460 PRACTICAL PHYSIOLOGY 



extracted with absolute alcohol, the extract filtered, again evapor- 

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

 being repeated until the evaporated residue is entirely dissolved in the 

 alcohol. The purified residue is now cooled by placing the dish con- 

 taining it on ice, and is mixed with one or two drops of pure nitric 

 acid, the mixture being allowed to stand on ice till next day when 

 it is examined for crystals of urea nitrate. 



The alcoholic extracts usually contain a considerable amount of fatty 

 acid which may mask the separation of urea nitrate. To remove this, 

 the first alcoholic extract should be mixed with a few drops of a 

 solution of basic lead acetate till no more precipitate is produced, after 

 which a few drops of a solution of ammonium carbonate are added to 

 cause the suspended precipitate of lead soaps to settle down. The 

 solution is then filtered, and the lead removed from the filtrate by 

 passing a stream of H 2 S gas through it. 



Estimation of Urea. Morner and Sjo'qvist's Method. Principle. 

 By the addition of certain reagents to a measured quantity of urine, 

 all the nitrogenous bodies except urea and ammonia are precipitated. 

 The precipitate is removed by filtration, and, after expelling the 

 ammonia by heat, the nitrogen of the filtrate is determined. This, 

 multiplied by 2*143, gives the amount of urea present. 



Solutions necessary. 



1. A saturated solution of chloride of barium containing 5 per 



cent, baryta. 



2. A mixture of 1 vol. ether and 2 vol. absolute alcohol. 



3. Apparatus, etc. for Kjeldahl's nitrogen determination. 

 Determination. 5 c.c. urine are mixed in a small stoppered flask 



with 5 c.c. of barium mixture and 100 c.c. alcohol-ether mixture, 

 whereby a copious precipitate falls down. The flask is corked and 

 left standing over night. The contents are then filtered through a 

 small filter paper (10 c.m. in diameter), the filtrate being collected in 

 a Kjeldahl combustion flask. When all the solution has passed through, 

 the precipitate is washed at least three times with alcohol-ether 

 mixture, and then the flask is placed on the water-bath heated to 60C., 

 a pinch of magnesium oxide being added to the contents so as to drive 

 off the last traces of ammonia from the solution. 1 When the volume of 

 fluid in the flask has reached about 10 c.c. concentrated sulphuric acid, 

 cupric sulphate and potassium sulphate are added to it, and the nitrogen 

 determined by Kjeldahl's process. 



In order to accelerate filtration the suction pump may be used, a 



1 This stage of the process can be much accelerated by carefully transferring 

 the contents of the flask to an evaporating dish which is then placed on a water 

 bath at 60 C. 



