OXYGEN GAS 19 



From these experiments it must therefore be concluded that 

 the air is not homogeneous, but consists of at least two gases 

 mixed together — (i) an active gas — oxygen — constituting about 

 one-fifth of the air, and (2) an inactive gas — nitrogen — consti- 

 tuting about four-fifths; and that when substances undergo 

 changes due to atmospheric action, such as rusting and burn- 

 ing, cheviical combi7iatio7i of the substance with the oxygen of 

 the air occurs, chemical compounds being formed having pro- 

 perties entirely different from the original substances. These 

 substances, if they be not themselves compounds, are termed 

 elements. The compounds of oxygen may be called oxides. 

 Thus, iron rust is oxide of iron, and the product of the com- 

 bustion of phosphorus is oxide of phosphorus. 



IV. OXYGEN GAS 



The nitrogen gas left behind when the oxygen has been 

 removed from air by burning phosphorus or rusting iron is 

 distinguished by its inability to support combustion or to 

 promote rusting. It is now necessary to try to procure the 

 oxygen gas of the air in the pure state, in order that its pro- 

 perties may also be studied. 



The oxide of iron (iron rust) already produced contains 

 oxygen derived from the air. Can the oxygen be obtained 

 from it ? Since heat decomposes many chemical compounds, 

 such as the constituents of wood and coal with the production 

 of gas, try heating some iron rust in a test-tube. Should no 

 decomposition occur at the highest temperature obtainable, try 

 heating the oxides of other metals, e.g. the black oxide of 

 copper, the red oxide of mercury, and the red oxide of lead 

 (red lead), all of which can be obtained by gently heating the 



