296 Mr. T. Graham on Liquid Diffusion applied to Analysis, 



of water were brought together, and the solution placed upon a 

 hoop dialyser, 10 inches in diameter. After four days the solu- 

 tion had increased to 1235 cub. centims. by the action of osmose, 

 colloid bodies being generally highly osmotic. The solution 

 now gave no precipitate with nitrate of silver, and contained 

 60-5 grammes of silica, 6*7 grammes of that substance having 

 been lost. The solution contained 4*9 per cent, of silicic 

 acid. 



The pure solution of silicic acid so obtained may be boiled in 

 a flask, and considerably concentrated, without change ; but when 

 heated in an open vessel, a ring of insoluble silica is apt to form 

 round the margin of the liquid, and soon causes the whole to 

 gelatinize. The pure solution of hydrated silicic acid is limpid 

 and colourless, and not in the least degree viscous, even with 14 

 per cent, of silicic acid. The solution is the more durable the 

 longer it has been dialysed and the purer it is. But this solution 

 is not easily preserved beyond a few days, unless considerably 

 diluted. It soon appears slightly opalescent, and after a time 

 the whole becomes pectous somewhat rapidly, forming a solid jelly 

 transparent and colourless, or slightly opalescent, and no longer 

 soluble in water. This jelly undergoes a contraction after a few 

 days, even in a close vessel, and pure water separates from it. 

 The coagulation of the silicic acid is effected in a few minutes by 

 a solution containing 10t o 00 th part of any alkaline or earthy 

 carbonate, but not by caustic ammonia, nor by neutral or acid 

 salts. Sulphuric, nitric, and acetic acids do not coagulate silicic 

 acid ■ but a few bubbles of carbonic acid passed through the solu- 

 tion produce that effect after the lapse of a certain time. Alcohol 

 and sugar, in large quantity even, do not act as precipitants • but 

 neither do they protect silicic acid from the action of alkaline 

 carbonates, nor from the effect of time in pectizing the fluid colloid. 

 Hydrochloric acid gives stability to the solution : so does a small 

 addition of caustic potash or soda. 



This pure water-glass is precipitated on the surface of a calca- 

 reous stone without penetrating, apparently from the coagulating 

 action of soluble lime-salts. The hydrated silicic acid then forms 

 a varnish, which is apt to scale off on drying. The solution of 

 hydrated silicic acid has an acid reaction, somewhat greater than 

 that of carbonic acid. It appears to be really tasteless (like most 

 colloids), although it occasions a disagreeable persistent sensation 

 in the mouth, after a time, probably from precipitation. 



Soluble hydrated silicic acid, when dried in the air-pump re- 

 ceiver, at 15°, formed a transparent glassy mass of great lustre, 

 which was no longer soluble in water. It retained 21*99 per cent, 

 of water after being kept two days over sulphuric acid. 



The colloidal solution of silicic acid is precipitated by certain 



