OXYGEN. 67 



oxygen takes place with greater facility and at a lower tempera- 

 ture than by heating potassium chlorate by itself. 



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 decomposed by 

 heat. A satisfactory explanation regarding this action of man- 

 ganese dioxide is yet wanting. 



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

 manganese dioxide, which then suffers a partial decomposition : 



3MnO 2 = Mti 3 O 4 + 2O. 



In this case but one-third of the total amount of oxygen present 

 is liberated, while two-thirds remain in combination with the 

 manganese. 



Other methods of obtaining oxygen are decomposition of 

 water by electricity, heating of chromates, nitrates, barium 

 dioxide, sulphuric acid, and other substances, which evolve a 

 portion of the oxygen present in the molecules. 



The quantity of oxygen liberated from a given quantity of a 

 substance may easily be calculated from the atomic and mole- 

 cular weights of the substances entering into decomposition. 

 For instance : 100 pounds of oxygen may be obtained from how 

 many pounds of potassium chlorate, or from how many pounds 

 of manganese dioxide ? 



The molecular weight of potassium chlorate is found by 

 adding up the weights of 1 atom of potassium = 39 + 1 atom 

 of chlorine = 35.5 + 3 atoms of oxygen = 48 ; total = 122.5. 

 Every 122.5 parts by weight of potassium chlorate liberate the 

 weight of 3 atoms or 48 parts by weight of oxygen. If 48 are 

 obtained from 122.5, 100 are obtained from 255.2. 



48 : 122.5 : : 100 : x 



x = 255.2. 



In a similar manner, it will be found that 815.6 pounds of 

 manganese dioxide are necessary to produce 100 pounds of 

 oxygen. Mn0 2 = 55 + 32 = 87. 3 MnO 2 = 3 X 87 = 261. 

 Every 261 parts furnish 2 X 16 = 32 parts of oxygen. 



32 : 261 : : 100 : x 



x = 815.6. 



Physical properties. Oxygen is a colorless, inodorous, tasteless 

 gas ; up to a few years ago it had been looked upon as a perma- 



