Mr. T. Graham on Liquid Diffusion applied to Analysis. 301 



element of instability in the extremely unequal diffusibility 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 and hydrochloric acid, in the proportion of 97*6 

 per cent, of the former to 2*4 per cent, of the latter. In nine- 

 teen 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 dif- 

 fusion on the dialyser. About one-half of the iron was lost by 

 a diffusion of eighteen days, in a particular experiment, leaving 

 on the dialyser a red liquid, in which ninety-four parts of per- 

 oxide of iron were still associated with six parts of acetic acid. 



Water 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 trans- 

 parent. Indeed the coagulation of this colloid is highly sug- 

 gestive of that of 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 having 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 mam miliary concretions, 

 is no doubt colloidal. 



Soluble Metaperoxide of Iron. — The soluble peroxide of iron 



of M. Pean de Saint-Gilles* appears to be the analogue of met- 



alumina. It was also prepared by the prolonged action of 



heat upon a pure solution of the acetate. The characteristic 



* Comptes Rendus, 1855, p. 568. 



