78 ANIMAL BASES. 



as completely as possible. Treat the residue with absolute alco- 

 hol till every thing which that liquor is capable of taking up is 

 dissolved. Distil the alcoholic solution over the steam-bath. 

 Dissolve the residue in a little water, and digest it with a little 

 animal charcoal, which will render it nearly colourless. Filter 

 the liquid. Heat it to 122, and dissolve in it as much oxalic 

 acid as it is capable of taking up at that temperature. On cool- 

 ing colourless crystals of oxalate of urea are deposited. By eva- 

 porating the residual liquid in a gentle heat we obtain more oxa- 

 late of urea. When it begins to thicken and has no longer a 

 strong acid taste, we may obtain a great deal more oxalate of 

 urea by heating it to 122, and adding a new dose of oxalic acid. 

 Collect the whole crystals thus obtained and wash them with a 

 little cold water. Then dissolve them in boiling water, adding 

 a small quantity of animal charcoal, filtrate and evaporate. Oxa- 

 late of urea is deposited in crystals as white as snow. Dissolve 

 these crystals in water and mix it with carbonate of lime in very 

 fine powder, which decomposes the oxalate of urea with efferves- 

 cence. When the liquor no longer reddens litmus-paper, let it 

 be filtered to get rid of the oxalate of lime, and evaporate the 

 clear liquid over the water-bath. We obtain a white mass of a 

 saline appearance, which is urea, but still mixed with an alkaline 

 oxalate. This oxalate is removed by digesting the saline mass 

 in absolute alcohol. Nothing is dissolved but pure urea. What 

 remains is a chemical combination of urea and an alkaline oxa- 

 late, usually oxalate of ammonia. 



Liebig has lately given another process, which he says is less 

 expensive, and which is merely the method used by Wohler to 

 convert cyanate of ammonia into urea.* 



Twenty-eight parts of dry prussiate of potash are mixed with 

 14 parts of peroxide of manganese in powder, and the mixture 

 is made as intimate as possible. This mixture is heated on a 

 plate of iron over a charcoal fire to a dull red heat It takes 

 fire, but is gradually extinguished, and it must be well stirred 

 while cooling to prevent agglutination and to facilitate the admis- 

 sion of air. When cold it is digested repeatedly in cold water, 

 and the solution is mixed with 20^ parts of sulphate of ammo- 

 nia. The first concentrated liquid obtained by washing the pre- 

 cipitate should be set aside, and the sulphate of ammonia dis- 



* Ann. der Pharm. xxxviii. 108. 



