92 A YEAR IN SCIENCE 



bottle and forces the water out. After the bottle is 

 filled with oxygen, it should be removed with a cover 

 over the mouth and kept right side up, because oxygen 

 is 1.1 times as heavy as air and if left inverted would 

 soon run out. 



Mercuric oxide is a compound sometimes used to 

 obtain oxygen. It is a red powdery substance which, 

 when thoroughly heated, separates into its component 

 parts, mercury and oxygen: 



Mercuric oxide = mercury and oxygen. 



In this connection mercuric oxide is also of his- 

 torical interest, for it was with this compound that in 

 1774 Joseph Priestley, an English scientist, carried on 

 experiments that led to the discovery of oxygen. 



When larger quantities of oxygen are desired potas- 

 sium chlorate, a compound containing a large amount 

 of oxygen, is used. It has been found that potassium 

 chlorate gives up its oxygen more quickly if a black, 

 finely powdered substance, manganese dioxide, is 

 mixed with it. Although manganese dioxide contains 

 oxygen, it gives up none of it in this process. The 

 sole function of the black powder is to hasten the 

 breaking up of the potassium chlorate. The change 

 which takes place may be expressed as follows : 



Potassium chlorate = potassium chloride + oxygen. 



Properties. On examining a bottle of pure oxygen, 

 it will be found colorless, odorless, and tasteless. It 

 is a very active element and combines readily with 

 many substances. With some it combines so actively 



