122 HOW CROPS GKOW. 



ed and seated, 14 per cent of anhydrous silica. This eo 

 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 Q , 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 separatioji 

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

 ignate as gelatinous silica. 



The addition of T oloo of an alkali or earthy carbonate, 

 or of a lew 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, M hich 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 

 uolc 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 silica which have been subjected to a 



