THE NITROGEN CYCLE 213 



mentioned as occurring under certain conditions in human 

 urine, and ornithin is so named from its occurrence in the 

 urine of birds. Skatol and indol are characteristic constituents 

 of faeces. In the case of flesh-eating mammals, however, by 

 far the greater proportion of the nitrogen, which is not used 

 up in adding to, or maintaining, the body substance, is 

 excreted in the form of urea contained in the urine. Urea 

 is a comparatively simple substance of which the chemical 

 formula is CO(NH 2 ) 2 ; chemically it is known as carbamide, 

 being the amide of carbonic acid, CO(OH) 2 . The proportion 

 of urea in the urine is, in fact, an index as to whether proper 

 physiological equilibrium is being maintained, and its deter- 

 mination in the urine is a routine test in medicine. Its 

 estimation depends on the fact that it is decomposed by 

 sodium hypobromite, with liberation of nitrogen, according to 

 the following equation : 



CO(NH 2 ) 2 + SNaBrO = C0 2 + N 2 + 2H 2 + 3NaBr 



Urea is also broken up in a similar manner by nitrous 

 acid, obtained by adding a mixture of sodium nitrite and 

 sulphuric acid to the solution containing the urea. In 

 this case nitrogen is evolved both from the nitrous acid 

 and the urea in equal proportion, according to the following 

 equation : 



CO(NH 2 ) 2 + 2HN0 2 = C0 2 + 3H 2 + 2N 2 



This reaction is of far-reaching importance, as it probably 

 represents one method by which the nitrogen, originally 

 consumed as albumin food, finally reappears as free nitrogen. 

 In the case of animals whose diet is wholly vegetable the 

 greater part of the nitrogen is excreted as so-called hippuric 

 acid or benzoyl glycocoll, which has the formula 



C 6 H 5 CONHCH 3 COOH 



