URINE 



109 



to 1050. The specific gravity has, however, been known to sink as 

 low as 1002 (after large potations, urina potus), or to rise as high as 

 1035 (after great sweating) in perfectly healthy persons. 



Composition. — The following table gives the average amounts of 

 the urinary constituents passed by a man in the twenty-four 

 hours : — 



The most abundant constituents of the urine are water, niea, and 

 sodium chloride. In the foregoing table the student must not be 

 misled by seeing the names of the acids and metals separated. The 

 acids and the bases are combined to form salts : — urates, chlorides, 

 sulphates, phosphates, &c. 



USEA 



Urea, or Carbamide, CO(NH2)2, is isomeric (that is, has the same 

 empirical, but not the same structural formula) with ammonium 

 cyanate (NH4)CN0, from which it was first prepared synthetically by 

 Wohler in 1828. Since then it has been prepared synthetically in 

 other ways. Wohler's observation derives interest from the fact that 

 this was the first organic substance which was prepared synthetically 

 by chemists. 



It may be crystaUised out from the urine, and it is then found to 

 be readily soluble both in water and in alcohol : it has a saltish taste, 

 and is neutral to Utmus paper. The form of its crystals is shown in 

 fig. 40. . 



^~^ ~¥i«en treated with nitric acid, nitrate of urea (CON2H4.HNO3) is 

 formed; this crystallises in octahedra, lozenge-shaped tablets, or 

 hexagons (fig. 41, a). When treated with oxalic acid, flat or pris- 

 matic crystals of urea oxalate (CON2H4.H,C204^-H20) are formed 

 (fig.41,&). 



