ALUMINUM. 289 



Aluminum chloride, A1C1 3 . This compound is of interest on account of 

 being the salt from which the metal was formerly obtained. Most chlorides 

 may be formed by dissolving the metal, its oxide, hydroxide, or carbonate in 

 hydrochloric acid. Accordingly aluminum chloride may be obtained in 

 solution : 



A1(OH) 3 + 3HC1 = A1C1 3 + 3H 2 O. 



On evaporating the solution to dryness, however, and heating the dry mass 

 further with the view of expelling all water, decomposition takes place, hydro- 

 chloric acid escapes, and aluminum oxide is left: 



2A1CL, + 3H 2 O = A1 2 3 -f 6HC1. 



Aluminum chloride, consequently, cannot be obtained in a pure state (free 

 from water) by this process, but it may be made by exposing to the action of 

 chlorine a heated mixture of aluminum oxide and carbon. Neither carbon 

 nor chlorine alone causes decomposition of the aluminum oxide, but by the 

 united efforts of these two substances decomposition is accomplished : 



A1 2 O 3 f 3C + GC1 = 3CO + 2A1CL,. 



Clay is the name applied to a large class of mineral substances, 

 differing considerably in composition, but possessing in common the 

 two characteristic features of plasticity and the predominance of 

 aluminum silicate in combination with water. A white clay, known as 

 kaolin, consists chiefly of a silicate of the composition 



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 disintegra- 

 tion of 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 burning these substances accom- 

 plishes the hardening by expelling 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 homoge- 

 neous mass, while in common earthenware the pores, produced by expelling 

 the moisture, remain unfilled. 



Glass is similar in composition to the better varieties of porcelain. 

 All varieties of glass are mixtures of fusible, insoluble silicates, made 

 by fusing silicic acid (white sand) with different metallic oxides or 

 carbonates, the silicic acid combining chemically with the metals. 

 Sodium and calcium are the chief metals in common glass, though 

 potassium, lead, and others also are frequently used. Color is im- 

 parted to the glass by the addition of certain metallic oxides, which 



19 



