418 PKACTICAL PHYSIOLOGY 



salt has the same empirical formula as urea, but its structural formula 

 is different : 



(Ammonium cyanate) (Urea) 



It was by this means that Wohler first showed that organic bodies 

 of animal origin could be formed from inorganic substances. 



PREPARATION OF UREA. 



1. From Urine- To about 400 c.c. urine add barium mixture (1 vol. saturated 

 barium nitrate solution mixed with 2 vol. baryta water) until there is no further 

 precipitate of sulphates and phosphates. Filter and evaporate the filtrate at 

 first over a free flame, afterwards on a water-bath to a thin syrup. Now mix 

 this syrup with about 100 c.c. methylated spirit, and, after allowing the mixture 

 to stand for about half an hour so that the precipitate of inorganic salts may 

 settle, filter the alcoholic extract into an evaporating dish and evaporate it nearly 

 to dryness on a water-bath. Allow the residue to cool, and then add to it about 

 double its volume of concentrated pure nitric acid, meanwhile placing the basin 

 in a dish of cold water, and stirring the contents with a glass rod so as to 

 accelerate the formation of the urea nitrate. After about half an hour the 

 crystals of urea nitrate are filtered off by means of a suction filter, sucked as 

 dry as possible, and then placed between several thicknesses of filter paper, 

 between which they are pressed so as to dry them. In order to convert the 

 nitrate into urea, the crystals are placed in an evaporating dish and dissolved in 

 as little water as possible ; the basin is then placed on a heated water-bath, and 

 powdered barium carbonate added with a penknife in small quantities until the 

 fluid reacts neutral. By this treatment the urea nitrate is decomposed, the nitric 

 acid combining with barium to form barium nitrate, and the urea being thereby 

 liberated. The mixture is now filtered, the filtrate evaporated to dryness and the 

 urea taken up from the residue by extracting with absolute alcohol, which does 

 not dissolve the barium nitrate. The alcoholic solution of urea is now evaporated 

 to dryness, when a mass of urea crystals is obtained. 



The above process may be considerably curtailed by omitting the preliminary 

 precipitation of phosphates, etc., with barium mixture, the evaporated urine being 

 simply mixed in a test tube with nitric acid, which is kept cool by immersing 

 it in a beaker of water. The crystals of urea nitrate are then filtered off, dried 

 between filter paper and treated with barium carbonate as above described. 1 



2. Separation of Urea from Blood, Serous Fluids or Watery Extracts 



of Tissues. About 100 c.c. of the fluid are mixed with four times its volume of 

 methylated spirit, vigorously shaken and allowed to stand over night. By this 

 treatment the proteins are coagulated, whereas the spirit dissolves the urea. The 

 coagulum is now filtered off, washed with spirit, and the washings are combined 

 with the filtrate, the whole being then evaporated to dryness on a water-bath. 

 The residue is extracted with absolute alcohol, the extract filtered, again evapo- 

 rated to dryness and re-extracted with absolute alcohol, this process being 



1 By adding powdered animal charcoal after barium carbonate, and boiling and 

 filtering, the final product will be rendered colourless. 



