80 ANIMAL BASES. 



oxide of silver is grey. This compound detonates when heated 

 and the oxide is reduced. But urea does not seem capable of 

 decomposing any of the metallic salts. We can only combine it 

 with the oxides by double decomposition. The best way of ob- 

 taining these compounds is to mix a solution of a metallic salt 

 with a concentrated solution of urea, and to add as much alkali 

 as will saturate the acid of the metallic salt. We may combine 

 urea with oxide of lead by digesting the oxide in a concentrated 

 solution of urea. 



Nitric acid forms with urea a compound which crystallizes in 

 large brilliant plates or transparent prisms ; though it is very 

 difficult to obtain the compound in regular crystals. These crys- 

 tals have an acid taste, and are not altered by exposure to the 

 air. At the temperature of 50 100 parts of water dissolve 19-7 

 parts of nitrate of urea. This salt was found by Dr Prout's ana- 

 lysis to be composed of, 



Nitric acid, . , 6-75 

 Urea, . . . *7-45 



14-2 



W r hen heated in a retort it gives out a combustible gas,f and is 

 converted into nitrate of ammonia. When heated rapidly on 

 platinum foil it detonates. A good deal of cold is produced 

 when nitrate of urea is dissolved in water. When the aqueous 

 solution is boiled an effervescence takes place and carbonic acid 

 is disengaged. There remains a solution of carbonate and ni- 

 trate of ammonia. 



Dr Prout discovered that oxalic acid forms a crystalline com- 

 pound with urea as well as nitric acid. Oxalate of urea is in 

 long slender plates. Its taste is cooling. When heated it melts 

 and boils, carbonate of ammonia is disengaged and cyanuric acid 

 is formed. Oxalate of urea dissolves in much greater quantity 

 in boiling than in cold water, and is deposited in crystals as the 



* There is probably another compound of nitric acid and urea. I obtained a 

 compound of nitric acid, 6-75 ; urea, 17-23, or rather more than twice the urea 

 stated in the text. In this case the urea was not deprived of its colouring mat- 

 ter, and therefore was heavier than it ought to have been. Had it been pure it 

 would in all likelihood not have exceeded fifteen, or double the quantity which 

 Prout obtained. 



j- Probably cyanogen. 



