ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
207 
Soluble Alumina . — We are indebted to Mr. Waltee Ceum for the interesting discovery 
that alumina may be held in solution by water alone in the absence of any acid. But 
two soluble modifications of alumina appear to exist, alumina and metalumina. The 
latter is Mr. Ceum’s substance. 
A solution of the neutral chloride of aluminium (Alg CI3), placed on the dialyser, 
appears to difiuse away without decomposition. But when an excess of hydrated alumina 
is previously dissolved in the chloride, the latter salt is found to escape by diffusion in 
a gradual manner, and the hydrated alumina, retaining little or no acid, to remain 
behind in a soluble state. A solution of alumina in chloride of aluminium, consisting 
at first of 52 parts of alumina to 48 of hydrochloric acid, after a dialysis of six days, 
contained 66 ’5 per cent, of alumina; after eleven days 76 ’5 per cent. ; after seventeen 
days 92‘4 per cent., and after twenty-five days the alumina appeared to be as nearly 
as possible free from acid, as traces only of hydrochloric acid were indicated by an acid 
solution of nitrate of silver. But in such experiments the alumina often pectizes in the 
dialyser before the hydrochloric acid has entirely escaped. 
Acetate of alumina with an excess of alumina gave similar results. The alumina 
remained fluid in the dialyser for twenty-one days, and when it pectized was found to 
retain 3-4 per cent, of acetic acid, which is in the proportion of 1 equivalent of acid 
to 28-2 equivalents of alumina. 
Soluble alumina is one of the most unstable of substances, a circumstance which fully 
accounts for the difficulty of preparing it in a state of purity. It is coagulated or 
pectized by portions, so minute as to be scarcely appreciable, of sulphate of potash and, 
I believe, by all other salts ; and also by ammonia. A solution containing 2 or 3 per 
cent, of alumina was coagulated by a few drops of well-water, and could not be trans- 
ferred from one glass to another, unless the glass was repeatedly washed out by distilled 
water, without gelatinizing. Acids in small quantity also cause coagulation ; but the 
precipitated alumina readily dissolves in an excess of the acid. The colloids gum and 
caramel also act as precipitants. 
This alumina is a mordant, and possesses indeed all the properties of the base of alum 
and the ordinai'y aluminous salts. A solution containing 0-5 per cent, of alumina may 
be boiled without gelatinizing, but when concentrated to half its bulk it suddenly 
coagulated. Soluble alumina gelatinizes when placed upon red litmus paper, and forms 
a faint blue ring about the drop, showing a feeble alkaline reaction. Soluble alumina 
is not precipitated by alcohol nor by sugar. No pure solution of alumina, although 
dilute, remained fluid for more than a few days. 
Like hydrated silicic acid, then, the colloid alumina may exist either fluid or pectous, 
or it has a soluble and insoluble form, the latter being the gelatinous alumina as pre- 
cipitated by bases. It is e\ident that the extraordinary coagulating action of salts upon 
hydrated alumina must prevent the latter substance from ever appearing in a soluble 
state when hberated from combination by means of a base. 
Colloidal alumina possesses also, I believe, a high atomic weight, like cosilicic acid. 
