422 PRACTICAL PHYSIOLOGY 



flask with a reflux condenser, keeping its contents just boiling for 1 hours. 

 After this time, and without permitting the flask to become cold, add distilled 

 water cautiously and transfer the contents to a Kjeldahl distilling flask ; 

 bring the volume of fluid up to 600 c.c. and allow 10 c.c. of 40 per cent. 

 NaOH solution to run down the side of the flask so as to form a layer under 

 the watery solution. Add some powdered pumice and distil the contents into 



a measured quantity of ^H 2 SO 4 as described in connection with Kjeldahl's 



method. The distillation requires much longer than in Kjeldahl's process 

 (li hours). 

 From the number of c.c. of y^NHo found must be deducted : 



(1) The c.c. TjrNH 3 present as such in the 5 c.c. of urine used (determined by 

 Folin's or Shaffer's method, p. 433). 



(2) The c.c. TjrNHg present in the reagents for MgCla always contains traces of 

 NH 3 . 



CHAPTER XVIII. 

 URIC ACID AND OTHER PURINE BODIES. 



IT will be remembered, from the description of the chemical structure 

 of nuclein (p. 310), that there exist among its decomposition products 

 several bodies belonging to the so-called purine group of chemical sub- 

 stances. Uric acid is also a member of this group. The group receives 

 its name because all the members of it contain, as their nucleus of 

 construction, a body called purine, which exists as a double ring of 

 carbon and nitrogen atoms. The various members of the group differ 

 from one another according to the nature and position of the atoms or 

 groups of atoms which are tacked on to this purine ring. In order to 

 make the relationships clear the structural formulae of the various 

 members should be studied side by side, thus : 



iN C 6 HN CO 



2 C 5 C _7M CO C NR 



\ II >CO; 



/ 



HN C NH 



Purine nucleus. Uric acid 



(2, 6, 8, trioxypurine). 



HN CO HN CO 



CO C NH CH C NH 



\ || >CH; X l! 



HN C N^ ^N C 



Xanthine Hypoxanthiue 



(2, 6, dioxypurine). (6, oxypurine). 



