THE ATMOSPHERE 



81 



Oxygen. Carl Wilhelm Scheele, a Swedish chemist, 

 first prepared oxygen in 1771 by heating mercuric oxide 

 and also by other methods. On August 1, 1774, Joseph 

 Priestley, an English experimenter, without knowing 

 anything of Scheele's work, obtained oxygen by heat- 

 ing mercuric oxide. Priestley's apparatus consisted of a 

 bottle of mercury inverted in 

 a bath of the same liquid. 

 A little mercuric oxide was 

 floated on top of the mercury 

 in the bottle. He then fo- 

 cused sunlight on the oxide 

 by means of a burning lens. 

 The heat from the sunlight 

 caused the oxide to disappear 

 and a colorless gas appeared 

 in its place. When he intro- 

 duced a glowing splinter into 

 the gas, the spark burst into 

 flames. He later put a live 

 mouse into the gas and it con- 

 tinued to live. 



Scheele called 



the gas, " Fire air " and 

 Priestley called it " Good air." 

 Both of these are appropriate names, as we shall soon 

 see. 



Although oxygen is one of the chief constituents of 

 the air, we have to use some means of entrapping it before 

 we can obtain it in its pure state. Oxygen unites with 

 many metals, and the compounds formed are the oxides 

 of the metals. We may obtain oxygen from many of its 

 compounds, but of these potassium chlorate is probably 

 the best for laboratory purposes. Potassium chlorate 



FIG. 82. Diving Bell. 



