1864.] 



and other analogous Colloidal Substances, 



337 



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 so- 

 luble sihcic 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 influence upon liquid silicic acid. But, on the other hand, none 

 of the liquids now referred to appear to conduce to the preservation of the 

 fluidity of the colloid, at least not more tban tbe 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 hydrate, whether that hydrate is in the liquid or 

 gelatinous condition, and give new substitution-products. 



A liquid compound of alcohol and silicic acid is obtained by adding 

 alcohol to aqueous silicic acid, and then employing proper means to with- 

 draw 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 dialyzing bag of parchment-paper con- 

 taining 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 

 sihcic acid should never be allowed to form more than 1 per cent, of the 

 alcoholic solution, otherwise it may gelatinize during the experiment. If 

 I may be allowed to distinguish the liquid and gelatinous 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 colour- 

 less liquid, not precipitated by water or salts, nor by contact with insolu- 

 able powders, probably from the small proportion of silicic acid present in 

 solution. It may be boiled and evaporated without change, but is gela- 

 tinized by a slight concentration. The alcohol is retained less strongly in 

 the alcosol of silicic acid than water is in the hydrosol, but with the same 

 varying force, a small portion of the alcohol being held so strongly as to 

 char 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 1 per cent, of the dry acid, in abso- 

 lute alcohol, and changing the latter repeatedly till the water of the hydrogel 

 is fully replaced by alcohol. The alcogel is generally slightly opalescent, 



