514 Mr. R. C. Campbell on the Ferrocyanides. 



that, heated in contact with the air, the products of the de- 

 composition are very different. The cyanide of potassium 

 takes oxygen from the air, and is thereby converted into cya- 

 nate of potash, while the cyanide of iron is decomposed, con- 

 verting the iron into an oxide. The absorption of oxygen is 

 caused by the presence of the cyanide of iron, for cyanide of 

 potassium, heated by itself in contact with the air, does not 

 become changed into cyanate of potash. I believe that a 

 process for cyanate of potash may be founded on the above 

 observation, preferable to the common one with oxide of 

 manganese ; for it has been proved that this oxide is not 

 essential in the process, and it not unfrequently happens that 

 a large quantity of the cyanide of potassium is converted into 

 carbonate of potash. Sometimes also a little manganese dis- 

 solves along with the cyanate. 



For the preparation of this salt, then, without the use of 

 manganese, the powdered and dried prussiate of potash is 

 heated almost to redness, in a flat iron vessel, with constant 

 stirring of the melted mass. Some ammonia is evolved, 

 which results from the action of the moisture of the air, for 

 the substance itself contains no hydrogen. The melted mass 

 should be taken out with an iron spatula, allowed to cool, 

 reduced to powder, and again fused ; because the melting 

 cyanate of potash is apt to protect little bits of the yellow 

 prussiate from the action of the air. If the heating be pro- 

 perly conducted, not a particle of cyanide of potassium will 

 be formed. The cyanate of potash is dissolved out with hot 

 alcohol filtered and crystallized. The undecomposed prus- 

 siate*of potash remains undissolved. 



The double salt of ferroprussiate of potash and lime acts 

 under heat exactly like the ferroprussiate of potash itself. 

 Well dried and set on fire, it continues to burn, until the 

 alkaline and earthy cyanides are converted into cyanates, and 

 the iron into oxide. The reason that the double salt con- 

 tinues to burn is to be found in the porous state of the mass, 

 which offers no obstacle to the free access of air ; whereas 

 the yellow prussiate alone fuses, and prevents the progress of 

 the combustion. 



The ferrocyanide of zinc, which always contains some ferro- 

 prussiate of potash in chemical combination, likewise con- 

 tinues to burn, and affords cyanate of potash and the oxides 

 of the metals. 



When the double salt of potash and lime is heated in the 

 air as above mentioned, and then dissolved in water, the solu- 

 tion possesses the remarkable property of becoming pink in 

 the sun's rays, and again becoming colourless in darkness. 



