290 METALS AND THEIR COMBINATIONS. 



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

 chromium green, etc. 



Cement or hydraulic mortar is the name given to a finely powdered 

 mineral, consisting chiefly of basic silicates of lime and alumina, and having 

 the power of forming an insoluble solid mass when mixed with water. Some 

 native limestones, containing also magnesium carbonate and aluminum silicate, 

 furnish cement after being heated to expel water and carbon dioxide. Other 

 cements are made by burning mixtures of limestone and clay of a suitable com- 

 position. The slag of iron furnaces also furnishes the material for cement. 



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. The approximate formula of the 

 blue compound is Na 2 S 2 .4NaAlSiO 4 . 



Tests for aluminum. 

 (Use about a 5 percent, solution of alum or aluminum sulphate.) 



1. To the solution add solution of potassium or sodium hydroxide : 

 a faintly bluish-white gelatinous precipitate of aluminum hydroxide, 

 A1(OH) 3 , is produced. The physical appearance of the precipitate is 

 characteristic. It is soluble in excess of the alkali, forming an 

 al-uminate, thus : 



A1(OH) 3 + 3NaOH = Al(ONa) 3 + 3H 2 O. 



This shows that A1(OH) 3 has weak acid character toward strong alka- 

 lies. It is reprecipitated on adding ammonium chloride and heating. 

 Aluminum hydroxide is soluble in acids, even acetic acid. 



2. To the solution add ammonia water : the same precipitate as 

 above is obtained, but it is insoluble in an excess of the reagent (dif- 

 ference from zinc) and also in ammonium chloride solution (difference 

 from magnesium). 



3. A solution of a carbonate produces the same precipitate as 

 above, with liberation of carbon dioxide, not very noticeable in dilute 

 solutions (see explanation in text). 



