238 ELEMENTARY PHYSIOLOGY. [XXIII. 



4. A small quantity of mucus derived from the 

 urinary passages is occasionally present in the 

 form of a faint cloudy precipitate. This may 

 be rendered more apparent by the addition of 

 acetic acid. 



5. Urea. Place a few crystals of urea in a watch- 

 glass, and dissolve them in a small quantity of 

 water. 



a. Mount a drop of the solution, and when it 

 has partially evaporated observe under a 

 high power the crystals of urea, consisting 

 of four-sided prisms 1 commonly ending in 

 two surfaces or in a single oblique surface ; 

 if the evaporation is rapid the urea crystal- 

 lises in long spicules. 



b. Add to another drop on a slide a drop of 

 pure, fairly strong, nitric acid; observe under 

 the microscope the rhombic and six-sided 

 tablets of nitrate of urea which crystallise out. 

 Note the strise frequently present in these 

 tablets. 



c. Repeat (b), using a concentrated solution of 

 oxalic acid instead of nitric acid. Oxalate of 

 urea will crystallise out in various forms, 

 prominent among which will probably be 

 long thin flat crystals often in bundles; 

 regular rhombic prisms, or tablets resembling 



1 Figures of the crystals described in this Lesson will be hung in 

 the laboratory. 



