130 . GOLD. 



iron to form oxide of iron, which unites with the acid 

 to form proto-sulphate of iron, whilst the hydrogen 

 escapes. Green vitriol is, however, made on a large 

 scale, principally from pyrites, in the manner just 

 described (285). 



290. Sulphate of iron, when pure, is a green trans- 

 parent salt, wholly soluble in water ; exposed to the 

 air it becomes brown and earthy-looking, being par- 

 tially decomposed, owing to the absorption of oxygen 

 and formation of peroxide; when strongly heated, it 

 is wholly decomposed, water and sulphuric acid being 

 given off, and oxide of iron left. When pure proto- 

 Bulphate of iron is decomposed by an alkali, a gray 

 or black precipitate is formed of the protoxide ; after 

 a short time this precipitate becomes red, having 

 absorbed oxygen, and become peroxide. 



291. The number of different metals known to 

 chemists is very considerable, amounting in all to 

 forty-six; but of these the greater number are com- 

 paratively rare, and of little importance, not being 

 used for any practical purpose, and consequently 

 chiefly interesting in a scientific point of view. It 

 will, therefore, be sufficient to consider briefly the 

 leading characters of eight of the most important of 

 them (19). 



292. Gold is found native in a pure and separate 

 state ; it has very little affinity for oxygen, never 

 tarnishing or showing any tendency to oxidize in the 

 air either at common temperature or when strongly 

 heated. It is insoluble in acids, except in a mixture 



