25 



Take 20 cc. of fresh, filtered dog or human urine and 

 add 20 cc. of baryta mixture to precipitate the phosphates 

 and sulphates. Filter, evaporate the filtrate to dryness, 

 and extract the residue 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 separate 

 out. Examine them under the microscope. (Fig. 5). 



Repeat in the hood, the experiment upon the urine of 

 the horse and compare results. 



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. 

 Medicines which increase the amount of urea are : urea 

 itself; uric acid, common salt, phosphoric acid, squill, theo- 

 bromine, colchicum, cubebs, atropine, cantharides, vegetable 

 acids, iron preparations, hyposulphite of soda, potassium chlor- 

 ide, ammonium chloride, coca, potassium permanganate, oxygen, 

 salicylic acid. 



Fig. 4. 

 Crystals of Nitrate of Urea. 



Fig. 5. 

 Crystals of Urea. 



Medicines which decrease the amount of urea are : digitalis, 

 alcohol, coffee, tea, potassium and sodium iodides, potassium 

 bromide, arsenic, turpentine, alkaline carbonates, mercury, anti- 

 pyrin, valerian, quinine sulphate, benzoic acid. 



