COMPOSITION OF THE BASIC OXIDES AND HYDRA TES 63 



XV. COMPOSITION OF THE BASIC OXIDES 

 AND HYDRATES 



The composition of the compound which the element 

 mercury forms with oxygen has been already determined. Of 

 the other metallic elements in the foregoing list, it has been 

 found by experiment that their most important oxides have the 

 following percentage compositions : — 



Metallic Oxide. 



Alumina 



Baryta 



Lime ....... 



Oxide of chromium .... 



Cuprous oxide (the red oxide of copper) . 

 Cupric oxide (the black oxide of copper) . 



Ferrous oxide 



Ferric oxide (the red oxide of iron) . 



Litharge . 



Magnesia 



Oxide of manganese 



Peroxide of manganese (the black oxide) . 



Red oxide of mercury 



Oxide of potassium . 



Oxide of sodium 



Strontia . 



Oxide of zinc . 



Calculate the number of combining weights of each element 

 in the compounds, and express the results by the simplest 

 possible formulae. For example, if the quantities of 

 magnesium and oxygen per cent, of magnesia be divided by 

 their respective combining weights, viz. 24 and 16, the 

 number of combining weights of the two elements will be 

 found to be equal, viz. 2.5 for each element. Hence the 

 simplest possible formula for magnesia is MgO. Having 



