m. T. GEAILA.M ON LIQUID DIFFUSION APPLIED TO ANALYSIS. 
205 
bodies being generally highly osmotic. The solution now gave no precipitate with nitrate 
of silver, and contained 60 ‘5 grammes of sihca, 6'7 grammes of that substance having 
been lost. The solution contained 4-9 per cent, of silicic acid. 
The pm’e solution of silicic acid so obtained may be boiled in a flask, and consider- 
ably concentrated, without change ; but when heated in an open vessel, a ring of inso- 
luble 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 pui’er 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 xo.oo 'qI'^^ of 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 solution 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 calcareous 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 lias 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 receiver, 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 other soluble colloids, 
such as gelatine, alumina, and peroxide of iron, but not by gum nor 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 allotropic 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 (Feitzsche). 
The amorphous silicic acid obtained by drying and calcining the jelly, and the vitreous 
