206 
ME. T. GEAHAM ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
acid obtained by igneous fusion, have both a specific gravity of about 2 ‘2, according to 
H. Eose and appear to be the same colloidal substance ; while the specific gravity of 
crystalloidal silicic acid (rock-crystal and quartz) is about 2 ’6. 
Soluble silicic acid forms a peculiar class of compounds, which like itself are colloidal, 
and difier entirely from the ordinary silicates. The new compounds are interesting 
from their analogy to organic substances, and from appearing to contain an acid of 
greatly higher atomic weight than ordinary silicic acid. Like gallo-tannic acid, gummic 
acid, and the other organic colloidal acids, silicic acid combines with gelatine ; the last 
substance appearing to possess basic properties. Silicate of gelatine falls as a flaky, white 
and opake precipitate, when the solution of silicic acid is gradually added to a solution 
of gelatine in excess. The precipitate is insoluble in water, and is not decomposed by 
washing. Silicate of gelatine prepared in the manner described, contains 100 silicic acid 
to about 92 gelatine. This is a greater proportion of gelatine than in the gallo-tannate 
of gelatine, and requires for soluble silicic acid a higher equivalent than that of gallo- 
tannic acid. In the humid state the gelatine of this compound does not putrefy. 
The acid reaction of 100 parts of soluble silicic acid is neutralized by 1'85 part of 
oxide of potassium, and by corresponding proportions of soda and ammonia. The colli- 
silicates or co-silicates thus formed are soluble and more durable than fluid silicic acid, 
but they are pectized by carbonic acid or by an alkaline carbonate, after standing for a 
few minutes. . The co-silicate of potash forms a transparent hydrated film on drying in 
vacuo, which is not decomposed by water, and appears to require about ten thousand 
parts of water to dissolve it. The silicate of soda which Forchhammer obtained by 
boiling freshly precipitated silicic acid with carbonate of soda, and collecting the pre- 
cipitate wLich falls on cooling, contains 2*74 per cent, of soda, and is represented by 
NaO-h 36810.2 (Gmelin). This silicate is probably a co-silicate of soda in the pectous 
condition. Soluble silicic acid produces a gelatinous precipitate in lime-water, contain- 
ing 6 per cent, and upwards of the basic earth. This and the other insoluble earthy 
co-silicates appear not to be easily obtained in a definite state. They gave out a more 
basic silicate to water on washing. The composition of these salts, and' that also of the 
co-silicate of gelatine, were found to vary according as the mode of preparation was modi- 
fied. When a solution of gelatine was poured into silicic acid in excess, the co-silicate 
of gelatine formed gave, upon analysis, 100 silicic acid with 56 gelatine, or little more 
than half the gelatine stated above as found in that compound prepared with the mode 
of mixing the solutions reversed. The gallo-tannate of gelatine is known to otfer the 
same variability in composition. 
The gelatine used in the preceding experiments was isinglass (colle de poisson), purified 
by solution in hydrochloric acid and subsequent dialysis. As the acid escapes by difiu- 
sion, a jelly is formed in the dialyser. This jelly is free from the earthy matter, amount- 
ing to about 0-4 per cent, in isinglass, and is not liable to putrefaction. 
Cosilicic acid also precipitates both alhuminic acid and pure casein, 
* Aunales de Chimie, 3 ser. t. Ivii. p. 163. 
