1246 PHYSIOLOGY 



is soluble in water and alcohol, and insoluble in ether. Its solutions 



are neutral in reaction, but it 

 forms crystalline salts with strong 

 acids. Thus urea nitrate, which 

 is produced by treating strong 

 solutions of urea with concen- 

 trated nitric acid, forms micro- 

 scopic rhombic plates which are 

 extremely insoluble, so that their 

 formation may be used as a test 

 for urea (Fig. 515). With oxalic 

 acid urea solutions yield an in- 

 soluble oxalate, also in typical 

 crystals. Urea when heated melts 

 at about 130 C. On further 

 heating, it undergoes decompo- 



sition, giving off ammonia and forming biuret, as follows : 



m 



FIG. 514. Urea. (FUNKE.) 



2 CO 



NH 2 \ CO 





NH 3 . 



CO 



NH 



At the same time a certain amount of cyanic acid is formed, and 

 this polymerises to cyanuric acid, H 3 C 3 N 3 3 . On heating with 

 alkalies or with strong acids urea undergoes hydrolysis, with the pro- 

 duction of carbon dioxide and ammonia : 



CON 2 H 4 + H 2 B* C0 2 + (NH 3 ) 2 . 



The same change is effected in urea by certain micro-organisms, e.g. 

 the micrococcus ureae, which is responsible for the ammoniacal change 

 which occurs in urine when exposed 

 to the air. 



Urea, being the chief nitrogenous 

 constituent of the urine, is the 

 most important index to the protein 

 metabolism of the body. As we 

 have seen, urea may be regarded 

 as partly exogenous, partly endo- 

 genous. The greater part of the 

 30 grm. excreted by a normal indi- 

 vidual in the course of the day is derived directly from the proteins of 

 the foods, as a result of the deamination of the amino-acids, which 

 occurs shortly after their absorption. The ammonia thus formed is 



FIG. 515. 

 Urea nitrate. Urea oxalate. 



