96 PHYSIOLOGICAL AND PATHOLOGICAL CHEMISTRY. 



4. Barium Chloride: white precipitate of barium phos- 

 phate and sulphate ; on the addition of hydrochloric acid the 

 barium phosphate dissolves and the quantity of the precipi- 

 tate diminishes perceptibly. 



5. Silver Nitrate : white precipitate of silver chloride and 

 silver phosphate; on the addition of nitric acid the latter 

 dissolves, silver chloride remains. 



6. Basic Lead Acetate : heavy precipitate, which consists 

 principally of lead chloride, lead phosphate, and lead sul- 

 phate, together with the greater part of the coloring matter 

 of the urine. The nitrate is colorless or almost colorless. The 

 precipitation with basic lead acetate is frequently used to 

 decolorize the urine. 



III. PREPARATION OF UREA. 



Two hundred to three hundred cubic centimeters of dog's 

 urine or double the quantity of human urine are treated with 

 baryta mixture (one volume of a saturated solution of barium 

 nitrate and two volumes of baryta- water), until a portion of 

 'the urine when taken out and filtered no longer gives a pre- 

 cipitate with the mixture; filter off the precipitate of barium 

 phosphate and sulphate, wash once with water (the precipi- 

 tate, after washing, may be thrown away), and evaporate 

 the filtrate to a sirup, at first over a free flame and then when 

 the volume amounts to about 200 cc. on the water-bath. 

 Precipitate with about 150 cc. of alcohol, and after half an 

 hour filter from the precipitate, which consists of salts and 

 extractive material. Evaporate the filtrate on the water- 

 bath as nearly to dryness as possible, and after cooling add 

 double the volume or somewhat more of nitric acid (one part 

 concentrated nitric acid to one part of water). The urea 

 nitrate is filtered off (preferably next day), washed with some 

 cold nitric acid, drained thoroughly, and dried on a porous clay 

 plate or on filter-paper. In order to convert the urea nitrate 



