Preparation of Urea — Arseniuretted Hydrogen. 185 



following new and easy method : — Twenty-eight parts of per- 

 fectly dried ferrocyanate of potash are mixed with fourteen 

 parts of the black oxide of manganese, both finely powdered, 

 the mixture heated on a smooth iron plate, (not in a crucible) 

 over a coal fire, to incipient redness, stirring frequently. The 

 mass on cooling is washed out with cold water, and the liquid 

 mixed with twenty and a half parts of dry sulphate of ammo- 

 nia. It is advantageous to place aside the first strong ley ob- 

 tained, and to dissolve the sulphate of ammonia at the com- 

 mon temperature in the last water, and to add this to the first 

 concentrated solution. Usually a strong precipitate of sul- 

 phate of potash is immediately formed, from which the liquid 

 is separated by decantation ; it is then evaporated in the water 

 bath or on some warm place, boiling being avoided, and the 

 liquid poured off, as long as it is possible, from the deposit of 

 sulphate of potash. It is then evaporated to dryness and 

 treated with boiling alcohol of 80-90 per cent., which dissolves 

 the urea, while the sulphate salts remain undissolved. In this 

 manner nearly four ounces of perfectly colourless and beauti- 

 fully crystallized urea may be obtained from a pound of the 

 ferrocyanate of potash. It frequently happens that the solu- 

 tion containing the sulphate of potash and urea is coloured 

 yellow by ferrocyanide of ammonium, or of potassium, which 

 dissolves in the alcohol, and gives to the crystals of urea a yel- 

 lowish colour; it may easily be separated by the addition of 

 a solution of copperas, after separation of the Prussian blue 

 carbonate of ammonia is added to the liquid, which decom- 

 poses the excess of iron salt, and the liquid becomes clear and 

 colourless, and may then be evaporated and treated as above. 

 {Arm. de Chem. und Pharm. vol. xxxviii. part 1.) 



Arseniuretted Hydrogen. 



It is well known that a solution of the perchloride of mer- 

 cury most easily decomposes arseniuretted hydrogen, on which 

 account it is employed not only to destroy every trace of this 

 gas, but likewise to indicate its presence. The composition of 

 this precipitate, which is yellow with a slight brownish tinge, 

 and is thus distinguished from the precipitate produced by the 

 action of perchloride of mercury on phosphuretted hydrogen, 

 is quite unknown ; Stromeyer seems to be the only person who 

 has examined it. According to him, arseniuretted hydrogen 

 forms, with a solution of perchloride of mercury, arsenious acid 

 and protochloride of mercury, and, finally, an amalgam of mer- 

 cury and arsenic. The precipitate decomposes by preserva- 

 tion in much water ; it becomes black, and consists at last only 

 of globules of mercury ; the liquid above it contains hydro- 



