50 ORGANIC CONSTITUENTS. 



it in the liquor amnii and in milk ; Kiilin and Lelimann in 

 bile and biliary concretions ; Golding Bird in sweat ; Wriglit in 

 saliva, Maclagan in tlie serous effusion into the ventricles in cer- 

 tain forms of fever ; and various chemists in dropsical fluids, &c. 

 Urea may be obtained from urine in a state of purity by 

 any of the following methods. 



a. The irrine must be evaporated to the consistence of a 

 syrup, and mixed when quite cold, with an equal volume of 

 pure nitric acid of specific gravity 1"42. If the evaporation 

 has been carried sufficiently far, the whole will form a thick 

 crystalline mass, consisting of a compound of nitric acid and 

 urea, which is sparingly soluble in nitric acid. All increase of 

 temperature must be carefully avoided lest the nitric acid 

 with the aid of heat, acting on the chlorides in the urine, 

 should develop chlorine and nitrous acid, both of which, as we 

 shall presently show, act powerfully in destroying urea. The 

 impure crystals of nitrate of urea are to be carefully washed in 

 dilute nitric acid, strongly pressed between folds of blotting paper, 

 dried on a porous tile, redissolved in warm water, and neu- 

 tralized with carbonate of lead. The residue after evaporation, 

 must be treated with alcohol. In this manner we obtain an 

 alcoholic solution of urea, from which sulphuretted hydrogen, 

 and animal charcoal, will suffice to remove any traces of lead 

 and colouring matter ; after due evaporation it will yield crystals 

 of nearly pure urea. 



b. O. Henry mixes the urine with basic acetate of lead, 

 and then adds sufficient sulphuric acid to convert all the 

 acetates into sulphates. After filtration through animal charcoal 

 the fluid will yield on evaporation crystals of nearly pure urea. 



c. Berzelius recommends that the alcohol -extract of urine 

 should be dissolved in water, treated with animal charcoal, 

 filtered, and warmed to about 120°, and that then as much 

 oxalic acid should be added as the warm fluid will dissolve. 

 Crystals form of sparingly soluble oxalate of urea, which must 

 be dissolved, filtered through animal charcoal, recrystallized, 

 and decomposed by carbonate of lime. 



Urea may also be obtained artificially by the decomposition 

 of certain cyanates. The following is the best method for 

 obtaining it in this manner on a large scale. TAventy-eight 

 parts of ferrocyanide of potassium, and 14 of peroxide of 



