258 PRACTICAL PHYSIOLOGY 



clip on the T-piece being open meanwhile, and water being added to, 

 or removed from, the outer vessel if necessary. The clip is now 

 applied, and the burette raised to ascertain that no leakage exists. 

 The two menisci are then readjusted, and the contents in the generat- 

 ing bottle mixed. The evolved N displaces the water in the burette. 

 After the reaction is complete, the generating flask is immersed in a 

 basin of water, so as to bring the temperature of the gas contained 

 in it to the same as that of the gas in the burette. After waiting 

 two minutes the two menisci are again brought to the same level, 

 and the number of c.c. of N read off. Do not hold or touch any part 

 of the glass of the apparatus with the bare hand. 



Calculation of Result. If no correction for temperature and 

 pressure be made it is very simple. 



42 '5 c.c. nitrogen were obtained from 5 c.c. urine tested and 

 35*4 c.c. are liberated from 0*1 grm. urea; therefore 42*5 c.c. equals 

 approximately 0*12 grm. urea, i.e. in 5 c.c. urine there is approxi- 

 mately 0'12 grm. urea, and therefore in 1,500 c.c., 0'12 x 300 

 36 grms. 



If correction for temperature and pressure be made this formula 

 is used : 



_ v' x273x(6-6') 

 (273 + t) X 760 



v' volume of gas evolved, b = barometric pressure, b' = tension 

 of watery vapour at t = temperature at time of estimation. 



For rapid clinical purposes quite satisfactory results may be 

 obtained by using the small Doremus ureometer. 



CHAPTER XV 



URINE (Continued] 



Purines, Creatinine and Ammonia 



Purines. There are always present in varying amount products 

 of the decomposition of nucleo -protein, mainly in the form of uric 

 acid and to a lesser extent as other purine bases. The amount of 

 these substances present is largely influenced by the nature of the 

 diet. After a meal rich in meat, more particularly one which has 

 contained material like sweetbread, the output of purines is high, 

 but they are also present although in diminished amount on a 

 purine-free diet. It is possible that a certain limited amount of the 

 purines may be of synthetic origin, i.e. not derived from the break- 

 down of nucleo-protein. In man uric acid may be regarded as the 

 principal end product of the catabolism of nucleo-protein. The 

 substances grouped under the heading of purine bases are the two 

 amino purines, Adenine and Guanine and the two oxypurins, 



