SILICA : SAND AND CLA Y 99 



Take two other portions of the solution, and dilute one of 

 them with ten times its bulk of water. Now add diluted 

 sulphuric acid (i to 4) to each portion. Note that no change 

 occurs in the weak solution, the silicic acid^ so far as it is 

 replaced by sulphuric acid in the sodium salt, remaining 

 dissolved, but that from the strong solution white gelatinous 

 silica is deposited, the silicic acid set free being unstable. 

 (Compare this action with that of dilute sulphuric acid on 

 weak and strong solutions of sulphite of sodium.) Evaporate 

 the weak solution containing silicic acid to dryness on the 

 water bath ; note the white residue of silica left. Add water 

 and hydrochloric acid ; note that in both it is insoluble. These 

 experiments show that silicic acid is a very unstable compound, 

 easily decomposing into silica and water ; and that silica, unlike 

 most anhydrides, is insoluble in water and incapable of recom- 

 bining with water to form silicic acid. They also provide a 

 method for distinguishing silicic acid or the silicates, as no 

 other substance would yield a white gelatinous precipitate 

 when hydrochloric acid is added to its solution, or a white 

 residue, insoluble in water and acid, when the acid mixture 

 is evaporated to dryness. 



Silica, the oxide of the element silicon, is th6 most abundant constituent 

 of the earth's crust. It occurs uncombined as quartz, sand, and flint, minerals 

 which, though geologically different, are chemically identical. Silica also 

 occurs combined with bases. Felspar and mica, two minerals which, mixed 

 with quartz, constitute granite, are double silicates of aluminium with 

 either calcium, magnesium, potassium, sodium, or iron, while clay, pro- 

 duced by the weathering and consequent disintegration of felspathic rocks, 

 is essentially silicate of aluminium, still retaining, however, variable 

 quantities of the other silicates. Both clay and sand are usually coloured 

 yellow, brown, or red, due to the presence of ferric oxide. Asbestos is a 

 basic silicate of calcium and magnesium ; meerschaum and steatite (talc, 

 soapstone, or french chalk) are acid silicates of magnesium. 



Like phosphoric anhydride silica combines with water and bases in more 

 than one proportion. Silicate of sodium has the formula Na4Si04 (that is, 

 SiOa-aNagO), and with this the soluble silicic acid probably corresponds, 

 H4Si04 'ox Si02.2H20). But the silicates occurring in nature more often 



