1254 



UREA. 



Mercuric nitrate yields a precipitate with asparagine which 

 may be used for its separation from vegetable extracts. Urea- 

 ferment converts it into succinic acid. 



THE UREA AND URIC ACID GROUP. 



1. Urea. (NH 2 ) 2 CO. (Carbamide.} 



This is the chief nitrogenous constituent of normal urine in 

 mammalia and some other animals. The urine of birds also 

 contains a small amount, more particularly on a meat diet. 

 Average normal human urine contains from 2*5 — 3*2 p.c, the 

 average total daily excretion varying from 22 — 35 grams or as 

 a mean 30 grams. It is also found in minute quantities in 

 normal blood ('025 p.c), serous fluids, lymph and aqueous 

 humour : it is not usually met with in the tissues except that 

 of the liver. It is never present in normal mammalian muscles, 

 but may make its appearance there under certain pathological 



Fig. 208. Urea Crystals separated by slow evaporation from aqueous 



solution. (After Funke.) 



conditions. Under ordinary conditions the amount of urea in 

 sweat is almost inappreciable, but the older statements of its 

 occurrence in this excretion have recently received confirma- 

 tion, and it appears that this source of nitrogenous loss to the 

 body may have to be taken into account. 



When pure it crystallizes from a concentrated solution in 

 the form of long, thin glittering needles. If deposited slowly 

 from dilute solutions, the form is that of four-sided prisms 

 with pyramidal ends ; these are always anhydrous. When the 

 separation occurs rapidly, as for instance from a strong alco- 

 holic solution on a glass-slide, the typical crystalline form is 



