170 On the Blue Compounds of Cyanogen and Iron. 



It is evident, from the quantity of water found, that less 

 hydrogen was in the substance than calculated, and that a 

 portion of peroxide of iron was contained in it. 



It is decomposed by potash in the same manner as Turn- 

 bull's blue, which has been dried in the air ; peroxide of iron 

 is set free, and hydroferrocyanic acid combines with the pot- 

 ash, as described under that compound. The cause of this 

 reaction, which seems irreconcileable with the composition 

 2Cfy3Fe, lies in an oxidation of the iron taking place by the 

 action of the air. We have seen above that Turnbull's blue 

 retains a considerable quantity of water, and mny assume that 

 its elements are divided between the radical and the iron. Its 

 formation is as follows : — 



2Cfy3iq 



3FeO [ = 2(Cfy H a ) Fe 3 4 + 3KO. 



4Aq : J 



The fresh precipitate behaves as such a compound ; but 

 after it has been dried in the air potash separates from it pure 

 peroxide of iron instead of the magnetic oxide. This proves 

 that, what indeed ensues with all protosalts of iron, oxidation 

 takes place on exposure to the air. A consideration of the 

 analytical results immediately shows that such an admixture 

 of peroxide of iron is contained in it. The process of oxida- 

 tion consists in 2 atoms of the cyanide taking up 1 atom of 

 oxygen : 



2{2(Cfy H 2 ) Fe 3 4 } + O = 4(Cfy H 2 ) + 3Fe 2 3 . 



This formula expresses the elements contained in the sub- 

 stance after drying in the air. A more probable expression 

 for its composition is however obtained, if the elements of 

 three such compound atoms are arranged to 4 atoms of prussian 

 blue and 1 atom peroxide of iron. An analogous compound 

 to the pure unoxidated Turnbull's blue is that which I de- 

 scribed under the name of ferridcy anide of iron and potassium, 

 but whicfl may as correctly be considered as a combination 

 of 2 atoms hydroferrocyanic acid with 1 peroxide of iron and 

 1 potash. A strong support of this view is the fact that it 

 contains 4 atoms of water, which are not expelled at 100°. 



I have also examined a great number of precipitates formed 

 by ferrocyanide of potassium with other metallic salts, and 

 universally found, that whatever may be the method of pre- 

 cipitation, it is not possible to replace all the potassium by the 

 metal made use of. Ferrocyanide of potassium was added to 

 a great excess of sulphate of copper ; the precipitate, which 

 had the well-known red- brown colour, was found after com- 

 plete washing to contain a considerable quantity of potassium. 



