Prof. Graham on the Properties of Silicic Acid fyc. 319 



silicic acid, the combined water (a crystalloid) leaves the acid (a col- 

 loid) to diffuse into alcohol ; and if the alcohol be repeatedly- 

 changed, the entire water is thus removed, alcohol (another crystal- 

 loid) at the same time taking the place of water in combination with 

 the silicic acid. The liquid in excess (here the alcohol) gains entire 

 possession of the silicic acid. The process is reversed if an alcogel 

 be placed in a considerable volume of water. Then alcohol separates 

 from combination, in consequence of the opportunity it possesses to 

 diffuse into water ; and water, which is now the liquid present in 

 excess, recovers possession of the silicic acid. Such changes illus- 

 trate the predominating influence of mass. 



Even the compounds of silicic acid with alkalies yield to the decom- 

 posing force of diffusion. The compound of silicic acid with 1 or 2 

 per cent, of soda is a colloidal solution, and, when placed in a dialyzer 

 over water in vacuo to exclude carbonic acid, suffers gradual decom- 

 position. The soda diffuses off slowly in the caustic state, and gives 

 the usual brown oxide of silver when tested with the nitrate of that 

 base. 



The pectization of liquid silicic acid and many other liquid col- 

 loids is effected by contact with minute quantities of salts in a way 

 which is not understood. On the other hand, the gelatinous acid 

 may again be liquefied and have its energy restored by contact with 

 a very moderate amount of alkali. The latter change is gradual, 1 

 part of caustic soda, dissolved in 10,000 water, liquefying 200 parts 

 of silicic acid (estimated dry) in 60 minutes at 100° C. Gelati- 

 nous stannic acid also is easily liquefied by a small proportion of 

 alkali, even at the ordinary temperature. The alkali, too, after 

 liquefying the gelatinous colloid, may be separated again from it by 

 diffusion into water upon a dialyzer. The solution of these colloids, 

 in such circumstances, may be looked upon as analogous to the solu- 

 tion of insoluble organic colloids witnessed in animal digestion, with 

 the difference that the solvent fluid here is not acid but alkaline. 

 Liquid silicic acid may be represented as the " peptone " of gelati- 

 nous silicic acid ; and the liquefaction of the latter by a trace of alkali 

 may be spoken of as the peptization of the jelly. The pure jellies of 

 alumina, peroxide of iron, and titanic acid, prepared by dialysis, are 

 assimilated more closely to albumen, being peptized by minute quan- 

 tities of hydrochloric acid. 



Liquid Stannic and Metastannic Acids. — Liquid stannic acid is 

 prepared by dialyzing the bichloride of tin with an addition of alkali, 

 or by dialyzing the stannate of soda with an addition of hydrochloric 

 acid. In both cases a jelly is first formed on the dialyzer ; but, as the 

 salts diffuse away, the jelly is again peptized by the small proportion 

 of free alkali remaining : the alkali itself may be removed by con- 

 tinued diffusion, a drop or two of the tincture of iodine facilitating 

 the separation. The liquid stannic acid is converted on heating it 

 into liquid metastannic acid. Both liquid acids are remarkable for 

 the facility with which they are pectized by a minute addition of 

 hydrochloric acid, as well as by salts. 



Liquid Titanic Acid is prepared by dissolving gelatinous titanic acid 



