122 ANIMAL CHEMISTRY LECTURE V. 



Again, when indigo is gently fused with caustic alkali it under- 

 goes a simultaneous hjdration and oxidation, whereby it is con- 

 verted into anthranilic acid, thus : 



Indigo Water Oxygen Anthranil. acid Carb-anhyd. 



C 7 H 5 (CN)0 + H,0 + 3 = C 7 H 5 (H 2 N)O a + C0 a 



When, however, the reaction takes place more violently, we get 

 salicic acid produced instead, thus : 



Carb- Am- 



Indigo Water Oxygen Salicic acid anhyd. monia 



C 7 H 5 (CN)0 + 2H Z + O z = C 7 H 5 (HO)O a + C0 a + H 3 N 



You observe that the anthranilic and salicic acids furnish us 

 with another instance of that relationship between amidated and 

 hydrated compounds to which I have so often adverted. 



Seeing from the observations and researches of Prout, Heller, 

 Debuyne, Hassall, Scheerer, Schunk, and others, that human urine 

 not unfrequently deposits indigo spontaneously, and contains ha- 

 bitually an indigo-yielding substance known as indican, which is 

 probably a glucoside of white or hydrogenised indigo, we come 

 to the conclusion that while a salicic residue in the form of tyro- 

 sine is a constant product of the natural and artificial oxidation of 

 nitrogenous tissue, another salicic residue in the form of indigo or 

 indican is a usual ingredient of that secretion by which the pro- 

 ducts of disintegrated nitrogenous tissue are principally discharged. 

 Moreover, this occurrence of indigo in urine further exemplifies a 

 point with which we must all, I believe, have been more or less 

 struck, both in this and previous lectures namely, the thorough 

 interdependence of vegetable and animal chemistry, as shown by 

 the frequent relationship and even identity of products formed in 

 vegetable and animal organisms. 



(131.) Another physiological point of interest connected with 

 salicic compounds is their occurrence in the urine in the form of 

 salisuric acid. It is well known that when benzoic aldehyd or 

 acid is taken internally, it makes its appearance in the urine in 



