ON THE PROPERTIES OF SILICIC ACID. 
65 
when the resulting jelly is rapidly distilled at a high temperature. Not a trace of 
silicic ether is found in any compound of this class. The jelly burns readily in the air, 
leaving the whole silicic acid in the form of a white ash. 
The alcogel, or solid compound, is readily prepared by placing masses of gelatinous 
silicic acid, containing 8 or 10 per cent, of the dry acid, in absolute alcohol, and chang¬ 
ing the latter repeatedly till the water of the hydrogel is fully replaced by alcohol. The 
alcogel is generally slightly opalescent, and is similar in aspect to the hydrogel, preserv¬ 
ing very nearly its original bulk. The following is the composition of an alcogel care¬ 
fully prepared from a hydrogel which contained 9*35 percent.of silicic acid :— 
Alcohol. 88T3 
Water ... 023 
Silicic acid . 11 ’64 
100-00 
Placed in water, the alcogel is gradually decomposed—alcohol diffusing out and water 
entering instead, so that a hydrogel is reproduced. 
Further, the alcogel may be made the starting-point in the formation of a great 
variety of other substitution jellies of analogous constitution, the only condition required 
appearing to be that the new liquid and alcohol should he intermiscible, that is, inter- 
diffusible bodies. Compounds of ether, benzole, and bisulphide of carbon have thus been 
produced. Again, from etherogel another series of silicic-acid jellies may be derived, 
containing fluids soluble in ether, such as the fixed oils. 
The preparation of the glycerine compound of silicic acid is facilitated by the com¬ 
parative fixity of that liquid. When hydrated silicic acid is first steeped in glycerine, 
and then boiled in the same liquid, water distils over, without any change in the ap¬ 
pearance of the jelly, except that when formerly opalescent it becomes now entirely 
colourless, and ceases to be visible when covered by the liquid. But a portion of the 
silicic acid is dissolved, and a glycerosol is produced at the same time as the glycerine 
jelly. A glycerogel prepared from a hydrate containing 9‘35 per cent, of silicic acid, 
was found by a combustion analysis to be composed of 
Glycerine . 87*44 
Water . 3-78 
Silicic acid . 8*95 
100-17 
The glycerogel has somewhat less bulk than the original hydrogel. When a glycerine 
jelly is distilled by heat, it does not fuse, but the whole of the glycerine comes over, 
with a slight amount of decomposition towards the end of the process. 
The compound of sulphuric acid, sulphagel, is also interesting from the facility of its 
formation, and the complete manner in which the water of the original hydrogel is re¬ 
moved. A mass of hydrated silicic acid may be preserved unbroken if it is first placed in 
sulphuric acid diluted with two or three volumes of water, and then transferred gradually 
to stronger acids, till at last it is placed in concentrated oil of vitriol. The sulphagel 
sinks in the latter fluid, and may be distilled with an excess of it for hours without 
losing its transparency or gelatinous character. It is always somewhat less in bulk than 
•the primary hydrogel, but not more, to the 'eye, than one-fifth or one-sixth part of the 
original volume. This sulphagel is transparent and colourless. When a sulphagel is 
heated strongly 4 in an open vessel, the last portions of the monohydrated sulphuric acid 
in combination are found to require a higher temperature for their expulsion than the 
boiling-point of the acid. The whole silicic acid remains behind, forming a white, 
opaque, porous mass, like pumice. A sulphagel placed in water is soon decomposed, and 
the original hydrogel reproduced. No permanent compound of sulphuric and silicic 
acids, of the nature of a salt, appears to be formed in any circumstances. A sulphagel 
placed in alcohol gives ultimately a pure alcogel. Similar jellies of silicic acid may 
readily be formed with the monohydrates of nitric, acetic, and formic acids, and are all 
perfectly transparent. 
The production of the compounds of silicic acid now described indicates the posses¬ 
sion of a wider rang*e of affinity by a colloid than could well be anticipated. The or¬ 
ganic colloids are no doubt invested with similar wide powers of combination, which 
may become of interest to the physiologist. The capacity of a mass of gelatinous silicic 
