in THE DISCOVERY OF OXYGEN 47 



Calcination of iron . 3Fe + 2O 2 = Fe 3 O 4 (blue-black scale or 



(Magnetic ca l x .) 

 oxide of 

 iron.) 



Preparation of red oxide of mercury 

 (a) " Mercurius calcinatus per se " 



(b) " Red precipitate " 



(Mercuric (Nitric 



nitrate.) oxide.) 



2Hg(N0 3 ) 2 = 2HgO + 4 N0 2 + 2 . 



(Mercuric (Nitrogen 

 oxide.) peroxide.) 



Decomposition of red oxide of mercury 



2HgO = 

 nitric acid 



4 

 nitre 



(Potassium (Potassium . 

 nitrate.) nitrite.) 



Red lead contains more oxygen than litharge, but has not a 

 definite composition, and cannot be represented by any exact 

 formula. Its preparation and decomposition may be shown 

 thus 



where x is a fraction which is always less than 0-5. 



