52 



XIX. 



246. Organic constituents of urine. Urea or carbaniid 

 (CON^H^) is the most important organic con.stituent in 

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

 nitrogenous con.stituents of the tissues and food. It has no 

 effect on litmus ; it is odorless, weak, cool, and has a bitter 

 taste like saltpetre. It is very soluble in water, and in 

 alcohol, and almost insoluble in ether. More than -^ of all 

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



247. Take 20 cc. of fresh, filtered omnivorous urine and 

 add 20 cc. of baryta mixture to precipitate the phosphates. 

 Filter, evaporate the filtrate to dryness and extract the resi- 

 due with a little boiling alcohol over the water bath very 

 carefully. Filter off the alcoholic solution, place some of it 

 on a slide and allow the crystals of urea, usually long, fine, 

 transparent needles, to ,=eparate out. Examine them under 

 the microscope. 



248. A simple method of detecting urea is to place a drop 

 or two of the urine upon a glass slide and after adding a 

 drop of nitric acid, gently warm over the flame. If urea be 

 present, upon evaporation, the micro.scope will show the 

 characteristic crystals of nitrate of urea, of rhombic or hex- 

 agonal form. 



249. To a few drops of the suspected fluid in a test-tube 

 add an equal quantity of solution of hypobromite of sodium, 

 and a rapid evolution of bubbles of nitrogen takes place if 

 urea be present. (Hypobromite of sodium is prepared by 

 mixing 2 cc. of bromine to 23 cc. of a 40% solution of caustic 

 soda. ) 



250. Heat some urea crystals in a test-tube. Biuret is 

 formed, and ammonia comes off. Add a trace of copper 

 sulphate solution and a few drops of 20% caustic potash. A 

 rose-red color is produced — the biuret reaction. 



251. Quantitative estimation of urea. I,yon's apparatus. 



