XVIII.] ORGANIC CONSTITUENTS OF THE URINE. 117 



LESSON XVIII. 



ORGANIC CONSTITUENTS OP THE URINE. 



1. Urea (CONoH^) is the most important organic constituent 

 in urine, and is the chief end-product of the oxidation of the 

 nitrogenous constituents of the tissues and food. It crystallises 

 in silken four-sided prisms, with obliquely-cut ends (rhombic 

 system), and when rapidly crystallised, in delicate white needles. 

 It has no effect on litmus ; odourless, weak cool-bitter taste, like 

 saltpetre. It is very soluble in water and in alcohol, and almost 

 insoluble in ether. It is isomeric with — z.e., it has the same empiri- 

 cal, but not the same structural formula as ammonium cyanate 

 (NHJCNO. It may 



be regarded as a diamid of CO2 or as carbamid = CO I tr-rT^* 



Urea represents the final stage of the metamorphosis of albu- 

 minous substances within the body. More than nine-tenths of all 

 the N taken in is excreted in the form of urea. 



2. Preparation from Urine. — Take 20 cc. of fresh filtered human 

 urine, add 20 cc. of baryta mixture — Lesson XIX. 12 (c.) — to preci- 

 pitate the phosphates. Filter, evaporate the filtrate to dryness in 

 an evaporating chamber, and extract the residue with boiling alco- 

 hol. Filter off the alcoholic solution, place some of it on a slide, 

 and allow the crystals of urea, usually long, fine, transparent 

 needles, to separate out. This is best done by allowing spontaneous 

 evaporation of the solution to go on in a warm place. Examine 

 them microscopically (fig. 60, a). 



3. Combinations. — Urea combines with acids, bases, and salts. 

 Evaporate human urine to one-sixth its bulk, and divide the residue 

 into two portions, using one for the preparation of nitrate, and the 

 other for oxalate of urea. 



4. Urea Nitrate (CII4N2O, HNO3). 



(a.) To the concentrated urine add strong pure nitric acd = a 

 precipitate of glancing scales of urea nitrate, which, being almost 

 insoluble in HNO.^^, separate out in rhombic plates or six-sided 

 tables, with a mother-of-pearl lustre, and often imbricate arrange- 

 ment. 



