UREA. 79 



solved in the succeeding weak liquids. A copious precipitate of 

 sulphate of potash falls. The supernatant liquid is decanted oft* 

 and evaporated over the water-bath. More sulphate of potash 

 falls, which is separated, and this is repeated as long as the sul- 

 phate continues to form. The liquid is now evaporated to dry- 

 ness, and the solid residue is digested in boiling alcohol of 80 or 

 90 per cent. The urea is dissolved. It crystallizes as the al- 

 cohol cools or is evaporated. By this process a pound of prus- 

 siate of potash will furnish one-third of a pound of urea, colour- 

 less and crystallized. 



The precipitate of potash when heated with black oxide of 

 manganese is converted into cyanate of potash, a salt very solu- 

 ble in water. When the solution of this salt is mixed with sul- 

 phate of ammonia, sulphate of potash and cyanate of ammonia 

 are formed, which last by a gentle heat, as Wohler first discover- 

 ed, is converted into urea. 



Urea when pure and in crystals is white and transparent. It 

 has no smell, but a cooling taste, and its lustre is pearly. When 

 deposited from a concentrated hot solution it is in the form of 

 fine needles ; but by spontaneous evaporation it assumes the 

 form of long, narrow four-sided prisms. It is best obtained in 

 crystals by allowing a boiling-hot saturated alcoholic solution to 

 cool slowly. It produces no change on vegetable blues. It is 

 not affected by exposure to the air, unless the atmosphere be very 

 moist, when it deliquesces slightly, but is not decomposed. When 

 heated it melts, one portion is decomposed and another sublimed 

 without any apparent change. The specific gravity of its crys- 

 tals, as determined by Prout, is 1 '350. 



At the temperature of 60, water dissolves more than its own 

 weight of urea. The solution exposed to the air for some months 

 underwent no alteration. Boiling water dissolves any quantity 

 whatever of urea, and the urea is not altered at that temperature. 



At the ordinary temperature alcohol of 0'816 dissolves the 

 fifth part of its weight of urea, and when boiling hot it dissolves 

 more than its weight of it. On cooling the additional quantity 

 is precipitated in crystals. It is hardly soluble in ether and oil 

 of turpentine, though it renders these liquids opaque. 



The fixed alkalies and the alkaline earths decompose urea, es- 

 pecially when assisted by heat and when water is present. It 

 combines with most of the metallic oxides. Its combination with 



