326 Intelligence and Miscellaneous Articles. 



even the oxalates. It does not when cold dissolve binoxide of mer- 

 cury ; but if it be heated the acid is decomposed into hydrocyanic 

 acid, which forms a cyanide with the mercury of the binoxide, and 

 into cyanide of iron, which is additionally oxidized at the expense of 

 a part of the binoxide of mercury, and metallic mercury separates. 



The ferrocyanic acid prepared by the process described is in the 

 state of a white powder, frequently with a slight blue or 31 ellow tint. 

 When it is perfectly dry it may be long exposed to the air without 

 alteration, when moist the decomposition takes place more rapidly ; 

 the acid becomes gradually blue, and is slowly and totally converted 

 into Prussian blue. 



It may be long exposed in a covered platina crucible to a tempe- 

 rature of 212°, and excluded from the air, without losing weight or 

 suffering any sensible change ; eventually, however, it is decomposed 

 under these circumstances. 



When it is more strongly heated, hydrocyanic acid is disengaged 

 and cyanide of iron remains, which is oxidized. If it be heated in a 

 current of carbonic acid gas, and the temperature be not raised above 

 212°, hydrocyanic acid is evolved and white cyanide of iron is left, 

 and this decomposes also at a temperature somewhat above 212°. It 

 is, as is well known, very soluble in water, and the solution submit- 

 ted to ebullition in contact with air becomes blue; but without the 

 presence of air it deposits, on the contrary, white cyanide of iron. 



Ferrocyanic acid is even more soluble in alcohol than in water. It 

 forms a syrupy, transparent solution, which decomposes either by 

 long exposure to the air or ebullition. This solution under the air- 

 pump, yields mammillated hard crystals of a yellow colour. 



The acid obtained as described is anhydrous, not losing, as already 

 mentioned, any weight at a temperature of 212°. Two analyses gave 

 the following results as the composition of this acid : — 



1. 11. Calculated. 



Cyanogen 72*71 73*33 73*09 



Hydrogen 1*99 2*27 1-84 



Iron 25*22 25*08 2506 



99*92 10068 99*99 



Journ. de Pharm. et de Chimie, Aout 1842. 



PREPARATION OF FERR1DCYANIDE OF POTASSIUM. 



M. Posselt remarks that it is well known with what facility an 

 excess of chlorine, when passed through a solution of ferrocyanide 

 of potassium, decomposes the ferridcyanide of potassium as it is 

 formed, and the difficulty which exists in completely separating the 

 green substance which is then produced, because it readily passes 

 through the filter. It is only by repeated crystallizations that the 

 crystals are completely freed from it, and these operations are always 

 attended with loss. 



The following process is stated by M. Posselt to give pure and 

 very fine crystals at once : — Pass chlorine gas through a very dilute 

 solution of ferrocyanide of potassium, and evaporate it when the 



