142 METALS AND THEIR COMBINATIONS. 



Glass is similar in composition to the better varieties of por- 

 celain. 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 chemi- 

 cally with the metals. Sodium and calcium are the chief metals 

 in common glass, though potassium, lead, and others are also 

 frequently used. The color is imparted to the glass by the ad- 

 dition 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 discovery of the artificial process for manu- 

 facturing it. 



Ultramarine is now manufactured on a very large scale by 

 heating a mixture 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 consti- 

 tution of it is yet unknown. 



Ultramarine is insoluble in water and is decomposed by acids 

 with liberation of sulphuretted hydrogen, which shows the 

 presence of sulphide of sodium. A green ultramarine is now 

 also manufactured. 



Analytical reactions. 



1. To solution of an aluminium salt add potassium or sodium 

 hydrate: a white gelatinous precipitate of aluminium hydrate 

 is produced, which is soluble in excess of the alkali. 



2. To aluminium solution add ammonium hydrate : the same 

 precipitate as above is obtained, but it is insoluble in an excess 

 of the reagent. 



3. The carbonates of ammonium, sodium, or potassium pro- 

 duce the same precipitate with liberation of carbon dioxide. 

 (See explanation above.) 



4. Ammonium sulphide produces the same precipitate with 

 liberation of sulphuretted hydrogen: 



A1 2 C1 6 -f 3(NH 4 ) 2 S 4- 6H 2 O = A1 2 GHO + 6NH 4 C1 + 3H 2 S. 



