378 PHYSIOLOGICAL CHEMISTRY. 



nitrogen increases the volume of air present in the apparatus. 

 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 grams of urea in 5 c.c. of urine. 



As the volume of a gas depends upon temperature and 

 pressure, corrections for these have to be made b}' using the 

 following formula : 



100. v b . 



p '~~ 760.870. a (I -t- 0.008666*). 



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 gram of urea. 



The above-described process for estimation of urea is for 

 various reasons far from being perfect (uric acid and kreatinin, 

 for instance, are also decomposed with liberation of nitrogen), 

 but it has been found that the results obtained are quite suf- 

 ficient for clinical purposes. 



Uric acid, H 2 C 5 H 2 N 4 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 proportions, 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 acid and urates. It is 

 also found in Peruvian guano. 



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

 substance, almost insoluble in water, requiring 1900 parts of 

 boiling and 15,000 parts of cold water for its solution; it is also 

 insoluble in alcohol and ether. The great insolubility of uric 

 acid causes to its separation in the solid state, both in the 

 bladder and in the tissues. 



Determination of uric acid. Uric acid may be recogjiized by 

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



