ALUMINA. 91 



gun-flints tiil they become brittle, then pulverize 

 them. Mix this powder with three or four times its 

 weight of carbonate of potash, and fuse the mixture 

 in a crucible, by a strong red heat. We shall thus 

 obtain a compound of alkali and siliceous earth : 

 dissolve it in water, and add to it diluted muriatic 

 or sulphuric acid ; a precipitation will take place, 

 which, when well washed, is pure silex. 



Siliceous earth, when pure, is white and tasteless. 

 It is infusible by itself, and insoluble in water. It 

 has a harsh feel, and does not form a cohesive mass 

 with water. 



No acid can act upon it, except the hydro- 

 fluoric, which dissolves it. When mixed with an 

 equal weight of carbonate of potash, and fused in a 

 strong furnace, it forms glass. With a larger pro- 

 portion of alkali it forms a substance soluble in 

 water, which has been called silicated alkali. The 

 solution of this was called liquor ofjiints. The silex 

 is precipitated from it in the state of a gelatinous 

 hydrate by acids. 



It is supposed that silica consists of oxygen 

 united to a certain base, which has been assumed 

 to be a metallic substance, and w^liich has been 

 called Silicium : but its real nature has not been 

 ascertained. It is imagined, however, that silicium 

 forms an alloy with iron, and that the properties of 

 some sorts of iron are owing to the addition of this 

 substance. 



ALUMINA. 



This earth forms a part of all clays, and hence has 

 been called argillaceous earth. It exists also in nu- 

 merous rocks, particularly slate, and even consti- 

 tutes some of the hardest gems and stones, as the 

 sapphire, ruby, and corundum. 



