210 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
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 prepared by diffusing away the 
acetic acid of the altered acetate of iron, than it is by boiling off that acid ; as the solu- 
tion is said to become precipitable by heat before the whole acetic acid is expelled. 
Ferrocyanide of Copper . — Many of the insoluble ferrocyanides are crystalline precipi- 
tates, but the compound above named, and the different varieties of prussian blue, appear 
to be strictly colloidal. 
Certain anomalous properties long observed in these compounds come thus to be 
explained. The ferrocyanide of copper precipitated from ferrocyanide of potassium and 
sulphate of copper, 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 exhibits 
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 Avater, gives a wine-red solution with no precipitate. This is the soluble 
condition of the colloid. When the red solution 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 ferrocyanide 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- 
broAATi colour, intermediate in tint beBveen 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 was observed by Peak de Saint-Gilles 
in his metaperoxide of iron. 
M hen a red solution, such as that described, was dialysed, the oxalate of ammonia 
came away in a gradual manner; 30 ‘6 per cent, of the oxalate of ammonia were found 
in the colourless diffusate of the first twenty-four hours ; 31 per cent, of the same salt 
in the diffusate of the next three days, and I8'2 per cent, in the diffusates of the folloAv- 
ing seA'en days, making altogether 79-8 per cent., or four-fifths of the Avhole oxalate 
of ammonia originally introduced. A small portion of the ammoniacal salt is retained 
with force, as might be expected from a feiTocyanide. Although the diffusate appeared 
colourless, it was found to contain a little oxide of copper, namely, 0'04I gramme (of 
which 0-022 gramme diffused out in the first twenty-four hours), from 2 grammes of 
ferrocyanide of copper placed in the dialyser. 
