64 
ON THE PROPERTIES OF SILICIC ACID. 
A dominating quality of colloids is the tendency of their particles to adhere, aggre¬ 
gate, and contract. This idio-attraction is obvious in the gradual thickening of the 
liquid, and when it advances leads to pectization. In the jelly itself, the specific con¬ 
traction in question, or synccresis, still proceeds, causing separation of water, with the 
division into a clot and serum ; and ending in the production of a hard stony mass, of 
vitreous structure, which may he anhydrous, or nearly so, when the water is allowed to 
escape by evaporation. The intense synawesis of isinglass dried in a glass dish over sul¬ 
phuric acid in vacuo, enables the contracting gelatin to tear up the surface of the glass. 
Glass itself is a colloid, and the adhesion of colloid to colloid appears to be more powerful 
than that of colloid to crystalloid. The gelatin, when dried in the manner described 
upon plates of calcspar and mica, did not adhere to the crystalline surface, hut detached 
itself on drying. Polished plates of. glass must not be left in contact, as is well known, 
owing to the risk of permanent adhesion between their surfaces. The adhesion of 
broken masses of glacial phosphoric acid to each other is an old illustration of colloidal 
synaeresis. 
Bearing in mind that the colloidal phasis of matter is the result of a peculiar attrac¬ 
tion and aggregation of molecules, properties never entirely absent from matter hut 
greatly more developed in some substances than in others, it is not surprising that col¬ 
loidal characters spread on both sides into the liquid and solid conditions. These 
characters appear in the viscidity of liquids, and in the softness and adhesiveness of cer¬ 
tain crystalline substances. Metaphosphate of soda, after fusion by heat, is a true 
glass or colloid; but when this glass is maintained for a few minutes at a temperature 
some degrees under its point of fusion, the glass assumes a crystalline structure without 
losing its transparency. Notwithstanding this change, the low diffusibility of the salt 
is preserved, with other characters of a colloid. Water in the form of ice has already 
been represented as a similar intermediate form, both colloid and crystalline, and in the 
first character adhesive and capable of reunion or “ regelation.” 
It is unnecessary to return here to the fact of the ready pectization of liquid silicic 
acid by alkaline salts, including some of very sparing solubility, such as carbonate of 
lime, beyond stating that the presence of carbonate of lime in water was observed to be 
incompatible with the coexistence of soluble silicic acid, till the proportion of the latter 
was reduced to nearly 1 in 10,000 water. 
Certain liquid substances differ from the salts in exercising little or no pectizing in¬ 
fluence upon liquid silicic acid. But, on the other hand, none of the liquids now re¬ 
ferred to appear to conduce to the preservation of the fluidity of the colloid, at least not 
more than the addition of water would do. Among these inactive diluents of silicic 
acid are found hydrochloric, nitric, acetic, and tartaric acids, syrup of sugar, glycerine, 
and alcohol. But all the liquid substances named, and many others, appear to possess 
an important relation to silicic acid, of a very different nature from the pectizing action 
of salts. They are capable of displacing the combined water of the silicic acid hy¬ 
drate, whether that hjdrate is in the liquid or gelatinous condition, and give new sub¬ 
stitution-products. 
A liquid compound of alcohol and silicic acid is obtained by adding alcohol to aqueous 
silicic acid, and then employing proper means to withdraw the water from the mixture. 
For that purpose the mixture contained in a cup may be placed over dry carbonate of 
potash or quicklime, within the receiver of an air-pump. Or a dialysing hag of parch¬ 
ment-paper containing the mixed alcohol and silicic acid may be suspended in a jar of 
alcohol: the water diffuses away, leaving in the bag a liquid composed of alcohol and 
silicic acid only. A point to be attended to is, that the silicic acid should never be 
allowed to form more than 1 per cent, of the alcoholic solution, otherwise it may gela¬ 
tinize during the experiment. If I may be allowed to distinguish the liquid and gela¬ 
tinous hydrates of silicic acid by the irregularly formed terms of hydrosol and hydrogel 
of silicic acid, the two corresponding alcoholic bodies now introduced may be named 
the alcosol and alcogel of silicic acid. 
The alcosol of silicic acid, containing 1 per cent, of the latter, is a colourless liquid, 
not precipitated by water or salts, nor by contact with insoluble powders, probably from 
the small proportion of silicic acid present in solution. It may be boiled and evaporated 
without change, but is gelatinized by a slight concentration. The alcohol is retained 
less strongly in the alcosol of silicic acid than water is in the hvdrosol, but with the 
same varying force, a small portion of the alcohol being held so strongly as to char 
