20S 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
The chloride of alumina with excess of alumina referred to above appears to be, either 
in whole or in part, a colloidal hydrochlorate of alumina, containing the latter sub- 
stance with its large colloidal equivalent, and may be really neutral in composition. 
The soluble basic persalts of iron, tin, &c. are likewise all colloidal, and have no doubt 
a similar constitution. Such colloidal salts are themselves slowly decomposed on the 
dialyser, being resolved into the crystalloidal acid which escapes and the colloidal oxide 
which remains behind. 
Soluble Metalumina. — Mr. Ceum first pointed out a singular relation of acetic acid to 
alumina, which has never been explained. Sulphate of alumina, when precipitated by 
acetate of lead or baryta, gives a binacetate of alumina, with one equivalent of free 
acetic acid ; the neutral teracetate of alumina not appearing to exist. It was further 
observed that, by keeping a solution of this binacetate in a close vessel at the boiling- 
point of water for several days, nearly the whole acetic acid came to be liberated, 
\vithout any precipitation of alumina occurring at the same time. Mr. Crum boiled off 
the free acetic acid, or the greater part of it, and thus obtained his soluble alumina. 
The same result may be arrived at by dialysing a solution of acetate of alumina that has 
been altered by heat. In three days the acetic acid was reduced on the dialyser to II per 
cent., gmngl equiv. acetic acid to 8 equivs. alumina; in six days to 7T7 percent, acid; 
in thu’teen days to 2-8 per cent, acid, or I equiv. acid to 33 equivs. alumina. The 
alumma exists in an allotropic condition, being no longer a mordant ; and forming, 
when precipitated, a jelly that is not dissolved by an excess of acid. Metalumina 
resembles alumina in being coagulated by minute proportions of acids, bases, and of most 
salts. Mr. Crum found the solution of metalumina to require larger quantities of 
acetates, nitrates, and chlorides to produce coagulation than of the former substances. 
The solution of metalumina is tasteless, and entirely neutral to test-paper, according to 
my oum obseiTation. 
Like alumina, the present colloid has therefore a fluid and a pectous form, the liquid 
soluble metalumina, and the gelatinous insoluble metalumina. 
Soluble Peroxide of Iron . — A solution of hydrated peroxide of iron may be obtained 
by a process exactly analogous to that for soluble alumina. Perchloride of iron in 
solution is fii’st saturated with hydrated peroxide of iron, added by small quantities at a 
time ; or carbonate of ammonia may be added in a gradual manner to perchloride of 
iron, so long as the precipitated oxide continues to be redissolved on stirring. These red 
solutions of iron have lately been carefully investigated by Mr. Ordway (Silltmajv’s 
-lournal, 3 ser. xxxix. 197), by M. Bechamp (Annales de Chimie, 3 ser. Mi. 293), and 
by M. Scheueer-Kesta^er (Ib. Iv. 330). It is observed that the act of solution of the 
hydrated peroxide by the chloride of iron is a gradual process, demanding time. The 
quantity of oxide taken up will go on increasing for a long time, if digestion in the cold 
is continued. Mr. Ordway found chloride of iron to take up so much as 18 equivalents 
of peroxide of iron in the course of five months. This slowness of action is highly cha- 
racteristic of colloids. Only monobasic acids, such as hydrochloric and nitric, serve for 
