168 PHYSIOLOGICAL AND PATHOLOGICAL CHEMISTRY. 



slight residue remains in the flask. A mixture of phenyl- 

 acetic acid and phenyl-propionic acid is thus obtained, from 

 which frequently, but not always, one of the two acids 

 separates. 



A quantitative method for the separation of these two 

 acids is not known at the present time. To detect the two 

 we may either make use of their conduct in the animal body 

 phenyl-propionic acid is converted into hippuric acid, phenyl- 

 acetic acid into phenaceturic acid, which are readily sepa- 

 rated l or of one of the other methods given. 2 If it is only 

 desired to prove the presence of the homologues of benzole 

 acid, Liicke's reaction is sufficient (see page 106). 



The solution left in the distilling-flask, after driving out 

 the volatile acids, which contains skatol-carbonic acid, oxy- 

 acids, and succinic acid, gradually becomes turbid on cooling 

 and deposits some resinous substance. It must be filtered 

 as soon as this material has settled so that filtration is possi- 

 ble (after some hours). From the clear filtrate there deposits 

 after twenty-four hours' standing in the cold, best in the 

 ice-chest, chalky- white grains of pure skatol-carbonic acid. 

 By evaporating the aqueous solution, separated from the 

 skatol-carbonic acid, to half of its volume, a new deposit of 

 skatol-carbonic acid is frequently obtained; the separation 

 is, however, never complete, a portion of the skatol-carbonic 

 acid always remaining in the aqueous solution, together with 

 the oxyacids and succinic acid. The complete separation of 

 the oxyacids from succinic acid has also not yet been accom- 

 plished. If the aqueous solution be shaken with pure ether, 

 the oxyacids, together with the skatol-carbonic acid still 

 present, are extracted, but some of the succinic acid is also 

 taken up by the ether, while the greater part remains in the 



1 Zeitschr. f. physiol. Chemie, 9, 503. 



3 Ibid. 10, 150, and H. Salkowski, Ber.d. d. chem. Ges., 18, 323. 



