ALUMINIUM. 141 



metal, its oxide, hydrate, or carbonate in hydrochloric acid. 

 Accordingly, aluminium chloride maybe obtained in solution : 



A1. 2 6HO + 6HC1 = A1 8 C1 6 + 6H 2 O. 



On evaporating the solution to dryness, however, and heating 

 the dry mass farther with the view of expelling all water, de- 

 composition takes place, hydrochloric acid escapes, and alu- 

 minium oxide is left : 



A1 2 C1 6 + 3H 2 = A1 2 S + 6HC1. 



Aluminium chloride cannot, consequently, be obtained in a 

 pure state (free from water) by this process, but it may be 

 made by exposing a heated mixture of aluminium oxide and 

 carbon to the action of chlorine. Neither carbon nor chlorine 

 alone causes any decomposition of the aluminium oxide, but by 

 the united efforts of these two substances decomposition is 

 accomplished : 



A1 2 O 3 -f 30 + 6C1 = 3CO + A1 2 C1 6 . 



Clay is the name applied to a large class of mineral sub- 

 stances, differing considerably in composition, but possessing in 

 common the two characteristic features of plasticity, and the 

 predominance of aluminium silicate in combination with water. 



The various kinds of clay have been formed in the course of 

 time from such double silicates as feldspar and others, by a 

 process which is partly of a mechanical, partly of a chemical 

 nature, and consists chiefly in the disintegration of these rocks 

 and a removal of potassium and sodium by the chemical action 

 of carbonic acid, water, and other agents. 



The various kinds of clay are used in the manufacture of 

 bricks, earthenware, stoneware, porcelain, etc. The process of burn- 

 ing these substances accomplishes the hardening by expelling 

 the water which is present in the clay. Pure clay is white ; the 

 red color of the common varieties is due to the presence of 

 ferric oxide. For china or porcelain clay is used containing 

 silicates of the alkalies which, in burning, melt, causing the 

 production of a more homogeneous mass, while in common 

 earthenware the pores, produced by expelling the moisture, 

 remain unfilled. 



