PRACTICAL PHYSIOLOGY. 



[XVIII 



(b.) Examine the crystals microscopically (fig. 60). 



(c.) If only traces of urea are present, concentrate the fluid 

 supposed to contain the urea, place a drop on a slide, put into 

 the drop one end of a thread, apply a cover-glass, and put a drop 

 of pure nitric acid on the free end of the thread. The acid will 

 pass into the fluid, and microscopic crystals of urea nitrate will 

 be formed on the thread. After a time examine the preparation 

 microscopically. 



5. Urea Oxalate (CH 4 N 2 0) 2 C 2 H 2 4 + H 2 0. 



(a.) To the other half of the concentrated urine add a concen- 

 trated solution of oxalic acid. After a time crystals of oxalate of 

 urea separate. 



(b.) Examine them microscopically (fig. 61). 



FlG. 60. a. Urea ; b. Hexagonal plates ; and r. Smaller scales, or rhombic 

 plates of urea nitiate. 



(c.) Add oxalic acid to a concentrated solution of urea = a preci- 

 pitate of urea oxalate, which may have many forms rhombic 

 plates, crystalline scales, easily soluble in water. 



(d.) Do the same test as described for urea nitrate (4, r.), but 

 substitute oxalic for the nitric acid. 



6. Urea and Mercuric Nitrate ( 2 CON 2 H 4 + Hg(NO a ) 2 + 3HgO). 



(a.) To urine (after removing the phosphates by baryta mixture) 

 or urea solution add mercuric nitrate = a white, cheesy precipitate, 

 a compound of urea and mercuric nitrate. Liebig's method for the 

 estimation of urea is founded on this reaction. 



