138 NON-METALS AND THEIR COMBINATIONS. 



takes place actually in two stages. At first potassium chlorate gives up but 

 one-fifth of its total oxygen, forming potassium chloride and perchlorate, 

 KC10 4 , thus : 



5KC1O 3 = 3KC1O, + 2KC1 + 3O. 



This part of the decomposition takes place at a comparatively low temper- 

 ature ; after it is complete, the temperature rises considerably and the decom- 

 position of the perchlorate begins : 



KC1O 4 = KC1 + 4O. 



If the potassium chlorate be mixed with 30-50 per cent, of man- 

 ganese dioxide, and this mixture be heated, the liberation of oxygen 

 takes place with greater facility and at a lower temperature than by 

 heating potassium chlorate alone. Apparently, the manganese dioxide 

 takes no active part in the decomposition, as its total amount is found 

 in an unaltered condition after all potassium chlorate has been decom- 

 posed by heat. A satisfactory explanation regarding this action of 

 manganese dioxide is yet wanting. 



A third method is to heat to redness, in an iron vessel, manganese 

 dioxide (MnO 2 ), which suffers then a partial decomposition : 

 3MnO 2 = Mn g O 4 -f 2O. 



In this case there is liberated but one-third of the total amount of 

 oxygen present, while two-thirds remain in combination with the 

 manganese. 



Other methods of obtaining oxygen are : Decomposition of water by elec- 

 tricity, heating of dichromates, nitrates, barium dioxide, and other substances, 

 which evolve a portion of the oxygen contained in the molecules. 



Heating a concentrated solution of bleaching powder with a small quantity 

 of a cobalt salt (cobaltous chloride) furnishes a liberal supply of oxygen, the 

 calcium hypochlorite of the bleaching powder being decomposed into calcium 

 chloride and oxygen : 



Ca(ClO) 2 = CaCl 2 + 2O. 



Oxygen may be obtained at the ordinary temperature by adding water to a 

 mixture of powdered potassium ferricyanide and barium dioxide, and also by 

 the decomposition of potassium permanganate and hydrogen dioxide in the 

 presence of dilute sulphuric acid. 



A commercial method operated largely in England is Erin's process, which 

 consists in pumping purified air under pressure over barium oxide contained 

 in a tube and heated to about 700 C., whereby barium dioxide is formed. The 

 accumulated nitrogen of the air escapes by a valve from the end of the tube. 

 When formation of barium dioxide is complete, the air-supply is cut off and 

 the pump is reversed, thus producing a partial vacuum in the tube. Under 

 this condition, although the same temperature is maintained as before, the 



