398 MANUAL OF PHYSIOLOGY. 



the amide of carbamic acid, with which it is considered by some 

 to be identical. 



On exposure to the air bacteria develop in the urine, and, act- 

 ing as a ferment, change the urea into ammonium carbonate, two 

 molecules of water being at the same time taken up, thus : 



CO(NH,),+ 2H 2 = (NH 4 ) 2 C0 3 . 



This gives rise to a change in the reaction of the urine, which 

 after a time becomes increasingly alkaline, and the change is 

 commonly spoken of as the alkaline fermentation of the urine. 

 This change is extremely slow in solutions of pure urea, which 

 do not support bacterial life. 



With nitric and oxalic acids urea forms sparingly soluble 

 salts a fact made use of in its preparation from urine. 



The amount of urea eliminated in the twenty-four hours is 

 about 500 grains (35 grammes). The amount varies (1.) in some 

 degree with the amount of urine secreted; an increase in the 

 amount of water being accompanied by a slight increase in the 

 urea eliminated. Some materials, such as common salt, increase 

 the water, and thereby also increase the urea. (2.) The character 

 and quantity of the diet influences most remarkably the quantity 

 of urea given off, the amount increasing in direct proportion to 

 the quantity of proteid consumed. Fasting causes a rapid fall in 

 the amount of urea ; even in the later days of starvation it con- 

 tinues to fall, but very slowly. (3.) The amount differs with age, 

 being relatively greater in childhood than in the adult (about half 

 as much again in proportion to the body weight). (4.) Many dis- 

 eases have a marked influence on the amount of urea. In most 

 febrile affections it increases with the intensity of the fever, while 

 in diseases of the liver it often notably decreases. 



In diabetes, if the consumption of food be very great, the daily 

 excretion of urea may reach, nearly 4 oz. (100 grammes), or 

 three times as much as normal. 



Preparation. To obtain urea from human urine it is evapo- 

 rated to one-sixth of its bulk, an excess of nitric acid is added, 

 and it is left to stand in a cool place. Impure nitrate of urea 

 separates from the fluid as a yellow crystallized precipitate. This 



