122 HOW CROPS GROW. 
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 yst00 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 §10,,2 H,O. At the temperature of 
212° ¥,, it loses half its water. Ata 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 18 to 14 parts 
of gelatinous silica. 
The hydrates of silica which have been subjected toa 
