]VIE. T. GEAHAM 01^’ LIQUID DIEFUSION APPLIED TO ANALYSIS. 
209 
preparing such solutions ; sulphuric and other polybasic acids giving insoluble subsalts 
with excess of ferric oxide, or of any other aluminous oxide. The red liquid so obtained 
is ah-eady a colloidal hydrochlorate of peroxide of iron, but requires to be dialysed for a 
sufficient time. Such a compound possesses an element of instability in the extremely 
unequal diffiisibility of its constituents. Beginning with perchloride of iron, containing 
five or six equivalents of peroxide in solution, the whole solid matter also amounting to 
4 or 5 per cent, of the liquid, and the latter forming a stratum of the usual depth of 
about half an inch in the dialyser, it was found that hydrochloric acid diffused out 
accompanied only by a small proportion of the iron. After eight days, the deep red 
solution in the dialyser contained peroxide of iron aud hydrochloric acid, in the propor- 
tion of 97'6 per cent, of the former to 2’4 per cent, of the latter. In nineteen days the 
hydrochloric acid was reduced to 1‘5 per cent., which gives 1 equiv. of acid to 30-3 
equivs. peroxide of iron. The last solution was transferred to a phial, in which it 
remained fluid for twenty days, and then spontaneously pectized. 
The peracetate of iron, prepared by double decomposition, is incapable of dissolving 
hydrated peroxide of iron, as is well known, but still may be made a source of soluble 
peroxide ; as the salt referred to is itself decomposed to a great extent by diffusion on 
the dialyser. About one-half of the hon was lost by a diffusion of eighteen days, in a 
particular experiment, lea\ing on the dialyser a red liquid, in which ninety-four parts of 
peroxide of iron were still associated wdth six parts of acetic acid. 
AVater containing about 1 per cent, of hydrated peroxide of iron in solution has the 
dark red colour of venous blood. The solution may be concentrated by boiling to a 
certain point, and then pectizes. The red solution is coagulated in the cold by traces 
of sulphuric acid, alkalies, alkaline carbonates, sulphates and neutral salts in general, 
but not by hydrochloric, nitric, and acetic acids, nor by alcohol or sugar. The coagulum 
is a deep red-coloured jelly, resembling the clot of blood, but more transparent. Indeed 
the coagulation of this colloid is higlily suggestive of that blood, from the feeble agencies 
which suffice to effect the change in question, as well as from the appearance of the 
product. The coagulum formed by a precipitant, or in the course of time, without any 
addition haring been made to the solution of peroxide of iron, is no longer soluble in 
water, hot or cold ; but it yields readily to dilute acids. It is, in short, the ordinary 
hydrated peroxide of iron. Here then, again, we have a soluble and insoluble form of 
the same colloidal substance. Native hematite, which presents itself in mammillary 
concretions, is no doubt colloidal. 
Soluble Metaperoxide of Iron . — The soluble peroxide of iron of M. Pean de Saift- 
Gilles * appears to be the analogue of metalumina. It was also prepared by the pro- 
longed action of heat upon a pure solution of the acetate. The characteristic properties 
of this substance, which indicate its allotropic nature, are the orange-red colour and the 
opalescent appearance of its solution. The metaperoxide of iron is entirely precipitated 
of a brown ochreous appearance by a trace of sulphuric acid, or of an alkaline salt, and 
* Comptea Eendus, 1855, p. 568. 
2 G 
MDCCCLXI. 
