THE ASH OF PLANTS. 137 



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

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

 transparent grains. This powder, if dried at ordinary 

 temperatures, has a composition nearly corresponding to 

 the formula H 4 Si 3 8 , or to a compound of 3 Si0 2 with 

 2 H 2 0. 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, 

 soluble in water. Fuchs and Bresser have found by ex- 

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

 parts of gelatinous silica. 



The hydrates of silica which have been subjected to a 

 heat of 212, or more, appear to be totally insoluble in 

 pure water. 



These hydrates of silica are readily soluble in solutions 

 of the alkalies and alkali carbonates, and readily unite 

 with moist, slaked lime, forming silicates. 



Exp. 58. Gelatinous Silica. Pour a small portion of the solution of 

 silicate potassium of Exp. 56 into strong hydrochloric acid. Gelatinous 

 silica separates and falls to the bottom, or the whole liquid becomes a 

 transparent jelly. 



EXP. 59. Conversion of soluble into insoluble hydrated silica. Evapo- 

 rate the solution of silica of Exp. 57, which contains free hydrochloric 

 acid, in a porcelain dish. As it becomes concentrated, it is very likely 

 to gelatinize, as happened in Exp. 58, on account of the removal of the 

 solvent. Evaporate to perfect dryiiess, finally on a water-bath (i. e., on 

 a vessel of boiling water which is covered by the dish containing the 

 solution). Add to the residue water, which dissolves away the chlo- 

 ride of potassium, and leaves insoluble hydrated silioa, 3 SiO 2 H 2 O, a 

 a gritty powder. 



In the ash of plants, silica is usually found in com- 

 bination with alkali-metals or calcium, owing to the 

 high temperature to which it has been subjected. 



In the plant, however, it exists chiefly, if not entirely, 

 in the free state. 



TITANIUM, an element which has many analogies with silicon, though 

 rarely occurring in large quantities, is yet often present in the form 

 of Titanic acid, TiO 2 , in rocks and soils, and, according to Salm-Hotst- 

 mar, may exist in the ashes of barley and oats. 



ARSENIC", in minute quantity, was found by Davy in turnips which 

 had been manured with a fertilizer (superphosphate), in whose prp- 

 aration arsenical oil of vitriol was employed. 



