Mr. T. Graham on Liquid Diffusion applied to Analysis. 297 



other soluble colloids, such as gelatine, alumina, and peroxide of 

 iron, but not by gum or caramel. As hydrated silicic acid, after 

 once gelatinizing, cannot be made soluble again by either water 

 or acids, it appears necessary to admit the existence of two allo- 

 tropic modifications of that substance, namely, soluble hydrated 

 silicic acid, and insoluble hydrated silicic acid, the fluid and 

 pectous forms of this colloid. 



The ordinary soluble silicate of soda is not at all colloidal, but 

 diffuses as readily through a septum as the sulphate of soda does. 

 Several crystalline hydrated silicates of soda are known (Fritzsche). 



The amorphous silicic acid obtained by drying and calcining 

 the jelly, and the vitreous acid obtained by igneous fusion, have 

 both a specific gravity of about 2*2, according to H. Rose*, 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 differ entirely from the ordinary sili- 

 cates. 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-taonate 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 com- 

 pound does not putrefy. 



The acid reaction of 100 parts of soluble silicic acid is neu- 

 tralized by 1*85 part of oxide of potassium, and by corresponding 

 proportions of soda and ammonia. The co//?-silicates or co-sili- 

 cates 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 precipitate 

 which falls on cooling, contains 2' 74 per cent, of soda, and is 

 * Phil. Mag. S. 4. vol. xix. p. 32. 



Phil Mag. S. 4. Vol. 23. No. 154. April 1862. X 



