ME. T. GEAHAM ON LIQUID DIUFUSION APPLIED TO ANALYSIS. 
213 
colloid bodies ; that two species of each of these hydrated oxides exist, of which alumina 
and metalumina are the types ; one derived from an unchanged salt, and the other from 
the heated acetate of the base ; further, that each of these species has two forms, one 
soluble and the other insoluble, or coagulated. This last species of duality should 
be well distinguished from the preceding allotropic variability of the same peroxide. 
The possession of a soluble and an insoluble (fluid and pectous) modification is not con- 
fined to hydrated silicic acid and the aluminous oxides, but appears to be very general, 
if not universal, among colloid substances. The double form is typified in the fibrin of 
blood. 
The precipitated and gelatinous j)eroa:uIe of tin is largely soluble in the bichloride of 
the same metal. Such a solution, when placed in the dialyser, allows the whole chlo- 
rine of the salt and a portion of the tin to diffuse away. Peroxide of tin, or stannic 
acid, remains behind, but not in a soluble state. It forms in the dialyser a semitrans- 
parent gelatinous cake, which after a few days is entirely free from chlorine. The 
original solution, containing excess of stannic acid, was diluted to various degrees, but 
was dialysed always with the same result. The coagulum was insoluble in hot or 
cold water, but dissolved readily in dilute acids. It was evidently the peroxide of tin 
unaltered. 
The metcistannic acid, or nitric acid peroxide of tin of Berzelius, forms a solid com- 
pound with a small quantity of hydrochloric acid. This compound is not dissolved 
by an excess of acid, but is soluble in pure water. The solution placed in the dialyser 
is readily decomposed, and leaves behind a semitransparent gelatinous mass of pure 
hydrated metastannic acid, insoluble both in water and acids. There appears, then, 
to be no soluble form of either hydrated stannic or metastannic acid, although both 
are coUoidal substances. 
Precipitated titanic acid was dissolved in hydrochloric acid and submitted to dialysis. 
The hydrochloric acid readily diffused away, leaving hydrated titanic acid, gelatinous 
and insoluble, upon the dialyser. The proportion of titanic acid, which escaped from 
the dialyser and was lost, amounted to 0-050 gramme out of 2-5 grammes. Titanic acid 
thus resembles stannic acid in not presenting itself in the form of a fluid colloid. 
Metallic protoxides are not soluble in their neutral salts, and cannot therefore be 
submitted to dialysis in the same conditions as the preceding peroxides. It was 
observed, however, that oxide of copper and oxide of zinc, when dissolved in ammonia, 
are capable of diffusing through a colloidal septum, and are therefore not colloids them- 
selves. The water outside the dialyser should be charged with ammonia in such an 
experiment. 
5. Dialysis of Organic Colloid Substances. 
Tannin . — The tannin employed was that extracted from gall-nuts by the ether process 
of Pelouze. a two per cent, solution of this substance, covering a surface of paper- 
parchment of the area of about of a square metre, or 15-6 square inches, to a 
