228 THE URISE. 



alcohol. Its formation is frequently observed when urine contain- 

 ing much urea is examined for albumin in the cold with nitric 

 acid. On standing, the nitrate may then separate out in crystalline 

 form. On heating, the substance is decomposed without leaving a 

 residue. 



Urea oxalate, CO(NH 2 ) 2 .C 2 H 2 O 4 , crystallizes in rhombic plates, or 

 hexagonal prisms, and is less soluble in water than the nitrate ; in 

 alcohol and in dilute solutions of oxalic acid it is nearly insoluble. 

 The substance is obtained in crystalline form on adding a saturated 

 solution of oxalic acid to a concentrated solution of urea. 



Urea also combines with various neutral salts, such as sodium 

 chloride and ammonium chloride, and also with the nitrates of 

 sodium and the oxides of silver and mercury, to form double salts. 

 With mercuric nitrate three different compounds result, according 

 to the concentration of the two solutions, viz., CO(NH 2 ) 2 .Hg 2 (NO 3 ) 4 , 

 CO(NH 2 ) 2 .Hg 3 (N0 3 ) 6 , and _[CO(NH 2 ) 2 ] 2 .Hg(NO 3 ) 2 + 3HgO. The 

 latter compound is of special interest, as Liebig's quantitative esti- 

 mation of urea, which was formerly much employed, was based 

 upon its formation. It results when a 2 per cent, solution of urea 

 is treated with a feebly acid solution of mercuric nitrate, and the 

 mixture is subsequently neutralized. 



Mercuric chloride precipitates urea in alkaline, but not in neutral 

 solutions. 



Very important, further, is the behavior of urea toward sodium 

 hypochlorite or hypobromite, as the most usual method of esti- 

 mating urea in the clinical laboratory is based upon the reaction 

 which here takes place. This reaction may be represented by the 

 equation : 



CO(NH 2 ) 2 -f SNaOBr = SNaBr + CO 2 + 2H 2 O -f 2N 



In practical work an alkaline solution of the hypobromite is 

 employed, so that the carbon dioxide which is liberated is at once 

 absorbed, while the nitrogen remains. It is to be noted, however, 

 that while 1 gramme of urea should theoretically give rise to the 

 formation of 372.7 c.c. of nitrogen, 354.3 c.c. are at best obtained 

 at C. and a pressure of 760 Hgmm. In clinical work this 

 difference is unimportant, and it is in a measure equalized by the 

 evolution of a small amount of nitrogen from some of the other 

 nitrogenous constituents which are at the same time present. On 

 hydrolysis urea is transformed into ammonium carbonate : 



CO(NH 2 ) 2 + 2M 2 = (NH 4 yO :i . 



This occurs during the process of ammoniacal fermentation, which 

 results when urine is exposed to the air, and is referable, as I have 

 pointed out, to the action of a specific bacterial enzyme. On boiling 

 with acids or alkalies the same result i.s primarily obtained, but the 

 -alt is then further decomposed, with the liberation of carbon dioxide 



