OXIDES. 87 



Galena (4e) is by far the most important ore of lead (lead 87, 

 sulphur 13). 



Copper-glance (3e) is a common ore of copper (copper 80, 

 sulphur 20). 



Cinnabar (3h) is the ore from which mercury or quicksilver 

 is obtained (mercury 86, sulphur 14). 



Pyrites (5d), one of the most common of minerals, is a com- 

 pound of iron and sulphur (iron 47, sulphur 53). 



Erubescite (5e), Copper pyrites (5f), and Tetrahedrite, or Grey 

 Copper-ore (7a), are valuable copper-ores. 



III. Common salt, the chloride of the metal sodium, is repre- Chlorides, etc. 

 sented in case 8f, and Fluor-spar, a compound belonging to the 



same division, begins at case 7e. 



IV. The next division consists of compounds of oxygen and Oxides, 

 includes most of the stony minerals : 



Cuprite (10a), an important ore of copper (copper 89, oxygen 

 11), is at first ruby-red in colour, but becomes blackened by 

 exposure to light. 



Spinel (lOe), in its transparent varieties, is one of the pre- 

 cious stones : the deep red is the Spinel Euby (less dense and 

 less hard than the true Euby), the rose-tinted is the Balas Euby, 

 and the yellow or orange-red the Eubicelle of the jewellers : 

 sometimes, it has a dark blue colour. On account of their 

 hardness, the less valuable specimens are used for the jewelling 

 of watches. 



Magnetite, or Magnetic Iron-ore (lOf), is the richest ore of iron, 

 of which it contains 72 per cent. It is the natural loadstone. 



Uraninite, or Pitchblende (lOh), consists mainly of oxygen 

 and uranium, but contains traces of helium and radium, of 

 which latter it is the source. 



Chrysoberyl (9e) is another of the precious stones. The beauti- 

 ful greenish-yellow variety, almost equal in lustre and hard- 

 ness to the Sapphire, is the Oriental Chrysolite of the jewellers ; 

 another variety, with a peculiar play of light, is the true Cat's- 

 eye ; while a third, green by sunlight, but red by candle- or 

 lamp-light, is known as Alexandrite. 



Corundum (9f ), when clear and of the proper colour, is, after 

 the Diamond, the most precious of stones. When pure it is colour- 

 less, but with minute traces of colouring ingredient it assumes 



