ALUMINUM. 163 



sition of the aluminum oxide, but by the united efforts of these two 

 substances decomposition is accomplished : 



A1 2 3 4- 30 + 6C1 = SCO + AL^CV 



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. 



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 homogo- 

 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 

 have a coloring effect, as, for instance, manganese violet, cobalt blue, 

 chromium green, etc. 



Ultramarine is a beautiful blue substance, found in nature as the mineral 

 " lapis lazuli," which was highly valued by artists as a color before the dis- 

 covery of the artificial process for manufacturing it. 



Ultramarine is now manufactured on a very large scale by heating a mix- 

 ture of clay, sodium sulphate and carbonate, sulphur, and charcoal in large 

 crucibles, when decomposition takes place and the beautiful blue compound 

 is obtained. As neither of the substances used in the manufacture has a ten- 

 dency to form colored compounds, the formation of this blue ultramarine is 

 rather surprising, and the true chemical constitution of it is yet unknown. 



Ultramarine is insoluble in water and is decomposed by acids with libera- 

 tion of hydrogen sulphide, which shows the presence of sodium sulphide. A 

 green ultramarine is now also manufactured. 



