122 HOW CEOPS GEOW. 



ed and heated, 14 per cent of anhydrous silica. This so- 

 lution was clear, colorless, and not viscid. It reddened 

 litmus paper like an acid. Though not sour to the taste, 

 it produced a peculiar feeling on the tongue. Evaporated 

 to dryness at a low temperature, it left a transparent, 

 glassy mass, which had the composition Si O^, H^O. This 

 dry residue was insoluble in water. These solutions of silica 

 in pure water are incapable of existing for a long time 

 without suffering a remarkable change. Even when pro- 

 tected from all external agencies, they sooner or later, usu- 

 ally in a few days or weeks, lose their fluidity and trans- 

 parency, and coagulate to a stiff jelly, from the separation 

 of a nearly insoluble hydrate of silica, which we shall des- 

 ignate as gelatinous silica. 



The addition of to^jo of an alkali or earthy carbonate, 

 or of a few bubbles of carbonic acid gas to the strong so- 

 lutions, occasions their immediate gelatinization. A mi- 

 nute quantity of potash or soda, or excess of chlorhydric 

 acid, prevents their coagulation. 



Gelatinous Silica. — This substance, which results from 

 the coagulation of the soluble silica just described, usually 

 appears also when the strong solution of a silicate has 

 strong chlorhydric acid added to it, or when a silicate is 

 decomposed by direct treatment with a concentrated acid. 



It is a white, opaline, or transparent jelly, which, on dry- 

 ing in the air, becomes a fine, white powder, or forms 

 transparent grains. This powder, if dried at ordiniary 

 temperatures, is 3 Si O^, 2 H^O. At the temperature of 

 213° F., it loses half its water. At a red heat it becomes 

 anhydrous. 



Gelatinous silica is distinctly, though very slightly, sol- 

 uble in water. Fuchs and Bresser have found by experi- 

 ment that 100,000 parts of water dissolve 13 to 14 parts 

 of gelatinous silica. 



The hydrates of siUca which have been subjected to a. 



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