PROPERTIES OP DIFFERENT EARTHS. 31 



The only acid which has been found to act upon silica 

 is the fluoric, and this will disengage it from glass, of 

 which it is one of the constituents. Hot alkaline lixivia 

 at;t slightly upon it. It is found abundantly in plants, 

 where it could only be introduced in a state of extreme 

 division, or by being dissolved in some alkali. 



This earth undergoes no change from the action of fire 

 or air, because it is saturated with oxygen ; according to 

 the analysis of Davy and Berzelius, it is composed of equal 

 parts of oxygen, and of a basis called silicium. 



According to my experiments, this earth, though dry 

 and impalpable, absorbs scarcely ^ of its own weight of 

 water, and permits it to escape by evaporation in ^ of the 

 time in which carbonate of lime, equally divided, parts with 

 it ; and in ^ of the time, in which it escapes from alumina 

 in the same state. 



All the compound primitive rocks contain alumina ; in 

 order to obtain this pure, it must be precipitated, by the 

 carbonate of ammonia, from a solution of alum, of which 

 it forms the basis ; the precipitate must be washed, and 

 ignited, and the residuum is perfectly pure alumina ; it is 

 always in the form of a white powder, and possesses the 

 following characteristics. 



It is very astringent.* 



Its specific gravity is from 2.2 to 2.3. 



It is hardened by fire, and undergoes, by the action of 

 it, a change which destroys its solubility in water. 



It absorbs water with great avidity, not being saturated 

 with less than 2.5 of its own weight, and retains it very 

 forcibly, especially when that which softened its surface is 

 evaporated ; yielding it entirely only at a temperature suf- 

 ficiently high to produce fusion. 



Alumina saturated with water forms a soft paste, smooth 

 to the touch, easily moulded, and receiving without diffi- 

 culty any form which one may wish to give it. 



According to the analysis of Berzelius, alumina consists 

 of 46.70 of oxygen, and 53.30 of a/wmmwrn. 



powder with 3 of potassa in a silver crucible, and evaporate to dry- 

 ness. Wash the mass in boiling distilled water, upon a filter, and the 

 white substance which remains is pure silica.t Its color is white ; its 

 specific gravity 2.66.— Tr.] 



[* Brande {Manual of Chymistry) says it is tastdess. — Tr.] 



I^t This is the usual process, hut the silica always retains potassa, and the earth ob- 

 tained by simply reducing the colorless rock crystal to powder is more pure. (Brande'i 

 Manual of Chymistry, p. 235.)— Th.] 



