122 HOW CHOPS GKOW. 



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 2 , H 2 O. This 

 dry residue was insoluble in water. These solutions of silica 

 in pure water are incapable of existing for a long tune 

 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 yo^o o 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 ordinary 

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

 212 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 arid Bresser have found by experi- 

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

 pf gelatinous silica. 



The hydrates of silica which have been subjected to a 



