402 MANUAL OF PHYSIOLOGY. 



method of Liebig, which depends on the power of mercuric ni- 

 trate to give a precipitate with it. The sulphates and phosphates 

 must be first removed by the addition of a mixture of 1 volume 

 saturated barium nitrate and 2 volumes saturated solution of 

 caustic baryta, to an equal volume of urine. This is filtered, 

 and from the filtrate an amount corresponding to 10 c.c. urine 

 is taken. Into this known volume of urine a standard solution 

 of mercuric nitrate (of which 1 c.c. corresponds to 1 centi- 

 gramme of urea) is dropped until a sample drop of the fluid, 

 mingled on a watch glass with a drop of concentrated sodium 

 carbonate solution, gives a yellow color, which indicates that 

 some free mercuric nitrate remains. For every cubic centimetre 

 of the standard mercuric solution used there will be 1 centi- 

 gramme of urea in the sample of urine ; a small reduction has 

 to be made for the chlorides, which are present in tolerably con- 

 stant amount. 



Another simple method consists in mixing together known 

 quantities of urine and sodium hypobromite (NaBrO) with ex- 

 cess of caustic soda. The urea is decomposed in the presence of 

 this salt, and free nitrogen evolved : 



CON 2 H 4 + 3 (NaBrO) + 2 (NaOH) = 3 NaBr + Na 2 CO 3 + 



The quantity of urea may be determined by ascertaining the 

 volume of nitrogen, which can be measured directly in a gradu- 

 ated tube. 



Uric acid, of which the formula is C 5 H 4 N 4 O 3 or C 3 H. 2 O 3 (NH.- 

 CN) 2 , is present only in extremely small quantities in the normal 

 urine of mammalia, but in birds, reptiles, and insects it forms 

 the chief ingredient of the renal secretion. It is sparingly soluble 

 in water, and insoluble in alcohol and ether. However, in solu- 

 tions of the neutral phosphates and carbonates of the alkalies 

 it combines with some of the base so as to form acid salts, and 

 at the same time converts the neutral into acid phosphates, to 

 which, as has been already stated, the urine owes its acid reac- 

 tion. These salts are more soluble in warm than in cold water, 

 and hence generally fall as a sediment when the urine cools. 



