92 CHEMICAL PHYSIOLOGY. 



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



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



concentrated solution of 

 oxalic acid. After a time 

 crystals of oxalate of urea 

 separate. 



(6.) Examine them micro- 

 scopically (Fig. 22). 



( c *) Add oxalic acid to a 

 concentrated solution of 

 urea = a precipitate of urea 

 oxalate, which may have 



cv rtrt - many forms rhombic 



Fig. 22. Crystals of oxalate of , , J . ,,. 



urea from urine. plates, crystalline scales, 



easily soluble in water. 



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

 but substitute oxalic for the nitric acid. 



6. Urea and Mercuric Nitrate (2U + Hg(NO 3 ) 2 + 3HgO). 



(a.) To urine 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. 



7. Other Reactions of Urea. 



Make a strong watery solution of urea, and with it perform the 

 following tests : 



(a.) Allow a drop to evaporate on a slide, and examine 

 the crystals which form (Fig. 21, a). 



(b.) Repeat, if you please, the exercises under Lesson 

 XV., 4. 



(c.) To a strong solution of urea add pure nitric acid = a 

 precipitate of urea nitrate (Fig. 21, b). 



