from the filtered 



' See Appendix I, Note 20. 



^ Erdmann has sometimes found hippuric, and at other times benzoic acid, in the 

 urine of the same horse. In all prohability an excess of nourishment favours the 

 production of this acid, for the urine of well-fed horses usually contains hippuric 

 acid, while only benzoic acid can be discovered in the urine of horses employed for 

 agricultural purposes : sometimes, however, the latter contains hippuric acid on some 

 days and not on others, without any perceptible cause. For Liebig's theory of tlie 

 origin of hippuric acid, see 'Animal Chemistrj',' pp. 82, 140. 



