128 



PRACTICAL PHYSIOLOGY. 



[xx. 



(a.) In a conical glass, add 5 parts of HC1 to 20 parts of urine, 

 put it in a cool place for twenty-four hours. Yellow or brownish- 

 coloured crystals of uric acid are deposited on the sides of the 

 glass, or form a pellicle on the surface of the fluid like fine grains 

 of cayenne-pepper. Both uric acid and its salts (urates), when 

 they occur as sediments in urine, are coloured, and the colour is 

 deeper the more coloured the urine. The slow separation of the 

 uric acid is probably due to the presence of phosphatic salts. 



(b.) Collect some of the crystals and examine them microscopi- 

 cally. The crystals assume many forms, but are chiefly rhombic. 

 They may be whetstone, lozenge-shaped, in rosettes, quadrilateral 



d 



FIG. 67. Uric Acid. a. Rhombic tables (whetstone form): b. Barrel f nil ; 

 c. Sheaves; d. Rosettes of whetstone crystals. 



prisms, &c. They are yelloioisli in colour, although their tint may 

 vary from yellow to red or reddish-brown, depending on the depth 

 of the colour of the urine (figs. 67, 68). 



(c.) The crystals are soluble in caustic soda or potash. Observe 

 this under the microscope. 



('/.) With the aid of heat dissolve some serpent's urine which 

 is solid, and consists chiefly of ammonium urate in a 10 per 

 cent, solution of caustic soda. Add water, and allow it to stand. 

 Pour off the clear fluid, and precipitate the uric acid with dilute 

 hydrochloric acid. Collect the deposit and use it for testing. 



3. Reactions and Tests. 



(a.) Murexide Test. Place uric acid in a porcelain capsule 

 add nitric acid, and heat gently, taking care that the temperature 



