302 Mr. T. Graham on Liquid Diffusion applied to Analysis. 



properties of this substance, which indicate its allotropic nature, 

 are the orange-red colour and the opalescent appearance of its 

 solution. The mctaperoxide of iron is entirely precipitated, of a 

 brown ochreous appearance, by a trace of sulphuric acid or of an 

 alkaline, salt, and is insoluble in all cold acids, even when the 

 latter are concentrated. The solubility of metaperoxide of iron 

 in water appears to be more precarious, if possible, than that of 

 the colloid alumina. It would no doubt be more safely pre- 

 pared by diffusing away the acetic acid of the altered acetate of 

 iron, than it is by boiling off that acid, as the solution is said 

 to become precipitable by heat before the whole acetic acid is 

 expelled. 



Ferrocyanide of Copper. — Many of the insoluble ferrocyanides 

 are crystalline precipitates ; but the compound above named, and 

 the different varieties of prussian blue, appear to be strictly col- 

 loidal. 



Certain anomalous properties long observed in these com- 

 pounds come thus to be explained. The ferrocyanide of copper 

 precipitated from ferrocyanide of potassium and sulphate of cop- 

 per, is a reddish-brown gelatinous precipitate, and carries down 

 a portion of the potash salt. It is obtained of greater purity, 

 like the other insoluble ferrocyanides, by the use of ferrocyanic 

 acid as the precipitant. Ferrocyanide of copper is then darker 

 in colour, and still more highly gelatinous. It is well known 

 that this substance appears as a transparent almost colourless 

 jelly when precipitated from strong solutions. This colloidal 

 matter assumes colour on the addition of water, in consequence 

 of further hydration, following in this respect the analogy of 

 the crystalloid salts of copper. The ferrocyanide of copper, 

 when once precipitated, may be washed without loss, and ex- 

 hibits no symptoms of solubility. But it has been remarked 

 that the same salt, when produced by mixing the precipitating 

 salts dissolved in not less than two or three thousand times their 

 weight of water, gives a wine-red solution with no precipitate. 

 This is the soluble condition of the colloid. When the red solu- 

 tion is placed in the dialyser, the salt of potash diffuses out and 

 the whole ferrocyanide of copper is retained behind in solution. 



Precipitated ferrocyanide of copper is not dissolved by oxalic 

 acid, nor by oxalate of potash, but dissolves freely in about one- 

 fourth of its weight of neutral oxalate of ammonia. The ferro- 

 cyanide of copper must be washed beforehand, to ensure solu- 

 bility. A solution holding 3 or 4 per cent, of ferrocyanide of 

 copper is of a dark reddish-brown colour, intermediate in tint 

 between the acetate and meconate of iron. The solution is 

 transparent, but assumes a peculiar appearance of opacity when 

 seen by light reflected from its surface. The same appearance 



