Mr. T. Graham on Liquid Diffusion applied to Analysis. 299 



of acetie 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 transferred from 

 one glass to another, unless the glass was repeatedly washed out 

 by distilled water, without gelatinizing. Acids in small quan- 

 tity also cause coagulation ; but the precipitated alumina readily 

 dissolves in an excess of the acid. The colloids gum and cara- 

 mel also act as precipitants. 



This alumina is a mordant, and possesses indeed all the pro- 

 perties of the base of alum and the ordinary 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 precipitated by 

 bases. It is evident that the extraordinary coagulating action 

 of salts upon hydrated alumina must prevent the latter sub- 

 stance from ever appearing in a soluble state when liberated 

 from combination by means of a base. 



Colloidal alumina possesses also, I believe, a high atomic weight, 

 like cosilicic acid. The chloride of aluminium with excess of alu- 

 mina 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 con- 

 stitution. 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. Crum 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 



X2 



