EXAMINATION OF THE URINE. 97 



into urea, dissolve it in water in a dish, add barium carbonate, 

 a little at a time, stirring thoroughly and heating, and con- 

 tinue adding the carbonate until the fluid no longer has an 

 acid reaction, then filter, and wash once. Decolorize the 

 nitrate, which is usually yellow-colored, by warming with 

 some bone-black, and filter again. We must now separate 

 the urea from the barium nitrate formed. This is done by 

 evaporating to dryness on the water-bath and extracting 

 the residue with alcohol, in which only the urea is soluble. 

 The alcoholic solution is then filtered and evaporated to 

 crystallization. The crystals of urea are left till next day, 

 separated from the mother-liquor, pressed dry between filter- 

 paper, and purified by recrystallizing from a small quantity 

 of absolute alcohol (by warming in a flask). 



MTT 



Urea, OC < -^jr 2 , the amide of carbonic acid, hence also- 

 called ' carbamide/ 7 crystallizes in long quadratic prisms or, 

 when crystallized rapidly, in needles; it is very readily sol- 

 uble in water (at 100 in every proportion), less readily, 

 though still quite soluble, in alcohol, and insoluble in anhy- 

 drous ether. It is not precipitated by metallic salts, with the 

 exception of mercuric nitrate. 



Reactions of Urea. 



1. Heat a small portion in a dry test-tube. It melts 

 (melting-point 132), giving a strong odor of ammonia, and 

 if the urea is not dry ammonium carbonate sublimes. Con- 

 tinue the heating till the melted mass just begins to solidify 

 (the re-formation of the solid is due to the change into cyan- 

 uric acid). When heated just beyond its melting-point urea 

 forms biuret: 



