URINE AND ITS NORMAL CONSTITUENTS. 457 



cylinder and the connection between the burette and the bottle made. 

 By now inclining the bottle so that the urine comes in contact with 

 the hypobromite, decomposition of urea takes place energetically. 

 The liberated carbon dioxide is absorbed by the sodium hydroxide, 

 while the nitrogen increases the volume of air present in the appa- 

 ratus. The burette is gradually raised as the nitrogen is evolved, 

 and the whole allowed to stand for half an hour. The cubic centi- 

 meters of nitrogen gas are read off (whilst the water in the burette 

 and cylinder are on a level), and give, multiplied by 0.0027, the 

 grammes of urea in 5 c.c. of urine. 



As the volume of gas depends upon temperature and pressure, 

 corrections for these have to be made by using the following formula : 



_ 100 y b 



P ~~ 760.370. a (1 + 0.003665^. 

 p = Weight of urea for 100 c.c. urine, 

 a = Volume of urine used, expressed in c.c. 

 v = Volume of nitrogen read off. 

 b = Barometric pressure in mm. 

 t = Temperature during the measurement of nitrogen. 



370 represents the c.c of nitrogen (at and 760 mm pressure) 

 obtained from one gramme of urea. 



The above described process for estimation of urea is, for various 

 reasons, far from being perfect (uric acid and kreatin, for instance, 

 are also decomposed with liberation of nitrogen ; but it has been 

 found that the results obtained are quite sufficient for clinical purposes. 



Experiment 70. Determine urea in urine by the method described above. 



Uric acid, H 2 C 5 H 2 N 4 O 3 . Uric acid is found in small quantities in 

 human urine, chiefly in combination with sodium, potassium, and 

 ammonium, but also with calcium and magnesium. In larger propor- 

 tions, uric acid is found in the excrement of birds, mollusks, insects, 

 and chiefly of serpents, the solid urine of the latter consisting almost 

 entirely of uric acids and urates. It is also found in Peruvian guano. 



Pure uric acid is a white, crystalline, tasteless, and odorless sub- 

 stance, almost insoluble in water, requiring 1900 parts of boiling and 

 15,000 parts of cold water for its solution ; it is also insoluble, or 

 nearly so, in alcohol and ether. The great insolubility of uric acid 

 causes its separation in the solid state, both in the bladder and in the 

 tissues. 



Reactions and determination of uric acid. Uric acid may be 

 recognized by its crystalline form, and by the murexid test, which is 



